Defrost control system

Abstract

 A method and apparatus for controlling the defrost of an evaporator coil (10) in a refrigeration or heat pump system comprises monitoring the humidity (H) in the coil environment together with monitoring the temperature (T) in the coil environment or the temperature (T_c) of the coil or both and defrosting the coil be selecting a defrost frequency profile based on the measured temperature or temperatures and humidity.

Description

[0001] The evaporator or outdoor heat exchanger coil of a heat pump operating in the heating mode, like an evaporator coil in a refrigerator or freezer, needs periodic defrosting in order to maintain an acceptable level of heat transfer capability. Currently used defrost initiation methods for heat pumps include using a fixed timer based on either clock time or compressor run time, such as, for example, initiating defrost every 90 minutes when the compressor is running and the outdoor temperature is below 9°C (48 °F). Alternate prior art methods involve initiating defrost after a predetermined pressure drop is reached across the evaporator coil or after the evaporator coil reaches a predetermined temperature below the outdoor temperature. A system of this kind is described in EP-A 31 946. It comprises a controller receiving input signals indicative of the temperature of the outdoor coil and the operation of the compressor. The controller has a timing function which is initiated upon the outdoor coil temperature being at or below a preselected value and the compressor being operated. The duration of the timing function is determined on a substantially continuous basis as a function of the magnitude of the outdoor coil temperature. The controller upon completion of the timing function, places the reverse cycle refrigeration apparatus into an outdoor coil defrost mode of operation.

[0002] It is the main object of the present invention to further improve the function and reliability of a defrost control system as described in the general portion of claims 1 and 5. The solution is characterized in the independent claims as attached to this specification. Preferred embodiments or modifications are described in the subclaims. According to the invention the measurement of outdoor humidity or, in the case of a refrigerator or freezer, the humidity in the coil environment is a key factor in determining when the evaporator coil should be defrosted. Thus, improvements in determining optimum defrost frequency are made by involving a function of

a) outdoor air temperature (or the temperature of coil environment) and outdoor humidity (or the humidity of coil environment), or

b) a function of outdoor or evaporator coil temperature and outdoor (or coil environment) humidity or

c) a function of a difference between the two temperatures and humidity.

[0003] The invention comprises monitoring the humidity in the coil environment together with monitoring the temperature in the coil environment or the temperature of the coil or both and defrosting the coil by selecting a defrost frequency profile based on the measured temperature or temperatures and humidity.

Brief Description of the Drawings

[0004] Figure 1 illustrates a heat pump together with the control system of the present invention.

[0005] Figure 2 shows examples of defrost frequency profiles, i.e. the number of defrost cycles per day C/D in relation to the outdoor temperature T for different values of relative humidity H.

Description of the Preferred Embodiment

[0006] A heat pump is a reversible refrigeration system wherein the functions of the heat exchangers are interchangeable, permitting the heat pump to be a heater during the cooler months and a cooler otherwise. Figure 1 illustrates a typical heat pump comprising first and second heat exchangers 10 and 20, a compressor 30, an expansion valve 40, and a four-way, two-position valve 50. Cold heat exchanger 10 (the one that absorbs the heat of vaporization from the air when the heat pump is operating in the heating mode) tends to frost over, particularly when it is drawing heat from cold winter air. As frost diminishes the effectiveness of heat exchanger 10, it is desirable to remove the frost. Typical systems that use the four-way two-position valve 50 accomplish this by reversing four-way valve 50 so that condensor 10 becomes an evaporator and the frost (on what was formerly the evaporator) melts away. As previously indicated, too frequent or too infrequent defrost cycles reduce the overall coefficient of performance of the heat pump (or of the refrigerator or freezer) and result in energy being wasted.

[0007] Figure 2 illustrates the defrost frequency in cycles per day C/D versus outdoor (or coil environment) temperature T and shows defrost frequency profiles for relative humidities H of 60, 70, 80 and 90%. The data of Figure 2 were computed for the Honeywell W89 Heat Pump Logic Control System using a validated computer program HFROST (see U. Bonne, R. D. Jacobson, A. Patani, D. A. Mueller and G. J. Rowley, "Electric Driven Heat Pump Systems: Simulations and Controls" (paper presented at the 4th Annual Heat Pump Technology Conference, 9-10 April, 1979) and D. A. Mueller and U. Bonne, "New Heat Pump Control Functions via Microelectronics" (paper presented at the 1st EPRI/RWE Conference on Technology and Application of the Electric Heat Pump, Dusseldorf, West Germany, 18-20 June, 1980, Proceedings, pg. 130). These profiles are based on the precondition that defrost operation is initiated on demand at 75% coil blockage.

