Patent Publication Number: US-6698218-B2

Title: Method for controlling multiple refrigeration units

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/896,610 filed Jun. 29, 2001 U.S. Pat. No. 6,564,563, the contents of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The present invention relates to software for a cooling and condensation control system. In particular, the present invention relates to a cooling system and condensation control system for computer logic modules. 
     One of the factors that limit processing speed in computer systems is the generation of excessive heat at higher clock speeds. Significant gains of speed and reliability have been achieved by cooling computer logic modules down to temperatures below ambient. Unfortunately, cooling a logic module to below ambient temperatures can result in the formation of condensation, which is undesirable in a computer system. 
     Prior attempts at providing a cooling system for a computer module have not been satisfactory for higher-end computing applications. For example, one approach has been to remove moisture from incoming air cooled to 5° C. This approach requires handling a tremendous amount of water, and does not prevent condensation in an application where refrigerant may be operating as cold as −40° Celsius. Another approach has been to simply apply a fixed high-power heater around an evaporator unit which surrounds the logic module. In this way, the surface temperature of the logic module housing remains above the dew point. Another approach relies on enclosing the logic module in a vacuum enclosure as a means of providing effective insulation. Unfortunately, these approaches cannot adequately ensure that there will be no condensation in the evaporator housing and are therefore not sufficiently reliable. 
     Another problem unresolved by prior art cooling systems relates to condensation formed on the opposite side of the circuit board. This problem has limited the temperatures to which the logic module can be cooled to avoid condensation. 
     SUMMARY 
     The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a refrigeration system including an evaporator housing including an evaporator block in thermal communication with the logic module. The evaporator housing includes a humidity sensor for detecting a humidity within the evaporator housing. The system further comprises a controller for controlling a refrigeration unit supplying cold refrigerant to the evaporator block in response to the operating conditions of the logic module and the temperature of the evaporator block. In another aspect of the invention, two modular refrigeration units are independently operable to cool the evaporator block, and each refrigeration unit is controllable in various modes of operation including an enabled mode in which it is ready to cool the evaporator and an on mode in which it is actively cooling the evaporator. In another aspect of the invention, the evaporator block and a heater on a reverse side of the circuit board are particularly controlled during concurrent repair operations. In another aspect of the invention, faulty sensors are recognized as such and an appropriate response is made. In another aspect of the invention, the system is shut down in a manner allowing rapid access to the logic module. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-discussed and other features and advantages will be made known from the following detailed description and accompanying drawings, wherein like elements are numbered alike, and in which: 
     FIG. 1 is a schematic representation of an exemplary embodiment of the hardware components of a refrigeration system; 
     FIG. 2 is a schematic representation of an exemplary refrigeration unit and evaporator block; 
     FIG. 3 is a cross-section view of an exemplary installed evaporator and heater; 
     FIG. 4 is a front perspective view of the evaporator of FIG. 3; 
     FIG. 5 is a rear perspective view of the evaporator of FIG. 3; 
     FIG. 6 is a schematic diagram depicting air flow management scheme in a processor cage; 
     FIG. 7 is a perspective view of an exemplary processor cage; 
     FIG. 8 schematically represents certain components of a refrigeration unit controller; and 
     FIGS. 9,  10 , and  11  show flow charts representing certain exemplary methods for operating or controlling a refrigeration system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram providing an overview of the hardware components of exemplary refrigeration system  10 . Refrigeration system  10  includes redundant refrigeration units  20  (two are shown) that sequentially or in unison provide coolant to evaporator  30  via refrigerant lines  26 . Although the cooling system will be described with reference to two refrigeration units  20 , it will be understood that the invention can be adapted for use with one or more such refrigeration units. Evaporator  30  is disposed over logic module  50  and is in thermal communication therewith for cooling logic module  50  to temperatures below ambient. Heat from logic module  50  is absorbed by coolant in evaporator  30 . Coolant is then passed back to refrigeration units  20  where it is compressed, condensed, and expanded in any known refrigeration cycle. 
