Related Applications

This application claims the benefit of U.S. Provisional Application No. 61/586,863 filed January 16, 2012, which is hereby incorporated herein by reference.

Field of Invention

The present invention relates generally to liquid cooling systems, and more particularly to pumped liquid cooling systems having parallel evaporator circuits.

Background

Electronic devices, such as computer servers, generate heat during operation, and therefore a cooling system is used to cool the electronic devices. For example, air may be blown over the electronic devices or a water-based fluid may be circulated through a heat exchanger coupled to the electronic devices to cool the devices.

In a data center that houses racks of servers, the amount of air or water- based fluid required for heating may vary from rack to rack. The varied heat may arise from some racks having a greater density of electronic equipment or electric equipment operating at higher levels.

Summary of Invention

The present invention provides a pumped loop system for cooling a heat- generating components without relying on a maximum heat load, the system including first and second evaporators in parallel with one another and first and second valves upstream of the first and second evaporators respectively, wherein the valves are controllable to vary the flow rate of fluid to the respective evaporator based on the amount of flow needed to control the respective heat- generating component. By cooling the heat-generating components without relying on the maximum heat load, adequate flow may be provided to an evaporator operating under a high heat low while reduced flow is provided to an evaporator operating under a low head load.

According to one aspect of the invention, a pumped loop system for cooling heat-generating components is provided including a first branch having a first evaporator for absorbing heat from a first heat-generating component having a variable heat load and a first valve upstream of the first evaporator for variably controlling the flow of fluid to the first branch, a second branch parallel to the first branch, the second branch having a second evaporator for absorbing heat from a second heat-generating component having a variable heat load and a second valve upstream of the second evaporator for variably controlling the flow of fluid to the second branch, a pump for pumping fluid to the first and second branches, and a condenser downstream of the first and second circuits and upstream of the pump for rejecting the heat absorbed by the fluid in the first and second branches.

The first and second valves are electronic stepper valves.

The system further includes at least one controller configured to control at least one of the first valve and the second valve.

The at least one controller controls fluid flow through the respective branch based upon an input that is indicative of the amount of flow needed to cool the heat-generating component.

The input is one or more of (i) the amount of electrical power being consumed by the heat-generating component; (ii) the temperature of the heat- generating component; (iii) the temperature of the heat-generating component as correlated to the amount of electrical power being consumed by the heat- generating component; and/or (iv) the amount of heat being generated by the heat-generating component.

The system further includes a sensor downstream of the first evaporator and a sensor downstream of the second evaporator, wherein the sensors sense a characteristic of the fluid flow through the first and second branches

respectively and communicate the characteristics to the controller to increase or decrease the fluid flow through the respective branches.

The characteristic is an amount of vapor in the fluid flow. According to another aspect of the invention, a method for cooling heat- generating components via a pumped loop system is provided, the system including a first branch having a first evaporator coupled to a first heat- generating component and a first valve upstream of the first evaporator, a second branch parallel to the first branch, the second branch having a second evaporator coupled to a second heat-generating component and a second valve upstream of the second evaporator, a pump upstream of the branches, and a condenser downstream of the branches. The method includes pumping fluid from the pump to the first and second branches, controlling the first and second valves via a controller to vary the flow of fluid to the first and second branches, and absorbing heat from the first and second heat-generating components via the evaporators.

According to still another aspect of the invention, a pumped loop system for cooling heat-generating components includes first and second evaporators in parallel with one another, first and second valves upstream of the first and second evaporators respectively, a pump configured to pump fluid to the first and second evaporators, and a condenser downstream of the first and second circuits and upstream of the pump, the condenser configured to reject the heat absorbed by the first and second evaporators, wherein the first and second valves are controllable to vary the flow rate of fluid to the respective evaporator based on the heat load at the respective evaporator.

