Abstract:
In a thermal control system of the type employing a two phase refrigerant that is first compressed and then is divided into a variable mass flow of refrigerant into a hot pressurized gas form and a differential remainder flow of cooled vapor derived from condensation and then thermal expansion, transitions between different temperature levels are enhanced by incremental variations of the mass flow at different control rates.

Description:
REFERENCE TO PRIOR FIELD APPLICATIONS  
       [0001]    This application relies for priority on previously filed provisional application 61/070,978 filed Mar. 25, 2008 by Kenneth W. Cowans et al and entitled “Thermal Control System with Advanced Temperature Capabilities”. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    U.S. Pat. No. 7,178,353, issued Feb. 20, 2007 and entitled “Thermal Control System and Method”, inventor Kenneth W. Cowans et al and assigned to Advanced Thermal Sciences Corporation, teaches a novel and widely applicable concept for precise and changeable temperature control of a thermal load. Among its departures from other known systems, the system circulates a two-phase refrigerant in direct thermal transfer relation to the load that is being controlled. To do this at different temperatures, it uses a controllable mix of pressurized refrigerant gas at high temperature together with a flow of the same refrigerant, after it has been condensed, then cooled by controlled expansion to provide a flow that is at least partially vapor. The mix then provides a refrigerant flow of predetermined pressure and temperature so that thermal exchange can be effected directly with the load, at a target temperature that can be adjusted up or down. This thermal control is directly effected with refrigerant alone and is therefore more efficient and responsive than most temperature control units, since both pressure and temperature can be controlled with facility, and no intermediate temperature stable media is required. 
         [0003]    Consequently, this thermal control technique, which has been descriptively called Transfer Direct of Saturated Fluid (TDSF) is of immediate benefit in a number of demanding applications and also of potentially general capability for a wide variety of temperature control systems. It is of particular promise for applications which require precision control of thermal loads at different temperature levels, along with capability for rapidly varying the temperature levels. 
         [0004]    When rapidly shifting between selected temperature levels, however, instabilities and offsets can be encountered since no significant time delays or averaging effects exist in the temperature loop. In systems using TDSF technology, the flow of hot gas controlled by a proportional valve is to be mixed with liquid refrigerant, partially expanded for cooling. While the proportional valve setting can be changed rapidly, imprecision and instability may be encountered because of delays in flow rate variations and system demands. The response times and amplitudes of changes have to be considered in system terms, which factors can be accounted for in accordance with the present invention. 
         [0005]    The above referenced patent to Cowans et al, U.S. Pat. No. 7,178,353, also discloses a number of advantageous features within the system, which enhance the ability to separately control pressurized hot gas in one flow path and cold expanding refrigerant in another, before mixing. The patent consequently also discloses a number of techniques for interrupting or modifying flows to increase or decrease temperature particularly rapidly under specified conditions. However, there is often a need for assuring that temperature changes take place at controlled transitional rates that limit overshoot or otherwise provide assurance that a new target has been reached at the thermal load. 
       SUMMARY OF THE INVENTION  
       [0006]    A TDSF system in accordance with the invention generally incorporates, as previously disclosed, separate flow paths for high temperature two-phase refrigerant and condensed, pressurized, partially expanded refrigerant at a lower temperature. Flow remainder in the high temperature path is controlled with a proportional valve and the temperature of the flow in the second path is controlled with a thermal expansion valve. A refrigerant mix of chosen pressure and temperature is thus provided for temperature control of a thermal load, the cycle being completed by recirculation of the two-phase refrigerant to the compressor. In accordance with the present invention, however, the rate of change of the hot gas flow as well as the final setting, are selectively varied by using access to stored control algorithms. This means that shifts of varying amounts from one flow rate to another can be effected stably, with due regard to system needs for response times varying the rate of change of the hot gas constituent. In one example, the proportioning valve is driven at a selectively variable frequency by a stepper motor that is responsive to the control algorithms. In a second example the control signals are varied in analog form, and an algorithm chosen signal amplitude controls the rate of change. 
