Abstract:
A system and method for maintaining the temperature of a thermal transfer fluid at a selectable level within a wide temperature range, so as to operate a process tool in a chosen mode employing at least two cascaded stages, each operating with a different fluid in a separate refrigeration cycle. By interrelating energy transfers between parts of upper and lower stages, thermal efficiency is maximized and a smooth continuum of temperature levels can be provided. The refrigerants advantageously have vaporization points below and above ambient, for upper and lower stages respectively, and employs the upper stage for a constant refrigeration capacity, controlling the final temperature with the lower stage. The system allows for a further extension of range because the thermal transfer fluid can be heated for some process tool modes as the refrigeration cycles are run at low loads.

Description:
FIELD OF THE INVENTION 
   This invention relates to temperature control systems which heat and/or cool separate process equipment by circulating thermal transfer fluid at a temperature which may be selected within a wide range but precisely maintained. 
   BACKGROUND OF THE INVENTION 
   Applicant has previously developed temperature control units utilizing pressurized liquid refrigerant, expansion valve devices, and heat exchangers/evaporators to provide the thermal capacity needed for cooling or heating thermal transfer fluid that flows within a process tool, in order to maintain the tool at a selected temperature level. The units function with high thermal efficiency, provide precise control, and meet the demanding needs of modern high-capital intensive industries, such as semiconductor industries using cluster tools. For such applications, long life and high reliability are essential, but the requirements also include compactness and small footprint because of the high costs of floor footage in such facilities. 
   These industries are continually evolving and developing more demanding applications which need more versatile temperature controls but at the same time at lower cost. More particularly, such installations now demand selectable refrigeration and optional heating of thermal transfer fluid in the range from about −80° C. to about +60° C., with precision and efficiency. It should be intuitively evident that such a wide temperature range cannot be met economically by conventional refrigeration systems. One approach to the problem of operating over a range of refrigeration temperatures is that proposed by Mizuno et al in U.S. Pat. No. 4,729,424 wherein a cascaded series of refrigeration units are employed. Each unit supplies its own refrigeration capacity as commanded by a central system, to provide stepwise refrigeration capability. Temperature levels between the different refrigeration increments are established by heating within the incremental range. The use of a number of refrigeration units (four in the Mizuno et al proposal) presents particular problems in terms of space requirements, efficiency and reliability. Also, refrigeration units, for long life, should not be run intermittently. Any specific refrigerant further imposes some inherent limitation, depending upon its critical temperature, on the range of operation. In addition efficiency is inherently reduced when heating must be employed to counteract over-cooling. 
   SUMMARY OF THE INVENTION 
   Systems and methods in accordance with the invention utilize an intercoupled cascaded arrangement of at least two modular refrigeration units, the first of which operates with a refrigerant having a relatively higher evaporation point to provide a refrigeration capacity predominantly for midrange operation. A second refrigeration unit, interacting in key respects with the first refrigeration unit, adds to the refrigeration capacity of the first unit while controlling the temperature of a thermal transfer fluid that circulates through the process tool. The second refrigeration unit, which uses a refrigerant having a lower evaporation point, can lower the temperature of the thermal transfer fluid to as low as −80° C. The system operates both refrigeration units efficiently in an integrated manner while providing a smooth continuum of operating temperature levels. When ambient or above ambient temperatures are needed, for transient or steady-state operation, a heater in the thermal transfer fluid loop to and from the process tool can be employed independently as the refrigeration units function at low loads. 
   The two refrigeration units are both designed in compact modular form, and for efficiency interchange thermal energy between the refrigeration cycles although having only limited connections between them. Different combinations of modules can be employed, for different applications, with functions being controlled by a digital control system. 
   The inter-relationship between the first and second refrigeration units includes one or more expansion valves in each unit, with the first unit supplying a controlled liquid/vapor mixture to an interchange heat exchanger/evaporator in the second unit which functions as a condenser in that unit. In the first unit, the gaseous pressurized output of the compressor is condensed, as by an air-cooled condenser arranged so that cooling air can also extract heat energy from compressed gaseous refrigerant in the adjacent second refrigeration unit. Chilled second refrigerant from the interchange heat exchange/evaporator is fed via a thermal expansion system that is precisely controllable and free of flood back propensity to a heat exchanger/evaporator that cools the thermal transfer fluid in the loop including the process tool. 