The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings. Brief Description of the Drawings

Fig. 1 is a schematic view of an exemplary cooling system according to the invention;

Fig. 2 is a control diagram of the cooling system of Fig. 1 ;

Fig. 3 is a schematic view of another exemplary cooling system according to the invention;

Fig. 4 is a control diagram of the cooling system of Fig. 3;

Fig. 5 is a schematic view of yet another exemplary cooling system according to the invention; Fig. 6 is a control diagram of the cooling system of Fig. 5;

Fig. 7 is a schematic view of still another exemplary cooling system according to the invention;

Fig. 8 is a control diagram of the cooling system of Fig. 7;

Fig. 9 is a schematic view of a further exemplary cooling system according to the invention;

Fig. 10 is a control diagram of the cooling system of Fig. 9;

Fig. 1 1 is a schematic view of another exemplary cooling system according to the invention;

Fig. 12 is a control diagram of the cooling system of Fig. 1 1 ;

Fig. 13 is a schematic view of yet another exemplary cooling system according to the invention; and

Fig. 14 is a control diagram of the cooling system of Fig. 13; Detailed Description

The principles of the present application have particular application to systems and methods for cooling heat-generating components such as racks of servers in data centers, and thus will be described below chiefly in this context. It will of course be appreciated, and also understood, that the principles of the invention may be useful in other cooling applications, such as cooling industrial equipment having a plurality of parallel drives.

Turning now in detail to the drawings and initially to Fig. 1 , a schematic representation of a pumped loop cooling system is illustrated generally at reference numeral 10. The pumped loop cooling system is provided to cool heat-generating components using a pumped fluid, such as a two-phase liquid refrigerant. When more than one heat-generating component is provided to be cooled, the pumped loop cooling system includes at least two branches in parallel, and in the illustrated embodiment a plurality of branches 12a-12n in parallel. Each branch has a respective evaporator 14a-14n for absorbing heat from a respective heat-generating component having a variable heat load. The evaporators may be any suitable heat absorbing device, such as a coiled tube surrounding the heat-generating component, a cold plate touching the heat- generating component, a liquid to refrigerant heat exchanger, an air to refrigerant evaporator, etc.

The liquid refrigerant is pumped to the evaporators'! 4a-14n by a pump 16 upstream of the evaporators. After absorbing heat from the heat-generating components, the refrigerant exits each evaporator 14a-14n, combines with the two-phase refrigerant from the other evaporators, and flows to a condenser 18 downstream of the evaporators'! 4a-14n and upstream of the pump 16 where the heat absorbed by the fluid is rejected and condensed back to a liquid. The fluid in the condenser 18 may be cooled in any suitable manner, such as by blowing air across the condenser 18 or flowing chilled water through the condenser 18. The fluid is then received in an accumulator 20 that delivers the fluid back to the pump 16. The accumulator 20 may act as a storage tank to compensate for varying volumes of the fluid in the system.

The flow of the liquid refrigerant to the evaporators 14a-14n is varied by respective valves 22a-22n upstream of the evaporators. The valves may be any suitable valve, such as electronic stepper valves configured to variably control the flow of fluid to the respective evaporators 14a-14n. The valves 22a-22n have a plurality of positions from closed to fully open to adjust the flow rate through each branch 12a-12n to balance flow to the respective evaporators 14a- 14n based on the amount of flow need to cool the respective heat-generating component. Thereby, if one of the branches is not operating at full heat load, the flow to that branch can be reduced.

The valves 22a-22n are controlled by at least one controller, and in the illustrated embodiment by a respective controller 24a-24n. The controllers 24a- 24n control the fluid flow through the respective branch 12a-12n based upon an input that is indicative of the amount of flow needed to cool the heat-generating component. The input may be determined based on one or more of the amount of electrical power being consumed by the heat-generating component, the temperature of the heat-generating component, the temperature of the heat- generating component as correlated to the amount of electrical power being consumed by the heat-generating component or to its percentage of the full load, the amount of heat being generated by the heat-generating component, etc. By cooling the heat-generating components without relying on the maximum heat load, adequate flow may be provided to a branch operating under a high heat low while reduced flow is provided to a branch operating under a low head load. During such an operation, the pump does not have to be operated constantly at full flow, thereby reducing energy usage, operating costs, and pump wear.

Factors other than the above described inputs may also affect the flow through the evaporators and thus can be communicated to the controllers 24a- 24n. These factors may include other valves within the circuit, pluggable rack configurations having varying evaporator design restrictions, etc. Based on the combination of factors, such as heat load and evaporator design, the controllers 24a-24n can determine whether the valves 22a-22n need to be adjusted.

