Patent Publication Number: US-6707987-B2

Title: Electric liquefied petroleum gas vaporizer

Description:
TECHNICAL FIELD 
     This invention relates to a vaporizer for vaporizing liquefied gases such as liquefied petroleum gas, and in particular, to heat exchangers used in liquefied gas vaporizers. 
     BACKGROUND OF THE INVENTION 
     Vaporizers for the controlled vaporization of liquefied gases are generally known. One electrically heated liquefied petroleum gas (LPG) vaporizer is disclosed in U.S. Pat. No. 4,255,646. Another liquefied gas vapor unit is disclosed in U.S. Pat. No. 4,645,904. Typically, the vaporizer includes a hollow, pressure vessel having a liquefied gas inlet near a lower end and a gas vapor outlet near a closed upper end remote from the liquefied gas inlet. A heating core is typically disposed within the pressure vessel, usually positioned close to the lower end. A plurality of resistive electric heating element may be embedded within the heating core. 
     Such vaporizers using electric heating elements often require the use of a temperature sensor coupled with a time proportional controller for applying power to the heating elements with a periodic on/off duty cycle determined by the deviation of the core temperature from a predetermined set point. An increase of the core temperature above the set point proportionately reduces the on time of the duty cycle, while a decrease of the core temperature below the set point proportionately increases the on time of the duty cycle. Control circuitry including switches are required. 
     The vaporizer may also have liquefied gas sensing means communicating with the interior of the pressure vessel near its upper end, below the gas vapor outlet. The liquefied gas sensing means is typically an overflow sensor or “float switch” for sensing the level of liquefied gas in the pressure vessel and controlling a valve that opens and closes to stop the flow of liquefied gas into the pressure vessel. Accordingly, the valve is controlled to open the pressurized flow of liquefied gas into the pressure vessel and to shut off the flow before the liquefied gas fills the gas vapor head space and liquefied gas floods through the outlet of the vaporizer. 
     A problem with such known vaporizers is the need to control the on/off duty cycle of the electric heater elements to prevent overheating. The circuitry required creates safety concerns, and in addition, maintenance and reliability concerns are created. Further, the circuitry increases the cost of manufacturing the vaporizer. 
     SUMMARY OF THE INVENTION 
     The present invention resides in a vaporizer for vaporizing a fluid with a heat exchanger having a mass of thermally conductive material and a tube embedded therein to transfer heat from the thermally conductive material to the contents of the tube, and a plurality of positive temperature coefficient heater elements thermally coupled to heat to the thermally conductive material. The tube has an inlet portion to receive the fluid to be vaporized and an outlet portion to discharge the vaporized fluid. 
     In one embodiment of the vaporizer, the heat exchanger has a block of thermally conductive material with a tube embedded therein and with a planar surface portion. The heater elements are each flat with a substantially planar surface arranged in coplanar parallel arrangement with the planar surface portion of the block. The block further includes an end surface, and the inlet and outlet portions of the tube project from the end surface of the block. 
     In this embodiment, the heater elements are electrically coupled in parallel and each has a cure temperature greater than the saturation temperature of the fluid to be vaporized. The heater elements are connectable directly to an electrical power source without regulation by the vaporizer of the power supplied by the power source. The tube extends within the block along a curved path. 
     In one embodiment, the vaporizer includes a first heat exchanger having a first block of thermally conductive material with a first tube embedded therein to transfer heat from the thermally conductive material of the first block to the contents of the first tube, with the first block having a surface portion. The first tube has an inlet portion to receive the fluid to be vaporized and an outlet portion to discharge the vaporized fluid. The vaporizer further includes a second heat exchanger having a second block of thermally conductive material with a second tube embedded therein to transfer heat from the thermally conductive material of the second block to the contents of the second tube, with the second block having a surface portion. The second tube has an inlet portion to receive the fluid to be vaporized and an outlet portion to discharge the vaporized fluid. The first and second blocks are arranged with the surface portions thereof facing each other, and the outlet portion of the first tube connected to the inlet portion of the second tube. This embodiment further includes a plurality of positive temperature coefficient heater elements. Each heater element is formed with first and second opposed surfaces. The heater elements are positioned between the first and second blocks with the first surfaces of the heater elements in thermal contact with the surface portion of the first block and with the second surfaces of the heater elements in thermal contact with the surface portion of the second block. 
     The inlet and outlet portions of the first and second tubes project from the respective first and second blocks. The vaporizer further includes at least one member holding the first and second blocks tightly together with the heater elements positioned therebetween clamped tightly between the surface portions of the first and second blocks. 
     In this embodiment, the heater elements may be arranged in a single row alignment. The heater elements are elongated and each is oriented with a longitudinal axis arranged transverse to a direction of the row, and every other one of the heater elements in the row is longitudinally offset from the adjacent heater elements. 
