Abstract:
A multi-compressor heat pump system configured to provide heating and cooling over a range of ambient temperatures. The compressors can be operated independently and alone or together in series for maximum output. Heat exchangers are selectively fluidically connected to the compressors to enable refrigerant flow between the compressors and at least two heat exchangers in a manner that enables the heat pump system to be selectively operable in various modes. Preferred aspects include selectively operating the compressors based on the ratio of the evaporating and condensing pressures of the refrigerant within the heat pump system, a mixing chamber between the compressors, and a lubricant management system to prevent the accumulation of a lubricant in one of the compressors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/685,302, filed May 27, 2005, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to heating and cooling systems, and more particularly to a heating and cooling system with multiple compressors. 
   Conventional heat pump systems utilize a reversible refrigerant flow to both heat and cool enclosed spaces, typically a building such as a house. In a heating cycle of a typical heap pump system, a compressor compresses a vaporized refrigerant to a high pressure and directs the resulting hot refrigerant vapor to an indoor heat exchanger functioning as a condenser. The indoor heat exchanger draws heat from the condensation of the refrigerant vapor to heat the house. The resulting cooled and liquid refrigerant is then directed to an expansion device and an outdoor heat exchanger where, under reduced pressure, heat is drawn from the outdoor environment to evaporate the liquid refrigerant. The resulting vaporized refrigerant is then directed back to the compressor where the refrigerant vapor is again compressed to continue the cycle. 
   To cool the house, the cycle is reversed. The compressor compresses the refrigerant vapor to a high pressure and directs the resulting hot refrigerant vapor to the outdoor heat exchanger, now functioning as the condenser, which releases heat to the outdoor environment from condensation of the refrigerant vapor. The cooled liquid refrigerant is than directed to the expansion device and the indoor heat exchanger where, under reduced pressure, heat is drawn from the house interior to evaporate the liquid refrigerant. The refrigerant vapor is then directed back to the compressor where the refrigerant vapor is again compressed to continue the cycle. 
   Conventional heat pumps have found widespread residential application due to their ease of installation and use. Conventional heat pumps are also economical to install and use, at least in milder climates, because the same components can be used for both heating in colder months and cooling in warmer months. However, in colder northern climates, the use of heat pumps presents additional challenges. One issue is that the performance of heat pump systems decreases in colder temperatures when heating capacity is most needed. Although heat pump systems that contain a single compressor may be designed to operate at very low ambient temperatures, such systems show decreased performance at higher temperatures. Also, the heating capacity of a single-compressor system will greatly exceed the cooling capacity of the system, providing an inefficient and wasteful heating-to-cooling capacity ratio. A system with excess heating capacity will also have to cycle on and off more frequently at higher ambient temperatures in order to reduce its capacity, leading to a reduced life span and decreased system efficiency. Proposed solutions include the use of variable speed compressors, parallel compressors, and variable displacement compressors. These solutions, however, increase the price of the system and eliminate the biggest advantage of the heat pump, namely, its low installation cost. 
   To provide increased heating capacity during the winter in northern climates, heat pumps have often been installed with a separate, backup heating system such as an electrical heating system. The supplemental heating system, however, reduces the desirability of a heat pump in the first place, and leads to significantly increased energy costs during the coldest months of the year. To address these issues, heat pump systems have been proposed that use compressors connected in series. A primary compressor is used for cooling the house during warmer months and heating the house in cooler months. During extremely cold conditions, a booster compressor is operated in series with the primary compressor to increase the system heating capacity. Multi-compressor heat pump systems are described in U.S. Pat. Nos. 5,927,088 and 6,276,148, both to Shaw. In the Shaw patents, compressor operation is determined by sensing the indoor and outdoor temperatures, and optionally the pressure immediately upstream of the primary compressor. In each of these patents, an economizer is used to increase the heating capacity of the system by bleeding a portion of the refrigerant flow from the main flow, expanding and cooling the bled portion, and then directing the bled portion through the economizer where it subcools the main flow of refrigerant flowing through the economizer to the evaporator. The bled refrigerant is then directed to the inlet of the primary compressor. 
   Although useful for increasing the heating capacity of the system, multiple compressors and an economizer present additional challenges in the design of an integrated heating and cooling system. To function properly, a compressor requires a lubricant that is typically entrained in the refrigerant delivered to the compressor, and may thus circulate through the system with the refrigerant. In systems with multiple compressors, the lubricant may migrate to one of the compressors, accumulating in the compressor and leading other compressors in the system to fail from lack of lubricant. U.S. Pat. No. 6,276,148 to Shaw addresses this issue with aspiration tubes in the compressors to draw lubricant from compressors with high lubricant levels. The lubricant drawn from a compressor is entrained in the refrigerant and circulated through the entire system to the other compressor. However, the entrained lubricant reduces the heating and cooling capacity of the system because the lubricant serves no purpose on the heat exchange side of the system. 
