US20130091874A1

US20130091874A1 - Variable Refrigerant Flow Cooling System

Info

Publication number: US20130091874A1
Application number: US13/438,171
Authority: US
Inventors: Stephen SILLATO; Timothy J. SCHRADER; Lou Monnier; John F. Judge
Original assignee: Liebert Corp
Current assignee: Vertiv Corp; Pixtronix Inc
Priority date: 2011-04-07
Filing date: 2012-04-03
Publication date: 2013-04-18
Also published as: GB201206177D0; GB2489825B; GB2489825A; CN202973639U

Abstract

A variable flow refrigerant system having a compressor and one or a plurality of evaporators. The suction at one or the plurality of evaporators for the input to the compressor is monitored and generally corresponds to the minimum pressure of the refrigerant. The pressure is associated with a temperature and is controlled to always be above the dew point temperature of the room.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/472,723 filed on Apr. 7, 2011. The entire disclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates to a variable refrigerant flow cooling system.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.
One way to broadly characterize technology for cooling room spaces is comfort cooling, typically for residential or office spaces occupied by persons, and industrial or electronics cooling, typically applicable to room spaces containing systems that generate significant amounts of heat. For example, electronic data centers can include server rooms, telecommunication rooms, or other spaces which house multiple electronics systems. Such systems are densely arranged with electronic equipment that generates significant amounts of heat. Cooling these spaces requires a substantially greater capacity than is typically required in a conventional residential or office space.
One configuration for comfort cooling applications, such as for residential or office spaces, utilizes variable refrigerant flow (VRF) technology. A conventional VRF system includes an outdoor unit and multiple indoor units. The outdoor unit can include compressors and condensers, while the indoor unit typically includes an expansion device, a heat exchanger, such as a microchannel heat exchanger, and a fan. The compressors are typically embodied as variable capacity compressors. Various techniques are used to control the overall capacity of the compressor based on the sum of the loads of the indoor units. The refrigerant flow is varied in order to minimize the load on the compressor and increase efficiency.
Cooling units configured to provide cooling to room spaces having high heat producing equipment are typically configured differently. In various configurations, the cooling systems may include two distinct circuits, each circuit utilizing different refrigerants and mechanical parts. A first circuit may be a pumped circuit that contains redundant circulating pumps, and in various configurations, a brazed plate heat exchanger along with accompanying valves and piping. A second circuit may be configured in a dual direct expansion circuit containing scroll compressors, expansion valves, brazed plate heat exchangers, and various piping. The brazed plate heat exchanger provides the interface between the two circuits. Heat rejection is accomplished by using condensers connected to the dual direct expansion circuit. In such conventional cooling units, dew point control is achieved by monitoring refrigerant temperature in the pumped circuit and backing off the compressors in the dual direct expansion circuit if the temperature gets too low. This effectively reduces the capacity of the dual direct expansion circuit in order to maintain a minimum refrigerant temperature in the pumped circuit. In other systems, the temperature of the pumped circuit refrigerant is controlled using the flow rate of the fluid that removes heat from the circuit. For example, various systems modulate a chilled water valve in order to maintain dew point margin of a refrigerant where the refrigerant is cooled by the building chilled water system.
In various configurations, a cooling system designed for high heat producing environments monitors room conditions and prevents condensation by maintaining the coolant being pumped to the cooling modules at a temperature above the dew point. In traditional cooling systems, the evaporator is housed in an enclosure that collects condensation and pumps it to a drain. In addition to cooling, these units are often used to dehumidify the space, in which case they are actually controlled to run at temperatures below the room dew point. These units are also usually in the area on the floor or in a mechanical room where condensation does not affect sensitive electronic equipment. In some applications for cooling concentrations of electronic equipment, the evaporators are in cooling modules located above the racks that house the electronic equipment. The piping to those cooling modules is located above the electronic equipment. Because of this, it is necessary to maintain the refrigerant temperature above the room dew point temperature so that condensation on the pipes and evaporator is prevented. Such systems are intended for sensible cooling. For at least this reason, traditional and VRF systems typically have not been considered for cooling high heat producing spaces because of the issues involved with controlling condensation.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
A variable flow refrigerant system is controlled to modulate the dew point temperature of the system in order to prevent condensation from forming in the room space to be cooled. Suction pressure of the system is monitored at one or a plurality of suction lines. The suction pressure corresponds to the lowest refrigerant temperature in the system and will indicate if the room dew point is being approached. If the load on one or more of the evaporators increases, the outlet superheat will increase and the corresponding expansion valve will open to allow more mass flow through the corresponding evaporator. When the expansion valve opens, suction pressure increases, and a variable compressor will respond by increasing capacity. If the load on one or more of the evaporators decreases, the superheat will decrease as well, causing the respective expansion valve to close. When the expansion valve closes, suction pressure decreases. If the decrease in suction pressure lowers the corresponding saturation temperature below the room dew point, the controller will direct the compressor to unload in order to raise the suction pressure back to a value with a corresponding saturation temperature above the dew point.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1. is a multiple evaporator VRF cooling system arranged according to various embodiments;

