U.S. patent application number 09/894979 was filed with the patent office on 2002-02-28 for high efficiency very-low temperature mixed refrigerant system with rapid cool down.
Invention is credited to Apparao, Tarrurisa, Bolarski, Mikhail, Flynn, Kevin, Podtchereniaev, Oleg.
Application Number | 20020023447 09/894979 |
Document ID | / |
Family ID | 26909118 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023447 |
Kind Code |
A1 |
Podtchereniaev, Oleg ; et
al. |
February 28, 2002 |
High efficiency very-low temperature mixed refrigerant system with
rapid cool down
Abstract
The present invention is a refrigeration system that uses a
nonflammable, nonchlorinated refrigerant mixture to achieve
very-low temperatures using a single compressor. The system is
characterized by both high efficiency and rapid cool down time.
Inventors: |
Podtchereniaev, Oleg;
(Novato, CA) ; Flynn, Kevin; (Novato, CA) ;
Apparao, Tarrurisa; (Fremont, CA) ; Bolarski,
Mikhail; (Macungie, PA) |
Correspondence
Address: |
ROSENMAN & COLIN LLP
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
26909118 |
Appl. No.: |
09/894979 |
Filed: |
June 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60214565 |
Jun 28, 2000 |
|
|
|
60214562 |
Jun 28, 2000 |
|
|
|
Current U.S.
Class: |
62/217 ;
62/513 |
Current CPC
Class: |
F25B 41/22 20210101;
F28D 7/024 20130101; F25B 39/02 20130101; C09K 5/045 20130101; F25B
2400/16 20130101; C09K 2205/22 20130101; F25B 9/006 20130101; C09K
2205/13 20130101; F25D 31/002 20130101; F25B 2400/18 20130101; C09K
2205/112 20130101 |
Class at
Publication: |
62/217 ;
62/513 |
International
Class: |
F25B 041/04; F25B
041/00 |
Claims
What is claimed is:
1. A low temperature closed cycle refrigeration system comprising:
a single compressor with an outlet delivering a refrigerant flow at
high pressure and an inlet receiving said refrigerant flow at low
pressure; a condenser unit receiving said high-pressure refrigerant
flow from said compressor and removing heat therefrom; an
evaporator receiving said refrigerant flow from said condenser unit
at low pressure and low temperature and returning said refrigerant
flow to said compressor inlet; a flow metering device between said
condenser unit and said evaporator to reduce said high pressure
from said compressor to said low pressure in said evaporator and to
lower the temperature of said refrigerant flow; and a refrigerant
circulating in said closed cycle without refrigerant freeze-out at
said low pressure, said refrigerant being a mixed refrigerant, the
components of said mixed refrigerant having a range of respective
normal boiling temperatures of at least 50.degree. C.
2. A refrigeration system as in claim 1, wherein said mixed
refrigerant is composed in mole percentage of the following: at
least one of Argon and nitrogen in a range of 0 to 20% R-14 in a
range of 20-60%; R-23 in a range of 10-40%; R-125 in a range of
4-30%; R-134a in a range of 0-30%; at least one of the following
high boiling components: R-236fa, R-245fa, and E-137.
3. A refrigeration system as in claim 1, wherein said mixed
refrigerant is 53 mole percent of R-14; 19 mole percent of R-23; 7
mole percent of R-125, and 21 percent of R-236fa.
4. The refrigeration system of claim 2, and further comprising a
control valve receiving said low pressure refrigerant flow from
said evaporator and outputting said refrigerant flow to said
compressor inlet, said control valve maintaining a selected level
of said low pressure at an inlet of said control valve and
controlling said evaporator pressure.
5. A refrigeration system as in claim 2, wherein said evaporator
includes a helical coil of finned tubing located in a first
cylinder, said finned tubing being wrapped around a second cylinder
within said first cylinder, said evaporator cooling a coolant that
flows within said first cylinder and over said finned tubing in a
cross-flow/counter flow arrangement, said refrigerant flow through
said finned tubing exchanges heat and cools said coolant.
6. The refrigeration system as in claim 5, further comprising: a
heat exchanger receiving refrigerant flow at high pressure from
said condenser and delivering said high pressure refrigerant to
said flow metering device, said flow refrigerant flow from said
evaporator being in heat exchange relationship with said high
pressure refrigerant flow in said heat exchanger.
7. Refrigeration system as in claim 6, wherein said control valve
is between said heat exchanger and said compressor inlet.
Description
[0001] This application claims the benefit of the following earlier
filed and pending U.S. provisional patent applications No.
60/214,565, and No. 60/214,562
BACKGROUND OF THE INVENTION
[0002] This invention is directed to the use of a highly efficient
very-low temperature mixed refrigerant system with rapid cool
down.
[0003] Refrigeration systems have been in existence since the early
1900s, when reliable sealed refrigeration systems were developed.
Since that time, improvements in refrigeration technology have
proven their utility in both residential and industrial settings.