     The first block further includes an end surface, and the inlet and outlet portions of the first tube project from the end surface of the first block. The second block further includes an end surface, and the inlet and outlet portions of the second tube project from the end surface of the second block. The end surfaces of the first and second blocks are arranged one adjacent to the other, and the outlet portion of the first tube is connected to the inlet portion of the second tube at a location adjacent to the adjacent end surfaces. 
     In some embodiments, the vaporizer includes a chamber with the thermally conductive material being a fluid contained within the chamber. The heater elements are immersed in the thermally conductive fluid. 
     In some embodiments the tube includes a coiled portion embedded in the thermally conductive material. The thermally conductive material may have a cylindrical shape with a longitudinal axis and the coiled portion of the tube may be arranged about the longitudinal axis. The heater elements may each include a rod shaped portion embedded in the thermally conductive material. 
     A method is also disclosed for forming a low-profile vaporizer with the foregoing constructions. 
     Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a liquefied gas vaporizer embodying the present invention having a heat exchanger comprised of two stacked heat exchanger blocks and a capacity control valve. 
     FIG. 2 is a schematic view of the vaporizer of FIG. 1 showing the capacity control valve used to control the inflow of liquefied gas to the heat exchanger in greater detail. 
     FIG. 3 is an isometric view of a vaporization tube used in each of the heat exchanger blocks of the vaporizer of FIG.  1 . 
     FIG. 4A is an isometric view of a positive temperature coefficient (PTC) heating element used to supply heat to the heat exchanger blocks of the vaporizer of FIG.  1 . 
     FIG. 4B is a front view of the heating element shown in FIG.  4 A. 
     FIG. 5 is a fragmentary isometric view of one of the heat exchanger blocks showing placement of four of the heating elements of the vaporizer of FIG.  1 . 
     FIG. 6 is an isometric view of the vaporizer of FIG. 1 shown partially assembled with one to the heat exchanger blocks show in phantom line to better illustrate the vaporization tube encased therein. 
     FIG. 7 is a cross-sectional side view of a second embodiment of a heat exchanger of a liquefied gas vaporizer embodying the present invention. 
     FIG. 8 is a cross-sectional end view taken substantially along line  8 — 8  of FIG.  7 . 
     FIG. 9 is a cross-sectional side view of a third embodiment of a heat exchanger of a liquefied gas vaporizer embodying the present invention. 
     FIG. 10 is a cross-sectional end view taken substantially along line  10 — 10  of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in a liquefied gas vaporizer  10 . The vaporizer  10  is shown in FIG. 1 as including a heat exchanger  12  comprised of two heat exchanger blocks  14  mounted face-to-face with eight positive temperature coefficient (PTC) heating elements  16  sandwiched between the heat exchanger blocks. In practice, ten PTC heating elements are used. One of the heat exchanger blocks is designated the first heat exchanger block and identified by reference numeral  14 A, and the other of the heat exchanger blocks is designated the second heat exchanger block and identified by reference numeral  14 B. 
     Each of the heat exchanger blocks  14  is formed of a rectangular casting of a thermally conductive material, such as aluminum, with an integral vaporization tube  18  encased therein, as best shown in FIGS. 3 and 6. Each of the vaporization tubes  18  has an inlet  20  and an outlet  22 . The vaporization tubes  18  of the heat exchanger blocks  14  are coupled together in series by a coupler tube  24  connecting the outlet  22  of the vaporization tube  18  of the first heat exchanger block  14 A and the inlet  20  of the vaporization tube  18  of the second heat exchanger block  14 B. 
     The heat exchanger blocks  14  are secured tightly together in face-to-face relation with the heating elements  16  sandwiched between them by a plurality of bolts  26 , or alternatively other fasteners or clamps. An alternating current electrical power supply  28 , operating at 110 to 240 volts, supplies electrical power to the heating elements  16 . A capacity control valve  30  is coupled to the inlet  20  of the vaporization tube  18  of the first heat exchanger block  14 A and controls the flow of liquefied gas from a liquefied gas source  32 , such as a liquefied petroleum gas storage tank, to the heat exchanger  12 . The vaporized gas exits through the outlet  22  of the vaporization tube  18  of the second heat exchanger block  14 B and is supplied to a gas vapor outlet tube  29 . 
     One of the PTC heating elements  16  used in the vaporizer  10  is shown by itself in FIGS. 4A and 4B. Such PTC heating elements are well known and include a pair of spaced-apart planar conductive plates  16   a  and  16   b  with a plurality of “stone” elements  16   c  positioned between the conductive plates. The PTC heating elements  16  have a flat, low side profile. An electrical lead  16   d  is attached to the end of one plate and an electrical lead  16   e  is attached to the end of the other plate to supply a voltage across the stones between the conductive plates. The stones  16   c  are arranged in a row between the conductive plates  16   a  and  16   b  with each stone having one face in electrical contact with one conductive plate and an opposite face in electrical contact with the other conductive plate. In the embodiment of the invention described, the PTC heating element is the EB style, using 5 stones sold by Dekko Enterprise of North Webster, Ind. 