To assist in flow balancing, the pumped loop system 10 also may include a mechanical pressure differential valve 26 downstream of the pump 16 before the valves22a-22n. The mechanical pressure differential valve 26 allows the system to maintain a constant pressure upstream of the valves 22a-22n regardless of the varying heat load conditions at the branches 12a-12n.

The mechanical pressure differential valve 26 includes an inlet that is connected to an outlet of the pump 16 and an outlet that is connected to the accumulator 20. The mechanical pressure differential valve 26 is spring biased to a closed position and is movable to an open position when the pressure differential between the pump outlet and pump inlet is a predetermined amount. For example, when the pump outlet is a predetermined amount greater than the pump inlet, such as when the heat load conditions at the branches 12a-12n is low, the pressure of the fluid exiting the pump moves the mechanical pressure differential valve 26 to the opened position. Thereby the flow from the pump that is not required by the branches 12a-12n flows to the accumulator 20. A filter 28, which may be any suitable filter, may be provided downstream of the pump to filter the fluid flowing from the pump 16 to the evaporators 14a-14n.

Turning now to Fig. 2, a control diagram illustrating the control of the pumped loop system 10 is shown generally at reference numeral 50. At block 52, a device exit quality setpoint from the evaporators 14a-14n is set, such as 70% vapor. At block 54, the device rejected power is provided, which is for example, the amount of power being rejected from the heat-generating

components. At block 56 a pump pressure differential setpoint is set, which is the desired pressure of the pump of the system. In the system 10 having the mechanical pressure differential value 26, the pump pressure differential setpoint is an opening pressure of the valve 26. The information from blocks 52-56 is fed into block 58, where the position of the valve is determined. Specifically, the device exit quality setpoint is compared to the device rejected power, and the comparison and the desired pump pressure flowing through the branches 12a- 12n is used to determine the valve position. If the device rejected power is low, to maintain the device exit quality, at block 60 the valve position can be maintained or at least partially closed, and then the valve position from block 60 is fed back into block 58. If the device rejected power is high, to maintain the device exit quality, at block 60 the valve position can be maintained or at least partially opened, and then the valve position from block 60 is fed back into block 58. If the pressure in the system is greater than the pump pressure differential setpoint, the excess pressure would be dumped to the accumulator.

Turning now to Figs. 3 and 4, an exemplary embodiment of the pumped loop cooling system is shown at 1 10. The pumped loop cooling system 1 10 is substantially the same as the above-referenced pumped loop cooling system 10, and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling system 10 is equally applicable to the pumped loop cooling system 1 10 except as noted below.

Moreover, it will be appreciated upon reading and understanding the

specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 3, to assist in flow balancing, the pumped loop system 1 10 may include an electronic variable control valve 126 downstream of the pump 1 16 before the valves 122a-122n for maintaining a constant pressure upstream of the valves 122a-122n. The electronic variable control valve 126 allows the system to maintain a constant pressure upstream of the valves 122a- 122n regardless of the varying heat load conditions at the branches 1 12a-1 12n. The electronic variable control valve 126 includes an inlet that is connected to an outlet of the pump 1 16 and an outlet that is connected to the accumulator 120. The electronic variable control valve 126 is also connected to a suitable controller 130 that controls the valve 126 to increase or decrease the fluid flowing through the valve from the pump 1 16. The controller 130 controls the electronic variable control valve 126 based on pressure readings from pressure sensors 132 and 134 operatively coupled to the controller 130. The pressure sensors 132 and 134 are upstream and downstream of the pump 1 16, respectively for measuring the pressure at the inlet and outlet of the pump. The controller calculates the pressure differential between the inlet and outlet of the pump and opens or closes the electronic variable control valve 126 based upon the differential pressure. The controller is also operatively coupled to the pump 1 16 and may optionally vary the speed of the pump based on the differential pressure.

If the difference in pressure reading measured by the pressure sensors

134 and 132 respectively is greater than the required setpoint, the controller, which is operatively coupled to the pump 1 16, may slow down the pump.

