Automatic defrosting control system

Abstract

An automatic defrosting control system for a refrigeration system having an evaporator or the like for absorbing heat from a zone to be cooled and a cold control to maintain the zone substantially at a preselected temperature, the evaporator being subject to the accretion of frost thereon which thereby increases the periods of energization of the evaporator in order to maintain the zone at the preselected temperature. The control system comprises means for causing defrosting of the evaporator, a thermostat and a thermal time-delay relay.

Description

BACKGROUND OF THE INVENTION

This invention relates to a defrost system for various types of refrigeration apparatus, the defrost system being operable in response to the build-up of frost on the cooling unit (e.g., the evaporator) of the refrigeration apparatus.

The accumulation or build-up of frost on the evaporator of a refrigerator or other refrigeration unit has long been a problem. Various automatic defrosting systems have been used and are well known in the art. Typically, an automatic defrost system is controlled by a timer which initiates operation of the defrost system at certain times of the day or after the compressor has run a predetermined length of time. The rate at which frost forms on the evaporator is a function of the amount of water vapor in the air passing over the evaporator, the greater the water content the faster the frost accumulates. In a refrigerator, the amount of water vapor within the air to be cooled depends a great deal on the ambient conditions (i.e., room temperature and relative humidity) outside the refrigerator because ambient air is introduced into the refrigerator each time the door is opened and closed, and water vapor sources (e.g., wet produce and open containers of liquids) within the refrigerator. With time-controlled defrost systems and with a slow build-up of frost, operation of the defrost system is sometimes initiated before any significant amount of frost has built up on the evaporator, thus resulting in a wastage of power to defrost the refrigerator when it is not required and exposing the items in the refrigerator to unnecessary defrost cycles. On the other hand, under heavy frost conditions, excessive frost may build up on the evaporator between the timed defrost cycles, thus reducing the efficiency of the refrigerator, increasng the power consumed thereby and warming foodstuff that should be kept cool, resulting in shorter shelf-life for refrigerated foods and possible contamination unknown to the user.

Another defrosting system is one in which the number of door openings are counted and a defrosting cycle is initiated after a selected number of openings occur. This arrangement is disadvantageous in that an unused or little used refrigerator would not be defrosted even though a substantial frost deposit has built up. Also, mechanical counters are relatively unreliable in continued use. Depressed temperature systems have also been utilized where defrosting cycles are initiated when the evaporator reaches a temperature much lower than its normal operating temperature. This depressed evaporator temperature occurs after ice forms on the evaporator, reducing its efficiency. Depressed temperatures systems have not been too successful because the low temperature varies from evaporator to evaporator due to production tolerances. Good sensing of the depressed temperature has been difficult due to inconsistency of heat transfer materials used between the evaporator and the sensing control. Depressed temperature systems have, as a rule, been more expensive than the systems in current usage.

Other systems utilized have been restricted air-flow methods with electronic sensors, but these are relatively expensive and difficult to build in production. Fluidic systems initiating defrost based on pressure changes in the refrigerating equipment are also expensive.

SUMMARY OF THE INVENTION

Among the several objects of this invention may be noted the provision of an automatic defrost system for refrigeration apparatus (e.g., a refrigerator, a freezer, a refrigerated vending machine, or an air conditioner) in which build-up of frost on the evaporator of the refrigeration apparatus initiates a defrosting cycle rather than having defrosting initiated by a clock or counter mechanism; the provision of a demand defrost system which conserves power and increases the operating efficiency of the refrigeration apparatus by eliminating unnecessary defrost cycles and by keeping the refrigeration apparatus free of excessive frost; the provision of such a defrost system which maintains better temperature control in the refrigeration system and which does not expose refrigerated or frozen items to unnecessary defrost cycles; the provision of defrost systems of the class described which permit a fast cool down of warm foods, fast freezing of foods and increased ice production; the provision of such a defrost system which is relatively simple and of economical construction and which will reliably operate regardless of ambient climatic conditions. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, an automatic defrosting control system of this invention comprises means for causing defrosting of the refrigeration system cooling means, a thermostat and a thermal time-delay relay. The thermostat is positioned adjacent and in heat-exchange relation with both the defrosting and cooling means and is connected in a control circuit. The thermostat has a first switching position for terminating energization of the defrosting means and a second switching position for enabling operation thereof. It switches from its first and its second position in response to its temperature falling to a lower predetermined level and switches to its first position in response to its temperature rising to a higher predetermined level. The thermal time-delay relay if positioned in heat-exchange relation with the cooling means and is also connected in the control circuit. The relay has a first switching position for permitting operation of the cooling means and a second switching position for energizing the defrosting means and preventing operation of the cooling means. The relay switches from its first position to its second position upon reaching a given temperature and switches from its second to its first position upon reaching a different preselected temperature. The thermostatic means responsive to the zone temperature periodically energizes the cooling means to cycle between an "on" mode and an "off" mode to maintain the zone substantially at the preselected temperature. Also provided are means responsive to the duration of one of said modes to supply heat to the relay whereby upon a sufficient accretion of frost forming on the cooling means the increased duration of the "on" mode will cause the temperature of the relay to reach its given temperature and switch to its second position and will cool the thermostat to cause its temperature to fall below its lower predetermined level and switch to its second position thereby energizing the defrosting means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in section of refrigeration apparatus employing a defrost system of this invention and illustrating the normal flow of air therethrough when the refrigeration apparatus is in operation;

