US3065608A - Defrost control - Google Patents

Description

Nov. 27, 1962 w. A. ARZBERGER DEFROST CONTROL Filed March 1, 1957 INVENTOR. WILLIAM A. ARZBERGER.

ATTORNEY.

FIG. 3

3,065,608 DEFROST CONTROL William A. Arzberger, North Syracuse, N.Y., assignor to Carrier Corporation, Syracuse, N.Y., a corporation of Delaware Filed Mar. 1, 1957, Ser. No. 643,397 4 Claims. (Cl. 62$ti) This invention relates to a defrosting control for an evaporator of a refrigeration system.

In the utilization of refrigeration systems of the compressor-condenser-evaporator type, one of the major problems involved in obtaining a high order of efficiency is that of removing the coating of frost formed on the evaporator as moisture deposited thereon freezes. For illustration the evaporator in a domestic refrigerator in cooling the air circulating within the refrigerator, has deposited on the surface thereof, a coating of frost during its operation. This frost occurs as moisture is precipitated from the air circulating within the compartment as the evaporator cools the air to a temperature below its dew-point temperature. The temperature of the evaporator being below the freezing point of Water, the moisture deposited on the evaporator surface freezes to form a frost formation. This frost is objectionable in this location because it acts as an insulator between the refrigerated evaporator and the air inside the cabinet. Various automatic devices are currently used for removing this frost. The arrangement proposed here consists of inactivating the refrigeration system evaporator when the evaporator temperature is reduced sufficiently to cool the air to a desired low temperature and reactivating the evaporator when all of the ice on the evaporator has been melted by means of the Warming air passing over it. This type of control of the evaporator is ordinarily accomplished by a thermostat having a control bulb attached to the evaporator surface.

Placement of the bulb on the evaporator has been found to have disadvantages. First, the cut-in temperature of the thermostat must be rather high to insure complete defrosting. Second, as this bulb is sensing metal temperatures and is not responsive to the air temperature, the cabinet air temperature varies considerably as the amount of heat leakage through the cabinet insulatioii varies in response to ambient changes. Third, in the use of combination cabinets with a frozen food section, and another section for normal above freezing storage, placement of the control bulb on the freezing section evaporator, results in excessively low refrigerator temperature, freezing ice or food in the refrigerator compartment.

This invention has for its chief object the provision of an automatically operable defrosting control for effectively removing the coating of frost from the evaporator during those periods when it is necessary to free the surface of the coil of the frost in the interest of obtaining more effective operation of the evaporator.

An additional object of the invention is the provision of a method of defrosting the evaporator of a refrigeration system which ensures a complete removal of the frost before the defrosting action is terminated.

In attaining the objects of this invention, it is proposed that a bulb, forming a part of a conventional thermalresponsive unit which controls the operation of a switch in a defrost circuit, be supported in spaced relation to the evaporator so as to be influenced by the run off of moisture that occurs during the defrosting operation. The thermal-responsive unit is used to sense a temperature of a value to indicate the necessity for defrost, and to institute the defrosting action. During the defrosting action, the frost on the surface of the evaporator coil is melted and flows from the evaporator through a path 3,065,608 Patented Nov. 27, 1962 that includes the bulb. Thus the bulb senses a temperature in the vicinity of 32 which is sufficiently low so that termination of the defrosting action and resumption of the normal operation of the evaporator is precluded. However, once the moisture has drained from the evaporator coil, the bulb senses an air temperature sufficiently high to indicate the need for termination of the defrosting cycle and resumption of the normal freezing cycle.

These objects and features of the invention as well as other objects and features will be apparent from a consideration of the ensuing specification and drawings in which FIGURE 1 is a diagrammatic view of a refrigeration system with which the defrosting control forming this invention may be used;

FIGURE 2 is an isometric view of an evaporator equipped with a portion of the defrost control;

FIGURE 3 is a detail illustrating one mannerin which the defrost control bulb of the thermal-responsive system may be connected to the evaporator; and

FIGURE 4 is a view similar to FIGURE 3 showing another arrangement for subjecting the defrost control bulb to the moisture flowing from the evaporator during the defrosting cycle.

Referring more particularly to the drawings, there is shown one embodiment of the invention merely for the purpose of describing the invention. FIGURE 1 is a diagrammatic View of a refrigeration system which may be employed in a refrigerator of the domestic type wherein there is provided a first compartment, for cooling foods, served by a first evaporator and a second compartment, for maintaining foods in a frozen condition, which is served by a second evaporator. In refrigeration systems of this type, the two refrigerators operate at different temperature levels. This invention is concerned with the defrost of the evaporator serving the compartment used for cooling the foods. This is normally referred to as the refrigerator evaporator.