Additionally or alternatively, if the difference in pressure measured by the pressure sensors 134 and 132 respectively is greater than the required setpoint, the controller 130 controls the electronic variable control valve 126 to open for the fluid to flow to the accumulator 120. This may occur, for example, when the pump 1 16 is at a minimum idle speed but flow at the outlet is still higher than flow at the inlet.

Turning now to Fig. 4, a control diagram illustrating the control of the pumped loop system 1 10 is shown generally at reference numeral 150. At block 152, a device exit quality setpoint from the evaporators 1 14a-1 14n is set, such as 70% vapor. At block 154, the device rejected power is provided, which is for example, the amount of power being rejected from the heat-generating components. At block 156 a pump pressure differential setpoint is set, which is the desired pressure of the pump of the system. The information from blocks 152-156 is fed into block 158, where the position of the valve is determined. Specifically, the device exit quality setpoint is compared to the device rejected power, and the comparison and the desired pump pressure flowing through the branches 12a-12n is used to determine the valve position.

If the device rejected power is low, to maintain the device exit quality, at block 160 the valve position is maintained or at least partially closed, and then the valve position from block 160 is fed back into block 158. If the device rejected power is high, to maintain the device exit quality, at block 160 the valve position is maintained or at least partially opened, and then the valve position from block 160 is fed back into block 158. At blocks 162 and 164, the pump inlet pressure and pump outlet pressure are measured, respectively. The inlet and outlet pressures are fed into block 166, which calculates the pressure differential. The pressure differential information is fed into block 168, along with the pump differential setpoint, and the pump speed is adjusted and/or excess pressure is dumped to the accumulator 120. Information from block 168 is then fed back into block 166.

Turning now to Figs. 5 and 6, an exemplary embodiment of the pumped loop cooling system is shown at 210. The pumped loop cooling system 210 is substantially the same as the above-referenced pumped loop cooling system 10, and consequently the same reference numerals but indexed by 200 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling system 10 is equally applicable to the pumped loop cooling system 210 except as noted below.

Moreover, it will be appreciated upon reading and understanding the

specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 5, the pumped loop cooling system 210 includes a common controller 240 coupled to the valves 222a-222n and the evaporators 214a-214n. The common controller 240 controls the position of each of the valves 222a-222n in a similar manner as the controllers 24a-24n control the valves 22a-22n. The common controller 240 may also be coupled to pressure sensors 232 and 234 that are upstream and downstream of the pump 216 respectively for measuring the pressure at the inlet and outlet of the pump. The information from the sensors 232 and 234, such as pressure differential, may be used in adjusting the valves 222a-222n. Turning now to Fig. 6, a control diagram illustrating the control of the pumped loop system 210 is shown generally at reference numeral 250. At block 252, a device exit quality setpoint from the evaporators 214a-214n is set, such as 70% vapor. At block 254, the device rejected power is provided, which is for example, the amount of power being rejected from the heat-generating components. At blocks 262 and 264, the pump inlet pressure and pump outlet pressure are measured, respectively. The inlet and outlet pressures are fed into block 266, which calculates the pressure differential. The information from blocks 252, 254, and 266 is fed into block 258 where the position of the valve is determined. Specifically, the device exit quality setpoint is compared to the device rejected power, and the comparison and the pump pressure differential is used to determine the valve position.

If the device rejected power is low, to maintain the device exit quality, at block 260 the valve position is maintained or at least partially closed, and then the valve position from block 260 is fed back into block 258 and 266. If the device rejected power is high, to maintain the device exit quality, at block 260 the valve position is maintained or at least partially opened, and then the valve position from block 260 is fed back into block 258 and 266.

Turning now to Figs. 7 and 8, an exemplary embodiment of the pumped loop cooling system is shown at 310. The pumped loop cooling system 310 is substantially the same as the above-referenced pumped loop cooling system 1 10, and consequently the same reference numerals but indexed by 200 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling system 10 and 1 10 is equally applicable to the pumped loop cooling system 310 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 7, the pumped loop cooling system 310 includes a common controller 340 coupled to the valves 322a-322n and the evaporators 314a-314n. The common controller 340 controls the position of each of the valves 322a-322n in a similar manner as the controllers 124a-124n control the valves 122a-122n. The common controller 340 may also be coupled to the pressure sensors 332 and 334 that are upstream and downstream of the pump 1 16, respectively for measuring the pressure at the inlet and outlet of the pump. The information from the sensors 332 and 334, such as pressure differential, may be used in adjusting the valves 222a-222n and the valve 326.

