U.S. patent application number 11/591465 was filed with the patent office on 2007-05-03 for thermal control system and method.
Invention is credited to Kenneth W. Cowans, William W. Cowans, Glenn W. Zubillaga.
Application Number | 20070095097 11/591465 |
Document ID | / |
Family ID | 37994541 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070095097 |
Kind Code |
A1 |
Cowans; Kenneth W. ; et
al. |
May 3, 2007 |
Thermal control system and method
Abstract
Improvements in systems and methods which employ a two-phase
fluid, such as a refrigerant, as a saturable fluid in direct heat
transfer relation to a thermal load, are realized by extraction of
vapor from the saturated fluid before heat exchange. Moreover,
automatic changing of the paths under command of a controller
enables charges to be effected between different modes at higher
rates than in other systems by employing the variety of modes
available in the direct transfer system.
Inventors: |
Cowans; Kenneth W.;
(Fullerton, CA) ; Cowans; William W.; (Fullerton,
CA) ; Zubillaga; Glenn W.; (Canyon Lake, CA) |
Correspondence
Address: |
JONES, TULLAR & COOPER, P.C.
P.O. BOX 2266 EADS STATION
ARLINGTON
VA
22202
US
|
Family ID: |
37994541 |
Appl. No.: |
11/591465 |
Filed: |
November 2, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60733078 |
Nov 3, 2005 |
|
|
|
Current U.S.
Class: |
62/512 ;
62/513 |
Current CPC
Class: |
F25B 40/00 20130101;
F25B 2400/01 20130101; F25B 2400/0403 20130101; F25B 41/00
20130101; F25B 2400/0411 20130101 |
Class at
Publication: |
062/512 ;
062/513 |
International
Class: |
F25B 43/00 20060101
F25B043/00; F25B 41/00 20060101 F25B041/00 |
Claims
1. A method for the efficient exchange of thermal energy between a
source and a load in which a two-phase medium comprising a
saturable fluid is utilized that has a controllable temperature,
vapor and pressure state comprising the steps of: providing a
pressurized saturable fluid at a temperature that is pressure
determined; before delivery of the saturable fluid to the load,
extracting vapor from the saturable fluid while maintaining the
pressure; exchanging thermal energy between the saturable fluid
after vapor extraction and the load; recombining the extracted
vapor with the fluid after thermal energy exchange and, restoring
the fluid to a selected pressure and temperature status.
2. The method as set forth in claim 1 further including the steps
of: compressing the fluid to a high pressure state; condensing a
portion of the high pressure fluid to substantially ambient
temperature; expanding the condensed fluid to a controlled
pressure-temperature state; blocking return flow of extracted vapor
into the expansion step; mixing the expanded fluid with compressed
uncondensed fluid in selected proportions to provide a final
temperature state for thermal energy exchange with the load;
exchanging thermal energy between the mixture and the load
thereafter; recombining the extracted vapor with the fluid after
thermal energy exchange with the load, and recompressing the
recombined fluid.
3. The method as set forth in claim 2 above, wherein the extraction
of vapor reduces the mass flow of saturable fluid through the load,
with more efficient thermal exchange, and the saturable fluid after
extraction is more efficiently recompressed.
4. The method as set forth in claim 2 above, further including the
steps of controlling the proportion of compressed uncondensed fluid
relative to the expanded fluid to determine the temperature
thereof, sensing the change of load temperature when full hot gas
flow is employed, and stabilizing the load temperature by diverting
a partial flow of compressed uncondensed fluid directly for
recompression.
5. The method of circulating a two-phase fluid through a thermal
exchange procedure so as to maintain efficiency over a wide
temperature range of operation comprising the steps of: compressing
the fluid to a high pressure hot gas state; condensing a portion of
the pressurized hot gas to a liquid; selectively expanding at least
some of the condensed liquid to a liquid/vapor phase; extracting
vapor from the liquid/vapor phase while maintaining the pressure
thereof; mixing a selected proportion of the pressurized hot gas
with the expanded liquid/vapor phase after extraction of vapor to
establish a chosen temperature/pressure status in the liquid/vapor
mixture prior to the thermal load; passing the mixture in thermal
exchange relation with the load, and recycling the fluid from the
load for recompression.
6. A method as set forth in claim 5 above, further including the
steps of accumulating refrigerant after thermal exchange with the
thermal load, and heating the accumulated refrigerant to establish
desired pressure and temperature balance for recompression, and
diverting a flow of pressurized hot gas directly to recompression
and redirecting the flow passed in thermal exchange with the
load.
