Heat pump with liquid refrigerant reservoir

Abstract: A refrigerant heat pump system (10) wherein the outdoor coil (16) is larger than the indoor coil (15) to provide for efficient operation when the system (10) is in a cooling mode. A refrigerant reservoir (35) is mounted in a refrigerant line that connects the two coils (15,16) during a heating mode which is arranged to collect liquid refrigerant, thus preventing the outdoor coil (16) from becoming overcharged with refrigerant during the heating process. Efficient operation of the system is thus maintained during both heating and cooling without the need for expensive equipment to regulate the flow of refrigerant between the heat exchanger coils (15,16).

Description: This invention relates to an improved compression refrigerant heat pump and, in particular, to alleviating problems associated with overcharging the outdoor coil of the heat pump with refrigerant when the heat pump is operating in the heating mode. A standard refrigerant air conditioning unit will contain an indoor coil for absorbing heat from a comfort zone into the refrigerant and an outdoor coil for rejecting the heat in the refrigerant to the surrounding ambient. In addition to handling the heat absorbed into the refrigerant during cooling, the outdoor coil must also handle the heat of compression developed by the unit compressor, and the heat generated by both the compressor motor and one or both of the coil fan motors. As a result, the outdoor coil of a standard air conditioning unit for providing cooling is considerably larger than the indoor coil simply because the outdoor coil must perform more work than the indoor coil. As is well known, the standard air conditioning system can be employed to provide heat to a comfort zone by thermodynamically reversing the cooling cycle to draw heat from the surrounding ambient and rejecting the absorbed heat into the comfort zone. To this end, a reversing valve is connected to the unit compressor to reverse the flow of refrigerant through the coils. When in a heating mode, the larger outdoor coil acts as an evaporator and the indoor coil as a condenser in the system. As can be seen, the functions of the outdoor and indoor coils are reversed when the system is in a heating mode. The condenser, which is now the indoor coil, is smaller than the evaporator or outdoor coil. Accordingly, the small condenser is not able to store as much liquid refrigerant as the outdoor coil, which acts as the condenser, than when the system is in the cooling mode. Because only a small amount of liquid refrigerant can be held back in the indoor coil, and thus an over-abundance of refrigerant is found in the outdoor coil producing an unwanted condition known as overcharging. This condition can be alleviated to some extent by use of regulated expansion devices such as thermal expansion valves for controlling the quality of return gas. These devices are relatively expensive and require the use of complex controls. Non regulated expansion devices such as capillary tubes and the like are less expensive and complex, however, they are incapable of regulating the return flow to the compressor when the heat pump is in a heating mode, and as a consequence, liquid or wet refrigerant may be delivered directly into the compressor pumping cavity thereby adversely effecting the operation of the system and compressor lifespan. Industry standards require that refrigerant entering the compressor be superheated to about 6°C (10°F) above saturation in order to protect the compressor components. Most systems employ an accumulator tank in the suction line of the compressor to prevent liquid refrigerant from entering the compressor. Although use of an accumulator insures that only superheated vapors enter the compressor when the system is up and running, cold refrigerant nevertheless can be drawn into the compressor outlet during start up. Along with the problem of low superheat, heat pumps employing rotary compressors encounter low oil temperatures whereupon the compressor discharge gas is able to condense into the oil. This, in turn, results in bearing problems and can lead to compressor failure. Since the cooling mode determines the heat pump system design, the heating cycle will always be overcharged with refrigerant unless special precautions are taken. As noted, non-regulated expansion devices cannot close down the refrigerant flow between coils to eliminate overcharging. In an ideally adjusted heat pump system, therefore, more refrigerant is required in the cooling mode than in the heating mode. Attempts to equal the charges at some average value lowers the cooling performance to an unacceptable level and does not totally solve the problems associated with overcharging when operating in the cooling mode. German patent application No. DE 2709343 A1 describes such a compressor-driven heat pump with an indoor coil for absorbing energy when the heat pump is in a cooling mode and for emitting heat when the heat pump is in a heating mode, and an outdoor coil for emitting heat when the heat pump is in a cooling mode and for absorbing energy when the heat pump is in a heating mode, wherein the compressor is equipped with a reversing valve that can be cycled when either a cooling or a heating mode of operation is selected. However, this heat pump requires a cumbersome separation of the liquid and gaseous phases of the refrigerant. This is achieved using an additional selective heat exchanger heating the refrigerant flowing towards the compressor when the heat pump is in a heating mode, but in the cooling mode thereof, the refrigerant is not heated. It is therefore an object of the present invention to improve refrigerant heat pumps. This object is achieved in an apparatus according to claim 1. A refrigerant heat pump utilizes the Camot cycle to expand refrigerant from the high pressure side of the system to the low pressure side as it is moved between coils. A first refrigerant line is arranged to carry refrigerant from the indoor coil to the outdoor coil when the system is operating in a heating mode and a second refrigerant line is arranged to carry refrigerant from the outdoor coil to the indoor coil when the system is operating in a cooling mode. The first line contains a check