Abstract:
A system and method for defrosting an air source heat pump are presented. A defrost conduit extends from the discharge from a system compressor to the outdoor coil. The defrost conduit includes a normally closed defrost valve. When the need for defrost exists, the defrost valve is opened, and warm refrigerant is delivered from the compressor discharge to the outdoor coil to effect defrost of the outdoor coil.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of heat pumps. More particularly, this invention relates to the field of defrost technology for air source heat pumps. 
       BACKGROUND OF THE INVENTION 
       [0002]    Air source heat pumps operate by extracting heat energy from outdoor air and delivering that heat energy to heat an interior space. In periods of high outdoor relative humidity combined with an outdoor temperature close to the freezing point of water, frost and/or ice will develop on the outdoor coil of the heat pump, thus impeding heat transfer to the coil surface areas. Defrost operation is required to remove the accumulated frost and/or ice. Defrosting may be accomplished on a timed schedule (time defrost), and/or by sensing the need to accomplish defrosting (demand defrost). 
         [0003]    Whether defrosting is accomplished on a time basis or a demand basis, defrosting is typically accomplished by reverse cycle defrosting. In reverse cycle defrosting, a four-way valve is operated so that the heat pump is, in effect, operated in a cooling mode of operation wherein heat energy is extracted from the previously heated interior space to heat the refrigerant fluid circulating in the system, and the heat energy is transported to the outdoor coil to melt and remove the frost/ice that has accumulated on the outdoor coil. The interior of the previously heated space is thus impacted in two ways. One is by removal of significant amounts of heat energy that has just been delivered to the interior space. The other is by creating an uncomfortable flow of cool air felt by occupants of the previously heated space. 
         [0004]    Another problem with reverse cycle defrosting is that the warm refrigerant is delivered to the top of the outdoor coil (the vapor outlet side in heating operation) and melts the frost/ice accumulation from the top of the coil downward. This can result in refreezing on the lower parts of the coil. 
         [0005]    Reverse cycle defrosting impairs the efficiency of operation of heat pump systems, especially as the frequency of defrost operation increases, and it also results in discomfort of the occupants of the interior space to be heated. 
       SUMMARY OF THE INVENTION 
       [0006]    The above-discussed problems of prior art heat pumps are overcome or alleviated by the present invention. In accordance with the present invention, an electrically operated flow control defrost valve is positioned downstream of the discharge of the compressor(s) of the heat exchange system, with the defrost valve being in a defrost refrigerant flow line connected between the compressor discharge and the bottom of the outdoor coil. The defrost valve is normally closed, so that refrigerant does not flow through the defrost line during normal (i.e. non-defrost operation) of the system. However, when defrost operation is desired, the defrost valve is opened, and warm refrigerant vapor is delivered from the discharge of the compressor(s) to the bottom of the outdoor coil to heat the outdoor coil and melt accumulated frost/ice. The defrost valve is closed when defrost operation is completed, and normal operation of the heat pump system is resumed. 
         [0007]    An important feature of this invention is that it eliminates the need for the conventional reverse cycle defrosting heretofore used in heat pump systems. Accordingly, this invention eliminates the need to extract energy from the previously heated interior space, since warm indoor air is no longer used as the source of energy for defrosting the outside coil, and cold drafts, which discomfort occupants of the interior space, are also eliminated. 
         [0008]    In the present invention, the frost/ice accumulation on the outdoor coil is melted from the bottom of the coil up to the top, thus eliminating or reducing the problem of possible refreezing of the coil when melted from the top down as in the prior art reverse cycle defrosting. 
         [0009]    Instead of extracting energy from the heated interior space for defrost operation, the energy source for defrost operation in the present invention is a combination of the electrical input to the compressor(s) and an accumulator heating element in the heat pump system. 
         [0010]    The present invention also incorporates a sensor to detect the build-up of frost/ice on the outdoor coil and initiate defrost operation when required. The sensor includes a light emitting diode (LED) and a photo-transistor secured between adjacent fins in the outdoor coil. A build-up of frost/ice between the LED and the photo-transistor partially blocks light transmission, and this blockage of light is used as a defrost initiating signal by the heat pump system microprocessor to initiate defrost operation for a predetermined time period, the time of defrost operation being a function of outdoor air temperature as sensed by the microprocessor at the initiation of defrost operation. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    Referring to the drawings, where like elements are numbered alike in the several figures: 
           [0012]      FIG. 1  is a schematic view of the system of the present invention in a heat pump system having heating and cooling capabilities. 