7. A method as set forth in claim 5 above, wherein the step of
condensation comprises supplying a condensate in thermal exchange
with the compressed fluid, sensing the temperature of the
condensate after the thermal exchange and adjusting the flow rate
of coolant to maintain the temperature of the condensate after
thermal exchange at a selected level, and wherein the step of
extracting vapor includes coalescing droplets of vapor,
accumulating a reservoir of liquid refrigerant therefrom, and
flowing refrigerant in excess of a given level to thermal exchange
with the load.
8. A method for improving the control stability of a system
utilizing saturable fluid in compressed, controlled temperature
phase in a vapor-compression cycle, for thermal exchange with a
load, comprising the steps of: compressing a saturable fluid to a
pressurized hot gas; condensing a flow of the pressurized hot gas;
expanding the condensed flow to a selected level; reducing the
vapor content of the expanded flow without changing the pressure;
mixing a selected proportion of pressurized hot gas with the
expanded flow; adding thermal energy to the mixture to reach a
target temperature; feeding the mixture to the load for thermal
exchange prior to return for recompression, and when the proportion
of hot gas to be provided to the load is at a high level, shunting
a selected fraction therefrom directly to compression to stabilize
the functions of controlling the expansion and controlling the
pressure.
9. A device for isothermally separating vapor from a liquid/vapor
fluid mixture at saturation to increase the proportion of liquid
therein comprising; a chamber having input and output ports for the
saturable fluid; a barrier section of selected porosity and high
surface area disposed within the chamber between the input and
output ports and intercepting the liquid/vapor mixture; a liquid
outlet at a lower region of the chamber for output of liquid
collecting within the chamber, and a valve including a surface
float disposed on the surface of liquid in the chamber and
including a closure member in operative relation to the liquid
outlet that is disposed to allow liquid in excess of a
predetermined level in the chamber to flow out of the chamber.
10. A device as set forth in claim 9 above, wherein the inlet port
has a flow impedance selected such that flow occurs from within the
chamber through the outlet vapor port and the outlet liquid port,
and wherein the inlet vapor port provides the saturable fluid and
the barrier is a fibrous structure substantially vertically
intercepting the flow path between the input and output ports and
further including a check valve coupled in the flow path to the
inlet port.
11. a system for controlling the temperature of at least one
thermal load unit using a circulating two-phase refrigerant
comprising: a vapor cycle refrigeration system including at least
one compressor and condenser series, and employing a two-phase
refrigerant as a temperature control medium, and arranged such that
the refrigerant is first compressed and then at least a portion
thereof is separately liquefied to a state of pressure saturation;
a proportioning device receiving a flow of compressed refrigerant
as a hot gas; an expansion device receiving a flow of saturated
liquid; a gas processor receiving the saturated liquefied state,
for extracting at least some vapor therefrom without substantially
altering the pressure/temperature equilibrium thereof, a mixing
junction coupled to receive the hot gas flow from the proportioning
device and the flow from the expansion device; a thermal load
comprising at least one heat exchange structure receiving the
vapor-extracted saturated liquid, and a controller system coupled
to operate the proportioning device and the expansion device to
control the temperature of the thermal load.
12. A system as set forth in claim 11 above, wherein the extracted
vapor from the gas processor is circulated to the compressor as
input and wherein the flow path to the compressor also includes an
accumulator receiving output from the load, and a
controller-operated coupling from the proportioning device to the
flow path to the compressor.
13. A system as set forth in claim 12 above, wherein the gas
processor comprises a vapor separator including a level-responsive
liquid-enriched flow of saturated fluid to the load, and thereafter
vapor to the compressor input, and where a check valve is coupled
into the input path to the vapor separator.
14. A system as set forth in claim 13 above, further including
controller operated shunt and bypass valves in circuit with the
proportioning device to enable abrupt commencement and cessation of
hot gas flow and controller operated shunt and bypass valves in
circuit with the expansion device to enable abrupt commencement and
cessation of flow of condensed fluid and a close on rise valve in
circuit with the compressor input and responsive to the input
pressure thereat.
15. A system as set forth in claim 13 above, wherein the system
includes means to extend the high temperature range of operation
comprising: a countercurrent heat exchanger coupled in circuit with
the load and exchanging thermal energy between input and output
flow to and from the load and a heater under command of the
controller for adding thermal energy to the input to the load.
