Abstract: The present invention discloses a control apparatus for a linear compressor which can vary a cooling force and prevent an inrush current. The control apparatus for the linear compressor includes a coil winding body laminated on the linear compressor, a first capacitor connected in series to the coil winding body, a capacitance varying unit being formed in a parallel structure to the first capacitor, and having a capacitor switch, and a control unit for inducing an output change of the linear compressor, by varying the whole capacitance of the control apparatus by controlling the capacitor switch.

Abstract: An air conditioning apparatus is provided. The apparatus includes: an engine; an outdoor unit including a compressor driven by the engine to inhale a refrigerant, and compress and discharge the inhaled refrigerant and an outdoor heat exchanger; an indoor unit including an indoor heat exchanger; a controlling unit which controls a rotational speed of the compressor to be lower than a predetermined upper limit rotational speed; an outdoor temperature detecting unit which detects an outdoor temperature; an in-box temperature detecting unit which detects an in-box temperature within an enclosure box which accommodates therein the engine and the compressor; a correcting unit which corrects an upper limit rotational speed of the compressor when the outdoor temperature exceeds a predetermined outdoor temperature; a canceling unit which cancels the correction of the upper limit rotational speed when the in-box temperature is lower than a predetermined in-box temperature.

Abstract: A method for controlling a multi-unit air conditioning system adapted to cool or heat room spaces while passing a refrigerant in one direction or in a reverse direction is disclosed. In the method, which is applied to a multi-unit air conditioning system including at least three compressors, when a compressor re-operation is repeatedly carried out after all of the compressors have been turned off, to turn on again at least one of the compressors, repetition of the compressor re-operation is carried out for a predetermined number of different orders in such a manner that at least one of the compressors is turned on first in an associated order of the repeated compressor re-operation.

Abstract: An air conditioner having two compressors enables, when one compressor is started, the phase advance capacitor for the other compressor to be temporarily separated and used in parallel with the phase advance capacitor for the one compressor. These compressors can have an increased starting torque without using a starting capacitor.

Abstract: A refrigeration cycle apparatus (100) includes: a first compressor (101); a second compressor (102) connected in parallel with the first compressor (101) in a refrigerant circuit (200); a radiator (103) for cooling a refrigerant compressed by the first and second compressors; an expander (104) coupled to a rotation shaft of the first compressor (101); an evaporator (105) for evaporating the refrigerant expanded by the expander (104); and a controller (115).

Abstract: A single speed compressor is provided with a switching device and control for turning the compressor drive motor ON and OFF in repeated succession at a selected ON time/OFF time ratio within a selected cycle time interval. The ON/OFF ratio and cycle time interval are selected to maintain desired temperature and/or humidity control within a conditioned space.

Abstract: A variable-capacity air conditioner includes a compressor for compressing refrigerant, an indoor heat-exchanger coupled to the compressor, an outdoor heat-exchanger coupled to the compressor, a piping for coupling the compressor, the indoor heat-exchanger, and the outdoor heat-exchanger, a first capillary tube provided in the piping, a second capillary tube provided in the piping in series with the first capillary tube, a by-pass pipe connected in parallel to the second capillary tube, a valve for opening and closing the by-pass pipe, and a controller for controlling the compressor and the valve. The compressor is operable at a first capacity and a second capacity less than the first capacity to compress the refrigerant. The air conditioner prevents the compressor from overload and allows the refrigerant to circulating at an optimal flow amount rate through a refrigeration cycle.

Abstract: A variable capacitance compressor is provided in a refrigeration cycle of a vehicular air conditioning system. The variable capacitance compressor controls discharge capacity of refrigerant by an external control pulse signal, and can substantially estimate compressor suction side pressure by the external control pulse signal and compressor discharge side pressure. A compressor torque calculator is provided to calculate torque based on evaporator upstream and downstream temperature difference data ?t, which is a temperature difference between inlet side air temperature of an evaporator and outlet side air temperature of the evaporator, compressor discharge side pressure data Pd, duty ratio data of a control pulse signal, and compressor's number of revolution data.

Abstract: To provide a low-cost and highly reliable indoor unit and an air-conditioning apparatus having the same by obtaining a dew point temperature at the outlet side without installing a humidity sensor and a dew point temperature detector at the outlet side.

