U.S. patent application number 12/785803 was filed with the patent office on 2011-11-24 for refrigeration system and process utilizing a heat pipe heat exchanger.
Invention is credited to Michael D. NEWMAN.
Application Number | 20110283716 12/785803 |
Document ID | / |
Family ID | 44971293 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110283716 |
Kind Code |
A1 |
NEWMAN; Michael D. |
November 24, 2011 |
REFRIGERATION SYSTEM AND PROCESS UTILIZING A HEAT PIPE HEAT
EXCHANGER
Abstract
A refrigeration system includes a cryogenic freezer and a
mechanical refrigerator, wherein the mechanical refrigerator
includes an enclosure containing an atmosphere, the cryogenic
freezer includes a cryogen exhaust conduit, and a heat pipe heat
exchanger including a warm end and a cold end, wherein the heat
pipe heat exchanger is in heat transfer relationship with the
mechanical refrigerator and the cryogenic freezer such that the
warm end of the heat pipe heat exchanger contacts the atmosphere
within the mechanical refrigerator enclosure, and the cold end of
the heat pipe heat exchanger is disposed within the cryogen exhaust
conduit of the cryogenic freezer. A refrigeration process includes
cooling an atmosphere within a mechanical refrigerator by exposing
the atmosphere to a warm end of a heat pipe heat exchanger, wherein
a cold end of the heat pipe heat exchanger is disposed within an
exhaust duct of a cryogenic freezer.
Inventors: |
NEWMAN; Michael D.;
(Hillsborough, NJ) |
Family ID: |
44971293 |
Appl. No.: |
12/785803 |
Filed: |
May 24, 2010 |
Current U.S.
Class: |
62/6 ;
165/104.26 |
Current CPC
Class: |
F28D 15/0275 20130101;
F25D 16/00 20130101; F25B 23/006 20130101 |
Class at
Publication: |
62/6 ;
165/104.26 |
International
Class: |
F25B 9/00 20060101
F25B009/00; F28D 15/02 20060101 F28D015/02 |
Claims
1. A refrigeration system, comprising a cryogenic freezer and a
mechanical refrigerator, wherein the mechanical refrigerator
comprises an enclosure containing an atmosphere, the cryogenic
freezer comprises a cryogen exhaust conduit, and a heat pipe heat
exchanger comprising a warm end and a cold end, wherein the heat
pipe heat exchanger is in heat transfer relationship with the
mechanical refrigerator and the cryogenic freezer such that the
warm end of the heat pipe heat exchanger contacts the atmosphere
within the mechanical refrigerator enclosure, and the cold end of
the heat pipe heat exchanger is disposed within the cryogen exhaust
conduit of the cryogenic freezer.
2. The refrigeration system of claim 1, wherein at least one fan is
disposed within the mechanical refrigerator for moving the
atmosphere over the warm end of the heat pipe heat exchanger.
3. The refrigeration system of claim 1, wherein the heat pipe heat
exchanger comprises a plurality of heat exchange pipes.
4. The refrigeration system of claim 3, further comprising a
plurality of fins associated with the plurality of heat exchange
pipes at the warm end of the heat pipe heat exchanger within the
mechanical refrigerator.
5. The refrigeration system of claim 1, wherein a working fluid
within the heat pipe heat exchanger comprises at least one of
water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane,
1,1,1,3,3-pentafluorobutane or perfluoropolyether.
6. The refrigeration system of claim 1, wherein a temperature
within the mechanical refrigerator is from about -40.degree. F.
(-40.degree. C.) to about 10.degree. F. (-12.2.degree. C.).
7. The refrigeration system of claim 1, where a temperature within
the cryogen exhaust conduit of the cryogenic freezer is from about
-120.degree. F. (-84.4.degree. C.) to about -80.degree. F.
(-62.2.degree. C.).
8. A refrigeration process, comprising cooling an atmosphere within
a mechanical refrigerator by exposing the atmosphere to a warm end
of a heat pipe heat exchanger, wherein a cold end of the heat pipe
heat exchanger is disposed within an exhaust duct of a cryogenic
freezer.
9. The refrigeration process of claim 8, further comprising
circulating the atmosphere within the mechanical refrigerator over
the warm end of the heat pipe heat exchanger.
10. The refrigeration process of claim 8, wherein the heat pipe
heat exchanger comprises a plurality of heat exchange pipes.
11. The refrigeration process of claim 10, further comprising a
plurality of fins coacting with the plurality of heat pipes on the
warm end of the heat pipe heat exchanger within the mechanical
refrigerator.
