Patent Publication Number: US-2022221197-A1

Title: Very low temperature refrigeration system with fast operation cycle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/IB2020/053398, filed Apr. 9, 2020, and published as WO 2020/208573 A1 on Oct. 15, 2020, the content of which is hereby incorporated by reference in its entirety and which claims priority of U.S. Provisional Application No. 62/833,563, filed on Apr. 12, 2019. The entire teachings of the above application are incorporated herein by reference. 
    
    
     BACKGROUND 
     Very low temperature refrigeration systems are used for a variety of different purposes, including removing water vapor and creating high vacuum environments for the coating industry. Such systems typically operate in three different modes: standby mode, in which the unit is recovering or is in readiness; cool mode, in which the system is cooling to serve the process or application need; and defrost mode, in which the system is regenerating the cryocoil. For a typical batch coating process, the system runs in cycles of these three modes. However, different applications have different constraints on how much time can be spent in each mode. Some applications require short time in the defrost mode, and can tolerate a long time in the standby mode after the defrost mode completes. Some applications require a fast cool down to a target supply temperature (for example, −110° C. or −120° C.), so that the coating process can start earlier. Some applications require a fast total cycle of defrost/standby/cool, which can put demands on shortening all three operating modes. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     Methods for shortening the cycle time in each of the defrost, standby and cool modes of operation of a very low temperature refrigeration system are taught herein. These methods can be used alone or in combination with one or more of each of the other techniques, including, for example, in a single very low temperature refrigeration system, to provide a fast total cycle of one, two or all three of the defrost, standby and cool modes. 
     A method of limiting peak operating pressure during startup of a very low temperature refrigeration system having a compressor, a plurality of heat exchangers, an expander and an evaporator comprises, during startup of the compressor, opening a defrost valve in a hot gas defrost circuit to bypass flow of a refrigerant around a high pressure side of the plurality of heat exchangers and the expander and to an evaporator inlet from which the refrigerant flows to the evaporator, thereby accommodating a volume of the refrigerant in the evaporator, to limit an initial increase in pressure of the refrigerant during the startup of the compressor. Subsequently the defrost valve is closed so that flow of the refrigerant proceeds through the high pressure side of the plurality of heat exchangers and the expander to the evaporator. 
     The refrigerant may be charged into the system at a refrigerant pressure that would create a peak pressure of the refrigerant that would exceed a design pressure of the very low temperature refrigeration system during startup of the compressor absent the opening of the defrost valve. The refrigerant flow volume of the evaporator may be greater than about 10 percent of a refrigerant system volume of the very low temperature refrigeration system. The refrigerant may comprise a mixture of a plurality of different refrigerant components. The mixture may comprise argon, R-14, R-23, R-125 and R-245fa. The refrigerant may be charged into the system such that the system has a balance pressure of between about 230 psig and about 300 psig. Subsequently closing the defrost valve may be performed after at least about 3 seconds from the startup of the compressor; and may be performed before at least about 6 seconds from the startup of the compressor. The method may comprise entering a standby mode of the system upon the subsequently closing the defrost valve, the standby mode comprising closing a cool valve to prevent flow of refrigerant from the high pressure side of the plurality of heat exchangers to the evaporator, while permitting flow of refrigerant through the high pressure side of the plurality of heat exchangers and a low pressure side of the plurality of heat exchangers. During the bypass flow of the refrigerant, a temperature of the refrigerant may be greater than about 25° C. The hot gas defrost circuit may bypass the refrigerant from a high pressure supply line of the compressor to the evaporator inlet from which the refrigerant flows to an evaporator. 
     A method of reducing time spent in a defrost mode of operation of a very low temperature refrigeration system comprises, in a defrost mode of operation of the system, (i) opening a defrost valve in a hot gas defrost circuit to bypass flow of a refrigerant around a high pressure side of a plurality of heat exchangers and to an evaporator inlet from which the refrigerant flows to an evaporator, to effect warming of the evaporator, and (ii) while opening the defrost valve, closing a cool valve so that the refrigerant does not flow from the high pressure side to the evaporator. Based on an input control signal, a value of a stored defrost completion set point temperature of a return temperature sensor on a low pressure side of the evaporator is set. During the warming of the evaporator, upon the return temperature sensor on the low pressure side of the evaporator reaching the stored defrost completion set point temperature of the return temperature sensor, the defrost valve is closed to prevent the refrigerant flowing to the evaporator. 
