Patent Publication Number: US-9851156-B2

Title: Molten-salt-heated indirect screw-type thermal processor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of thermal processing of materials, more particularly to thermal processing by indirectly heating a process material in a processor, and especially to molten-salt-heated indirect screw-type thermal processors. 
     2. General Background and State of the Art 
     U.S. Pat. No. 8,739,963 describes one of many available screw-type thermal processors. 
     Chinese Utility Model CN203479118U discloses a molten salt energy storage system capable of gravity flow salt evacuation without dependence on a salt evacuation pump. 
     INVENTION SUMMARY 
     It is an object of the present invention to provide superior performance and cost-effectiveness in the indirect thermal processing of materials. 
     It is also an object to provide a fluid-heated indirect thermal processor from which a hot heat transfer fluid, which would solidify or become unworkably viscous upon cooling to ambient temperature, will drain passively before solidifying in the event that its circulation through the processor is interrupted. 
     It is also an object to provide a fluid-heated indirect thermal processor which operates safely and economically, with heat transfer fluid circulating at temperatures in excess of 800° F., as high as 1100° F., and even higher, should salts usable at such temperatures become available to the apparatus, even when the heat transfer fluid is corrosive and has a solidification temperature as high as 480° F. 
     It is also an object to provide a fluid-heated indirect thermal processor in which the thermal processor—the portion of the apparatus which transfers heat from a heat transfer fluid to a feed material while conveying the feed material from a feed inlet to a feed outlet—is constructed and operated without any need of ASME pressure boundary certification. For example, in some cases the processor should heat transfer fluid at a pressure very near ambient pressure, i.e., at less than 1 bar, and should not be exposed directly to any source of high-pressure such as a heat transfer fluid circulation pump or the like. 
     It is also an object to provide a fluid-heated indirect thermal processor that operates safely and economically without dependency on a high-powered electric heating system in addition to a combustion heating system. Such independence in some cases avoids costs associated with access to a regional power grid and costs associated with redundant heating systems. 
     It is also an object to provide a fluid-heated indirect thermal processor that operates safely and economically without any need for a heat trace on the piping or on the processor itself. 
     It is also an object to provide a fluid-heated indirect thermal processor which operates safely and economically when started with cold, dehydrated salt, with cold, hydrated salt, or with hot hydrated or dehydrated salt. The apparatus should be able to hydrate a salt on-site, starting with hot or cold dehydrated salt. The apparatus also should be able to reach operating temperature gradually enough to avoid thermal shock to any part of the apparatus, especially the thermal processor, even if both hydration and dehydration of a salt are called for in an operating cycle. The apparatus should be capable of controlled shutdown with prompt, safe, passive disposal of hot salt at a low point in the system followed, if desired, by gradual hydration of hot salt for storage and for later re-starting from ambient temperature without prolonged melting. 
     In accordance with these objects and with others which will be described and which will become apparent, an exemplary embodiment of molten-salt-indirectly heated screw-type thermal processing apparatus has an indirectly heated screw-type thermal processor; a heater; a rundown tank; and a pump. The apparatus requires an operating volume of a heat transfer fluid for transferring heat from the heater to the thermal processor. The thermal processor has a heat transfer fluid inlet fluidly communicating with the heater and a heat transfer fluid outlet fluidly communicating with the rundown tank. The rundown tank has a fluid-containing portion dimensioned to hold at least the operating volume and has a rundown tank headspace portion above the fluid-containing portion. The rundown tank headspace portion is equipped to relieve a pressure differential between the rundown tank and the ambient environment. 
     The pump, the heater, the thermal processor and the rundown tank are operatively connected so as, when the pump is active, to establish a heat transfer circulation loop through the heater and the thermal processor. 
     The pump, the heater, the thermal processor and the rundown tank are operatively connected so as, when the pump is inactive, to establish the fluid-containing portion as the fluid passive drainage destination relative to the pump, the heater and the thermal processor. 
     Another exemplary embodiment has a gravity tube, a gravity tube upper drain, a gravity tube gas orifice, and a gravity tube lower drain. The gravity tube fluidly communicates with the heat transfer fluid inlet at a first height. The gravity tube fluidly communicates with the heater at a second height, the second height being above the first height. 
     The gravity tube upper drain also fluidly communicates with the gravity tube at a third height, the third height being above the second height. The gravity tube upper drain fluidly communicates with the rundown tank. The gravity tube gas orifice fluidly communicates with the gravity tube at a fourth height, the fourth height being above the third height. The gravity tube gas orifice also fluidly communicates with the rundown tank headspace portion (and may be regarded as having, e.g., a connector tube running to the rundown tank headspace portion for this purpose). 
     The gravity tube lower drain fluidly communicates with the gravity tube at a fifth height, the fifth height being below the first height. The gravity tube lower drain fluidly communicates with the rundown tank at a sixth height, the sixth height being below the fifth height. 
     Another exemplary embodiment has a restrictor located in the gravity tube lower drain at a seventh height, the seventh height between the fifth height and the sixth height. The restrictor is dimensioned to restrict fluid conducting capacity of the gravity tube lower drain, thereby assuring that most of the fluid flows through the processor while enough fluid flows through the restrictor to keep the gravity tube lower drain hot, preventing solidification and obstruction. 
     In another exemplary embodiment, the gravity tube has a fill tank and the gravity tube upper drain has a stem pipe, the fill tank communicating with the gravity tube at the second height, the stem pipe fluidly communicating with the fill tank at the third height and fluidly communicating with the gravity tube upper drain. 
     Another exemplary embodiment is selectively configurable to establish a preheating fluid circulation loop through the rundown tank and the heater and to interrupt the heat transfer circulation loop, so that such processes as melting, heating and hydration can be conducted within the rundown tank or within a loop between a heater and the rundown tank while the fluid is not circulating through the processor. Preferably, at least one heater is situated in the preheating fluid circulation loop. 
     In another exemplary embodiment, the rundown tank is equipped with a heat trace, the heat trace being coextensive with the preheating fluid circulation loop through the rundown tank, so that a path is available for circulating fluid, even when most of the fluid in the rundown tank has cooled and solidified. 
     Another exemplary embodiment has a fluid hydrator and a hydration fluid supply. The fluid hydrator fluidly communicates with the processor preheating fluid circulation loop and with the hydration fluid supply. The hydration fluid supply is selected from among a supply of water, a supply of steam, and a supply of a hydrating liquid solution. 
     In another exemplary embodiment, the fluid hydrator has a nozzle, located in the rundown tank headspace portion, configured to gently deposit a hydration fluid in the rundown tank, so that, for example, a molten salt can be hydrated without disrupting the fluid surface and splattering the fluid in the tank. 
     In another exemplary embodiment, the fluid hydrator has a sparge tube located in the fluid-containing portion of the rundown tank. Alternatively, the fluid hydrator has an eductor located somewhere in the preheating fluid circulation loop or in the heat transfer circulation loop. 
     Another exemplary embodiment is adapted for a heat transfer fluid having a melting point and a density. The first height and the second height are selected such that a column of the heat transfer fluid extending vertically from the first height to the second height exerts pressure at the second height no greater than 14.9 PSIG when the fluid is at the melting point. 
     Another exemplary embodiment has at least one external heater located at least partially outside the rundown tank, the preheating fluid circulation loop passing through the external heater. 
     Another exemplary embodiment has a pressure sensor proximate the heat transfer fluid inlet of the thermal processor and a pump variable speed control, the pump variable speed control being operatively coupled with the pressure sensor so as to slow the pump when the pressure sensor reports a pressure approaching 14.9 PSIG. 
     In an exemplary embodiment, the heat transfer fluid outlet is located above the first height and a path is provided from the heat transfer fluid inlet for passive drainage to the rundown tank when the pump is inactive. 
     In an exemplary embodiment, a vacuum breaker fluidly communicates with the heat transfer fluid outlet, preventing vacuum lock interference with drainage of fluid from the processor and preventing vapor lock interference with entry of fluid into the processor. 
     In an exemplary embodiment, the rundown tank has a sump and a heat trace; the pump has a pump inlet located in the sump or near enough thereto to ingest fluid heated by the heat trace. 
     In an exemplary embodiment, underpressurization of the rundown tank is relieved by introduction of a padding gas, preferably an inert padding gas such as nitrogen to avoid contaminating a molten salt with carbonates formed from atmospheric carbon dioxide. 
     Also in accordance with the above objects, a method of operating a molten-salt-indirectly heated screw-type thermal processor includes the steps of: 
     providing a molten-salt-indirectly heated screw-type thermal processor, the thermal processor having an operating heat transfer fluid temperature range, an operating heat transfer fluid flow rate range and an operating heat transfer fluid pressure range; 
     providing a body of heat transfer fluid, a heater, and a rundown tank, 
     the heat transfer fluid being capable of conveying heat from the heater to the thermal processor at a temperature within the operating heat transfer fluid temperature range while flowing into the thermal processor at a heat transfer fluid flow rate within the operating heat transfer fluid flow rate range at a pressure within the operating heat transfer fluid pressure range, 
     the heater being capable of heating the heat transfer fluid sufficiently at the flow rate and temperature, 
     the body of heat transfer fluid having volume at least sufficient to operate with the heater and the thermal processor, 
     the rundown tank having capacity more than sufficient to contain all of the body of heat transfer fluid; 
     delivering the heat transfer fluid from the heater to the thermal processor at the temperature, the flow rate and the pressure while delivering the heat transfer fluid from the thermal processor to the heater; and 
     after the steps of delivering, passively disposing the body of heat transfer fluid in the rundown tank. 
     An exemplary instance of the method may include, before the step of disposing, a step of producing the heat transfer fluid by melting a solid. While melting the solid, the fluid generated by melting can in some exemplary instances of the method be circulated within or through the rundown tank. In some exemplary instances, this circulation is done while interrupting the heat transfer circulation loop, so that the processor is not exposed to the fluid at that time. 
     Also in an exemplary instance of the method, the method may be carried out with a heat transfer fluid comprising a melting-point-altering material selected from among: water, a hydrating fluid, a dopant. Such a fluid could, for example, be delivered ready-made or produced nearby for introduction into the apparatus. 
     In an exemplary variation, before the step of disposing, there is a step of producing the heat transfer fluid by adding to a salt a melting-point-altering material selected from among: water, a hydrating fluid, a dopant. A preliminary step of melting a solid may be added if the fluid has solidified. 
     In an exemplary instance, after the step of disposing, there is a step of adding to the heat transfer fluid a melting-point-altering material selected from among: water, a hydrating fluid, a dopant, so that the fluid can be stored as a hydrated liquid. This is convenient when, for example, a stored solidified dehydrated salt would require prolonged steps of melting and hydration the next time the apparatus is operated. 
     In a variation of the method, the heat transfer fluid comprises a salt which is at least partially hydrated and which is at least partly dehydrated before the step of disposing the solution in the rundown tank. It is often preferable to dehydrate the salt fluid before or during the step of delivering the fluid to the processor. 
     The step of dehydrating and the step of delivering may be at least partially simultaneous. In an exemplary method, the thermal processor has a predetermined maximum tolerable rate of temperature increase, the salt has a melting temperature which increases with decreasing hydration, and the step of dehydrating occurs at a rate such that the thermal processor is warmed at a rate no greater than the maximum tolerable rate of temperature increase. 
     An exemplary method may be carried out with the step of delivering including steps of measuring the pressure, computing a correction of the pressure, and delivering the heat transfer fluid to the thermal processor at a flow rate adjusted to effect the correction of the pressure. 
     Preferably, in an exemplary method, the step of delivering includes a step of elevating the heat transfer fluid relative to the thermal processor so as to establish a gravity fluid pressure head with the heat transfer fluid entering the thermal processor at a pressure at least within the operating heat transfer fluid pressure range. 
     In a highly preferred method, the step of delivering includes a step of passively diverting the heat transfer fluid to bypass the thermal processor in an amount sufficient to prevent the pressure exceeding the operating heat transfer fluid pressure range. 
     In an exemplary instance, a fill tank fluidly communicates with the heater and with the thermal processor; a stem pipe fluidly communicates with the fill tank and with the rundown tank; the step of elevating includes accumulating the fluid in the fill tank; and the step of passively diverting includes directing the fluid via the stem pipe to the rundown tank. 
     An exemplary instance is carried out with a thermal processor having a heat transfer fluid inlet at a lower elevation than the heat transfer fluid outlet. In the step of delivering, the fluid enters the thermal processor via the heat transfer fluid inlet and exits the thermal processor via the heat transfer fluid outlet. In the step of disposing, the fluid exits the thermal processor via the heat transfer fluid inlet. 
     In an exemplary instance, a vacuum breaker fluidly communicates with the heat transfer fluid outlet. In the step of disposing, a gas enters the thermal processor via the vacuum breaker. 
     In an exemplary instance, the rundown tank has a rundown tank headspace portion and the vacuum breaker fluidly communicates with the rundown tank headspace portion. 
     In an exemplary instance, the fill tank has a fill tank headspace portion; the rundown tank has a rundown tank headspace portion; and a headspace connector fluidly communicates with the fill tank headspace portion and the rundown tank headspace portion. 
     In an exemplary instance, the operating heat transfer fluid pressure range is from −12 PSIG to 14.9 PSIG, inclusive. 
     An exemplary instance has, during the step of melting, a step of measuring a temperature of the solid being melted and a step of initiating the step of delivering when the temperature has reached a predetermined value. 
     In an exemplary instance, during the step of melting, there is a step of measuring a temperature of the solid being melted and a step of initiating the step of adding when the temperature has reached a predetermined value. 
     In an exemplary instance, during the step of dehydrating, steam is vented from the rundown tank headspace portion. 
     In an exemplary instance, the rundown tank has a rundown tank headspace portion and there is a step of supplying a padding gas to the rundown tank headspace portion when the rundown tank headspace portion is underpressurized relative to the ambient environment and a step of venting a gas from the rundown tank headspace portion when the rundown tank is overpressurized relative to the ambient environment. In an exemplary instance, during the step of dehydrating, steam is vented from the rundown tank headspace portion. 
     In an exemplary instance, during the step of disposing, there is a step of conducting a gas from the rundown tank headspace portion to the thermal processor fluid outlet via the vacuum breaker. 
     In an exemplary instance, the rundown tank has a rundown tank headspace portion. During the step of delivering, there is a step of conducting a gas from the thermal processor to the rundown tank headspace portion. 
     In an exemplary instance, during the step of adding, there is a step of measuring a melting-point-altering material content of the heat transfer fluid and a step of initiating the step of delivering when the material content has reached a predetermined value. 
     In an exemplary, although not necessarily preferred instance of the method, a step of hydrating a dehydrated salt heat transfer fluid is begun while the fluid is circulating in the heat transfer circulation loop, rather than after the fluid has passively drained to the rundown tank. 
     From experience with the dangers of rupturing a vessel containing molten heat transfer fluid, and mindful of the cost and operational limitations encountered when designing and building thermal processing apparatus with the types of steel that are certifiable for use as pressure boundary materials, the inventors sought a low-cost, high-reliability method of ensuring that heat transfer fluid is never delivered to the thermal processor at a pressure requiring a pressure boundary material. The present invention assures that the fluid is delivered to the thermal processor from a source which derives its pressure from a fluid column height under the influence of gravity and not directly from a pump or other source which could deliver higher pressures. The present invention also assures that, when pumping ceases, fluid located in the gravity tube and fluid located in the thermal processor will drain passively, rather than remaining in place and solidifying in place. 
     Mindful of a customer&#39;s interest in lowering costs of operating in remote rural locations, the inventors sought to avoid any dependency on power in or on a fail-safe pumping system to prevent costly and expensive solidification of molten salt heat transfer fluid inside the thermal processor or inside the piping to or from a heater. The present invention ensures prompt, passive draining of heat transfer fluid from the thermal processor, heater and gravity tube whenever pumping ceases. In the present invention, the rundown tank provides a reservoir at a low point in the apparatus. The rundown tank can receive fluid on its way from the thermal processor back to the heater. The rundown tank can receive drainage from any part of the apparatus at any time drainage is desired. The rundown tank can have a headspace which serves as a gas reservoir and which can be fluidly connected with, e.g., the heater, the fill tank, and the thermal processor. 
