Patent Publication Number: US-9885503-B2

Title: Refrigeration system with free-cooling

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 14/205,829 filed Mar. 12, 2014, entitled “REFRIGERATION SYSTEM WITH FREE-COOLING,” which claims priority to U.S. Provisional Application Ser. No. 61/791,193 filed Mar. 15, 2013, entitled “HEAT EXCHANGER WITH FREE-COOLING,” which are hereby incorporated by reference in their entirety. 
    
    
     FIELD 
     The invention relates generally to a refrigeration system with free-cooling cooling. In particular, the invention relates to free-cooling systems which when operating in free-cooling mode the free-cooling loop can directly cool the liquid cooling fluid and when operating in full mechanical cooling mode the free cooling loop can reject heat from a second refrigerant circuit used to cool the cooling fluid. 
     BACKGROUND 
     Many applications exist for refrigeration systems including residential, commercial, and industrial applications. For example, a commercial refrigeration system may be used to cool an enclosed space such as a data center, laboratory, supermarket, or freezer. Very generally, refrigeration systems may include circulating a fluid through a closed loop between an evaporator where the fluid absorbs heat and a condenser where the fluid releases heat. The fluid flowing within the closed loop is generally formulated to undergo phase changes within the normal operating temperatures and pressures of the system so that considerable quantities of heat can be exchanged by virtue of the latent heat of condensation and vaporization of the fluid. 
     Refrigeration systems may operate with a free-cooling system or loop when ambient temperatures are low. The free-cooling system may exploit the low temperature of the ambient air to provide cooling without the need for an additional energy input from, for example, a compressor, a thermoelectric device, or a heat source. Typically, free-cooling systems may employ a separate heat exchanger or portion of a heat exchanger coil when operating in a free-cooling mode. When free-cooling is not desired, or feasible, the separate heat exchanger or coil portion may not be utilized. 
     In an air-cooled condenser, the refrigerant flowing through the condenser can exchange heat with circulating air generated by an air moving device such as a fan or blower. Since circulating air is used for heat exchange in an air-cooled condenser, the performance and efficiency of the condenser, and ultimately the HVAC&amp;R system, is subject to the ambient temperature of the air that is being circulated through the condenser. As the ambient air temperature increases, the condensing temperature (and pressure) of the refrigerant in the condenser also increases. At very high ambient air temperatures, i.e., air temperatures greater than 110 degrees Fahrenheit (° F.), the performance and efficiency of the HVAC&amp;R system can decrease due to higher condensing temperatures (and pressures) caused by the very high ambient air temperatures. 
     Some projects require chilled water year round (data centers, process applications) at a relatively warm chilled water temperature (between 7° C. and 15° C.). When ambient temperature is lower than required chilled water temperatures, free-cooling becomes a more efficient solution than mechanical cooling. It would be beneficial for a free-cooling option to offer the possibility to operate the chiller in mechanical cooling mode with no loss of efficiency, allowing the free cooling loop to draw heat from a mechanical refrigeration loop. It would also be beneficial to use the free-cooling loop as the unique source of cooling when working in free-cooling only mode. 
     SUMMARY 
     An embodiment is directed to a system for cooling air for use with a liquid cooling fluid loop. The system includes a first refrigerant circuit with an first condenser, a second refrigerant circuit with a second condenser, and a free-cooling loop. A control device is provided for controlling the operation of the system between first mode, a second mode, and a third mode. When operating in the first mode, only the free-cooling loop cooperates directly with liquid cooling fluid in the liquid cooling fluid loop to cool the liquid cooling fluid, when operating in the second mode, the second refrigerant circuit is not engaged, and when operating in the third mode, the free-cooling loop interacts with the second refrigerant circuit to reject heat of the second refrigerant circuit through the free-cooling loop. 
     An embodiment is directed to a system for cooling air for use with a liquid cooling fluid loop. The system includes a first refrigerant circuit with an first condenser, a second refrigerant circuit with a second condenser, and a free-cooling loop with a valve that directs free-cooling liquid of the free-cooling loop. A control device is provided for controlling the operation of the system between a free-cooling-only mode, a free-cooling-plus-mechanical-cooling mode, and a full mechanical cooling mode. When operating in the free-cooling-only mode, the valve directs the free-cooling liquid to a heat exchanger which is positioned in-line with the liquid cooling fluid loop, and when operating in the full mechanical cooling mode the valve directs free-cooling liquid to the liquid-cooled condenser of the second refrigerant circuit. 
