Frost sensor for use in defrost controls for refrigeration

• US 6,467,282 B1
• Filed: 09/18/2001
• Issued: 10/22/2002
• Est. Priority Date: 09/27/2000
• Status: Expired due to Fees

• Current Assignee: ADA Technologies, Inc.
• Original Assignee: ADA Technologies, Inc.
• Inventors: Veatch, Bradley D., French, Patrick D., Butz, James R., OʼConnor, Michael W.
• Primary Examiner(s): Esquivel, Denise L.
• Assistant Examiner(s): NORMAN, MARC E
• Application Number:
  US09/957,169
• Time in Patent Office: 399 Days
• Field of Search: 621/40, 621/51, 621/56, 621/71, 621/80, 622/594, 623/05, 165/104.33, 374/29, 374/30
• US Class Current: 62/140
• CPC Class Codes: F25B 2700/11 Sensor to detect if defrost is necessary
• CPC Class Codes: F25D 21/006 with electronic control circuits
• CPC Class Codes: F25D 21/02 Detecting the presence of frost or condensate
• CPC Class Codes: G01N 25/18 by investigating thermal conductivity by calorimetry G01N25/20; by measuring change of resistance of an electrically-heated body G01N27/18

Abstract

An apparatus and method for measuring the total thermal resistance to heat flow from the air to the evaporative cooler fins of a refrigeration system. The apparatus is a frost sensor that measures the reduction in heat flow due to the added thermal resistance of ice (reduced conduction) as well as the reduction in heat flow due to the blockage of airflow (reduced convection) from excessive ice formation. The sensor triggers a defrost cycle when needed, instead of on a timed interval. The invention is also a method for control of frost in a system that transfers heat from air to a refrigerant along a thermal path. The method involves measuring the thermal conductivity of the thermal path from the air to the refrigerant, recognizing a reduction in thermal conductivity due to the thermal insulation effect of the frost and due to the loss of airflow from excessive ice formation; and controlling the defrosting of the system.

Claims

1. A frost sensor for mounting on an evaporative heat exchanger that is exposed to an airflow, said frost sensor comprising:

a printed circuit board comprising an electrical circuit;

a first temperature sensor that is mounted on said printed circuit board and that is electrically connected to said electrical circuit;

a copper clip;

a heat flux sensor that is mounted on said copper clip and that is electrically connected to said electrical circuit;

a spring clip that is operative to hold said heat flux sensor or said copper clip and said first temperature sensor against a first cooling fin of said evaporative heat exchanger and to hold a part of said printed circuit board against said first cooling fin on the side of said cooling fin opposite the side against which said heat flux sensor or said copper clip is being held;

a second temperature sensor that is mounted on said printed circuit board in such a manner that the airflow impinges on the second temperature sensor before it impinges on said first cooling fin; and

an electrical connector that is mounted on said printed circuit board and electrically connected to said electrical circuit.

2. The frost sensor of claim 1 wherein said heat flux sensor is mounted on said copper clip by means of an adhesive and said copper clip is held against said first cooling fin.

3. The frost sensor of claim 1 further comprising:

a third temperature sensor that is in thermal contact with said evaporative coil at a location where ice tends to melt the slowest and that is thermally insulated from said airflow.

4. A defrost control system that comprises a controller and an evaporative heat exchanger to which is attached the frost sensor of claim 1.

5. A refrigeration system that comprises a compressor, a condenser and an evaporative heat exchanger to which is attached the frost sensor of claim 1.

6. A device for sensing frost on a cooling fin of an evaporative heat exchanger that is subjected to an airflow, said device comprising:

a heat flux sensor that is in thermal contact with and located on one side of said fin;

a thermal insulator that is in thermal contact with said fin and located on the other side of said fin, opposite the location of said heat flux sensor;

a first temperature sensor that is in contact with said airflow before said airflow is in contact with said fin; and

a second temperature sensor that is in thermal contact with said fin and that is thermally insulated from said airflow.

7. The device of claim 6 further comprising:

a third temperature sensor that is in thermal contact with said evaporative heat exchanger at a location where ice tends to melt slower than said ice melts in the vicinity of said fin and that is thermally insulated from said airflow.

8. The device of claim 6 wherein said heat flux sensor is a differential thermopile.

9. The device of claim 6 in which said thermal insulator is at least a portion of a printed circuit board.

10. A sensor for characterizing the heat-transfer effectiveness of an evaporative coil that comprises a plurality of cooling fins that are subject to insulating accumulations, said sensor comprising:

means for measuring the heat flow from an airflow to a first cooling fin that produces a first signal;

means for measuring the temperature of said first cooling fin that produces a second signal; and

means for measuring the temperature of said airflow that produces a third signal;

wherein said three signals are used to quantify the total thermal conductivity of said accumulations.

11. The sensor of claim 10 wherein said third signal is used to determine when said coil has been adequately defrosted during a defrost cycle.

12. The sensor of claim 10 further comprising:

means for scheduling the defrost cycle for said evaporative coil.

13. The sensor of claim 10 further comprising:

means for measuring the temperature of a second cooling fin that produces a fourth signal, said second cooling fin being located where ice tends to melt more slowly than it does on said first cooling fin during a defrost cycle.

14. The sensor of claim 12 wherein said fourth signal is used to determine when said coil has been adequately defrosted during a defrost cycle.