valve to pass flow from the indoor coil to the outdoor coil and a non-regulated expansion device positioned between the check valve and the outdoor coil for throttling refrigerant from the high pressure side of the system to the low pressure side. A liquid reservoir is also positioned in this line between the indoor coil and the reversing valve. The second refrigerant line contains a second check valve arranged to pass flow from the outdoor coil to the indoor coil and a second non-regulated expansion device positioned in the line between the outdoor coil and the indoor coil. The second line is connected into the first line between the indoor coil and the refrigerant reservoir whereby refrigerant in the liquid phase is collected in the reservoir when the system is operating in the heating mode and vapor is collected in the reservoir when the system is operating in the cooling mode. For a better understanding of these and other objects of the present invention, reference shall be made herein to the following detailed description of the invention which is to be read in association with the accompanying drawings, wherein Fig. 1 is a schematic drawing illustrating a heat pump embodying the teachings of the present invention showing the heat pump operating in a cooling mode, Fig. 2 is a schematic drawing illustrating the heat pump of Fig. 1 showing the heat pump operating in a heating mode, and Fig. 3 is a diagram comparing oil to discharge condensation temperatures in a rotary compressor utilized in the present system over a one hour period of time. Referring to Fig. 1 there is illustrated schematically a refrigerant heat pump system 10 embodying the teachings of the present invention. The system employs the well known Carnot vapor compression cycle to provide either heating or cooling to a comfort region which, in this case, is an indoor zone 11 located on the inside of a building 13. The system includes an indoor heat exchanger 15 situated in the comfort zone and an outdoor heat exchanger 16 located on the outside of the building in heat transfer relation with the surrounding ambient. Also located on the outside of the structure is the system compressor 17 (mandated for noise reduction), which, in this case, is a rotary compressor as normally employed in this type of system. The compressor is equipped with a reversing valve 18 that can be cycled when either a cooling or a heating mode of operation is selected. When, as in Fig. 1, the cooling mode is selected, the compressor discharge port is connected to the outdoor coil and the suction port is connected to the indoor coil. The outdoor coil thus serves as a condenser and the indoor coil as an evaporator in the system. As will be explained in greater detail below, refrigerant is expanded or throttled between the high pressure condenser and the low pressure evaporator by means of a non-regulated expansion device such as a capillary tube 19. Other examples of non-regulated expansion devices are throttling valves, and plate orifices which may also be used in the system without departing from the teachings of the present invention. The capillary tube 19 is contained in a first refrigerant line 20 extending between the two coils. A check valve 21 is also operatively mounted in the refrigerant line and is arranged to permit refrigerant to flow in one direction only from the outdoor coil to the indoor coil. The check valve, as shown in Fig. 1, is positioned on the upstream side of the capillary tube. As indicated by the arrows in Fig. 1, high pressure, high temperature refrigerant in vapor form is discharged form the compressor into the outdoor coil where the refrigerant is reduced to a liquid at a constant temperature. During this time, heat is given up to the surrounding ambient. The liquid refrigerant leaving the outdoor coil 16 is then expanded through a capillary tube 19 to a lower temperature and pressure as it moves through line 41 between the outdoor and indoor coils. Refrigerant enters the indoor coil as a wet mixture and is further evaporated to saturation as it absorbs energy (heat) from the comfort zone. The saturated vapor leaving the indoor coil is superheated in the suction line 24 under the influence of the compressor. By design, the coils are sized to handle the desired cooling load without overcharging the system when it is in the cooling mode. As a result, the system operates efficiently in the cooling mode without the need for an accumulator or a regulated expansion device. A second refrigerant line 28 is also provided in the system which serves to deliver refrigerant from the indoor coil to the outdoor coil when the system is switched from the cooling mode described above into the heating mode as shown in Fig. 2. The refrigerant line 28 also contains a check valve 30 that permits refrigerant to flow in the line only in one direction from the indoor coil to the outdoor coil to throttle refrigerant as it moves between the coils. Here again, a capillary tube 32 is positioned in the line between the check valve and the outdoor coil. A refrigerant reservoir 35 is also mounted in the line 28 upstream from the check valve between the check valve 30 and the indoor coil 15. It should be noted that one end of the refrigerant line 28 enters the other refrigerant line 20 at connector 40 which is located between check valve 21 and the outdoor coil 16. The other end of line 28 is also attached to line 20 via a second connector 41 located between the refrigerant reservoir 35 and the indoor coil 15. As can be seen, the direction of flow through the conjoined refrigerant line is controlled by the two check valves. Upon switching the system to a heating mode, the reversing valve is cycled to direct refrigerant gas at a high temperature and pressure from the compressor discharge port to the indoor coil. The gas is reduced to a liquid phase in the indoor coil which is now acting as a condenser and the heat of condensation is rejected into the comfort zone. Liquid refrigerant leaves the indoor coil and due to the arrangement of the two check valves, is conducted through refrigerant line 28 to the outdoor coil. As a result, the refrigerant reservoir 35 is filled to capacity with liquid high pressure refrigerant. The liquid refrigerant is expanded to the