Abstract: An HVAC controller, a method of operating a constant air volume (CAV) HVAC unit and a CAV HVAC system are disclosed herein. In one embodiment, the HVAC controller includes: (1) an interface configured to receive both a latent cooling demand and a sensible cooling demand and (2) a processor configured to direct both a dehumidification function and a discharge air temperature control function for a CAV HVAC system employing the latent cooling demand and the sensible cooling demand.

Abstract: An air conditioner has a controller that controls the operation of a refrigerant circuit that has a compressor, a condenser, an expansion valve or orifice tube, and an evaporator. The condenser receives a compressed refrigerant from the compressor and condenses the refrigerant to either a liquid phase or a saturated liquid-vapor phase. The condensed refrigerant is then passed through the expansion valve or orifice tube to expand the refrigerant and to delivery the refrigerant to the evaporator. When the compressor is first started, various sounds and vibrations are created that may be unpleasant to humans. Also, if the engine is cold, then the compressor may have liquid refrigerant that can increase the torque needed to start the compressor. The controller pulses the compressor between ON and OFF operating states to reduce or eliminate these sounds and/or manage the start up torque of the compressor.

Abstract: A system and method are provided to control the output capacity of a compressor in response to the outdoor ambient temperature. The outdoor ambient temperature is measured by a sensor. The measured outdoor ambient temperature is then used to control the output capacity of the compressor. The compressor is operated at less than 100% capacity in response to the outdoor ambient temperature being less than or equal to 95° F. If the outdoor ambient temperature reaches a predetermined temperature greater than 95° F., the compressor is then operated at 100% capacity.

Abstract: A system and method of controlling components of a refrigeration system includes determining a desired rate of change of a refrigeration system operating parameter, using historical data to predict an expected rate of change of the operating parameter for cycling each component, calculating an appropriateness factor for each component corresponding to a difference between the expected rate of change for the component and the desired rate of change, ranking the components based on the appropriateness factor, and selectively cycling at least one component based on the ranking.

Abstract: An air conditioner has a heat exchanger, an absorbing agent, and a controller. The absorbing agent performs an absorbing operation for absorbing moisture in passing air flowing through the heat exchanger functioning as an evaporator and a regenerating operation for desorbing moisture from passing air heated by the heat exchanger functioning as a condenser. The controller performs control such that the absorbing operation and the regenerating operation are switched at a predetermined switching time interval. The controller also performs control of a capacity of a compressor and control for changing the predetermined switching time interval such that a predetermined load out of a total heat load, which is the sum of the latent heat load and the sensible heat load, is preferentially processed.

Abstract: A compressor, which is susceptible to protective shutdowns when certain operating conditions are sensed, includes a control feature wherein, when the compressor is restarted after a period of time, it may be caused to operate in an unloaded mode such that a reoccurrence of the shutdowns is less likely.

Abstract: An air conditioning control device is used to control an air conditioner having a utilization unit and a heat source unit. The air conditioning control device includes a state detection unit and a mitigation control unit. The state detection unit is configured to detect an increased energy state where a space temperature of an air conditioning target space of the utilization unit is frequently below a set temperature of the utilization unit during cooling operation or frequently exceeds the set temperature of the utilization unit during heating operation. The mitigation control unit is configured to control the air conditioner so as to mitigate the increased energy state when the state detection unit detects the increased energy state.

Abstract: Methods, systems, and apparatus for linearizing control of a commercial refrigeration system. In an embodiment of the invention, a controller is configured to receive a non-linear sensed suction pressure and convert the suction pressure to a linear temperature equivalent. The linear temperature equivalent is used by the controller to achieve efficient system operation over an entire range of operating temperatures.

Abstract: The invention relates to a method for regulating a coolant circuit (2) of a vehicle air conditioning system (4). According to said method, a target value (SW(VT)) for an evaporator temperature (VT) is predetermined in a base control circuit. Said value is used to determine a control value (U) for controlling the evaporator temperature (VT) by means of an evaporator temperature regulator (16). Said control value is also fed to a ventilator fan controller (26).

Abstract: A fuel-powered engine is provided to drive a mobile air conditioning or refrigeration unit such as a portable chiller, a container refrigeration unit, a portable packaged system, a transport tractor-trailer or truck refrigeration unit, etc. A control rapidly changes the engine speed between the predefined set of discreet engine speeds to precisely adjust the capacity of the refrigerant system. If the engine operates only at a single speed, then the control cycles the engine between this operating speed and a zero speed (the engine is shut off). If the engine can operate at multiple discreet speeds, then the control can cycle the engine between any of these speeds (including a zero speed). When a lower capacity is desirable, the engine operates for a longer time interval at a lower speed, and when a higher capacity is desirable, the engine operates for a longer period of time at a higher speed. The cycle rate is selected to control the comfort (e.g.