   U.S. Pat. No. 4,586,351 to Igarashi discloses a lubricant management system for a multi-compressor heat pump system that prevents the circulation of lubricant to the heat exchange side of the heat pump system. Lubricant entrained in the refrigerant leaving the compressors is separated from the refrigerant in a lubricant separator. The lubricant is then redirected to an accumulator that mixes the lubricant with the refrigerant returning to the inlet side of the compressors. Although useful for preventing the circulation of lubricant on the heat exchange side of the system, Igarashi does not appear to address the problems inherent in attempting to balance the lubricant level between two compressors connected in series and operating at different pressure levels. 
   The use of an economizer also presents certain challenges. After being bled from the main refrigerant line and allowed to expand, the refrigerant circulated through the economizer and returned to the compressors is typically in a two-phase state of both liquid and vapor. To some degree, the two-phase refrigerant from the economizer mixes with the refrigerant vapor from the evaporator before entering the compressors. However, liquid refrigerant can impair the operation of a compressor, and the prior art appears to lack means for ensuring adequate mixing of the two-phase refrigerant from the economizer with the refrigerant vapor from the evaporator. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a multi-compressor heat pump system configured to provide efficient heating and cooling over a wide range of ambient temperatures. 
   According to a first aspect of the invention, the compressors can be operated independently, either alone or together in series for maximum output. In this embodiment, at least two compressors are part of a compressor section of the heat pump system. First and second heat exchangers are selectively fluidically connected to the compressor section to enable flow of a refrigerant between the compressor section and the first heat exchanger, between the first and second heat exchangers, and between the compressor section and the second heat exchanger. Valves control the flow of the refrigerant through the compressors and the first and second heat exchangers. The valves are controlled so that the heat pump system is selectively operable in each of the following modes: the compressors operate in series wherein the a first compressor operates as a low stage compressor and a second compressor operates as a high stage compressor; the first compressor operates independently and the second compressor is bypassed by the refrigerant; and the second compressor operates independently and the first compressor is bypassed by the refrigerant. 
   According to this aspect of the invention, the heat pump system provides increased flexibility while allowing for the use of relatively lowcost fixed-speed compressors. Alternatively, one of the compressors may be a variable capacity compressor with a high and low setting to provide additional flexibility in the capacity of the system. An economizer may also be used to provide still further flexibility and increased total output for the system. According to a preferred aspect of the invention, one or both of the compressors of the heat pump system can be selectively caused to operate based on a ratio of the evaporating and condensing pressures of the refrigerant within the heat pump system, as opposed to sensing temperatures to control the system. With this approach, only one of the compressors is operated if the ratio is less than a predetermined value for the ratio, and both compressors are operated if the ratio is greater than the predetermined value. As such, if the pressure ratio were to rise to a level at which the compressors could be damaged if operated individually, the other compressor is started to provide a two-stage operating mode. 
   According to another aspect of the invention, a heat pump system is provided having a compressor section with at least two compressors, first and second heat exchangers selectively fluidically connected to the compressor section to enable flow of a refrigerant between the compressor section and the first heat exchanger, between the first and second heat exchangers, and between the compressor section and the second heat exchanger, and valves for controlling the flow of the refrigerant through the compressors and the first and second heat exchangers, wherein the valves are controlled so that the heat pump system is selectively operable in a first mode in which the compressors operate in series and a second mode in which only one of the compressors operates independently and the other compressor(s) is bypassed by the refrigerant. According to this embodiment, the heat pumping system includes a mixing chamber fluidically connected to the outlet of a first of the compressors and to the inlet of a second of the compressors, and an economizer fluidically connected to the first heat exchanger, fluidically connected to the second heat exchanger, and selectively fluidically connected to the mixing chamber. A first portion of the refrigerant flowing between the first and second heat exchangers is selectively delivered to the mixing chamber for mixing with a second portion of the refrigerant flowing into the mixing chamber from the outlet of the first compressor if the first and second compressors are operating in series. The first portion of the refrigerant is not delivered to the mixing chamber if the first and second compressors are not operating in series. In this manner, liquid refrigerant that may be entrained in the first portion of the refrigerant leaving the economizer can be thoroughly dispersed in the vapor leaving the first compressor before entering the second compressor when both compressors are operated, but is prevented from entering the second compressor if only the second compressor is operating. 
