LNG (liquefied natural gas) and LIN (liquid nitrogen) in transit refrigeration heat exchange system

A heat exchanger includes a housing disposed in a first atmosphere and having an upstream end, a downstream end and a chamber within the housing; a metallic block disposed in the chamber and having a passageway therethrough and through which a cryogen can flow; and a heat pipe assembly in contact with the metallic block and extending to a second atmosphere which is separate from the first atmosphere for providing heat transfer at the second atmosphere.

BACKGROUND

The present embodiments relate to heat transfer for refrigerating spaces such as for example spaces that are in transit.

In transit refrigeration (ITR) systems are known and may include cryogenic ITR systems which use fin tube heat exchangers for liquid nitrogen and carbon dioxide chilled or frozen applications, or a snow bunker for solid CO2snow (dry ice) chilled or frozen applications. Such known systems experience problems of safety, temperature control, cold down rates, dual temperature zone control, efficiency and fouling.

DETAILED DESCRIPTION OF THE INVENTION

Heat pipes can be used instead of known fin tube heat exchangers to achieve comparable heat transfer with minimal air surface contact area, thereby eliminating issues resulting from snow accumulation on heat exchanger fins. In addition, the thermal conductivity of heat pipes can be adjusted to deliver precise heat transfer rates to the system by using variable conductivity heat pipes.

Referring toFIGS. 1-2, a cryogen heat exchanger embodiment is shown generally at10. The heat exchanger10is mounted for use with a compartment having a sidewall12defining a space14in the compartment. The heat exchanger10can be mounted to the sidewall12by mechanical fasteners16, such as for example brackets. The sidewall12may be insulated or vacuum jacketed.

The heat exchanger10includes a housing18. The housing18includes an insulated sidewall20defining an internal chamber22in the housing. An inlet24and an outlet26at the sidewall are in communication with the internal chamber22. A solid conductive metallic block28is disposed in the internal chamber22.

The metallic block28can have a rectangular cross section as shown inFIGS. 1-2, or can be formed with a cross section having another shape. Copper is one type of material which may be used for forming the metallic block28by way of example only, as other metals or alloys may be used, provided such are highly conductive and have sufficient heat transfer capabilities, i.e. highly thermally conductive. An internal area of the block28is formed with a plurality of bores30, channels or passages as shown in particular inFIG. 1. The plurality of passages30form a continuous internal flow path in a serpentine pattern within the block28. A “serpentine pattern” as used herein refers to a pattern that is winding or turning one way and another. Tubes32interconnect adjacent ones of the plurality of passages30, thereby providing for the continuous internal flow path. It may be from the construction of the metallic block28that the tubes32are observable from an exterior of the apparatus10, thereby providing an indication of the plurality of passages30within the block28, although this is not required for operation of the apparatus10.

A liquid cryogen, such as liquid nitrogen (LIN), is provided through a cryogen inlet pipe34to the inlet24in communication with one of the passages30in the block28, as indicated by arrow36. The liquid cryogen enters one end of the block28and flows through the internal flow path to an opposite or terminating end of the flow path, where it is discharged through the outlet26as a cryogenic gas or vapor38through a vapor outlet pipe40in communication with the outlet26. In this example, the liquid nitrogen would be discharged as gaseous nitrogen from the outlet pipe40. This is the case the liquid nitrogen changes to a gas phase as it is warmed during its flow thorough the plurality of the passages30of the metallic block28. The outlet pipe40may include a modulating type valve41which is used to control the mass flow rate of cryogen flowing through the block28.

Referring toFIG. 1, the sidewall12of the compartment space14is formed with holes42extending therethrough, such that when the apparatus10is mounted to the wall12each one of the holes42will receive a corresponding one of a plurality of heat pipes44extending from within the metallic block28through the holes42and into the space14of the compartment. The heat pipes44may be provided as shown in an assembly or in an array. Seals46or gasketing in the sidewall12prevent leakage or seepage of cryogen liquid and vapour into the compartment space14. Seals or gasketing is required if the heat pipes44penetrate into one of many of the passages30in the metallic block28. If the heat pipes44terminate in the solid block28only, then there is little if any possibility of cryogen liquid and vapor entering the compartment space14.

By way of example only, any number of heat pipes44may be used, depending upon the chilling or freezing application to be employed within the space14, the products in the space and the volume of the space. By way of example only, 25-100 heat pipes may be used. Each one of the heat pipes44extends approximately 6″-12″ into the space14. The positioning of the heat pipes44is such that an end portion of each one of the heat pipes is embedded in the block28, while an opposite end portion of each one of the heat pipes is exposed to the atmosphere of the space14. Accordingly, the extreme cold of the liquid cryogen is transferred by conduction from the metallic block28through each heat pipe44to an opposite end of each one of the heat pipes exposed to the space14atmosphere, such that heat is transferred from the space14atmosphere to the cryogen36where it experiences a phase change and boils off. The gaseous or cryogen vapor38is vented or exhausted through the outlet pipe40to the atmosphere external to the apparatus10.