     The stones  16   c  are composed of a thermally sensitive semiconductor resistor material that generates heat in response to a voltage applied across it by the conductive plates  16   a  and  16   b , and have the characteristic of producing substantially the same heat output regardless of the voltage applied across it. As such, the PTC heating elements  16  produce a very constant heat output independent of the voltage used for the electrical power supply  28 . This avoids having to carefully and accurately regulate the power source for the PTC heating elements  16  as is required in conventional electrical heater vaporizers so as to produce the desired heat. This produces a simpler and less expensive vaporizer. It also reduces the need and expenses incurred with conventional vaporizers requiring highly regulated power when adapting them for use in other countries that have very different power supply systems. The PTC heating elements  16  allow wide use without regard for the power supply system providing the electrical power for the heating elements. For example, a sample of the EB style, 5 stone PTC heating elements being used produces a surface temperature ranging from 103 to 117 degrees Centigrade when the voltage ranges from 120 volts to 230 volts, respectively. 
     Other advantages are realized by using the PTC heating elements  16 . As noted, the stones  16   c  are arranged in a row between the conductive plates  16   a  and  16   b  so that if one stone fails, the other stones between the conductive plates continue to operate and produce heat, thus making the heating element resistant to total failures. In this regard, as shown in FIG. 1, the leads  16   d  of the heating elements  16  are connected together, and the leads  16   e  of the heating elements are connected together, such that the heating elements are connected in parallel to the electrical power supply  28 . With this arrangement, should one of the heating elements  16  fail completely, the other heating elements will continue to have power supplied and to operate. A large enough number of heating elements  16  are used such that should some of the stones fail in several of the heating elements, or even several of the heating elements completely fail, the other heating elements will still provide enough heat to accomplish the desired vaporization of the liquefied gas supplied to the heat exchanger  12 . 
     Another advantage results from the fact that the PTC heating elements  16  are self-regulating in that they have a cure temperature at which they operate and they will reduce the heat they generate if the temperature of the environment in which they are operating starts to go above their cure temperature. Thus, even though the maximum heat production of the number of PTC heating elements  16  used in the heat exchanger  12  may be more than needed, there is no need to use control circuitry to regulate the supply of power using a varying duty cycling or other control technique for temperature control purposes. The electrical power supplied by the electrical power supply  28  is simply connected directly to the PTC heating elements  16  without fear of producing a dangerous overheated situation where the temperature increases without control. This eliminates the need for expensive heating element temperature control circuitry as required for conventional resistive heating elements and eliminates the fear of overheating. By selecting PTC heating elements with a cure temperature that is just above the saturation temperature of the liquefied gas for which the vaporizer  10  is designed to vaporize, the heat exchanger  12  tends to operate at the selected temperature at all times without a need for power regulation to control the heat generated. As such, there is also no need for a high limit safety circuit as a fail-safe as required in a conventional vaporizer to cut off power to the heating elements should even the heating element temperature control circuitry fail to avoid overheating. 
     Using the PTC heating elements  16  ensures a self-regulated temperature that, when properly selected, cannot exceed the auto-ignition temperature of gas vapor being produced by the vaporizer  10 . The self-regulated temperature is supplied constantly without power cycling that might otherwise generate sparks. 
     Each of the PTC heating elements  16  is packaged in an electrically isolating jacket  17  formed of a material having a high coefficient of thermal conductivity. The jacket  17  is shown in FIG. 4A partially removed to reveal the conductive plates  16   a  and  16   b  of the PTC heating element  16 . Thus, when the PTC heating elements  16  are tightly sandwiched between the conductive metal heat exchanger blocks, to promote good thermal conductivity therewith, the jacket  17  prevents the conductive plates  16   a  and  16   b  of the heating element from making electrical contact with the heat exchanger blocks while at the same time permitting the efficient transfer of the heat generated by the heating element through the jacket to the heat exchanger blocks. The electrically isolating, heat conductive jacket  17  of the PTC heating elements  16  used is made of KAPTON®, a polyamide film presently available from du Pont de Nemours and Company of Wilmington Del. The PTC heating element is shown fully inside its jacket  17  in FIG.  4 B. 