   According to yet another aspect of the invention, a heat pump system is provided with a lubricant management system to prevent the accumulation of a lubricant in one of the compressors of the heat pump system. As with the previous embodiments, the heat pump system has a compressor section with at least two compressors, first and second heat exchangers selectively fluidically connected to the compressor section to enable flow of a refrigerant between the compressor section and the first heat exchanger, between the first and second heat exchangers, and between the compressor section and the second heat exchanger, and valves for controlling the flow of the refrigerant through the compressors and the first and second heat exchangers, wherein the valves are controlled so that the heat pump system is selectively operable in a first mode in which the compressors operate in series and a second mode in which only one of the compressors operates independently and the other of the first and second compressors is bypassed by the refrigerant. According to this embodiment, the heat pumping system includes a lubricant equalization conduit fluidically coupled to the compressors, and a valve for selectively fluidically connecting the compressors through the lubricant equalization conduit and for selectively controlling flow of the lubricant through the lubricant equalization conduit to provide for equalization of levels of the lubricant in the compressors when the compressors are not operating. This approach also preferably employs a lubricant separator to remove the lubricant from the refrigerant leaving the compressor section and return the removed lubricant back to the inlets of the compressors. 
   In view of the above, the present invention provides a multi-compressor heat pump system capable of being operated without a backup heating system in colder climates, yet can be economical to install and use. The multiple compressors can be operated independently to provide variable capacity or operated in series to provide maximum capacity, and optionally with an economizer to provide increased total capacity for the system and increased flexibility in the system capacity. According to preferred aspects of the invention, the compressors can be independently operated, with or without an economizer, while avoiding certain complications associated with multi-compressor heat pump systems that utilize economizers. In particular, when operated with an economizer, the heat pump system preferably utilizes a mixing chamber to ensure effective mixing of liquid-containing refrigerant from the economizer and vaporized refrigerant prior to entering a compressor. Furthermore, the heat pump system preferably includes a lubricant management system that prevents lubricant from circulating with the refrigerant in the heat exchangers of the system, and effectively equalizes the lubricant level between the compressors connected when operated in series at different pressure levels. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically represents a multi-compressor heat pump system in accordance with a preferred embodiment of this invention. 
       FIG. 2  schematically represents a two-stage heating mode for the fluid-carrying portion of the heat pump system of  FIG. 1 . 
       FIG. 3  schematically represents a single-stage heating mode for the fluid-carrying portion of the heat pump system of  FIG. 1 . 
       FIG. 4  schematically represents a cooling mode for the fluid-carrying portion of the heat pump system of  FIG. 1 . 
       FIG. 5  schematically represents a defrost mode for the fluid-carrying portion of the heat pump system of  FIG. 1 . 
       FIG. 6  is a graph plotting the heat demand and heat output versus ambient temperature characteristic of the heat pump system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A heat pump system  10  in accordance with a preferred embodiment of the present invention is schematically represented in  FIG. 1 . The heat pump system  10  will be described with particular reference to residential house applications in nordic climates, though it will be understood that the system  10  of this invention can find use in other applications and operating environments. 
   As shown in  FIG. 1 , the heat pump system  10  includes a low-stage compressor  12 , a high stage compressor  14 , an indoor refrigerant-water heat exchanger  16 , an indoor refrigerant-air heat exchanger  18  for delivering heating and cooling air to the interior of the house (not shown), an outdoor refrigerant-air heat exchanger  20 , a closed economizer  22 , a four-way reversing valve  24 , a lubricant separator  26 , a suction gas accumulator  28 , check valves  30  and  32  in fluidic parallel with the compressors  12  and  14 , respectively, a control valve  34  for adjusting the refrigerant flow rate through the heat exchanger  18 , a solenoid valve  36  to block refrigerant flow to the heat exchanger  16 , expansion devices  38 ,  40 ,  42 , and  44  to control refrigerant flow through the heat exchanger  20 , heat exchanger  18 , economizer  22 , and heat exchanger  16 , respectively, a solenoid valves  46  and  48  for controlling refrigerant flow and lubricant flow, respectively, in the compressors  12  and  14 , a mixing chamber  50  for mixing two-phase refrigerant from the economizer  22  and refrigerant vapor from the compressor  12 , outdoor, indoor, and floor temperature sensors  52 ,  54 , and  56 , pressure sensors  58  and  60  for sensing the evaporating and condensing pressures within the heat pump system  10 , and a control unit  62  for controlling the heat pump system  10  and its various components. As described above, the heat pump system  10  is configured for use with an interior hydronic system (not shown) coupled with the indoor refrigerant-water heat exchanger in combination with a forced air heating/cooling system coupled with the indoor refrigerant-air heat exchanger  18 . However, preferred aspects of the heat pump system  10  may also be used with any conventional interior heat exchange system, such as a conventional forced air heating/air-conditioning system. 
