Refrigeration – Automatic control – By accumulation on freezing surface – e.g. – ice
Reexamination Certificate
2001-09-18
2002-10-22
Esquivel, Denise L. (Department: 3744)
Refrigeration
Automatic control
By accumulation on freezing surface, e.g., ice
C062S151000
Reexamination Certificate
active
06467282
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a frost sensor for use in defrost controls for refrigeration and a system that incorporates the frost sensor. In particular, the invention relates to an apparatus and method for sensing the buildup of insulating accumulations or materials (e.g., ice or dirt) on the evaporative heat exchanger of a refrigeration system and for controlling the operation of the system.
The evaporative coils of commercial and industrial refrigeration systems form frost because the evaporative coil surface is below freezing and warmer moist air is blown across the coil in order to cool the refrigerated space. This frost deposition accumulates over time to form ice deposits that reduce the efficiency of the refrigeration system. The efficiency is reduced because the ice acts as a thermal insulator of the cooling coil and also blocks airflow over the cooling fins of the coil. It is therefore necessary to defrost the evaporative coils periodically in order to maintain cooling capacity. Current defrost controls are predominantly time clocks which are typically set to defrost too often and for too long a period.
Significant power savings can be realized by defrosting the heat-exchanger in refrigeration systems only when necessary, and only for as long as necessary. On the order of 5 percent to 12 percent of the total power consumed in refrigeration can be saved by the proper implementation of a demand-defrost system. For a commercial system to be successful, it must: (1) use a rugged sensor (having a longer life than the refrigeration unit, with zero maintenance), (2) have universal application to different heat-exchangers, (3) add no complexity to system operation, and (4) be low-cost in terms of hardware and installation. A variety of types of demand-defrost systems have been proposed.
Optically-based sensors have been installed to detect frost buildup. When frost reaches a preset thickness, a defrost cycle is initiated. Several sensors of this type have been marketed with limited success, primarily because the sensors trigger unnecessary defrosts due to dirty environments and require intensive maintenance. Because sensors have to be located at the display-case evaporator, replacement of a failed sensor is very difficult, often requiring removal of refrigerated product from the case. For this reason, failed sensors are generally replaced with standard timers.
A temperature-sensor-based demand-defrost controller was developed and introduced by Honeywell. The controller employed two temperature sensors located at the inlet and outlet of the display case evaporator. Defrost was initiated when the temperature difference between the two sensors reached a defined setpoint. This controller did not gain wide acceptance because it was prone to triggering defrosts unnecessarily.
A system that was developed but was never made commercially available used the temperature difference between the inlet to the heat-exchanger fan and the air temperature after blowing over the heat-exchanger. Application of this system was limited to systems with specific hardware configurations. Identifying the actual trigger point for defrosting for most systems added too much complexity for the average operator. In addition, the formation of ice on the heat-exchanger blocked much of the air flow and thereby produced a lower flow of air that still had a large enough temperature drop so that defrost was not initiated even after the heat-exchanger frosted.
Pressure-sensor technology measures the change in refrigerant pressure through the evaporator as frost is deposited on the surface and can signal when defrost is required. The magnitude of this pressure difference is very small (a fraction of an inch of water) which is difficult to measure accurately. Moreover, the cost of accurate pressure sensors is prohibitively high, which makes them unattractive for defrost control.
A number of systems that rely on some type of adaptive control have been proposed, with a moderate degree of technical success. These systems were commercial failures because of the complexity of troubleshooting. Since the triggering is not based on a simple, single measurement, the operators cannot easily identify the problem. In virtually all cases, the system is immediately bypassed, reverting back to timing clocks. As a practical matter, once a system has been bypassed, it is seldom repaired.
The background art is characterized by U.S. Pat. Nos. 3,525,648; 3,854,915; 3,945,217; 4,142,374; 4,156,350; 4,173,871; 4,305,259; 4,329,682; 4,344,294; 4,345,441; 4,347,709; 4,348,870; 4,409,795; 4,481,785; 4,528,821; 4,530,218; 4,532,806; 4,608,832; 4,671,072; 4,882,908; 4,993,233; 5,051,645; 5,319,943; 5,345,775; 5,493,867; 5,522,232; 5,692,385; 6,038,872; and 6,092,925; the disclosures of which patents are incorporated by reference as if fully set forth herein.
Poppendiek in U.S. Pat. No. 3,525,648 discloses a thermoelectric heat flow responsive device. An embodiment of this device is incorporated into the invention of U.S. Pat. No. 4,608,832, discussed below. This device is limited in that it appears to be relatively thick, possibly over ⅙ inch in thickness. Moreover, the device has a substantial resistance to heat flow due to the relatively high number of interfaces between the layers that make up the device and due to the thickness of the device. This suggests that a temperature drop of several degrees would occur across the device. This substantial resistance limits the applications of the device to other than frost sensing in a refrigeration system. If the air-exposed side of a heat flux sensor is several degrees warmer than the adjacent fin surface, ice will preferentially form on the fin and not on the heat flux sensor, rendering a defrost control system that incorporates such a heat flux sensor inoperative.
Schulze-Berge et al. in U.S. Pat. No. 3,854,915 disclose a demand defrost system. This invention is limited in that a periodic switch device is required to initiate defrost cycles in response to relative humidity values ambient to a refrigeration system.
Barshark in U.S. Pat. No. 3,945,217 discloses a refrigeration system defrost control. This invention is limited in that it includes a sensor that exhibits a change in resistance as a function of the amount of moisture it absorbs. When the sensor has absorbed a certain amount of water, a defrost cycle is initiated. The sensor must be dried during the defrost cycle.
Ansted et al. in U.S. Pat. No. 4,142,374 disclose a demand defrost time clock control circuit. This invention is limited in that the construction and method of operation of its frost sensor appears to be a conventional optical frost sensor, preferably produced by Altech, Inc.
Elliott et al. in U.S. Pat. No. 4,156,350 disclose a demand defrost control system and method. This invention is limited in that it bases the interval between future defrosting operations on the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrosting operation.
Brooks in U.S. Pat. No. 4,173,871 discloses a demand defrost control system and method. This invention is limited in that it bases the interval between future defrosting operations on the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrosting operation.
Jaeschke in U.S. Pat. No. 4,305,259 discloses a frost sensor employing a self-heating thermistor as a sensor element. This invention is limited in that current must be applied to the thermistor to cause it to increase in temperature during a frost measurement.
Baker in U.S. Pat. No. 4,329,682 discloses a method and apparatus for providing a warning of icing conditions in an aircraft air conditioning system. This invention is limited in that a thermo-electric heat pump is used to maintain a constant difference between the temperature of a cold surface in an air stream and temperature of the air stream.
Gelbard in U.S. Pat. No. 4,344,294 discloses a thermal delay
Butz James R.
French Patrick D.
O'Connor Michael W.
Veatch Bradley D.
Esquivel Denise L.
Hunter Robert M
Norman Marc
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