     An embodiment is directed to a refrigeration system for use with a liquid cooling fluid loop. The refrigeration system includes a first refrigerant circuit with a first condenser, a second refrigerant circuit with a second condenser and a free-cooling loop with free-cooling liquid. A control device is provided for controlling the operation of the system between a free-cooling-only mode, a free-cooling-plus-mechanical-cooling cooling mode and a full mechanical cooling mode. A heat exchanger cooperates with the free-cooling loop and the liquid cooling fluid loop. A cooler cooperates with the first refrigerant circuit, the second refrigerant circuit and the liquid cooling fluid loop. When operating in the free-cooling-only mode, the free-cooling liquid is directed to a heat exchanger and when operating in the full mechanical cooling mode the free-cooling liquid is directed to the second condenser. 
     In some embodiments the first condenser of the first refrigerant circuit is an air-cooled condenser. In some embodiments the second condenser of the second refrigerant circuit is a liquid-cooled condenser. 
     An advantage of the present application is a system which is compatible with water or other cooling fluids in the building loop. 
     Another advantage of the present application is increased system capacity at very high ambient air temperatures. 
     Still another advantage of the present application is the system can operate in a regular cooling mode (with no free-cooling) with no decrease in efficiency and no other penalties. 
     A further advantage of the present application is the ability to use small pump power to provide efficient cooling. 
     Yet a further advantage of the present application is the system requires no new heat exchangers and can be used with no change in footprint from existing systems. 
     Other advantages of the present application will be apparent from the drawings and the detailed description of the illustrative embodiments provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an illustrative embodiment for a heating, ventilation, air conditioning and refrigeration system. 
         FIG. 2  illustrates an illustrative free-cooling system with two refrigerant circuits and a glycol loop. 
         FIG. 3  is a graph of design capacity relative to return water temperature. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     Referring to  FIG. 1 , an illustrative environment for a heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) system  10  in a building  12  for a typical commercial setting is shown. HVAC&amp;R systems  10  may include a compressor incorporated into a chiller or rooftop unit  14  that may supply a chilled liquid that may be used to cool building  12 . HVAC&amp;R system  10  may also include a boiler  16  to supply a heated liquid that may be used to heat building  12 , and an air distribution system that circulates air through building  12 . The air distribution system may include an air return duct  18 , an air supply duct  20  and an air handler  22 . Air handler  22  may include a heat exchanger (not shown) that is connected to boiler  16  and rooftop unit  14  by conduits  24 . The heat exchanger (not shown) in air handler  22  may receive either heated liquid from boiler  16  or chilled liquid from rooftop unit  14  depending on the mode of operation of HVAC&amp;R system  10 . HVAC&amp;R system  10  is shown with a separate air handler  22  on each floor of building  12 . However, several air handlers  22  may service more than one floor, or one air handler may service all of the floors. 
     As shown in  FIG. 2 , the chiller  14  includes a cooling system  120  which includes a first refrigerant circuit  135 , a second refrigerant circuit  126 , and a free-cooling loop or glycol loop  127 . As noted above with respect to  FIG. 1 , chiller  14  is housed within a single structure and may be located outside of a building or environment, for example on a roof top. Chiller  14  includes a portion of a liquid cooling fluid loop  124  that circulates a liquid cooling fluid, such as chilled water, an ethylene glycol-water solution, or propylene glycol-water solution, brine, or the like, to a cooling load, such as a building, piece of equipment, or environment. In certain embodiments, the liquid cooling fluid may circulate within the liquid cooling fluid loop  124  to a cooling load, such as a research laboratory, computer room, office building, hospital, molding and extrusion plant, food processing plant, industrial facility, machine, or any other environments or devices in need of cooling. 
     The first refrigerant circuit  135  includes a first compressor  200 , an air-cooled condenser or condenser coil  202 , an expansion device  206 , and a cooler  208  which cools chilled water. The cooler  208  cools the fluid in the liquid cooling fluid loop  124 . The cooler  208  includes two evaporators, one which is positioned in the first refrigerant circuit  135  and one which is positioned in the second refrigerant circuit  126 . 