   The system  10  can be generally and preferably physically separated into three main units, as indicated in  FIG. 1 . An indoor heating/cooling unit of the system  10  includes the heat exchanger  18  (with fan), expansion device  40 , and temperature sensor  54 , all of which can be located inside a heating/cooling duct system of the house. An outdoor unit of the system  10  includes the heat exchanger  20  (with fan), expansion device  38 , and temperature sensor  52 , all of which can be located outside the house for absorbing heat from and dissipating heat to the ambient outside environment outside the house. Finally, the main unit of the system  10  contains the remaining system components, including the compressors  12  and  14 , economizer  22 , heat exchanger  16 , control unit  62 , etc. 
   The operation of the system  10  will now be described in reference to the following modes of operation: a two-stage heating mode for very cold ambient temperatures; a single-stage heating mode for cooler ambient temperatures; an air-conditioning (cooling) mode for warm to hot ambient temperatures; and a defrosting mode for defrosting the coils of the outdoor heat exchanger  20  during winter. 
     FIG. 2  schematically depicts the heat pump system  10  of  FIG. 1  (the electrical components are omitted for clarity) in its two-stage heating mode for very cold ambient temperatures. As shown in  FIG. 2 , both compressors  12  and  14  are operated in series to meet the heating demands of a very cold ambient temperature. In this scenario, both the low stage compressor  12  and the high stage compressor  14  are activated and valves  30  and  32  are closed. The compressor  12  compresses the refrigerant from a low pressure to an intermediate pressure. The hot refrigerant vapor discharged from the compressor  12  then flows to the mixing chamber  50  where it is mixed with the two-phase refrigerant from the economizer  22  in order to reach a suitable lower inlet temperature for the high stage compressor  14 . The temperature at the inlet of the compressor  14  is regulated with the expansion valve  42 , which controls the refrigerant temperature at the inlet to the compressor  14  by adjusting the refrigerant flow rate through the economizer  22 . 
   After compressing the refrigerant from the intermediate pressure achieved with the compressor  12  to the higher pressure achieved with the compressor  14 , the gaseous refrigerant passes through a lubricant separator  26  where the lubricant (oil) is separated from the refrigerant. The lubricant is fed to the conduit connected to the inlet of the compressor  12 , and is therefore drawn into the compressor  12  so that the lubrication of both compressors  12  and  14  is provided under all operating conditions. 
   After leaving the lubricant separator  26 , the refrigerant vapor passes through the reversing valve  24  and enters the heat exchanger  16  (operating as a water-cooled condenser). At this point, two different interior heating scenarios are provided. In a first scenario, if the air heating to the house is switched off, the control valve  34  is closed and all of the refrigerant passes through the heat exchanger  16  where it is liquefied and transfers heat to the return water of the hydronic system, after which the resulting liquid refrigerant passes through the bypass valves of  44  and  36 . In the second scenario, if air heating to the house is switched on, the control valve  34  is open and part of the refrigerant will pass through the air-cooled condenser  18  and release heat to the air of the forced air heating/cooling system of the house. The capacity of the heat exchanger  18  is preferably smaller than the capacity of the heat exchanger  16 , so that at least a portion of the refrigerant passes through the heat exchanger  16 . The control valve  34  is controlled by the surface temperature of the heat exchanger  18  and the outlet temperature of the heat exchanger  18  to ascertain a certain subcooling of the refrigerant. 
   The liquid refrigerant exiting the heat exchanger  18  joins the refrigerant stream from the solenoid valve  36  and then flows through the economizer  22 , where heat is extracted, as understood by those skilled in the art and discussed below. After passing through the economizer  22 , the refrigerant is separated into two portions. A smaller portion of the refrigerant passes through the expansion valve  42  to the economizer  22 , where it is partly evaporated. The partially evaporated (two-phase) refrigerant then passes though the solenoid valve  46  and into the mixing chamber  50 , where it is mixed with the hot discharge vapor from the compressor  12  as described above. The remaining and larger portion of the refrigerant passes through the expansion device  38 , which is controlled by the superheat of the heat exchanger  20 . The refrigerant then passes through the heat exchanger  20 , where it evaporates using heat drawn from the ambient air outside the house. Afterwards, the refrigerant flows through the reversing valve  24  to the suction gas accumulator  28 , which regulates the refrigerant flow to the compressors  12  and  14  and thus protects the compressor  12  from damage, especially during the startup of the system  10 . The refrigerant vapor leaving the suction gas accumulator  28  is then mixed with the lubricant leaving the lubricant separator  26  and enters the compressor  12 , at which point the cycle repeats. 