FIG. 2 is a circuit diagram of a defrost system of this invention;

FIG. 3 is a view, partially in section, of a thermal time-delay relay utilized in a defrost system of this invention; and

FIG. 4 is a graphical representation of the temperatures of various components of the refrigeration system, the thermostat and the relay during operation beginning with a frost-free condition after a defrost period, through normal operation and frost accumulation to a point where the frost deposit is substantial enough to initiate defrosting.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings, a defrost system of this invention, as indicated generally at 1, is shown installed in a conventional two-door refrigerator-freezer 3. The refrigerator-freezer includes a cabinet 5 having a top 7, side walls 9, a back wall 11, a bottom wall (not shown) and a partition 13 dividing the interior of the cabinet and defining a freezer compartment 15 and a food compartment 16, these compartments constituting refrigerated zones. A freezer door 17 and refrigerator door 19 close the front of the cabinet. The refrigerator includes a conventional refrigeration system including a compressor driven by an electric compressor motor C (see FIGS. 3 and 5-7), a condenser (now shown) and a cooling unit or evaporator generally indicated at 23. The evaporator includes a plurality of refrigerant lines 25 constituting a coil, this coil being subject to frost build-up. A flow path generally indicated at 27 provides for the intake of air from both the freezer and food compartments, for the passage of this air over the evaporator for absorbing heat from the air and thus chilling the air, and for the discharge of the chilled air into the refrigerated compartments or zones. A blower or fan 29 is provided for forcing air through the flow path. While the defrost system of this invention is depicted as installed in a two-compartment refrigerator-freezer, it will be understood that it may be installed in other refrigeration apparatus, such as a single-compartment refrigerator, a freezer, a refrigerated vending machine, or an air conditioner.

More particularly, flow path 27 is, in part, defined by partition 13 and by a horizontal panel 31 in freezer compartment 15 spaced above the partition and thus forming a main passage 33 between the horizontal panel and the partition. An opening 35 is provided in partition 13 for the intake of air into the passage from food compartment 16 and an opening 37 is provided in panel 31 for the intake of air from the freezer compartment. Evaporator 23 is located within passage 33 for chilling air from the food and freezer compartments as it passes thereover. An inner vertical panel 39 is spaced from back wall 11, thereby to provide a return or outlet passage 41 for the discharge of chilled air into the food compartment via an outlet 43. A vertical wall 45 extends up from panel 31 and a fan shroud 47 is disposed between vertical wall 45 and panel 39, thereby to define a fan intake chamber 49 and a discharge chamber 51, with the upper end of the fan intake chamber being closed by a cap 53. An opening 55 in panel 31 provides communication between main passage 33 and the fan inlet chamber. A baffle 57 directs and divides the chilled air discharged from the fan into outlet passage 41 for discharge into the food compartment and into freezer compartment 15 via openings 58 and 59.

The defrost system 1 of this invention comprises a defrost termination thermostat T1 and thermal time-delay relay TR which serves as a defrost initiation thermostat, both of which are connected in a circuit for controlling energization of a heater DH for melting and removal of frost from evaporator 23 upon a predetermined build-up of frost thereon. Thermostat T1 is a conventional temperature-responsive wide-differential switch, such as any of the widely used bimetallic disk-actuated types in which the contacts are abruptly moved from one switching position to the other when heated and cooled. Relay TR is also a wide-differential type switching unit, again preferably a bimetallic disk-actuated type.

T1 is mounted on or adjacent evaporator 23 preferably on the most frost-prone portion thereof. It is also in close heat-exchange relationship with defroster heater DH. Thermal relay TR is preferably mounted out of the air flow across the evaporator to minimize convection cooling and may be conveniently mounted within an enclosed well in the side wall of the refrigerator.