         [0007]    A further feature of this invention is the introduction of a flow control circuit including a fast acting control valve between the hot gas line subsequent to the proportioning valve and a return line to the compressor input, but before processor elements in that input line. When it is desired to cool the load virtually immediately, a valve in this bypass line, which may be a solenoid expansion valve, is opened to shunt all the hot gas flow back to the input of the compressor, so that only the cooling flow is applied to the load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]    A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a block diagram of a TDSF system with flow rate control in accordance with the invention that uses a digital control scheme; 
           [0010]      FIG. 2  is a block diagram of a part of a TDSF system that presents an alternative control scheme for use in the system of  FIG. 1 , and 
           [0011]      FIG. 3  comprises timing diagrams labeled A, B and C showing variations in response rates in systems in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring now to  FIG. 1 , a TDSF system  10  includes, as described in the above-identified Cowans et al &#39;353 patent, a refrigeration loop for a two-phase refrigerant which loop includes a compressor  12  feeding a first part of its pressurized output to a first high pressure gas path  32  and a second remaining part of its output  26  to a condenser  14 . The condenser  14  is cooled with a flow of ambient temperature water from a facility  18 . The water for cooling is fed to a heat exchanger  16  disposed in thermal contact relation to the condenser  14 , the flow is further controllable by a control valve  20 . Other fluid systems, or gas, may be used for cooling the condenser  14  to ambient temperature. The output from the condenser  14  is directed as one input to mixing circuits  22  that include a thermal expansion valve  28 , hereafter TXV, for receiving and modulating the second flow. The output of the TXV  28  within the mixing circuits  22  is propagated through a pressure dropping (ΔP) valve  30  for reasons previously expanded in the Cowans patent which need not be repeated here. The first flow path  32  from the output of the compressor  12  is directed first to a shut off valve  34 , which feeds a separate input to the mixing circuits  22 . However the first flow is modulated at a variable rate by a valve  42  whose setting in this example is controlled by a stepper control circuit  44  commanded by a system controller  40 . The valve  42  is of the type known as a proportioning valve and provides a variable flow of pressurized hot gas to the mixing circuits  22 . To change its setting, the controller  40  provides commands to the stepper control circuits  44  that generate a sequence of pulses supplied at a predetermined rate by a variable frequency control  48  to drive the proportioning valve  42  open or closed. In the controller system  40  stored programs  46  contain suitable control algorithms supplying any of a variety of integrating and/or differential functions, as described in the Antoniou and Christofferson patent entitled “Systems and Methods for Controlling Temperatures of Process Tools”, U.S. Pat. No. 6,783,080. The chosen algorithm determines the rate at which the variable frequency control  48  feeds pulses to operate the proportioning valve  42 . This actuation varies the response rate of the valve  42 , consequently the mass of hot gas that is supplied in response. 
         [0013]    The flows in the first flow path  26  and second flow path  32 , after modulation are subsequently combined in a mixing tee  50  within the mixing circuits  22 , after the hot gas flow has been passed through a check valve  52 . The output from the mixing circuits  22  is then applied to the load  54 , and its output is returned, via other circuits to the input to the compressor  12 . 
         [0014]    The TXV  28  is a well known device and is externally equalized by pressure communicated from a bulb  56  in operative relation (thermal interchange) with the return line  57  from the load  54 . The bulb  56  generates a pressure level in the gas it contains that is applied via a coupling line  58  to the TXV  28 , for equalization of the TXV setting to the load  54  output. The return line  57  from the load  54  passes serially through a Close-on-Rise (COR) regulator valve  70 , toward the compressor  12  input. Before that input, however, it branches off at a shunt line  76  including a desuperheater valve (DSV)  72  of conventional purpose, that is externally equalized by pressure in a conduit  74 ′ from a bulb  74  responsive the input temperature to the compressor  12 . The shunt line  76  that includes the DSV  72  couples from the output of the condenser  14  to the return line  57  that leads to the compressor  12 . A separate shunt line  77  couples the output of the compressor  12  back to the compressor input line  57 , to a hot gas bypass valve (HGBV)  78  which responds to temperature levels at the compressor  12  input as detected by a temperature sensor  79  in that region of the shunt line  76 . A temperature sensor  79  input is provided to the controller  40 , which also provides a control output to a heater  82  in the compressor input line  57 , the heater  82  serving to insure that the compressor  12  receives gaseous input only. 
         [0015]    For purposes of rapid cooling, when operating independently of typical load temperature changes, the system  10  also includes a bypass line  60  starting at between the hot gas flow path after the proportioning valve  42  and extending to the return line  57  (the input to the compressor  12 ), the junction being made at a point prior to the COR regulator valve  70 . This bypass line  60  includes a solenoid expansion valve (SXV)  62 , followed by an orifice  63  so that when the SXV  62  is abruptly closed no hot gas is supplied to the mixing circuit  22 , the SXV  62  is controlled by the stored program circuits  64  responsive to the controller  40 . The hot gas flow path  32  can also be closed by the shut off valve  34  before the proportioning valve  42 . 