Abstract: A refrigerant system comprising a compressor, a condenser, an electronic expansion valve, and an evaporator is controlled in a normal operating mode to meet moderate cooling loads; however, when the load approaches that which is sufficient to induce liquid refrigerant carryover from the evaporator to the compressor, the system is controlled in a capped operating mode to limit a certain thermodynamic variable rather than controlled to meet the high load. In the normal mode, the compressor and/or the expansion valve might be controlled in response to the amount of superheat of the refrigerant leaving the evaporator or the level of liquid refrigerant in the evaporator. In the capped operating mode, the compressor and/or the expansion valve might be controlled to limit a variable such as the compressor's capacity, the saturated pressure or dynamic pressure of the refrigerant entering the compressor, or the refrigerant's mass flow rate.

Abstract: Relatively constant temperatures are maintained in a space by varying the capacity of heating or cooling equipment. A proportional band is used to adjust capacity to compensate for changes in heating or cooling load, and the proportional band is adjusted to bring the actual temperature within the space closer to the set point temperature. Such an adjustment may be made by determining the difference between the present temperature and the set point temperature, and adjusting a floating temperature set point by a percentage (e.g., ten percent) of the difference. Such a process may be repeated, for example, at regular intervals, such as once per minute. As a result, the temperature within the space approaches (e.g., asymptotically) the set point temperature whether the heating or cooling load is high or low. Methods, systems, and apparatuses are contemplated, including HVAC units and controls, including for residential applications, including using variable-speed drives.

Abstract: An air-conditioning apparatus includes first and second outdoor units having first and second outdoor heat exchangers and first and second heat source-side degree of subcooling adjustment devices configured to adjust first and second degrees of subcooling in outlet sides of the first and second outdoor heat exchangers, respectively. First and second outdoor-side determination units are configured to determine first and second degrees of subcooling, respectively. A controller is configured to control the first and second heat source-side degree of subcooling adjustment devices, respectively, such that a difference between the first degree of subcooling and the second degree of subcooling is reduced when refrigerant is charged into a refrigerant circuit having the first outdoor heat exchanger and the second outdoor heat exchanger.

Abstract: An exhaust system includes an entrapment vacuum pump equipped with a refrigerator, a master compressor which is driven in a driving frequency band to make constant a pressure difference between gas recovered from the refrigerator via a low-pressure gas recovery pipe and compressed gas supplied to the refrigerator via a high-pressure gas supply pipe, and a first sub-compressor which is driven at a predetermined driving frequency to supply compressed gas at a first gas volume to the refrigerator via a high-pressure gas supply pipe from gas recovered from the refrigerator via a low-pressure gas recovery pipe.

Abstract: A plurality of scroll compressors with different fixed volume indexes are connected in fluid parallel circuit and configured to selectively operate to maximize isentropic efficiency at different condensing temperatures. Different quantities of scroll compressors of different volume indexes may be selected based upon typical climate or geographic location environmental conditions to attempt to maximize efficiency. A controller may selectively operate different combinations of the compressors of different volume indexes bases up load demands and condensing temperature conditions, which may be determined in a variety of ways.

Abstract: A variable capacity air conditioner compressor is controlled by setting a car target indoor temperature; sensing car indoor and outdoor temperatures and solar radiation with sensors at predetermined car positions; calculating a vent target discharge temperature using the target temperature, sensed car temperatures and solar radiation; calculating a target evaporator temperature and blower voltage based on target discharge temperature; calculating a control duty based on the target evaporator temperature; calculating target evaporator temperature and blower voltage change rates; determining whether or not the compressor conies under a sudden variable condition based on the control duty, and target evaporator temperature and blower voltage change rates; and setting a control duty change rate maximum value to an accelerative slew rate greater than and equal to a basic slew rate when sudden and not sudden variable conditions are respectively determined.