FIG. 2. is a multiple evaporator VRF cooling system arranged according to various embodiments;

FIG. 3. is a multiple evaporator VRF cooling system arranged according to various embodiments;

FIG. 4. is a multiple evaporator VRF cooling system arranged according to various embodiments;

FIG. 5. is a multiple evaporator VRF cooling system arranged according to various embodiments; and

FIG. 6 is an example enthalpy diagram detailing operation of an example VRF cooling system; and

FIG. 7 is a block diagram indicating for depicting control of the VRF cooling system.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying drawings.
Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.
The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.
When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.
Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
A multiple evaporator VRF system 110 according to various embodiments is described in accordance with FIG. 1. VRF system 110 includes an indoor unit 112 placed in proximity to the room space to be cooled and a condenser unit 114 placed remotely from the room space to be cooled. In various embodiments, remote unit 114 is placed outside of the building housing the space to be cooled. Indoor unit 112 in various embodiments is located in or above a row of computer server racks. A control unit 119 includes a compressor and a control circuit. Control unit 119 is located indoors. In various embodiments, control unit 119 could be located outdoors. In various embodiments, control unit 119 could be located in proximity to or remotely from indoor unit 112 or condenser unit 114.
As shown in FIG. 1, indoor unit 112 includes multiple evaporator circuits, shown in FIG. 1 as evaporator circuits 116 a, 116 b, . . . , 116 n−1, 116 n. Each evaporator circuit includes a respective expansion valve 118 a, 118 b, . . . , 118 n−1, 118 n. Expansion valves 118 control the application of cooling fluid to respective evaporators 120 a, 120 b, . . . , 120 n−1, 120 n. Expansion valves 118 operate either mechanically as thermal expansion valves or electrically powered via a dedicated controller. The expansion valves 118 respond to control evaporator outlet superheat. If the load on an evaporator 120 increases, the superheat will increase, and the expansion valve 118 will respond by opening to allow more refrigerant flow, which will return the superheat to the desired level. Each evaporator 120 a, 120 b, . . . , 120 n connects to a respective suction line 122 a, 122 b, . . . , 122 n−1, 122 n which connects to a central suction line 124. Evaporators 120, according to various embodiments, are implemented using, by way of nonlimiting example, conventional fin and tube evaporators or microchannel heat exchangers as the evaporating coil. Microchannel heat exchangers offer improved heat removal per unit volume, providing a space savings. Microchannel heat exchangers require less internal volume which facilitates refrigerant charge management because smaller refrigerant volumes are required. This enables the use of smaller liquid receivers and suction accumulators.
Central suction line 124 connects to a suction accumulator 126. Suction accumulator 126 accumulates liquid refrigerant. To maintain suction on central suction line 124, suction is applied to suction accumulator 126 through a return or suction input line 128 which connects to an inlet 130 of a compressor 132, such as a variable capacity compressor, arranged in control unit 119 of VRF system 110. Variable capacity compressor 132 receives mechanical drive through input shaft 134. Variable capacity compressor 132 generates suction on suction input line 128. Variable capacity compressor 132 also includes an outlet 136 which connects to a discharge line 138. Discharge line 138 provides fluid at pressure to a condenser 140.