     The second refrigerant circuit  126  includes a second compressor  210  which supplies pressurized refrigerant vapor to a condenser  212 . The condenser  212  cools and condenses the refrigerant. High pressure liquid refrigerant flows through an expansion device  214  where it is converted to low pressure fluid, usually in the form of gas/liquid mixture, or mist. The cooler  208  cools the fluid in the liquid cooling fluid loop  124  as the low pressure fluid evaporates, absorbing heat from the fluid in the liquid cooling fluid loop  124 . 
     The free-cooling loop or glycol loop  127  includes a glycol coil  220 , a glycol pump  222 , and a three-way valve  216  that connects the outlet of the pump  222  to either a glycol-water heat exchanger  218  or the condenser  212 . A fan  204  moves air through the glycol coil  220  and the condenser coil  202 . The glycol coil  220  is located upstream of the condenser coil  202  of the first refrigerant circuit  135 . 
     The liquid cooling fluid loop  124  includes a second pump  234  which moves return fluid  230  through the glycol-water heat exchanger  218 . The chilled water  236  which exits the heat exchanger  218  flows from the glycol-water heat exchanger  218  through the cooler  208  to become supply fluid  232  for the liquid cooling fluid loop  124 . 
     In one illustrative embodiment, the air-cooled heat exchanger or condenser which includes the condenser coil  202  and the glycol coil  220  can be set up, configured or arranged to have one or more portions with substantially planar sections or coils arranged or positioned in a V-shape. The sections or coils can be stacked or nested and operated at different condensing temperatures, condensing pressure and/or in different refrigerant circuits. The stacked sections or coils can be arranged or positioned so that the air exiting one section or coil enters the other section or coil. Stated differently, the air flow through the sections or coils of the portion of the condenser can be in a series configuration or arrangement. In another illustrative embodiment, the condenser may have portions with both stacked sections and coils operating at different condensing temperatures or pressures and single sections or coils operating at a single condensing temperature or pressure. 
     In one illustrative embodiment, the condenser coil  202  and/or the glycol coil  220  can be implemented with microchannel or multichannel coils or heat exchangers. Microchannel or multichannel coils can have the advantage of compact size, light weight, low air-side pressure drop, and low material cost. The microchannel or multichannel coils or sections can circulate refrigerant through two or more tube sections, each of which has two more tubes, passageways or channels for the flow of refrigerant. The tube section can have a cross-sectional shape in the form of a rectangle, parallelogram, trapezoid, ellipse, oval or other similar geometric shape. The tubes in the tube section can have a cross-sectional shape in the form of a rectangle, square, circle, oval, ellipse, triangle, trapezoid, parallelogram or other suitable geometric shape. In one embodiment, the tubes in the tube section can have a size, e.g., width or diameter, of between about a half (0.5) millimeter (mm) to about a three (3) millimeters (mm). In another embodiment, the tubes in the tube section can have a size, e.g., width or diameter, of about one (1) millimeter (mm). 
     In another illustrative embodiment, the condenser coil  202  and/or the glycol coil  220  can be implemented with round-tube plate-fin coils. One illustrative configuration for round-tube plate-fin coils is to split the fins so that there is no conduction path between the two refrigerant circuits or coils, but to use a common tube sheet. The result is two separate coils from a thermal standpoint, but mechanically they appear as a single unit. Another illustrative configuration is to make a round-tube coil where the refrigerant circuits share the fins. However, there may be conduction through the fins between the two circuits or coils that may be limited by the inclusion of a thermal break (such as a slit) in the fin design. In still another illustrative embodiment, the round-tube coil condensers can be configured to have the desuperheating sections downstream of both condensing sections and the subcooling sections upstream of both condensing sections to provide the optimum thermal performance. 
     The glycol loop  127  is separate from the liquid cooling fluid loop  124 , thereby allowing the glycol or other fluid having similar properties to be exposed to the ambient air to be independent from the liquid cooling fluid circulating within liquid cooling fluid loop  124 . In general, the fluid circulating within glycol loop  127  may have a lower freezing point temperature than the liquid cooling fluid circulating within liquid cooling fluid loop  124 . In certain embodiments, the fluid circulating within glycol loop  127  is a freeze-protected fluid, such as glycol or brine with a high glycol concentration, to inhibit freezing during periods of low ambient temperatures. However, freeze-protected fluids may have a higher cost, higher viscosity (which may result in increased pumping power), and/or a lower heat transfer rate when compared to other cooling fluids, such as water. By circulating the freeze-protected fluid through a relatively small glycol loop  127 , a relatively small amount of freeze-protected fluid may be employed, which in turn may improve efficiency of chiller  14  and/or reduce costs. 