           [0013]      FIG. 2  is a view showing the frost/ice accumulation detector of the present invention. 
           [0014]      FIG. 3  is a schematic view of the system of the present invention in a heat only heat pump system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring to  FIG. 1 , a heat pump system is shown which is capable of heating and cooling operation. The heat pump system preferably is a boosted air source system having a primary compressor  10  and a booster compressor  12  connected for operation in series. As shown, primary compressor  10  is operating, i.e., on, and booster compressor  12  is inoperative, i.e., off. However, it will be understood that both compressors can be operating, i.e., on, if desired. The system also has an outdoor coil  14  that functions as an evaporator in the heating mode of system operation to extract heat from the outside air, an indoor coil  16  that functions as a condenser in the heating mode of operation to deliver heat to a interior space to be heated, and a 4 way flow control valve  18  that functions to switch the system between the heating to the cooling modes of operation depending on the position of the 4 way valve. The solid line position of 4 way valve  18  is the heating position; the position shown in the dashed lines is the cooling position. Outdoor coil  14  can be, but need not be, an inverted “A” coil. 
         [0016]    The system also has a refrigerant circulation conduit system including: a conduit segment  20  which receives compressed refrigerant from the discharge from primary compressor  10  and delivers the refrigerant through 4 way valve  18  as shown; a conduit segment  22  which delivers the compressed refrigerant to indoor coil  16  to heat the indoor space by heat transfer exchange with air (indicated by arrows flowing over the coil) supplied by a fan  16 A; a conduit segment  24 A that delivers the refrigerant through a check valve  24 B and around a closed thermal expansion valve (TXV)  25  to a conduit segment  26 ; a conduit segment  26  including a thermal expansion valve  27  through which the refrigerant flows to a refrigerant distributor  14 B and then to the bottom of outdoor coil  14  and through outdoor coil  14  to extract heat from the outdoor air flowing over coil  14  (indicated by arrows flowing over the coil) supplied by fan  14 A; a conduit segment  28  that delivers the refrigerant through 4 way valve  18  as shown to conduit segment  29  and then to accumulator  30  where oil entrained in the refrigerant is separated for return to the compressor sumps (separation and return of the oil not shown); and the refrigerant is then delivered to conduit segments  31  and  32  and through check valve  34 ; and the refrigerant then flows to conduit segments  36  and  38  which receive the refrigerant from check valve  34  and deliver the refrigerant to the inlet to primary compressor  10 . The flow of refrigerant through the conduit system for the heating mode of operation is indicated by the arrows in the various conduit segments. 
         [0017]    If booster compressor  12  is also operating, check valve  34  is closed by the discharge pressure from booster compressor  12 , and the refrigerant flows from branch conduit  31  to branch conduit  40 , and then to the inlet to booster compressor  12 ; and the refrigerant discharged from booster compressor  12  is delivered via branch conduit  42  and branch conduit  38  to the inlet to primary compressor  10 . 
         [0018]    For operation of the system in the cooling mode, 4 way valve  18  is moved to the position shown in by the dashed lines, whereby the direction of flow of refrigerant in the system is reversed. In that case, the system operates as in a cooling mode. Heat is extracted from the indoor space at indoor coil  16 , and the heated refrigerant is delivered through 4 way valve  18  and conduit segment  29  to accumulator  30  and to conduit segments  31 ,  32 ,  34 ,  36  and  38  to the inlet to primary compressor  10  if primary compressor  10  is on and booster compressor  12  is off (or if both compressors are on, the refrigerant from indoor coil  16  is delivered via conduit segment  22  and through 4 way valve  18  to conduit segment  29  and accumulator  30  and conduit segment  40  to the inlet to booster compressor  12 , and then from the discharge from booster compressor  12  to the inlet to primary compressor  10 ). The compressed refrigerant discharged from primary compressor  10  then flows via conduit segment  20  and through 4 way valve  18  and via conduit segment  28  to the top of outdoor coil  14  and through coil  14  to discharge heat at the outdoor coil. The refrigerant then flows around closed TXV  27  via bypass line  26 A and check valve  41  to conduit segments  26  and  24  to TXV  25  and to indoor coil  16  where heat is removed from the space to be cooled. The refrigerant is then delivered by conduit segment  22 , 4 way valve  18 , conduit segment  29 , accumulator  30  and conduit segments  31 ,  32 ,  36  and  38  and check valve  34  to the inlet to primary compressor  10 . 