     By setting the first and second heights to limit pressure at the processor fluid inlet to no greater than 14.9 PSIG, the present invention tailors the processor fluid inlet pressure to the object of avoiding the necessity of using ASME pressure boundary materials and construction. 
     The inventors chose to equip the rundown tank to vent a gas to the ambient environment and to receive a gas from a source selected from other parts of the apparatus or from a padding gas supply. Being familiar with the special requirements of various heat transfer media, they sought to avoid deleterious effects of carbon dioxide and oxygen absorbed from the atmosphere. When the heat transfer fluid is chemically and physically compatible with the constituents of the Earth&#39;s atmosphere, such as when the molten salts are below 850° F. in the environment in question, air can be admitted through the gravity tube gas orifice. When it is preferable to close the system, a connector can be used to assure fluid communication between the gravity tube gas orifice and a headspace of another component of the apparatus—preferably, the rundown tank, also in some cases a fill tank. Pressure differentials between the respective headspaces of such components as a thermal processor, a fill tank and a heater can be relieved in this manner, relieving local and systemic pressure differentials while in many cases avoiding loss of gas to the environment or intrusion of atmosphere from the environment. When the operational cycle of the apparatus at times requires the introduction of a gas to compensate for an overall pressure reduction in the apparatus, a padding gas, e.g., nitrogen, is introduced via the gravity tube gas orifice or through a similar orifice in, e.g., a headspace of a rundown tank or thermal processor. 
     Exemplary embodiments of the apparatus in accordance with the present invention include a gravity tube and a gravity tube upper drain. Some embodiments also include a fill tank and a stem pipe. The inventors wished to provide a consistent source of heat transfer fluid to the thermal processor, even when the rate of delivery of such fluid from the heater fluctuates. This arrangement delivers heat transfer fluid from an elevated reservoir at a head of pressure proportional to the difference between the height of the fluid level in the fill tank and the height of the heat transfer fluid inlet of the thermal processor. As long as enough fluid is being delivered to keep the gravity tube fed, the pressure at the thermal processor fluid inlet will be within a narrow range. The total column height from the stem pipe opening to the heat transfer fluid inlet sets the upper limit of the range; the total column height between the bottom of the fill tank and the heat transfer fluid inlet  38  sets the lower limit. When a gross excess of fluid is delivered to the fill tank, the stem pipe efficiently drains the excess to the rundown tank. When pumping ceases, fluid in the fill tank and fluid that has already entered or passed through the gravity tube will drain passively, one way or another, under the influence of gravity. 
     In some exemplary embodiments the rundown tank disposes the fluid volume so as to provide a fluid upper surface suitable for hydration by gentle deposition of water mist on the surface. The inventors found sufficient surface area for hydration to be important, because it facilitates the use of salts which have high operating temperatures and correspondingly high melting temperatures. The inventors, wishing to avoid splattering molten salt during hydration, arranged for the hydration water dispenser to provide a mist fine enough not to disrupt the surface of the salt. 
     The rundown tank has a headspace. Gas may flow from the rundown tank headspace to the gravity tube gas orifice, so that air and its carbon dioxide and oxygen constituents are not drawn into the system. The inventors sought to avoid the formation of carbonates in the salt. 
     In an embodiment having a rundown tank, the rundown tank is equipped to heat the fluid, and the pump and the rundown tank are configured selectively to circulate the fluid between the pump and the rundown tank. The inventors were aware of difficulties that attend the operation of fluid-heated indirect thermal processors. With a heat transfer fluid which is a salt that solidifies at a temperature well above ambient, such as 288° F. or 448° F., it may be necessary to start the apparatus after the salt has cooled and solidified in the rundown tank. Sometimes, it is desirable to heat a portion of the rundown tank surrounding a pump located there until a small volume of salt has liquefied, start the pump, and recirculate the salt to the rundown tank via the bypass branch. When enough salt has liquefied, the heater can be started and liquefied salt can be delivered to the thermal processor. 
     In some applications, the inventors contemplate the use of a salt which has a high melting point—so high, that a cold thermal processor would not withstand the temperature gradients caused by the abrupt introduction of the melted salt. The inventors solved this problem by recognizing that the in some cases the salt may be hydrated, lowering the temperature at which it liquefies. To provide water for hydration, the rundown tank has a set of water misting nozzles. The process of hydrating a salt may be started with hot salt at a time when the salt is circulating in the apparatus at a high temperature after warming cold salt in the rundown tank. However, it is preferable to hydrate the salt in the rundown tank, using the water misting nozzles while recirculating the salt to the rundown tank. At shutdown, hot dehydrated salt is drained to the rundown tank; if rehydration is desired, it is done by recirculating fluid to the rundown tank with the rundown tank vent open and the nozzles activated, beginning hydration at about 300° F. for some salts and about 500° F. for others, and continuing until the salt is fully hydrated at a temperature close to ambient. 
     An exemplary embodiment of the apparatus has a restrictor located in the gravity tube lower drain at a seventh height below the first height and above the fifth height. The inventors intended that only enough fluid would drain through the restrictor to keep this path heated, thereby keeping it open. An additional requirement, however, was that passive drainage be accomplished before salt in the apparatus has time to solidify. In the present invention, the restrictor directs most of the fluid flow to the thermal processor. Only a small fraction of the fluid flow drains through the restrictor. Nevertheless, this fraction is large enough to permit passive drainage of molten salt to be completed in about 30 minutes. 
     In an exemplary embodiment of the apparatus, the heat transfer fluid outlet is located above the first height, i.e., the level of the processor fluid inlet. The processor is inclined or otherwise so constructed that the fluid drains passively out the processor fluid inlet when pumping has stopped. With a vacuum breaker or a headspace connector fluidly communicating with the heat transfer fluid outlet at a relative high point, any vapor lock during filling or vacuum lock during drainage can be relieved. 
     The method is practicable even when it includes, while the pump is active, a step of circulating the fluid in the apparatus at a temperature in excess of 1000° F. 
     Also in accordance with the present invention, an exemplary embodiment of a phase-separating pressure modulator for molten-salt-indirectly heated screw-type thermal processing apparatus comprises a fill tank having a fill tank bottom portion; a heater output tube fluidly communicating with the fill tank at the fill tank bottom portion; a gravity tube fluidly communicating with the fill tank at the fill tank bottom portion and fluidly communicating with a fluid delivery destination; a stem pipe fluidly communicating with the fill tank at an elevation above the fill tank bottom portion; a fill tank headspace portion defined as a portion of the fill tank above the elevation; and a fill tank headspace vent fluidly communicating with the fill tank headspace portion and with a fluid drainage destination. Preferably, the drainage destination is a rundown tank and the fluid delivery destination is a thermal processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the objects and advantages of the present invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawing, in which like parts are given like reference numbers and wherein: 
         FIG. 1  is a schematic representation of A FIRST EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention; 
         FIG. 2  is a schematic representation of SECOND and THIRD EXEMPLARY EMBODIMENTS thereof; 
         FIG. 3  is a schematic representation of A FOURTH EXEMPLARY EMBODIMENT thereof; 
         FIG. 4  is a schematic representation of A FIFTH EXEMPLARY EMBODIMENT thereof; 
         FIG. 5  is a schematic representation of A SIXTH EXEMPLARY EMBODIMENT thereof; 
         FIG. 6  is a schematic representation of A SEVENTH EXEMPLARY EMBODIMENT thereof; and 
         FIG. 7  is a schematic representation of AN EIGHTH EXEMPLARY EMBODIMENT thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described.  FIG. 1  illustrates in schematic view A FIRST EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , having fluidly interconnected a thermal processor  22 , a rundown tank  24 , a pump  26 , and a heater  28 . 
     The thermal processor  22  has heat transfer fluid spaces  32  with heat transfer fluid inlets  38  and heat transfer fluid outlets  84 . The thermal processor  22  has a process space  34  with a process material inlet  82  and a process material outlet  86 . A fluid outlet drain tube  85  fluidly connects the heat transfer fluid outlets  84  to the rundown tank  24 . The thermal processor  22  is configured to transfer heat between the heat transfer fluid spaces  32  and the process space  34 . A body of heat transfer fluid  25  (fluid  25 ) is shown in the rundown tank  24 . 
     A processor fluid inlet tube  87  fluidly connects the heat transfer fluid inlets  38  with the heater output tube  80  of the heater  28 . Heat transfer fluid  25  flows from the heater  28 , enters through the heat transfer fluid inlets  38 , flows through the heat transfer fluid spaces  32 , and exits through the heat transfer fluid outlets  84 . The heat transfer fluid spaces  32  are passively drainable. The heat transfer fluid spaces  32  are configured to deliver fluid  25  passively to the rundown tank  24  via the fluid outlet drain tube  85 . 
     A conveyor  90  is disposed in the process space  34 . Process material enters through the process material inlets  82 , receives heat from the heat transfer fluid spaces  32  while the conveyor  90  moves it through the process space  34 , and exits through the process material outlet  86 . As illustrated, the conveyor  90 , which is disposed within the process space  34 , includes one of the heat transfer fluid spaces  32 . Another  32  surrounds the process space  34 . 
     The pump  26  is configured to propel fluid  25  from the rundown tank  24  to the heater  28 . 
     The heater  28  is passively drainable. The heater  28  is configured to guide the fluid  25  upwardly while heating the fluid  25  and then to deliver the fluid  25  to the thermal processor  22 . 
     With reference to  FIG. 1 , where the apparatus preferably is arranged in a gravitational field, the processor fluid inlets  38  are at higher elevations in the apparatus; the rundown tank  24  is lowermost; and the heater  28  and thermal processor  22  are at intermediate elevations. 
     The rundown tank  24  has a rundown tank fluid containing portion  41  with capacity to hold the entire volume of fluid  25  required by the apparatus, and a gas-accommodating rundown tank headspace portion  40  above the rundown tank fluid containing portion  41 . The fluid  25  is shown occupying the rundown tank fluid-containing portion  41 . The rundown tank headspace portion  40  is equipped with a rundown tank headspace vent  71  providing the ability to relieve a pressure differential relative to the ambient environment, and with a padding valve  64  providing the ability to admit a padding gas to the rundown tank headspace portion  40  to relieve underpressure when air is to be excluded. Additionally, it often is desirable to fluidly connect the rundown tank headspace portion  40  with other gas-containing spaces in the apparatus, e.g., the  32  of the thermal processor  22 , to equalize pressure differentials between such spaces when one of them is filling and another is emptying. Such structure, e.g. tubing, is explicitly drawn and described elsewhere herein. 
     With continued reference to  FIG. 1 , in a first mode of operation, associated with the thermal processing of a process material, the apparatus transfers heat continuously from the heater  28  to the thermal processor  22 . The pump  26  urges the fluid  25  to flow in a heat transfer circulation loop through the heater  28  and the thermal processor  22 , i.e., through the heater  28 , where the fluid  25  is heated, to the thermal processor  22 , where the fluid  25  deposits heat, and back to the heater  28 . In  FIG. 1 , the rundown tank  24  is drawn as being in this heat transfer circulation loop. It should be understood that a tube conveying fluid  25  from the thermal processor  22  to the heater  28  might have sufficient capacity to be regarded as being the rundown tank  24  for purposes of the FIRST exemplary embodiment. However, certain other embodiments described herein will require the rundown tank  24  to be equipped with sensors and to create a fluid surface suitable for hydration. 
     With continued reference to  FIG. 1 , it also should be understood that the location and interconnection of the pump  26  may vary, so long as it urges the fluid  25  to travel in the heat transfer circulation loop and so long as the pump  26  drains passively when it is not activated. In this regard, a pump has the advantage that, properly oriented and connected, it allows unimpeded passive drainage when it is not pumping. 
     The heater  28  heats the fluid  25 . The fluid  25  then flows via the heater outlet tube  80 , through the heat transfer fluid inlet  38  to the heat transfer fluid spaces  32  of the thermal processor  22 . Heat flows from the heat transfer fluid spaces  32  to the process space  34  of the thermal processor  22 . The fluid  25  occupies the heat transfer fluid space  32  and then flows from the heat transfer fluid space  32  via the heat transfer fluid outlet  84  via the heat transfer fluid outlet drain tube  85  to the rundown tank  24 . While this mode of operation continues, the pump  26  once again urges the fluid  25  to flow in the heat transfer circulation loop. 
     With continued reference to  FIG. 1 , in a second mode of operation, the flow of fluid  25  in the heat transfer circulation loop abruptly or unexpectedly ceases—as might occur if the pump  26  stops or the heat transfer circulation loop, heater  28 , thermal processor  22 , or rundown tank  24  loses integrity while fluid  25  is flowing in the apparatus (see first mode of operation, above), or if for any reason it is desired to stop the apparatus. 
     After the pump  26  has stopped, fluid  25  in the heater  28  tends to flow backwards from the heater  28 , through the pump  26 , into the rundown tank  24 . As mentioned, the heater  28  is passively drainable: no pumping is necessary in order for fluid  25  in the heater  28  to drain out of the heater  28 . 
     After the pump  26  has stopped, heat transfer fluid  25  in the heater output tube  80  tends to flow either backward to the heater  28  or forward into the heat transfer fluid spaces  32  of the thermal processor  22 . In this circumstance, fluid  25  in the heat transfer fluid spaces  32  of the thermal processor  22  tends to flow to the rundown tank  24 . As mentioned, the heat transfer fluid spaces  32  are passively drainable: no pumping is necessary in order for fluid  25  in the heat transfer fluid spaces  32  to drain out of the heat transfer fluid space  32 . With particular reference to  FIG. 1 , as the apparatus is drawn in this figure, the heat transfer fluid  25  enters the heat transfer fluid space  32  from above and exits the heat transfer fluid space  32  to below. Alternatively, the relative elevations of the heat transfer fluid inlet  38  and heat transfer fluid outlet  84  may be approximately equal, and the fluid  25  would nevertheless drain from the heat transfer fluid space  32  to the rundown tank  24 . 
     Because the heater  28  and heat transfer fluid space  32  are passively drainable, it is practicable to configure these structures such that, in the event that the pump  26  abruptly or unexpectedly stops while fluid is in the heat transfer circulation loop, the fluid  25  will drain down to the rundown tank  24  passively, rather than remaining elsewhere in the heat transfer circulation loop. Preferably, the fluid  25  that drains passively is collected in the rundown tank  24 ; however, as mentioned above, a different structure, not strictly regarded as a tank but suitably dimensioned and equipped and located at a low elevation in the heat transfer circulation loop, may serve adequately in this FIRST embodiment. 
       FIG. 2  is a schematic view of A SECOND EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , having a thermal processor  22  with heat transfer fluid spaces  32 , heat transfer fluid inlets  38 , heat transfer fluid outlets  84 , process space  34 , conveyor  90 , process material inlet  82  and process material outlet  86 ; a rundown tank  24  with rundown tank fluid-containing portion  41  (containing a fluid  25 ) and rundown tank headspace portion  40 ; a pump  26 , and a heater  28 . The heater  28  has a heater outlet  78  and a heater output tube  80 . 
     A gravity tube  204  fluidly communicates with the heat transfer fluid inlet  38  at a first height  211  and with the heater output tube  80  at a second height  212 . The second height  212  is above the first height  211 . 
     A gravity tube upper drain  206  fluidly communicates with the gravity tube  204  at a third height  213 . The third height  213  is above the second height  212 . The gravity tube upper drain  206  also fluidly communicates with the rundown tank headspace portion  40 . 
     A gravity tube gas orifice  208  fluidly communicates with the gravity tube  204  at a fourth height  214 . The fourth height  214  is above the third height  213 . The gravity tube gas orifice  208  fluidly communicates with the rundown tank headspace portion  40 . 