VRF system 110 also includes a controller 142 that communicates with compressor 132 via a signal line 144. Similarly, controller 142 communicates with each of respective evaporators 120 a, 120 b, . . . , 120 n−1, 120 n via respective signal lines 156 a, 156 b, . . . , 156 n−1, 156 n.
Condenser 140 connects to an output or liquid line 150 which is routed to suction accumulator 126 to provide further cooling of fluid flowing through liquid line 150. At the output of suction accumulator 126, liquid line 152 connects to each of individual liquid lines 154 a, 154 b, . . . , 154 n−1, 154 n. Individual liquid lines are input to respective evaporator valves 118 a, 118 b, . . . , 118 n−1, 118 n.
VRF system 110 is controlled to maintain the refrigerant temperature above the room dew point temperature in order to prevent condensation from forming in the room space to be cooled. In operation, suction pressure of the system is monitored at one or any of suction lines 122, central suction lines 124, or return suction input line 128. The suction pressure corresponds to the lowest refrigerant temperature in the system and will indicate if the dew point of the system is being approached. If the load on any one or more of evaporators 120 increases, the outlet superheat will increase, and the respective expansion valves 118 will open to allow more mass flow through that particular evaporator 120. With the expansion valve opening, suction pressure will increase, and variable capacity compressor 132 will respond by increasing capacity.
If the load on one or more of the evaporators 120 decreases, the superheat will decrease as well, causing the respective expansion valve 118 to close. When the respective expansion valve 118 closes, suction pressure will decrease. If the decrease in suction pressure lowers the corresponding saturation temperature below the room dew point temperature, the controller 142 will direct compressor 132 to unload in order to raise the suction pressure back to a value with a corresponding saturation temperature above the dew point. In various embodiments, the actual evaporator pressure can be monitored via signal lines 156 a, 156 b, . . . , 156 n−1, 156 n for each of respective evaporators 120 a, 120 b, . . . , 120 n−1, 120 n. Controller 142 in various embodiments can control the unloading of compressor 132 via signal line 144. Thus, in various embodiments, suction pressure is monitored in order to maintain a minimum pressure, and resultant temperature, of the refrigerant in the circuit.
FIG. 2 is arranged similarly to FIG. 1 with modifications described herein. Throughout the description, items similarly configured in the figures will be referred to using similar reference numbers. For example, item 112 of FIG. 1 will be referred to as item 212 in FIG. 2. Likewise, for example, condenser 140 of FIG. 1 will be referred to as condenser 240 in FIG. 2 and condenser 340 in FIG. 3, etc. Items in succeeding figures which are configured similarly to prior figures will not be described in detail, and the operation of similarly configured items will not be described in detail. However, differing items or operations will be described in greater detail as necessary.
FIG. 2 depicts a VRF system 210. VRF system 210 includes a liquid receiver 260 disposed in liquid line 250 which is shown as an input line 250′ to liquid receiver 260 and an output line 250″ at the output of liquid receiver 260. Output line 250″ connects to the input of suction accumulator 226. In FIG. 2, the refrigerant charge is managed with a suction line accumulator 226 and liquid receiver 260. The liquid line 250, which is a high pressure and high temperature line, is routed through suction accumulator 226 in order to heat and evaporate any liquid that might be in the accumulator. Liquid receiver 260 provides a reservoir that accumulates cooling fluid.