   More specifically the expansion valve system in the second refrigeration unit includes a variable duty cycle solenoid expansion valve having a relatively large orifice. Varying the duty cycle integrates the flow to establish a chosen average level, while the orifice area is capable of supplying large flows for high demand conditions. The output of the solenoid expansion valve is fed to a thermal expansion valve having a variable orifice and incorporating a feedback input reflecting the temperature at the output of the interchange heat exchange/evaporator. Both the solenoid expansion valve and the thermal expansion valve in the second refrigeration unit as well as the expansion valve in the first refrigeration unit are responsive to command inputs which control the refrigeration capacity supplied by each subsystem. 
   The modular construction is such that each refrigeration unit can be used independently, with minimal connections between them being easily engaged when needed. In addition the first or upper refrigeration unit can employ a water-cooled condenser, if desired—in this case the first unit will also usually have a separate fan for extracting heat energy from the compressed gas conduit in the second or lower stage refrigeration unit. 
   A number of features are included in these modules to improve useful life, increase reliability and provide assurances against catastrophic failures. The refrigerant unit in the second refrigeration unit presents theoretical problems because of gas pressure buildup, due to the low boiling point, but this is obviated by the use of an excess gas chamber as well as a preset pressure burst disks. The thermal transfer loop is substantially confined within the second lower stage module, but nonetheless includes a storage reservoir, a differential pressure regulation system, and a gas purge system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be had by reference to the following description taken in conjunction with the accompanying drawings, in which 
       FIG. 1  is a block diagram of a system in accordance with the invention including an associated control system and a process tool, and also showing how separate modules and units depicted in  FIGS. 2A ,  2 B and  2 C are interchangeable; 
       FIG. 2  is a set of four drawings in block diagram form, including respectively the composite system view in some detail ( FIG. 2  alone), with more detailed views of the upper stage module ( FIG. 2A ), the lower stage module ( FIG. 2B ) and a final module including the thermal transfer loop and process tool ( FIG. 2C ); 
       FIG. 3  is a detailed view of a portion of the lower stage module showing an alternate form of expansion valve system that may be used in the lower stage module. 
       FIG. 4  is a perspective view of the exterior of a practical example of one combination of an upper stage module including an air cooled condenser, and a lower stage module with the exterior walls removed to show a part of the interior; 
       FIG. 5  is a perspective view of an implementation of the two modules of  FIG. 3 , as seen from a different angle, and 
       FIG. 6  is a perspective view of the practical implementation of the lower stage module presented at a different angle than in  FIGS. 3 and 4  to show a different part of the interior. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Systems and methods in accordance with the invention are founded on the apparatus shown in  FIGS. 1 and 2 , to which reference is now made. The primary units are, as seen in  FIG. 1 , an upper (temperature) stage module  10  using a first refrigerant, and a lower (temperature) stage module  12  employing a different refrigerant and interchanging thermal energy with the upper stage module  10  in various ways. The lower stage module  12  exchanges thermal energy, at a final temperature level that is at, above or below ambient, with a thermal transfer fluid that feeds through a process tool  14  in a loop, via a supply line  16  and a return line  18 . Because of the number of individual units that are employed in the stages, details are depicted in added Figures by subdividing some principal elements of  FIG. 1  into the composite system of  FIG. 2 , then providing diagrams which delineate details of the two modules (upper and lower stage, respectively), as separate  FIGS. 2A and 2B  and the final thermal transfer loop of  FIG. 2C .  FIG. 1  also depicts a control system  20  that receives inputs from an operator, and from sensors and transducers in the system, and that provides control signals to controllable elements in the temperature control system. A control system which may advantageously be employed is that described by Matthew Antoniou et al in a pending patent application dated May 16, 2003 Ser. No. 10/439,299 and entitled “Systems and Methods of Controlling Temperatures of Process Tools”. 