     A gravity tube lower drain  210  fluidly communicates with the gravity tube  204  at a fifth height  215 . The fifth height  215  is below the first height  211 . The gravity tube lower drain  210  fluidly communicates with the rundown tank headspace portion  40  at a sixth height  216 . The sixth height  216  is below the fifth height  215 . 
     The gravity tube  204  feeds fluid  25  to the thermal processor  22 , and also to the rundown tank headspace portion  40  via the gravity tube lower drain  210 . The gravity tube upper drain  206  conducts excess fluid flow to the rundown tank headspace portion  40  as will be discussed in more detail below. The gravity tube gas orifice  208  relieves pressure differentials should they develop between the gravity tube  204  and the rundown tank headspace portion  40 . The gravity tube lower drain  210  provides for passive drainage from the thermal processor  22  to the rundown tank  24  should fluid  25  cease to flow in the heat transfer circulation loop and, while fluid  25  is flowing, maintains enough fluid flow to avoid being obstructed by solidifying cooled fluid  25 . 
     With regard to the thermal processor  22  as drawn in  FIG. 2 , notably, the heat transfer fluid inlets  38  are at elevations below the elevations of the respective heat transfer fluid outlets  84 . The thermal processor  22  has a thermal processor low portion  35  and a thermal processor high portion  37 . The heat transfer fluid inlets  38  are located on the thermal processor low portion  35  on the conveyor  90 . The heat transfer fluid outlets  84  are located on the thermal processor  22  high portion  37  and on the conveyor  90 . A fluid outlet drain tube  85  fluidly connects the heat transfer fluid outlets  84  to the rundown tank headspace portion  40 . The fluid outlet drain tube  85  incorporates a vacuum breaker  92 . A vacuum breaker connector tube  93  fluidly connects the vacuum breaker  92  with the rundown tank headspace portion  40 . The vacuum breaker  92  fluidly communicates with the heat transfer fluid outlets  84  and the vacuum breaker connector tube  93  at an eighth height  218  within a range between the first height  211  and the second height  212 . 
       FIG. 2  is now referenced digressively to introduce A THIRD EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, having a restrictor  46  located in the gravity tube lower drain  210  at a seventh height  217 . The seventh height  217  is between the fifth height  215  and the sixth height  216 . The restrictor  46  has a restrictor cross-section, the heat transfer fluid outlet  84  of the thermal processor  22  has a combined heat transfer fluid outlet cross-section, and the restrictor cross-section is a predetermined fraction of the heat transfer fluid outlet cross-section. 
     With continued reference to  FIG. 2  and to the SECOND and THIRD exemplary embodiments, in the previously discussed first mode of operation, with the pump  26  activated and a body of fluid  25  provided for the system, the fluid  25  flows from the heater outlet  78 , via the heater output tube  80 , to the gravity tube  204 , where it flows downward under the influence of gravity. A major portion of the fluid  25  passing through the gravity tube  204  flows through the heat transfer fluid inlets  38  at the thermal processor low portion  35 , into the heat transfer fluid spaces  32 , where it fills the heat transfer fluid spaces  32  and eventually exits the heat transfer fluid outlets  84  at the thermal processor high portion  37  and flows through the fluid outlet drain tube  85  to the rundown tank headspace portion  40 . A minor portion of the fluid  25  passing through the gravity tube  204  passes through the restrictor  46  to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 2 , in the previously discussed second mode of operation, after the pump  26  has stopped, fluid  25  in the heater output tube  80  and gravity tube  204  tends to flow either backwards through the heater outlet  78  into the heater  28  and from there ultimately into the rundown tank  24 , or down the gravity tube  204 , into the gravity tube lower drain  210 , through the restrictor  46 , into the rundown tank headspace portion  40 . Fluid  25  in the heat transfer fluid spaces  32  tends to flow backward through heat transfer fluid inlets  38  at the thermal processor low portion  35 , into the gravity tube lower drain  210 , through the restrictor  46 , to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 2 , in a third mode of operation, while the pump  26  is active and fluid  25  is flowing in the apparatus, the rate at which fluid  25  passes from the heater output tube  80  to the gravity tube  204  exceeds the rate at which fluid  25  enters the heat transfer fluid inlets  38 . One way in which this could occur is for the heat transfer fluid  25  to flow slowly through heat transfer fluid spaces  32  of the thermal processor  22  due to increased viscosity there during a cold startup or due to some other impedance of flow. In any event, in this third mode of operation, the fluid  25  tends to accumulate in the gravity tube  204 . In this situation, it may be desirable to establish equilibrium of fluid inflow and outflow of the gravity tube  204 . For example, it may be desirable to do so by reducing the output of the pump  26  or heater  28 . However, it might transpire that the pump output is not reduced or not sufficiently reduced. Alternatively, or simultaneously, it may be desirable that the fluid overflows without detriment through gravity tube upper drain  206  at height  213 . In any event, the apparatus tolerates fluid  25  exiting the heater output tube  80  at a rate greater than the rate at which fluid  25  is entering the heat transfer fluid space  32  of the thermal processor  22 , by accommodating the excess flow. 
     A first means of accommodating excess flow from the heater output tube  80  to the gravity tube  204  in this manner is for the gravity tube  204  to be configured to deliver fluid  25  to the rundown tank  24  while also delivering fluid  25  to the heat transfer fluid space  32 . As excess fluid  25  accumulates in the gravity tube  204 , its pressure due to gravity increases and the combined rates at which the fluid  25  flows from the gravity tube  204  to the heat transfer fluid space  32  and from the gravity tube  204  through the gravity tube lower drain  210  through the restrictor  46  to the rundown tank headspace portion  40  may approach the rate at which fluid  25  is flowing from the heater  28  to the gravity tube  204 . Accomplishing this simultaneous flow is one motivation the inventors had for fluidly connecting the gravity tube  204  to the rundown tank headspace portion  40  as well as to the heat transfer fluid inlets  38 . The restrictor  46  serves to establish the heat transfer fluid space  32  of the thermal processor  22  as a favored path of flow, directing a major portion of the fluid flow into that path, while at the same time assuring continuous flow of a minor portion of the fluid flow through the restrictor  46  and into the rundown tank headspace portion  40 , so that solidification and consequent blockage do not occur there. 
     A second means of accommodating this excess flow from the heater  28  into the gravity tube  204  by increasing its rate of exit from the gravity tube  204  is for the gravity tube  204  to be configured such that fluid  25  passively drains to the rundown tank headspace portion  40  via yet another path when, despite flowing both via the heat transfer fluid spaces  32  and via the gravity tube lower drain  210 , the heat transfer fluid  25  continues to accumulate in the gravity tube  204 . To make this happen, the gravity tube upper drain  206  fluidly connects the gravity tube  204  to the rundown tank headspace portion  40 . When fluid  25  in the gravity tube  204  rises to the third height  213 , the gravity tube upper drain  206  carries excess fluid  25  directly to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 2 , in a fourth mode of operation, the apparatus is started from a rest condition in which most or all of the fluid  25  is in the rundown tank  24  and little or none of the fluid  25  is in the heater  28  or thermal processor  22 . Different conditions may prevail at such a time; they will be discussed separately in various parts of this detailed description. Here we describe a first manner of starting the apparatus from such a rest condition. 
     The rundown tank  24  has a rundown tank fluid-containing portion  41  and the pump  26 , wherever located, is fluidly connected so as to be able to urge fluid  25  in the rundown tank fluid-containing portion  41  into and through the heater, wherever the heater  28  is located, and into the gravity tube  204 . One suitable variety of pump  26  is a centrifugal pump having an impeller configured to capture fluid  25  centrally and accelerate fluid  25  radially within a housing which directs radially accelerated fluid  25  to, or draws ingestible fluid  25  from, the heater  28 . One suitable location for the pump  26  is at a relatively low elevation within the rundown tank fluid-containing portion  41 ; another is outside the rundown tank  24  and connected to draw fluid  25  therefrom; another is downstream of the heater  28  and upstream of the gravity flow tube. The heater  28  may be located within the rundown tank  24 , outside the rundown tank  24 , or part inside and part outside, either upstream or downstream of the pump. Whatever the spatial relationships between pump, heater  28  and rundown tank  24 , the result is the provision of fluid  25  via the heater output tube  80  to the gravity flow tube  204 . 
     The pump  26  starts. The pump  26  causes fluid  25  to emerge from the heater outlet  78  and flow through the heater output tube  80  into the gravity tube  204 . The fluid  25  enters the gravity tube  204 , possibly accumulating in the gravity tube  204 , building up pressure in the gravity tube  204 . Under this pressure, the fluid  25  flows through the gravity tube  204  to the heat transfer fluid spaces  32  of the thermal processor  22 . Heat flows from the heat transfer fluid spaces  32  to the process space  34 . As the fluid  25  accumulates in the heat transfer fluid spaces  32 , it builds up pressure there. When the fluid  25  reaches the respective heights of the heat transfer fluid outlets  84 , it flows via the fluid outlet drain tube  85  to the rundown tank  24 . While this mode of operation continues, the pump  26  once again pressurizes the fluid  25  and the fluid  25  once again flows to the heater  28 . The heat transfer circulation loop is established. 
       FIG. 3  is a schematic view of A FOURTH EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , having structure and interconnection as described for the THIRD exemplary embodiment with reference to  FIG. 2 , but for the presence of a fill tank  30  fluidly communicating with the gravity tube  204  at the second height  212  and with the gravity tube upper drain  206  at the third height  213 , and the presence of a stem pipe  48  fluidly communicating with the fill tank  30  at the third height  213  and also with the gravity tube upper drain  206  (the stem pipe  48  provides the means by which the gravity tube upper drain  206  opens upwardly within the fill tank  30 ). 
     The apparatus of this FOURTH exemplary embodiment operates largely as described for the THIRD exemplary embodiment with reference to  FIG. 2 , with a few exceptions. The fill tank  30  provides an increased capacity—a reservoir—of fluid. The stem pipe  48  has an opening in the fill tank  30  at the third height  213  and directs fluid  25  into gravity tube upper drain  206 , from which the fluid  25  flows to the rundown tank headspace  40  whenever the fluid level in the fill tank  30  rises above the third height  213 . The gravity tube gas orifice  208  fluidly communicates with the fill tank  30  at a higher level, which could be regarded as being within a fill tank headspace portion  72 —high enough that it is exposed to gas, but not to liquid, and thus remains open to relieve gas pressure differentials between the rundown tank headspace portion  40  and the fill tank  30 . 
       FIG. 4  is a schematic view of A FIFTH EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 . This FIFTH exemplary embodiment is selectively configurable to establish a preheating fluid circulation loop through the rundown tank  24  and the heater  28  and to interrupt the heat transfer circulation loop. The apparatus has a thermal processor  22  with heat transfer fluid spaces  32 , heat transfer fluid inlets  38 , heat transfer fluid outlets  84 , process space  34 , conveyor  90 , process material inlet  82  and process material outlet  86 ; a rundown tank  24  with rundown tank fluid-containing portion  41  (containing a fluid  25 ) and rundown tank headspace portion  40 ; a pump  26 , and a heater  28 . The heater  28  has a heater outlet  78  and a heater output tube  80 . A fill tank  30 —specifically, a fill tank bottom portion  36 , fluidly communicates with the heater output tube  80  and with a gravity tube  204  at a second height  212  and with a gravity tube upper drain  206  at a third height  213 . The fill tank  30  has a fill tank headspace portion  72 . A stem pipe  48  fluidly communicates with the fill tank headspace portion  72  at the third height  213  and also fluidly communicates with the gravity tube upper drain  206 . 
     The fill tank  30  provides an increased capacity—a reservoir—of fluid. The stem pipe  48  has an opening in the fill tank  30  at the third height  213  and directs fluid  25  into gravity tube upper drain  206  communicating to the rundown tank headspace  40  whenever the fluid level in the fill tank  30  exceeds the third height  213 . Gravity tube  204  directs fluid  25  to the gravity tube lower drain  210  and to the heat transfer fluid inlets  38 . The gravity tube gas orifice  208  fluidly communicates with the fill tank headspace  72  at the fourth height  214 —high enough that it is exposed to gas, but not to liquid, and thus remains open to relieve gas pressure differentials between the rundown tank headspace portion  40  and the fill tank  30 . 
     The gravity tube  204  fluidly communicates with the heat transfer fluid inlets  38  at a first height  211  and with the fill tank bottom portion  36  at the second height  212 . The second height  212  is above the first height  211 . 
     The gravity tube upper drain  206  fluidly communicates with the fill tank headspace portion  72  at the third height  213 . The third height  213  is above the second height  212 . The gravity tube upper drain  206  also fluidly communicates with the rundown tank headspace portion  40 . 
     A gravity tube gas orifice  208  fluidly communicates with the gravity tube  204  at a fourth height  214 . The fourth height  214  is above the third height  213 . The gravity tube gas orifice  208  fluidly communicates with the rundown tank headspace portion  40 . 
     The gravity tube lower drain  210  fluidly communicates with the gravity tube  204  at a fifth height  215 . The fifth height  215  is below the first height  211 . The gravity tube lower drain  210  fluidly communicates with the rundown tank headspace portion  40  at a sixth height  216 . The sixth height  216  is below the fifth height  215 . 
     The gravity tube  204  feeds fluid  25  to the thermal processor  22  via the heat transfer fluid inlets  38  and also feeds fluid  25  to the rundown tank headspace portion  40  via the gravity tube lower drain  210 . The gravity tube upper drain  206  conducts excess fluid flow to the rundown tank headspace portion  40  as will be discussed in more detail below. The gravity tube gas orifice  208  relieves pressure differentials should they develop between the fill tank  30  and the rundown tank headspace portion  40 . The gravity tube lower drain  210  provides for passive drainage from the thermal processor  22  to the rundown tank  24  should fluid  25  cease to flow in the heat transfer circulation loop and, while fluid  25  is flowing, maintains enough fluid flow to avoid being obstructed by solidifying cooled fluid. 
     The heat transfer fluid inlets  38  are at elevations below those of the respective heat transfer fluid outlets  84 . The thermal processor  22  has a thermal processor low portion  35  and a thermal processor high portion  37 . The heat transfer fluid inlets  38  are located on the thermal processor  22  low portion  35 . The heat transfer fluid outlets  84  are located on the thermal processor  22  high portion  37 . A fluid outlet drain tube  85  fluidly connects the heat transfer fluid outlets  84  to the rundown tank headspace portion  40 . The fluid outlet drain tube  85  incorporates a vacuum breaker  92 . A vacuum breaker connector tube  93  fluidly connects the vacuum breaker  92  with the rundown tank headspace portion  40 . The vacuum breaker  92  fluidly communicates with the heat transfer fluid outlets  84  and the vacuum breaker connector tube  93  at an eighth height  218 . The eighth height  218  is above the first height  211  and below the second height  212 . 
     A restrictor  46  is located in the gravity tube lower drain  210  at a seventh height  217 . The seventh height  217  is between the fifth height  215  and the sixth height  216 . The restrictor  46  has a restrictor cross-section, the heat transfer fluid outlet of the thermal processor  22  has a heat transfer fluid outlet cross-section, and the restrictor cross-section is a predetermined fraction of the heat transfer fluid outlet cross-section. 