In the system shown, refrigerant is discharged in the gaseous phase from motor compressor 10 to the condenser 12 through a discharge line 11. Gaseous refrigerant is liquefied in the condenser 12 and flows through line 13 to a freezer capillary 14 and into freezer evaporator 15. Refrigerant flowing from the 'line 13 also flows through a refrigerator capillary 16 in series with a refrigerator evaporator 17. From each evaporator, refrigerant in the gaseous and/ or liquid phase flows to the accumulator 18 where the gaseous portion of the refrigerant flow is delivered to the compressor through suction line 19.

Refrigerant flows continuously through the circuit described so that heat sufficient to cause vaporization of the refrigerant in the two evaporator sections is absorbed from the air flowing within the compartment being served by each evaporator. The air circulated within the refrigerator compartment or the compartment served by the evaporator 17 is cooled below its dew-point so that moisture from the air is deposited on the surface of the evaporator unit 17. The moisture so deposited is con verted into frost because of the surface temperature of the evaporator which is below 32.

This accumulation of frost builds up over a certain length of time to a point where the operating efficiency of the evaporator is seriously impaired. In order to restore the evaporator to its original efiicient level of operation it is necessary to remove the frost in an action normally referred to as a defrost action. This invention contemplates the removal of this frost automatically and likewise, contemplates the resumption of the operation of the refrigerant evaporator once the coating of frost has been removed.

To this end, there is located in close proximity to the evaporator, a trough 19; serving to collect the moisture flowing by gravity from the evaporator 17 during the defrosting action. The trough is shown as being substantially U-shaped and having a pitch or inclination such that moisture flows down the opposed legs of the trough to the connecting portion and from the connecting portion through a drain 20" where it is disposed of in any one of a number of conventional manners. 'Referring toFIGURE 2, there is shown a type of evaporator unit that is used in the cooling compartments of a domestic refrigerator of the kind contemplated. The evaporator coil 17 is secured to the back portion of a plate 38 which in turn may be secured to the walls of a compartment of a domestic refrigerator. In the interests of clarity, the refrigerator construction has not been illustrated. However, it will be evident that a conventional refrigerator having one or more compartments for the reception of food may be employed. The compartment normally contains opposed side walls as well as top, bottom and rear walls. The plate 38 is secured in fixed relation to the side walls and back of the compartment. Likewise, the trough 19' is also connected as by fastening elements 20 and insulating bumper means 21 to the walls of the compartment.

It is contemplated that the defrosting of the evaporator 17 occur by' virtue. of the operation of a solenoid valve located in the line connecting the evaporator with the accumulator 18. When a condition is sensed necessitating a need for defrost action, solenoid valve 40 is closed so as to prevent the flow of refrigerant through coil 17. Thisinactivates the coil and permits the temperature of the air in the compartment being served by the coil to rise. Oncethe temperature in the compartment rises above 32, the fro-st accumulation on the coil will melt and drain off of the evaporator into the, trough 19 and be carried away through the drain 20'.

Referring to FIGURE 1, it will be noted that the coil of solenoid valve 40 is disposed in a circuit including a switch 26 operable under the influence of a thermalresponsive system 22. The thermal-responsive system includes a bulb 23, a bellows 24 and a capillary 25 conmeeting the bulb and the bellows. The system, as described, is a conventional unit wherein a thermal-responsivfe'fill is enclosed within the three elements 23, 24 and 25.

In order to effectively attain the objects of this invention, it is proposed that the bulb 23 of the thermal system 22 be placed in relation to the evaporator coil so that it'is subject to the run off or drain of moisture from the 'coil. This may be accomplished in a manner illustrated in FIGURE 3 wherein a strap 28, fixed to the evaporator plate, 38, supports the bulb in such a manner that it is subjectedto the temperature of the air circulating about the evaporator as well as the moisture which drains 013? of the evaporator. Another arrangement for accomplishing this is shown in FIGURE 4. Here, strut numbers 30 support the bulb in space relation to the evaporator coil. The struts 30 are supported from the upper ends of the wall portions of the trough 19.

In refrigeration systems of the type described here, there is normally included an arrangement for defrosting or limiting the temperature of the freezer evaporator. Accordingly, a second thermal-responsive unit 32, including a bulb 33, secured to the freezer evaporator, a bellows 34, and a connecting capillary 35, is shown. When the temperature of the freezer coil drops to a predetermined low'leve l, indicating the existence of a coating of frost on the freezer evaporator coil, bellows 34 contracts to open switch 36 and interrupt a circuit through the compressor motor 10.