Turning now to Fig. 8, a control diagram illustrating the control of the pumped loop system 310 is shown generally at reference numeral 350. At block 352, a device exit quality setpoint from the evaporators 314a-314n is set, such as 70% vapor. At block 354, the device rejected power is provided, which is for example, the amount of power being rejected from the heat-generating components. At blocks 362 and 364, the pump inlet pressure and pump outlet pressure are measured, respectively. The inlet and outlet pressures are fed into block 366, which calculates the pressure differential. The information from blocks 352, 354, and 366 is fed into block 358 where the position of the valve is determined. Specifically, the pressure differential and a comparison of the device exit quality setpoint and the device rejected power are used to open/close the valves 322a-322n.

If the device rejected power is low, to maintain the device exit quality, at block 360 the valve position is maintained or at least partially closed. If the device rejected power is high, to maintain the device exit quality, at block 360 the valve is maintained or at least partially opened. The information from block 360 is then fed into block 370 where the pump differential pressure is calculated again, and then the information from block 370 is fed into block 368 along with the pump differential setpoint from block 356. At block 368 the pump speed is adjusted and/or excess pressure is dumped to the accumulator 320, and then information from block 368 is then fed back into block 366.

Turning now to Figs. 9 and 10, an exemplary embodiment of the pumped loop cooling system is shown at 410. The pumped loop cooling system 410 is substantially the same as the above-referenced pumped loop cooling system 310, and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling systems 10 and 310 is equally applicable to the pumped loop cooling system 410 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 9, the pumped loop cooling system 410 include a pressure sensor 442 that is upstream of the valves 422a-422n and downstream of the pump 416, and a pressure sensor 444 that is downstream of the evaporators 414a-414n and upstream of the condenser 418. The pressure sensors 442 and 444 are coupled to the common controller 440 and are configured to measure the pressure of the fluid at a common manifold before the fluid reaches the valves 422a-422n and at a common manifold after the fluid has absorbed the heat from the heat-generating devices. The valves 422a-422n are adjusted as discussed above regarding Fig. 1 , and may be further adjusted based on an available pressure differential from the manifolds calculated by the controller 240. For example, the pressure from the pump outlet may not remain constant upstream of the valves 422a-422n, for example when components upstream of the valves change the pressure of the fluid. To ensure that the valves are open/closed sufficient amounts to maintain cooling, the pressure from the pressure sensors 442 and 444 is fed to the common controller 440 and the controller 400 adjusts the valves 422a-422n based on the pressure in addition to the above described inputs.

Turning now to Fig. 10, a control diagram illustrating the control of the pumped loop system 410 is shown generally at reference numeral 450. At block 452, a device exit quality setpoint from the evaporators 414a-414n is set, such as 70% vapor. At block 454, the device rejected power is provided, which is for example, the amount of power being rejected from the heat-generating components. At blocks 472 and 474, the inlet and outlet manifold pressure is measured, which is the pressure measured by pressure sensors 442 and 444. The inlet and outlet manifold pressures are fed into block 476, which calculates the pressure differential. The information from blocks 452, 454, and 476 is fed into block 458 where the position of the valve is determined. Specifically, the pressure differential and a comparison of the device exit quality setpoint and the device rejected power are used to open/close the valves 422a-422n.

If the device rejected power is low, to maintain the device exit quality, at block 460 the valve position is maintained or at least partially closed. If the device rejected power is high, to maintain the device exit quality, at block 460 the valve is maintained or at least partially opened. At blocks 462 and 464, the pump inlet pressure and pump outlet pressure are measured, respectively, such as by pressure sensors 432 and 434. The inlet and outlet pressures are fed into block 466, which calculates the pressure differential. The information from block 466 is then fed into block 470, where the pump differential pressure is calculated again, and then the information from block 470 is fed into block 468 along with the pump differential setpoint from block 456. At block 468 the pump speed is adjusted and/or excess pressure is dumped to the accumulator 420, and then information from block 468 is then fed back into block 476.