     With continued reference to  FIG. 4 , in the previously discussed first mode of operation, with the pump  26  activated and a body of fluid  25  provided for the system, the fluid  25  flows from the heater outlet  78 , via the heater output tube  80 , to the fill tank bottom portion  36 , where it accumulates and builds pressure. Meanwhile, from the fill tank bottom portion  36 , the fluid  25  flows into the gravity tube  204  and downward therein under the influence of gravity. A major portion of the fluid  25  passing through the gravity tube  204  flows through the heat transfer fluid inlets  38  at the thermal processor low portion  35 , into the heat transfer fluid spaces  32 , where it fills the heat transfer fluid spaces  32  and eventually exits the heat transfer fluid outlets  84  at the thermal processor high portion  37 , and flows through the fluid outlet drain tube  85  to the rundown tank headspace portion  40 . A minor portion of the fluid  25  passing through the gravity tube  204  passes through the gravity tube lower drain  210  and the restrictor  46  to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 4 , in the previously discussed second mode of operation, after the pump  26  has stopped, fluid  25  in the heater output tube  80 , fill tank bottom portion  36 , and gravity tube  204  tends to flow either backwards through the heater outlet  78  into the heater  28  and from there ultimately into the rundown tank  24 , or down the gravity tube  204 , into the gravity tube lower drain  210 , through the restrictor  46 , into the rundown tank headspace portion  40 . Fluid  25  in the heat transfer fluid spaces  32  tends to flow backward through heat transfer fluid inlets  38  at the thermal processor low portion  35 , into the gravity tube lower drain  210 , through the restrictor  46 , to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 4 , in a third mode of operation, while the pump  26  is active and fluid  25  is flowing in the apparatus, the rate at which fluid  25  passes from the heater output tube  80  to the fill tank bottom portion  36  exceeds the rate at which fluid  25  enters the heat transfer fluid inlets  38  of the thermal processor  22 . One way in which this could occur is for the transfer fluid  25  to flow slowly through heat transfer fluid spaces  32  of the thermal processor  22  due to increased viscosity there during a cold startup or due to some other impedance of flow. However this third mode of operation occurs, the fluid tends to accumulate in the fill tank bottom portion  36 . In this situation, it may be desirable to establish equilibrium of fluid inflow and outflow of the fill tank bottom portion  36 . For example, it may be desirable to do so by reducing the output of the pump  26  and heater  28 . However, it might transpire that the pump output is not reduced or not sufficiently reduced. Alternatively, or simultaneously, it may be desirable for fluid  25  to exit the heater output tube  80  at a rate greater than the rate at which fluid  25  is entering the heat transfer fluid spaces  32  of the thermal processor  22 , in which case the excess flow must be accommodated. 
     A first means of accommodating excess flow from the heater output tube  80  to the fill tank  30  and then into the gravity tube  204  in this manner is for the gravity tube  204  to be configured to deliver fluid  25  to the rundown tank  24  while also delivering fluid  25  to the heat transfer fluid space  32 . As excess fluid  25  accumulates in the fill tank  30 , its level rises, its pressure increases, and the combined rates at which the fluid  25  flows from the gravity tube  204  to the heat transfer fluid spaces  32  and from the gravity tube  204  through the gravity tube lower drain  210  through the restrictor  46  to the rundown tank headspace portion  40  may approach the rate at which fluid  25  is flowing from the heater  28  to the gravity tube  204 . Accomplishing this simultaneous flow is one motivation the inventors had for fluidly connecting the gravity tube  204  to the rundown tank headspace portion  40  via the gravity tube lower drain  210  as well as to the thermal processor  22  via the heat transfer fluid inlets  38 . The restrictor  46  serves to establish the heat transfer fluid space  32  of the thermal processor  22  as a favored path of flow, directing a major portion of the fluid flow into that path, while at the same time allowing flow of a minor portion of the fluid flow through the restrictor  46  and into the rundown tank headspace portion  40  as long as fluid  25  is flowing from the fill tank  30  to the gravity tube  204 , so that solidification and consequent blockage does not prevent drainage to the rundown tank headspace portion  40 . 
     A second means of accommodating this excess flow from the heater  28  into the fill tank  30  by increasing its rate of exit from the fill tank  30  is for the fill tank  30  to be configured such that fluid  25  passively drains to the rundown tank headspace portion  40  via yet another path when, despite flowing both via the heat transfer fluid space  32  and via the gravity tube lower drain  210 , the heat transfer fluid  25  continues to accumulate in the fill tank  30 . To make this happen, the stem pipe  48  fluidly communicates with the gravity tube upper drain  206 , which is fluidly connected to the rundown tank headspace portion  40 . When fluid  25  in the fill tank  30  rises to the third height  213 , the stem pipe  48  conducts excess fluid  25  into the gravity tube upper drain  206 , where it flows directly to the rundown tank headspace portion  40 . 
     With continued reference to  FIG. 4 , in a fourth mode of operation, the apparatus is started from a rest condition in which most or all of the fluid  25  is in the rundown tank  24  and little or none of the fluid  25  is in the heater  28 , fill tank  30  or thermal processor  22 . Different conditions may prevail at such a time; they will be discussed separately in various parts of this detailed description. Here we describe a first manner of starting the apparatus from such a rest condition. 
     The rundown tank  24  has a rundown tank fluid-containing portion  41  and the pump  26 , wherever located, is fluidly connected so as to be able to urge fluid  25  in the rundown tank fluid-containing portion  41  into and through the heater  28 , wherever the heater  28  is located, and into the fill tank  30 . One suitable variety of pump  26  is a centrifugal pump having an impeller configured to capture fluid  25  centrally and accelerate fluid  25  radially within a housing which directs radially accelerated fluid  25  to, or draws ingestible fluid  25  from, the heater  28 . One suitable location for the pump  26  is at a relatively low elevation within the rundown tank fluid-containing portion  41 ; another is outside the rundown tank  24  and connected to draw fluid  25  therefrom; another is downstream of the heater  28  and upstream of the fill tank  30 . The heater  28  may be located within the rundown tank  24 , outside the rundown tank  24 , or part inside and part outside, either upstream or downstream of the pump  26 . Whatever the spatial relationships between pump  26 , heater  28  and rundown tank  24 , the result is the provision of fluid  25  via the heater output tube  80  to the fill tank  30  and from there to the gravity tube  204 . 
     The pump  26  starts. The pump  26  causes fluid  25  to emerge from the heater outlet  78  and flow through the heater output tube  80  into the fill tank  30 . The fluid  25  flows from the fill tank bottom portion  36  into the gravity tube  204 , building up a head of pressure in the gravity tube  204 . Under this pressure, the fluid  25  flows from the gravity tube  204  through the heat transfer fluid inlets  38  to the heat transfer fluid spaces  32  of the thermal processor  22 . Heat flows from the heat transfer fluid spaces  32  to the process space  34 . As the fluid  25  accumulates in the heat transfer fluid spaces  32 , it builds up pressure there due to its depth. When the fluid  25  reaches the height of the heat transfer fluid outlets  84 , it flows out the heat transfer fluid outlets  84 , through the fluid outlet drain tube  85 , to the rundown tank  24 . While this mode of operation continues, the pump  26  once again pressurizes the fluid  25  and the fluid  25  once again flows to the heater  28 . The heat transfer fluid circulation loop is established. 
     The immediately previous description of the FIFTH embodiment with reference to  FIG. 4  addresses cases in which the fluid  25  already is in a condition to flow through the apparatus—i.e., cases in which the fluid  25  is not too viscous to flow in a manner approximating that described with reference to the first mode of operation, continuous transfer of heat. That description can be called a first manner of starting the apparatus. However, there are other situations that must be dealt with. 
     To address cases in which the fluid  25  must be rendered less viscous by heating in order for it to flow in a manner approximating that described with reference to the first mode of operation, continuous transfer of heat, a second manner of starting the apparatus is described, accounting separately for how to establish the described flow of fluid  25  through the rundown tank  24 , through the pump  26 , through the heater  28 , through the fill tank  30 , and through the heat transfer fluid spaces  32  of the thermal processor  22 . A major portion, if not all, of the fluid  25  is located in the rundown tank fluid-containing portion  41 . The fluid  25  is too viscous to flow efficiently through the apparatus. The fluid  25  is of a type whose viscosity decreases with increasing temperature. It is necessary to render the fluid  25  less viscous by heating it. 
     The rundown tank  24  has a rundown tank headspace portion  40 . The rundown tank  24  has a sump  77  that is equipped with a heat trace  66  and has pedestal  68  supporting the pump  26 . The rundown tank  24  is equipped with heating elements  184 . The rundown tank  24  is equipped with a relief valve  62  and also with a padding valve  64  that is connected to a padding gas supply tube  182 . The rundown tank  24  is equipped with a sump temperature sensor  186 , a rundown tank hydration measuring device  188 , and a rundown tank temperature sensor  190 . A set of hydration fluid misting nozzles  60  is located in the rundown tank headspace portion  40  and is connected to a supply of a hydration fluid. 
     The heater  28 , as drawn in  FIG. 4 , is external to the rundown tank  24 . The heater  28  has a heater outlet  78 , a heater output tube  80 , and a heater inlet  54 . 
     The pump  26  has a pump outlet  52 . A pump output tube  55  fluidly connects the pump outlet  52  to the heater inlet  54 . A bypass valve  56  is located in the pump output tube  55 . A bypass branch tube  58  fluidly connects the bypass valve  56  to the rundown tank headspace portion  40 . 
     A heat trace  66  is located in the rundown tank  24 . The heat trace  66  warms the fluid  25  along a path, which may be called the heat trace path, originating in the rundown tank fluid-containing portion  41  and including a portion thereof, proximate the pump  26 , from which the pump  26  may draw fluid. When the heat trace  66  is activated to heat the fluid  25  along the heat trace path, the fluid  25  in the heat trace path is warmed until it is capable of flowing into the pump  26 . 
     The impeller of the pump  26  draws fluid  25  from the heat trace path and accelerates the fluid  25  within the pump  26  housing. The pump  26  housing directs the accelerated fluid  25  upward through the pump outlet  52 , into to the pump output tube  55 , through the heater inlet  54 . The fluid  25  enters the heater  28 , is heated as it moves through the heater  28 , and flows from the heater  28  to the fill tank  30 , thence to the thermal processor  22  and to the rundown tank  24 , as described herein previously with reference to the first mode of operation. 
     It may be deemed necessary to warm a substantial portion of the fluid  25  in the rundown tank  24  before beginning to introduce the fluid  25  to the heater  28 . To accomplish this objective, the bypass valve  56  (which may be regarded as a selector valve) is operated to allow the pump output tube  55  to fluidly communicate with the bypass branch tube  58  and not with the heater inlet  54 . With the heat trace  66  activated, when a sufficient amount of material is sufficiently warmed in the rundown tank fluid-containing portion  41  to provide fluid  25  to be circulated, the pump  26  is activated, delivering fluid  25  back to the rundown tank  24  via the bypass branch tube  58 . This process is continued until a sufficient volume of fluid  25  is warmed in the rundown tank  24  to support function of the heater. Then, the bypass valve  56  is operated to allow the pump output tube  55  to fluidly communicate with the heater inlet  54  and not with the bypass branch tube  58 . With the pump  26  continuing to run, the fluid  25  flows from the pump  26  to the heater  28 . When such condition is reached, the heat trace  66  becomes unnecessary and may be deactivated. 
     At other times, the fluid  25  must be rendered less viscous by the admixture of another material. For such times, a third manner of starting the apparatus is described. Once again, as discussed above, a major volume of the fluid  25  is located in the rundown tank  24 , is too viscous to flow efficiently through the apparatus, and is of a type whose viscosity decreases with increasing temperature. However, in this case, in order to avoid thermally shocking the processor  22 , it is necessary to gradually warm the processor  22  by circulating and gradually heating the heat transfer fluid  25 . With some types of heat transfer fluids, it may be that the viscosity of the fluid  25  being used is not low enough to flow efficiently until its temperature is so high that it could not be introduced into a cold thermal processor  22  without unacceptable thermal shock. Although the thermal processor  22  could be heated electrically or by other means, in some circumstances the inventors find it preferable to gradually warm the thermal processor  22  by circulating a mixture of fluids through the heat transfer circulation loop including the rundown tank  24 , pump  26 , heater  28 , fill tank  30 , and thermal processor  22 . 
     To accomplish this gradual warming of the thermal processor  22  requires several considerations. First to be considered, the fluid mixture is obtained from outside or is made within the apparatus. Second to be considered, the fluid mixture is first circulated through the apparatus at a temperature low enough to be safely introduced into the cold thermal processor  22 . Third to be considered, the fluid mixture is heated while being circulated through the apparatus. Fourth to be considered, when the circulating heated fluid mixture has heated the thermal processor  22  to a temperature at which the thermal processor  22  can safely receive a fluid  25  at the temperature at which continuous heating is to be performed, the fluid mixture circulating in the apparatus can be replaced by the fluid  25  that accomplishes continuous heating. 
     First, the fluid mixture is obtained from outside or is made within the circuit of the apparatus. Generating the fluid mixture is unnecessary if the fluid mixture is delivered as, for example, a hydrated salt solution. Where the fluid mixture must be generated on-site, one approach to generating the fluid mixture is to mist water onto the top of a volume of hot fluid  25  (a melted salt, to be more specific) in the rundown tank  24  and vent steam from the rundown tank headspace portion  40  to the ambient environment. A set of water-misting nozzles  60  is located in the rundown tank headspace portion  40 . The rundown tank  24  has a sump  77  that is equipped with a heat trace  66  and has pedestal  68  supporting the pump  26 . The rundown tank  24  is equipped with heating elements  184 . The rundown tank  24  is equipped with a relief valve  62  and also with a padding valve  64  which is connected to a padding gas supply tube  182 . The rundown tank  24  is equipped with a sump temperature sensor  186 , a rundown tank hydration-measuring device  188 , and a rundown tank temperature sensor  190 . 
     When starting with solidified salt occupying the rundown tank fluid-containing portion  41 , the fluid mixture is generated by heating the salt in the rundown tank  24  as described previously with reference to the second manner of starting the apparatus, then slowly misting water onto the melted salt fluid  25  and venting excess steam from the rundown tank  24  to the ambient environment while continuing to recirculate the melted salt through the rundown tank  24  via the bypass valve  56  and the bypass branch tube  58 . When the salt is sufficiently hydrated and its temperature is low enough to be introduced safely into the cold thermal processor  22 , the nozzles  60  are deactivated, the bypass valve  56  is closed and the next step can begin, namely, circulating the fluid mixture through the heat transfer circulation loop of the apparatus. 
     Another approach to generating the fluid mixture is to locate an eductor in the preheating fluid circulation loop.  FIG. 5 , a schematic representation of a SIXTH EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , shows apparatus resembling the FIFTH exemplary embodiment but differing in how it provides an additive to the fluid  25 . An eductor  232  is located in the rundown tank fluid-containing portion  41  and is fluidly connected to an additive supply tube  230 . The eductor  232  is immersed in or filled with the fluid  25 . The additive, e.g., water, steam or another hydrating fluid, enters from the additive supply tube  230  and mixes with the fluid  25  as the fluid  25  passes through the eductor  232 . As with the FIFTH exemplary embodiment, either the salt in the rundown tank  24  already is melted, or it must be melted by heating the salt in the rundown tank  24  as described previously with reference to the second manner of starting the apparatus. Once melted salt is available, the eductor  232  is operated and steam is vented from the rundown tank  24  to the ambient environment while the melted salt is recirculated to the rundown tank  24  via the bypass valve  56  and bypass branch tube  58 . When the salt is sufficiently hydrated and its temperature is low enough to be introduced safely into the cold thermal processor  22 , the eductor  232  is deactivated, the bypass valve  56  is closed and subsequent steps can begin, such as circulating the fluid mixture through the heat transfer circulation loop of the apparatus, adding heat to the hydrated salt, and dehydrating the hydrated salt. 
     Yet another approach to generating the fluid mixture is to locate a sparge tube  234  in the rundown tank fluid-containing portion  41 .  FIG. 6 , a schematic representation of a SEVENTH EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , shows apparatus resembling the SIXTH exemplary embodiment but once again differing in how it provides an additive to the fluid. A sparge tube  234  is located in the rundown tank fluid-containing portion  41  and is fluidly connected to an additive supply tube  230 . The sparge tube  234  is immersed in the fluid. The hydration fluid enters from the additive supply tube  230 , exits the finely perforated sparge tube  234 , and mixes with the fluid in the rundown tank fluid-containing portion  41 . As with the FIFTH and SIXTH exemplary embodiments, either the salt in the rundown tank  24  already is melted, or it must be melted by heating the salt in the rundown tank  24  as described previously with reference to the second manner of starting the apparatus. Once melted salt is available, the sparge tube  234  is operated and steam is vented from the rundown tank  24  to the ambient environment while the melted salt is recirculated to the rundown tank  24  via the bypass valve  56  and bypass branch tube  58 . When the salt is sufficiently hydrated and its temperature is low enough to be introduced safely into the cold thermal processor  22 , the sparge tube  234  is deactivated, the bypass valve  56  is closed and subsequent steps can begin, such as circulating the fluid mixture through the heat transfer circulation loop of the apparatus, adding heat to the hydrated salt, and dehydrating the hydrated salt. 