     Logic module  50  is preferably a multi-chip module, as are generally known in the art, but cooling system  10  is applicable to other concentrated heat generating devices that are advantageously maintained at below ambient temperatures. Logic module  50  is electrically and mechanically attached to a first side  58  of circuit board  55 . Both logic module  50  and circuit board  55  are relatively flat structures having heights that are substantially less than their widths or lengths, and are shown in FIG. 1 in profile view (i.e., disposed perpendicular to the page). To prevent condensation on a second side  59  of circuit board  55  from forming, a heater  40  is disposed on second side  59  of circuit board  55 . Heater  40  includes a heat spreader plate  41  in thermal communication with circuit board  55 . Thermal pads  52  (FIG. 3) enhance thermal coupling between heat spreader plate  41  and plated through-hole connections (not shown) of circuit board  55 , effectively warming circuit board  55 . Heater  40  further includes redundant heat cartridges  44  to provide heat to heat spreader plate  41 . Each heat cartridge  44  includes a resistive heat element (not shown) for converting electrical energy into heat energy. In a preferred embodiment, each heat cartridge is rated at 300 W. In addition to heat cartridges, other heat source types, such as flat resistive heaters, are contemplated. 
     Each refrigeration unit  20  includes a respective controller  24  in communication with a primary cage controller  60  and secondary cage controller  65 , the latter being available in case of failure of primary cage controller  60 , via two-way communication lines  64 . Controller  24  operates in response to instructions from primary cage controller  60  or secondary cage controller  65  to cool logic module  50  to a desired temperature. In aid of this objective, controller  24  is equipped with various outputs and inputs to control refrigeration equipment  22  and to monitor the actual temperature of the evaporator. In addition to cooling logic module  50 , controller  24  is responsible for preventing condensation within evaporator  30  and on circuit board  55 , monitoring the temperature and relative humidity in the evaporator cavity (to be described in more detail below) and the temperature of heater  40 , to ensure that it is greater than the dew point within the processor cage (FIGS. 6,  7 ). 
     Controller  24  controls refrigeration equipment  22  within refrigeration unit  20  via signal lines  23 , and controls heat cartridge  44  via lines  29 . Evaporator plate temperature is provided via lines  36  and evaporator cavity temperature and relative humidity are provided via lines  33  and  34 , respectively. Finally, lines  42  provide heater temperature feedback. 
     Refrigeration units  20  are modular in nature, i.e., they are interchangeable and provide redundancy or backup capabilities to evaporator  30 . Refrigerant lines  26  and refrigerant return lines  27  are connected to refrigeration unit  20  via respective quick-disconnect couplings  25  so that either refrigeration unit  20  may be quickly swapped out for maintenance or replacement once another refrigeration unit is on line. Refrigeration units  20  each include a condenser and compressor, as is known in the refrigeration art, and passes pressurized refrigerant, which may exist as a liquid, gas, or mixed-phase fluid to evaporator  30  via refrigerant lines  26 . In a preferred embodiment, the compressor is a Maneurop LT-22 compressor, but any rotary, reciprocating, or scroll compressor of sufficient cooling capacity could be used. The precise refrigeration cycle and working fluid is determined based on a number of factors, such as the desired resultant temperature, environmental/regulatory considerations, desired coefficient of performance, desired size of condenser, cost, etc. 
     Referring now to FIG. 2, an exemplary cooling cycle using R507 (AZ50) refrigerant provides temperatures as cold as −40° C. and colder. As would be appreciated by those skilled in the art, the target temperature may vary with the compression ratio and other factors, including various control measures as will be hereinafter described. 
     In the exemplary arrangement shown, a mixture of gas and liquid refrigerant are passed to evaporator block  31  via refrigerant line  26 . Evaporator block  31  comprises copper, aluminum, or other heat conductive material and includes independent serpentine paths  61  and  62  formed therein. Serpentine paths  61  and  62  are circuitous pathways that may be formed by any known method, such as by cutting paths into a center layer that is then sealed between upper and lower layers of the material. Exemplary evaporator block structures are shown and described in commonly assigned U.S. Pat. No. 5,970,731 to Hare et al, issued Oct. 26, 1999 and U.S. Pat. No. 6,035,655 also to Hare et al., issued Mar. 14, 2000, both of which are herein incorporated by reference. Evaporator block  31  includes an additional serpentine path  62  for connection with a second refrigeration unit  20 . Serpentine paths  61  and  62  are separated and are not in fluid communication with each other so that one refrigeration unit  20  may be disconnected while the other continues to operate. Evaporator block  31  is disposed in thermal communication with logic module  50  within evaporator  30  (FIGS. 1,  3 ) so that heat from logic module  50  is readily absorbed by refrigerant in serpentine paths  61  and  62 . 
     Upon absorbing heat from logic module  50  (FIG. 1) the refrigerant vaporizes and is passed back to refrigeration unit  20  via refrigerant return line  27 . This gaseous refrigerant is passed to compressor  70  via line  63 , compressed therein, and then passed to condenser  71  via line  67 , wherein it is condensed back into a liquid or mixed phase fluid. Blower  72  is driven by a variable A.C. motor or D.C. motor and pulls air through electronic controller  24  and condenser  71  in a path generally represented by arrow  88  for absorbing the heat from the refrigerant. 