FIG. 3 depicts a VRF system 310 arranged in accordance with another embodiment of the present invention. In FIG. 3, the suction accumulator 126, 226 of respective FIGS. 1 and 2 has been removed, and central suction line 324 forms a continuous return to input 330 of compressor 332. In FIG. 3, the refrigerant charge is managed via liquid receiver 360. In FIG. 3, the liquid line 350″ output from liquid receiver 360 is in thermal contact with suction line 324 in order to heat the vapor in central suction line 324 in order to evaporate any liquid that might be in central suction line 324. This reduces the liquid that could reach the input 330 of compressor 332.
FIG. 4 depicts a VRF system 410 arranged in accordance with another embodiment. In FIG. 4, the refrigerant charge is managed with suction accumulator 426 and liquid receiver 460. Liquid receiver 460 has an output directly to liquid line 452. In this manner, liquid receiver 460 does not have an output connected to the input of the heat exchanger of suction accumulator 426. The liquid output from liquid receiver 460 does not provide a heat exchange function for suction accumulator 426. In various embodiments, suction accumulator 426 could include a supplemental heater 462, such as a strap-on heater, which could be operated when the liquid in return line 424 reaches a predetermined level.
FIG. 5 depicts VRF system 510 arranged in accordance with another embodiment. In the embodiment of FIG. 5, refrigerant charge is managed using only liquid receiver 560. Suction line 524 communicates with input 530 of compressor 532 and is applied directly thereto. In the embodiment of FIG. 5, no suction accumulator is shown, which differs from the embodiments of FIGS. 1-2 and 4. In this manner, only liquid receiver 260 manages the refrigerant charge.
FIG. 6 depicts one example of an enthalpy diagram for demonstrating an exemplary operation of the various embodiments described herein. As can be seen in FIG. 6, a saturation curve 670 can be used to demonstrate the state of the refrigerant. As can be seen, suction pressure is monitored because this pressure corresponds to the lowest refrigerant temperature in the system as indicated by the saturated liquid line 672. If the load on an evaporator increases, the expansion valve will open to allow more mass flow through the evaporator. With the expansion valve opening, the suction pressure will increase and the compressor will respond by increasing capacity.
FIG. 7 depicts a block diagram 780 for controlling operation of a VRF cooling system in connection with FIG. 1. Control begins at start block 782 and proceeds to block 784. Block 784 monitors the suction pressure. Control then proceeds to block 786. At block 786, control proceeds based on the suction pressure. If the suction pressure is determined to be high, control proceeds to block 788 which increases the compressor output in order to compensate for the high suction pressure. Control then proceeds to block 784. If the suction pressure is determined to be low at block 786, then control proceeds to block 790 which decreases the compressor output. Control then proceeds to block 784. Returning to blocks 786, if the suction pressure is unchanged, control returns to block 784.
Returning to block 790, a decrease in suction pressure can lower the corresponding saturation temperature below the room dew point temperature. In various configurations, having the saturation temperature below the dew point temperature can generate undesired condensation. Accordingly, the suction pressure is monitored and associated with a corresponding saturation temperature. If the value of the suction pressure indicates that the corresponding saturation temperature is below room dew point temperature, the compressor output is decreased in order to maintain the suction pressure so that the corresponding saturation temperature stays above the room dew point temperature.
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