     To facilitate good thermal transfer from the PTC heating elements  16  to the heat exchanger blocks  14 A and  14 B, each of the heat exchanger blocks has a face  15  which is machined flat and the heat exchanger  12  is assembled with the flat faces  15  of the two heat exchanger blocks facing toward each other with the PTC heating elements  16  oriented with one of the conductive plates  16   a  and  16   b  toward the flat face of one of the heat exchanger blocks and the other of the conductive plates toward the flat face of the other heat exchanger blocks. Thus, the heat exchanger blocks  14 A and  14 B when bolted together using the bolts  26 , are separated by only the thickness of one of the PTC heating elements  16  to provide a low side profile to the heat exchanger  12  and a compact design. The flat faces  15  also provide good surface contact with nearly the entire flat exterior surfaces of both faces of the PTC heating elements  16  to facilitate maximum heat transfer to the heat exchanger blocks  14 A and  14 B. To further facilitate good heat transfer, a heat transfer grease  19  or other medium is applied so it is positioned between the faces of the PTC heating element and the flat face  15  of each of the heat exchanger blocks  14 A and  14 B, as shown for one heat exchanger block  14 B in FIG.  5 . While not illustrated in the drawings, to better distribute the heat generated by the heating elements  16 , every other heating element is shifted toward one or the other longitudinal edges of the heat exchanger blocks  14 A and  14 B, such that adjacent heating elements are longitudinally offset from each other. 
     While the vaporizer  10  shown and described has included two heat exchanger blocks  14 A and  14 B, it is to be understood that a vaporizer according to the present invention can be constructed using more that two heat exchanger blocks stacked atop each other with PTC heating elements  16  therebetween. As such, a vaporizer can be constructed using a modular approach by stacking together the necessary number of heat exchanger blocks with PTC heating elements therbetween to provide the vaporizer with the desired operating characteristics. Alternatively, a vaporizer can be constructed using only a single heat exchanger block with the PTC heating elements  16  mounted thereon. The vaporizer  10  and alternative constructions using the present invention have a very low profile and compact size, and can be inexpensively manufactured using off the shelf PTC heating elements  16  and other components. 
     The construction of the vaporizer  10  lends itself to mass manufacture and eliminates much of the expensive control and safety circuitry and other components previously required with vaporizers using electric heating elements. For example, the vaporizer  10  uses no thermostats, control boards, relays or high limit controls. Since the switching elements and circuitry used in conventional electric heater vaporizers have been eliminated, the vaporizer  10  is safer, more reliable and requires less maintenance. The construction of the heat exchanger blocks  14  using a casting with the vaporizer tube  18  formed integrally therein is inherently economical and maintenance free. Further, the vaporizer  10  has a potentially wider applicability since it is simpler and easier to use. It requires few, if any, adjustments or attention by the user so it can be safely used in applications even where a knowledgeable operator is not present. 
     The shape of the vaporizer tube  18  used in each of the heat exchanger blocks  14  is best seen in FIGS. 3 and 6. The vaporizer tube  18  extends within the heat exchanger block  14  in which embedded with a first portion extending from the end at which its inlet  20  is located with a generally serpentine pattern toward the opposite end of the heat exchanger block, and then turns back on itself with a second portion extending above the first portion with a generally serpentine pattern back towards the same end. The vaporizer  18  has its inlet  20  and outlet  22  at the same end of the heat exchanger block. This arrangement facilitates use of the coupler tube  24  to connect the outlet  22  of the vaporizer tube  18  of one heat exchanger block with the inlet  20  of the vaporizer tube of another heat exchanger block stacked on the first when connecting a plurality of heat exchanger blocks together in series. 
     The operation of the vaporizer  10  will now be described. As best shown in FIG. 2, the capacity control valve  30  includes a value inlet  34  connected to a liquefied gas inlet tube  36 , which is coupled to and receives liquefied gas from the liquefied gas source  32 . The capacity control valve  30  further includes a valve outlet  38  connected to a liquefied gas inlet tube  39 , which extends to the inlet  20  of the first heat exchanger block  14 A. The capacity control valve  30  is constructed generally the same as a thermal expansion valve (TEX), such as commonly used in air conditioning systems. However, the capacity control valve  30  is operated in reverse of the operation of a thermal expansion valve in an air conditioning system to perform a different function, as will be describe below. 
     The capacity control valve  30  includes a valve body  40  having a thermal expansion chamber  42 , a liquefied gas inlet chamber  44  and a liquefied gas outlet chamber  46 . A diaphragm  48  divides the thermal expansion chamber  42  from the liquefied gas inlet chamber  44 . In the illustrated embodiment, the diaphragm is a flexible, thin metal disk of conventional design. A thermal sensing bulb  50  is positioned in thermal contact with the gas vapor outlet tube  29  connected to the outlet  22  of the second heat exchanger block  14 B, which carries the vaporized gas from the heat exchanger  12 , at a location reasonably close to the heat exchanger outlet  22 . The thermal sensing bulb  50  is connected by a tube  52  to the thermal expansion chamber  42 . When the vaporizer  10  is implemented for use with liquefied petroleum gas as being described herein, the sensing bulb  50  is charged with an expansion fluid  54  having saturation properties similar to those of liquefied petroleum gas. The tube  52  provides fluid communication of the fluid  54  between the sensing bulb  50  and the thermal expansion chamber  42 . 