     Second, the fluid mixture is circulated through the heat transfer circulation loop of the apparatus at a first temperature low enough not to damage the cold thermal processor  22 . The pump  26  ingests the fluid mixture from the rundown tank  24  and propels the fluid mixture to the heater  28 . The fluid mixture flows from the heater  28  to the fill tank  30  and from the fill tank  30  to the heat transfer fluid spaces  32  of the thermal processor  22  and then to the rundown tank  24 . 
     Third, the heater  28  is activated and the heat transfer fluid is heated while being circulated through the apparatus. The temperature of the heat transfer fluid increases. The heat transfer fluid  25  warms the thermal processor  22 . As the temperature of the heat transfer fluid  25  increases, the degree of hydration decreases and steam is released. Dehydration of the fluid  25  has begun. 
     Fourth, dehydration of the fluid  25  continues: the fluid  25  is heated and steam is vented until the fluid  25  is suitably dehydrated (in many situations, anhydrous), and is further heated if necessary until it reaches a predetermined temperature at which continuous heating of process material in the thermal processor  22  is to be performed. Thus, the thermal processor  22  is safely and gradually heated from ambient temperature to operating temperature and heat shock is avoided. 
     An effect of heating the hydrated salt to a temperature at which it begins to dehydrate is the evolution of steam. Steam must be vented from the apparatus as the body of fluid  25  is dehydrated. In one approach to venting steam, the fill tank  30  has a fill tank steam vent  73 . A mixture of partially dehydrated salt and steam exits the heater  28  and passes via the heater output tube  80  to the fill tank bottom portion  36 . In the fill tank  30 , steam rises to the fill tank headspace portion  72  and exits via the fill tank steam vent  73 . Alternatively or additionally, exits the fill tank headspace portion  72 , via the gravity tube gas orifice  208  and passes to the rundown tank headspace portion  40 , which is vented to the ambient environment. In a fifth mode of operation, the apparatus has been operating in the first mode and now is to be shut down in a controlled manner. The manner of shutting the apparatus down depends largely on whether the fluid being used must be rehydrated in order to be used again to restart the apparatus—either because the salt being used melts at a temperature high enough to thermally shock the thermal processor  22 , or because the salt melts at a temperature higher than ambient temperature and melting it again when restarting the apparatus would be inefficient or inconvenient. 
     If the fluid will be usable to restart the apparatus without hydration, then a first manner of shutting down the apparatus is usable—this entails shutting down the input of external energy to the heater  28  and shutting down the pump  26 . As discussed previously with reference to the second mode of operation, which includes unintentional or unexpected interruption of the continuous heat transfer process, the fluid  25  drains passively from the heater  28 , the fill tank  30  and the thermal processor  22 , preferably into the rundown tank  24 . 
     If the fluid will require rehydration before being used again to restart the apparatus, a second manner of shutting down the apparatus is used, which entails interrupting input of external energy to the heater  28  and stopping the pump, so that the fluid  25  in the heat transfer circulation loop passively drains to the rundown tank  24 . Next, the procedure for hydrating a melted salt occupying the rundown tank  24  is performed, with the result that, at temperature equilibrium, the rundown tank fluid-containing portion  41  will be occupied by a hydrated salt solution which is liquid at ambient temperature. 
     As previously described, the procedure for hydrating a melted dehydrated salt in the rundown tank  24  entails interrupting the heat transfer circulation loop in the apparatus, establishing the preheating fluid circulation loop (the same loop that was used for preheating in the rundown tank), and then hydrating the salt. The approaches to hydrating the salt—more generally, mixing a melting-point-reducing additive into the salt—were discussed above and include the addition of a hydration fluid by means of such options as misting nozzles  60 , an eductor  232 , and a sparge tube  234 . The addition process continues until the freezing point of the circulating fluid  25  drops to a temperature low enough to permit circulation of the fluid  25  in the heat transfer circulation loop the next time the apparatus is to be operated. 
       FIG. 7 , a schematic representation of an EIGHTH EXEMPLARY EMBODIMENT of a molten-salt-indirectly heated screw-type thermal processing apparatus in accordance with the present invention, shown generally at  20 , shows apparatus resembling the FIFTH exemplary embodiment, differing chiefly in that heating is performed substantially within the rundown tank  24 , rather than by an external heater (see reference number  28  in previous figures). Externally-fed and exhausted transverse fire tubes  236  are located in the rundown tank fluid-containing portion  41 . 
     In the aforementioned first mode of operation, continuous processing, the fire tubes  236  and pump  26  are activated and fluid  25  flows in the heat transfer circulation loop. 
     In the second mode, shutdown, the fire tubes  236  and pump  26  are deactivated and the fluid  25  drains passively to the rundown tank fluid-containing portion  41 , which in this EIGHTH embodiment includes the fire tubes  236 . Subsequent rehydration may be done if a hydrated salt has been dehydrated during operation and it is desired to store the salt in a hydrated form. 
     When the apparatus is to be started and the fluid  25  has solidified in the rundown tank fluid-containing portion  41  and in the heater, the solidified fluid  25  must be melted. The fire tubes  236  are activated. As soon as a temperature measured by the rundown tank temperature sensor  190  proximate the pump  26  indicates that a usable amount of material has melted, the pump  26  may safely be started. The bypass valve  56  is operated to interrupt the heat transfer circulation loop and to establish the pre preheating fluid circulation loop—in this EIGHTH exemplary embodiment, the fluid  25  enters the pump  26  from portions of the fire tubes  236  near the pump, exits the pump outlet  52  and flows via the pump output tube  55  back to the rundown tank  24  via the bypass branch tube  58 . Eventually, the entire body of fluid  25  in the rundown tank  24  will melt. Alternatively, with the pump  26  in active, convection will eventually accomplish melting of the entire body of fluid  25 . 
     When the apparatus is to be started and it is deemed necessary to hydrate the fluid  25  before circulating it to the thermal processor  22 , the previously described steps for performing hydration are performed. With this EIGHTH exemplary embodiment, with melted salt is present in the rundown tank  24 , the pump  26  is activate, the fire tubes  236  are deactivated, the rundown tank  24  is vented and a hydration fluid is mixed with the fluid  25 . When the fluid temperature measured by the rundown tank temperature sensor  190  indicates a temperature consistent with hydration, or when the water content measured by the hydration measuring device  188  indicates a predetermined acceptable water content, the bypass valve  56  is operated to interrupt the preheating fluid circulation loop and establish the heat transfer circulation loop. Hydrated fluid  25  flows through the thermal processor  22 . With the pump  26  and fire tubes  236  activated and the rundown tank  24  vented, the temperature of the fluid  25  increases gradually until the operating temperature is reached, at which point the apparatus is operating in the first mode. 
     A variety of hydration fluids are usable to lower the melting point of a heat transfer fluid such as a salt. These include water, steam, and hydrating solutions containing other salts or metal salts—lithium salts, for example. 
     The salt contemplated for an exemplary embodiment in accordance with the present invention can be operated at up to 800° F. without an inert blanket and at up to 1100° F. with a nitrogen gas blanket to protect it from atmospheric carbon dioxide. The salt is a solid below 300° F. to 500° F. and must be melted prior to circulation. The salt has an advantage of moving more heat per unit volume pumped than other heat transfer fluids. 
     The rundown tank  24  is located below the processor and all other equipment, allowing gravity draining. The rundown tank  24  can be fitted internally, externally or a combination of both, with fire tubes  236 , electric heating, auxiliary heat transfer fluid tubing, thermal fluid jacketing, or steam coils to melt the salt and keep it molten for an extended time or even to be the heater. In substantially all applications, it is necessary to locate at least one heat source in or on the rundown tank  24 . Such equipment may, indeed, be used to heat the fluid  25  in the system, such that, for some applications, a heater  28  located outside of the rundown tank  24  may be unnecessary. Advantageously, all pumps for moving the heat transfer fluid  25  through the apparatus can be located in the rundown tank  24 , where they are immersed in the fluid  25  and where a reservoir of hotter, less viscous fluid  25  is likely to be available. 
     In a preferred embodiment of the apparatus in accordance with the present invention, a turbine pump or a centrifugal pump is employed. The drive motor is above the rundown tank  24 . When the pump  26  stops, fluid  25  in the pump output tube  55  can drain backwards through the pump  26  into the rundown tank  24 . 
     The inventors evaluated many molten salts for use in accordance with the present invention. Two salts commonly used in heat transfer applications, both sold by Coastal Chemical Company, are HITEC® brand eutectic salt mixture and HITEC SOLAR® brand salt mixture. HITEC SOLAR® is used mostly for heat storage, because it is less costly. Both of these two salt products have the same heat capacity for heat transfer: approximately 4.9 to 5.75 Btu/gallon/degree ° F. See, http://www.skyscrubber.com/MSR%20%20HITEC%20Heat%20Transfer%20Salt.pdf. 
     HITEC® brand heat transfer salt, formerly known as “HTS,” is a eutectic mixture of potassium nitrate, sodium nitrite, and sodium nitrate. It is used as a heat transfer medium because of its low melting point of 288° F., its high heat transfer coefficient and its low cost. It can be used with carbon steel up to 850° F. and with 304SS above that temperature. Its viscosity at 350° F. is 10 cP and at 850° F. its viscosity is 1.4 cP. It is completely chemically stable up to 850° F. From 850° F. to 1000° F., it slowly degenerates over a period of years. At temperatures above 850° F., it should be under nitrogen gas padding to protect it from oxygen in the air, because oxygen will slowly oxidize the nitrite, producing a mixture with an undesirable elevated melting point. Its thermal conductivity coefficient is 0.33 to 0.35 Btu/(hr·ft·° F.), independent of temperature. Its specific heat is 0.32 to 0.35 Btu/lb/° F. Its density varies with temperature from ˜16 lbs/gallon at ˜450° F. with 5.6 Btu/gal per ° F. to ˜14 lbs/gallon at 1000° F. with 4.9 Btu/gal per ° F. 
     HITEC SOLAR® brand salt mixture is a higher service temperature salt. It is a two-part mixture of sodium nitrate and potassium nitrate salts. It is thermally equivalent to the eutectic salt but has a higher melting point and service temperature. It is useful up to 1100° F. Its specific heat is 0.37 Btu/lb/° F. Its melting point is 431° F. For practical purposes, the temperature needs to be 500-550° F. before the salt is run through the apparatus. For this reason, HITEC SOLAR® brand salt mixture is first hydrated, so that the melting point of the hydrate can allow the molten salt hydrate to be circulated at a much lower temperature of about 300° F. Its coefficient of thermal conductivity is 0.31 Btu/(hr·ft·° F.). Its heat transfer coefficient is 1164 Btu/h/ft 2  per ° F. Its viscosity is 2.1 cP. Its density is about 14 to 16 lbs/ft 2 . Its specific heat is up to 5.75 Btu/gal/° F. See, http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.175.2487&amp;rep=rep1&amp;type=pdf 
     Other materials are often also present in the apparatus and, when they are, in accordance with the present invention often must be managed toward the object of effectively filling the structures of the apparatus with fluid, the object of effectively draining fluid  25  from those structures, the object of appropriately modulating pressures within those structures, one relative to another and collectively relative to ambient pressure, and the object of preserving desirable or necessary chemical or physical properties of the fluid  25  or fluid mixture. Notably in accordance with the present invention, where the apparatus is located on the surface of the Earth, the presence and pressure of the Earth&#39;s atmosphere and its various constituents are taken into account. Also notably in accordance with the present invention, when water is added to a salt heat-transfer material in the apparatus, water vapor may be generated and may require attention and management. 
     The fluid  25  or fluid mixture may change in properties other than temperature and viscosity. Such other properties of the fluid  25  or fluid mixture are important in operating the apparatus in accordance with the present invention. Notably in accordance with the present invention, the fluid  25  or fluid mixture expands when heated. In the case of at least one salt used as the heat-transfer fluid  25  in accordance with the present invention, the fluid  25  expands approximately 25% when heated from ambient temperature to the temperature at which continuous heating is accomplished in the apparatus. In accordance with the present invention, fluid expansion and contraction are accommodated toward the object of avoiding counterproductive pressure differences between the several structures of the apparatus and between those structures and the ambient-pressure environment, and toward the object of avoiding undesired fluid flow behavior. The management of the behavior of the heat-transfer fluid  25  and other materials in the apparatus is next described. 
     When a portion of the fluid  25  or fluid mixture expands as it is heated, it displaces any other material with which it shares a nearby portion of the apparatus. Either the volume of a portion of the apparatus must change to accommodate such displaced material, or such other material must contract, or such other material must move to another portion of the apparatus or exit the apparatus. Where the apparatus is constructed of substantially rigid vessels and tubes, as is often the case, expansion of the apparatus is impracticable. Where the fluid  25  or fluid mixture is not elastically compressible—usually such is the case with liquids and semisolids—contraction of other portions of the fluid  25  or fluid mixture to accommodate displacement is not to be expected. Where a gas shares a portion of the apparatus with the fluid  25  or fluid mixture, the displacement due to expansion of the heated fluid  25  or fluid mixture may be accommodated by compression of the gas or by other volume-reducing phenomena such as condensation, deposition, adsorption or chemical reaction. However, in a given application, it may be that none of these accommodations is practicable or preferable. Then, a portion of the fluid  25  or fluid mixture or a portion of a material with which the fluid  25  or fluid mixture shares the apparatus must flow within the apparatus or exit the apparatus to accommodate the displacement due to expansion. In accordance with the present invention, the apparatus is equipped to relieve pressure and to allow flow of fluid  25  or fluid mixture or other materials within the apparatus and exit of same from the apparatus, as is now described. 
     With reference to  FIG. 4 , in an exemplary embodiment of the apparatus accordance with the present invention, fluid  25  or fluid mixture partially fills the rundown tank  24 . Unless the rundown tank  24  is designed to withstand a partial vacuum relative to ambient pressure—often a design requirement that is undesirable and costly to meet—the balance of the volume of the rundown tank  24  (this volume is known as a headspace) is occupied by gas. As the fluid  25  or fluid mixture occupying any portion of the rundown tank  24  expands with increasing temperature, the volume available to this gas is reduced, pressurizing the gas. The rundown tank  24  is equipped with a relief valve  62  fluidly communicating with the rundown tank headspace portion  40 . When pressure in the rundown tank headspace portion  40  exceeds ambient pressure by more than a predetermined tolerable value, the relief valve  62  opens and allows gas to escape from the rundown tank headspace portion  40 . Conversely, when the fluid  25  or fluid mixture contracts or the rundown tank headspace portion  40  pressure falls below ambient pressure by more than a predetermined tolerable value, the relief valve  62  opens and allows gas to flow into the rundown tank headspace portion  40 , reducing the pressure differential. A specific example of a gas which may be managed in this manner is air or preferably nitrogen, which is allowed to escape to the atmosphere or enter from the atmosphere through the relief valve  62  as needed to reduce the pressure differential. 