     After being condensed in condenser  71 , the coolant is received in filter/drier tank  78  via line  76 , then passed to expansion valve  85  where a portion of the coolant is vaporized and its temperature is reduced. The coolant is then once again made available to evaporator  30  via line  87 . A bypass line  79  is provided between lines  67  and  87  for bypassing the condenser and expansion valve in order to moderate the coolant&#39;s temperature. Bypass line  79  includes hot gas bypass valve  80 , actuated by solenoid  82 . 
     Controller  24  includes a processor and memory, as will be further described, for executing machine code to control the operation of refrigeration unit  20 . Control unit  24  includes various inputs for temperature and humidity sensors in evaporator  30 , to be described in more detail below, and outputs to control compressor  70 , bypass valve  80 , and blower  72  via lines  66 ,  83 , and  73 , respectively. Any of lines  66 ,  83 , and  73  may carry control signals for actuating a relay or motor controller (not shown) which switches on and off power to the respective component, or such relay/motor controller may be internal to controller  24 , and lines  66 ,  73 , and  83  carry power to the respective components, in the known manner. The power supply and power lines are not illustrated for sake of clarity. 
     Additionally, refrigeration unit  20  includes outputs for controlling heat cartridge  44  (FIGS. 1,  3 ) and an input for receiving electrical power for supplying energy to controller  24 , as well as compressor  51 , blower  61 , and solenoid  82 . 
     FIG. 3 shows a cross section view of an exemplary implementation of an evaporator  30  and heater  40  assembled with a logic module  50  and circuit board  55 . Circuit board  55  is provided with stiffeners  97  on either side to aid in supporting and attaching evaporator  30  and heater  40  to circuit board  55 . Evaporator  30  is attached to a first side  58  of circuit board  55  and heater  40  is attached to a second side of circuit board  55  by screws  43  connected to stiffener  97  as shown. Refrigerant line  26  and refrigerant return line  27  supplies one of two serpentine paths  61  and  62  (FIG.  2 ). A second pair of refrigerant and refrigerant return lines lie directly behind lines  26  and  27  shown, allowing connection to an additional refrigeration unit  20 , as described above. 
     Refrigerant line  26  and refrigerant return line  27  are connected to evaporator block  31  which is in thermal communication with logic module  50 . Evaporator block and logic module  50  are sealed in an evaporator housing  32  by gasket  94 . After assembly, Evaporator cavity  35  is filled with insulation, such as an injected polymer foam insulation, to reduce the amount of heat absorbed from evaporator housing  32 , thus preventing the temperature of evaporator housing  32  from dropping below dew point. Grommet  48  is formed of insulating material and includes holes for passing refrigerant lines  26  and refrigerant return lines  27  into evaporator housing  32 . Also extending into housing  32  is evaporator block temperature probe  39 , which includes a sensor extending into evaporator block  31  for detecting the temperature of the evaporator block, and providing feedback information to refrigeration units  20  as previously described. Evaporator block temperature probe  39  preferably comprises dual thermistors for providing independent temperature detection for each of two refrigeration units  20 . Evaporator housing  32  is attached to circuit board  55  in the known manner, with evaporator block  31  being biased against logic module  50  using biasing elements  49 , which may comprise metal springs or elastomeric blocks. 
     Heater  40  comprises heat spreader plate  41  which includes accommodations for two heat cartridges  44  and dual temperature sensors  46  for detecting the temperature of heat spreader plate  41  and providing feedback to respective refrigeration units  20 . 
     The internal atmosphere of evaporator  30  will be described with reference now to FIGS. 1 and 3. Desiccant canister  45  is connected at its inlet to a source of pressurized air. Dry air flows out of desiccant canister  45  through capillary tube  47  to heat spreader plate  41 . Heat spreader plate  41  and circuit board  55  include an aligned through-hole  95  for conducting dry air from said capillary tube to the first side  58  of circuit board  55 . Logic module  50  comprises a zero-insertion force connector as is known in the art which includes a small air space  57  (FIG. 3) between logic module  50  and circuit board  55 . This arrangement raises the air pressure in evaporator housing  32  to slightly above ambient. Thus, any small amount of leakage or diffusion will only result in dry air infiltrating into evaporator housing  32 . 
     Evaporator  30  also includes a desiccant slot  37  housing a desiccant bag  92  which absorbs any remaining moisture in evaporator cavity  35 , such as might occur upon replacement or servicing of dual humidity and temperature sensor  38 . Arrows  96  shows free movement of air between evaporator cavity  35  and desiccant slot  37 . Desiccant slot  37  also houses dual humidity and temperature sensor  38 , which includes redundant humidity and temperatures sensors. In a preferred embodiment, humidity and temperature sensor is or is similar to one available from Honeywell, part number HIH-3602-C and includes two independent humidity sensors and two independent thermistors. Capillary tube  47  is sufficiently long or otherwise includes an airflow resistor, such as an orifice, to prevent excessive air flow through capillary tube  47  in the case of a leak or while dual humidity and temperature sensor  38  is being serviced or replaced. 