     The stored defrost completion set point temperature of the return temperature sensor may be about 0° C. or lower, depending on the application. The return temperature sensor may comprise a thermocouple on the low pressure side of the evaporator. The method may comprise closing the defrost valve when a controller receives a temperature control signal from the return temperature sensor that is at least as warm as the stored defrost completion set point temperature of the return temperature sensor, the stored defrost completion set point temperature being stored in a memory of the controller. 
     A method of reducing recovery time after defrost of a very low temperature refrigeration system comprises, based on an input control signal, setting a value of a stored bypass control set point temperature of a return temperature sensor on a low pressure side of the evaporator. Upon a return temperature sensor on a low pressure side of the evaporator warming to be at or above the stored bypass control set point temperature of the return temperature sensor, the method comprises (i) closing a return valve to prevent refrigerant flow through a low pressure side of a plurality of heat exchangers, and (ii) opening a bypass valve to bypass flow of the refrigerant around the low pressure side of the plurality of heat exchangers and to a suction line that enters a low pressure side of a compressor; and warming the bypassed flow of the refrigerant in the suction line before it enters the low pressure side of a compressor. 
     Warming the bypassed flow of the refrigerant in the suction line may comprise warming the bypassed flow using a heat exchanger that exchanges heat between the suction line and a high pressure side of the plurality of heat exchangers. Warming the bypassed flow of the refrigerant in the suction line may comprise using a heater to heat the suction line. The stored bypass control set point temperature of the return temperature sensor may be less than the compressor&#39;s rated input flow temperature, for example, less than about −40° C. For example, the stored bypass control set point temperature of the return temperature sensor may be between about −40° C. and about −70° C. 
     A method of reducing a cool down time of a very low temperature refrigeration system comprises, during a cooling mode of operation of the system, flowing refrigerant through a high pressure side of a plurality of heat exchangers, through a flow metering device and a cool valve with which the flow metering device is in a series flow connection to an inlet of an evaporator, through the evaporator and through a low pressure side of the plurality of heat exchangers. Upon a discharge pressure of a compressor of the system being at least as high as a stored set point of a maximum discharge pressure during the cooling mode of operation, an unload valve is opened that permits refrigerant flow to bypass around the flow metering device and to the cool valve, until the discharge pressure reduces to be less than the stored set point of the maximum discharge pressure. 
     An inlet of the evaporator or an outlet of the evaporator may be at a temperature of less than about −110° C. The stored set point of the maximum discharge pressure may be less than an activation pressure of a buffer solenoid valve of the system. 
     A very low temperature refrigeration system comprises a compressor, a plurality of heat exchangers, an expander, and a controller comprising a processor and a memory. The controller is configured to, (i) during startup of the compressor, control a defrost valve in a hot gas defrost circuit to open to bypass flow of a refrigerant around a high pressure side of the plurality of heat exchangers and the expander and to an evaporator inlet from which the refrigerant flows to an evaporator, thereby accommodating a volume of the refrigerant in the evaporator to limit an initial increase in pressure of the refrigerant during the startup of the compressor; and (ii) to subsequently control the defrost valve to close so that flow of the refrigerant proceeds through the high pressure side of the plurality of heat exchangers and the expander to the evaporator. 
     Another very low temperature refrigeration system comprises a compressor, a plurality of heat exchangers, an expander; and a controller comprising a processor and a memory. The controller is configured to, in a defrost mode of operation of the system, (i) control a defrost valve in a hot gas defrost circuit to open to bypass flow of a refrigerant around a high pressure side of the plurality of heat exchangers and to an evaporator inlet from which the refrigerant flows to an evaporator, to effect warming of the evaporator, and (ii) while controlling the defrost valve to open, control a cool valve to close so that the refrigerant does not flow from the high pressure side to the evaporator. The controller is further configured to, (i) based on an input control signal, set a value of a stored defrost completion set point temperature of a return temperature sensor on a low pressure side of the evaporator; and (ii) during the warming of the evaporator, upon the return temperature sensor on the low pressure side of the evaporator reaching the stored defrost completion set point temperature of the return temperature sensor, control the defrost valve to close to prevent the refrigerant flowing to the evaporator. 