What is claimed is:

1. A variable refrigerant flow cooling system comprising:

a compressor having an inlet and an outlet, the inlet generating a first pressure, and the outlet generating a second pressure higher than the first pressure;

a condenser having an inlet and an outlet, the inlet of the condenser communicating with the outlet of the compressor, the condenser receiving fluid provided by the outlet of the compressor and removing heat from the fluid;

an expansion valve having an inlet and an outlet, the inlet of the expansion valve communicating with the outlet of the condenser, the expansion valve enabling expansion of the liquid at its outlet and varying a flow of liquid between its inlet and outlet;

an evaporator having an inlet and an outlet, the inlet of the evaporator communicating with the outlet of the expansion valve, the evaporator absorbing heat into the fluid as the fluid passes from its inlet to outlet; and

a controller monitoring a pressure at the outlet of the evaporator and varying the output of the compressor in accordance with the monitored pressure, wherein the monitored pressure indicates a saturation temperature and the control maintains the saturation temperature above a dew point temperature.

2. The variable refrigerant flow system of claim 1 wherein the expansion valve is responsive to the superheat of the evaporator.

3. The variable refrigerant flow system of claim 1 further comprising a suction line communicating with the outlet of the evaporator at a first end and the inlet of the compressor at a second end for fluid flow between the outlet of the evaporator and the inlet of the compressor.

4. The variable refrigerant flow system of claim 3 further comprising an accumulator interposed in the suction line between the evaporator and the compressor.

5. The variable refrigerant flow system of claim 4 wherein fluid flowing from the outlet of the condenser to the inlet of the expansion valve passes through the accumulator in order to reduce the temperature of the liquid flowing therethrough.

6. The variable refrigerant flow system of claim 1 further comprising a liquid receiver having an inlet and an outlet, the inlet of the liquid receiver communicating with the outlet of the condenser and the outlet of the liquid receiver communicating with the expansion valve.

7. The variable refrigerant flow system of claim 6 wherein the outlet of the liquid receiver is in thermal communication with a suction line interconnecting the output of the evaporator with the input of the compressor, thereby cooling the liquid output by the liquid receiver prior to input to the expansion valve.

8. The variable flow refrigerant system of claim 1 further comprising:

a plurality of expansion valves, each having an inlet and an outlet, the inlet of the each expansion valve communicating with the outlet of the condenser, each expansion valve enabling expansion of the liquid at its outlet and varying a flow of liquid between its inlet and outlet; and

a plurality of evaporators, each evaporator having an inlet and an outlet, the inlet of each evaporator communicating with a respective outlet of the expansion valve, each evaporator absorbing heat into the fluid as the fluid passes from its inlet to outlet.

9. A variable refrigerant flow cooling system comprising:

a compressor generating a first pressure and a second pressure higher than the first pressure;

a condenser communicating with the second pressure, the condenser receiving fluid provided at the second pressure and reducing the temperature of the fluid;

a liquid line communicating with the condenser and receiving the liquid;

an expansion valve communicating with the liquid line, the expansion valve enabling expansion of the fluid in the liquid line;

an evaporator communicating with the fluid output by the expansion valve, the fluid absorbing heat as it passes through the evaporator;

a vapor line communicating with the evaporator and returning fluid output from the evaporator to the compressor; and

a controller monitoring a pressure in the vapor line and varying the output of the compressor in accordance with the monitored pressure, wherein the monitored pressure indicates a saturation temperature of the vapor line, and the output of the compressor is varied to maintain the saturation temperature above a dew point temperature for the vapor line.

10. The variable refrigerant flow system of claim 9 wherein the expansion valve is responsive to the superheat of the evaporator.

11. The variable refrigerant flow system of claim 9 further comprising an accumulator interposed in the vapor line between the evaporator and the compressor.

12. The variable refrigerant flow system of claim 11 wherein liquid line passes through the accumulator in order to reduce the temperature of the fluid flowing therethrough.

13. The variable refrigerant flow system of claim 9 further comprising a liquid receiver interposed in the liquid line.

14. The variable refrigerant flow system of claim 13 wherein the output of the liquid receiver is in thermal communication with the vapor line, thereby cooling the liquid output by the liquid receiver prior to input to the expansion valve.

15. The variable flow refrigerant system of claim 9 further comprising:

a plurality of expansion valves communicating with the liquid line, each expansion valve enabling expansion of the fluid in the liquid line; and

a plurality of evaporators communicating with the fluid output by a respective expansion valve, the fluid absorbing heat as it passes through the evaporator.

16. A method for controlling an output of a compressor in a cooling system comprising:

providing an evaporator;

monitoring a suction pressure at the output of the evaporator;

comparing the suction pressure against a predetermined threshold, wherein the predetermined threshold is determined in accordance with a saturation temperature of a fluid output by the evaporator; and

decreasing the output of the compressor if the suction pressure is below the predetermined threshold.

17. The method of claim 16 further comprising increasing the output of the compressor if the suction pressure is above a predetermined threshold.

18. The method of claim 17 further comprising maintaining the suction pressure unchanged if the suction pressure is equal to the predetermined threshold.

19. The method of claim 16 further comprising maintaining the suction pressure unchanged if the suction pressure is equal to the predetermined threshold.