In particular, low-temperature refrigeration systems currently
provide essential industrial functions in biomedical applications,
cryoelectronics, coating operations, and semiconductor
manufacturing applications. Conventional refrigeration systems have
historically utilized chlorinated refrigerants, which have been
determined to be detrimental to the environment and are known to
contribute to ozone depletion. Thus, increasingly restrictive
environmental regulations have driven the refrigeration industry
away from chlorinated fluorocarbons (CFCs) to hydrochlorinated
fluorocarbons (HCFCs), and more recently, to hydroflourocarbons
(HFCS) due to a European Union law banning the use of HCFCs in
refrigeration systems as of Jan. 1, 2001.
[0004] Traditional refrigeration systems that achieve very-low
temperature cooling (-50 C and -200 C) using a single compressor
include the Missimer, Kleemenko, and single expansion device
varieties. Each of these systems employs refrigerant mixtures that
are either flammable or chlorinated. The use of chlorinated
refrigerants is increasingly restricted and, while flammable
refrigerants are environmentally safe, they pose certain fire risks
that make their inclusion into a system less desirable. What is
needed is a way to achieve very-low temperature cooling using a
nonflammable and nonchlorinated refrigerant mixture.
[0005] Providing refrigeration at temperatures below 223 K (-50 C)
have many important applications, especially in industrial
manufacturing and test applications. This invention relates to
refrigeration systems which provide refrigeration at temperatures
between 223 K and 73 K (-50 C and -200 C). The temperatures
encompassed in this range are variously referred to as low, ultra
low and cryogenic. For purposes of this Patent the term "very low"
or very low temperature will be used to mean the temperature range
of 223 K and 73 K (-50 C and -200 C).
[0006] There are many vacuum processes that have the need for such
very low temperature cooling. The chief use is to provide water
vapor cryopumping for vacuum systems. The very low temperature
surface captures and holds water vapor molecules at a much higher
rate than they are released. The net effect is to quickly and
significantly lower the chamber's water vapor partial pressure.
This process of water vapor cryopumping is very useful for many
physical vapor deposition processes in the vacuum coating industry
for electronic storage media, optical reflectors, metallized parts,
etc.
[0007] Another application involves thermal radiation shielding. In
this application large panels are cooled to very low temperatures.
These cooled panels intercept radiant heat from vacuum chamber
surfaces and heaters. This can reduce the heat load on surfaces
being cooled to lower temperatures than the panels. Yet another
application is the removal of heat from objects being manufactured.
In some applications the object is an aluminum disc for a computer
hard drive, a silicon wafer for an integrated circuit, or the
material such as glass or plastic for a flat panel display. In
these cases the very low temperature provides a means for removing
heat from these objects more rapidly, even though the object's
final temperature at the end of the process step may be higher than
room temperature. Further, some applications involving, hard disc
drive media, silicon wafers, or flat panel display material,
involve the deposition of material onto these objects. In such
cases heat is released from the object as a result of the
deposition and this heat must be removed while maintaining the
object within prescribed temperatures. Cooling a surface like a
platen is the typical means of removing heat from such objects. In
all these cases an interface between the refrigeration system and
the object to be cooled is proceeding in the evaporator where the
refrigerant is removing heat from the object at very low
temperatures.
[0008] In traditional refrigeration systems, temperatures colder
than -50.degree. C. are usually achieved by cascade refrigeration.
Cascade systems require at least two compressors, each with
separate refrigeration loops, with the evaporator of the warmer
stage used to condense the refrigerant of the cooler stage. The
lower temperature cooling achieved with these systems unfortunately
is coupled with a greater power demand and, subsequently, a loss in
system efficiency, as efficiency is directly proportional to the
amount of heat removed from a substance and inversely proportional
to the amount of power input to the system. What is needed is a way
to improve the efficiency of a very-low temperature refrigeration
system.
[0009] Industrial applications that require very-low temperature
cooling often specify using liquid coolants. A common
characteristic of these coolants is that they become highly viscous
at such temperatures. Increased viscosity at lower temperatures is
a common characteristic of most liquids. These particular liquids
tend to be very viscous. As a liquid coolant is pumped through a
closed loop system, the pressure drop experienced by the coolant as
it flows through the evaporator affects the heat load on the
refrigeration system, as higher coolant pressure drop requires
higher input powers. Higher input powers to achieve a given amount
of heat removal results in lower efficiency. What is needed is a
way to achieve very-low temperature cooling without a high coolant
pressure drop.
[0010] When designing refrigeration systems for a customer, the
customer dictates certain parameters that must be met in order to
fit the applications for which the system is intended. One such
parameter is rapid cool down. When the customer demands that a
system achieve the desired heat removal within 30 minutes of
initiating the system, a refrigeration system with rapid cool down
capabilities is required. Additionally, the physical size of the
refrigerant evaporator has a direct effect on its cool down rate.
The greater the mass of copper material that is present within a
refrigeration system the more time is needed for the system to
reach steady state refrigeration during cool down. Therefore, what
is needed is a way to achieve rapid cool down in large
refrigeration systems.
[0011] Certain applications find it desirable to control the
coldest temperature supplied to a liquid coolant, as many
industrial coolants become highly viscous and may even freeze out
which makes it difficult or impossible to pump the liquids. What is
needed is a way to control the coldest temperature supplied by a
refrigeration system to a coolant.