low pressure side of the system as it passes through capillary tube 32 prior to entering the outdoor coil. The outdoor coil now acts as an evaporator to absorb energy from the surrounding ambient thus evaporating the refrigerant to a saturated vapor. Again, under the influence of the compressor, the saturated vapor leaving the evaporator is superheated as it moves through the suction line 24 of the compressor. As can be seen, because of the configuration of the two return lines and the positioning of the check valves, the refrigerant reservoir 35 acts as a receiver for liquid refrigerant only when the system is operating in a heating mode. The reservoir will receive only refrigerant vapor when the system is operating in a cooling mode. Because liquid refrigerant is far denser than the vapor, the reservoir holds considerably more refrigerant during heating than cooling. The reservoir is sized so that the system will not be overcharged with refrigerant during the heating cycle yet will be operating at peak efficiency during the cooling cycle. For example, a 9500 kJ (9,000 btu) unit will require about thirty ounces of liquid refrigerant for efficient operation during the cooling mode. However,-only 770 ml (twenty-six ounces) of liquid refrigerant is required for efficient operation during the heating mode. Under these conditions, the refrigerant reservoir is sized to store about 118 ml (four ounces) of liquid refrigerant when the system is in the heating mode, thereby preventing overcharging of the outdoor coil during cooling operations. When the system is switched to a cooling mode, the reservoir will fill with only a negligible amount of refrigerant gas thus freeing up additional refrigerant for use in the cooling mode. The diagram of Fig. 3 compares the operation of the present system in a heating mode with a similar system that is not equipped with a liquid reservoir. The diagram plots the condensation temperature of refrigerant in the compression chamber of a notary compressor with the compressor oil temperature. As illustrated, the oil temperature in the compressor of a system equipped with the present refrigerant reservoir will remain about 20°F above the condensation temperature of the refrigerant in the compressor, thus assuring that the discharge gas in the compressor will not condense in the compressor when the system is called upon to produce heating. In comparison, a heat pump of similar construction that is not equipped with the refrigerant reservoir of the present invention will become overcharged with refrigerant when running in the heating mode. As a consequence, the outdoor coil becomes flooded and the returning vapor to the compressor will not be superheated. This, in turn, causes the oil temperature in the compressor to drop below the condensation temperature of the refrigerant whereupon the oil in the compressor becomes diluted leading to bearing problems, ineffective heating, and early compressor failures.

Claims: A refrigerant heat pump (10) that is characterized by : an indoor coil (15) for absorbing energy from a comfort zone (11) when the heat pump (10) is in a cooling mode and rejecting heat into the comfort zone (11) when the heat pump (10) is in a heating mode, an outdoor coil (16) for rejecting energy to the surrounding ambient when the heat pump (10) is in a cooling mode and absorbing energy when the heat pump (10) is in a heating mode, said outdoor coil (16) being larger than said indoor coil (15) to reject excess energy when the heat pump is in the cooling mode, a compressor (17) having a discharge port and a suction port, a reversing valve (18) for selectively connecting the discharge port of the compressor to the indoor coil (15) and the suction port of the compressor to the outdoor coil (16) when the heat pump (10) is in the heating mode, and the discharge port to the outdoor coil (16) and the suction port to the indoor coil (15) when the system is in the cooling mode, a first refrigerant line (28) for connecting the indoor coil (15) to the outdoor coil (16), a first valve means (30) in the line for passing refrigerant in one direction from the indoor coil (15) to the outdoor coil (16) when the heat pump is operating in a heating mode, a first non-regulated expansion device (32) positioned in the line between the first valve means (30) and the outdoor coil (16), and a refrigerant reservoir (35) in said line (28) the refrigerant reservoir (35) being positioned between the first valve means (30) and the indoor coil (15), and sized to prevent overcharging the outdoor coil (16) with refrigerant when the heat pump (10) is operating in a heating mode, a second refrigerant line (20) for connecting the outdoor coil (16) to the indoor coil (15), a second valve means (21) in the second line for passing refrigerant in one direction from the outdoor coil (16) to the indoor coil (15) when the heat pump is operating in a cooling mode, and a second non-regulated expansion device (19) positioned in said second line (20) between the second valve means (21) and the indoor coil (15), whereby a predetermined amount of liquid refrigerant is collected in said reservoir (35) when the heat pump (10) is operating in the heating mode. The heat pump (10) of claim 1 wherein said valve means (30, 21) are check valves. The heat pump (10) of claim 1 wherein said non-regulated expansion devices (32, 19) are capillary tubes. The heat pump (10) of claim 1 that further includes a first connector means (41) for coupling the second refrigerant line (20) to the first refrigerant line (28) between the indoor coil (15) and the refrigerant reservoir (35). The heat pump (10) of claim 4 that further includes a second connector means (40) for coupling the first refrigerant line (28) into the second refrigerant line (20) between the outdoor coil (16) and the second valve means (21). The heat pump (10) of claim 1 wherein said compressor (17) is a rotary compressor and the refrigerant reservoir (35) is sized so that the oil temperature in the compressor remains about 11°C (20°F) above the condensation temperature of the refrigerant in said compressor.

Inventors: MEYER KENNETH J, MEYER, KENNETH J.
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 1996
Main classification: F25B13
IPC codes: F25B13, F24D15