Abstract: A system and method in a commercial refrigeration system for compensating for events. A commercial refrigeration system includes a controller that predicts a first change in cooling capacity required based on at least one anticipated system event. The controller also predicts a second change in cooling capacity required based on at least one anticipated system event during a period of time. The controller compares the first and second changes in cooling capacity and implements a change in cooling capacity based on a relationship between the first and second changes in cooling capacity.

Abstract: A refrigeration cycle controls a second evaporator while a first evaporator is operating. The refrigeration cycle of the present invention makes a differential pressure between an inlet refrigerant pressure and an outlet refrigerant pressure of a second evaporator solenoid valve when starting or stopping the second evaporator during operation of the first evaporator smaller than the differential pressure when operating only the first evaporator, then opening or closing the second evaporator solenoid valve.

Abstract: A variable speed control system is provided for use with a heat pump heating/cooling system having a variable speed compressor. The control system includes a remote transceiver, a second transceiver to communicate with the first transceiver, and an AC power module to communicate with the second transceiver. The AC power module further includes multiple current limiting devices and multiple temperature sensors and varies the output of the variable speed compressor by comparing the readings from the temperature sensors to a pre-determined temperature setting.

Abstract: A control device for an air-conditioning system of a vehicle comprises an evaporator, a compressor with externally controlled variable displacement, and an expansion valve. The control device comprises a control block that receives a reference temperature indicating a desired temperature of the air downstream of the evaporator, and an effective temperature of the air present downstream of the evaporator. The control device-supplies a control signal for the compressor to bring the effective temperature substantially equal to the reference temperature. An observer module receives the control signal and supplies a temperature disturbance indicating an estimate of the oscillatory effect generated by an expansion valve on the temperature of air downstream of the evaporator when a compressor is driven by the control signal. An adder block removes from the effective temperature the temperature disturbance so as to eliminate the oscillatory effect on the effective temperature downstream of the evaporator.

Abstract: A refrigerant compressor assembly for a refrigeration circuit controls the temperature within a temperature controlled space. The refrigerant compressor assembly includes a first unloading valve, a first valve actuator, and a first valve control system that adjusts the first valve actuator via a pulse-width-modulated signal, a second unloading valve, a second valve actuator, and a second valve control system that adjusts the second valve actuator via a pulse-width-modulated signal. The refrigerant compressor assembly also includes a third unloading valve. The first valve actuator is coupled to the first and third unloading valves and controlled by the first valve control system. The unloading valves selectively allow or resist fluid flow from higher to lower pressure areas within the refrigerant compressor assembly.

Abstract: An apparatus for summing capacities of outdoor units in a multiple air conditioner comprises self-capacity storage devices mounted in outdoor units, respectively, for storing self-capacities of the outdoor units and outputting the self-capacities of the outdoor units when necessary, an outdoor unit capacity summing part mounted in one outdoor unit of the outdoor units for receiving the capacities of the outdoor units outputted from the respective self-capacity storage devices to calculate the total capacity of all the outdoor units, and a control device for receiving the result of the calculation performed by the outdoor unit capacity summing part to control the operation of the multiple air conditioner. The possibility of incorrect input of the capacities of the outdoor units is eliminated, and therefore, more accurate control of the multiple air conditioner is accomplished.

Abstract: A wild frequency avionic refrigeration system and a controller therefore are provided. One embodiment of the refrigeration system includes: a refrigeration LRU including a vapor cycle system with a brushless DC compressor motor, a brushless DC condenser motor, a brushless DC evaporator motor and a plurality of sensors configured to output operating parameter data relative to the vapor cycle system; a power module configured to convert a wild frequency AC input voltage to at least one DC output voltage; a motor control module in communication with the brushless DC compressor, condenser and evaporator motors; and a processing module in communication with the plurality of sensors and the motor control module, wherein the processing module, according to the operating parameter data, outputs control signals to the motor control module for independently driving the brushless DC compressor, condenser and evaporator motors.

Abstract: An apparatus for calculating torque of a variable capacity compressor, including: a sensor configured to detect internal and external states of an air conditioner; an OFF-torque calculator configured to calculate and store a steady-state torque according to a state detected by the sensor just before a clutch is turned off; a start torque calculator configured to calculate a start torque according to a state detected by the sensor after the clutch is turned on; a steady-state full-stroke calculator configured to calculate a steady-state full-stroke torque based on an assumption that the compressor was in a full-stroke-state according to a state detected by the sensor after the clutch is turned on; and a determiner configured to provide, when an elapsed time after the clutch is turned on is less than a predetermined time, a maximum one of the torque values calculated by the OFF-torque calculator, start torque calculator, and steady-state full-stroke calculator.