     The cooling system  120  may operate in different modes of operation depending on the requirements of the cooling load and the temperature of the ambient air. A control device  300  may govern operation of chiller  14  and cooling system  120  to cool the fluid within the liquid cooling fluid loop  124  to a prescribed temperature or prescribed range of temperatures. For example, control device  300  may switch cooling system  120  between a free-cooling mode, a conventional cooling mode and a full mechanical cooling mode. 
     When the outside air temperature is low, for example, during winter in northern climates and equivalent seasons in the southern hemisphere, cooling system  120  may operate in a first mode or free-cooling mode. In this mode of operation, the second refrigerant circuit  126  does not operate. The three-way valve  216  of the glycol loop  127  is positioned to direct glycol to the glycol-water heat exchanger  218 . In this mode, heat from the liquid cooling fluid  230  in the liquid cooling fluid loop  124  is transferred to the glycol or freeze-protected fluid circulating within the glycol loop  127 . The glycol loop  127  then circulates the freeze-protected fluid through glycol coil  220  to expel the heat to the low temperature outdoor air. 
     The cooling system  120  may operate in the first mode, or free-cooling-only mode, of operation when the ambient air temperature is sufficiently low enough to provide free-cooling. For example, chiller  14  may operate in the free-cooling-only mode during the winter when outside temperatures are below approximately 15 degrees Celsius. However, in other embodiments, the cooling mode determination may depend on a variety of factors such as the cooling requirement of the cooling load, the outside temperature and/or humidity, the type of cooling fluid, and the cooling capacity of the chiller  14  among other things. 
     In a second mode or free-cooling-plus-mechanical-cooling cooling mode, the first refrigerant circuit  135  may operate to supply additional mechanical cooling if required. If needed, the first refrigerant circuit  135  runs as a conventional refrigeration system. 
     In this second mode of operation, the liquid cooling fluid may first be cooled by the freeze-protected fluid as the liquid cooling fluid circulates. Specifically, as the liquid cooling fluid  230  of the liquid cooling fluid loop  124  flows through heat exchanger  218 , the liquid cooling fluid  230  may transfer heat to the freeze-protected fluid flowing through heat exchanger  218  from the glycol loop  127 . After exiting the heat exchanger the liquid cooling fluid  236  may undergo further cooling by transferring heat to a refrigerant flowing within the first refrigerant circuit  135 . Specifically, as the liquid cooling fluid  236  flows through the cooler  208 , the liquid cooling fluid  236  may transfer heat to the refrigerant flowing within first refrigerant circuit  135 . 
     When the first refrigerant circuit  135  is engaged with the glycol loop  127 , the outside air temperature has increased and/or the outside air temperature is not cool enough to provide sufficient cooling to the cooling load. In this mode of operation, the liquid cooling fluid within liquid cooling fluid loop  124  may be cooled by both the glycol loop  127  and the first refrigerant circuit  135 . Specifically, the liquid cooling fluid of the liquid cooling fluid loop  124  may transfer heat to the freeze-protected fluid circulating within glycol loop  127 . The freeze-protected fluid may then release the heat absorbed from the cooling fluid to ambient air as the freeze-protected fluid flows through glycol coil  220 . After the liquid cooling fluid  230  has been cooled by the freeze-protected fluid within heat exchanger  218 , the liquid cooling fluid  236  may then flow through cooler  208  of the first refrigerant circuit  135  where first refrigerant circuit  135  may further remove heat from the liquid cooling fluid  236  by absorbing additional heat from the liquid cooling fluid  236  by the refrigerant in first refrigerant circuit  135 . In this manner, both free-cooling loop or glycol loop  127  and first refrigerant circuit  135  may be used to provide cooling capacity during this mode of operation. 
     When required, the cooling system  120  operates in a third mode or full mechanical cooling mode which utilizes full mechanical cooling. In this mode, the first refrigerant circuit  135  runs as a conventional refrigeration system. If the required load can be satisfied by the operation of only the first refrigerant circuit  135 , the second refrigerant circuit  126  will not be engaged. As only the first refrigerant circuit  135  is operating, a lower condensing temperature is achieved. In addition, as the second refrigerant circuit  126  does not operate, operation of the glycol pump is not required. 