12. The refrigeration process of claim 8, wherein a working fluid
within the heat pipe heat exchanger comprises at least one of
water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane,
1,1,1,3,3-pentafluorobutane or perfluoropolyether.
13. The refrigeration process of claim 8, wherein a temperature
within the mechanical refrigerator is from about -40.degree. F.
(-40.degree. C.) to about 10.degree. F. (-12.2.degree. C.).
14. The refrigeration process of claim 8, where a temperature
within the cryogen exhaust conduit of the cryogenic freezer is from
about -120.degree. F. (-84.4.degree. C.) to about -80.degree. F.
(-62.2.degree. C.).
Description
[0001] The present embodiments are directed to a refrigeration
system and/or process for removing heat from a mechanical
refrigerator using a heat pipe heat exchanger in heat transfer
relationship with the mechanical refrigerator and a cryogenic
freezer exhaust.
[0002] Mechanical refrigerators are often used to store products
cooled or frozen by a cryogenic process. In these instances, the
cryogenic process may include a tunnel freezer, spiral freezer,
impingement freezer or immersion freezer in which a cryogenic fluid
is sprayed or otherwise distributed within the freezer in order to
cool or freeze the products passing through the tunnel freezer. The
products will typically pass into a storage area, which may be kept
cold by a mechanical refrigeration process.
[0003] Because the mechanical refrigeration storage area may be
very large, it is desirable to increase the efficiency of the
mechanical refrigeration. The cryogenic process produces large
amounts of cryogenic exhaust at a very low temperature, and
therefore still contains usable heat transfer capability such as
cooling power. In many processes, the cryogenic exhaust is merely
wasted to an external atmosphere, and its excess cooling power is
lost.
[0004] What is therefore needed is a refrigeration system and/or
process which is capable of utilizing exhaust cryogen from a
cryogenic freezer to provide additional cooling to the mechanical
refrigerator, in order to increase the efficiency of the mechanical
refrigerator and reduce expenses associated with the refrigeration
system and/or process.
[0005] For a more complete understanding of the present mechanical
refrigeration process and apparatus embodiments, reference may be
made to the following description taken in conjunction with the
following drawings, of which:
[0006] FIG. 1 is a schematic representation of one embodiment of
the refrigeration system and process.
[0007] FIG. 2 is a schematic representation of a portion of the
embodiment of FIG. 1.
[0008] A first embodiment of the subject system and process
comprises a refrigeration system comprising a cryogenic freezer and
a mechanical refrigerator, wherein the mechanical refrigerator
comprises an enclosure containing an atmosphere, the cryogenic
freezer comprises a cryogen exhaust conduit, and a heat pipe heat
exchanger comprising a warm end and a cold end, wherein the heat
pipe heat exchanger is engaged in heat transfer relationship with
the mechanical refrigerator and the cryogenic freezer such that the
warm end of the heat pipe heat exchanger contacts the atmosphere
within the mechanical refrigerator enclosure, and the cold end of
the heat pipe heat exchanger is disposed within the cryogen exhaust
conduit of the cryogenic freezer.
[0009] A second embodiment of the subject system and process
comprises a refrigeration process comprising cooling an atmosphere
within a mechanical refrigerator by exposing the atmosphere to a
warm end of a heat pipe heat exchanger, wherein a cold end of the
heat pipe heat exchanger is disposed within an exhaust duct of a
cryogenic freezer.
[0010] Either or both of the first and second embodiments may
further comprise, in addition to or in the alternative, circulating
the atmosphere within the mechanical refrigerator over the warm end
of the heat pipe heat exchanger. In certain embodiments, this may
be accomplished via at least one fan disposed within the mechanical
refrigerator which is capable of moving the atmosphere over the
warm end of the heat pipe heat exchanger.
[0011] Any or all of the preceding embodiments may further include,
in addition to or in the alternative, that the heat pipe heat
exchanger comprises a plurality of heat exchange pipes.
[0012] The preceding embodiment may further comprise, in addition
to or in the alternative, a plurality of fins engaged with the
plurality of heat exchange pipes on the warm end of the heat pipe
heat exchanger within the mechanical refrigerator.
[0013] Any or all of the preceding embodiments may further
comprise, in addition to or in the alternative, that a working
fluid within the heat pipe heat exchanger comprises at least one of
water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane,
1,1,1,3,3-pentafluorobutane or perfluoropolyether.
[0014] Any or all of the preceding embodiments may further
comprise, in addition to or in the alternative, that a temperature
within the mechanical refrigerator is from about -40.degree. F.