     FIG. 3  represents the single-stage heating mode of the heat pump system  10  suitable for operation in cool ambient temperatures. In this mode, only one of the compressors  12  and  14  need be operated (with the other bypassed) to meet the lower heating requirements of the cool ambient temperatures. The check valve  30  in fluidic parallel with the compressor  12  is open so that the low stage compressor  12  is bypassed, and the check valve  32  is closed so that the refrigerant is directed exclusively to the high stage compressor  14 , which therefore is operated independently to compress the refrigerant from low pressure to high pressure. Alternatively, the check valve  30  could be closed and the check valve  32  opened so that the refrigerant flow is directed exclusively to the low stage compressor  12 , with the high stage compressor  14  being bypassed. If the compressor  12  is operated independently, the refrigerant is first compressed by the compressor  12  before entering the mixing chamber  50 , which is inactive as a result of the solenoid valve  46  being closed, as discussed below. From the mixing chamber  50 , the refrigerant bypasses the compressor  14  by using the flow path through the check valve  32  and enters the lubricant separator  26 . Thereafter, the refrigerant circuit functions essentially the same as the two-stage mode described above with reference to  FIG. 2 . When the refrigerant vapor passes through the lubricant separator  26 , the lubricant is separated from the refrigerant and added back to the low pressure line to the compressors  12  and  14  in order to guarantee lubrication for the compressors  12  and  14 . After leaving the lubricant separator  26 , the refrigerant passes through the reversing valve  24  and enters both the heat exchanger (water-cooled condenser)  16  and the control valve  34 , at which point the two different scenarios for water-heating only and combined air-heating and water-heating can be carried out, as described above. 
   After the liquid refrigerant leaves the indoor heat exchangers  16  and/or  18 , it enters the inactive economizer without changing its state since flow through the injection line to the economizer  22  is prevented by the closed solenoid valve  46 . Because of the solenoid valve  46  is closed, the refrigerant stream is not split into the two above-noted portions after leaving the economizer  22 . Instead, the entire refrigerant volume is expanded by the expansion device  38 , which is again controlled by the superheat of the outdoor heat exchanger  20 . The refrigerant then passes through the heat exchanger  20 , where it evaporates using heat drawn from the ambient air outside the house. Thereafter, the refrigerant flows through the reversing valve  24  to the suction gas accumulator  28 , which regulates the refrigerant flow to the compressors  12  and  14  as discussed above. The refrigerant vapor leaving the suction gas accumulator  28  is then mixed with the lubricant leaving the lubricant separator  26 , at which point the cycle repeats. 
     FIG. 4  represents the air-conditioning (cooling) mode suitable for warm to hot ambient temperatures. As shown in  FIG. 4 , to operate in the air-conditioning mode, the solenoid valves  36  and  46  are closed, the control valve  34  is opened, and the reversing valve  24  is actuated to an air-conditioning position. The low pressure refrigerant vapor bypasses the compressor  12  by using the flow path through the check valve  30 , and is compressed in the compressor  14 . The refrigerant vapor then flows to the lubricant separator  26 , where the lubricant is separated from the refrigerant and added back to the low pressure line to the compressors  12  and  14  as discussed previously. After leaving the lubricant separator  26 , the refrigerant vapor passes through the reversing valve  24  to enter the outdoor heat exchanger  20  (now acting as a condenser), where the refrigerant condenses by dissipating heat to the ambient air drawn by the fan through the heat exchanger  20 . The liquid refrigerant then flows through the economizer  22 , which again is inactive as a result of the valve  46  being closed. As a result, the state of the refrigerant remains unchanged. In addition, the solenoid valve  36  is closed, causing the refrigerant to enter the indoor heat exchanger  18  after passing through the expansion device  40 , which is controlled by the evaporating pressure and outlet temperature of the heat exchanger  18 . While passing through the heat exchanger  18 , the refrigerant evaporates by absorbing heat from the air stream drawn from the house interior, thus cooling the indoor air. Finally, the refrigerant flows through the open control valve  34  and through the reversing valve  24  to the suction gas accumulator  28 , is mixed with the lubricant leaving the lubricant separator  26 , and proceeds to the valve  30 , at which point the cycle repeats. 
     FIG. 5  represents the defrosting mode of the system  10  for defrosting the outdoor coil of the heat exchanger  20 , as need from time to time during winter as a result of ice buildup exceeding a predetermined limit. The ice buildup, caused by freezing of the moisture of the ambient air at evaporating temperatures below the freezing point, decreases the efficiency of the system  10  because the airflow across the coil of the heat exchanger  20  is reduced and the evaporating temperature of the heat pump decreases. 
   In order to enter the defrost mode, the compressor  12  is turned off and the solenoid valve  46  is closed while compressor  14  is running. The reversing valve  24  is then changed to air-conditioning mode, the solenoid valve  36  is kept open, and the control valve  34  is closed. This condition is held as long as the defrosting cycle lasts, which can be terminated either by a timer or a temperature control on the coil of the heat exchanger  20 . The outdoor fan can also be turned off in order to decrease the heat loss over the coil and, therefore, reduce the defrost time. To leave the defrosting mode, the reversing valve  24  is switched back to heating mode and the other valves are switched back to their positions before entering the defrost mode. 