     Another very low temperature system comprises a compressor, a plurality of heat exchangers, an expander, and a controller comprising a processor and a memory. The controller is configured to, based on an input control signal, set a value of a stored bypass control set point temperature of a return temperature sensor on a low pressure side of an evaporator. The controller is further configured to, upon a return temperature sensor on a low pressure side of the evaporator warming to be at or above the stored bypass control set point temperature of the return temperature sensor, (i) control a return valve to close to prevent refrigerant flow through a low pressure side of the plurality of heat exchangers, and (ii) control a bypass valve to open to bypass flow of the refrigerant around the low pressure side of the plurality of heat exchangers and to a suction line that enters a low pressure side of the compressor. The system is configured to warm the bypassed flow of the refrigerant in the suction line before it enters the low pressure side of the compressor. 
     The system may comprise a heat exchanger that exchanges heat between the suction line and a high pressure side of the plurality of heat exchangers. The system may comprise a heater to heat the suction line. 
     Another very low temperature refrigeration system comprises a compressor, a plurality of heat exchangers, an expander, a flow metering device and a cool valve with which the flow metering device is in a series flow connection to an inlet of an evaporator, and a controller comprising a processor and a memory. The controller is configured to, upon a discharge pressure of a compressor of the system being at least as high as a stored set point of a maximum discharge pressure during the cooling mode of operation, control an unload valve to open to permits refrigerant flow to bypass around the flow metering device and to the cool valve, until the discharge pressure reduces to be less than the stored set point of the maximum discharge pressure. 
     Systems may be configured to implement any or all of the methods taught herein. 
     The summary is provided to introduce a selection of concepts in a simplified form that are further described in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments. 
         FIG. 1  is a schematic diagram of a very low temperature refrigeration system. 
         FIG. 2  is a graph showing defrost time for a system with increased flow through capillary tubes. 
         FIG. 3  is a graph of test data showing the reduction of defrost time with an increased refrigerant charge, using a technique of limiting the peak operating pressure during startup. 
         FIG. 4  is a graph showing improved recovery time of a very low temperature refrigeration system using different return valve set points. 
         FIG. 5  is a simplified schematic block diagram of a controller. 
     
    
    
     DETAILED DESCRIPTION 
     A description of example embodiments follows. 
     Methods for shortening the cycle time in each of the defrost, standby and cool modes of operation of a very low temperature refrigeration system are taught herein. As used herein, “very low temperature” means the temperature range from 90 K to 203 K. The methods can be used alone or in combination with one or more of each of the other techniques, including, for example, in a single very low temperature refrigeration system, to provide a fast total cycle of one, two or all three of the defrost, standby and cool modes. 
       FIG. 1  is a schematic diagram of a very low temperature refrigeration system. The system can, for example, be an auto-cascade refrigeration system  100 . Such systems use a mixture of two or more refrigerants in which the difference between the normal boiling points from the warmest boiling component to the coldest boiling component is at least 50 K or 100 K or 150 K or 200 K. Such systems can include a refrigeration compressor  101 , a condenser  102  or desuperheater heat exchanger for rejecting heat, a series of two or more heat exchangers  103  (also referred to herein as a “heat exchanger array” or “refrigeration process”), one or more expanders  104 , such as a throttle or flow metering device  104 , and an evaporator  105  for heat removal from an application process. In addition, such systems can include phase separators  106 ,  107  which are positioned on the discharge side between heat exchangers and remove liquid phase refrigerant for use in an internal recycle loop. Such systems can have the ability to operate in different operating modes, including cool mode in which the evaporator  105  is cooled, defrost mode in which hot gas from the compressor  101  is supplied to the evaporator  105 , and standby mode in which neither cold refrigerant nor hot refrigerant is delivered to the evaporator  105 . Flow through various flow loops within the system can be controlled via a series of capillary tubes  108 ,  109 ,  110  and  111  which restrict flow and allow for expansion and thus cooling of refrigerant, and/or via on/off solenoid valves, such as cool solenoid valve  112 , bypass solenoid valve  113 , return solenoid valve  114 , buffer solenoid valve  116 , defrost solenoid valve  123 , and unload solenoid valve  130 . In the embodiment shown in  FIG. 1 , capillary tubes  108 ,  109 ,  110  and  111  are not associated with any solenoid valves, while capillary tube  104  is in parallel with unload solenoid valve  130  and in series with cool solenoid valve  112 . Other arrangements of capillary tubes and solenoid valves can be used. The capillary tubes and/or the solenoid valves can be replaced with a proportional valve such as a thermal expansion valve, or a pressure actuated or stepper motor actuated valve. Such systems can also contain an expansion tank  115  which is used to manage high evaporation and expansion of the liquefied refrigerants once the system is turned off and warmed to room temperature. Further, such systems with expansion tanks  115  can also have a buffer solenoid valve  116  which allows high pressure gas to be directed to the expansion tank  115 . Such a buffer solenoid valve  116  allows the amount of refrigerant gas in circulation to be reduced, which in turn reduces compressor discharge and suction pressures. For example, any of the methods can be used that are disclosed in U.S. Pat. No. 6,574,978 B2 of Flynn et al., the entire disclosure of which is hereby incorporated herein by reference. Systems as described in this patent enable additional operating modes such as controlled cool down and warm up processes, and extended operation in a hot gas flow mode, or bakeout mode, in which a portion of the hot gas exiting the compressor is continuously circulated from the compressor to the evaporator coil and then back to the compressor, while another portion of the refrigerant exiting the compressor continuously flows through the condenser and then the heat exchanger array and then returns to the compressor. 
     The buffer solenoid valve  116  is a connection between the discharge (or high pressure) side of the unit and one or more expansion tanks  115 . When a high pressure condition exists the controller opens this buffer solenoid valve  116  and allows a portion of the refrigerant to be stored in the expansion tanks  115 , thereby reducing the discharge pressure. This can prevent an excessive discharge pressure fault condition. 
     A hot gas defrost system  121  of the very low temperature system can be used to achieve warming of the evaporator  105 . The hot gas defrost system  121  includes a defrost hand shut-off valve  122  and a defrost solenoid valve  123 , and directs hot gas from a high pressure supply line  192  of the compressor  101  to the evaporator inlet  124  of the evaporator feed line which sequentially flows through the feed line, the evaporator  105  (also known as cryocoil or cryosurface), the return line  125  and then through the low pressure side of the heat exchanger array  103 . There is an oil separator  138  in the high pressure supply line  192  downstream of compressor  101  for separating oil from the flow and returning it to the compressor  101 . 
     The possibility of freezeout of refrigerant that is discharged from the compressor, or another warmer point in the system, and that is being directed to a colder point in the system, can be addressed. Such refrigerant that is being discharged from the compressor may have a higher risk of freezeout because it has not yet passed through the phase separators in the system, and therefore has a different composition than later in the refrigeration process, and thus may have a warmer freezing point and be more likely to freezeout when directed to a colder point in the system. To prevent such freezeout, a freezeout prevention circuit or temperature control circuit can be used, which uses a controlled bypass flow to warm the lowest temperature refrigerant in the system, to warm the stack sufficiently that the refrigerant discharged from the compressor (or another warmer point) does not freezeout when redirected to a colder point in the system. For example, any of the freezeout prevention circuits or temperature control circuits can be used that are disclosed in U.S. Pat. No. 7,478,540 B2 of Flynn et al., the entire disclosure of which is hereby incorporated herein by reference. In the example in  FIG. 1 , a freezeout prevention valve  131  directs refrigerant that is exiting phase separator  107  to the low pressure inlet  117  of the subcooler  118 , which is positioned closer to the evaporator than the next-coldest heat exchanger  119  in the heat exchanger array  103 . 
     The refrigeration system can include a series of internal return paths  108 ,  109 ,  110  from the high pressure side of the system to the low pressure side in addition to the return path via the evaporator  105 . Typically, the internal return paths  108 ,  109 ,  110  are throttle devices. Example throttle devices are capillary tubes and thermal expansion valves. In other scenarios, turbo expanders or other means to reduce the pressure of the refrigerant are used. In a typical defrost warming process the internal throttle devices  108 ,  109 ,  110  are allowed to have flow. In other scenarios their flow rate is stopped or controlled. In one example, capillary tubes can be used for the internal throttle devices  108 ,  109 ,  110  with no upstream valves. As a result, these throttle devices continue to permit flow during the defrost warming process. 