Description
REFERENCE TO PRIOR APPLICATIONS
[0001] This invention relies for priority on previously filed
application Ser. No. 11/057,383 of Kenneth W. Cowans et al, filed
Feb. 15, 2005 and entitled "Thermal Control System and Method", and
on provisional application 60/733,078 filed Nov. 4, 2005 by Kenneth
W. Cowans et al and entitled "Thermal Control System and
Method".
BACKGROUND OF THE INVENTION
[0002] The above mentioned utility application of Cowans et al
discloses a system and method of temperature control in which the
temperature of a load is varied by thermal exchange with a
saturated fluid directly, without requiring an intervening heat
transfer fluid. Typically the saturated fluid is a refrigerant used
in pure gas phase, or pure liquid phase, or more often in a mixed
or saturated phase. In accordance with the invention, the
vaporizable refrigerant is processed through substantially
conventional compression and condensation steps as provided by
commercially available equipment. However, compressed but not
condensed refrigerant from the compressor is separately controlled,
later to be mixed with condensed and selectively expanded
refrigerant. The saturated refrigerant after mixing is at a
pressure which determines its temperature, and its thermal energy
is transferred directly with the thermal load which is to be
controlled in temperature. Used in this way, the saturated fluid
can provide a wide range of temperatures at the thermal load.
However this range can be extended, because at one extreme the
thermal load can be heated by using the hot gas phase alone, and at
the other extreme the load can be chilled using only refrigerant on
the condensed phase, after expansion. In this direct transfer
system, temperature changes can be rapid and set points can be
controlled very precisely. Equipment costs are substantially
reduced because the system does not require use of an intermediate
thermal transfer fluid, pumps, or a heat exchanger for thermal
transfer fluid.
[0003] The system and method present unique challenges, as well as
possibilities. Processor components in a refrigeration system must
operate satisfactorily through all phases in the compression and
heat exchange cycle, so that the refrigerant must be at proper
temperatures and pressures so that different functions can be
performed. For example, the input to a compressor system should be
free of liquid, and in a particular pressure range, or efficiency
will be lost or the compressor damaged, or both. Maintaining
efficiency throughout can present other problems that require
solutions consistent with overall system requirements. For example,
if the fluid is in a saturated state, the compressed gas component
contributes relatively very little to chilling the heat load during
thermal transfer. Thus when the load temperature is to be dropped
to a minimum, the presence of the gas limits heat exchange
efficiency. Another factor affecting efficiency occurs in a
different temperature range, resulting from limitations on the
energy of compression that can be applied to the refrigerant. When
the refrigerant is to be used for heating, the compressor brings
the hot gas to a given level, such as 120.degree. C. However, if
substantial heating energy is needed at the load, a much higher
temperature level must be reached. The return flow, after heat
exchange with the load, cannot however be at levels that disrupt
the pressure/temperature/enthalpy balance needed with a vaporizable
refrigerant. It is highly desirable to eliminate such problems
without introducing thermodynamic conditions which affect the
integrity of the refrigeration cycle.
[0004] The response capabilities of systems in accordance with the
invention can be employed to meet the operative requirements of
many different temperature control combinations. Where time is of
the essence in bringing a thermal load to a target temperature,
anticipatory manipulation of the controller can be useful.
SUMMARY OF THE INVENTION
[0005] In a transfer direct saturated fluid (TDSF) thermal control
system, a number of local variations in control loops and
components improve efficiency and stability across a range of load
temperatures. Cooling efficiency, at very low temperatures, for
example, is markedly improved at the load by extracting at least
part of the vapor components in the condensed fluid after expansion
before mixing with compressed gas. The lowered mass flow through
the line then returning from the load to the compressor reduces the
pressure drop in said return line. To this end, a vapor separator
is disposed in the line transporting liquid/vapor product before
the mixing junction, and the extracted vapor is fed back into the
refrigerant returning from the load to compressor while a higher
proportion of liquid is fed to the load. Thus heat transfer is more
efficient without affecting the temperature of the mix.
[0006] Other aspects of the invention are concerned with
maintenance of efficiency and improvement of temperature limits
when using the high temperature capability of the saturated fluid
system. A counter-current heat exchanger may be positioned in the
flow path to the load to interchange thermal energy between the
incoming input, and the out-going refrigerant passing from the load
back to the suction input of the compressor. This energy
interchange both increases the temperature level of the input to
the load and reduces the temperature of the return fluid. The
return fluid may thereafter be brought to a level compatible with
the demands of compressor operation. For further heating the input
to the load can be passed through a heater operated by the
controller and included in the input stream to the load to thereby
raise the temperature well above the compressor capability if
desired.