   Referring now to  FIGS. 1 and 2 , together with the more detailed views of  FIGS. 2A ,  2 B and  2 C, the upper stage module  10  includes a compressor  22 , here of nominally 7.5 kW capacity to meet the needs of a specific practical application. The compressor  22  pressurizes a refrigerant having a relatively high boiling point, such as R-507, raising its temperature. R-507 is a liquid at ambient pressure and temperature and after compression and condensation the refrigerant again becomes liquefied for use in a liquid/vapor state. After thermal energy exchange within the user system, expanded R-507 refrigerant in vapor state is returned to an input accumulator  24  at the suction input of the compressor  22 . An input valve, such as a Schrader valve  26  (“S.V” in the drawings), couples into the suction input line so that refrigerant volume can be restored if needed. A different Schrader valve  28  is also included in the pressurized output line from the compressor  22 . 
   In this example the compressed gaseous refrigerant in the upper stage  10  is liquefied in an air cooled condenser  30 . The condenser  30  is compact, such as 5″×12″×24″, and so configured relative to the compressor  20  and other elements as to fit within a standard form factor upper stage module  10  of 10″×24″×35″. The modular installation concept is described in a co-pending application of Kenneth W. Cowans entitled “Systems and Methods for Temperature Control”, Ser. No. 10/079,592 filed Feb. 22, 2002. As shown in that application, it is highly advantageous to be able to deploy modules of different capabilities with form factors that are either standard, or integral multiples of the standard. Such modules, mounted replaceably in a support frame, can then be used in different combinations to provide a variety of functions and meet a number of operative requirements that may change with time. In this example, both the upper stage module  10  and the lower stage module  12  are standard width units, fitting replaceably within receptacles in a standard frame or enclosure to form a double width assembly. 
   The air cooled condenser  30  includes a large fan  32  which blows cooling air across interior heat conductive conduits  33  transporting the compressed refrigerant gas from the compressor  22 , thus extracting sufficient thermal energy to condense it to a pressurized liquid. The cooling air flow, exterior to the upper stage module  10 , also flows into the adjacent lower stage module  12  ( FIG. 2B ) to pass over a finned conduit desuperheater heat exchanger  34  within that module  12 . The conduit  34  within the heat exchanger transfers the compressed gas refrigerant into the lower stage module  12 , so that substantial thermal energy is extracted by this means from the second refrigerant. Approximately 1250 watts of thermal energy is taken out in this example by cooling the gas exiting the low temperature stage compressor to a temperature not much warmer than the temperature of the ambient air. 
   At the input to the air cooled condenser  30  in the upper stage module  10 , referring again to  FIG. 2A , a coupler  36  provides an additional shunt path to a conventional (Danfoss) hot gas bypass valve  38  which is responsive to the suction input pressure at the compressor  22 . When the input pressure is too low, the hot gas bypass valve  38  opens to add a flow of compressed gas into the chilled liquid/vapor refrigerant output that is fed from the upper stage module  10  to the lower stage module  12  ( FIG. 2B ). The output flow from the air cooled condenser  30  feeds into a refrigerant output loop  40  in the upper stage which includes, serially, conventional elements such as a high pressure switch  41 , a filter drier  42  and a sightglass  43 . The refrigerant then enters one input to a subcooler heat exchanger having a body  44  which internally receives expanded low temperature refrigerant that is being returned to the compressor  22  from the lower stage  12 . A coil  45  wrapped about the body  44  transports the pressurized and liquefied refrigerant from the condenser  30 , to further chill the refrigerant before it is controllably expanded by a thermal expansion valve (TXV)  48 , such as is described in the W. W. Cowans U.S. Pat. No. 6,446,446 issued Sep. 10, 2002 and entitled “Efficient Cooling System and Method”. The TXV  48  is responsive to pressure variations influencing the position of an internal diaphragm as determined by the temperature of the returning refrigerant. The gas of the latter temperature, which is detected at a sensor bulb  49  disposed before the gas refrigerant input to the subcooler body  44  communicates a pressure that may modify the effective size of the orifice in the TXV  48 . The output flow from the TXV  48  is a liquid/vapor mixture, in a ratio determined by the TXV  48  responsively to the input from the bulb  49 . There may also be a supplemental gas input, when the hot gas bypass valve  38  is open, via a T-coupling  50 . The injection of compressed gas via the hot gas bypass valve  38  and coupler  50  affects the temperature of the liquid/vapor output by raising the pressure of the liquid/vapor to a minimum value above that is predetermined by the setting of the hot gas bypass valve  38 . 