     In addition, in the embodiment of  FIG. 1 , there is also a low pressure side bypass circuit, which includes a bypass solenoid valve  113 . This bypasses refrigerant around the low pressure side of the heat exchanger array  103  when the temperature of refrigerant returning from the evaporator  105  is at or above a stored temperature that can be set based on an input control signal. Such a temperature of returning refrigerant can be measured at the location indicated by Tc in  FIG. 1 , at the low pressure side of the evaporator  105 , and can be detected by a temperature sensor, such as, for example, a thermocouple in that location. A sensed temperature signal from the thermocouple can, for example, be provided to a controller, which can compare the sensed temperature signal with the stored temperature based on the input control signal. When the sensed return temperature is at or above the stored temperature based on the input control signal, a return solenoid valve  114  can be shut off by the controller, while the bypass solenoid valve  113  is opened. This permits refrigerant to be bypassed around the low pressure side of the array of heat exchangers  103 , in order to prevent overloading the heat exchanger array  103  with returning refrigerant that is too warm. 
     The system of the embodiment of  FIG. 1  also includes a suction line heat exchanger  132 , the operation of which will be described further, below. It is connected with its high pressure side in series fluid connection with the outlet  120  of the condenser  102 , and with its low pressure side in series fluid connection with the low pressure side of the heat exchanger array  103 . 
     The system of  FIG. 1  also includes a control module or controller  180 , which is described further, below, with reference to  FIG. 5 . 
     In one embodiment, a defrost mode of operation of the system  100  is made faster by increasing the flow through capillary tubes in the system, such as  108 ,  109 ,  110  and  104 , by using larger diameter and/or shorter length capillary tubes than would otherwise be used, and keeping approximately the same flow ratio for any two of the capillary tubes. For example, an existing set of capillary tubes can be duplicated and connected in parallel. This decreases the system&#39;s flow resistance, and increases the maximum suction pressure from a range of about 40-50 psig to a range of about 50-70 psig during the defrost mode of operation.  FIG. 2  is a graph showing defrost time for a system with increase flow through capillary tubes, here, by having duplicated capillary tubes. Defrost time was reduced by more than 15%, although with the tradeoff of reducing cooling capacity somewhat. In an alternative embodiment, capillary tubes such as  108 ,  109 ,  110  and  104  can be adjustable flow metering devices (such as proportional valves or stepper motor expansion valves) instead of capillary tubes, and flow increase can be achieved by adjusting the valve opening. 
     In another embodiment, a method is provided of limiting the peak operating pressure during startup of the very low temperature system. One method of increasing the speed of defrost of the system is to increase the quantity of refrigerant charged to the system. However, with more charge mass, the system will have a higher balance pressure (for example, between about 230 and about 300 psig), and the compressor may have difficulty starting because the peak pressure will exceed the design pressure limit, which results in the system shutting off automatically. In order to avoid this, a method is used that limits the peak operating pressure during startup. In this method, the system is started in defrost mode, so that the defrost refrigerant line and evaporator  105  can be used as an additional volume to expand the gas and reduce the peak pressure during startup. This peak pressure during startup typically lasts between about 3 seconds and about 5 seconds. Once the system overcomes the peak pressure, the system can switch back to the standby mode of operation.  FIG. 3  is a graph of test data showing the reduction of defrost time with an increased refrigerant charge, using such a technique of limiting the peak operating pressure during startup. Here, the defrost time was reduced by more than 20%. When the increased refrigerant charge is used, a freezeout prevention circuit (as described above) can be used to prevent freezeout. 
     As used herein the “balance pressure” means a pressure achieved when the high pressure and low pressure of the system are equal, or approximately equal, for example when the stack is warmed such that the average heat exchanger array temperature is at least as warm as a temperature from the group consisting of −5° C., 0° C., 5° C., 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., 40° C.; or for example when the heat exchanger array is warmed such that the range of temperatures in the stack is from at least −5° C. up to 40° C., or is a smaller range within the range of −5° C. to 40° C. 