Abstract: A compressor is controlled by generating and storing a compressor operation log. In addition, a compressor operation is selected from the compressor operation log in response to a sensor failure. Furthermore, the compressor is modulated according to the selected compressor operation in response to the sensor failure.

Abstract: A vehicle air conditioner that can increase the range of energy-saving operation by stopping an engine without impairing the air conditioning comfort and a method for controlling the vehicle air conditioner are provided.

Abstract: In an air-conditioner control unit, when pollen mode control is set by a vehicle occupant to perform the pollen mode control that can provide a defogging effect while maintaining the pollen removal effect, the opening of an air inlet door is forcibly set to an inside-air introduction mode (S330) when the following two conditions hold: ambient temperature Tam is higher than a threshold (5 to 6° C.) set in a low-temperature region (S320), and a compressor system is able to operate normally without causing an abnormal pressure (S330). If even one of these conditions does not hold, the opening of the air inlet door is not forcibly set to the inside-air introduction mode, and the air inlet door is operated as in normal control. In this way, in the pollen mode control, since the pollen removal effect in the inside-air introduction mode is enabled only when the ambient air is relatively high, the occurrence of windshield fogging can be suppressed.

Abstract: An improved air-source heat pump for residential and commercial use, employing two closed refrigerant systems having different refrigerants in cascade relationship to each other to address efficiency and space concerns, with the first closed refrigerant system partitionable into a first sub-system and a second sub-system, with the first sub-system working in conjunction with the second closed refrigerant system in heating mode and the second sub-system working independently in cooling mode.

Abstract: A compressor drive torque estimating device suppresses a discrepancy between an estimated drive torque, which is based on a torque estimating device of a compressor, and an actual drive torque of the compressor. The discrepancy is due to delay in switching between employment of the torque estimating device and the actual drive torque of the compressor in judging the compressor torque. Thus, even in a transitional state immediately after the start of compression, the idling speed of an engine driving the compressor is controlled based on an accurate estimate of the compressor torque, to improve the stability of the engine idle speed.

Abstract: A refrigerant system is provided that includes a compressor, a motor for driving the compressor, a refrigerant system component, and a controller. The refrigerant system component can operate in a pulse width modulated mode having a loaded phase and an unloaded phase. The compressor motor is loaded in the loaded phase but unloaded in the unloaded phase. The controller applies a first voltage to the compressor motor in the loaded phase and a second voltage to the compressor motor in the unloaded phase. Here, the second voltage is less than the first voltage.

Abstract: A compressor capacity control operation mechanism includes a pilot valve (flow channel switching valve) having capillary tubes, a suction branching pipe, an intermediate pipe, and a discharge branching pipe. The suction branching pipe is connected to a first capillary tube and branches off from a compressor suction pipe. The intermediate pipe is connected to a second capillary tube and a compressor cylinder intermediate part. The discharge branching pipe is connected to a third capillary tube and branches off from a compressor discharge pipe. Preferably, the suction, intermediate and discharge branching pipes have larger diameters than the first second and third capillary tubes. Also, the pilot valve preferably has a flow channel configuration switchable between first and second states in which the first, second and third capillary tubes are connected differently in the first and second states.

Abstract: A motor vehicle having a solar air conditioning system which is entirely independent of the vehicle propulsion system. The vehicle comprises a body, at least two axles, a propulsive power plant, and a first electrical system serving the power plant, lighting, and other support functions of the motor vehicle. The air conditioning system has a refrigerant compressor including an electric motor, and a second electrical system comprising a battery, a photovoltaic element which charges the battery of the air conditioning system, and an on-off switch controlling the motor of the refrigerant compressor. The first electrical system is electrically unconnected to the second electrical system of the motor vehicle. The battery of the solar air conditioning system may be located proximate one axle and the refrigerant compressor may be located proximate the other axle.

Abstract: A vehicle air conditioner is controlled so a target control value of a pressure control valve controlling tilt angle of a swash-plate of a swash-plate type variable capacity compressor is variably controlled according to temperature deviation between target evaporator temperature and actual evaporator temperature, thereby controlling the compressor discharge capacity. Target heat discharge quantity is calculated. Target heat discharge quantity is calculated for high thermal load control or low thermal load control. The control value of the pressure control valve is compulsively controlled. Then the control value is normally controlled. The actual evaporator temperature can reach the target evaporator temperature at an initial operating stage, and temperature stability is secured.

