Control system and method for defrosting the outdoor coil of a heat pump

• US 4,563,877 A
• Filed: 06/12/1984
• Issued: 01/14/1986
• Est. Priority Date: 06/12/1984
• Status: Expired due to Term

• Current Assignee: York International Corporation (Johnson Controls International plc)
• Original Assignee: BorgWarner Incorporated
• Inventors: Harnish, James R.
• Primary Examiner(s): TANNER, HARRY B
• Application Number:
  US06/619,957
• Time in Patent Office: 581 Days
• Field of Search: 62/156, 62/155, 62/151, 62/140, 62/128, 62/80
• US Class Current: 62/80
• CPC Class Codes: F25D 21/002 Defroster control

Abstract

The current outdoor ambient temperature and outdoor coil temperature in a heat pump are sensed when the heat pump'"'"'s outdoor coil is clean and frost-free, and from those current temperatures the split or difference that will later exist between the temperatures, when sufficient frost has built up on the outdoor coil to necessitate defrosting, may be determined. When that defrost temperature split, called the Defrost Value or DV, is reached, defrost is initiated and the frost that has accumulated on the coil is melted. Before defrost occurs, however, changing weather conditions (namely, changing outdoor temperature and/or changing outdoor relative humidity) may effectively invalidate the previously determined defrost temperature split or DV, and a frost condition may be reached at a substantially different temperature split, either greater or smaller than that previously calculated. To ensure that the heat pump is switched to a defrost mode only and always when defrost is needed, the defrost control system continually monitors the outdoor ambient and outdoor coil temperatures and from those temperatures any significant weather condition change may be detected and a new defrost temperature split, that will exist when defrosting becomes necessary under the new weather conditions, will be calculated from the sensed temperatures. When the new defrost temperature split or DV is attained, defrost takes place. Hence, the temperature differential, where defrosting will be required, is effectively updated or adjusted between defrost modes in response to changing weather conditions, thereby optimizing the efficiency of the heat pump and conserving energy.

Claims

1. In a heat pump having an outdoor coil through which refrigerant flows and absorbs heat from outdoor ambient air, a defrost control system for the outdoor coil comprising:

a first temperature sensor for sensing the outdoor ambient temperature;

a second temperature sensor for sensing a temperature which is related to the temperature of the outdoor coil;

control means responsive to said first and second temperature sensors under clean coil conditions for determining a Defrost Value which is the difference that will exist between the two sensed temperatures under frosted coil conditions when defrosting will be required;

and defrost means, controlled by said control means, for defrosting the outdoor coil when the Defrost Value is reached,

said control means functioning, before defrosting occurs, to recalculate the Defrost Value any time there is a predetermined change in sensed temperatures, which will be the result of changes in weather conditions.

2. In a heat pump having a compressor, an indoor coil, and an outdoor coil in thermal communication with outdoor ambient air, and which heat pump may be switched from a heating mode to a defrost mode to defrost the outdoor coil, a defrost control system for the outdoor coil comprising:

a first temperature sensor for sensing the temperature of the outdoor ambient air;

a second temperature sensor for sensing a temperature which is related to the outdoor coil temperature;

control means responsive to said first and second temperature sensors for determining, from the currently sensed temperatures under clean outdoor coil conditions, a Defrost Value which is the difference that will later exist between the two sensed temperatures under frosted coil conditions when defrosting will be necessary;

and defrost means, controlled by said control means, for establishing the heat pump in its defrost mode to defrost the outdoor coil when the Defrost Value is reached by the sensed temperatures,

said control means responding to the two sensed temperatures and functioning, after the Defrost Value has been determined under clean coil conditions but before defrosting occurs, to recalculate the Defrost Value any time there is a predetermined change in sensed temperatures, which will be the result of changes in weather conditions, thereby effectively updating and adjusting the Defrost Value between defrost modes as weather conditions vary so that defrost will occur only and always when it is needed and the efficiency of the heat pump will be optimized.