     FIGS. 4 and 5 show front and rear perspective views of evaporator  30 , respectively. Refrigerant lines  26  and refrigerant return lines  27  are surrounded with thermal insulation to prevent condensation and loss of efficiency. Desiccant slot cover  28  (FIG. 4) encloses desiccant slot  37  and supports “D” connectors  98  which connect dual humidity and temperature sensor  38  to respective refrigeration units  20 . Similarly, “D” connectors  99  are provided to connect evaporation block temperature sensor  39  to respective refrigeration units  20 . 
     FIG. 6 shows a schematic representation of airflow management within processor cage  100 . Air is drawn into processor cage  100  at air inlet  102 . Air flows past memory books  105  and redundant power supplies (not shown) along path  103 . Air is then forced though one of a plurality of blowers  110  (only one shown). Blower  110  includes louvers  11  which operate as check valves to prevent back flow through blower  110  when fewer than all blowers are operating. Air flows generally along path  107  by memory books  106  and along path  108  by heater  40 . A majority of this air exits at exit  119 , however some air flows through orifice  112  and then past evaporator  30  and then recirculates through one or more of blowers  110 . In this manner, evaporator housing  32  is warmed by air previously warmed by absorbing heat from the power supply (not shown), memory books  105 ,  106 , blowers  110 , and heater  40 . This warmed air passes heat energy to evaporator housing  32 , thereby increasing its surface temperature to above dew point, ensuring that no condensation will form thereon. 
     FIG. 7 shows a perspective view of cage controller  100 . A source of pressurized air supplies air through air hose  113  to desiccant canister  45  (FIG. 1) which resides in canister housing  115 . A view-window  116  is provided so that an operator can see when the desiccant changes color, indicating saturation. Canister housing  115  outputs dry air to capillary tube  104  which is sufficiently long to restrict the air flow as previously described. Capillary tube  104  feeds into heat spreader plate  41  as shown. Cavity  101  receives modular redundant power supplies which provide power to logic module  50  (FIG.  1 ). 
     Referring to FIG. 8, each refrigeration unit controller  24  includes a processor  120 , an analog-to-digital converter  122 , random access memory  124 , non-volatile memory  126 , communication port  128 , and output  132 . Controller  24  also includes other necessary ancillary components such as power supply, clock, etc., which are not shown for sake of clarity. Non-volatile memory  126  may be any type of machine readable media such as ROM, PROM, EPROM, EEPROM, Flash, magnetic media, optical media, or other known type of non-volatile memory. Communications port  128  is preferably a standard serial port, as are generally known in the industry, such as the RS422 serial port. Each of these components are in communication with processor  120  via one or more data busses  130  (only one shown). 
     During operation of controller  24 , software stored in non-volatile memory  126  causes processor  120  to perform various operations on input and to generate outputs accordingly. As shown in FIG. 1, controllers  24  are in communication with primary and secondary cage controllers  60  and  65 . Additionally, cage controllers  60  and  65 , which are also intelligent devices, are in communication with other controllers and sensors via Ethernet (see line  68  in FIG. 1) as generally known and understood in the art. Cage controllers  60  and  65  are responsible for monitoring and regulating the power supply, cooling fans, internal processor cage temperature, and other environmental aspects of the processor cage to ensure proper functioning of the system and the various internal components. 
     The use of an intelligent refrigeration unit controller allows the refrigeration unit to cooperate with the cage controller and larger system to maximize the performance of the logic module without sacrificing reliability. In this regard the controller can, in response to instructions from cage controller  60 , precisely control the temperature of the evaporation block by controlling the speed of compressor  70  and by actuating hot gas bypass valve  80  (FIG. 2) in response to temperature readings from evaporator block temperature probe  39 . The temperature to which the evaporator block is controlled may vary according to the current condition or operation of the logic module operating mode or power consumption, as well as the expected condition or operation of the logic module. In addition, the refrigeration unit controller is capable of reacting to component failures and notifying the cage controller of such failures. This provides improved reliability by allowing the cage controller to then switch to a second refrigeration unit and alert system administrators of the failure for repair or replacement of the defective component and/or refrigeration unit. Furthermore, by integrating the refrigeration unit with the cage controller, smooth transitions from one refrigeration unit to another can be easily achieved. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 SYSTEM 
                 RU 1 
                 RU 2 
               