Considering the operation of this invention the refrigeration system operates in its normal intended manner until a predetermined coating of frost appears on the evaporator. This coating of frost is determined by the temperature sensed by the bulb 23, in a following man.-

ner. As shown in FIGURE 3, the bulb 23, is subject not only to the air temperature in the immediate vicinity of the evaporator but is also subject to the temperature of the refrigerator by conduction through the connecting support clip 28. This latter effect, however, is of a secondary significance because the strap is composed of a material having a high resistance of heat transfer. When the bulb senses a temperature of about 13 F., the thermal responsive fill in the thermal system cools sufficiently to reduce the pressure in the unit and cause the bellows to contract and close the switch 26 completing a circuit through the coil of the solenoid valve 40. This action closes the solenoid valve which in turn stops the flow of the refrigerant through the coil 17. This action has the effect of de-activating coil 17 causing the temperature within the compartment being served by the evaporator to rise. Once the evaporator temperature rises over 32, the frost on the coil melts and drains by gravity into the trough 19 flowing over bulb 23 in the process. It will be apparent that the air temperature of the evaporator compartment will be above 32. Inasmuch as the cut-in point of the bulb 23 may be set for a temperature of about 37 F., it is obvious that the bulb must be subjected to some temperature other than air temperature in order to insure complete defrosting of the coil. With the construction shown, it is obvious that the coil is being, subjected to a flow of moisture having a temperature below the cut-in point of 37. Once the coil has been completely defrosted, and the flow of water has been terminated, the bulb 23 then senses the air temperature within the compartment. By this time, the temperature of the air flowing over the bulb from the evaporator has risen above 37 causing the thermal-responsive fill in the unit 22 to expand and expand the bellows 24 to open the circuit and de-energize the coil in solenoid valve 30. Once again refrigerant is free to flow through the evaporator 17 and the system once again resumes a normal operation.

Thus by placing the bulb in the position shown in FIG- URE 3, the moisture draining from the coil during the defrosting operation determines the temperature sensed by the bulb. Once the moisture flow terminates, the air within the compartment flows over the bulb and its temperature determines the temperature sensed by the bulb. Flow of air within the compartment over the evaporator obtains by virtue of convection. When the bulbis placed as shown it is subjected to the coldest air (direct flow from the evaporator) in the compartment. The arrows on FIGURE 3 indicate the direction of a portion of air flow in the compartment. Under this arrangement a wider temperature differential between the cut-in and cut-out points may be selected. With such a range a less expensive thermal unit may be used. Also the bulb is located where the air is moving relatively fast as compared to the movement of the air stream in other parts of the compartment. This allows a faster response to be obtained, an important feature when a load is placed on the freezer evaporator 15. Under these latter circumstances the back pressure in the suction line rises causing a corresponding rise in suction temperature. This rise in temperature likewise causes a rise in the evaporator temperature so that when the bulb 23 is placed directly in contact with the evaporator surface, the air temperature could decrease while the evaporator temperature was still above the defrost cut-out point.

Thus it will be obvious that an automatic defrost control arrangement is provided for insuring complete removal of frostfrom the coil before the defrosting action is terminated and the normal operation of the evaporator is restored.

Other arrangements and constructions capable of performing this invention will suggest themselves to those skilled in the art. The described operation is equally applicable to a single compartment refrigerator,

wherein. the compressorstarts and stops by this thermoe,ees,eos

stat. However, the above description is offered for the purpose of illustrating the operation of an embodiment of the invention and is not intended to limit the invention as defined in the appended claims.

I claim:

1. In a refrigeration system for cooling air circulating within an enclosure and including a compressor, a condenser and an evaporator over which said air circulates, means for removing frost from the surface of said evaporator; trough means disposed beneath said evaporator to receive moisture draining therefrom during defrost; and control means comprising a thermal responsive element supported in spaced relation to said evaporator so as to be responsive to the temperature of the air circulating within the enclosure as well as the flow of moisture from the evaporator to the trough as the evaporator defrosts whereby said control means is operative to effect defrost as long as moisture drains from said evaporator and is operative to terminate defrost in the absence of drainage and in the presence of the relatively warm air circulating over the evaporator as drainage ceases.

2. The invention set forth in claim 1 wherein said thermal responsive element is suspended from said evaporator by a strap formed of a material having relatively high resistance to the transfer of heat.

3. The invention set forth in claim 1 wherein struts are provided supporting said thermal responsive element in spaced relation to said evaporator.

4. The method of defrosting an evaporator of a re frigeration system employed to cool air circulating within an enclosure over said evaporator which consists in the steps of interposing a thermal control element in spaced relation to said evaporator in the air stream fiowing over said evaporator, arranging said element so as to be responsive to the flow of moisture from the evaporator as said evaporator defrosts, terminating flow of liquid refrigerant through the evaporator in response to a predetermined air temperature to raise the temperature of the evaporator to defrost same, continuing the defrost operation as long as the fiow of moisture over said thermal control element ensues, and terminating defrost of the evaporator in the absence of drainage of moisture from the coil and in the presence of an air temperature of a predetermined maximum value.

References Cited in the file of this patent UNITED STATES PATENTS 1,516,739 Ketterer Nov. 25, 1924 2,135,875 Morse Nov. 8, 1938 2,479,011 Kernper Aug. 16, 1949 2,687,020 Staeb-ler et al. Aug. 24, 1954 2,716,867 Jacobs Sept. 6, 1955 2,729,074 Monroe Jan. 3, 1956 2,773,354 Tillman Dec. 11, 1956 2,867,093 Simmons Jan. 6, 1959

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