     The diaphragm  48  is configured to respond to a pressure differential between the thermal expansion chamber  42  and the liquefied gas inlet chamber  44 . At equilibrium, when the pressure in both chambers  42  and  44  is equal, the diaphragm  48  is balanced in an “at rest” position between the chambers  42  and  44 . A pressure difference between the thermal expansion chamber  42  and the liquefied gas inlet chamber  44  causes the diaphragm  48  to move or flex into the one of the chambers  42  and  44  having the lesser pressure therein. The degree of expansion, i.e., the distance that the diaphragm  48  moves into the lower pressure chamber, is a function of the difference in pressure between the chambers  42  and  44 : the greater the pressure differential, the farther the diaphragm  48  moves. Thus, the diaphragm  48  moves along a continuum that is infinitely variable in response to changes in the pressure differential between the thermal expansion chamber  42  and the liquefied gas inlet chamber  44 . 
     The valve inlet  34  of the capacity control valve  30  supplies the liquefied gas carried by the liquefied gas inlet tube  36  to the liquefied gas inlet chamber  44 . The valve outlet  38  discharges the liquefied gas in the liquefied gas outlet chamber  46  to the liquefied gas inlet tube  39  to supply the liquefied gas to the inlet  20  of the first heat exchanger block  14 A for vaporization by the heat exchanger  12 . An annular wall  56  with a central orifice  58  divides the liquefied gas inlet chamber  44  from the liquefied gas outlet chamber  46 . A valve seat  60  is formed on an underside of the annular wall  56 , about the orifice  58 , and a valve  62  is positioned below the annular wall and is operatively movable between a fully closed position with the valve seating in the valve seat, and a fully open position with the valve moved downward, substantially away from the valve seat. The valve  62  is positionable at all positions between the fully closed and fully open positions, as will be described in greater detail below. 
     When the valve  62  is in the fully closed position, in seated arrangement with the valve seat  60 , the valve blocks the flow of liquefied gas from the liquefied gas inlet chamber  44  into the liquefied gas outlet chamber  46 , and hence blocks the flow of liquefied gas to the heat exchanger  12 . As the valve  62  opens and moves downward progressively farther away from the valve seat  60 , the flow of liquefied gas from the liquefied gas inlet chamber  44  into the liquefied gas outlet chamber  46  progressively increases, as does the flow of liquefied gas to the heat exchanger  12 . As the open valve  62  moves upward progressively closer to the valve seat  60 , the flow of liquefied gas from the liquefied gas inlet chamber  44  into the liquefied gas outlet chamber  46  progressively decreases, as does the flow of liquefied gas to the heat exchanger  12 . 
     The movement of the valve  62  is principally controlled by the movement of the diaphragm  48  using a rigid valve stem  64 , which couples the valve  62  to the diaphragm  48  for movement therewith. An upper end of the valve stem  64  is attached to a central portion of the diaphragm  48 , and a lower end of the valve stem is attached to a central portion the valve  62 . When a pressure differential exists between the thermal expansion chamber  42  and the liquefied gas inlet chamber  44 , the diaphragm  48  moves toward the chamber with the lesser pressure therein, and the valve stem  64  causes the valve  62  to move in the same direction and by the same amount relative to the valve seat  60 . 
     In operation, the movements of the diaphragm  48  open and close the valve  62  as the relative pressures of the liquefied gas in the liquefied gas inlet chamber  44  and the liquid  54  in the thermal expansion chamber  42  change. If the pressure P BULB  of the liquid  54  in the thermal expansion chamber  42  should decrease, as a result of the sensing bulb  50  sensing the temperature of the gas vapor in the gas vapor outlet tube  20  decreasing, the diaphragm  48  will move upward into the thermal expansion chamber  42  and the valve stem  64  will drive the valve  62  upward. With sufficient upward movement the valve  62  will reach the fully closed position, with the valve seated in the valve seat  60  and the flow of liquefied gas to the heat exchanger  12  completely blocked. Of course, the direction and amount of movement of the valve  62  results from the amount and direction of the differential pressure experienced by the diaphragm  48 . If the pressure P IN  of the liquefied gas in the liquefied gas inlet chamber  44  should also increase or decrease, the valve  62  will move upward in a different amount, and could even move in the downward direction. 