Abstract: The present invention related to cooling systems. More specifically, the present invention relates to a system for cooling one or more parts of a building and or processes comprising a compressor, a condenser having an inlet and an outlet and a plurality of condenser cooling fans, a receiver, a liquid refrigerant pump, an expansion device, an evaporator, the evaporator being surrounded by a chiller barrel having a cooled water return and a chilled water supply line, and refrigerant line means interconnecting the compressor, condenser, expansion device and evaporator in series, in a closed loop for circulating refrigerant therethrough; means for switching the cooling system between free cooling and mechanical cooling and means for actively floating the head pressure in both mechanical and free cooling modes.

Abstract: A turbo chiller equipped with a high-efficiency two-stage turbo compressor is provided. In a turbo chiller including a control unit for controlling the degrees of opening of first inlet guide vanes of a first impeller and second inlet guide vanes of a second impeller, the control unit has a slave mode in which the second inlet guide vanes are operated so as to be dependent on the first inlet guide vanes in a slave-mode priority region, and an independent mode in which the degree of opening of the second inlet guide vanes is increased independently of the first inlet guide vanes in an independent-mode priority region.

Abstract: A vapor compression refrigeration system including components capable of determining the superheat at a compressor inlet is provided. The vapor compression refrigeration system may include sensors capable of making measurements from which the superheat at the compressor inlet may be determined. The vapor compression refrigeration system may be operable to compare the determined superheat level at the inlet to the compressor to a desired superheat level and generate a new evaporator discharge superheat level target for one or more evaporators operatively interconnected to the compressor to affect the superheat at the compressor inlet. The vapor compression refrigeration system may be operable to broadcast the new evaporator discharge superheat level target to the one or more evaporators over a communications bus. The vapor compression refrigeration system may update and broadcast the evaporator discharge superheat level target at programmed intervals.

Abstract: A system includes a scroll compressor having a first scroll member interleaved with a second scroll member to define at least two moving fluid pockets that decrease in size as they move from a radially outer position to a radially inner position. A valve in fluid communication with the compression chamber is movable between an open state allowing fluid to the compression chamber and a closed state preventing fluid to the compression chamber. A controller continuously cycles the valve between the open state and the closed state at a rate that is faster than a thermal time constant of a load on the system.

Abstract: A motor controller (101) according to the invention comprises an inverter circuit (3) for driving a brushless motor (4) and a control unit for controlling the rotational speed of the brushless motor (4) by controlling the phase of the motor current of the brushless motor (4) through the inverter circuit (3).

Abstract: In an automotive air-conditioning system, when pollen mode control is effected by a vehicle occupant, air outlet mode is forcibly set to a face mode. Further, the air flow amount determined from the air flow amount Vm used in the usual auto mode and the air flow amount f(Tw) used during warm-up control or the air flow amount determined by the air flow amount correction value f(Tam) having a relatively small upper limit value when ambient temperature is higher than a predetermined temperature (15° C.), whichever is larger, is taken as the set value for the air flow amount in the pollen mode control. With this arrangement, when coolant temperature and ambient temperature are low, a relatively small amount of air can be blown to the upper part of the occupant's body, and thus, while maintaining the pollen removal effect, occupant discomfort can be reduced by preventing a fast flow of air from being blown toward the occupant's face, etc.

Abstract: A cooling device for printing machines, includes a refrigerating circuit (10) and a cooling water circuit (12), in which the refrigerating circuit (10)-is provided with a compressor (14) having a compressor drive and a regulator (34). The compressor is configured such that the compressor output of the compressor (14) can be controlled via the regulator independently of the rotational speed of the compressor drive.

Abstract: A diagnostic system includes a controller adapted for coupling to a compressor or electronic stepper regulator valve. The controller produces a variable duty cycle control signal to adjust the capacity of the compressor or valve position of the electronic stepper regulator valve as a function of demand for cooling. The diagnostic system further includes a diagnostic module coupled to the controller for monitoring and comparing the duty cycle with at least one predetermined fault value indicative of a system fault condition and an alert module responsive to the diagnostic module for issuing an alert signal when the duty cycle bears a predetermined relationship to the fault value.