     With reference to  FIG. 1 , by way of illustration, in one embodiment, a method of limiting peak operating pressure during startup of a very low temperature refrigeration system  100  comprises, during startup of a compressor  101 , opening a defrost valve  123  in a hot gas defrost circuit  121  to bypass flow of a refrigerant around a refrigerant circuit of a high pressure side of a plurality of heat exchangers  103  and to an evaporator inlet  124  from which the refrigerant flows to an evaporator  105 , thereby accommodating a volume of the refrigerant in the evaporator  105 , to limit an initial increase in pressure of the refrigerant during the startup of the compressor  101 . Subsequently the defrost valve  123  is closed so that flow of the refrigerant proceeds through the refrigerant circuit of the high pressure side of the plurality of heat exchangers  103 . The refrigerant can be charged into the system  100  at a refrigerant pressure that would cause peak pressure to exceed a design pressure of the very low temperature refrigeration system during startup of the compressor absent the opening of the defrost valve; for example, the refrigerant can be charged to reach a pressure of about 230 to about 300 psig, which would normally create peak pressures higher than the system&#39;s design pressure during startup. The refrigerant flow volume of the evaporator  105  can, for example, be greater than about 10 percent of a refrigerant system volume of the entire very low temperature refrigeration system  100 , or even greater than about 15 percent. The volume increase will depend on the size of the evaporator  105  in the system. The refrigerant can be a mixture of a plurality of different refrigerant components, and can, for example, consist of argon, R-14, R-23, R-125 and R-245fa. 
     The refrigerant can be charged into the system such that the system has a balance pressure of between about 230 psig and about 300 psig. Subsequently closing the defrost valve  123  can be performed after at least about 3 seconds from the startup of the compressor  101 ; and can be performed before at least about 6 seconds from the startup of the compressor. The method can comprise entering a standby mode of the system upon the subsequently closing the defrost valve  123 . The standby mode comprises closing the cool valve  112  that permits flow of refrigerant from the refrigerant circuit of the high pressure side of the plurality of heat exchangers  103  to the evaporator  105 , while permitting flow of refrigerant through the high pressure side of the plurality of heat exchangers and a low pressure side of the plurality of heat exchangers. During the bypass flow of the refrigerant, a temperature of the refrigerant can, for example, be greater than about 25° C. The hot gas defrost circuit can, for example, bypass the refrigerant from a high pressure supply line  192  of the compressor  101  to the evaporator inlet  124  from which the refrigerant flows to the evaporator  105 . 
     A defrost completion set point can be set to 0° C. or lower, which differs from a conventional fixed setting of 20° C. This provides not only a fast defrost, but also a fast recovery, because less heat is absorbed in the refrigerant supply line and the application cryocoil. In applications where the chamber containing the cryocoil is evacuated during defrost, the chamber pressure can stay below the triple point of water. If so, the ice on the cryocoil may sublime to vapor directly at a lower temperature, hence negating the need for a higher defrost temperature setting. 
     By way of illustration, with reference to  FIG. 1 , an embodiment of a method of reducing time spent in a defrost mode of operation of a very low temperature refrigeration system  100  comprises, in a defrost mode of operation of the system, (i) opening a defrost valve  123  in a hot gas defrost circuit  121  to bypass flow of a refrigerant around a refrigerant circuit of a high pressure side of a plurality of heat exchangers  103  and to the evaporator inlet  124  from which the refrigerant flows to an evaporator  105 , to effect warming of the evaporator  105 , and (ii) while opening the defrost valve  123 , closing a cool valve  112  so that the refrigerant does not flow from the refrigerant circuit of the high pressure side to the evaporator  105 . A value of a stored defrost completion set point temperature of a return temperature sensor  133  on a low pressure side of the evaporator  105  is set based on an input control signal. During the warming of the evaporator  105 , upon the return temperature sensor  133  on the low pressure side of the evaporator  105  reaching the stored defrost completion set point temperature of the return temperature sensor  133 , the defrost valve  123  is closed to prevent the refrigerant flowing to the evaporator  105 . The stored defrost completion set point temperature of the return temperature sensor  133  can be about 0° C. or lower. The return temperature sensor  133  can be, for example, a thermocouple on the low pressure side of the evaporator  105 . The method can include closing the defrost valve  123  when a controller receives a temperature control signal from the return temperature sensor  133  that is at least as warm as the stored defrost completion set point temperature of the return temperature sensor  133 , the stored defrost completion set point temperature being stored in a memory of the controller. 