Referring now to FIG. 2, thermostat T1 and relay TR are schematically shown connected in a control circuit for selectively energizing defrost heater DH and periodically actuating compressor motor C in response to the temperature sensed by a thermostat T3, a conventional adjustable cold control typically positioned in refrigerator food compartment 16. Included in the control circuit is a heater comprising a resistor H together with a second heater R, also constituted by a resistor. Resistor H is preferably a self-regulating, self-heating positive temperature coefficient resistor which has a relatively low resistance when deenergized at ambient temperature but which will increase in resistance abruptly as its temperature rises above a given level. Heaters H and R are positioned in close heat-exchange relation with TR (as indicated by the dashed lines therebetween indicating a thermal link) and are preferably enclosed within the housing thereof.

FIG. 3 illustrates the physical arrangement of thermal relay TR with the single-pole double-throw switching components and the thermal-actuating disk being enclosed within a housing 61. PTC heater H is secured to one surface of a heat sink 63 of metal such as copper, which acts as a thermal capacitor, and heater resistor R is positioned in sink 63. Sink 63 and housing 61 are in facial contact for efficient heat transfer therebetween. An insulating case 65 of phenolic resin or the like encloses the componentS of TR and the entire package is surrounded by a good thermal insulation material 67 such as foamed polyurethane.

T1 will move from a first (solid-line) switching position, in which operation of the defrost heater DH is terminated, to its second (broken-line) position when its temperature falls to a level of about 0.degree.-10.degree.F., for example, and will not switch back to its first position until its temperature rises to say 65.degree.F. Similarly TR will move from its first (solid-line) switching position, in which the compressor may be energized, to its second (broken-line) position in which the heater DH may be energized only when its temperature falls to a low level in the order of 10.degree.-20.degree.F., remaining there until its temperature rises to a high level, e.g., 70.degree.-100.degree.F., whereupon it abruptly reverts to its first position.

As illustrated in FIG. 2, and with thermostat T1 and relay T2 both in their first or solid-line positions, the defrost heater DH is disabled and compressor C will be periodically energized each time cold control thermostat T3 moves to its solid-line position in response to the food compartment's temperature rising above a selected control temperature level. Resistor heater H will be energized to heat TR's thermal actuator during compressor "off" modes when T3 is open and will be deenergized during compressor "on" modes by the shunting action of TR and T3, when closed. R, which is arelatively high-resistance low-wattage (e.g., 2 watts) heater will also supply heat to TR as R will be directly connected across an a.c. power source indicated at L1, L2 when T1 is in its solid-line position. This is the normal operational mode of the circuit between termination of one defrost mode and the initiation of the next.

FIG. 4 illustrates the temperatures of the various system components, each noted parenthetically in relation to its respective temperature curve, beginning with the termination of a defrost mode. At that moment T3 will be closed, TR will be in its second or broken-line position, and T1 will have just switched to its solid-line position (thereby disabling heater DH). However, as R was deenergized during defrosting and it will require a finite time for TR to be heated by R and H to its high actuation temperature a few minutes are allowed for draining melted frost ("soaking") from the evaporator before TR is heated to a high enough temperature to move to its first (solid-line) position and permit reenergization etc. The evaporator tubing 25, T1, food compartment 16 and freezer compartment 15 will all be at their maximum temperatures to which they were heated by DH. TR's temperature will continue to rise briefly until it reaches its high level whereupon it switches to its solid-line position and permits reenergization of compressor C. All components then cool, including TR, because heater H will be deenergized during the long initial pull-down compressor "on" mode. However, heater R will prevent TR from cooling to its low temperature during this pull-down. Even if T1 is cooled sufficiently during such pull-down so that it moves to its second (dashed-line) position thereby deenergizing R, TR will not cool sufficiently during initial pull-down or immediately thereafter to reinitiate defrosting. The normal operational mode described above will continue with each indicated "on" mode of the compressor causing the illustrated temperature decreases in coils 25 and each "off" mode energizing heater H to increase or ratchet it temperature upwardly.

As normal operational mode continues and frost accumulates on the evaporator coils, a cooling of T1 occurs until it will reach its lower temperature, e.g. 20.degree.F., and switch to its second (broken-line) position. However, while this enables operation of heater DH and deenergizes heater R, energization of DH will not occur as TR remains in its first (solid-line) position with its temperature still above its lower actuation temperature. However, as frost accumulates the cooling efficiency drops and in order to maintain the preselected zone temperatures the durations of the "on" compressor mode continue to increase while the "off" mode periods decrease. Thus, heater H will be energized shorter increments of time so that gradually, as indicated in FIG. 4, the temperature of TR will fall until its temperature drops to its lower actuation level of 10.degree.-20.degree.F. as indicated at X. TR will then switch to its second (broken-line) position to energize heater DH thereby initiating a defrost cycle. It should be understood that the curves of FIG. 4 are merely representative or illustrative and that many "on" and "off" cycles of the compressor would take place between a termination of one defrost mode and the initiation of the next one. During defrosting, heater H will heat TR but without the additional heat supplied by R, which serves as a lock-out heater during initial pull-down, TR will not reactuate to its solid-line position.