     If the pressure P BULB  of the liquid  54  in the thermal expansion chamber  42  should increase, as a result of the sensing bulb  50  sensing the temperature of the gas vapor in the gas vapor outlet tube  29  increasing, the diaphragm  48  will move downward into the liquefied gas inlet chamber  44  and the valve stem  64  will drive the valve  62  downward. With sufficient downward movement the valve  62  will reach the fully open position, with the valve spaced far from the valve seat  60  and the flow of liquefied gas to the heat exchanger  12  substantially uninhibited. The more the movement opens the valve  62 , the larger the flow of liquefied gas to the heat exchanger. If the pressure P IN  of the liquefied gas in the liquefied gas inlet chamber  44  should also increase or decrease, the valve  62  will move downward in a different amount, and could even move in the upward direction. Again, the direction and amount of movement of the valve  62  results from the amount and direction of the differential pressure experienced by the diaphragm  48 , the differential pressure being the difference between the pressure of the liquid  54  in the thermal expansion chamber  42  (which is dependent on the temperature of the gas vapor in the gas vapor outlet tube  29  being measured by the sensing bulb  50 ) and the pressure of the liquefied gas in the liquefied gas inlet chamber  44  (which is dependent on the pressure of the liquefied gas being supplied to the vaporizer  10  by the liquefied gas source  32 ). 
     The pressure of the liquefied gas in the liquefied gas inlet chamber  44  is the inlet pressure of the liquefied gas supplied to the vaporizer  10  by the liquefied gas source  32 . This vaporizer inlet pressure changes with the conditions experienced by the liquefied gas source  32 , such as the temperature of the source, and the vaporizer inlet pressure tends to follow the saturation pressure of the input gas. Thus, the capacity control valve  30  controls the input flow of liquefied gas to the heat exchanger  12  based upon both the temperature of the gas vapor in the gas vapor outlet tube  29  and the inlet pressure of the liquefied gas supplied to the vaporizer  10  by the liquefied gas source  32 , unlike some prior art vaporizers which only controlled the input flow based upon the temperature of the gas vapor produced without concern for the inlet pressure of the liquefied gas being supplied to the vaporizer. As such, these prior art vaporizers do not adequately respond to the changing conditions of the liquefied gas input to the vaporizer. 
     As noted above, the amount and direction of the movement of the diaphragm  48 , and hence the amount and direction of movement of the valve  62  and the amount of liquefied gas that the valve allows to flow through the capacity control valve  30  into the inlet tube  39  of the heat exchanger  12 , are a function of the pressure differential between the thermal expansion chamber  42  and the liquefied gas inlet chamber  44 . Accordingly, a pressure within the liquefied gas inlet chamber  44  that is greater than the pressure in the thermal expansion chamber  42  will cause the diaphragm  48  to move upward and the valve stem  64  to move the valve  62  toward the valve seat  60  and the fully closed position, thereby progressively reducing the flow of liquefied gas to the heat exchanger  12 . Conversely, a pressure within the thermal expansion chamber  42  that is greater than the pressure of the liquefied gas inlet chamber  44  will cause the diaphragm  48  to move downward and the valve stem  64  to move the valve  62  away from the valve seat  60  and toward the fully open position, thereby progressively increasing the flow of liquefied gas to the heat exchanger  12 . Preferably, the valve  62 , the valve seat  60 , and the valve stem  64  are configured in combination with the diaphragm  48  such that when at equilibrium, with the pressure across the diaphragm balanced and the diaphragm  48  in the “at rest” position, the valve  62  is at a distance away from the valve seat  60  such that the pressurized flow of liquefied gas passing through the capacity control valve  30  and into the heat exchanger  12  is at a predetermined flow rate selected to provide the desired rated output of gas vapor in the outlet tube  29  at a desired superheated temperature under normal operation of the vaporizer  10 . 
     As discussed, the pressure differential across the diaphragm  48  is the difference between the inlet liquefied gas pressure P IN  within the liquefied gas inlet chamber  44  and the pressure P BULB  of the liquid  54  in the thermal expansion chamber  42 . Change in the temperature of the gas vapor exiting the heat exchanger  12  through the outlet tube  29  is indicative of a change in the operating condition occurring inside the heat exchanger  12 , with the liquid  54  within the sensing bulb  50  communicating that change of gas vapor temperature to the thermal expansion chamber  42 . As noted above, the sensing bulb  50  is charged with a fluid having saturation properties similar to those of the liquefied gas for which the vaporizer  10  of the invention is implemented, such as liquid petroleum gas for the embodiment described herein. Similarly, a change in the condition experienced by the liquefied gas source  32  is communicated to the liquefied gas inlet chamber  44  via the valve inlet  34 . In operation, the net result of these changes is movement of the diaphragm  48  and hence adjustment by the capacity control valve  30  of the liquefied gas supplied to the heat exchanger  12 . 