[0008] A similar relationship may be developed using the temperature T of evaporator coil 10 rather than the outdoor (or coil environment) temperature T; alternately, a differential between the outdoor temperature and the coil temperature may be used. The level of particularity of the profiles may also be substantially increased, such as by providing many defrost frequency profiles, e.g., at 5 percent intervals of relative humidity H rather than at the 10 percent intervals as shown.

[0009] In accordance with the present invention, Figure 1 illustrates a controller 60 comprising a standard digital computer. Controller 60 is coupled to a temperature sensor 70 for monitoring the temperature T of the evaporator coil, a sensor 80 for monitoring the coil environment temperature T, and a sensor 90 for monitoring the coil environment humidity H. Controller 60 is programmed with the defrost frequency profile data in order to reverse valve 50 in accordance with defrost frequency profiles such as illustrated in Figure 2 or in accordance with similar profiles utilizing humidity together with coil temperature or a differential between coil temperature and coil environment temperature. As previously indicated, controller 60 may be programmed with any level of particularity desired.

[0010] The key to the present invention and to the improved coefficient of performance of systems incorporating the present invention is the monitoring of the evaporator coil environment humidity in order to initiate defrost as related to not only coil environment temperature or coil temperature or both, but also as related to actual humidity as measured in the environment of concern.

[0011] In the case of a heat pump system operating in the heating mode, heat exchanger 10 is the outdoor heat exchanger and heat exchanger 20 is located inside the building which is to be heated. In a refrigerator or freezer heat exchanger 10 is the evaporator located inside the space or cabinet which is to be cooled. If the heat pump is operated for cooling a building or space,heat exchanger 10 is the evaporator cooling the air in the building or space. If a heat pump system can alternately be used for heating or cooling a building, the indoor heat exchanger and the out-. door heat exchanger might be provided with sensors 70, 80 and 90 in order to control defrost operation of the indoor heat exchanger during cooling operation in the summer and to control defrost operation of the outdoor heat exchanger during heating operation in the winter. Controller 60 then has inputs for both groups of sensors and the memory associated with its microprocessor has stored defrost frequency profiles for both modes of operation.

Claims

1. Apparatus for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized by:

sensor means (80) for monitoring the temperature (T) in the coil environment;

sensor means (90) for monitoring the humidity (H) in the coil environment; and

means (60) to control defrosting of the coil (10) by selecting a defrost frequency profile based on the measured temperature and humidity.

2. Apparatus for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized in that :

sensor means (70) for monitoring the coil temperature (Tc);

sensor means (90) for monitoring the humidity (H) in the coil environment; and

means (60) to control defrosting of the coil (10) by selecting a defrost frequency profile based on the measured temperature and humidity.

3. Apparatus for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized in that:

sensor means (70) for monitoring the coil temperature (T_c);

sensor means (80) for monitoring the temperature (T) in the coil environment;

sensor means (90) for monitoring the humidity (H) in the coil environment; and

means (60) to control defrosting of the coil (10) by selecting a defrost frequency profile based on the
measured temperatures (T, T ) and the measured humidity (H).

4. Apparatus according to one of claims 1 to 3 for a reversible heat pump system, characterized by sensor means (90; 70 and/or 80) associated with each of the coils (10, 20) and connected to the control means (60).

5. Method for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized by:

monitoring the temperature (T) in the coil environment;

monitoring the humidity (H) in the coil environment; and

defrosting the coil (10)'by selecting a defrost frequency profile based on the measured temperature and humidity.

6. Method for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized by:

monitoring the coil temperature (T_c);

monitoring the humidity (H) in the coil environment; and

defrosting the coil (H) by selecting a defrost frequency profile based on the measured temperature and humidity.

7. Method for controlling the defrost operation of an evaporator coil (10) in a refrigeration system or of the heat absorbing coil of a heat pump system, characterized by:

monitoring the coil temperature (T );

monitoring the temperature (T) in the coil environment;

monitoring the humidity (H) in the coil environment; and

defrosting the coil (10) by selecting a defrost frequency profile based on the measured temperatures and the measured humidity.

Drawing