   In  FIG. 5 , only the high stage compressor  14  is indicated as running in the defrost mode. The low pressure refrigerant from the suction side bypasses the compressor  12  by using the flow path through the check valve  30 , and is then compressed by the compressor  14  before flowing to the lubricant separator  26 , whose operation is the same as that described above for the other operating modes. After leaving the lubricant separator  26 , the refrigerant passes through the active reversing valve  24  and enters the heat exchanger  20 , where it condenses by dissipating heat to the heat exchanger  20 . If the outdoor air fan is turned off, most of the refrigerant heat is used to melt the frost and, therefore, defrost the outdoor coil. After leaving the heat exchanger  20 , the liquid refrigerant flows through the device  38  and flows through the economizer  22 , which is again inactive because the valve  46  is closed. The refrigerant, whose state is unchanged by the inactive economizer  22 , flows to only the heat exchanger  16  as a result of the valve  34  being closed and the valve  36  being open. Before entering the heat exchanger  16 , the refrigerant is expanded by the expansion device  44 , after which the expanded refrigerant is evaporated in the heat exchanger  16  using the heat of the hydronic system, which has an almost imperceptible effect on the hydronic system since the thermal mass of the hydronic system is high and the duration of the defrost mode is short. After leaving the heat exchanger  16 , the liquid refrigerant proceeds to the reversing valve  24  and then the suction gas accumulator  28 , where the remaining cycle is the same as described for the other modes of operation. 
   As known in the art, because of the inherently different lubricant circulation rates of the two compressors  12  and  14 , the system  10  will experience lubricant migration from one compressor to the other depending on the operating conditions of the system  10 . If the level of the lubricant sump of one compressor is too low, lubrication cannot be guaranteed for that compressor and the reliability of the compressor will decrease drastically. Therefore, lubricant equalization is essential to keep the system  10  running. To address this issue, the heat pump system  10  of this invention preferably incorporates a lubricant equalization subsystem that operates when both compressors  12  and  14  are not operating, since the difference in suction pressure of the compressors  12  and  14  is much higher than the static pressure difference of the different heights in lubricant level. Lubricant equalization is accomplished by opening the solenoid valve  48  (shown closed in  FIGS. 2 through 5 ) whenever both compressors  12  and  14  are not operating. To ensure the lubricant will equalize when the valve  48  is opened, both compressors  12  and  14  are preferably mounted at the same level so that the static pressure difference between the lubricant level of the compressors  12  and  14  is negligible. Lubricant equalization can be initiated by the control unit  62  whenever the heat pump system  10  is turned off, and is automatically terminated by the control unit  62  before starting either of the compressors  12  or  14 . 
   During colder months, the system  10  can be operated in either of the two-stage or single-stage heating modes. According to a preferred aspect of the invention, the single-stage heating mode is preferably used whenever the pressure ratio between the evaporating and condensing pressures is small. If the pressure ratio between the evaporating and condensing pressures rises to a predetermined level at which the compressors  12  and  14  could be damaged when operating in the single-stage mode, the control unit  62  causes the other compressor  12  or  14  to start, and the heat pump system  10  begins operating in the two-stage mode. As represented in  FIGS. 1 through 5 , the evaporating and condensing pressures can be measured directly using the pressure sensors  58  and  60 , respectively, which are shown at preferred locations within the system  10 , though other locations are possible as long as one of the sensors  58  or  60  is on the high pressure side and the other on the low pressure side of the system  10 . Alternatively, these pressures could be calculated from the evaporating and condensing temperatures as measured by temperature sensors. However, this approach would require placement of temperature sensors directly at the heat exchangers  16 ,  18 , and  20 . After obtaining the evaporating and condensing pressures using either method, the ratio of the evaporating pressure to the condensing pressure can be calculated by the control unit  62 . 
   The two-stage operation of the heat pump system  10  can also be initiated if the required heat load is higher than a predetermined limit exceeding the heat output of a single compressor. To decide which compressor  12  or  14  is running in single-stage mode, two conditions can be applied. First, damage to the compressors  12  and  14  must be prevented by not exceeding the operation limits of either compressor  12  and  14 . Second, the particular compressor  12  or  14  to be operated in the single-stage mode may be selected based on the heat demand. The states of the different valves have already been defined in the description of the four different operating modes depicted in  FIGS. 2 through 5 . The temperatures to detect the heat demand and the limiting conditions of the compressors  12  and  14  are preferably measured at the outdoor coil of the heat exchanger  20 , on the indoor coil of the heat exchanger  18 , in the ambient air, and at the floor or indoor air of the house. Additional sensors may also be used to improve the control and allow for greater flexibility. 
   A schematic of the heat output of the described heat pump system  10  and the heat demand for a residential house versus the ambient temperature is plotted in  FIG. 6 . As described above, the compressors  12  and  14  are capable of being operated independently or in series for maximum output, depending on the demands of the environmental conditions. The compressors  12  and  14 , which can be lowcost fixed-speed compressors of known design, can also be operated with or without the economizer  22 , depending on the required heating capacity. Thus, as shown in Table I below, six separate operating capacities may be achieved with the heat pump system  10  using fixed-speed compressors. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE I 
             