     Also with continued reference to  FIG. 4 , in another exemplary embodiment of the apparatus in accordance with the present invention, air is undesirable as the gas in the rundown tank headspace portion  40 . This may be the case because, for example, a constituent of air is carbon dioxide, which reacts with a salt heat-transfer fluid to produce a carbonate which undesirably elevates the melting point of the salt. In such an embodiment, nitrogen may be employed as a padding gas to reduce pressure differentials while excluding air from the rundown tank headspace portion  40 . Rather than a relief valve  62 , the rundown tank  24  is equipped with a padding valve  64  connected to a nitrogen reservoir. Nitrogen is introduced to the rundown tank headspace portion  40  to displace air. When headspace pressure is high enough or low enough, relative to ambient pressure, to require relief, the padding valve  64  allows nitrogen to flow to the ambient environment or from the nitrogen reservoir as needed to reduce the pressure differential. 
     With continued reference to  FIG. 4 , in an exemplary embodiment of the apparatus in accordance with the present invention, at start-up, a gas occupies the apparatus but for the portion of the rundown tank  24  occupied by fluid. As the fluid  25  moves from the pump  26  into the pump output tube  55  and enters the heater  28 , the fluid  25  displaces the gas from the pump output tube  55 , forcing the gas through the heater  28  and toward the fill tank  30 . As the fluid  25  passes through the heater  28 , it expands, accelerating the rate of displacement of gas toward the fill tank  30 . This displaced gas moves into the fill tank  30  and, in doing so, will build pressure if not relieved. Such gas pressure could oppose the flow of fluid  25  into and through the heater  28  and into the fill tank  30 . To relieve this gas pressure, the fill tank headspace portion  72  should be vented and, more particularly, is vented by the gravity tube gas orifice  208  via the fill tank headspace connector  75  to the rundown tank headspace portion  40 . Thus, gas back-pressure that might oppose flow of fluid  25  from the pump  26  through the heater  28  to the fill tank  30  is relieved. Conversely, at shut-down or during a cessation of operation, the fill tank headspace connector  75  conducts gas from the rundown tank headspace portion  40  to the fill tank headspace portion  72  to relieve any partial vacuum that could oppose passive drainage of fluid  25  from the fill tank  30  and heater  28  to the rundown tank  24 . Pressure differentials between the rundown tank  24  and the are relieved via the relief-pad-depad valve  64  of the rundown tank  24 . 
     During startup of the apparatus with a hydrated heat transfer fluid  25  being increased from ambient temperature to continuous heating temperature, water vapor leaves the fluid mixture and a substantial volume of steam evolves there and would pressurize the apparatus and oppose flow from the heater  28  to the fill tank  30  were it not relieved. To relieve steam pressure, the fill tank  30  is equipped with a fill tank steam vent  73 , which is opened to allow the steam evolving from the fluid mixture to escape, relieving any excess steam that could oppose flow of fluid  25  from the heater  28  to the fill tank  30  or overpressurize the apparatus. 
     Likewise, during shutdown of the apparatus, if water is added to a salt heat-transfer fluid  25  to produce a mixture which circulates efficiently at temperatures between ambient temperature and continuous heating temperature, steam is generated. In an exemplary apparatus in accordance with the present invention, the rundown tank  24  is equipped with a set of misting nozzles  60  located in the rundown tank headspace portion  40 . When planned, intentional shutdown is desired, the feeding of process material to the thermal processor  22  is interrupted and the process material in the thermal processor  22  is conveyed out of the thermal processor  22 . The supply of outside energy to the heater  28  is interrupted. The bypass valve  56  is operated to interrupt the heat transfer circulation loop and to establish the preheating fluid circulation loop. The pump  26  is operated. Hydration fluid is supplied to the hydrator  60  in the rundown tank headspace portion  40  and is misted a fine spray onto the liquid salt heat-transfer fluid  25  in the rundown tank  24 . An intended effect of misting the water onto the salt fluid  25  is to hydrate the salt, producing a mixture having low viscosity at temperatures between continuous heating temperature and ambient temperature. An additional effect of misting the hydration fluid onto the salt fluid  25  is the production of steam, whenever the salt is at a temperature high enough to boil the water in the rundown tank  24  under the existing conditions. This steam would overpressurize the rundown tank  24  were it not relieved. To prepare for the steam, the pad-depad valve  64  on the rundown tank headspace  40  is opened. With the pad-depad valve  64  open, the steam generated in the rundown tank  24  escapes through the pad-depad valve  64 , avoiding over pressurization of the apparatus. In one exemplary embodiment, misting continues until the freezing point of the hydrated salt fluid mixture reaches 60° F. The pad-depad valve  64  will have been closed after steam no longer needs to escape. 
     As so far described, the heater  28 , the fill tank  30  and the heat transfer fluid space  32  of the thermal processor  22  are described as passively drainable. The structures and interrelations rendering these structures and the apparatus overall passively drainable are now further described. 
     With continued reference to  FIG. 4 , in an exemplary embodiment of the apparatus in accordance with the present invention, the rundown tank  24  is configured to receive fluid  25  directly from the pump  26  at the pedestal  68  should the fluid  25  flow backward into the pump  26  from the heater  28 . The rundown tank headspace portion  40  is fluidly connected with the gravity tube gas orifice  208  on the fill tank headspace portion  72  (as will be discussed shortly). The rundown tank  24  is configured to receive fluid  25  from the heat transfer fluid spaces  32  of the thermal processor  22  via the processor fluid outlet drain tube  85 . 
     In an exemplary embodiment of the apparatus in accordance with the present invention, the rundown tank  24  has a sump  77  formed with a pedestal  68 . The pump  26  is a centrifugal-type pedestal pump  26  resting on the pedestal  68  in the sump  77 . A shaft connected to a motor located atop the rundown tank  24  drives the pump  26 . The pump  26  has no seals or check-valves capable of halting backward flow of material from the heater  28  through the pump  26  into the rundown tank  24 . The openness of the pump  26  to such backward flow advantageously allows fluid  25  draining from the heater  28  to flow quickly into the rundown tank  24 , facilitating passive drainage of the apparatus. 
     In an exemplary embodiment of the apparatus in accordance with the present invention, the heater  28  is formed to provide a continuously inclined fluid flow path from the heater inlet  54  to the heater outlet  78 . The pump output tube  55  connects the heater inlet  54  to the pump outlet  52 . The heater output tube  80  connects the heater outlet  78  to the fill tank bottom portion  36 . Thus, the fill tank  30  and heater  28  are passively drainable to the rundown tank  24  via the pump  26  under the influence of gravity. 
     With reference to  FIGS. 3-7 , the fill tank  30  also is equipped with a stem pipe  48  opening upwardly at a high portion of the fill tank  30  and communicating with the fill tank headspace portion  72 . The stem pipe  48  is connected to the rundown tank headspace portion  40 . Fluid  25  will not flow from the fill tank  30  into the stem pipe  48  unless the level of fluid  25  in the fill tank  30  exceeds the level at which the stem pipe  48  opens. Thus, the level at which the stem pipe  48  opens sets the maximum depth of fluid  25  in the fill tank  30  and, by doing so, sets an upper limit on the range of pressures the fluid  25  flowing out of the low portion of the fill tank  30  will exert when received at the thermal processor  22 . As long as the level of fluid  25  in the fill tank  30  is above the level of the gravity tube  204 , fluid  25  will tend to flow out of the fill tank  30 , through the gravity tube  204 , and through both the restrictor  46  and the thermal processor  22  to the rundown tank  24 . When the pump  26  ceases to drive fluid  25  up through the heater  28 , fluid  25  flows back rapidly from the fill tank bottom portion  36 , through the heater output tube  80 , through the heater  28  and pump output tube  55  to the rundown tank  24  and, simultaneously, more slowly through the gravity tube  204  through the restrictor  46  to the rundown tank  24 . 
     With continued reference to  FIG. 4 , the thermal processor  22  is now discussed, first with attention to how it is interrelated with the fill tank  30 , the restrictor  46  and the rundown tank  24 ; second, with attention to how it is formed and configured to be completely fillable and passively drainable; third, with attention to how the thermal processor  22 , restrictor  46  and fill tank  30  cooperate to accomplish the important object of managing fluid  25  flow during startup of the apparatus. 
     The thermal processor  22  has at least one thermal processor low portion  35  with at least one heat transfer fluid inlet  38  and at least one thermal processor high portion  37  with at least one heat transfer fluid outlet  84 . The thermal processor  22  has at least one heat transfer fluid space  32  fluidly communicating with the transfer fluid inlet  38  and transfer fluid outlet  84 , and has a process space  34  with process material inlet  82  proximate the thermal processor low portion and process material outlet  86  proximate the thermal processor high portion. In some embodiments, the heat transfer fluid space  32  is inclined. A conveyor  90  is disposed within the process space  34  and is driven by a mover associated with the thermal processor  22 . When the thermal processor  22  is at a processing temperature, process material is received at the process material inlet  82 . The conveyor  90  is activated and urges the process material toward the process material outlet  86 . 
     The thermal processor  22  may require the input of a substantial amount of heat in order to reach operating temperature. Thus, in operating the apparatus in accordance with the present invention, it is observed that during startup the rise in temperature of the fluid  25  in the heat transfer fluid space  32  of the thermal processor  22  lags the rise in temperature of the fluid  25  flowing through the restrictor  46  to the rundown tank headspace portion  40 . Thus, the fluid  25  in the heat transfer fluid space  32 , being cooler, is more viscous and flows more slowly. Consequently, during startup, the total flow of fluid  25  out of the low portion of the fill tank  30  is reduced. However, there might not be any mechanism in place to adjust the output of the pump  26  and heater  28  to account for this reduced flow. With the pump  26  and heater  28  delivering fluid  25  at a rate higher than the restrictor  46  and the thermal processor  22  together can accept fluid, the fluid  25  begins to accumulate in the fill tank  30  (e.g., FOURTH exemplary embodiment) or gravity tube  204  (e.g., THIRD exemplary embodiment). As the level of the fluid  25  in the fill tank  30  rises, the fluid  25  pressure increases at the restrictor  46  and at the heat transfer fluid inlet  38 , increasing the rate of flow through these two structures somewhat. This increase of flow rate might suffice to manage the excess. 
     If the flow from the heater  28  into the fill tank  30  (or gravity tube  204 , THIRD exemplary embodiment) continues to exceed the flow out from the fill tank  30 , the level of fluid  25  in the fill tank  30  rises until fluid  25  begins to flow through the stem pipe  48  (or gravity tube upper drain  206 , THIRD exemplary embodiment) to the rundown tank headspace portion  40 , limiting any further increase in pressure at the restrictor  46  and the heat transfer fluid inlets  38 , even if the pump  26  and heater  28  continue to deliver fluid  25  to the fill tank  30  (or gravity tube  204 ) at an excessive rate. 
     As the thermal processor  22  warms up, the fluid  25  in the transfer fluid  25  flows more easily and equilibrium may be achieved with the fluid level stabilized at a level intermediate that of the low portion of the fill tank  30  and that of the stem pipe  48  orifice. In any event, the restrictor  46  and thermal processor  22  are never exposed to a fluid pressure greater than can be exerted by a column of fluid  25  extending between the height of the heat transfer fluid  25  inlet and the height of the stem pipe  48  in the fill tank  30  (or the height at which the gravity tube  204  fluidly communicates with the gravity tube upper drain  206 ). 
     In accordance with the present invention, when molten salt is used as a heat transfer fluid, materials for surfaces in contact with the salt are selected based on temperature tolerance and the ability to withstand corrosion. Carbon steel is usable for temperatures up to 800° F., 304 SS for temperatures up to 1000° F., and 347, among other SS for temperatures above 1000° F. 
     Piping and vessels are insulated to conserve heat. 
     The rundown tank  24  is at the lowest elevation in the system. The highest fill level of the rundown tank  24  is at a lower elevation than the thermal processor  22 , fill tank  30  and heater  28 . The rundown tank  24  has sufficient capacity to hold 100% of the volume of heat transfer fluid  25  contained by the apparatus, plus an additional 30% for expansion and an additional 20% for safety. It is common for a preferred embodiment of the apparatus in accordance with the present invention to have a minimum of a 150 gallon rundown tank  24  with an approximate diameter of 36 inches and an approximate length of 48″ with a sump  77  projecting approximately 12 inches downward. 
     The pump  26  for a preferred embodiment of the apparatus in accordance with the present invention is a pump designed for the intended temperature range. The pump  26  has a pump curve specific to the required pressure determined by the head pressure for a given installation, i.e., pressure sufficient to deliver fluid  25  through the heater  28  to the fill tank  30  at a sufficient rate to keep the fill tank  30  at a desired fill level. A preferred pump configuration uses a vertical shaft pit pump without a seal. The drive is above the rundown tank  24 . 
     The fill tank bottom portion  36  is at an elevation higher than the elevation of the heat transfer fluid inlet  38  of the thermal processor  22  and higher than the elevation of the rundown tank  24 . The fill tank  30  has a volume of approximately 5% of the volume of heat transfer fluid  25  needed to run the apparatus during operation. In a preferred embodiment, the tank has a volume of about 6 gallons, although a much larger fill tank  30  may be more practical in some circumstances. 
     In a preferred embodiment of the apparatus in accordance with the present invention, the thermal processor  22  has a length of 26 feet, a width of 4 feet, a height of 4 feet, with conveying screws 20 feet in length and 14 inches in diameter. Material thicknesses are approximately 5/16 inch. 
     In a preferred embodiment of the apparatus in accordance with the present invention, the heater  28  is about 4 feet in diameter and 7 feet in height. 
     In some embodiments, the rundown tank  24  has a rupture disk (see  192  in  FIG. 4 ) for safe rapid release of vapor in the event of gross overpressurization. 
     The tubing in which the heat transfer fluid  25  circulates is constructed of materials capable of tolerating the anticipated temperatures, pressures and chemical conditions. 347 SS is a preferred material for high temperatures and can be used when, as is described herein, the use of a pressure boundary material is not required for safety or certification. 
     The rundown tank  24  has capacity for a volume of the heat transfer fluid  25  sufficient to operate the apparatus, capacity for expansion of the fluid  25  volume, and capacity for a volume of gas above the fluid  25  volume. With reference to  FIG. 4 , in the FIFTH exemplary embodiment, when the rundown tank  24  is occupied by a volume of fluid, it disposes the fluid volume so as to provide a fluid upper surface  240  at a fluid upper surface height below the top of the rundown tank  24 , i.e., allowing vacant space accounted for by the rundown tank headspace portion  40 . Capacity for the volume of fluid  25  (i.e., liquefied heat transfer fluid) sufficient to operate the apparatus is important, because the entire volume of thermal transfer fluid  25  in the apparatus should be able to drain into the rundown tank  24  when it is not being heated and circulated, thereby avoiding retention of fluid  25  in the thermal processor  22  or elsewhere outside the rundown tank  24  when temperatures fall below the melting point of the fluid. Capacity for expansion of the fluid  25  is important, because the fluid  25  expands up to 25% over the range of temperatures at which it is used. Capacity for a volume of gas above the fluid  25  is important for accommodating the expansion of the fluid  25  during heating, for accommodating the steam generated both during hydration and during dehydration, and for equalizing pressure differentials relative to the transfer fluid space  32  of the thermal processor  22  and the fill tank headspace  72 . The displacement of a gas is important in managing a liquid or solid which is in contact with the gas while managing the pressure within a portion of the apparatus relative to the environment. A fluid upper surface  240  of the volume of fluid  25  is important, because it is preferable for the pump  26  to ingest the fluid  25  at a location below the fluid upper surface  240  and because the fluid upper surface  240  creates a liquid-gas interface at which material and heat are transferrable. 
     With reference to  FIG. 4  and the FIFTH exemplary embodiment, the gravity tube  204  accepts heat transfer fluid  25  from the heater outlet  78  and feeds the fluid  25  to the processor fluid inlet  38  under the influence of gravity at a pressure determined largely by the height differential between the first height  211  and the second height  212 , and not determined by the pressure or flow rate of the pump  26 . Passively limiting pressure to within a predetermined range in this manner is advantageous for its simplicity and reliability. The gravity tube upper drain  206  receives the fluid  25  flow, if any, that exceeds the fluid  25  flow entering the gravity tube  204  and flowing toward the processor fluid  25  inlet  38 . The gravity tube gas orifice  208  admits gas to the gravity tube  204  at times when a partial vacuum might develop in the gravity tube  204 , e.g., when fluid  25  is entering the gravity tube  204  from the heater outlet  78  more slowly than it is flowing into the gravity tube  204  toward the processor fluid  25  inlet  38 . Conversely, the gravity tube gas orifice  208  allows gas to escape the gravity tube  204  when a vapor lock might develop there, e.g., when fluid  25  is entering the gravity tube  204  from the heater outlet  78  more rapidly than it is flowing into the gravity tube  204  toward the processor fluid  25  inlet  38 . This arrangement facilitates and assures passive drainage when the pump  26  stops. 