               
                   
               
             
            
               
                 N mode 
                 On 
                 Off 
               
               
                   
                   
                 Deactivated 
               
               
                 N + 1 mode 
                 On 
                 Enabled 
               
               
                 Switchover mode 
                 Switchover-from 
                 Switchover-to 
               
               
                   
               
            
           
         
       
     
     The system and the individual refrigeration units are capable of entering several different modes of operation to accomplish this task as summarized by Table 1. These modes will now be described with reference to an exemplary system capable of being connected to only two refrigeration units at any one time, though it should be understood that the system may be adapted to accommodate any number of refrigeration units. In the “N-mode,” one refrigeration unit is on while the other refrigeration unit is either off or deactivated. When in the off-mode, the refrigeration unit is not operating to cool the logic module, but it is actively communicating with cage controller  60 . When in the deactivated-mode, the refrigeration unit is off-line or disconnected. In the N+1 mode, the first refrigeration unit is on, while the second is “enabled”. In the enabled mode, a refrigeration unit is not functioning to cool the logic module, and it is in communication with cage controller  60 . What distinguishes “enabled” from “off” is that when enabled, the refrigeration unit is ready to step in and turn on, by itself and without instruction from the cage controller, if necessary. The conditions under which an enabled refrigeration unit may turn on by itself include sensing evaporator over-temperature and the cage controller didn&#39;t turn it on. A third mode of the system is the “switchover” mode. In a switchover mode, one refrigeration unit is designated the “switchover-from” unit, indicating that it is in a shut-down sequence, and one refrigeration unit is in a “switchover-to” mode, indicating that it is in a start-up sequence. The shutdown and startup sequences vary depending on whether the switchover-from unit is experiencing a fatal error or whether the switchover-from unit is running normally or experiencing only a minor error. A minor error is one that can be compensated, an example of which will be described below with reference to FIG.  11 . 
     Each controller  24  makes status data available to the cage controller  60 , and the cage controller  60  periodically transmits status data and instructions to the refrigeration unit controllers  24 . This periodic data transmission is called “stuffage” and allows the refrigeration units to react to failures of the cage controllers, while the cage controllers&#39; monitoring of refrigeration unit data allows them to react to failures of the refrigeration units in a logical manner. In the exemplary embodiment, refrigeration unit controller  24  sends posts the value of all the sensors of the refrigeration unit, particularly the evaporator and heater block temperatures, flags indicating the mode the refrigeration unit is currently in, and fault data, such as evaporator cavity over-humidity, heater block over-temperature, etc. This information is available to the cage controller to read. Every 7 seconds or so, the cage controller  60  stuffs the refrigeration unit with data including the current power of the logic module, ambient temperature, control commands such as turn on/off, prepare for power on, prepare for self test, enter enable mode, enter/exit switch-over-to mode, enter/exit switch-over-from mode, fault flags control commands to set or clear fault flags in the refrigeration unit controller. 
     During operation, cage controller  60  determines the set point for the refrigeration units. The set point is the desired temperature of evaporator block, and is dependent on the requirements of the logic module and the mode of operation and power draw of the logic module. For example, during start up operation, the set point is initially set to a high temperature of, e.g., 0° C. which may be designated a standby temperature. During self-test or periods of low current draw, the set point is set to an intermediate temperature to avoid condensation, e.g., −10° C. During normal operation, the set point is set to a low temperature, e.g., −20° C. It may be necessary to raise the set point during periods of low power dissipation in the logic module to ensure the surface temperature of the portion of the circuit board outside the sealed environment does not rise above the dew point. 