(-40.degree. C.) to about 10.degree. F. (-12.2.degree. C.).
[0015] Any or all of the preceding embodiments may further
comprise, in addition to or in the alternative, that a temperature
within the cryogen exhaust conduit of the cryogenic freezer is from
about -120.degree. F. (-84.4.degree. C.) to about -80.degree. F.
(-62.2.degree. C.).
[0016] A heat pipe heat exchanger is a heat transfer apparatus that
combines the principles of both thermal conductivity and phase
transition to manage the transfer of heat between two interfaces. A
heat pipe heat exchanger may consist of a single conduit or
multiple conduits, and typically will contain a "bank" of conduits,
pipes or tubes. For ease of understanding, the conduit or conduits
which make up the heat pipe heat exchanger will be referred to in
the plural, but it will be understood that there may be only a
single conduit which makes up the heat pipe heat exchanger.
[0017] The environment within each of the conduits which make up
the heat pipe heat exchanger may be at very low pressure, and may
comprise a partial or substantially complete vacuum prior to
inserting a working fluid into the conduits. At the "warm end" of
the heat pipe heat exchanger, the working fluid evaporates upon
contact with the surface of the conduits by absorbing the latent
heat of that surface. The working fluid then travels through the
conduits to the "cold end" of the heat pipe heat exchanger, where
it condenses, releasing the latent heat absorbed from the warm end
of the heat pipe heat exchanger. The working fluid then travels
back to the warm end, creating a heat transfer cycle within the
conduits.
[0018] When the pressure within the conduits is very low, the
working fluid will travel at or about atomic speeds (discussed
further below) within the conduit, increasing the efficiency of the
heat pipe heat exchanger. The pressure within the conduits may be
adjusted with respect to the physical properties of the working
fluid, so that the working fluid will properly evaporate and
condense at the temperatures present at the warm end and cold end
of the heat pipe heat exchanger, respectively.
[0019] In certain embodiments, the conduits each independently
consist of a sealed conduit, pipe or tube made of a material with
high thermal conductivity, such as copper or aluminum. A vacuum
pump may be used to remove all air from the empty conduits, and the
working fluid is added to the conduits. The working fluid may be
chosen to match the operating temperatures present at the warm end
and the cold end of the heat pipe heat exchanger. Because of the
partial vacuum within the conduits that may be near or below the
vapor pressure of the working fluid, some of the fluid may be in
the liquid phase, and some may be in the gas phase. The use of a
vacuum eliminates the need for the gas phase of the working fluid
to diffuse through any other gas, so that the bulk transfer of the
vapor to the cold end of the heat pipe heat exchanger is at the
speed of the moving molecules, known as the atomic speed. Thus, the
only practical limitation on the rate of heat transfer of the heat
pipe heat exchanger may be the speed with which the gas can be
condensed to a liquid at the cold end of the heat pipe heat
exchanger.
[0020] The working fluid within the heat pipe heat exchanger may
comprise at least one of water, methanol, ethanol, ammonia,
1,1,1,2-tetrafluoroethane, 1,1,1,3,3-pentafluorobutane or
perfluoropolyether.
[0021] In order to increase the efficiency of heat transfer through
the walls of the conduits, pieces of a thermally conductive
material, known as "fins", may be engaged to an exterior of the
conduits to coact with same, so that heat from the exterior
environment may be more efficiently transferred to or away from the
walls of the conduits. Also in order to increase the efficiency of
heat transfer to or away from the walls of the conduits, fans may
be utilized to increase the rate at which the exterior environment
passes over the cold end or warm end of the heat pipe heat
exchanger.
[0022] In the alternative, it may be desirable to leave the
conduits bare, i.e. without fins, so that the adverse effects on
heat transfer of any contaminants from the mechanical refrigerator
or cryogen exhaust adhering to the exterior of the conduits may be
minimized. For example, the exhaust from the cryogenic freezer may
contain ice crystals or food particles, which are byproducts of a
food freezing process. If the cold end of the conduits are left
bare, ice crystals and food particles will not build up as readily
or as quickly on the exterior surfaces of the conduits, and the
conduits may be more easily cleaned.
[0023] FIG. 1 is a schematic representation of one illustrative
embodiment of the refrigeration system and process 10. A cryogenic
fluid represented by arrow 16 is introduced into a cryogenic
freezer 12, where the fluid 16 may be utilized to flash freeze
products within the cryogenic freezer 12. The cryogenic fluid 16
the cryogenic freezer 12 as a cryogen exhaust fluid 18, via
cryogenic exhaust conduit 40. A heat pipe heat exchanger 20 is
disposed such that a cold end 32 of the heat pipe heat exchanger 20
contacts the exhaust fluid 18 within exhaust conduit 40. The warmed
exhaust cryogen 24 may be vented to the atmosphere, may be utilized
in other processes or may be recycled.