   Where fabrication facilities utilize tools that are to be temperature controlled by systems in accordance with the invention and that permit the use of water as a cooling fluid, a different modular construction may be used for the upper stage module  10 , as shown schematically in dotted line outline in  FIG. 2A . In this example, the finned conduits  34  for SUVA  95  refrigerant are still employed in the lower stage module  10 , along with a small fan  32 ′ in the upper portion of the upper stage module  12 , and air flow slots in the sidewall. This arrangement enables a common lower stage module  12  to be used with either type of condenser in a modular system. 
   In the lower stage module  12  as seen in  FIG. 2B , a compressor  62 , again of approximately 7.5 kW nominal capacity in this example, pressurizes a different refrigerant, such as SUVA  95 . This refrigerant has a substantially lower boiling point than R-507 and is a gas at ambient temperature and pressure. To assure reliability, therefore, special expedients are used to maintain unrestricted flow and protect against overpressure. The lower stage compressor  62  receives suction input flows via an accumulator  64  and provides pressurized output flows via an oil separator  66 . The oil that is filtered out by the separator  66  is returned by a shunt line through the accumulator  64  to the lower stage compressor  62  input. The oil separator  66  is useful because a refrigerant such as SUVA  95  used at temperatures as low as −55° C. or lower can be clogged with high viscosity lubricating oil if subsequent quantities of this oil are present at low temperature. The mass of SUVA  95  fluid may be supplemented via a Schrader valve  68  in the output line from the oil separator  66 . The SUVA  95  output line from the finned desuperheater exchanger  34  feeds a separate hot gas bypass valve  70  via a T-coupling  72  which initiates a hot gas bypass loop that includes the valve  70 . When the hot gas bypass valve  70  is opened in response to compressor input, the flow is directed through a shunt line  76  to the suction input to the lower stage compressor  62 . The shunt line  76  output from the valve  70  also includes a Schrader valve  74 . The same suction input line  76  containing SUVA  95  connects through a flow restricting orifice  78  to an excess volume cylinder  80  through a branch line  76   a , the volumetric capacity of which helps to assure that the internal gas pressure of the refrigerant does not become excessive during periods of time when the system is inoperative. A high pressure switch  73  in the return line from the exchanger  34  is used to protect the compressor  62  in the case of an excessively high pressure occurring in the compressor output line during operation. 
   The principal flow path of the compressed gaseous SUVA  95  refrigerant after the compressor  62 , oil separator  66  and finned heat exchanger  34  is to an interchange heat exchanger/evaporator  84 . Heat energy is extracted from gaseous SUVA  95  after the compressor  62  by air flowing from the fan  32  ( FIG. 2A ) past finned heat exchanger  34  to cool the refrigerant. Further thermal energy is extracted by exchange in the interchange HEX unit  84  with the controllably expanded liquid-vapor output from the TXV  48  of the upper stage module  10 . The evaporative cooling of the R-507 refrigerant in the HEX  84  assures efficient thermal energy extraction to at least partially liquefy the SUVA  95  refrigerant in the HEX  84 . In the lower stage module  12 , a subcooler body  86  receives the liquid SUVA  95  output from the interchange heat exchanger/evaporator  84 . Expanded gaseous R-507 from the interchange heat exchanger  84  is returned through the subcooler body  44  in the upper stage module  10  ( FIG. 2A ) to the compressor  22  suction input in that module  10 . 
   In  FIG. 2B , the output of liquefied SUVA  95  is transported within a subcooler coil  90  disposed in thermal exchange relation about the subcooler body  86 , in which interior counterflow of returning and expanded SUVA  95  aids in further chilling of the refrigerant. 
   There are two potential methods of control that are used in the lower stage module  12  subsystem. Both employ liquid/vapor expansion to current temperature settings. In one approach, as seen in  FIG. 2B , an SXV  107  (solenoid expansion valve) regulates the flow of expanding pressurized liquid SUVA  95  at the command of the control (module  20  of  FIG. 1 ). A liquid thermistor  102  in the SUVA  95  flow path after the subcooler coil  90  senses the temperature in the suction line exiting evaporator  84  and provides a corresponding signal to the control circuits  20 , of  FIG. 1  Whenever thermistor  102  senses that liquid SUVA  95  is in this line a signal is sent to control module  20  which causes SXV  107  to be shut. 