           
           
             
                 
                 
             
             
                 
               COMPRES- 
                 
             
             
                 
               SORS 
               ECONOMIZER 
             
           
        
         
             
               OPERATING MODE 
               12 
               14 
               22 
             
             
                 
             
             
               2-Stage Heating with Economizer 22 
               ON 
               ON 
               ON 
             
             
               2-Stage Heating without Economizer 22 
               ON 
               ON 
               OFF 
             
             
               1-Stage heating with Compressor 12 
               ON 
               OFF 
               OFF 
             
             
               1-Stage heating with Compressor 14 
               OFF 
               ON 
               OFF 
             
             
               1-Stage cooling with Compressor 12 
               ON 
               OFF 
               OFF 
             
             
               1-Stage cooling with Compressor 14 
               OFF 
               ON 
               OFF 
             
             
                 
             
           
        
       
     
   
   Alternatively, a variable speed compressor may be used as the high-stage compressor  14  to achieve still further flexibility in heating capacity. As shown in the following chart, ten separate operating capacities may be achieved with this approach. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE II 
             
           
           
             
                 
                 
             
             
                 
                 
               ECONO- 
             
             
                 
               COMPRESSORS 
               MIZER 
             
           
        
         
             
               OPERATING MODE 
               12 
               14 
               22 
             
             
                 
             
             
               2-Stage Heating with Economizer 22 
               ON 
               HIGH 
               ON 
             
             
               2-Stage Heating without Economizer 22 
               ON 
               HIGH 
               OFF 
             