Turning now to Figs. 1 1 and 12, an exemplary embodiment of the pumped loop cooling system is shown at 510. The pumped loop cooling system 510 is substantially the same as the above-referenced pumped loop cooling system 10, and consequently the same reference numerals but indexed by 500 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling systems 10 is equally applicable to the pumped loop cooling system 510 except as noted below.

Moreover, it will be appreciated upon reading and understanding the

specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 1 1 , the pumped loop cooling system 510 includes respective sensors 546a-546n downstream of the respective evaporators 514a- 514n. The sensor may be any suitable sensor, such as a sensor configured to monitor a percent of vapor in the fluid output from the evaporators utilizing, for example, one or more optical, ultrasonic, temperature or pressure sensors. The sensors 546a-546n are coupled to the respective controllers 524a-524n and configured to sense a characteristic of the fluid flow through the respective branches 512a-512n, such as an amount of vapor in the fluid flow, and communicate the characteristics to the respective controllers 524a-524n to adjust the valves based on a feedback loop to increase or decrease the fluid flow through the respective branches 512a-512n.

If the amount of vapor in a branch 512a-512n is greater than a

predetermined amount, the respective controller 524a-524n controls the respective valve 522a-522n to open a predetermined amount to allow for more flow through the respective evaporator 514a-514n. If the amount of vapor is less than a predetermined amount, the respective controller 524a-524n controls the respective valve 522a-522n to close a predetermined amount to allow for less flow through the respective evaporator 514a-514n.

Turning now to Fig. 12, a control diagram illustrating the control of the pumped loop system 510 is shown generally at reference numeral 550. At block 552, a device exit quality setpoint from the evaporators 514a-514n is set, such as 70% vapor. At block 554, the device exit quality from the sensors 546a-546n is provided. The information from blocks 552 and 554 is fed into block 558, where the device exit quality setpoint is compared to the device exit quality and the position of the valve is determined. If the device exit quality is higher than the device exit quality setpoint, at block 560 the valve is at least partially opened, and then the valve position from block 560 is fed back into block 558. If the device exit quality is lower than the device exit quality setpoint, at block 560 the valve is at least partially closed, and then the valve position from block 560 is fed back into block 558.

Turning now to Figs. 13 and 14, an exemplary embodiment of the pumped loop cooling system is shown at 610. The pumped loop cooling system 610 is substantially the same as the above-referenced pumped loop cooling system 510, and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the pumped loop cooling systems 1 10 and 510 are equally applicable to the pumped loop cooling system 610 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable.

Referring now to Fig. 13, to assist in flow balancing, the pumped loop system 610 includes an electronic variable control valve 626, similar to the electronic control valve 126, downstream of the pump 616 and upstream of the valves 622a-622n for maintaining a constant pressure upstream of the valves 622a-622n. Turning now to Fig. 14, a control diagram illustrating the control of the pumped loop system 610 is shown generally at reference numeral 650. At block 652, a device exit quality setpoint from the evaporators 514a-514n is set, such as 70% vapor. At block 654, the device exit quality from the sensors 546a-546n is provided. The information from blocks 652 and 654 is fed into block 658, where the device exit quality setpoint is compared to the device exit quality and the position of the valve is determined. If the device exit quality is higher than the device exit quality setpoint, at block 660 the valve is at least partially opened, and then the valve position from block 660 is fed back into block 658. If the device exit quality is lower than the device exit quality setpoint, at block 660 the valve is at least partially closed, and then the valve position from block 660 is fed back into block 658.

At block 656 a pump pressure differential setpoint is set, which is the desired pressure of the pump of the system. At blocks 662 and 664, the pump inlet pressure and pump outlet pressure are measured, respectively. The inlet and outlet pressures are fed into block 666, which calculates the pressure differential. The pressure differential information is fed into block 668, along with the pump differential setpoint, and the pump speed is adjusted and/or excess pressure is dumped to the accumulator 620. Information from block 668 is then fed back into block 666.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.