3. A defrost control system according to claim 2 wherein an initial Defrost Value is calculated after the defrost control system has been powered up and after the compressor has been running, with heating requested, for at least a preset time period following power up of the control system.

4. A defrost control system according to claim 2 wherein, after the outdoor coil has been defrosted, a new Defrost Value, based on the current outdoor ambient temperature and outdoor coil temperature, is not calculated until a given time interval has elapsed since the end of defrost.

5. A defrost control system according to claim 2 wherein the sensed temperatures are averaged over a given time interval before the defrost control system responds to those temperatures.

6. A defrost control system according to claim 2 wherein once a defrost mode has been initiated, the mode will be terminated when the temperature of the outdoor coil increases to a given value.

7. A defrost control system according to claim 2 wherein once a defrost mode has been initiated, the mode will be terminated when a preset time period has elapsed since the start of defrost.

8. A defrost control system according to claim 2 for use in a heat pump where the refrigerant flows, during the heating mode, to the outdoor coil through the heat pump'"'"'s liquid line, said second temperature sensor sensing the refrigerant temperature in the liquid line, which liquid line temperature is essentially the same as the outdoor coil temperature.

9. A defrost control system according to claim 8 wherein the Defrost Value is calculated by adding k₁ to the current outdoor temperature and then subtracting, from the sum, the product of k₂ and the current liquid line temperature, where k₁ and k₂ are constants.

10. A defrost control system according to claim 2 wherein after a Defrost Value has been calculated, and before defrosting occurs, a recalculation is subsequently made if the difference between the current outdoor ambiient temperature and outdoor coil temperature decreases by a predetermined amount from the difference between those temperatures existing at the time of the last calculation.

11. A defrost control system according to claim 2 wherein after a Defrost Value has been calculated, and before defrosting occurs, a recalculation is subsequently made if the current outdoor coil temperature increases by a predetermined amount from the outdoor coil temperature existing at the time of the last calculation.

12. A defrost control system according to claim 2 wherein a defrost operating mode cannot be initiated if the outdoor coil temperature is above a preselected level.

13. A defrost control system according to claim 2 wherein once a defrost mode has been initiated, the mode will be terminated when the instantaneous outdoor coil temperature increases to a given value.

14. A defrost control system according to claim 2 for use in a heat pump having a reversing valve for reversing refrigerant flow between the indoor and outdoor coils to switch the operation of the heat pump from a heating mode to a defrost mode, the reversing valve being controlled by said defrost means.

15. In a heat pump having an outdoor coil through which refrigerant flows and absorbs heat from outdoor ambient air which flows across the outdoor coil, a defrost control system for the outdoor coil comprising:

a first temperature sensor for sensing the outdoor ambient air temperature;

a second temperature sensor for sensing the temperature of the air leaving the outdoor coil;

control means responsive to said first and second temperature sensors under clean coil conditions for determining a Defrost Value which is the difference that will exist between the two sensed temperatures under frosted coil conditions when defrosting will be required;

and defrost means, controlled by said control means, for defrosting the outdoor coil when the Defrost Value is reached,

said control means functioning, before defrosting occurs, to recalculate the Defrost Value any time there is a predetermined change in sensed temperatures, which will be the result of changes in weather conditions.

16. In a heat pump having a compressor, an indoor coil, and an outdoor coil in thermal communication with outdoor ambient air, a method for defrosting the outdoor coil to melt the frost accumulated thereon during operation of the heat pump in its heating mode, comprising the steps of:

sensing the temperature of the outdoor ambient air;

sensing the temperature of the outdoor coil;

initially determining, from the two sensed temperatures under clean outdoor coil conditions when the coil is devoid of frost, an initial Defrost Value which is the difference that will later exist between the two sensed temperatures under frosted conditions when defrosting is necessary;

before the initial Defrost Value is reached, continually updating and adjusting the Defrost Value, based on the current outdoor air and outdoor coil temperatures, in the event that weather conditions change by a predetermined extent;

and defrosting the outdoor coil when the Defrost Value is attained by the two sensed temperatures.