[0012] In semiconductor applications many aspects of a
refrigeration product are tightly constrained. Typically,
limitations are placed on system floor space requirements, system
height, input power, and cost. The ability to produce a system that
meets all of these limitations is not obvious. For example,
providing a short cool down time may be readily achieved with a
large refrigeration system. However, such a system will require
more input power and more floor space than allowed. Similarly, a
fast cool down time could be enabled by a very compact heat
exchanger which would produce a high pressure drop and would in
turn increase the thermal load on the system (due to increased pump
work on the coolant) and require excessive input power. Therefore,
being able to meet all of the many performance requirements is
difficult.
BACKGROUND PATENTS
[0013] U.S. Pat. No. 6,112,534, "Refrigeration and heating cycle
system and method," assigned to Carrier Corporation (Syracuse,
N.Y.), describes an improved refrigeration system and
heating/defrost cycle. The system, for heating circulating air and
defrosting an enclosed area, includes a refrigerant, an evaporator
using said refrigerant for heating the circulating air; and a
compressor for receiving the refrigerant from the evaporator and
compressing the refrigerant to a higher temperature and pressure.
Advantageously, the system further includes the combination of an
expansion valve positioned between the compressor and the
evaporator for forming a partially expanded refrigerant, a
controller for sensing system parameters, and a mechanism
responsive to said controller, based on the sensed parameters, for
increasing temperature differential between the refrigerant and the
circulating air, for improving system efficiency and for optimizing
system capacity during heating and defrost cycles.
[0014] U.S. Pat. No. 6,089,033, "High-speed evaporator defrost
system," assigned to Dube, Serge (Quebec, Canada), describes a
high-speed evaporator defrost system comprised of a defrost conduit
circuit connected to the discharge line of one or more compressors
and back to the suction header through an auxiliary reservoir
capable of storing the entire refrigerant load of the refrigeration
system. Auxiliary reservoir is at low pressure and is automatically
flushed into the main reservoir when liquid refrigerant accumulates
to a predetermined level. The auxiliary reservoir of the defrost
circuit creates a pressure differential across the refrigeration
coil of the evaporators sufficient to accelerate the hot high
pressure refrigerant gas in the discharge line through the
refrigeration coil of the evaporator to quickly defrost the
refrigeration coil even at low compressor head pressures and
wherein the pressure differential across the coil is in the range
of from about 30 psi to 200 psi U.S. Pat. No. 6,076,372, "Variable
load refrigeration system particularly for cryogenic temperatures,"
assigned to Praxair Technology, Inc. (Danbury, Conn.), describes a
method for generating refrigeration, especially over a wide
temperature range including cryogenic temperatures, wherein a
non-toxic, non-flammable and low or non-ozone-depleting mixture is
formed from defined components and maintained in variable load form
through compression, cooling, expansion, and warming steps in a
refrigeration cycle.
[0015] U.S. Pat. No. 5,749,243, "Low-temperature refrigeration
system with precise temperature control," assigned to Redstone
Engineering (Carbondale, Colo.), describes a low-temperature
refrigeration system for accurately maintaining an instrument with
a time varying heat output at a substantially constant
predetermined cryogenic temperature. The refrigeration system
controls the temperature of the instrument by accurately adjusting
the pressure of coolant at a heat exchanger interface associated
with the instrument. The pressure and flow of coolant is adjusted
through the use of one or two circulation loops and/or a
non-mechanical flow regulator including a heater. The refrigeration
system further provides a thermal capacitor that allows for
variation of the cooling output of the system relative to a cooling
output provided by a cooling source.
[0016] U.S. Pat. No. 5,396,777, "Defrost controller," assigned to
General Cryogenics Incorporated (Dallas, Tex.), describes a method
and apparatus to refrigerate air in a compartment wherein liquid
CO.sub.2 is delivered through a first primary heat exchanger such
that sufficient heat is absorbed to evaporate the liquid carbon
dioxide to form pressurized vapor. The pressurized vapor is heated
in a gas-fired heater to prevent solidification of the pressurized
carbon dioxide when it is depressurized to provide isentropic
expansion of the vapor through pneumatically driven fan motors into
a secondary heat exchanger. Orifices in inlets to the fan motors
and solenoid valves in flow lines to the fan motors keep the vapor
pressurized while the heater supplies sufficient heat to prevent
solidification when the CO.sub.2 vapor expands through the motors.
CO.sub.2 vapor is routed from the second heat exchanger to chill
surfaces in a dehumidifier to condense moisture from a stream of
air before it flows to the heat exchangers.
SUMMARY OF THE INVENTION
[0017] The present invention is a refrigeration system that uses a
nonflammable, nonchlorinated refrigerant mixture to achieve
very-low temperatures using a single compressor. The system is
characterized by both high efficiency and rapid cool down time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] For better understanding of the invention, reference is had
to the following description taken in connection with the
accompanying drawings, in which:
[0019] FIG. 1 is a block diagram of a refrigeration system which
removes heat from a coolant in accordance with the invention;
[0020] FIG. 2 is a drawing of the refrigerant evaporator of FIG. 1
which exchanges heat with and cools the coolant, in accordance with
the invention.