   The liquid output of SUVA  95  from the interchange heat exchanger  84  is passed through a filter drier  98  and a T-coupler  100  to the subcooler coil  90  for further cooling. The T-coupler  100  also has a side port communicating with a TXV functioning as a desuperheater valve  104  which is responsive to the temperature in the suction line input to the compressor  62 , as detected by a sensor bulb  106 . Opening of the desuperheater valve  104  injects liquid vapor refrigerant into the cold side input to the subcooler body  86  via a T-coupler  105 . The output from the external subcooler coil  90  about the subcooler body  86  is pressurized liquid refrigerant (SUVA  95 ) at a temperature level determined by the operative parameters of both the upper and lower stages  10 , 12 , respectively. This liquefied refrigerant may flow by a burst disk (not shown) coupled to the line, and set at 500 psi for release of overpressure. 
   In the second control method, shown in  FIG. 3 , a SXV  201  controlled by control box  20  is used in series with a TXV  202  as shown in  FIG. 6 . The use of a TXV, with its inherent feedback via the bulb  203  replaces the function of liquid thermistor  102  as described above. 
   In the example of  FIG. 3 , the liquefied SUVA  95  is fed successively for controlled expansion through a solenoid expansion valve (SXV)  201 , which has a fixed orifice size and operates with a varying duty cycle under control signals from the control system  20 , and then a second, serially coupled thermal expansion valve (TXV)  202 . The second valve or TXV  202  has a variable orifice size to introduce an analog flow variation, determined by electrical signals from the control system  20 , which sets the temperature level of output provided to a second heat exchanger/evaporator  114  which controls system output temperature. The temperature of that output is sensed by a closed bulb element  203  ( FIG. 3 ) that converts the temperature to a variable pressure via a conduit  110  to the second valve or TXV  202 . The serially combined expansion valve functions have important operative advantages for evaporative thermal control units, as noted before. 
   When the SXV is used in conjunction with a TXV for control, the liquid thermistor  88  of  FIG. 2B  is not used. When only the SXV is used to regulate flow and thereby control the liquid thermistor is needed to prevent liquid exiting from the evaporator  114 . 
   The serial SXV  201  and TXV  202  combination of expansion valves shown in  FIG. 3  is advantageous not only in achieving control of liquid/vapor flow but also in more general system terms. It is desirable in general to employ an expansion valve having a large orifice capability in order to meet maximum flow demands. A large orifice size, however, carries with it the danger of transferring some liquid refrigerant into the post-expansion line, because such a flooding condition introduces control instabilities, and the likelihood of compressor mechanism damage. To prevent or limit flooding, systems have been designed which sense the presence of liquid refrigerant in the compressor input, or regulate the capacity of the refrigeration loop. In the present system, however, a large orifice can be employed in the SXV  201 , making available increased cooling power at temperature levels above minimum. This feature enables the system to cool down rapidly. Flooding does not occur, and control is maintained, however, because the TXV  202  functions in an analog fashion limiting the amount of flow as necessary with a variable orifice. Feedback of a corrective pressure from the temperature responsive sensor bulb  203  to the TXV  202  assures maintenance of an opening optimized for the control setting. Consequently, the liquid-vapor mix fed into the second or output heat exchanger/evaporator  114  is boiled off in efficient heat exchange relation with the process fluid, while maintaining the temperature desired, and with no flooding under transient conditions. 
   The liquid-vapor SUVA  95  input from the SXV  107  of  FIG. 2B  (or, in the case of the control system shown in  FIG. 3 , from SXV  201  and TXV  202 ), is supplied to the second heat exchanger/evaporator  114 . This is a selectively controlled flow for chilling the counter-flowing thermal transfer fluid, such as Galden HT-70. 
   The system also includes a thermal transfer fluid loop physically contained principally within the housing of the lower stage module  12  of  FIG. 2B , but extending externally to the tool  14 , as shown schematically in  FIG. 2C . The temperature controlled thermal transfer fluid output from the evaporative heat exchanger  114  is coupled via the supply line  16  to the tool  14  by way of a T-coupling  118 , a sideport of which leads to a pressure relief line  120  that terminates at an adjustable pressure relief valve  122 . Signals indicating the pressure of the thermal transfer fluid are provided to the control system  20  via a pressure transducer  132  open to the supply line  16 . 