     For example, assuming that the diaphragm  48  was in the “at rest” position and the valve  62  was in a correspondingly open position, if a condition occurs such that the temperature of the vaporized gas in the outlet tube  29  goes down, the liquid  54  in the sensing bulb  50  contracts and the pressure in the thermal expansion chamber  42  decreases. This might result because the heat exchanger  12  is receiving a larger flow of liquefied gas than the heating elements  16  can vaporize with the desired gas vapor temperature. Assuming that there is no change also occurring in the condition of the liquefied gas source  32 , this will cause the valve  62  to move upward and reduce the flow of liquefied gas to the heat exchanger  12 . As the flow of liquefied gas to the heat exchanger  12  decreases, the heat produced by the heating elements  16  will be transferred to the now smaller flow of liquefied gas into the vaporization tube  18 . As a result, the temperature of the vaporized gas exiting the outlet  22  of the second heat exchanger block  14 B will begin to increase compared to the temperature of the vaporized gas the electric heater had been producing at the higher flow rate. As the temperature of the gas vapor in the outlet tube  29  sensed by the sensing bulb  50  rises, the liquid  54  will begin to expand and the pressure in the thermal expansion chamber  42  will increase. This will cause the valve  62  to move downward and further open the valve  62  to increase the flow of liquefied gas to the heat exchanger  12  until the flow rate through the vaporization tube  18  allows the heating elements  16  to produce gas vapor in the outlet  22  of the second heat exchanger  14 B at the desired temperature. 
     This operation also insures that only gas vapor, and not liquefied gas flows out the outlet  22  of the second heat exchanger block  14 B since should the heat exchanger  12  start flooding with liquefied gas, the gas vapor being produced will become very saturated and its temperature will drop, thus moving the valve  62  toward the fully closed position and restricting or even cutting off the flow to and from the heat exchanger  12  until the temperature of the gas vapor in the outlet tube  29  rises to the desired temperature. However, since the diaphragm  48  is responsive to the pressure P IN  of the liquefied gas in the liquefied gas inlet chamber  44  (i.e., the inlet pressure of the liquefied gas supplied to the vaporizer  10  by the liquefied gas source  32 ), and not just the temperature of the gas vapor in the outlet tube  29 , should a change in the inlet pressure be occurring at the same time, the operation of the capacity control valve  30  takes that into account. For example, if the inlet pressure is rising, the valve  30  will be closed even further, but if the inlet pressure is falling, the valve will not be closed as far, thereby producing overall better results than if only the temperature of the gas vapor in the outlet tube  29  was used to control the operation of the capacity control valve. Thus, the flow of liquefied gas into the heat exchanger  12  will be more accurately controlled to provide gas vapor at the desired temperature and the flow of liquefied gas into the heat exchanger  12  will not exceed the vaporization ability of the heating elements  16 . 
     In contrast to the flooding condition just discussed, should gas vapor in the outlet tube  29  increase in the temperature beyond the desired superheated temperature, the liquid  54  in the sensing bulb  50  will expand and the pressure in the thermal expansion chamber  42  increase. This might result because the heat exchanger  12  is receiving a smaller flow of liquefied gas than the heating elements  16  can vaporize with the desired gas vapor temperature, thus overheating the gas that is vaporized. Assuming that there is no change also occurring in the condition of the liquefied gas source  32 , this will cause the valve  62  to move downward and increase the flow of liquefied gas to the heat exchanger  12 . As the flow of liquefied gas to the heat exchanger  12  increases, the heat produced by the heating elements  16  will be transferred to the now larger flow of liquefied gas into the vaporization tube  18 . As a result, the temperature of the vaporized gas exiting the outlet  22  of the second heat exchanger block  14 B will begin to decrease compared to the excessive temperature of the vaporized gas the heating elements had been producing at the lower flow rate. As the temperature of the gas vapor in the outlet tube  29  sensed by the sensing bulb  50  lowers, the liquid  54  will begin to contract and the pressure in the thermal expansion chamber  42  will decrease. This will cause the valve  62  to move upward and further close the valve  62  to decrease the flow of liquefied gas to the heat exchanger  12  until the flow rate through the vaporization tube  22  allows the electric heater  12  to produce gas vapor in the outlet tube  20  at the desired temperature. As a result, the vaporizer  10  is self-regulating to always produce gas vapor at its maximum design capacity and at the desired temperature. 
     Again, since the diaphragm  48  is responsive to the pressure P IN  of the liquefied gas in the liquefied gas inlet chamber  44  (i.e., the inlet pressure of the liquefied gas supplied to the vaporizer  10  by the liquefied gas source  32 ), and not just the temperature of the gas vapor in the outlet tube  29 , should a change in the inlet pressure be occurring at the same time, the operation of the capacity control valve  30  takes that into account. For example, if the inlet pressure is falling, the valve  62  will be opened even further, but if the inlet pressure is rising, the valve will not be opened as far, thereby producing overall better results than if only the temperature of the gas vapor in the outlet tube  29  was used to control the operation of the capacity control valve. Thus, the flow of liquefied gas into the heat exchanger  12  will be more accurately controlled to provide gas vapor at the desired temperature. 