[0021] FIG. 3 is a block diagram of an alternative portion of the
refrigeration process in accordance with the invention.
[0022] FIG. 4 is Table 1 which lists refrigerant compositions in
accordance with the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0023] In a first embodiment, FIG. 1 is a diagram of a
refrigeration system 100 in accordance with the invention.
Refrigeration system 100 includes a compressor 102 feeding an
optional oil separator 104 that feeds a condenser 106. An outlet of
condenser 106 feeds a filter-drier 108 that subsequently feeds an
inlet of a buffer tank 110 whose outlet feeds a buffer tank valve
112 feeding a supply inlet of a refrigeration process 116 via a
liquid line 114. A return outlet of refrigeration process 116
closes the loop back to compressor 102 via a compressor suction
line 118. In compressor suction line 118 between refrigeration
process 116 and compressor 102 is an evaporator pressure regulating
(EPR) valve 120. Refrigeration system 100 further includes a safety
circuit 122. Refrigeration process 116 is also referred to as a
cryoblock.
[0024] Refrigeration process 116 further includes a regenerative
heat exchanger 126 whose supply inlet is fed by liquid line 114. A
supply outlet of regenerative heat exchanger 126 feeds a flow
metering device (FMD) 128 that feeds a supply inlet of an
evaporator 130. A return outlet of evaporator 130 feeds a return
inlet of regenerative heat exchanger 126 whose return outlet feeds
compressor suction line 118. A secondary coolant loop (not shown in
its entirety) accesses evaporator 130 via a coolant inlet 132 and a
coolant outlet 134. Additionally, a liquid drain valve 136 is
connected to coolant inlet 132 of evaporator 130.
[0025] Coolant hereafter refers to the substance from which heat is
to be removed by refrigeration system 100, while refrigerant refers
to the substance that removes heat from the coolant by being warmed
and or vaporizing. The coolant for which refrigeration system 100
was originally designed is the highly viscous HT 70 by Galden,
although those skilled in the art will readily perceive that
refrigeration system 100 lends itself to a wide array of industrial
coolants.
[0026] Compressor assembly 102 pumps refrigerant and maintains an
adequate pressure, to force enough refrigerant to flow to meet the
cooling requirements of refrigeration system 100. Compressor
assembly 102 is a conventional compressor that takes low-pressure,
low-temperature refrigerant gas from refrigeration process 116 and
compresses it to high-pressure, high-temperature gas that is fed to
oil separator 104 to be cooled and partially liquefied by condenser
106. The typical operating pressure of refrigerant leaving
compressor assembly 102 is between 10.0 and 20.7 bars, at a typical
temperature of +105 C. When exiting the condenser, the refrigerant
pressure is typically unchanged e.g. a pressure drop of 1 bar, and
the temperature is typically 20 C to 35 C.
[0027] Optional oil separator 104 is a conventional 1.4-liter oil
separator with an maximum working pressure of 31.0 bars, in which
the compressed mass flow from compressor assembly 102 enters into a
larger separator that removes most of the oil from the refrigerant
gas. The removed oil is returned to compressor 102 via compressor
suction line 118. The mass flow from oil separator 104, minus the
oil removed, continues to flow onward to condenser 106 and
subsequently to refrigeration process 116. The oil separator is not
needed if the compressor is oil free. If an oil lubricated
compressor is used, an oil separator is not needed when the amount
of the oil in the refrigerant gas exiting the compressor is low
enough to work reliably in the refrigeration system (i.e. without
separating and freezing out at the lowest temperature of the
evaporator).
[0028] Condenser 106 is a conventional air-cooled or water-cooled
condenser, and is the part of the system where the heat is rejected
by condensation. As the hot gas travels through condenser 106, it
is cooled by air or water passing over it. As the hot gas
refrigerant cools, drops of liquid refrigerant form within its
coil. Eventually, when the gas reaches the end of condenser 106, it
has condensed partially; that is, liquid and vapor refrigerant are
present. In order for condenser 106 to function correctly, the
fluid passing through the condenser 106 must be cooler than the
working fluid of the system.
[0029] Filter-drier 108 is a conventional filter drier that traps
any moisture contained in the refrigerant mixture, thereby
preventing moisture that may cause icing from propagating to
downstream elements of refrigeration system 100, such as to FMD
128.
[0030] Buffer tank 110 is a relatively small tank (for example, 2.8
liter and maximum working pressure equal to 34.4 bars) that stores
gases and controls the pressure of liquid line 114. Buffer tank 110
is particularly important during the start up of refrigeration
system 100. Receiver tank valve 112 is typically locked open to
allow continuous flow of refrigerant from buffer tank 110 to
refrigeration process 116.
[0031] Refrigeration process 116 includes components that remove
heat from a coolant by exchange with the refrigerant. Regenerative
heat exchanger 126 is a heat exchanger that warms refrigerant
exiting evaporator 130 to near room temperature and cools
refrigerant exiting condenser 106.