   The return line  18  for process (i.e., thermal transfer) fluid from the tool  14  includes a check valve  134  which blocks flow in the reverse direction toward the tool  14  but allows flow of process fluid through a flow meter  136  that provides flow rate signals to the control system  20 . The return line  18  feeds through a T-coupling  138  into a reservoir  140  for the process fluid. Return flow is via a diverging internal cone or nozzle  142  that, in a reversible manner, reduces the flow velocity present in input flow within the enclosed reservoir  140 . The cone transfers almost all the velocity energy in the input flow to pressure energy, thus minimizing overflow effects. A level sensor  146  within the reservoir  140  and a pressure transducer  148  open to the reservoir signal the values of these parameters to the control system  20 . The reservoir  140  also is coupled to a pressure relief valve  150  which provides security against over-pressurization. Independently, as seen in  FIG. 2B , a Schrader valve  152  to pressurize the reservoir  140  is coupled in common to a T-coupler  156  open to the reservoir  140  interior. 
   In the thermal transfer loop shown primarily in  FIG. 2C , the outlet from the reservoir  140  feeds a pump  160 , typically of the regenerative turbine type, which inputs the process thermal transfer fluid to the second heat exchanger/evaporator  114  through a heater  162 , typically of the electrical resistive type. A cap tube bleed line  164  is coupled from the upper-most region of the reservoir  140  to a downstream location relative to the pump  160  and before the input to the evaporative heat exchanger  114 . A drain valve  166  ( FIG. 2B  only), which may be of the Schrader type, is at the remote end of a separate bypass from the heater  162  outlet and at a lower elevation, to permit the entire system to be drained as desired. 
   The system of  FIGS. 1 and 2 , in operation, provides continuous temperature control of the process tool  14  in the range from −80° C. to +60° C., and to higher levels above ambient if desired. Both upper and lower stage modules  10 ,  12  operate continuously, as is needed for reliable, very long term precision performance, even though the cooling loads may be very low, as when the heating capability is being used. In most operative situations that require heat, short term heating is employed to restore temperature so that the process tool  14  can shift to another mode, as is done with semiconductor cluster tools. At times, steady state operation at above ambient is maintained for some duration to effect particular process sequences. 
   The upper stage  10 , operating with R-507 refrigerant, absorbs all of the heat of the lower stage load, insulation losses and all the power supplied to the lower stage refrigerator subsystem. The upper stage then pumps this heat to a higher temperature in order to reject it to the surrounding ambient cooling, shown as air cooling in the current example. As shown in dotted lines in  FIG. 2A , the fan  32  and air cooled condenser  33  can alternatively be replaced by a supply of facility cooling water using a cascade chiller and a liquid-to-refrigerant heat exchanger/condenser of conventional design. When this mode of absorbing the condensing heat of the R-507 refrigerant is used, a small fan is employed to provide a flow of cooling air to pass by fined tube exchanger  34 . 
   In effecting this function of absorbing the heat output of lower stage  12 , expanded liquid-vapor R-507 mixture flows to one counterflow input of the interchange HEX/evaporator  84  in the lower stage  12 . The opposite counterflow input receives minimally chilled gaseous SUVA  95  refrigerant from the compressor  62  in the lower stage  12  after being partially desuperheated in finned tube exchanger  34 . After thermal energy exchange, the SUVA  95  is liquefied and passed to the entrance of subcooler coil  90  at the same temperature as the expanded R-507 that is returned to the upper stage module  10 . The SXV  107  (or in the alternate control system shown in  FIG. 3  the SXV  201  and TXV  202 ) under command input from the control system  20 , then adjusts the liquid/vapor flow in the SUVA  95  through the evaporator heat exchanger  114 , to provide enough cooling to set the temperature level to which the process fluid is to be brought in the second heat exchanger/evaporator  114 . 
   The system can be considered both a chiller and heater with a controlled output that can cool or heat a flow of pumped liquid so as to control the temperature of that liquid. Heat is supplied by an electrical heater  162  as needed to raise the temperature of the pumped liquid. 
   Energy efficiency is enhanced by using air flow from the fan  32  in the upper module  10  to convectively cool the finned conduits  34  in the adjacent lower stage module  12 . This type of interchange eliminates two fluid/gas connections between the modules that would be needed if gaseous SUVA  95  from the output of compressor  62  were to be cooled of its superheat in the upper stage module  10 . 