     A return solenoid valve  114  is used to control flow through a return side of the heat exchanger array  103 . There is a return hand shut off valve  136  in the return side line to the heat exchanger array  103  and a bypass hand shut off valve  137  in the low pressure bypass line. The return valve  114  and low pressure bypass valve  113  are controlled by a control scheme that is based on the temperature Tc in the return location. Only one of the valves  114  and  113  is activated, depending on how the temperature Tc compares with the set point temperature stored in the controller. For example, when the temperature Tc is at or higher than the set point temperature, the bypass valve  113  is opened and the return valve  114  is closed, so that flow is bypassed through the low pressure bypass around the low pressure side of the heat exchanger array  103 ; but when the temperature Tc is lower than the set point temperature, then the return valve  114  is opened and the bypass valve  113  is closed, so that flow proceeds through the low pressure side of the heat exchanger array  103 . In addition, the set point range of the control temperature for the return valve  114  and bypass valve  113  can be set lower than a previous conventional limit of −40° C., which was previously used because it represents the lower operating limit of compressor  101 .  FIG. 4  is a graph showing improved recovery time of a very low temperature refrigeration system using different return valve set points. To be able to set the return valve set point lower than −40° C., a suction line heat exchanger  132  is added between the discharge (high pressure) and suction (low pressure) refrigerant lines in the heat exchanger array  103 . The suction line heat exchanger  132  uses refrigerant at the liquid line temperature (for example, between about 14° C. and about 40° C.) to warm up the suction temperature to protect the compressor, so that the setpoint can be set lower than −40° C. At the same time, it lowers the liquid line temperature and helps the system to recover to be colder in the standby mode of operation. It also improves the overall system efficiency due to the use of internal heat transfer. In another embodiment, the function of the suction line heat exchanger  132  can be performed using, or supplemented with, a heater, such as an electrical resistance heater. Using a heater such as an electrical resistance heater can, for example, permit enhanced control of the temperature of flow to the compressor. 
     By way of illustration, with reference to  FIG. 1 , an embodiment of a method of reducing recovery time after defrost of a very low temperature refrigeration system  100  comprises, upon a return temperature sensor  133  on a low pressure side of the evaporator  105  warming to be at or above a stored bypass control set point temperature of the return temperature sensor  133 , (i) closing a return valve  114  to prevent refrigerant flow through a refrigerant circuit on a low pressure side of a plurality of heat exchangers  103 , and (ii) opening a bypass valve  113  to bypass flow of the refrigerant around the refrigerant circuit on the low pressure side of the plurality of heat exchangers  103  and to a suction line  134  that enters a low pressure side of a compressor  101 ; and warming the bypassed flow of the refrigerant in the suction line  134  before it enters the low pressure side of the compressor  101 . The bypassed flow of the refrigerant in the suction line  134  can be warmed using a heat exchanger  132  that exchanges heat between the suction line  134  and a refrigerant circuit on a high pressure side of the plurality of heat exchangers. Alternatively, a heater can be used to heat the suction line. The stored bypass control set point temperature of the return temperature sensor  133  can be less than about −40° C. For example, the stored bypass control set point temperature of the return temperature sensor  133  can be between about −40° C. and about −50° C., between about −50° and about −60° C., or between about −60° about −70° C. 