At a position where the heat pipes44protrude into the space14there is provided a shield48or shroud to protect the heat pipes from any products within or shifting about the space14of the compartment. The shroud48also facilitates air flow, represented generally by arrows50created by a circulation device52, such as a fan for example, or a plurality of fans, across the heat pipes44for a higher heat transfer rate proximate the heat pipes. Accordingly, the temperature of the air flow downstream of the heat pipes44at a position generally represented at54is lower than a temperature of the air flow upstream of the heat pipes proximate the fan52. The shroud48may be fabricated from metal. A plurality of fans52may be used to increase net heat transfer effect.

The fan52or plurality of fans are mounted at a shroud inlet56for drawing air from the space14into the inlet and moving the air through a shroud space58or channel for discharge back into the space, as indicated by the arrows50showing said air flow through the shroud. An outlet60of the shroud may have a curved or arcuate portion, as shown inFIG. 2, to direct the airflow50back to a more centralized region of the space14.

Heat from the warm air drawn in by the fans52is transferred via the heat pipes44to the colder solid metallic block28in which is contained the flow of cryogen. The thermal conductivity of the heat pipes44can be adjusted by selecting different sizes of heat pipes or different materials from which the heat pipes are fabricated, and/or adjusting the fan speed to match the required refrigeration load of the heat exchanger embodiment10. In addition, variable conductivity heat pipes can be used for the pipes for active control of the heat flux or heat transfer to provide a wide range of heat flux and temperature gradients at the pipes44and to the airflow50. A sensor62mounted at the sidewall12for example is used to sense temperature of the space14downstream of the shroud outlet60.

As mentioned above, the temperature of the space14can be controlled by varying the rate of the air flow across the heat pipes44. That is, if for example, the space14is to maintain a chilled temperature, such as for a vegetable food product for example, the fan(s) speed can be adjusted to thereby effect the heat transfer rate of the heat pipes44and controlling internal temperature of the space14. If a frozen food product is in the space14, then the fan speed would be adjusted to provide a higher heat transfer rate of the air flow50across the heat pipes44.

FIG. 3shows another embodiment101of the heat exchange apparatus for use with for example an ITR truck or other intermodal transportation vehicle. Elements illustrated inFIGS. 3 and 4which correspond to the elements described above with respect toFIGS. 1-2have been designated by corresponding reference numerals increased by 100, respectively. The embodiments ofFIGS. 3 and 4are designed for use in the same manner as the embodiment ofFIGS. 1 and 2, unless otherwise stated.

The embodiment101includes a housing118with an internal chamber122sized and shaped to receive a pair of metallic blocks128,129. The metallic block128is similar to that described above with respect to the embodiment ofFIGS. 1-2. The metallic block129can also be of a similar metallic construction as that of block128, however the block129will receive liquid natural gas at an inlet pipe135which will phase shift to a gas during its flow through passageway131, which can also have a serpentine pattern, to be discharged at outlet pipe137as natural gas.

The metallic blocks128,129are adjacent each other or nested together in the internal chamber122of the housing118. The heat pipes144which coact with the metallic block128can be disposed such that an end portion of the heat pipes144can terminate either in the metallic block128andfor in the passages130. In contrast, heat pipes147which are disposed for coaction with the metallic block129all have an end portion which terminates within the metallic block129. That is, none of the heat pipes129terminate in or are in contact with the passages131.

As shown inFIG. 3, liquid nitrogen can be provided to the inlet pipe134for said liquid nitrogen to be provided to the passages130of the metallic block128. The heat transfer which occurs with respect to the heat pipes144causes the liquid nitrogen to phase to gas such that gaseous nitrogen is exhausted through the outlet pipe140.

Liquid natural gas may be provided by the inlet pipe135for introduction to the passages131of the metallic block129. The liquid natural gas experiences a phase change and is exhausted as natural gas through outlet pipe137. The use of the heat pipes144,147with their corresponding metallic blocks128,129, respectively, enable two separate refrigerated liquids to be introduced and used in series such that the LNG block129may be used first for example, followed by the liquid nitrogen block128. Therefore, the air flow150is cooled or refrigerated first by exposure to the heat pipes147coacting with the metallic block129, afterwhich further cooling or refrigeration of the air flow150occurs upon contact with the heat pipes144coacting with the metallic block128.