   When operating in the temperature range above 20° C., the refrigeration capacity of the lower stage compressor  62  is called upon only to a limited extent. In the event that the return suction pressure as the lower stage compressor  62  is too low for proper compressor operation, the hot gas bypass valve  70  opens to supply more gaseous refrigerant into the suction line, preventing damage to the associated compressor  62 . As the output of valve  70  is warmer than the input of compressor  62  can effectively accept, the desuperheater valve  104  provides enough expanded SUVA  95  to maintain the input to compressor  62  at acceptable levels. In the variation of  FIG. 3 , sensor bulb  204  is used to sense temperature input to the compressor and supply adequate liquid refrigerant to maintain correct temperature. 
   The reservoir  140  and the principal functioning elements of the process fluid supply and return system are contained within the lower stage module  12 , which also is designed to be sufficiently compact to fit within a standard width module is 10″×24″×35″. The thermal transfer fluid, here Galden HT-70, is fed from the reservoir  140  by the pump  160  and through the second heat exchange/evaporator  114  to be lowered to the temperature needed for maintaining the tool  14  at its then-desired temperature. The supply line  16  and return line  18  outside the lower stage module  12  can be, within limits imposed by flow impedance, an arbitrary length. External connections of these lines  16 ,  18  can be made at input and output manifolds (not shown in  FIG. 1  or  2 ) in the lower stage module  12 . After being circulated through the tool  14 , the thermal transfer fluid is transported on the return line  18  to be injected via the feeder cone  142  into the reservoir  140 . 
   In the lower level cooling range, for refrigeration to −80° C., the refrigeration capacity of the lower stage compressor  62  is utilized, up to a maximum. The upper stage module  10  continues to function as previously described to provide the regulated liquid-vapor mix of R507 to the lower stage module  12 . Compressed SUVA  95  refrigerant is first desuperheated by air cooling in the finned conduit  34  segment in the line adjacent the first module  10  and then fully condensed in the interchange heat exchanger/evaporator  84 . The SUVA  95  liquid/vapor input mixture, as modulated by the expansion valves  107 , or  201 ,  202 , is applied to the second heat exchanger/evaporator  114  along with the oppositely flowing “Galden HT-70”. Cascading in this fashion employs the individual properties of the two different refrigerants to best advantage, and without anomalies or dead zones anywhere in the range of controllable temperatures. When heating the thermal transfer fluid to or above ambient temperature.-both the upper stage module  10  and the lower stage module  12  continuously operate but with minimal chilling. Heating of a process tool is most often utilized, as in semiconductor cluster tools, to restore temperature after a period of operation in a refrigeration cycle. It can, however, also be utilized to maintain the thermal transfer fluid and the process tool  14  at an elevated temperature for a period of time for a specific tool function. The level of heating achievable, and the rage of heating, are dependent upon the wattage rating of the heater  162  which can be arbitrarily selected. Typically, the heater  162  is an electrical resistance device of approximately 1000–1500 watts capacity. 
   The system includes a substantial number of sensing and command elements which operate in conjunction with the control system  20  of  FIG. 1  to provide the desired control of tool  14  temperature. The pump  160  provides a given flow rate of thermal transfer fluid, although the rate can be varied if desired by using a variable speed driver. The tool  14  itself conventionally has its own control system which specifies the fluid temperature that is needed to maintain the tool  14  at a chosen level given a known flow rate for the thermal transfer fluid. Thus it is only required to assure that the supply line  16  or the tool  14  be at a given temperature, which may be sensed by a conventional transducer or transducers and supplied to the control system. 
   In response to the operative setting that is chosen, the control system  20  determines the refrigerant temperature levels that are to be established within the lower stage, and/or the heat to be added. The load on the lower stage will influence the temperature of the upper stage by means of the action of TXV  48  under the influence of sensor bulb  49 . Consequently, the input from the controller  20  is to the SXV  107   FIG. 2B  (or  201  and TXV  202  of  FIG. 3 ) in the lower stage  12 , or to the heater  162  to introduce a desired thermal transfer fluid increase in temperature. The heater  162  may also be used for the only control at above ambient temperature if no cooling is required of the system or even for vernier adjustments of temperature when the cooling system has slightly over-cooled the thermal transfer fluid. 