         [0016]    Inasmuch as the general operation of the TDSF system is adequately described in U.S. Pat. No. 7,178,353, those portions which are not essential to the inventive features herein will only be briefly described. The flow of pressurized hot gas from the compressor  12  is fed into the hot gas pressurized flow line via the first flow path  32 . The proportioning valve  42  is operated by the controller  40 , usually in relation to any cooled expanded flow in the second flow path  26  so as to provide, from the mixing circuit  22 , a predetermined output to the load  34  for the temperature and pressure conditions specified by the controller  40 . Consequently, in the mixing circuit  22 , the second flow in the second path  26  has been controllably expanded by the TXV  28  and applied to the separate input to the mixing tee  50  after passing the ΔP valve  30 . Consequently, a combined flow at a predetermined pressure and temperature is available at the input to the load  54 . Because the concept facilitates rapid pressure and temperature changes, and because the two-phase refrigerant is used directly in thermal exchange with the load  54 , the system has unique operative capabilities and cost advantages. 
         [0017]    Further uniqueness is now provided via the controller  40  in relation to the operation of the proportioning valve  42 , and also the bypass line  60 , in relation to the operation of the SXV  62 . The controller  40  includes what may be called a variable frequency control board  48  that with stored PLC algorithms to operate the stepper circuits  44  for control of the degree of opening of the proportioning valve  42 . The hard wired stored programs supply the controller  40  with instructions for commanding the stepper  44  to move the proportioning valve  42  open or closed at a selected rate to a desired final position. 
         [0018]    Consequently, when a change in the setting of the proportioning valve  42  is indicated, as a new temperature level is chosen for the system  10 , the controller  40  accesses the stored PLC algorithms in the storage  46  indicate the rate of change as well as the limit position to be reached. The necessary number of stepper increments are supplied at a chosen rate, and the stepper control circuits impulse the proportioning valve  42  accordingly. This consequently adapts the proportion and the rate of change of the hot gas flow to assure that the new setting is both precise and achieved with stability. 
         [0019]    The advantages of this approach can perhaps better be appreciated by referring to the operative diagram of  FIG. 3 , illustrating in curve (A) sharp transition commands, as when fully off to fully on. The dotted line curve shows the resulting flow changes, with delay in response on opening and overshoot on reaching target flow. This may be followed by oscillations about the target level. In waveform (B), illustrating by a solid line a sudden nominal change from full open to fully closed, a reciprocal instability condition occurs for a period of time as the valve is fully closed, as seen in the dotted line waveform which depicts typical actual flow conditions in response to sudden change. In contrast, in waveform (C) the incrementally changing slope of the valve change in opening (solid line) is very closely followed by the flow change (dotted line) and there is no overshoot. With the modulated stepper motor approach the angle of the slope can be varied arbitrarily. 
         [0020]    There are some operating conditions in which it is desired or necessary to transition to a cooler temperature as quickly as possible, by passing the rate control. For this purpose, the SXV  62  in the bypass line  60  is driven by the PLC algorithm in the stored programs  61  to close virtually instantaneously, enabling the expanded coolant in the first flow line  26  to be operatively effective without delay. This line, which includes an orifice  63 , coupled to the return line  57  which goes into the compressor  12  input, at a point prior to the COR regulator valve  70 . Consequently, this feature provides a rapid response characteristic that supplements those already mentioned in the aforementioned Cowans et al patent. 
         [0021]    In some systems it may be desired or necessary to use an analog system for changing the opening of the proportioning valve  42 , and  FIG. 2 , to which reference is made, shows only the signal generating and motor driving parts of such a system, the remainder of the system of  FIG. 1  being applicable and therefore not shown. Here the controller  40 ′ provides a variable amplitude signal indicating a new target position for the proportioning valve  42 , and selects one of a number of timing circuits  90  to supply a drive signal of the needed slope to actuate the analog drive circuit  92  which moves the proportioning valve  42 . Again, a controlled rate of transition between the prior and new flow set points is achieved. 
         [0022]    Although various forms and alternatives have been shown or described, utilizing the teachings of the invention, it should be appreciated that the invention is not limited thereto but encompasses all expedients and variations within the scope of the appended claims.