Referring toFIG. 4, the cryogen heat pipe heat exchanger embodiment101is mounted to a compartment or trailer of a truck64or other in transit vehicle or mode of transportation to provide ITR. Although the heat pipe heat exchanger may be mounted anywhere along the sidewall112of the compartment space114, a top (as shown) or side mounted embodiment is more desirable because the shroud148and heat pipes144,147protruding into the compartment will be exposed to and consume valuable floor space for pallets (not shown) or other products that would be deposited on a floor of the compartment. Mounting the cryogen heat pipe heat exchanger to the top of the compartment, as opposed to the bottom of the compartment, will also protect the shroud and heat pipes extending into the compartment from being damaged due to products or pallets shifting within the compartment.

As shown inFIG. 4, for the embodiment101ofFIG. 3mounted to the top of the compartment of the truck, pipe(s) would be used to connect tanks of liquid nitrogen and liquid natural gas for this embodiment.

The cryogen heat pipe heat exchanger101shown mounted to the top of the compartment space114is constructed and arranged to be provided with liquid cryogen through pipes72,74connected to liquid cryogen storage vessels66,68. In this embodiment, the vessel66contains liquid nitrogen, and the vessel68contains liquid natural gas. The vessels66,68are the source for the liquid cryogen during for example ITR. The vessels66,68may be mounted for operation beneath a bottom70of the compartment space114. The vessels66,68have sidewalls which are vacuum jacketed or surrounded by insulation material, and the pipes72,74distributing the liquid cryogen to the exchanger101may also be insulated or vacuum jacketed. The vessels66,68are maintained under a pressure at a range from of 2 to 8 Barg to force the liquid cryogen from the vessels through the pipes72,74and into the heat exchanger101.

A heat pipe76extends between the vessels66,68with one end75of the heat pipe76in communication with liquid nitrogen in the vessel66, and an opposite end77of the heat pipe76in communication with liquid natural gas in vessel68. The heat pipe76may be a variable conductance heat pipe having the opposed ends75,77disposed in the liquid storage vessels66,68. Since liquid nitrogen (LIN) is colder than liquid natural gas (LNG), heat can be transferred from the LNG vessel to the LIN vessel, thereby recondensing any gaseous LNG in the vessel68. The heat pipe76may be disposed in a head space (vapor area) of each of the vessels66,68, or for a more effective heat phase change, the end75of the heat pipe76may be disposed in the liquid nitrogen, while the end77of the heat pipe76may be disposed in the head space (vapor area) of the vessel68.

A sensor80is mounted for sensing the temperature in the space114and can be connected to a control panel (not shown) for receiving a signal of the temperature sensed and then adjusting the amount of liquid cryogen flow necessary from each one of the vessels66,68, depending upon the temperature that must be obtained and maintained in the space. Sensor probes, such as for example capacitance probes (not shown), may also be mounted to each one of the corresponding vessels66,68to sense the level of the cryogen liquid in the corresponding vessel and generate a signal of same which is transmitted to the control panel (not shown). Temperature in the vessels66,68is not controlled, but rather the heat pipe76is used to phase change the vapor in the head space of the tank68so that no LNG needs to be vented to the atmosphere. This provides for a stable, constant pressure in the vessel68so that LNG does not have to be vented. There is however, no problem with venting the LNG from the tank66. Temperatures in the compartment space114can also be maintained by adjusting the pressure in the vessel66or with the use of variable conductance heat pipes as discussed above. As shown inFIG. 4, a door78provides access to the compartment114.

A pipe82may be connected to the exhaust pipe137to direct the natural gas to an engine84of the truck64. The pipe82can be jacketed or insulated, although not necessary. The gaseous LNG from the heat exchanger101is fed directly to the engine84to power the truck64, while the gaseous nitrogen is discharged or vented by the pipe140to the atmosphere. The demand by the engine84will determine the demand upon the amount of LNG to be provided from the heat exchanger101through the pipe82to the engine84.

The pipes72,74can also be insulated or jacketed if disposed at an exterior of the sidewall112. Alternatively, the pipes72,74can be disposed inside the compartment114or possibly embedded in the wall112of the compartment.

All of the embodiments discussed above with respect toFIGS. 2-4also provide for gasketing or seals such as those called for inFIG. 1, where the heat pipes extend through the wall of the tank and the wall of the compartment.

The compartment ofFIG. 4may be mounted or constructed as a part of the truck64, trailer, automobile, railcar, flatbed, barge, shipping container or other floating vessel, etc., hence the ability to provide in-transit refrigeration (ITR).

It will be understood that the embodiments described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result.