     However, if required, the first refrigerant circuit  135  and the second refrigerant circuit  126  can operate at the same time. In this mode, the first refrigerant circuit  135  runs as a conventional refrigeration system. The second refrigerant circuit  126  may implement a vapor-compression cycle, or other type of cooling cycle, such as absorption or a thermoelectric cycle, to provide additional cooling for the cooling load. The heat generated by the second refrigerant circuit  126  is rejected through the glycol loop  127 . 
     In this third mode the three-way valve  216  of the glycol loop  127  directs glycol to the condenser  212  and prevents flow to the glycol-water heat exchanger  218 . This allows the second refrigerant circuit  126  to make use of the glycol coils  220  and the lower air temperatures in the upstream location of the coils to minimize condensing temperature. 
     Accordingly, during this third mode of operation, heat exchanger  218  is used to transfer heat from second refrigerant circuit  126  to glycol loop  127 . Specifically, glycol loop  127  circulates the freeze-protected fluid from heat exchanger  218  to glycol coil  220  to expel the heat into the environment. In this manner, glycol coil  220  may be used by cooling system  120  to remove heat from the system even when the system is not operating in a free-cooling mode. For example, glycol loop  127  may be used to remove heat from second refrigerant circuit  126  even when environmental air temperatures may be higher than the fluid temperature. Specifically, even though the ambient air temperature may be high, for example above 21 degrees Celsius, the ambient air temperature still may be lower than the temperature of the high pressure and temperature refrigerant flowing within the second refrigerant circuit  126 . This temperature difference may enable glycol coil  220  to transfer heat from second refrigerant circuit  126  to the environment, thereby increasing the cooling capacity of cooling system  120 . 
     Regardless of the mode of operation, chiller  14  may function to cool the liquid cooling fluid circulating to and from the cooling load. The liquid cooling fluid may enter chiller  14  through a return line  238  that is in fluid communication with the cooling load. A pump  234  circulates the cooling fluid through liquid cooling fluid loop  124 . The pump may be any suitable type of pump such as a positive displacement pump, centrifugal pump, or the like. 
     In the illustrative embodiments, control devices  300  govern operation of the chiller  14  and the cooling system  120  to control the temperature of, including, but not limited to; the fluid in the cooling system as it enters and exits the chiller  14 ; the freeze-protected fluid entering or leaving the glycol coils  220 ; and/or the temperature of the fluid entering or leaving the condenser coils  202 . For example, the temperature of the freeze-protected fluid entering heat exchanger  218  may be maintained at a certain temperature above freezing to inhibit freezing of the liquid cooling fluid  230  of the liquid cooling fluid loop  124  also circulating within heat exchanger  218 . In a specific example, control devices  300  may turn off a motor that drives fan  204  to cease airflow through air-to-liquid glycol coils  220 , which in turn may increase the temperature of the freeze-protected fluid entering heat exchanger  218 . Control devices  300  may govern operation of the components of the cooling system  120  based on ambient air temperature, temperature of the freeze-protected fluid, temperature of the cooling fluid, time of day, operating times, calendar days, or combinations thereof, among others. Further, the control devices  300  may be coupled to valves, pumps and/or other such equipment and may use information received from sensors to determine when to operate pumps and when to switch positions of valves. Control devices  300  may include local or remote command devices, computer systems and processors, and/or mechanical, electrical, and electromechanical devices that manually or automatically set a temperature related signal that a system receives. 
     The  300  includes control circuitry, which may include one or more processors with supporting memory circuitry and/or firmware that stores routines carried out by the processor. The processor may be of any suitable type, including, but not limited to, microprocessors, field programmable gate arrays, processors of special purpose and general purpose computers. Similarly, memory might include, but is not limited to, random access memory, flash memory, read only memory. The control devices  300  may also include or be associated with input/output circuitry for receiving sensed signals and interface circuitry for outputting control signals. 
     Control devices  300  may be configured to switch chiller  14  and the cooling system  120  between the modes of operation based on input received from temperature sensors and the like. A respective temperature sensor may sense the temperature of the ambient outside air and another temperature sensor, which may be disposed within the cooling loop, may sense the temperature of the liquid cooling fluid returning from the building. In certain embodiments, when the ambient air temperature sensed by the first sensor is below the liquid cooling fluid temperature sensed by second sensor, control devices  300  may set cooling system  120  to operate in a mode of operation that employs only free-cooling as described above. Control devices  300  may operate cooling system  120  in the free-cooling mode of operation until the temperature of the ambient air reaches a specified value or is a certain amount above the temperature of the cooling fluid or some other threshold is reached. 