             
               2-Stage Heating with Economizer 22 
               ON 
               LOW 
               ON 
             
             
               2-Stage Heating without Economizer 22 
               ON 
               LOW 
               OFF 
             
             
               1-Stage heating with Compressor 12 
               ON 
               OFF 
               OFF 
             
             
               1-Stage heating with Compressor 14 
               OFF 
               HIGH 
               OFF 
             
             
               1-Stage heating with Compressor 14 
               OFF 
               LOW 
               OFF 
             
             
               1-Stage cooling with Compressor 12 
               ON 
               OFF 
               OFF 
             
             
               1-Stage cooling with Compressor 14 
               OFF 
               HIGH 
               OFF 
             
             
               1-Stage cooling with Compressor 14 
               OFF 
               LOW 
               OFF 
             
             
                 
             
           
        
       
     
   
   In view of the above, the present heat pump system  10  offers various advantages over existing heat pump systems. The flexible configuration of the compressors  12  and  14  and economizer  22  allows for the generated heat to closely follow and quickly respond to ambient conditions and heat demands, improving the thermal comfort of the interior of a house and decreasing the costs of operation. System performance is increased because the system  10  does not need to cycle on and off as frequently as prior art systems. The multiple compressor configuration of the present invention can also be easily adapted to existing air handling and heat exchange systems, allowing the present invention to be easily adapted to existing systems. 
   The lubricant management system is configured to accommodate the needs of multiple compressors configured to operate in series, and provides better performance by preventing lubricant flow through the heat exchangers  16 ,  18  and  20 . Specifically, performance is increased because the thermal resistance of the heat exchangers  16 ,  18 , and  20  is lower if a lubricant film is not present on their tube walls. Also, the lubricant equalization between the compressors  12  and  14  ensures more even lubrication of the compressors  12  and  14  to improve system performance and reliability of the compressor section of the heat pump system  10 . The flow path on the suction side of the compressors  12  and  14  also inhibits lubricant from migrating to the inactive compressor  12 / 14  while the system  10  is running in the single-stage mode, further improving the reliability of the system  10 . 
   Another advantage is that the system  10  can be configured to be physically separated into three units: an indoor air unit containing the heat exchanger  18  (and accessories); an outdoor air unit containing the heat exchanger  20  (and accessories); and a main unit containing the compressors  12  and  14 , heat exchanger  16 , control unit  62 , economizer  22  (optional), and accessories. Because of the lower weight and smaller size of each component of the system  10 , transport of the components is easier and less expensive. The separation of the compressors  12  and  14  from the heat exchangers  18  and  20  is advantageous because the outdoor unit (containing the heat exchanger  20 ) does not include the compressors  12  and  14 , allowing for a greater degree of freedom for the design of the heat exchanger  20 , with the potential for increased performance, more economical construction, and optimization of the drainage of condensate from the heat exchanger  20 . Furthermore the compressors  12  and  14  can be installed to be more easily serviced since they are not required to be surrounded by any of the heat exchangers  16 ,  18 , and  20 . The compressors  12  and  14  can also be housed in a noise-damping enclosure since openings for air coils are not needed. Because the compressors  12  and  14  are the loudest part of the system  10 , the noise level of the overall system  10  can thus be reduced. The compressors  12  and  14  may also be located indoors, eliminating the need for crankcase heating on startup and providing better performance, lower running cost, and increased reliability. 
   While a particular embodiment has been described and represented in the Figures, various modifications are also within the scope of the invention. For example, though the heat pump system  10  has been described for use as a residential heating and cooling system, the present invention is not limited to residential applications, but could also be used in commercial and industrial applications and accommodations. In addition, the expansion valve  42  and solenoid valve  46  could be replaced by a single electronic expansion valve to provide more accurate control of the refrigerant flow and to eliminate the need to use two valves. A bypass solenoid valve could be installed parallel to the compressor  12  and its bypass valve  30  to more quickly equalize the suction pressures of the compressors  12  and  14 . Such a modification can more rapid lubricant equalization immediately after the compressors  12  and  14  are turned off and the bypass solenoid valve is opened. Another possible modification is to rely on natural defrosting at ambient temperatures above, for example, 2° C., since air at such temperatures can provide enough heat to defrost the coil of the heat exchanger  20  by providing air flow through the outdoor coil. 
   In view of the above, though the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.