[0032] FMD 128 is any conventional flow metering device, such as a
capillary tube, orifice, ventori , proportional valve with
feedback, porous plug or any restrictive element that produces a
throttling effect of the refrigerant and causes a refrigeration
effect. It is sized or controlled to limit flow and achieve the
desired operating refrigerant pressures of the system when
operating in combination with the compressor, condenser, and
regernerative heat exchanger.
[0033] Evaporator 130 is a compact efficient shell and finned coil
heat exchanger as described herein and in an application filed
concurrently herewith. Refrigerant evaporates and absorbs heat from
the coolant within evaporator 130.
[0034] Coolant inlet 132 is a line through which warm coolant is
pumped into evaporator 130 from the secondary coolant loop. Coolant
outlet 134 is a line through which cooled coolant is pumped out of
evaporator 130 into the secondary coolant loop.
[0035] Liquid drain valve 136 is a normally closed valve located at
the bottom of the liquid loop at coolant inlet 132 and is not
usually used during operation. This valve is only used when it is
required that liquid coolant be filled or drained out of
refrigeration system 100. Effective draining of the coolant is
important due to the high cost of liquid coolants. Further, liquid
drain valve 136 provides a port that is used to fill the coolant
passages of refrigeration system 100. This is also very important
because trapped air or gas in a liquid loop decreases pump
displacement. Therefore liquid drain valve 136 must be the lowest
point of the coolant loop and must have direct plumbing that is
continuously downward to prevent the possibility of having any
trapped air or gas.
[0036] In an alternative arrangement, the refrigeration process 116
shown in FIG. 1 could be modified. One modification includes at
least one phase separator and may include additional heat
exchangers. Such an arrangement is shown in FIG. 3 which is an
alternative construction of the regenerative heat exchanger
126.
[0037] FIG. 3 shows a single phase separator arrangement 300, in
accordance with the invention, which has the identical refrigerant
flow inputs and outputs as heat exchanger 126. High pressure
refrigerant enters optional heat exchanger 302 via liquid line 114.
High pressure refrigerant exits via line 310 which feeds FMD 128.
Low pressure refrigerant returning from evaporator 130 enters via
line 312. Low pressure refrigerant exits via line 316. The flow
between the various components is as follows. Liquid line
refrigerant enters optional heat exchanger 302 via line 114 and
exits into liquid-vapor phase separator 304. Liquid-vapor phase
separator 304 separates the flow into two streams, one that is
mainly liquid and the other that is mainly vapor. The vapor stream
feeds heat exchanger 306 which in turn feeds heat exchanger 308
which in turn exits via line 310. Low pressure refrigerant enters
via line 312 which feeds heat exchanger 308 which in turn feeds
heat exchanger 306 which in turn feeds heat exchanger 302 which
exits via line 316. The liquid stream separated by phase separator
304 is throttled to low pressure by FMD 314 and is blended with low
pressure refrigerant at a node between heat exchangers 308 and 306.
The purpose of heat exchangers 302, 306 and 308 is to exchange heat
between the high pressure and low pressure refrigerant streams
flowing through each heat exchanger.
[0038] To achieve high efficiency, the temperature of phase
separator 304 needs to be near room temperature or slightly chilled
below room temperature. Therefore heat exchanger 302 is optional.
Its need will be determined by the specific application.
[0039] Heat exchanger 306 is required to return the throttled
refrigerant exiting FMD 314 to suction refrigerant of the
appropriate temperature to maximize the heat transfer potential of
the low pressure stream. It is a required heat exchanger if heat
exchanger 302 is not used. If heat exchanger 302 is used, heat
exchanger 306 is optional. Its need will be determined by the
operating characteristics of heat exchangers 302 and 308.
[0040] FIG. 3 illustrates one possible arrangement. Many other
similar arrangements are possible, including various arrangements
of FMD's, heat exchangers and phase separators. An arrangement (not
shown) with multiple phase separators is also considered within the
scope of this invention. However, a preferred embodiment has to use
no phase separator, or a single phase separator. The goal in each
of these arrangements is to achieve superior refrigeration system
efficiency, especially relative to the traditional cascade
refrigeration system, with each stage containing a single
refrigerant, or refrigerant blend with components having closely
spaced normal boiling points. Such systems have inherent
inefficiencies. The intent of this invention is to provide superior
performance for a given amount of heat removal at a specified
temperature. This considers that such a comparison should be based
on compressors of similar construction and efficiency, and that the
mixed refrigerant systems using a single compressor will benefit
from a slightly higher efficiency by virtue of having a large
displacement.
[0041] FIG. 1 associates with a single refrigerant evaporator
cooling a single coolant flow path. Cooling of multiple coolant
paths with one or more evaporators is also possible. In the general
case, refrigerant from regenerative heat exchanger 126 feeds two or
more FMD's arranged in parallel. These in turn feed two or more
independent evaporators arranged in parallel, each of which cools
independent coolant flow paths. The return refrigerant from each
evaporator is mixed together and returned to the low pressure inlet
of the regenerative heat exchanger 226 (and alternatively to
arrangement 300). In other arrangements, the independent coolant
flow paths are cooled by evaporators arranged in series. In one
such arrangement refrigerant exiting from one evaporator feeds into
the next. The final evaporator returns its refrigerant to
regenerative heat exchanger 226 (and alternatively to arrangement
300). Finally, various combinations of series and parallel
evaporator arrangements are possible.