[0007] A further feature of the system is the incorporation of a
computer controlled solenoid bypass between the hot gas shunt that
leads to the mixing circuit, and the return line from the load to
the suction input of the compressor. This bypass is operated to
remove some of the flow from the hot gas line when full hot gas
flow might tend to make the loop gain of the servo system unstable
and/or reduce the amount of cooling available at temperatures
within the desired range. The result introduces a time delay in
cooling the load to enable closer control of temperature.
[0008] Control circuit adaptations may be introduced to realize
further benefits from the concept. Where fast reaction to commands
requiring fast temperature changes are needed, temporary and short
term commands can be utilized to shorten response times. If, for
example, the active part of a thermal load comprises the surface of
a wafer holding chuck in a semiconductor processing system, and the
heat exchange region is physically spaced apart from the upper
surface of the chuck, some time may be needed to bring the chuck
surface to a specified temperature. By introducing control
algorithms which bypass the need for accumulation of tracking data,
the wafer can be much more rapidly stabilized at a target
temperature, increasing yield rates and lowering costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A better understanding of the invention may be had by
reference to the following description taken in conjunction with
the accompanying drawings, in which:
[0010] FIG. 1 is a block diagram representation of a temperature
control system using direct transfer of saturated fluids in
accordance with the invention;
[0011] FIG. 2 is a generalized schematic diagram of one form of a
vapor separator such as may be used in the system of FIG. 1;
[0012] FIG. 3 is a graphical representation of the improved control
stability achieved by utilizing a supplemental hot gas bypass
coupling as shown in FIG. 1; and
[0013] FIG. 4 is a graphical representation of high temperature
response in a system utilizing a hot gas heater and heat exchanger
as shown in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Referring now specifically to FIG. 1, a temperature control
system is shown utilizing a novel refrigeration/heating cycle in
accordance with the above-referenced previously filed Cowans et al
application Ser. No. 11/057,383. The present system integrally
incorporates features in this context for achieving improved
stability and/or efficiency when directly transferring thermal
energy using saturated fluids.
[0015] In FIG. 1, a compressor 10, which may use a conventional
refrigerant such as R507, or a different refrigerant depending upon
the application, feeds compressed hot gas into a condenser 12 which
also is arranged as part of a pressure regulating system. The
compressor 10 output is fed into a hot gas line 14 which is
separately split to a condensed refrigerant line 16, the flow
proportions being controlled by a digital controller system 20, as
described in the previously filed application. The hot gas line 14
includes a proportional valve 22 operated by the controller 20 so
as to provide a pressure modulated flow, responsive to system
requirements. In the separate line 16, condensed refrigerant flow
is fed through and regulated by a thermal expansion valve (TXV) 24
which reduces the pressure of the condensed input, and expands the
volume so as to lower the temperature, again in accordance with
vapor-cycle operation and operating objectives. These two separate
lines come together in a mixing circuit 26 and as described in the
prior case, in the principal range of operation the relative flows
are adjusted so as to provide an output temperature determined by
the pressure.
[0016] The output flow from the mixing circuit 26 then cools or
heats the thermal load 28, such as a semiconductor tool, after
which the refrigerant is transferred on a return line 30 back to
the suction input to the compressor 10. The return line 30 includes
a serial accumulator chamber 32, in which a heater 34 operated by
the controller 20 restores the temperature level, as and if
necessary.
[0017] Operation of the system has, among other things, the
advantage of providing a very wide potential temperature control
range from hot to cold (e.g. from .+-.120.degree. C. to -60.degree.
C.), extremely fast temperature adjustments, and also high
precision (e.g. .+-.1.degree. C.). In addition, since no
intermediate heat transfer system or medium is needed, these unique
capabilities can be provided with substantial cost savings.