     Because the processor fluid  25  outlet  84 , the gravity tube  204  and the gravity tube upper drain  206  fluidly communicate with the rundown tank  24 , fluid  25  from these three paths converges and can again be circulated to the gravity tube  204  via the heater  28 . Because the gravity tube gas orifice  208  and the vacuum breaker connector tube  93  fluidly communicate with the rundown tank  24 , gas is transferrable among the rundown tank  24 , the thermal processor  22 , and the gravity tube  204  (portion thereof proximate heater output tube  80 ) or fill tank headspace portion  72 . Gas transfer is important, because the entry of thermal transfer fluid  25  into any one of these three structures may be facilitated by allowing the gas that the fluid  25  displaces to pass to another of these three structures. Additionally, temperature changes often produce expansion or contraction of the heat transfer fluid  25  or of a gas, requiring gas transfer to avoid creating a troublesome or dangerous local overpressure or partial vacuum. 
     In an exemplary embodiment, the first height  211  and the second height  212  are selected such that a column of the fluid  25  extending vertically from the first height  211  to the second height  212  exerts pressure at the processor fluid  25  inlet  38  no greater than 14.9 PSIG. When pressures are kept at or below this limit, structural loads and rupture hazards are mitigated and engineering standards calling for costly ASME pressure boundary materials and construction are not implicated. As an additional benefit, 347 SS, which is not an ASME-recognized pressure boundary material, is preferred in the construction of some embodiments of the apparatus in accordance with the present invention. 
     In an exemplary embodiment, the rundown tank  24  is equipped to vent a gas to the ambient environment and to receive a gas from a source selected from among the ambient environment of the apparatus and a padding gas supply tube  182 . This arrangement, commonly a pad-depad valve  64  fluidly connected with a supply of inert padding gas such as nitrogen, allows overpressure in the rundown tank  24  to be vented to the atmosphere and compensates for under pressure in the rundown tank  24  by admitting inert gas to the rundown tank headspace portion  40 . 
     The fill tank  30  (or the fluid communication of the gravity tube  204  with the heater output tube) is located uppermost in the apparatus. Heat transfer fluid  25  passing from the heater outlet  78  to the fill tank bottom portion  36  at the second height  212  is free to enter the gravity tube  204  and flow downward toward the processor fluid inlets  38 . Heat transfer fluid  25  which has accumulated in the fill tank  30  and has risen to the third height  213  in the fill tank headspace portion  72  (or in the gravity tube  204 , see THIRD exemplary embodiment) enters the stem pipe  48  ((e.g., FOURTH exemplary embodiment) and flows down the gravity tube upper drain  206  toward the rundown tank  24 . 
     With reference to  FIG. 4 , in an exemplary embodiment, the rundown tank  24  disposes the fluid volume so as to provide a fluid upper surface  240  at least a portion of which is accessible and suitable for hydration. The rundown tank  24  has hydration water-dispensing nozzles  60  and a rundown tank headspace vent  71 . Preferably, the hydration water-dispensing nozzles  60  are configured to deposit a water mist gently onto the fluid upper surface  240  without disrupting it. The fluid upper surface  240  should be large enough to permit the hydration nozzles  60  to efficiently add water to a body of dehydrated salt in the rundown tank  24 . It is usually undesirable to cause a molten salt to erupt and splatter in the rundown tank  24 . Therefore, when hydrating hot molten salt, the hydration water nozzles  60  should apply water in such a manner that the water absorbs heat from the hot salt, the water and the salt combine to form hydrated salt, and yet the water is never injected into the salt in a manner that could cause water to flash beneath the fluid upper surface  240 . Preferably, a fine mist of water is injected above the salt and gently and uniformly settles onto the fluid upper surface  240 , whereupon some of the water vaporizes and some of the water combines with salt. Initially, flash steam may require an efficient path to the environment, such as may be provided by an amply dimensioned pad-depad valve  64  on the rundown tank headspace portion  40 . Gradually, as the volume of hot dehydrated salt is replaced by a volume of cooler hydrated salt, the temperature and pressure in the rundown tank  24  will decrease, at which time padding gas may be admitted through the pad-depad valve  64  to compensate for any underpressure as the remaining steam condenses. 
     In an exemplary embodiment, the rundown tank  24  is equipped to heat the fluid  25  and the pump  26  and the rundown tank  24  are configured selectively to circulate the fluid  25  between the pump  26  and the rundown tank  24 . When solidified salt occupies the rundown tank  24 , the salt must be liquefied in order for it to circulate. Whether cold solidified salt is warmed in order to liquefy it, or it is hydrated in order to produce a hydrated salt liquid (which is later dehydrated), it is preferable initially to circulate the salt in a short loop including the rundown tank  24  and the pump  26 . In one embodiment, see  FIG. 4 , a bypass branch  58  fluidly connects the pump  26  output to the rundown tank  24  at a level above the fluid-gas boundary of the fluid upper surface  240 , allowing fluid  25  to flow over and through the solid salt toward the pump  26  in the rundown tank  24 . During this period, it may be preferable to equip the rundown tank  24  with a heating element  184  which is activated to begin liquefying the salt. The pump  26  may be positioned in a sump  77  in the rundown tank  24 , in which case a heat trace  66  is installed on the sump  77  and is activated at this time. After a sufficient amount of salt is liquefied, the heat trace  66  or heating element  184  is deactivated and the fluid  25  is routed to the heater  28  for circulation through the apparatus. 
     In the exemplary embodiments of  FIGS. 2-7 , for example, a restrictor  46  is located in the gravity tube lower drain  210  at a seventh height  217  below the first height  211  and above the six height. The restrictor  46  allows a minor portion, such as 2-4%, of the fluid  25  flowing into the gravity tube  204  to flow to the rundown tank  24 , while a major portion flows to the thermal processor  22  fluid  25  inlet. While the pump  26  is active, the major portion of the fluid  25  flow serves to transfer heat to the thermal processor  22 , while the minor portion of the fluid  25  flow serves to keep the gravity tube  204  hot so that the gravity tube  204  remains capable of carrying fluid  25  to the rundown tank  24 . When the pump  26  is inactive or for any other reason fluid  25  is no longer being delivered at sufficient rate to cause it to flow through the thermal processor  22 , the fluid  25  is free to flow out of the thermal processor  22  through the processor fluid  25  inlet  38 , back to the gravity tube  204 , and through the restrictor  46   214  to the rundown tank  24 . 
     In an exemplary embodiment, the heat transfer fluid outlet  84  of the thermal processor  22  is located above the first height  211 . When the pump  26  stops, fluid  25  in the thermal processor  22  flows out through the processor fluid  25  inlet  38  and ultimately to the rundown tank  24 , while fluid  25  which has left via the processor fluid  25  outlet  84  also flows to the rundown tank  24 . 
     With reference to  FIG. 4 , an exemplary method of operating a fluid-heated indirect thermal processing apparatus in accordance with the present invention is described for use with a salt having a melting point below 300° F. This method is carried out in a closed loop thermal heating system with nitrogen padding with equipment designed to accommodate differential thermal expansion of the housing and screw conveyor. As its thermal fluid  25  heat transfer fluid, this method uses a molten salt. This exemplary method is accomplished without hydrating the salt. As previously described with reference to the apparatus of  FIG. 4 , a rundown tank  24  is located at a low point of the apparatus to facilitate gravity draining of the thermal fluid  25  into the rundown tank  24  during shutdown. The rundown tank headspace portion  40  is nitrogen padded—i.e., nitrogen blanketed—to protect the thermal fluid  25 —in this exemplary method, a molten salt—from the atmosphere and to balance the headspace pressures (in the rundown tank  24 , thermal processor  22 , heater  28  and fill tank  30 ) throughout the apparatus. The rundown tank  24  serves as a container for the inventory of thermal fluid  25 , an expansion tank as the thermal fluid  25  heats, as a receiver for the return flow from the thermal processor  22 , as a common headspace for pressure compensation, as a sump for drain-down at time of shutdown or at time of plant failure such as a power failure and as a surface for hydration/dehydration when that is advisable. The pad-depad valve  64  releases air or padding gas (nitrogen, in this exemplary method) from the system as pressure increases due to expansion of the molten salt when heated. During shutdown when the molten salt cools/solidifies, the pad-depad valve  64  admits nitrogen from a nitrogen supply, filling the rundown tank headspace  40  with nitrogen. The rundown tank  24  contains a pump  26  which is submerged in the rundown tank  24  and is located on a pedestal  68  in a sump  77  located at a low point of the rundown tank  24 . The pump  26  is employed to circulate the thermal fluid  25  through the apparatus. The sump  77  has an electrical heat trace  66 . Alternately, preferably additionally, the rundown tank  24  is equipped with submersion heating elements  184 . 
     During startup, if the salt is solidified in the rundown tank  24 , the heat trace  66  or heating elements  184  are activated to melt the salt around the pump  26  and the heating elements  184  melt the salt in the rundown tank  24 . A flow path is created in the rundown tank  24  into the sump  77 . During a cold startup, the path from the pump outlet  52  to the heater inlet  54  is closed and the bypass branch tube  58  from the pump output back to the rundown tank  24  is open. Once the salt is heated to melting around the pump  26 , as detected by a temperature sensor in the sump  77 , the pump motor can be started. The pump motor can be variable frequency controlled and typically is operated at a low flow rate, with the melting salt circulating from the sump  77 , through the pump  26 , through the bypass branch  58 , into the rundown tank headspace portion  40 , and down through the flow paths created by the heating elements  184 , where it returns to the sump  77  for recirculation. When the salt in the rundown tank  24  is fully melted, as detected by a temperature sensor in the rundown tank  24 , the path from the pump outlet  52  to the heater inlet  54  is opened and the bypass branch  58  is closed. 
     Molten salt rises from the pump outlet  52  to the heater inlet  54  and into the bottom of the thermal fluid heater  28 . The heater  28  preferably is formed to provide a continuously inclined fluid flow path from the heater inlet  54  to the heater outlet  78  without valves in the thermal fluid loop. Valves can be used, but in this method are not required. The thermal fluid characteristics of density and viscosity change significantly as the thermal fluid  25  is heated. The density and viscosity of molten salt change dramatically with temperature. Therefore, for a particular pump  26 , the flow rate changes at different operating temperatures. In the embodiment utilized for this method, the flow rate is always more than the required by the thermal processor  22 . Having reached a desired temperature in the heater  28 , the molten salt exits through the heater outlet  78  located at the top of the heater  28  and flows into the fill tank  30 . 
     The fill tank  30  is located at the highest point in the apparatus and sets the head pressure on the fluid flow into the thermal processor  22 . As the fill tank  30  fills with molten salt, the fill tank headspace connector  75  allows the air or nitrogen inside the fill tank  30  to be displaced into the rundown tank headspace portion  40 . The salt exits the fill tank bottom portion  36  through the fill tank gravity flow tube  42  and then through the thermal processor  22  branch tube  44  to the thermal processor  22 . As mentioned previously, the flow rate of molten salt from the heater  28  exceeds the flow requirements of the thermal processor  22 . Consequently, the level of the molten salt in the fill tank  30  rises. A stem pipe  48  inside the fill tank  30  sets the maximum fill level of the tank. The balance of excess flow discharges into the rundown tank headspace portion  40  after entering the stem pipe  48 . 
     The thermal processor  22  in this embodiment is a heat exchanger that transfers heat from the thermal fluid  25  to the feed passing through the thermal processor  22 . The thermal processor  22  can be one of several different types. In the thermal processor  22 s used in accordance with this exemplary method, the supply of heated thermal fluid  25  enters the thermal processor  22  at a low portion thereof and generally flows up through the flow paths of the processor  22  and exits a high portion. Thus, if the supply of thermal fluid  25  is cut off, the flow reverses under the influence of gravity and exits the processor  22  at the low portion. The thermal fluid  25  is allowed to gravity drain back to the rundown tank  24  when the pump  26  shuts off. With the pump  26  operating, thermal fluid  25  exists the heat transfer fluid outlet  84  and flows to a vacuum breaker  92 . 
     The vacuum breaker connector tube  93  fluidly connects the vacuum breaker  92  to the rundown tank headspace portion  40 . This prevents air-locking on startup or (vacuum-locking) during drain down by venting the gases in the flow paths of the thermal processor  22  to the rundown tank headspace portion  40 . Thermal fluid  25  returns to the rundown tank  24  from the vacuum breaker  92  and is recirculated by the pump  26 . 
     Another feature of this apparatus is the ability to passively limit the pressure on the fluid  25  to a few psi and therefore not requiring the heat exchanger or any of the tanks to be an ASME pressure vessel. This is particularly important in extremely high temperature applications where the use of molten salt as the heat exchange media is employed. The preferred metallurgy for the salt, a commercial heat transfer salt, at 1100° F. is 347SS, which is not an ASME-recognized pressure boundary material. 
     An important design feature of all piping and equipment for the use of molten salt as the heat transfer fluid  25  is the gravity draining during shutdown and when the pump  26  stops. Because the two most common molten salts used for heat transfer fluids, HITEC® and SOLAR SALT®, freeze at 288° F. and 448° F. respectively, it is necessary to drain all of the equipment and piping dry of molten salt on shutdown or power failure. During a shutdown of the system, the molten salt drains by gravity down to the rundown tank  24 . Molten salt gravity drains from the fill tank  30 , both back to the heater  28  (and from there back through the pump  26 ) and also through the gravity tube  204 , to the rundown tank  24 . As the lines drain, the vent line to the rundown tank headspace portion  40  supplies nitrogen from the padding system to fill the void created by the draining thermal fluid. The flow from the heater  28  drains down through the pump  26  to the rundown tank  24 . The transfer fluid spaces  32  of the thermal processor  22  are self-draining to the rundown tank  24 . The apparatus passively drains when the pump  26  is not operating. 
     Occupying relative high points of the apparatus are the fluid communication of the gravity tube  204  with the heater output tube  80 , gravity tube upper drain  206  and gravity tube gas orifice  208  (see  FIG. 2 ), the fill tank  30  ( FIGS. 3-7 ), and the vacuum breaker  92  ( FIGS. 2-7 ). The gravity tube gas orifice  208  and the vacuum breaker connector tube  93  gravity tube lower drain  210  are both fluidly connected to the rundown tank headspace portion  40 . This is important, because on shutdown, all the heat transfer fluid flow paths empty and fill with air or padding gas, which must be displaced at startup. 
     For molten salts with melting temperatures greater than 300° F., the molten salt must be hydrated during startup to avoid thermally shocking the apparatus. The two major HTF salts, HITEC® and SOLAR SALT®, melt at 288° F. and 448° F. respectively. Damage to the equipment could occur if hot (&gt;300° F.) molten salt were circulated through a cold system. It is necessary to warm the system gradually. In the event that a salt is used with a melt temperature above 300° F., the salt must be hydrated during each startup and dehydrated during each shutdown. The process of hydrating the salt involves adding water to the salt as it cools until it becomes a saturated salt solution that remains in the liquid state. During startup the water is boiled out of the solution. 
     The primary differences between this exemplary method and the method without hydration are the addition of a salt hydration system and a vent to remove the steam during dehydration. All other aspects are fundamentally the same and should be inferred from the previous description of a method without hydration. The following method will be employed during each startup and shutdown to hydrate and dehydrate the salt solution. 