   Other sensed parameters are input to the controller  20  from the pressure transducer  124  in the supply line to the tool  14 , and the flow meter  136  in the return line  18 . These signals are used to indicate that the thermal transfer fluid is flowing without obstruction or leakage. For reliability, also, the level sensor  146  and the pressure transducer  148  at the reservoir  140  for thermal transfer fluid generate signals that warn of present or incipient problems. 
   Other operative features that are employed in the system are of practical importance to system life and reliability. Because SUVA  95  has characteristics that are optimized for lowest temperature operation it has a low boiling point and is above its critical temperature at ambient temperature. Its pressure can therefore build to a relatively high level when average system temperatures rise. In order to prevent catastrophic failure in the event of overpressure, gas in the suction line to the lower stage compressor  62  ( FIG. 2B ) is shunted through a small orifice  78  into the excess volume cylinder  80  which is of adequate strength to withstand high pressure and this path can also counterflow SUVA  95  gas to the compressor  62  if the input pressure drops. The burst disk  102  set to be actuated at 500 psi provides further assurance that internal damage will not occur. 
   The fluid characteristics of SUVA  95  are such that compressor  62  operation requires oil in the refrigerant, although the presence of substantial amounts of oil in the heat exchangers at very low temperatures is not desirable. Accordingly, the oil separator  66  extracts oil almost immediately from the pressurized compressor  62  output and returns the oil to the suction input manifold  64  to the compressor  62 . 
   As seen in  FIGS. 2B and 2C , the lower stage module  12  includes a shunt line between the supply line  16  and the return line  18 , this shunt line  120  incorporating an adjustable pressure relief valve  122  which may correspond to the configuration described in the K. W. Cowans application entitled “Systems and Methods for Temperature Control”, Ser. No. 10/079,542 filed Feb. 22, 2002. In the event of a pressure imbalance, the pumped fluid is lowered in pressure in accordance with the adjustable setting of the relief valve  122 , which couples into the input cone  142  in the reservoir  140 . 
   Different views of parts of a practical exemplification of the system of  FIGS. 1 and 2  are shown in  FIGS. 4 ,  5  and  6  which depict, in different perspectives two side-by-side modules with housings containing the upper stage  10  and lower stage  12 , and illustrating the air-cooled condenser version. In some process tool installations, water as a cooling medium must be avoided. Thus the air-cooled condenser with a fan  32  mounted on a transverse rotational axis, as seen in  FIGS. 3 and 5 , provides air flow across conventional internal refrigerant flow conduits (not seen in  FIGS. 4 ,  5  and  6 ) toward an outlet screen extending across the module width. This fan  32  is also deployed to direct air centrifugally outward and laterally toward the lower stage module  12  through air slots in the housing well. Inasmuch as the internal configuration of the upper stage module  10  can be in accordance with the teaching of K. W. Cowans patent application Ser. No. 10/079,542, referred to above, these details are not described herein. However, the slots in the sidewall of the upper module  10  that faces the lower stage module  12  provide a flow cooling air transversely between the two modules  10 ,  12  and over the finned conduits  34  for the SUVA  95  lines from the lower stage compressor  62  that can be seen adjacent these orifices. 
     FIGS. 4 ,  5  and  6  also demonstrate that there are only two direct refrigerant couplings between the sidewall of the upper stage module  10  and the facing side of the lower stage module  12 . Furthermore, the modules  10 ,  12  are also sufficiently compact, with this design, to meet the standard form factor. The compressor  62 , reservoir  140 , excess volume reservoir  80  and pump  60  are the largest volumetric elements within the lower stage module  12 . Manifolds or accumulators for coupling thermal transfer fluid to and from the supply and return lines  14 ,  16  are disposed adjacent one end of the structure, and the electrical heater  162  is disposed adjacent the base of the unit and in communication with the output manifold. 
   Another advantage of this approach is that the modules can also function separately, if desired, although modifications would be employed for thermal energy interchange with the thermal transfer fluid and tool in each case. 
   Another advantage of the modular configuration described is that the two modules can be mounted in a vertical assembly with the high temperature module  10  mounted above the lower stage module  12 . This is desirable in some installations wherein a smaller footprint may be needed and height is acceptable. 
   Although a number of forms and variations have been described it will be appreciated by those skilled in the art that the invention is not limited thereto but encompasses all alternatives and expedites within the scope of the appended claims.