         [0019]    In prior art heat pump systems when operating in the heating mode, frost and/or ice can accumulate on the outdoor coil  14 , necessitating the need to effect a defrost operation. That defrost operation typically involves reversing the operation of the system to the cooling mode, whereby warm refrigerant is delivered to flow from the top of outdoor coil  14  to the bottom of outdoor coil  14  to melt to frost and/or ice accumulated on the coil. 
         [0020]    This is sometimes referred to as “reverse cycle defrost” operation. However, this reverse cycle defrost operation has several problems well known in the art. Perhaps foremost among these problems is that heat energy is extracted from the previously heated space by indoor coil  16  to heat the refrigerant flowing through the system, and that heat energy is transported to outdoor coil  14  to defrost the accumulated frost/ice on outdoor coil  14 . The interior of the previously heated indoor space is thus negatively impacted in two ways. One is by removal of significant amounts of heat energy that has just been delivered to the interior space; the other is by creating an uncomfortable flow of cool air felt by occupants of the previously heated space. Another problem with the prior art reverse cycle defrosting is that the warm refrigerant is delivered to the top of the outdoor coil (the vapor outlet side in heating operation) and melts the frost/ice accumulation from the top of the coil downward. This can result in refreezing on the lower parts of the coil. Especially in situations of high humidity and a temperature at or near freezing for the outside air, frequent defrosting can be required, and system efficiency is impaired. These problems of the prior art are eliminated or substantially reduced in the present invention. 
         [0021]    In accordance with the present invention, a defrost branch conduit  50  is connected from the discharge from primary compressor  10  to refrigerant distributor  14 B at the bottom of outdoor coil  14 . An electrically operated flow control defrost valve  52  in conduit  50  controls the flow of refrigerant in conduit  50  to outdoor coil  14 . Valve  52  is a combination back pressure regulator/on-off control valve. When called upon to open, it will prevent the compressor discharge pressure from dropping below the equivalent of about 70 degrees F., thus ensuring that indoor coil  16  remains sufficiently pressurized to quickly resume heating upon completion of defrost. Defrost valve  52  is closed during normal heating operation of the heat pump system, so no refrigerant flows through conduit  50  to outdoor coil  14  during normal heating operation of the system. However, when frost and/or ice accumulates on outdoor coil  14  requiring defrost, a signal is delivered from the system microprocessor controller  54  to open defrost valve  52 , whereby warm refrigerant vapor discharged from primary compressor  10  is delivered to distributor  14 B and then to the bottom of outdoor coil  14  and flows through outdoor coil  14  to melt the accumulated ice and/or frost. The outlet from distributor  14 B connects to each one of the circuits in outdoor coil  14  to evenly distribute the flowing refrigerant to the outdoor coil circuits. Distributor  14 B has an internal orifice at its entry which further expands the refrigerant exiting TXV  27  during heating operation. Defrost conduit  50  is connected to distributor  14 B downstream (in the direction of refrigerant flow during heating operation) of the internal orifice in distributor  14 B so that the internal orifice does not restrict the flow of defrost refrigerant to outdoor coil  14 . After passing through outdoor coil  14 , the defrost refrigerant passes through conduit segment  28  and through  4  way valve  18  to conduit segment  29  and through accumulator  30  and is delivered to the inlet of primary compressor  10  (or to the inlet to booster compressor  12  if both compressors are on) for compression and delivery of warm refrigerant vapor through defrost conduit  50  to outdoor coil  14  to continue the defrost cycle. 