     FIG. 9 provides a flow chart diagram describing an exemplary start-up sequence. This represents code in refrigeration unit controller  24  and is particularly directed towards starting up the compressor without drawing excessive current from the power supply. The procedure begins at start block  152  and proceeds immediately to block  154  which instructs the hot gas bypass valve  80  (FIG. 2) to open to unload the compressor. Compressor  70  (FIG. 2) is driven to a relatively slow speed of about 2700 r.p.m. The hot gas bypass valve  80  is then pulsed by opening, closing, then opening again, to ensure that it is functioning properly. By employing pulse width modulation, the hot gas bypass valve is ramped to 100% closed at block  162 . At about 81 seconds from start-up, the compressor speed is advanced to 50 Hz, which is equivalent to 3000 rpm at block  164 . In block  166  PID (proportional, integral, derivative) control of the evaporator block temperature then begins at about 86 seconds after startup and the heater is powered to 60% on (e.g., using pulse width modulation with about 2 second cycle time). Block  168  begins thermal regulation and at block  170 , the logic thread terminates. 
     The cage controller prepares for changes in logic state and sets the set point prior to the change, but not for so long as to create condensation. For example, in order for the logic module to function at optimum cycle time, it should already be chilled to its planned temperature condition. Therefore, the logic code must first request a low temperature state, then wait for the cage controller and refrigeration unit to provide that state, e.g., by changing the set point. Once the set point is achieved, the refrigeration unit sets a status bit for the cage controller to read, thus indicating that it has achieved status. The status bit may be indicative of an evaporator block temperature being within a selected range, e.g., 2° C. or 5° C., of the set point. Once the cage controller notices that the status bit is set, it gives permission to the logic code to operate at the faster cycle time. If the cage controller or refrigeration unit notices that the logic module failed to increase its cycle time within a selected time frame, e.g., anywhere from 20 seconds to 2 minutes, it will reset the set point to the high or medium temperature, to avoid condensation. 
     FIG. 10 represents code in cage controller  60  that is executed prior to “clocks-on” mode, i.e., prior to running logic module at its optimum speed. This procedure begins with block  172  and proceeds immediately to block  174  wherein one of refrigeration units  20  is turned on. The unit to be turned on may be selected at random, or otherwise. At block  176 , the set point temperature T is initially set to high, which may correspond to 0° C. At block  178 , cage controller  60  then waits for the status bit in refrigeration unit  20  as described above. If all is well, cage controller  60  sets the set point to a medium value, e.g., −10° C. as block  180  provides. At block  182 , cage controller  60  then waits once again for the status bit in refrigeration unit  20 . At block  184 , with the logic module cooled to its intermediate value, cage controller  60  signals higher-level code to commence its self test. This step may be skipped at the user&#39;s option. If all is well, cage controller continues to block  186  where the set point is finally set to the low temperature, e.g., −20° C. At block  188 , cage controller  60  again waits for the status bit. Then, at block  190 , cage controller gives the higher-level code the go-ahead for clocks-on, and the procedure terminates at block  192 . If clocks-on fails to commence within a selected time-frame, e.g., 2 minutes, then the set point is returned to the intermediate value to prevent over-cooling of the circuit board. 
     Each refrigeration unit controller  24  monitors its individual humidity sensor or its independent output of dual humidity and temperature sensor  38  to ensure that it remains within normal limits. If the sensor output is outside the normal limits of the sensor probe, controller  24  sets an error flag indicating that the sensor is “insane”. Higher level system code monitors the error flags from refrigerator unit  20 . Upon detecting the error, the higher level system code will message a system operator that the humidity sensor needs to be replaced, as described below. 