[0042] Table 1 (FIG. 4) is a listing of the refrigerant mixtures of
the present invention, including Argon or Nitrogen, along with
R-14, R-23, R-125, R-134a, R-236fa, R-245fa, E-347.
[0043] Table 1 lists the ingredients to refrigerant mixture of the
present invention that enable the refrigeration system to
accomplish a high-efficiency, very-low temperature refrigeration
cycle while using a nonflammable, nonchlorinated refrigerant
mixture. With the exception of E-347, all refrigerants listed are
designated in accordance with American Society of Heating and
Refrigeration and Air Conditioning Engineering (ASHRAE) standard
No. 34.
[0044] E-347 is known as
1-(methoxy)-1,1,2,2,3,3,3-heptafluoropropane (also
CH3--O--CF2--CF2--CF3), 3 M product reference Hydrofluoroether
301.
[0045] The Example column of Table 1 lists the specific composition
applied with favorable results in a system developed in accordance
with the invention. The specific system developed was arranged as
shown in FIG. 1. An overall blend composition in the column labeled
"Range" in Table 1 identifies ranges of compositions that fall
within the scope of this invention. Within these ranges the number
of compositions and their performances are potentially
infinite.
[0046] The warmer boiling components, R-236fa, R-245fa, and E-137
are considered to be interchangeable with each other. These three
components may not be used at all in a mixture. In this case, most
systems will not have any condensed refrigerant formed in a room
temperature condenser. This limits the amount of heat that can be
rejected by the system and thereby reduces amount of heat that can
removed by the evaporator. Although this may be beneficial in rare
circumstances, the preferred embodiment contains enough high
boiling components to enable condensed refrigerant to form in the
condenser since this will generally produce higher system
efficiency.
[0047] Further, in accordance with the invention, another component
may be added to the above compositions provided that the ratios of
the listed components (Table 1) remain in the same proportions
relative to each other.
[0048] Another feature, in accordance with the invention, an EPR
valve 120 used to regulate the lowest refrigerant temperature (in
combination with other system elements) and thus prevent the
coolant from becoming excessively cold. EPR valve 120 is a standard
refrigeration valve, normally used for single component
refrigerants or blends of refrigerants having closely spaced
boiling points and, thus, is unique in its application to
refrigeration system 100, in which a refrigeration mixture with
widely spaced boiling points is incorporated. The refrigerant
composition is disclosed in additional detail in Poly 17.490. The
primary function of this type of valve is to prevent the pressure
of evaporator 130 from falling below a predetermined value to which
the valve has been set. When properly adjusted for a particular
system, EPR 120 keeps the temperature of the refrigerant from
falling below a certain lower limit. That temperature limit is
therefore able to be influenced by adjustment of EPR valve 120.
Examples of this commercially available valve are the ORIT valves
manufactured by Sporlan Valve Company (Washington, Mo.), or similar
valves made by Alco (St. Louis, Mo.) and Danfoss (Nordburg,
Denmark). The use of EPR valve 120 is further described in a
concurrently filed application (Docket POLY 18.764).
[0049] In operation, EPR 120 regulates the refrigerant pressure
within evaporator 130 and thereby prevents the coolant from falling
below -100 C, although EPR 120 may be adjusted to a wide range of
pressure set points. EPR 120 is also used to prevent freeze-out of
the refrigerant components by acting to maintain a minimum suction
pressure, thereby keeping the suction pressure of refrigeration
system 100 from dropping too low. As the suction pressure gets
lower the temperature tends to drop, if a freeze out condition
occurs the suction pressure tends to drop creating positive
feedback and further reducing the temperature, causing even more
freeze out.
[0050] Another feature of EPR 120 is the regulation of the minimum
coolant temperature. By setting the EPR to limit the refrigerant
temperature the coldest refrigerant temperature is limited, thereby
limiting the temperature to which the coolant can be cooled.
[0051] Conventional refrigeration systems use a single component
refrigerant or refrigerant components with closely spaced boiling
points so as to emulate the properties of single refrigerants. The
mixed refrigerants disclosed in this application have a range of
boiling points that is at least 50 C, and more, typically 100 C or
more. The result is that there is no close relationship between
temperature and pressure as with single component refrigerants.
Therefore, setting the pressure by itself is insufficient to know
what the controlled temperature will be. Additionally, the standard
application of the EPR valve has it located at the outlet of the
evaporator. The temperatures produced by these systems are colder
than -50 C and are typically colder than the valves are rated for,
especially if they use elastomer seals.
[0052] The use of the EPR valve as shown in this application
addresses the low temperature exposure issue by locating it on the
suction line where the refrigerant temperature is always warm
enough (typically 0 C or warmer) to meet the design requirements of
the valve, provided that the system is properly designed. The valve
is set when there is a minimal load on refrigeration system 100,
that is, when there is no applied thermal load on the evaporator.