         [0022]    Most importantly, defrost of outdoor coil  14  is accomplished without moving 4 way valve  18  to the cooling position, and without extracting heat energy from the interior space being heated (as happens in the prior art reverse cycle defrost operation), so the major problems of the prior art reverse cycle defrost operation are eliminated. Also, the warm refrigerant vapor delivered to outdoor coil  14  for defrost is delivered to the bottom of the coil (which previously received liquid refrigerant in the heating cycle), so the frost/ice on outdoor coil  14  is melted from the bottom to the top of the coil, thus reducing or eliminating the prior art problem of refreezing of the coil during defrost operation. 
         [0023]    Accumulator  30  has a heating element  56  which is activated by controller  54  to provide additional heat input to the defrost refrigerant if the warm refrigerant vapor discharged from the compressor(s) is not sufficient to accomplish defrost operation. In effect, the electrical input to drive the primary compressor (or the electrical input to drive both compressors if both are operating) plus the electrical input to the heating element  56  in accumulator  30  is the energy source for defrost operation. It should be noted that signals from controller  54  to fans  14 A and  16 A stop the operation of those fans during defrost operation. Some prior art accumulators have included a heating element used to boil off liquid refrigerant in the accumulator to return the refrigerant to circulation in the system. In the present invention, the heating element in the accumulator performs the function of adding heat energy to the refrigerant to increase the heat energy content of the refrigerant vapor for defrost operation. 
         [0024]    Referring now to  FIG. 2 , a detector system for detecting the need for demand defrost operation is shown.  FIG. 2  shows a partial detail of the outdoor coil  14  of  FIG. 1 . Outdoor coil  14  has a series of refrigerant tubes  60 , with heat exchange coil fins  62  mounted on the tubes  60 . A defrost sensor  66  is mounted between a pair of adjacent fins to detect the accumulation of frost/ice on outdoor coil  14  and trigger operation of the defrost cycle. Defrost sensor  66  includes a light emitting diode (LED)  68  and a phototransistor  70  for sensing the light emitted by LED  68 . Prior to the accumulation of frost/ice sufficient to require defrost operation, enough of the light emitted by LED  68  is sensed by phototransistor  70 , and phototransistor  70  delivers a signal to microprocessor  54  to keep defrost valve  52  closed However, when an amount of frost/ice accumulates on outdoor coil  14  sufficient to require defrost operation, the light received at phototransistor  70  drops below a threshold level, and the signal from the phototransistor  70  to microprocessor  54  also drops below a threshold level. When this happens, microprocessor  54  delivers a signal to defrost valve  52  to open valve  52  and initiates defrost operation. Microprocessor  54  can be programmed either to terminate defrost operation after a predetermined period of time, or to continue defrost operation until the light received at phototransistor  70  from LED  68  is sufficient to indicate that defrost operation has been completed, at which time microprocessor  54  closes defrost valve  52  to terminate defrost operation. A similar detector for ice build-up in aircraft carburetors is the “Iceman” probe of Lamar Technologies Corporation. 
         [0025]    With the use of the detector system of  FIG. 2 , defrost operation is initiated early in the frost/ice accumulation process. Accordingly, the deposited frost/ice is removed quickly from the outdoor coil before any significant impeding of heat transfer from outdoor air into the coil is encountered. 
         [0026]    Operation of the defrost system of the present invention can, if desired, be modulated by sensing the humidity and temperature of the outdoor air to adjust the operation of the defrost system for conditions of high humidity and low temperature 
         [0027]    Referring now to  FIG. 3 , an embodiment of the present invention is shown for a “heating only” system, i.e., a system which has the capacity to heat, but does not have the capacity to cool. The components of the system of  FIG. 3  are numbered as in  FIG. 1 . “Heating only systems are known in the prior art. However, even though such systems do not need a 4 way valve to switch to the cooling mode of operation, most prior art “heating only” systems still require the presence of a 4 way valve to reverse the flow of refrigerant to accomplish reverse cycle defrosting. However, with the presence of defrost conduit  50  and defrost valve  52  to deliver warm refrigerant vapor to defrost outdoor coil  14 , a 4 way valve is not required for the heating only system of  FIG. 3 . 
         [0028]    While preferred embodiments of the present invention have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described and shown by way of illustration and not limitation.