     Sometimes, a humidity sensor will slowly drift from the correct reading, rather than returning an insane value. To detect a faulty sensor that is merely inaccurate, rather than insane, cage controller  60  compares the actual sensor values together when one of refrigeration units  20  indicates an over humidity condition. If the difference between the readings is beyond a miscompare limit, e.g., 5% relative humidity, the higher-reading humidity sensor is flagged as defective and the operator is requested to replace the dual humidity and temperature sensor  38 . If the difference between the readings is within the miscompare limit, then a dry air breach warning is surfaced and a repair is requested of this condition. In an alternate embodiment, dual humidity and temperature sensor  38  is replaced with a pressure sensor. This would protect against failures of the seal around evaporator cavity  35 , but would not insure against moisture diffusion through the various elastomeric membranes. 
     It should be noted that the output of the thermistors in dual humidity and temperature sensor  38  are used to correct the sensed relative humidity value for evaporator cavity  35  in accordance with the manufacturer&#39;s specifications. It is this temperature-corrected relative humidity value that is used for fault detection and isolation. The correction is required since the local air temperature around dual humidity and temperature sensor  38  varies with ambient as well as with the temperature set point in evaporator  30 . 
     Concurrent maintenance, i.e., maintenance during continued operation of the logic module, of the dual humidity and temperature sensor  38  (humidity sensor) is accomplished by deactivating the humidity sensor. In this case, “deactivating” the humidity sensor means causing heat cartridge  44  to thermostatically control the board temperature to a higher than normal value to prevent moisture from forming thereon when evaporator cavity  35  is opened for the short time needed to replace humidity sensor  38 . Refrigeration unit controller  24  sets a timer when the humidity sensor is deactivated and gives a warning signal if the repair procedure takes too long. For example, if the evaporator cavity is opened for more than 10 minutes, an audible alarm is produced, or a warning message is sent to cage controller  60 . If the amount of dry air supplied from desiccant canister  45  is increased while evaporator cavity  35  is opened, then this time may be increased. For example, capillary tube  47  may be replaced with a pair of differently-sized orifices, with a valve and actuating mechanism, to allow increased air flow while the humidity sensor is being serviced. 
     While dual humidity and temperature sensor  38  is being replaced, the internal desiccant bag is also replaced so that any moisture that enters evaporator cavity  35  during this procedure is removed. Once dual humidity and temperature sensor  38  is replaced, it is “activated” and the heaters are returned to normal power. Refrigeration unit  20  then resumes normal monitoring of the relative humidity levels in evaporator cavity  35 . 
     Temperature sensors  46  are monitored for sanity in a manner similar to the way the humidity sensors are monitored. For example, if the heaters are more than 10° C. apart in their temperature measurements, a miscompare fault flag is set, and one or both sensors are replaced. 
     Outputs of temperature sensors  46  are also compared with over and under temperature limits. These limits are a function of other system states or environmental conditions. A temperature outside the range defined by the over and under limits, could indicate a faulty heater, connection, or drive circuit. A defect in the running refrigeration unit triggers a switchover to the other (good) refrigeration unit. The defective heat cartridge and thermistor is concurrently replaced by first deactivating the heater, causing the good running refrigeration unit to thermostatically control the heater to a lower, touchable temperature. As with the humidity sensor, a timer is set as a result of the heater deactivating command. Sufficient time is allotted for replacing heat cartridge  44  and thermistor  46 , but not so much time as to permit moisture to form on critical surfaces. For example, the timer may be set for 10 minutes, after which an alarm sounds. 
     Each refrigeration unit  20  controls its own heat cartridge  44 , and the heat value is controlled from no heat to full heat, up to 300 W, if heat cartridge  44  is a 300 W heater. Heat cartridge  44  is controlled by pulse width modulation at a frequency fast enough compared to the thermal mass of heat cartridge  44  that prevents temperature cycling and related heater failure, e.g., 2 Hz has been found to be a sufficiently fast frequency. Unique heat values are used for particular power and environmental states. In a method described by FIG. 11, the heater power is reduced for high ambient air temperature conditions. Ambient temperature is measured within the refrigeration unit enclosure at the inlet to condenser  71  (FIG. 2) using a thermistor. This