     Hydrated salt can be already liquid at ambient temperature. It may be only partially hydrated, which lowers the melting point, in which case only a slight heating can easily melt the solid. To start the apparatus with liquid hydrated salt, the pump  26  is turned on and hydrated salt is circulated through the system. The heater  28  is activated and the salt is heated at ramp rate of approximately 5° F. per minute to 215° F.-220° F. At this temperature, the salt will begin to dehydrate, releasing evaporated water. Depending on the volume of molten salt in the system, the temperature can be held at 220° F. until the rate of steam release begins to subside, or a new ramp rate of 2° F. per min can be imposed immediately after reaching 220° F. The steam in the fill tank  30  exits through a rundown tank headspace vent  71  that is open during the startup. Steam from the thermal processor  22  vents to the rundown tank  24  and vents to the rundown tank headspace vent  71 , out to the environment. At 480° F., the molten salt eutectic becomes anhydrous and the heating ramp rate can be set to 5° F. per minute until the final desired temperature has been reached. The Solar salt requires more than 600 F for full dehydration. 
     During shutdown, the salt must be hydrated if solidification is to be prevented when it cools. At least partial hydration is required if circulation of the salt is to begin after melting without first having to heat the salt to too high a temperature to begin circulation without unacceptable thermal shock. The heater  28  is deactivated and circulation of the salt is continued for hydration during shut down. Meanwhile, feed is emptied from the process space  34  of the processor  22 . When the molten salt has cooled to near its melting temperature, the pad-depad valve  64  is opened to the atmosphere for venting of water vapor. Hydration fluid is misted onto the top of the melted salt in the rundown tank  24  via the water misting nozzles  60 . The salt is hydrated until its freezing point drops preferably to below 60° F. Once the salt has become fully hydrated, the system functions in the manner of the method described above without hydration, except that the rundown tank remains vented for dehydration up to at least 600° F.; beginning at that temperature, the system is operated as a blanketed closed system. 
     To start the apparatus with solidified salt, a portion of the salt around the pump must be melted. The melting salt expands. Provision is made for the electric or other heat sources to melt a vertical passage to the surface of the solidified salt for the expanding liquid. Once a sufficient volume of liquid is melted the pump can be started. The rundown tank  24  is an insulated tank of sufficient volume to contain the whole volume of salt in the apparatus with room for expansion on heating and an adequate surface area for hydration. The rundown tank headspace portion  40  is connected to all headspaces (fill tank  30 , heater  28 , and processor  22 ) within the molten salt loop to maintain a common pressure and inert atmosphere and to break any siphon effects. The rundown tank  24  includes a sump  77  with a heat trace  66  which the pump  26  sits in. The pump  26  is VFD driven, capable of sufficient pressure to circulate the salt through the system. In the event the salt solidifies, the heat trace  66  melts the salt around the pump  26  and the heating elements  184  melt the salt in the rundown tank  24 . Alternatively, steam or thermal oil could be circulated through tubes in the rundown tank  24  as a means of heating the salt. As the salt melts, the heating elements  184  provide vertical and horizontal melt channels communicating with the sump  77  so that molten salt can flow up to the surface of the salt in the rundown tank  24  and back to the pump  26 . The pump  26  is started with the bypass valve  56  operated so that the bypass branch  58  open. The molten salt is recirculated into the rundown tank  24 . The molten salt continuously expands the molten volume by convection until the whole mass of salt in the rundown tank  24  is melted. Once the salt is sufficiently melted, as detected by a temperature sensor, with the mass reaching approximately 480° F., the hydration of the salt can begin. The misting nozzles  60  are activated. The hot molten salt is circulated in the rundown tank  24  while hydration fluid is added. Some of the hydration fluid is absorbed by the salt, while some vaporizes. Steam vents through the pad-depad valve  64  to the atmosphere. A rupture disk  192  protects the rundown tank  24  from any gross over pressurization. As the temperature of the salt cools due to the addition of water, the salt absorbs water more rapidly. Once the salt is adequately hydrated by being a liquid at a safe temperature for starting circulation without unacceptable thermal shock to the thermal processor  22 , the hydration fluid supply is turned off. At this point, a startup with hydrated salt can commence. 
     With reference to  FIG. 1 , a FIRST exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out with a molten-salt-indirectly heated screw-type thermal processor  22  which has an operating heat transfer fluid  25  temperature range, an operating heat transfer fluid  25  flow rate range and an operating heat transfer fluid  25  pressure range. A heater, and a rundown tank are operatively connected to the thermal processor  22  as previously discussed. 
     With continued reference to  FIG. 1 , a heat transfer fluid  25  is provided. The fluid  25  is capable of conveying heat from the heater  28  to the thermal processor  22  at a temperature within the processor&#39;s operating heat transfer fluid  25  temperature range while flowing into the thermal processor  22  at a heat transfer fluid  25  flow rate within the processor&#39;s operating heat transfer fluid  25  flow rate range at a pressure within the processor&#39;s operating heat transfer fluid  25  pressure range. The volume of fluid  25  that is provided is at least sufficient to operate with the heater  28  and thermal processor  22 . The heater  28  is capable of heating the heat transfer fluid  25  sufficiently at the aforementioned flow rate and temperature. The rundown tank  24  has capacity more than sufficient to contain all of the heat transfer fluid  25  that is added. The pump  26  is activated and delivers the heat transfer fluid  25  from the heater  28  to the thermal processor  22  at the temperature, the flow rate and the pressure while delivering the heat transfer fluid  25  from the thermal processor  22  to the heater. When it is desired to cease processing, the pump  26  is stopped. The heat transfer fluid  25  flows passively into in the rundown tank  24 . 
     With reference to  FIG. 4 , a SECOND exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out by using one or more heaters to produce the heat transfer fluid  25  by melting a solid. A solid material such as a salt is located in the rundown tank  24 . A small portion of the material is heated until it melts, making fluid  25  available to the pump. The heat transfer  25  circulation loop is interrupted. The preheating circulation loop is established. The pump  26  is activated. When a rundown tank temperature sensor indicates that the body of salt in the rundown tank  24  has melted, the preheating circulation loop is interrupted and the processing circulation loop is established. 
     It is often advantageous to begin circulating a fully hydrated salt in the heat transfer  25  circulation loop and at least partially dehydrate the salt while circulating it. With reference to  FIG. 4 , a THIRD exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out with a heat transfer fluid  25  comprising a hydrating fluid. Such a method is useful when a cold thermal processor must be protected from sudden exposure to a very hot heat transfer fluid. With the heat transfer  25  circulation loop established, the pump  26  activated and the rundown tank  24  vented, the heater  28  is activated, gradually warming and dehydrating the fluid  25  as the fluid  25  gradually warms the thermal processor  22 . Simultaneously dehydrating the salt and delivering the salt to the thermal processor  22  has the advantage of gradual change of processor temperature and gradual removal of water (vented as steam). A thermal processor  22  has a predetermined maximum tolerable rate of temperature increase, the step of dehydrating is performed slowly enough that the thermal processor  22  is warmed at a rate no greater than that maximum tolerable rate. 
     With reference to  FIG. 4 , a FOURTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention entails making the melting-point-reduced material, or hydrated salt, as the case may be, on-site in the rundown tank  24 . A body of molten salt is provided in the rundown tank  24 , melting the salt in accordance with the aforementioned SECOND exemplary method if necessary. With the heat transfer  25  circulation loop interrupted, the preheating circulation loop established, the rundown tank  24  vented and the pump  26  activated, a melting-point-altering material selected from among water, a hydrating fluid, and steam is added to the fluid. When a rundown tank temperature sensor indicates that the body of salt in the rundown tank  24  is at a temperature consistent with sufficient hydration, or when a rundown tank hydration measuring device  188  indicates sufficient hydration, the preheating circulation loop is interrupted and the processing circulation loop is established, and the aforementioned THIRD exemplary method can be practiced. 
     With reference to  FIG. 4 , a FIFTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention entails rehydrating the dehydrated salt, or re-making the melting-point-reduced material, as the case may be, on-site in the rundown tank  24  after ceasing processing and draining the fluid  25  to the rundown tank  24 . This method is useful when it is preferred to store liquid hydrated salt instead of letting dehydrated salt solidify. The salt is rehydrated in accordance with the aforementioned FOURTH exemplary method, using the molten salt that was drained into the rundown tank  24 . 
     With reference to any of  FIGS. 1-7 , a SIXTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out by carefully managing the performance of the pump  26  to avoid excess pressure. This may be done by measuring the pressure of fluid  25  arriving at the thermal processor  22 , and delivering the heat transfer fluid  25  to the thermal processor  22  at a flow rate adjusted to effect the correction. 
     With reference to  FIGS. 2-7 , a SEVENTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out by elevating the heat transfer fluid  25  relative to the thermal processor  22  so as to establish a gravity fluid  25  pressure head with the heat transfer fluid  25  entering the thermal processor  22  at a pressure at least within the operating heat transfer fluid  25  pressure range. Thus, it becomes unnecessary to modulate pump performance in order to ensure that fluid  25  is delivered at adequate pressure to the thermal processor  22 . As long as the fluid  25  is capable of flowing and is being made available to flow downward from the correct height into the heat transfer fluid inlet  38  of the thermal processor  22 , there will be a predetermined pressure. 
     Also with reference to  FIGS. 2-7 , an EIGHTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out in accordance with the aforementioned SEVENTH exemplary method, with the step, while delivering the fluid  25  to the thermal processor  22 , of passively diverting the heat transfer fluid  25  to bypass the thermal processor  22  in an amount sufficient to prevent the pressure exceeding the operating heat transfer fluid  25  pressure range. The gravity tube upper drain  206  fluidly communicating with the gravity tube  204  accomplishes this as do the stem pipe  48  (fluidly connected to the gravity tube upper drain  206 ) and fill tank  30  (fluidly connected to the gravity tube  204 ). 
     When these methods are practiced with structure including a fill tank  30  and a stem pipe  48 , greater capacity is available to supply fluid  25  to the thermal processor  22  during, e.g., a momentary interruption of pumping. 
     Positioning the heat transfer fluid inlets  38  at elevations lower than those of the heat transfer fluid outlets  84  facilitates passive drainage of the heat transfer fluid space  32  at shutdown and clearance of gas from the heat transfer fluid space  32  when fluid  25  fills the heat transfer fluid space  32 . 
     With a vacuum breaker  92  fluidly communicating with the heat transfer fluid outlet  84  at a high elevation relative to the heat transfer fluid space  32 , and a vacuum breaker connector tube  93  fluidly connecting the vacuum breaker  92  with the rundown tank headspace portion  40 , gas pressure and vacuum will not impede drainage of the heat transfer fluid space  32 . 
     Fluidly connecting the fill tank headspace portion  72  to the rundown tank headspace portion  40  avoids pressure differentials interfering with filling and passive drainage of the fill tank  30  and the heater. 
     Limiting the operating heat transfer fluid  25  pressure range of the thermal processor  22  to between −12 PSIG and +14.9 PSIG, inclusive, avoids the need to fabricate the thermal processor  22  with a certified pressure boundary material. 
     Timing of delivery of fluid  25  to the thermal processor  22  is facilitated, during melting, by measuring a temperature of the melting material and starting to deliver the fluid  25  via the heat transfer  25  circulation loop when the measured temperature has reached a predetermined value, and alternatively by measuring the water content of the fluid  25  and starting delivery when the measured water content has reached a predetermined value. 
     Timing of the addition of a hydration fluid to the heat transfer fluid  25  is facilitated, during melting, by measuring a temperature of the melting material and beginning to add the hydration fluid when the measured temperature has reached a predetermined value. 
     Steam must be vented from the heater  28  and rundown tank  24  during hydration, and from the fill tank  30 , via the gravity tube gas orifice  208  to the rundown tank  24 , during dehydration. 
     A NINTH exemplary method of operating a molten-salt-indirectly heated screw-type thermal processor in accordance with the present invention is carried out by supplying a padding gas to the rundown tank headspace portion  40  when the rundown tank headspace portion  40  is underpressurized relative to the ambient environment, and venting a gas from the rundown tank head space portion  40  when the rundown tank headspace portion  40  is overpressurized relative to the ambient environment. This management of gas pressure differentials prevents vapor lock, vacuum lock and rupture. 
     At shutdown, when the fluid  25  is passively draining to the rundown tank  24 , conducting a gas from the rundown tank headspace portion  40  to the thermal processor  22  fluid  25  outlet via the vacuum breaker  92  relieves vacuum lock in the thermal processor  22 . 
     During start-up and continuous operation, conducting a gas from the thermal processor  22  head space  32  to the rundown tank headspace portion  40  relieves vapor lock which could impede filling of the thermal processor  22 . 
     Also in accordance with the present invention, with reference to  FIG. 4 , AN EXEMPLARY EMBODIMENT of a phase-separating pressure modulator for molten-salt-indirectly heated screw-type thermal processing apparatus comprises a fill tank  30  having a fill tank bottom portion  36 ; a heater output tube  80  fluidly communicating with the fill tank  30  at the fill tank bottom portion  36 ; a gravity tube  204  fluidly communicating with the fill tank  30  at the fill tank bottom portion  36  and fluidly communicating with a fluid delivery destination such as the rundown tank  24 ; a stem pipe  48  fluidly communicating with the fill tank  30  at an elevation above the fill tank bottom portion  36 ; a fill tank headspace portion  72  defined as a portion of the fill tank above the elevation where the stem pipe  48  fluidly communicates with the fill tank  30 ; and a fill tank headspace vent  73  or gravity tube gas orifice  208  fluidly communicating with the fill tank headspace portion  72  and with a fluid drainage destination such as the thermal processor  22 . Advantageously, during dehydration, when a mixture of molten salt and water vapor passes from the heater  28  to the fill tank  30 , the fill tank separates the steam from the molten salt under the influence of gravity, enabling the steam to escape via the fill tank headspace vent  73  or gravity tube gas orifice  208  while the molten salt fluid  25  flows to the thermal processor  22 , rundown tank  24  or both. While this function is performed, the previously described function of regulating the pressure of the fluid  25  at the heat transfer fluid inlets  38  of the thermal processor  22  is also performed. 
     A pump  26  may refer to any energetic mechanism for urging or circulating a material within the apparatus. A gravity tube gas orifice  208  preferably fluidly communicates with the rundown tank headspace portion  40 , as, e.g., via a connector tube. While exemplary apparatus and methods in accordance with the present invention may be claimed with recitation of a molten-salt-indirectly heated screw-type thermal processor, the advantages of embodiments of the apparatus and instances of the method in accordance with the present invention are applicable without strict limitation as to the composition of the heat transfer fluid or as to the type of conveyance if any used in the thermal processor. The apparatus shown in any of  FIGS. 1-3  and  FIGS. 5-6  should be regarded as being capable of having one or more heating elements  184  as shown in  FIG. 4  when and as needed. In most situations, the presence of fire tubes  236  reduces the need for auxiliary heating of the rundown tank fluid containing portion  41 . The apparatus shown in  FIG. 3  should be regarded as being capable of having a fill tank headspace vent  73  (often useful to vent steam during heating) as shown in, e.g.,  FIG. 4 . 
     As can be seen from the drawing figures and from the description, each embodiment of the apparatus and method for fluid-heated indirect thermal processing in accordance with the present invention solves a problem by addressing the need for safe, cost-effective, efficient, simple, reliable structure and steps in the thermal processing of materials. 
     While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that any arrangement calculated to achieve same purposes can be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments of the invention. It is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Combinations of the above embodiments, and other embodiments not specifically described herein will be apparent to those of skill in the art upon reviewing the above description. The scope of various embodiments of the invention includes any other applications in which the above structures and methods are used. Therefore, the scope of various embodiments of the invention should be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled. 
     In the foregoing description, if various features are grouped together in a single embodiment for the purpose of streamlining the disclosure, this method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims, and such other claims as may later be added, are hereby incorporated into the description of the embodiments of the invention, with each claim standing on its own as a separate preferred embodiment.