[0018] However, the use of a saturated fluid in different phases
(liquid, vapor, and saturated liquid/vapor phase), introduces a
number of operative problems or conditions that should be accounted
for to realize system potential more fully. In some applications it
is desirable to effect rapid changes between operating modes at
different temperature levels. For abrupt cessation of flow in the
hot gas line 14, a shutoff valve 40 is disposed in series with the
proportional valve 22. For virtually immediate or assured full flow
of hot gas, the shutoff valve 40 is bypassed by a shunt valve 46
which is in parallel with it. For rapid control of the condensed
refrigerant flow, the condensed refrigerant line 16 includes a
shutoff valve 42 and a shunt valve 44, which bypasses the TXV 24,
all valves being operated by the controller 20. The condenser
system includes a condenser heat exchanger 50 which is cooled by
water from a conventional source 52, although other cooling fluids
may be used. In a configuration known in the prior art, the water
flow rate is governed by a flow control system 54, so as to
maintain the output pressure from the compressor 10. To facilitate
maximum cooling, a bypass valve 56 is disposed in parallel with the
coolant flow control.
[0019] Expedients are also used to improve system response and
reliability in terms of thermodynamic efficiency. A subcooler heat
exchanger 60 is disposed in the return line 30 leading to the
suction input to the compressor 10. The subcooler heat exchanger 60
operates as a counterflow device, cooling the outgoing flow from
the condenser 12 with returning fluid, which in most modes will be
expanded and cooled gases, directed back to the compressor 10. In
accordance with the W. W. Cowans Pat. No. 6,446,446 referenced in
the predecessor parent application, shunt loop 62 about the
subcooler heat exchanger 60 includes a desuperheater valve 64
responsive to a temperature sensor 66 at the suction input to the
compressor 10. If the input pressure to the compressor 10 falls too
low the flow is augmented by opening the desuperheater valve 64. In
the shunt loop 62, these flows are derived from a T-junction 68 at
the condenser 12 output.
[0020] In order to preserve pressure and temperature balance in the
closed loop compression/heat exchange system, the return line from
the load 28 is passed through an accumulator 37 which includes a
heater 34 operated by the controller 20. The system can thus act in
response to temperature signals provided from a temperature sensor
78 associated with the load 28 to restore or equalize the
temperature of the fluid in the return line. Also, a conduit to the
TXV 24 from sensor bulb 74 in communication with the return line 30
is used for external equalization of the TXV 24.
[0021] Another feature cooperates with these elements and
relationships to overcome different potential problems. If the
pressure in the return suction line to the compressor 10 becomes
too high, it is automatically lowered by an included crankcase
pressure regulator valve, also known as a "close on rise" (COR)
valve 76.
[0022] Flows at different points in the circulating loop must often
be brought into predetermined pressure and temperature ranges for
components to work properly. For example, the compressor 10 input
must be maintained above a selected pressure range. This is
accomplished by a hot gas bypass valve 82 responsive to a pressure
sensor 80 at the input to the compressor 10. The hot gas bypass
valve 82 feeds back a portion of the compressor 10 output flow to
the suction input in the event the input pressure is too low.
[0023] The system as thus far described operates as described and
in practice validates the concept and its advantages, but it also
possesses certain advantages and potentials not immediately
evident. For example, in the high temperature mode, the system can
be operated with the proportional valve 22 alone providing
temperature modulation, and with the refrigeration line 16 being
shut off by the valve 42. The input to the load 12 is then solely
the high temperature gaseous flow, but the temperature of the input
to the load 28 can be further raised to an even higher level,
suitably compensating for anticipated major heat losses at the load
28 at these temperatures. For this purpose, a counterflow heat
exchanger 86 and a serially coupled electrical heater 88 are
disposed between the mixing circuit 26 and the load 28. The input
temperature to the load 28 is detected by a sensor 90, so that
actuating signals can be applied from the controller 20 to the
heater 88 subsequent to the mixing circuit 26. Separately, a heater
92 is provided in the hot gas line 14 prior to the mixing circuit
26, to be energized by the controller 20 to provide further
heating. Reverse flow back toward the control valves is blocked by
a suitably placed check valve 94. The counterflow heat exchanger 86
keeps the ? temperature level down to a predetermined range in the
suction line to the compressor 10. The sequence of temperature
changes in this mode is shown graphically in FIG. 4, wherein the
hot gas temperature is successively increased from the level (A)
provided by the full open proportional valve to the higher level
(B) at the HEX 86 output and then the final highest level, from the
heater 88.
