System and method for controlling fluid flow and temperature within a pumped two-phase cooling distribution unit

Abstract

A control system for a cooling system utilizing a two-phase refrigerant includes a cooling distribution unit (CDU) having a CDU refrigerant supply line for supplying the refrigerant to at least one evaporator, and a CDU two-phase return line for returning the refrigerant to the CDU. A controller is operatively coupled to the CDU, the controller including a first control loop configured to maintain a constant pressure differential across the CDU refrigerant supply line and the CDU two-phase return line.

1 . A control system for a cooling system utilizing a two-phase refrigerant, comprising:
 a cooling distribution unit (CDU) including a CDU refrigerant supply line ( 19   b ) for supplying the refrigerant to at least one evaporator, CDU two-phase return line for returning the refrigerant to the CDU, a refrigerant pumping portion having at least two pumps, and a bypass valve, wherein the refrigerant pumping portion and bypass valve are in fluid communication with each other, and the bypass valve is operative to divert refrigerant flow from the at least two pumps away from the at least one evaporator; and   a controller operatively coupled to the CDU, the controller comprising a first control loop configured to maintain a constant pressure differential across the CDU refrigerant supply line and the CDU two-phase return line by adjusting a pump speed of the at least two pumps and a position of the bypass valve, said pump speed of each of the at least two pumps being controlled via a single PID output, wherein a transfer equation corresponding to a number of the at least two pumps is applied to the single PID output to limit a speed of each of the at least two pumps.   
     
     
         2 . The control system according to  claim 1 , wherein the CDU comprises:
 an accumulator tank in fluid communication with the   refrigerant pumping portion and bypass valve,   wherein the bypass valve is operative to divert refrigerant flow from the at least one two pumps away from the at least one evaporator and into the accumulator tank,   
     
     
         3 . The control system according to  claim 1 , wherein when refrigerant demand is below a predetermine threshold, the controller is configured to command the at least two pumps to operate at a predetermined minimum speed, and to regulate the CDU pressure differential via the bypass valve. 
     
     
         4 . The control system according to  claim 3 , wherein when the bypass valve is commanded to bypass a minimum amount of refrigerant the controller is configured to command the at least two pumps to vary a speed to regulate the CDU pressure differential. 
     
     
         5 . The control system according to  claim 3 , wherein the controller is configured to place one of the plurality of pumps in a standby mode and use another of the plurality of pumps to regulate the CDU pressure differential. 
     
     
         6 . The control system according to  claim 3 , wherein the controller is configured to command each of the plurality of pumps to operate at a reduced speed to regulate the CDU pressure differential. 
     
     
         7 . The control system according to  claim 6 , wherein the controller is configured to use an output from a single PID controller to command the plurality of fluid pumps. 
     
     
         8 . The control system according to  claim 2 , wherein the controller further comprises a second control loop configured to
 detect cavitation at the at least two pumps, and   automatically vary a speed of the at least two pumps upon detection of cavitation.   
     
     
         9 . The control system according to  claim 8 , wherein the second control loop is configured to detect cavitation by:
 converting refrigerant pressure at an inlet of the at least two pumps to a saturation temperature;   comparing the saturation temperature to an actual refrigerant temperature at the inlet of the at least two pumps to acquire an amount of subcool in the refrigerant;   determining a minimum amount of subcool in the refrigerant that provides cavitation-free operation for a given pump speed; and   determining cavitation is present based on a comparison of the amount of subcool in the refrigerant and the minimum amount of subcool.   
     
     
         10 . The control system according to  claim 1 , further comprising:
 a condenser in fluid communication with the two-phase return line;   a fluid valve in fluid communication with the condenser and operative to provide a coolant to the condenser for cooling the refrigerant,   wherein the controller includes a third control loop configured to regulate a temperature of the refrigerant by varying a coolant flow through the condenser.   
     
     
         11 . The control system according to  claim 1 , further comprising:
 a condenser in fluid communication with the two-phase return line;   a cooling device operative to provide a cooling medium to the condenser for cooling the refrigerant,   wherein the controller includes a third control loop configured to regulate a temperature of the refrigerant by varying a flow of the cooling medium through the condenser.   
     
     
         12 . The control system according to  claim 10 , wherein the third control loop is configured to:
 generate a refrigerant temperature reference based on a refrigerant temperature setpoint and a tolerance value;   convert the refrigerant temperature reference to a corresponding pressure reference; and   adjust coolant flow through the fluid valve based on a comparison of the corresponding pressure reference with an actual pressure at the CDU two-phase return line.   
     
     
         13 . The control system according to  claim 12 , wherein the controller comprises a fourth control loop configured to vary the refrigerant temperature setpoint based on a comparison of an ambient dew point and an actual refrigerant temperature. 
     
     
         14 . The control system according to  claim 13 , wherein the fourth control loop is configured to:
 calculate the ambient dew point based on ambient temperature and ambient humidity;   calculate a dew point reference based on a predetermined threshold value added to the calculated ambient dew point; and   activate a dew point fault when the fluid temperature is below the dew point reference.   
     
     
         15 . The control system according to  claim 14 , wherein when a dew point fault is active, the fourth control loop is configured to increase the refrigerant temperature setpoint by a predetermined value, and recalculate the ambient dew point. 
     
     
         16 . The control system according to  claim 15 , wherein when a dew point fault is active and the refrigerant temperature is above the dew point reference, the fourth control loop is configured to decrease the refrigerant temperature setpoint by a predetermined value, and recalculate the ambient dew point. 
     
     
         17 . A method for controlling a cooling system utilizing a two-phase refrigerant, the cooling system including a cooling distribution unit (CDU) having a CDU refrigerant supply line for supplying the refrigerant to at least one evaporator, a CDU two-phase return line for returning the refrigerant to the CDU, a refrigerant pumping portion having at least two pumps, and a bypass valve operative to divert refrigerant flow from the at least two pumps away from the at least one evaporator, the method comprising maintaining a constant pressure differential across the CDU refrigerant supply line and the CDU two-phase return line by adjusting a speed of the at least two pumps and a position of the bypass valve, wherein adjusting a speed of the at least two pumps includes generating a single PID output for controlling a speed of each of the at least two pumps and applying a transfer equation corresponding to a number of the at least two pumps to the PID output to limit a speed of each of the at least two pumps. 
     
     
         18 . (canceled) 
     
     
         19 . The method according to  claim 17  and any other claim, further comprising when refrigerant demand is below a predetermine threshold, operating the at least two pumps at a predetermined minimum speed using the bypass valve to regulate the CDU pressure differential. 
     
     
         20 . The method according to  claim 17  and any other claim, further comprising:
 detecting the occurrence of cavitation at the at least two pumps; and 
 automatically varying a speed of the at least two pumps upon detection of cavitation, wherein automatically varying the speed includes 
 converting refrigerant pressure at an inlet of the at least two pumps to a saturation temperature; 
 comparing the saturation temperature to an actual refrigerant temperature at the inlet of the at least two pumps to acquire an amount of subcool in the refrigerant; 
 determining a minimum amount of subcool in the refrigerant that provides cavitation-free operation for a given pump speed; and 
 determining cavitation is present based on a comparison of the amount of subcool in the refrigerant and the minimum amount of subcool. 
 
     
     
         21 . A controller for controlling a two-phase cooling system, comprising:
 a processor and memory; and   logic stored in the memory and executable by the processor, the logic when executed by the processor configured to cause the processor to carry out the method according to  claim 17 .