     A cool down time of a very low temperature system can be reduced. Typically, a coating application requires a certain target temperature, such as from about −110° C. to about −120° C. at the evaporator in ( 124 ) or evaporator out, based on the maximum water vapor pressure permissible. The time required to achieve this target process temperature is an important factor in the throughput of the coating process. During cool down, it is often observed that the buffer solenoid valve  116  is triggered due to high discharge pressure. When this valve opens, some of the high pressure gas is diverted to an expansion tank  115 , and this flow diversion, however temporary, can result in a delay in the cool down time, due to a decrease in refrigerant flow to the cryocoil. In order to reduce the cool down time, activation of the buffer solenoid valve  116  is reduced or eliminated by removing the occurrence of high pressure faults during cooling. An unloader solenoid valve  130  is placed in parallel with flow metering device  104  in the cooling path to the evaporator  105 . There is also a cool hand shutoff valve  135  and a cool solenoid valve  112  in the cooling path to the evaporator  105 . During cool down, when the discharge pressure exceeds a setpoint, for example 415 psig, this unloader solenoid valve  130  will open so as to allow refrigerant to bypass the flow meter device  104  for a few seconds. This allows the system pressure to be reduced below the limit, without diverting any flow away from the application cryocoil. 
     By way of illustration, with reference to  FIG. 1 , an embodiment of a method of reducing a cool down time of a very low temperature refrigeration system  100  comprises, during a cooling mode of operation of the system, flowing refrigerant through a high pressure side of a plurality of heat exchangers  103 , through a flow metering device  104 , a cool hand shutoff valve  135  and a cool valve  112  with which the flow metering device  104  is in a series flow connection, to an inlet of an evaporator  105 , through the evaporator  105  and through a low pressure side of the plurality of heat exchangers  103 . Upon a discharge pressure of a compressor  101  of the system being at least as high as a stored set point of a maximum discharge pressure during the cooling mode of operation, an unload valve  130  is opened that permits refrigerant flow to bypass around the flow metering device  104  and to the cool valve  112 , until the discharge pressure reduces to be less than the stored set point of the maximum discharge pressure. An inlet  124  of the evaporator  105  or an outlet (at  133 ) of the evaporator  105  can be at a temperature of less than about −110° C. The stored set point of the maximum discharge pressure can be less than an activation pressure of a buffer solenoid valve  116  of the system. 
     One or more sensors, such as  133 , can be used to provide sensed temperature signals that can be provided to a controller, to be compared with one or more stored temperature setpoints, which can be stored in the memory of the controller. The sensors can, for example, be thermocouples brazed onto one or more locations (such as  133 ) in the system. For example, the discharge inlet to or discharge outlet from one or more heat exchangers, or the suction inlet to or suction outlet from one or more heat exchangers, can be used as locations for temperature sensors. Also, temperatures at any of the solenoid valves can be used, or inlets or outlets to solenoid valves. In one example, a temperature Tc at the low pressure side of the evaporator, and at the inlet of the return solenoid valve  114 , at  133 , is sensed. In another example, other temperature sensors can be used in place of, or in addition to, thermocouples, such as silicon diodes or other similar devices. 
     Various techniques set forth herein are implemented using a controller, and can include computer implemented components. 
       FIG. 5  is a simplified schematic block diagram of a controller that can be used, for example, as the controller  180  of  FIG. 1 . Control techniques discussed herein can be implemented using hardware, such as a controller  580  that includes one or more processors  581 , which can for example include one or more Application Specific Integrated Circuits (ASICs)  582 ,  583 ; application software running on one or more processors  581  of the controller  580 ; sensor lines  584 ,  585  delivering electronic signals from sensors that are coupled to systems set forth herein (such as sensor lines from temperature sensor  133  and any pressure sensors) to the controller  580 ; and actuator lines  586 - 588  delivering electronic signals to actuated components within systems set forth herein (such as actuator lines delivering electronic signals to actuated valves or other controlled components). The controller  580  can also include user input module  589 , which can include components (such as a keyboard, touch pad, and associated electronics in connection with the processor  581  and memory  590 ) to receive user input to provide set point temperatures, such as control signals that set the stored defrost completion set point temperature, the stored bypass control set point temperature or the stored set point of the maximum discharge pressure. The controller  580  can also include a memory  590  to store such set point temperatures, and to implement procedures under control of computer hardware and software. It will be appreciated that other control hardware may be used, including control hardware that is at least in part pneumatic. 
     Portions of the above-described methods and systems can be implemented using one or more computer systems, for example to permit automated implementation of control techniques for refrigeration systems and related components discussed herein. For example, techniques can be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers. 
     The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
     While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims. 
     Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.