[0024] At the opposite (cold) end of the operable temperature
range, there are limitations on the low range of temperature
possible, depending on the proportion of liquid in the refrigerant
mix that is fed to the load. The presence of gas in the saturated
mix employed on cooling adversely affects performance by increasing
the pressure drop between load 28 and input to compressor 10. The
temperature of a mix of liquid and vapor at any point is equal to
the saturation temperature of the liquid at the pressure
experienced by the mix at that particular point. In these systems,
with respect to flow in the return line from the load 28 to the
compressor 10 input, the pressure drop is proportional to the
square of the mass flow of the refrigerant. Since the cooling
output power of a vapor cycle system is proportional to the
compressor input pressure, it is advantageous to reduce the mass
flow return to the compressor. Specifically, cutting the mass flow
in half reduces the pressure drop four-fold, since pressure drop in
a flowing gas is about proportional to the square of the mass of
the gas flow.
[0025] With these factors in mind, the condensed refrigerant line
16 includes, subsequent to the TXV 24, a check valve 98 and a vapor
separator 100, an example of which is seen in more detail in FIG.
2. Vapor which is collected in the separator 100 is directed along
a vapor line to the return line 30 to the compressor 10. For
maximum cooling effect, the liquid proportion is accentuated by the
vapor separator 100, and increases the efficiency of the compressor
10.
[0026] FIG. 2 shows a schematic cross-sectional diagram of the
liquid separator 100. Somewhat similar devices are commonly used in
vapor-cycle refrigeration systems to separate high pressure
refrigerant gas emerging from the compressor from oil mist carried
along with the refrigerant but this use in a thermodynamic function
is novel. Gas and liquid enter the separator body 150 at entry port
153 as indicated by arrow 154 on FIG. 2. The separator 100
functions by using a finely divided metal wool barrier 156 placed
in the path of the gas such that all the refrigerant must pass
through the wool 156 toward an outlet. Droplets coalesce on the
surfaces of the metal wool barrier 156 and descend, under the
influence of gravity, to the bottom of the separator cavity. When a
sufficient level of liquid builds up in the bottom it lifts a float
160 that is connected to a valve 162 fitted into an exit port 164
and thus allows liquid to flow from the system through the liquid
exit port to the load 28. The gaseous refrigerant that passes the
barrier 156 flows toward the compressor 10 input from a gas exit
port 166 near the top of the separator cavity. Sufficient pressure
drop from the inside of the separator 150 to the outside must be
maintained in order to drive the fluids through their respective
openings. This is provided for by the use of an input orifice 153,
whose impedance is chosen to be high enough to provide the needed
level of driving pressure across the liquid port 164 and gas or
vapor port 166.
[0027] When liquid only is fed to the mixing tee 22 to combine with
hot gas regulated in flow by proportional valve 22 the combined
mixture flowing from mixing tee 26 will simply have less gas than
if the separator 100 were not present. When the maximum amount of
cooling is demanded at the lowest possible load temperature a
condition is encountered that should be noted. Proportional valve
22 would be shut in this mode and the flow through the system shown
in FIG. 1 would be almost pure liquid entering and leaving mixing
tee 26. Under some conditions, such as when the refrigerant
condenses at 60.degree. C. and provides cooling at -20.degree. C.,
without the separator 100 there would only be about 40% of the
total mass flow traveling out of mixing tee 22. This means that the
total pressure drop over the loop from the mixing tee 26 to the
exit line will be only about 16% of what it would be if the full
mass flow of gas and liquid were to be passed through the system.
This can provide a significant improvement in system performance. A
typical pressure drop of such a system, measured from the supply
line to the exit line, would be of 12 psig (measured cooling 5 KW
at a set point of -40.degree. C.) but would be less than 4 psig
with the improved system shown in FIG. 1. Temperature measured at
load would therefore drop about 12.degree. C. when tested under
these same conditions.
[0028] The basic TDSF system enables providing useful heat to
maintain the load in the range of 90.degree. C. to 120.degree. C.
by delivering high pressure gas to the load at temperatures that
are sometimes well in excess of the required load temperature
level. Thus, for example, to provide 5 KW of heat to a load which
is to be heated to 120.degree. C. with a flow of 200 grams/second
of R507 gas requires that the gas be heated to about 28.5.degree.
C. or more above 120.degree. C. In giving up heat to the load by
cooling 28.5.degree. C. this gas flow will bring the load to the
target temperature. Somewhat more than this amount is needed to
provide drive for the needed transfer of heat across whatever heat
exchanger is used. A temperature of 120.degree. C. is as high as a
typical commercial compressor readily withstands, whereas the
improvement of FIG. 1 can provide gas at temperatures approaching
200.degree. C. if the structural members used can support such
levels.