[0024] The cryogenic fluid 16 may be at least one of nitrogen
(N.sub.2), carbon dioxide (CO.sub.2) or air.
[0025] A warm end 30 of heat pipe heat exchanger 20 is disposed
within a mechanical refrigerator 14, which is used to store the
products such as food products (not shown) on or in for example
storage racks 25, said products frozen in the cryogenic freezer 12
or in other cryogenic or mechanical refrigerators (not shown).
Mechanical refrigerant 22 is present in coils 26 within an
evaporator 27 within the mechanical refrigerator 14, and an
atmosphere 42 within mechanical refrigerator 14 is circulated over
the coils 26, providing a cooling effect within mechanical
refrigerator 14. Heat pipe heat exchanger 20 provides additional
cooling to the mechanical refrigerator 14 by transferring heat
extracted from the atmosphere 42 to the exhaust cryogen fluid
18.
[0026] The mechanical refrigerant 22 may be ammonia or any other
conventional refrigerant. The atmosphere 42 may be air, N.sub.2,
CO.sub.2 and/or any other desirable gas.
[0027] FIG. 2 is a schematic representation of the heat pipe heat
exchanger 20 of the refrigeration system and process 10 of FIG. 1.
Heat pipe heat exchanger 20 includes heat exchange pipes 28, the
warm end 30 and the cold end 32. The working fluid within the heat
exchange pipes 28 functions as described above, i.e. evaporating at
the warm end 30, traveling to the cold end 32 where it condenses,
and then returning to the warm end 30 to complete the heat transfer
cycle. The warm end 30, which may have the fins 36 being disposed
within the mechanical refrigerator 14, draws heat from the
mechanical refrigerator 14 and, via the working fluid, transfers it
to the cold end 32, disposed within the exhaust conduit 40, thereby
warming the cryogen exhaust fluid 18 but also cooling the working
fluid. Fans 34 move or circulate the atmosphere 42 within the
mechanical refrigerator 14, thereby increasing the efficiency of
the heat transfer process by increasing the rate of heat transfer
on the exterior surfaces of heat exchange pipes 28.
EXAMPLE
[0028] A cryogenic food freezing system using liquid nitrogen will
consume approximately 5000 lb/hr of nitrogen liquid. A central
exhaust for such a freezer system will remove 80% of this mass flow
as a cold cryogenic gas at -80.degree. F. (-62.2.degree. C.).
Therefore, 4000 lb/hr of nitrogen would be removed from the process
and passed through the heat pipe heat exchanger of the present
embodiments. It is desirable to warm the cold cryogen gas from
-80.degree. F. (-62.2.degree. C.) to -30.degree. F. (-34.4.degree.
C.). The specific heat of nitrogen gas is 0.24 btu/lb/F.
Accordingly, the amount of heat transferred into the cold nitrogen
gas would be Q=4000 lb.times.0.24 btu/lb/F.times.(50 F delta
T)=48,000 btu/hr or 14057 W of energy.
[0029] For this Example, we use heat exchange pipes 28 containing
an ammonia mix for the working fluid. The heat exchange pipes 28
are constructed of stainless steel. The diameter of each of the
heat exchange pipes is 10 mm, and the length is 150 mm by way of
example only. My Example shows that this heat exchange pipe
configuration will achieve an axial heat flux of 0.295 kW/cm.sup.2.
The cross sectional area of a 10 mm diameter heat exchange pipe is
0.785 cm.sup.2, which results in a heat transfer rate of 230 W
(watts) per pipe. If we now divide the energy requirement of 14,057
W by 230 W per pipe, we therefore know that we will need 61 heat
exchange pipes 28 for this application.
[0030] In a cryogenic cooling or freezing process, where cooled or
frozen products, such as food products, are stored in a
mechanically refrigerated storage area after exiting the cryogenic
process, energy savings and increased efficiency of the mechanical
refrigerator are realized by utilization of the present system and
process. In particular, greater refrigeration efficiency, increased
cooling capacity, and/or lower energy consumption may be realized
by the present system and process.
[0031] It will be understood that the embodiments described herein
are merely exemplary and that a person skilled in the art may make
many 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 present embodiments as
described and claimed herein. It should be understood that the
embodiments described above are not only in the alternative, but
may be combined.
* * * * *