15. A method for defrosting a refrigeration system that includes an evaporative heat exchanger having a fin that is exposed to an airflow and that is subject to an insulating accumulation, said method comprising:

measuring the heat flux from the airflow into the fin, the temperature of the fin and the temperature of the airflow;

calculating the total thermal conductivity or total thermal resistance value of said insulating accumulation; and

initiating a defrost cycle when the total thermal conductivity or total thermal resistance value reaches a setpoint.

16. The method of claim 15 further comprising:

terminating said defrost cycle when the temperature of the fin reaches a target temperature.

17. The method of claim 15 further comprising:

comparing heat flux data collected at a previous time with heat flux data collected at a subsequent time; and

initiating an alarm procedure if the comparison indicates that the measured heat flux is trending downward.

18. A technique for operating a refrigeration system that includes an evaporative heat exchanger having a fin that is exposed to an airflow and that is subject to an insulating accumulation, said technique comprising:

measuring the heat flux from the airflow into the fin, the temperature of the fin and the temperature of the airflow;

determining whether the heat flux is below an expected value;

if the heat flux is not below the expected value, calculating the total thermal conductivity or total thermal resistance value of said insulating accumulation and applying a setpoint adjustment factor;

resetting the system timers;

determining whether the system is in an auto defrost mode;

if the system is in an auto defrost mode, determining whether the heat flux or the or the total thermal conductivity or total thermal resistance value has reached a setpoint;

if the system is not in an auto defrost mode, determining whether the time until defrost has expired; and

if either the heat flux or the total thermal conductivity or total thermal resistance value has reached a setpoint or the time until defrost has expired, initiating a defrost cycle.

19. The technique of claim 18 further comprising:

if the heat flux is below the expected value, determining if the heat flux is downward trending;

if the heat flux is downward trending, concluding that an evaporator dirt accumulation is indicated, sending an evaporator dirt accumulation alarm message and setting the system to a timer mode; and

if the heat flux is not downward trending, concluding that an equipment failure is indicated, sending an equipment failure alarm message and setting the system to a time mode.

20. The technique of claim 18 further comprising:

if a defrost cycle has been initiated and the system is in an auto defrost mode, determining whether the fin temperature has reached a fin temperature target;

if the fin temperature has reached the fin temperature target, comparing the actual defrost time to the target defrost time;

if the actual defrost time is less than the target defrost time, decreasing the setpoint adjust factor;

if the actual defrost time is greater than the target defrost time, increasing the setpoint adjust factor; and

if the system is not in the auto defrost mode, determining whether the time until defrost has expired.

21. A process for control of frost in a system that transfers heat from air to a refrigerant along a thermal path that passes through a surface of a fin, said process comprising:

measuring the thermal conductivity of the thermal path from the air into a single portion of the surface of the fin by means of a sensor that is capable of measuring the heat flux from said air into the single portion of the surface;

recognizing a reduction in thermal conductivity due to the thermal insulation effect of the frost and due to the loss of airflow from excessive ice formation; and

controlling the defrosting of the system.

22. A refrigeration system comprising a compressor, a condenser and an evaporative heat exchanger, said system being operated in accordance with the process of claim 21.

23. A method for defrosting a refrigeration system that includes an evaporative heat exchanger having a fin with a surface that is exposed to an airflow and that is subject to an insulating accumulation, said method comprising:

measuring the heat flux from the airflow essentially perpendicularly into the surface of the fin;

calculating the thermal conductivity or thermal resistance value of said insulating accumulation; and

initiating a defrost cycle when the thermal conductivity or total thermal resistance value reaches a setpoint.

24. A device for sensing frost on a cooling fin of an evaporative heat exchanger that is exposed to an airflow, said device comprising:

a heat flux sensor having a side that is exposed to said airflow and another side that is in thermal contact with a first portion of said fin and being configured to measure the heat flow from said airflow essentially perpendicularly into said first portion of said fin.

25. The device of claim 24 wherein the temperature of said side exposed to said airflow does not differ from the temperature of a second portion of said fin adjacent to said heat flux sensor by more than about three degrees Fahrenheit.

26. A method for defrosting a refrigeration system that includes an evaporative heat exchanger having a fin with a surface that is exposed to an airflow and that is subject to an insulating accumulation, said fin having attached thereto a heat flux sensor, said method comprising:

measuring the rate at which heat is flowing from the airflow along a path that passes through any insulating accumulation, through the heat flux sensor and into the surface of the fin;

calculating the thermal conductivity or thermal resistance value of any insulating accumulation; and

initiating a defrost cycle when the thermal conductivity or total thermal resistance value reaches a setpoint.

27. A device for sensing frost on a cooling fin of an evaporative heat exchanger that is exposed to an airflow, said device comprising:

a heat flux sensor having a side that is exposed to said airflow and another side that is in thermal contact with a first portion of said fin, said heat flux sensor being configured to measure the heat flow from said airflow through said heat flux sensor and into said first portion of said fin.

28. A device for sensing frost on a cooling fin of an evaporative heat exchanger that is exposed to an airflow, said device comprising:

a heat flux sensor having a side that is exposed to said airflow and another side that is in thermal contact with a first portion of said fin, said heat flux sensor being comprised of a thin film with hot junctions being spread over one of said sides and cold junctions being spread over the other of said sides to measure the heat flow from said airflow through said heat flux sensor and into said first portion of said fin.