thermistor is tested for sanity in the same manner as temperature sensor  46  described above. Starting with block  194  refrigeration unit controller  24  proceeds immediately to block  196  and determines whether the thermistor at the inlet of condenser  71  is insane. If not, controller  24  proceeds to block  198  wherein the condenser inlet temperature is taken as the ambient temperature, and controller  24  proceeds to block  202 . If the thermistor is insane, then the ambient temperature is set to the ambient temperature from a thermistor mounted on the system frame, which is provided in stuffage from cage controller  60 . A compensation value is added to this value since the frame temperature will typically be several degrees cooler than the air entering condenser  71 , due to the environment in refrigeration unit  20 . Controller  24  then proceeds to block  202 . 
     At block  202 , controller  24  tests whether the ambient temperature is less than 30° C. If so, heat cartridge  44  is set to 60% of maximum and blower  72  (FIG. 2) is set to 2,000 rpm. Then, the procedure terminates at block  208 . If the ambient temperature is greater than 30° C., the heat cartridge is set to only 30% of maximum and the blower is set to 2,800 rpm, to compensate for warmer ambient air. 
     Switchover from one refrigeration unit  20  to another refrigeration unit  20  can occur on a scheduled basis, for example, every 160 hours. In addition, switchover can be for recovery purposes, i.e., when one refrigeration unit is faulty. In particular, a recovery switchover may be in response to such failures as double communications fault to the on refrigeration unit, temperature sensor  46  insane, over temperature, or under temperature, or dual humidity and temperature sensor insane or over humidity. A double communication fault occurs when both cage controller  60  and secondary cage controller  65  lose communication with the on refrigeration unit. 
     In a switch-over mode, cage controller  60  posts switching-from and switching-to bits in stuffage to respective refrigeration units  20 , then posts a “switchover” function bit. The switching-from refrigeration unit compressor is set to maximum speed and its hot gas bypass valve is closed (0%) to prepare for turning on the switching-to refrigeration unit. After a short pause, the switching-to refrigeration unit is turned on, causing significant heat to be passed to evaporator block  31  during startup. The switching-to unit set point is then set a small amount lower (e.g., 1° C.) than the set point of the switching-from refrigeration unit, causing the switching-to unit to be taxed to a greater extent as the switching-from unit dumps heat through its hot gas bypass valve in an effort to bring the temperature up to its set point. Switching-to and switching-from refrigeration units are designed to be capable of maintaining this “switch-over” mode indefinitely. However, the switching-from refrigeration unit is kept on for a selected period of time, e.g., 4 minutes. The switching-from refrigeration unit is then turned off only after the switching-to unit exhibits no fault or warning conditions. After good status, the cage controller shuts down the switching-from refrigeration unit, and places it in enabled mode, then clears the switchover mode and function bit. 
     In the case of a recovery switchover, there is no pause in bringing the switching-to refrigeration unit to the set point temperature, and once achieved, the switching from refrigeration unit is immediately shut down. 
     When a system is powered off after the logic module has been chilled to below zero, there is a danger that condensation will form when the evaporator is removed to service the logic module. To prevent such condensation from forming, cage controller  60  sends the running refrigeration unit  20  a “prepare-for-power-off” command, causing it to open its hot gas bypass valve  80 , causing high enthalpy refrigerant to pass directly into the evaporator and thus rapidly heating the evaporator, as well as the logic module it is attached to. To further accelerate the heating of the chilled logic unit and circuit board, heat cartridge  44  is left on. These steps enable logic module  50  to be serviced in an acceptable timeframe without danger of condensation being formed on the vulnerable areas. 
     While the invention has been described with reference to specific embodiments thereof, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted as illustrative and not limiting in nature. Various modifications of the disclosed embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art upon reference to this description, or may be made without departing from the spirit and scope of the invention as defined in the appended claims.