[0029] Basically, this is achieved by using the counter-current HEX
86 together with the extra electrical heater 88 in the input path
to the load 28, after the mixer 26. During operation, when the
system is supplying temperatures less than about 60.degree. C. the
system functions substantially the same as does the prior system.
When temperatures above this level are required the hot gas from
the compressor 10 first provides its maximum level [(A) in FIG. 4.]
which is raised to a higher level by flowing through the
countercurrent HEX [(B) in FIG. 4]. Finally, the heater 88 is
activated by electronic controller 20 to provide adequate heat to
raise the temperature of the refrigerant to the desired final value
[(C) in FIG. 5]. The counter-current HEX 86 isolates the bulk of
the TDSF system from any adverse effects of the high temperature
because the fluid emerging on the return line from the
counter-current HEX 86 will be not much hotter than the fluid
emerging from the high pressure outlet of the compressor.
[0030] The system includes a further improvement providing adequate
control during times when the TDSF system is closely controlling
the temperature of an object that is being temperature controlled
by the TDSF. A full flow of gas from proportional valve 22
overwhelms the controller function if the entire flow is mixed with
the flow from TXV 24. This condition is illustrated by the graph of
FIG. 3, which shows the instability that exists when there is full
hot gas flow. The effect of a small change in flow from valve 22 is
then such as to change the total thermal output of the mixture to
an excessive degree, and the system can tend to become unstable. In
essence, the loop gain of the servo system which includes the
combined output of the valves 22 and 24 is too high if the full
flow from 22 is used to mix with the flow from valve 24. This
causes problems when the temperature of the load is being
controlled to close tolerances: A swing of temperature around the
control point results, particularly when the controlled load is
located at a distance from the point of application of cooling or
heating. This condition introduces a time delay between the
application of cooling at the load 28 and the reaction of any
temperature sensor located at the load 28.
[0031] The solution used is to employ a bypass line 103 including a
hot gas bypass (solenoid) valve 104 as shown in FIG. 1. The hot gas
bypass valve 104 is responsive to the controller 20. When close
control is needed, the valve 104 is opened, allowing some of the
gas output of proportional valve 22 to bypass directly to the input
of compressor 10, which has the effect of reducing flow through the
cooled load 28. A check valve 106 in the bypass line prevents any
flow from the TXV 24 from being bypassed when the proportional
valve 22 is closed. Thus the pressure drop through the load 28 is
reduced and concomitantly the temperature difference across the
load is also reduced. Control is enhanced because the overall loop
gain of the control servo circuit is reduced and thus easier to
control.
[0032] These expedients all contribute in a highly integrated
fashion to assuring greater reliability and extended range for TDSF
systems. Practical applications of this concept can use the
potential for fast response and precise control afforded by the
system to achieve superior results for particular situations. Some
testing and instrumentation systems, for example, test a
multiplicity of parts or products sequentially at a series of
different temperatures, which may vary widely. The capability of a
TDSF system for changing rapidly between temperature levels can
save much time and money and increase throughout in these
inspection applications.
[0033] It has been found that TDSF systems can respond to needed
temperature changes even faster than the electronic controllers,
when the controllers have to store a series of readings before
establishing reaching a steady state condition. In a typical
controller using proportional and derivative functions, for
example, the entry of a new set point can initiate a time consuming
sequence in which, while transitioning to a new target value, a
succession of readings are required. Where a TDSF system has a
faster response it has been found useful to enter an artificial and
temporary temperature reading into the controller. A new sequence
of readings is not needed because previously taken temperature
measurements are retained and the controller operates without
interrupting the prior sequence. This enables final temperature
adjustment of the saturated fluid much more rapidly. In a specific
example, the artificial temperature input is used to compensate for
thermal delays that are inherent in the design of a tool. For the
semiconductor application, there is a physical distance between the
top of a chuck, on which the semiconductor wafer rests, and base
region where thermal transfer with the refrigerant takes place. By
altering the input temperature artificially in step-wise fashion
before starting application of power, control of the chuck
temperature is both more rapid and precise. Other empirically
derived artificial inputs may be used in other situations, for
start-up or shut-down sequences.
[0034] While a number of forms and alternatives have been described
above, it will be appreciated that the invention is not limited
thereto but includes all variants and alternatives within the scope
of the appended claims.
* * * * *