Patent Application
20130008186
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 known fin tube heat exchangers for liquid nitrogen and carbon dioxide chilled or frozen applications, or a snow bunker for solid CO2 snow (dry ice) chilled or frozen applications. Such known systems experience problems of safety, temperature control, cool down rates, dual temperature zone control, efficiency and fouling.
For a more complete understanding of the present inventive embodiments, reference may be had to the following drawing figures taken in conjunction with the description of the embodiments, of which:
Heat plates (flat 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 plates can be adjusted to deliver precise heat transfer rates to the system by using variable conductivity heat plates.
Referring to
The heat exchanger 10 includes a housing 18 which functions as a shroud. The housing 18 may be referred to herein as a housing 18, shroud or shroud housing. The shroud housing 18 includes an inlet 20 in communication with an internal chamber 22 of the housing, which in turn is in communication with an outlet 24 or discharge end of the housing. A fan 26 or plurality of fans are mounted at the inlet 20 for drawing air 27 from the space 14 into the inlet 20 and moving the air through the internal chamber 22 for discharge at the outlet 24 into the space 14, as indicated by arrows 28 showing an air flow through the housing 18. The outlet 24 may have a curved or arcuate portion 25 to direct the airflow 28 to a more centralized region of the space 14.
Another housing which may be constructed as a solid conductive metal block 30 is disposed in the internal chamber 22 and exposed to the airflow 28. The metallic block 30 can have a rectangular cross section as shown in
Liquid cryogen, such as liquid nitrogen (N2) or liquid carbon dioxide (CO2), is provided as indicated by arrow 37 through a cryogen inlet pipe 36 in communication with one of the passages 32 in the block 30. A modulating type value 38 may also be installed for use with the inlet pipe 36. The liquid cryogen enters one end of the block 30 and is transferred through the internal flow path to an opposite or terminating end of the flow path where it is discharged as a cryogenic gas or vapor 39 through the cryogen vapor outlet pipe 40. The cryogen vapor outlet pipe 40 may include a modulating type valve 42 which is used to control the mass flow rate of cryogen flowing through the block 30.
A heat plate assembly includes a heat plate 44 fabricated from for example copper or stainless steel and is disposed at one side, such as for example a top or an upper side, of the metallic block 30. The heat plate 44 is exposed to the airflow 28 in the internal chamber 22. A heat plate 46 fabricated from for example copper or stainless steel is mounted to another side, such as for example a bottom or opposed side, of the metallic block 30 and is exposed as well to the airflow 28 within the internal chamber 22.
An airflow separator 48 or airfoil is disposed at an upstream end 50 of the metallic block 30 proximate the inlet 20. The air foil is disposed such that it is positioned at the leading edge or the upstream end 50 of the metallic block 30 to separate the airflow 28 at the inlet 20, such that approximately fifty percent (50%) of the airflow moves along and contacts the heat plate 44, while approximately the other fifty percent (50%) of the airflow 28 moves along and contacts the heat plate 46 at the bottom of the metallic plate 30. Heat flux or heat transfer occurs at an interface at the heat plates 44,46 and the airflow 28.
The airflow separator 48 may have a triangular shape cross-section for example, to bifurcate the airflow 28 to move along upper and lower sides of the metallic block 30, or alternatively have a frustoconical shape to guide the airflow 28 along all sides of the block 30. In most constructions of the heat exchanger apparatus 10, the internal chamber 22 is sized and shaped so that the metallic block 30 takes up or uses most of the volume of said chamber, except where the heat plates 44,46 are located so that the air flow 28 is substantially directed along sides of the metallic block where the heat plates are exposed for contact with said air flow.
The heat from the warm air drawn in by the fans 26 is transferred via the heat plates 44,46 to the colder solid metallic block 30 in which is contained a flow of liquid cryogen 37. The thermal conductivity of the heat plates 44,46 can be adjusted by selecting different sizes of heat plates or different materials from which the heat plates are fabricated, and/or adjusting the fan speed to match the required refrigeration load of the heat exchanger embodiment 10. In addition, variable conductivity heat plates can be used for the plates 44,46 for active control of the heat flux or heat transfer to provide a wide range of heat flux and temperature gradients at the plates and to the airflow 28. Warmer cryogen vapor or gas is discharged from the cryogen vapor outlet pipe 40 for subsequent use or exhaust to the external atmosphere.
The airflow 27 introduced at the inlet 20 is substantially cooled upon exposure to the heat plates 44,46 for discharge at the outlet 24 downstream 52 of the metallic block 30. The airflow 28 cools at the interface of said airflow and the heat plates 44,46. A sensor 41 senses temperature of the space at least upstream of the shroud housing 18.
The metallic block 30 can be mounted in the internal chamber 22 by use of mechanical fasteners 54 or brackets connecting the metallic block to the housing 18.
The cryogen heat plate heat exchanger 10 can be used for example in the compartments of trucks, barges and train flatbeds.
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.