During the defrost mode, T3 continues to remain in its closed position and TR in its broken-line position. After the accretion of frost is melted by DH, the temperature of the evaporator and T1 will rise until the temperature of the latter increases to 65.degree.F. This termination thermostat, which also provides a safety function inpreventing overheating of the unit in case of a component or circuitry fault or failure, will then switch to its first or solid-line position thereby completing one full cycle of operation of the automatic defrosting system of this embodiment of the invention.

It will be noted that the particular switching temperatures referred to above are merely illustrative and they may be varied widely within broad limits. It will be understood that the wattage, type and placement of optional resistor heaters R and H may be changed to vary the temperature gradients. Typically the run or "on" modes of the compressor will increase from say 18-20 minutes, with a relatively frost-free evaporator, to 40-45 minutes when the evaporator becomes frosted to the extent that it requires defrosting. TR will have a cooling time constant that generally matches that of the compressor run time, so that under normal operation its temperature will cycle as indicated in FIG. 4, but with the increased length of the run cycle caused by frosting and with both heaters H and R deenergized TR will cool in about 45 minutes so as to enter the defrost initiation "window" and initiate defrosting. This system is, in effect, a thermal analog timer that montors run time which has a close relationship to the cooling efficiency of the refrigerator. And this efficiency in turn is closely related to the accumulation of frost on the evaporator which causes long run periods. Thus, the demand defrost thermal analog system of this invention integrates freezer temperature and the ratio of compressor "on-off" time to initiate defrosting when the refrigeration system begins to lose its heat-exchange efficiency, and it does so economically by the use of a thermostatic switch and a thermal time-delay relay.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. An automatic defrosting control system for a refrigeration system having cooling means for absorbing heat from a zone to be cooled and thermostatic means for periodically energizing the cooling means to maintain the zone substantially at a preselected temperature, said cooling means being subject to the accretion of frost thereon which thereby increases the periods of energization of the cooling means in order to maintain the zone at the preselected temperature, said control system comprising:

means for causing defrosting of said cooling means;

a thermostat adapted to be positioned in heat-exchange relation with both said cooling means and said defrosting means and adapted for connection in a control circuit, said thermostat having a first switching position for terminating energization of said defrosting means and a second switching position for enabling operation thereof, said thermostat switching from its first to its second position in response to its temperature falling to a lower predetermined level and switching to its first position in response to its temperature rising to a higher predetermined level;

a thermal time-delay adapted to be positioned in heat-exchange relation with said cooling means and adapted for connection in said control circuit, said relay having a first switching position for permitting operation of said cooling means and a second switching position for energizing said defrosting means and preventing operation of said cooling means, said relay switching from its first position to its second position upon reaching a given temperature and switching from its second to its first position upon reaching a different preselected temperature;

said thermostatic means being responsive to the temperature in said zone to periodically energize said cooling means to cycle between an "on" mode and an "off" mode to maintain the zone substantially at said preselected temperature; and

means responsive to the duration of one of said modes to supply heat to said relay whereby upon a sufficient accretion of frost forming on the cooling means the increased duration of the "on" mode will cause the temperature of the relay to reach its given temperature and switch to its second position and will cool said thermostat to cause its temperature to fall below its lower predetermined level and switch to its second position thereby energizing said defrosting means.

2. A system as set forth in claim 1 in which said heat-supplying means is an electric resistance heater adapted to be energized during the "off" mode of the cooling means whereby as the accretion of frost increases the duration of the "on" mode lengthens and the duration of the "off" mode shortens thereby to decrease the heat supplied to the relay and permit it to cool to a lower given temperature and switch to its second position to energize said defrosting means.

3. A system as set forth in claim 2 wherein the thermostatic means and the relay when in its first position are serially connected with the cooling means and the heater is shunt-connected across the relay and thermostatic means whereby the heater is deenergized only when the relay is in its first position and the thermostatic means is closed.

4. A system as set forth in claim 3 wherein said heater is a self-regulating, self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.

5. A system as set forth in claim 4 which further includes another heater in heat-exchange relation to said relay and adapted for connection in the control circuit when the thermostat is in its first position to supply heat to said relay.