     Control devices  300  may then set cooling system  120  to operate in the mode of operation that employs first refrigerant circuit  135 , in addition to circulating the cooling fluid through the glycol loop  127 . Control devices  300  may operate cooling system  120  in this mode of operation until the ambient air temperature reaches another specified value or amount above the liquid cooling fluid temperature or until the liquid cooling fluid temperature rises above a certain threshold. Further, control devices  300  may operate cooling system  120  in this mode of operation until the temperature of the freeze-protected fluid exceeds or approaches the temperature of the cooling fluid. Control devices  300  may then switch cooling system  120  to the fully mechanical mode of operation that employs first refrigerant circuit  135  or a combination of the first refrigerant circuit  135  and the second refrigerant circuit  126 . 
     The control devices  300  may be based on various types of control logic that uses input from temperature sensors. Control devices  300  also may control other valves and pumps included within the chiller  12 . Further, additional inputs such as flow rates, pressures, and other temperatures may be used in controlling the operation of chiller  14 . For example, other devices may be included in chiller  14 , such as additional pressure and/or temperature transducers or switches that sense temperatures and pressures of the refrigerant and cooling fluid, the heat exchangers, the inlet and outlet air, and so forth. Further, the examples provided for determining the mode of operation are not intended to be limiting. Other values and set points based on a variety of factors such as system capacity, cooling load, and the like may be used to switch chiller  14  between the first, second, and third modes of operation. 
     The configuration of the cooling system  120  is shown by way of example only and is not intended to be limiting. For example, the locations and types of pumps, valves and other components may vary. 
       FIG. 3  shows the appropriate use of the various modes of operation based on percentage of design capacity relative to ambient/return water temperature. This graph illustrates when it is efficient to use the different modes of operation. 
     Other variations are possible. For example in cases where glycol is in the chilled water loop, the glycol-water heat exchanger  218  in  FIG. 2  may be eliminated and the glycol loop can be connected directly to the liquid cooling fluid loop  124 . In this case there would be two connections: one would draw glycol for the glycol coil  220 , and the other would return glycol from the coil  220  to a location downstream of the first in free-cooling mode. In mechanical cooling mode the valve  216  would direct glycol flow to the condenser  212  and prevent flow of warm glycol into the liquid cooling fluid loop  124 . Additional valves may be included to eliminate any remaining mixing of warm glycol to the cold glycol in mechanical cooling mode. Another variation includes moving the glycol pump  222  to a position upstream of the glycol coil  220 . 
     Other options are possible in the fluids. While the glycol loop would normally use an ethylene or propylene glycol and water solution, water can be used instead of glycol in locations where freezing is not an issue. In that case the glycol-water heat exchanger may be eliminated as described above. Other possible fluids such as calcium chloride or sodium chloride brine or non-aqueous materials such as d-limonene are possible and can be used in the glycol loop or cooling fluid loops depending on the requirements of a particular application. 
     Many variations are possible in piping for the glycol loop to achieve similar control. For example two two-way valves can replace three-way valve  216  to direct flow to either the glycol to water heat exchanger  216  or the condenser  212 . Alternatively, a single one-way valve may be used on the line to the heat exchanger  216  to prevent flow of warm glycol during operation of compressor  210 . The location of the valve(s) can be upstream of the heat exchanger  218  and condenser  212  as shown or they can be downstream. The pump  222  may be upstream of the coil or downstream. 
     If a reversible pump is used for pump  222 , then simple check valves can control flow so that pumping in one direction moves glycol through heat exchanger  218  and pumping in the opposite direction moves flow through the condenser  212 . Glycol connections for heat exchanger  218  and condenser  212  should be configured to provide counter flow when glycol is moving through them. 
     Yet another alternative is to use multiple glycol pumps. For example a first pump could be located in the line for heat exchanger  218  and a second pump can be located in the line for the condenser  212 . Only one of these two pumps operates a given time. Check valves in the lines can prevent backflow of glycol through pumps when the other pump is operating, although some types of pumps do not allow backflow. 
     Only certain features and embodiments of the invention have been shown and described in the application and many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the illustrative embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.