     The capacity control valve  30  includes a biasing spring  66  positioned between the valve  62  and an adjustment screw  68 , to apply an upward biasing force or spring pressure P SPR  on the valve tending to urge the valve toward the fully closed position. The biasing spring  66  is arranged directly below the valve  62 , in coaxial alignment with the valve stem  64 , and provides a resistance force against downward movement of the valve which must be overcome by the pressure P BULB  of the liquid  54  in the thermal expansion chamber  42 , in addition to the pressure P IN  within the liquefied gas inlet chamber  44 , to move the valve downward toward the fully open position. If the pressure P BULB  of the liquid  54  in the thermal expansion chamber  42  minus the sum of the pressure P IN  within the liquefied gas inlet chamber  44  and the spring pressure P SPR  is greater than zero, then the valve  62  will open (i.e., if: P BULB −[P IN +P SPR ]&gt;0, then the valve will open). 
     The adjustment screw  68  is located to engage and selectively adjustably move upward or downward the lower end of the biasing spring  66 . This is accomplished by rotating the adjustment screw to threadably move it inward or outward to increase or decrease, respectively, the amount of upward force the biasing spring  66  applies to the valve, which sets the “at rest” position of the diaphragm  48 , i.e., the position the diaphragm will assume if the pressure in both the chambers  42  and  44  is equal. The effect is to set the superheated temperature to which the heat exchanger  12  will heat the gas vapor in the outlet tube  29  under normal operation of the vaporizer  10 . The capacity control valve  30  thus prevents liquefied gas (in the illustrated embodiment LPG liquid) carryover into outlet tube  29  by ensuring a minimum amount of superheat within the heat exchanger  12 . 
     A second embodiment of a heat exchanger  100  according to the present invention is shown in FIGS. 7 and 8. In this embodiment, the heat exchanger  100  includes a solid cylindrical body  102  of cast aluminum or another suitable rigid material with a coiled vaporization tube  104  encased therein. The coil of the vaporization tube  104  is wound about a longitudinal axis of the cylindrical body  102 . The vaporization tube  104  has an inlet  106  to receive the liquefied gas from a liquefied gas source  32  (see FIG.  1 ), such as a liquefied petroleum gas storage tank, using a capacity control valve  30  (see FIG. 1) or otherwise. The vaporization tube  104  also has an outlet  108  from which the gas vapor exits the heat exchanger  100 . The inlet  106  and the outlet  108  project from a sidewall of the cylindrical body  102 . The inlet  106  is located toward a first end  110  of the cylindrical body  102  and the outlet  108  is located toward a second end  112  of the cylindrical body. The second end  112  of the cylindrical body  102  has a threaded end portion  114  to removably receive a threaded end cap  116 , which when threaded onto the threaded end portion of the cylindrical body defines an chamber  118  within the end cap. 
     In this second embodiment, the heat exchanger  100  includes four rod shaped heating elements  120  made of a positive temperature coefficient (PTC) material. Each of the heating elements  120  is positioned in one of four elongated, round apertures  122  in the cylindrical body  102  extending fully through the second end  112  of the cylindrical body and in communication with the chamber  118 , and toward the first end  110  of the cylindrical body but not extending outward of the cylindrical body at the first end. The apertures  122  can be made as part of the casting process, drilled, reamed or in another suitable manner. When in position in the aperture  122 , an end portion  123  of the heating element  120  projects out of the aperture and into the chamber  118 . A pair of electrical leads  124  is attached to the end portion  123  of each heating element  120  for supplying electrical power to the heating element. The electrical leads  124  extend into the chamber  118  and exit the chamber through a wire conduit  126  formed in the cylindrical body  102  which extends between the second end  112  of the cylindrical body, at a position within the chamber and covered by the end cap  116 , and a port  128  in the sidewall of the cylindrical body at a location toward the second end. The end cap  116  serves to protect both the heating elements  120  and the electrical leads  124  from damage. 
     In yet a third embodiment shown in FIGS. 9 and 10, very similar to that of FIGS. 7 and 8, the cylindrical body  102  has a body chamber  130  filled with water or another suitable heat transfer media. The heating elements  120  extend into the body chamber  130  and are in thermal contact with the heat transfer media therein. 
     The heating elements  120  used in the second and third embodiments, and the design of the heat exchanger  100  generally, provide the self-regulating heat and other benefits discussed above for the first described embodiment. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.