This provides an empirical way to set the valve and adjust it
further until the desired refrigerant evaporator temperatures are
achieved. The factors effecting the equilibrium temperature are the
pressure setting, the refrigerant mixture, the heat load on the
evaporator, and the sizing of the regenerative heat exchanger 126
(or the sizing of alternative heat exchangers as described in FIG.
3). In contrast, a conventional application of the EPR valve
requires only knowledge of the refrigerant properties to determine
the required pressure setting.
[0053] There are many control devices included within refrigeration
system 100, which are for simplicity not shown in FIG. 1. Safety
circuit 122 provides control to, and receives feedback from, the
plurality of control devices included within refrigeration system
100, such as pressure switches and thermally coupled temperature
switches, via control lines 124. Control lines 124 feeding the
control devices are electrical in nature. Likewise, the feedback
from the various sensing devices via control lines 124 of safety
circuit 122, are electrical in nature.
[0054] Refrigeration system 100 is designed to remove heat from
industrial coolants such as Galden HT 70, which is widely used in
applications such as in the semiconductor industry. The industrial
coolant enters refrigeration system 100 via coolant inlet 132 and
exits refrigeration system 100 as a very low temperature liquid
between -50 C and -125 C through coolant outlet 134. The
nonflammable, nonchlorinated refrigerant mixture used in the
present invention is disclosed in provisional applications
60/214,562 and (Docket Poly 18.720P, filed Jun. 1, 2001).
[0055] Further examples of industrial coolants include, but are not
limited to: Galden HT-200 (Ausimont USA, Inc., Thorofare, N.J.),
Galden HT 55 (Ausimont USA, Inc.), Novec HFE-7100 (3M company, St.
Paul, Minn.), Novec HFE-7200 (3M company), Novec HFE-8401 HT (3M
company), FC 77 (3M company), Galden HT-200. Note: Galden is a
registered trademark of Ausimont, and Novec is a trademark of
3M.
[0056] For purposes of this application these highly viscous fluids
are characterized by having a kinematic viscosity of at least 0.2
centistokes at 25 C. This is a typical temperature that viscosity
data is available for. The viscosity becomes much higher at very
low temperatures.
[0057] The discharge and suction pressure for refrigeration system
100 are maintained within the normal operating range of compressor
102. Typical suction pressures range from 2 to 6 bar absolute.
Typical discharge pressures range from 10 to 30 bar absolute. While
the present invention is a single stage system (no phase
separators), a two-stage system (single phase separator) may be
attained by the present invention with the inclusion of a phase
separator and an additional expansion device. However, the
inclusion of a phase separator decreases the simplicity of the
system but if the separator temperature is 10 C or warmer, the
efficiency of the system 100 will be improved. Such a phase
separator can obviate the need for oil separator 104.
[0058] In addition to the special refrigerant mixture disclosed in
Poly 17.490, filed concurrently, and Poly 18.720P, certain key
elements included in refrigeration system 100 contribute to the
high efficiency of refrigeration system 100. Temperatures below -50
K normally are achieved by cascade systems, in which a separate
compressor is included for each refrigerant; however, the present
invention implements a single-compressor design that accommodates
the entire refrigerant mixture to achieve the same temperatures.
The size of a compressor is closely related to its efficiency, with
larger compressors having greater efficiencies. The compressors
commonly used in cascade systems are relatively small, and thus,
have a low efficiency associated with them. The inclusion of a
plurality of such low efficiency compressors results in an even
greater overall system inefficiency. Compressor 102 is larger than
the compressors used in plurality in cascade systems, and helps
achieve a simpler, more compact system design while achieving
increased system efficiency. Further, since cascade refrigeration
systems require additional heat exchangers which result in lower
thermodynamic efficiencies, (due to the required temperature
difference to cause heat transfer, and due to the pressure drop in
the heat exchangers), they are inherently less efficient.
[0059] To establish a reference for refrigeration efficiency a
comparison relative to the best that is thermodynamically possible
will be used. For refrigeration systems the comparison is between
the actual coefficient of performance (COP) compared to the
coefficient of performance for an ideal cycle (COP ideal). An
advantage of this method is that it takes into account the fact
that providing refrigeration at lower temperatures is more
difficult than at warmer temperatures.
[0060] The COP is defined as the ratio between heat removed by the
evaporator to the heat rejected by the condenser. The COP ideal is
defined as the ratio of evaporator temperature and the difference
between the temperature of the cooling media used to cool the
condenser and the temperature of the evaporator. For purposes of
this application, the evaporator temperature is defined by the
average of the refrigerant temperature at the inlet and outlet of
the evaporator. For calculations of the COP, the temperatures must
be in absolute units (i.e. Kelvin). In the current example system
the refrigerant inlet temperature is about -81 C and the
refrigerant outlet temperature is about 74 C. For an air
temperature of 30 C (used to cool the condenser), the COP ideal has
a value of 1.8 (195.5 K/(303 K 195.5 K). The actual COP is 0.278
(500 W/1800 W). The efficiency relative to ideal is 15.4%.
[0061] These are the results achieved at the design condition for a
small system (1.5 horsepower) with a relatively low efficiency
compressor. Larger scale systems will realize higher efficiencies
due to the use of large, more efficient compressors. However, in
comparison to cascade systems, mixed refrigerant systems will be
more efficient relative to the COP ideal.
[0062] As shown in FIG. 2, in accordance with the invention, a
special, low pressure drop evaporator design is used to cool the
coolant. An important element to the increased efficiency of
refrigeration system 100 is the unique evaporator 130. Evaporator
130 is a low mass, low volume evaporator that achieves very-low
temperature cooling while maintaining a low coolant pressure drop.
A low coolant pressure drop is required for high efficiency because
the pump work imparted to the coolant increases the heat that must
be removed by refrigeration system 100. Therefore a low pressure
drop for the coolant heat exchanger is an important factor in
achieving a high efficiency. Achieving a low coolant pressure drop
is challenging since many of the industrial coolants that are used
have very high viscosity at low temperatures.
[0063] As a specific example, Galden HT 70 has a viscosity of 10
centistokes at -80 C. The uniqueness of evaporator 130 includes a
helically wound evaporator coil with brazed copper, aluminum, or
stainless steel fins. The flow of coolant occurs crosswise to the
tube length and roughly crossflow/counterflow to the direction of
the refrigerant flow. As a result of the small size of evaporator
130, there is always a small volume of refrigerant in evaporator
130, thereby reducing the required size of buffer tank 110. A
smaller buffer tank 110 lends itself nicely to achieving a more
simplistic system design and a compact form, and the size of
evaporator 130 helps to achieve a shorter system start-up time due
to the smaller mass of copper requiring cool down. In contrast,
alternative designs can provide effective heat transfer with low
pressure drop. However, these designs required additional
refrigerant volume, which required additional buffer volume, which
resulted in an unacceptably large system size.
[0064] Referring to FIG. 2, the main elements are as follows.
Refrigerant inlet 206 feeds coiled tubing 214 which in turn feeds
refrigerant outlet 208. As refrigerant flows through coiled tubing
214 it receives heat from the coolant. Coolant flows into the heat
exchanger through coolant inlet 202 (which corresponds to coolant
inlet 132 of FIG. 1). In the heat exchanger it is cooled by the
refrigerant and exits from coolant outlet 204 (which corresponds to
coolant outlet 134 of FIG. 1). The heat exchanger shell is formed
by external cylinder 210 and two caps 216,224. Inner cylinder 212
is capped at two ends by caps 218. Only one cap 218 is shown, but a
second identical cap is used at the opposite end of cylinder.
Support 222 is used to fix the inner cylinder's position relative
to the caps of the outer cylinder. Heat transfer between the
coolant and heat exchanger is made efficient by the use of fins 220
attached to coiled tubing 214.
[0065] The fins 220 are made of a conductive metal that is
thermally bonded to the tubing. The fins have a constant height
relative to the tube and are formed by a metal ribbon that is
wrapped around the tubing in a helix. The width of the ribbon
extends perpendicular to (generally radially from) the tube and is
referred to as the fin height. When viewing a straight length of
finned tubing along the tube axis the fins are viewed as circles,
with an outside diameter equaling the diameter of the tube plus
twice the fin height (ribbon width). The fins are spaced at
specific interval, typically 0.3 to 0.06 inches.
[0066] In one specific example the requirement was to limit input
power to 1800 W, provide initial cooling within 30 minutes of
start, and remove 500 Watts from a stream of Galden HT 70 flowing
at 200 gram/sec to achieve a final coolant temperature of 31 74 C
with a coolant pressure drop not to exceed 2 psi. The specific
system that enabled this capability was a system with a 1.5
horse-power compressor using a refrigerant mixture circulating with
a composition as shown in Table 1. The evaporator had a total mass
of about 12 pounds without coolant This mass consisted entirely of
copper. The total coolant volume was about 100 cubic inches. The
inner cylinder and caps minimized the volume of coolant in the
evaporator. This limited the coolant initially cooled during system
start up. The inner cylinder, along with the inner surface of
cylinder 210, prevented coolant flow from bypassing the finned
tubing. Coiled tubing 214 has a diameter of 0.375 inches. The fins
220 attached to the tubing are 0.25 inch high, have a thickness of
0.015 inch, and have a spacing of about 0.06 to 0.07 inches. The
finned tubing is of a commercially available type such as that
provided by Heat Exchange Applied Technology (Orrville, Ohio).
Inner cylinder 212 has a diameter of about 2.13 inches and outer
cylinder 210 has a diameter of about 4 inches and a length of about
12 inches. The total heat transfer surface area of the heat
exchanger (provided mainly by the finned tubing) is about 16 square
feet.
[0067] The evaporator 130 design meets all of the important
requirements for this refrigeration system.
[0068] It will thus thus been seen that the objects set forth
above, among those made apparent from the preceding description,
are efficiently attained, and since certain changes amy be made in
the above refrigerant blend without departing from the spirit and
scope of the invention, it is intended that all matter contained in
the above description shall be interpreted as illustrative and not
in a limiting sense.
[0069] It is also to be understood that the following claims are
intended to cover all of the generic and specific features of the
invention herein described, and all statements of the scope of the
invention which might be said to fall therebetween.
* * * * *