US3845637A

US3845637A - Defrost cycle initiation system

Info

Publication number: US3845637A
Application number: US00394947A
Authority: US
Inventors: G Shepherd
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1973-09-06
Filing date: 1973-09-06
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05

Abstract

A defrost cycle initiation system for refrigeration apparatus (e.g., a refrigerator, a refrigerator-freezer, or a room air conditioner) in which operation of the defrost system is initiated by frost build-up on the refrigeration apparatus. Generally, the refrigeration apparatus has cooling means for absorbing heat from a refrigerated zone, this cooling means being subject to the build-up of frost. The refrigeration apparatus has a main flow path for the intake of air from the refrigerated zone and for passage of the air over the cooling means to chill the air, and for discharge of the chilled air into the refrigerated zone, and a blower for forcing air through the main flow path. The defrost cycle initiation system of this invention includes an auxiliary flow path providing communication between the main flow path and the refrigerated zone, air normally flowing in the auxiliary flow path from the refrigerated zone to the main flow path when the refrigeration apparatus is in its normal mode of operation. The system further includes means for sensing the temperature of air flowing through the auxiliary flow path. The sensing means initiates a defrost cycle upon sensing a predetermined initiation temperature of air. Means are also provided for effecting reversal of flow of air in the auxiliary flow path in response to build-up of frost on the cooling means thereby to direct air having a temperature less than the abovementioned predetermined initiation temperature through the auxiliary flow path to lower the temperature of the air sensed by the sensing means below the predetermined temperature and for reestablishing the flow of air in the auxiliary flow path from the refrigerated zone to the main flow path upon the clearing of the cooling means of frost.

Description

United States Patent [191 Shepherd 3,845,637 Nov. 5, 1974 DEFROST CYCLE lNITIATlON SYSTEM Glen C. Shepherd, Garland, Tex.

Texas Instruments Incorporated, Dallas, Tex.

Filed: Sept. 6, 1973 Appl. No.: 394,947 I lnventor:

Assignee:

[52] US. Cl 62/155, 62/156, 62/180,

62/186, 62/419 Int. Cl. F25d 21/06 Field of Search 62/140, 156, 155, 180,

References Cited UNITED STATES PATENTS 12/1966 Kelly..... 62/180 3/1967 Jacobus 2/1972 Harrison. 1/1974 Schrader 62/262 Primary ExaminerMeyer Perlin Attorney, Agent, or Firm.lohn A. Haug; James P. McAndrews; Edward J. Connors, Jr.

[5 7 ABSTRACT eration apparatus. Generally, the refrigeration apparatus has cooling means for absorbing heat from a refrigerated zone, this cooling means being subject to the build-up of frost. The refrigeration apparatus has a main flow path for the intake of air from the refrigerated zone and for passage of the air over the cooling means to chill the air, and for discharge of the chilled air into the refrigerated zone, and a blower for forcing air through the main flow path. The defrost cycle initiation system of this invention includes an auxiliary flow path providing communication between the main flow path and the refrigerated zone, air normally flowing in the auxiliary flow path from the refrigerated zone to the main flow path when the refrigeration apparatus is in its normal mode of operation. The system further includes means for sensing the temperature of air flowing through the auxiliary flow path. The sensing means initiates a defrost cycle upon sensing a predetermined initiation temperature of air. Means are also provided for effecting reversal of flow of air in the auxiliary flow path in response to build-up of frost on the cooling means thereby to direct air having a temperature less than the above-mentioned predetermined initiation temperature through the auxiliary flow path to lower the temperature of the air sensed by the sensing means below the predetermined temperature and for reestablishing the flow of air in the auxiliary flow path from the refrigerated zone to the main flow path upon the clearing of the cooling means of frost.

13 Claims, 15 Drawing Figures PATENIEMV 51924 $845637 I SEE! k (If 6 FHQIO FIG.7

1 DEFROST CYCLE INITIATION SYSTEM BACKGROUND OF THE INVENTION This invention relates to a defrost cycle initiation system for various types of refrigeration apparatus, and more particularly to such systems which are operable in response to the actual build-up of frost on the cooling unit (e.g., the evaporator) thereof.

The accumulation or build-up of frost on the evaporator of refrigeration apparatus and its efficient re moval have long presented serious problems. Various automatic defrost cycle initiation systems have been used and are well known in the art but have not satisfactorily resolved these problems. For example, in room air conditioners the rate of frost build-up on the evaporator depends on a number of factors, including the room ambient temperature and humidity conditions. Under severe frost conditions, air flow through the evaporator of an air conditioner may become blocked (or partially blocked) thus choking the flow of chilled air into the room, reducing the heat transfer efficiency of the evaporator to the point where the room cannot be adequately cooled, and reducing the operating efficiency of the unit such that excessive power may be consumed. In some room air conditioners, however, no automatic defrost system as such is provided, but rather the normal on-off cycling of the unit under the control of the room temperature cold control allows the evaporator to clear itself of frost when the compressor is not operating. These units may work satisfactorily under certain conditions, but if the evaporator cannot clear itself of frost between compressor run cycles, frost may continue to build-up on the evaporator and may eventually block the flow of air through the evaporator. In the event this happens, the compressor will continue to run because the room can no longer be adequately cooled and the only recourse is to manually shut off the air conditioner until the frost melts from the evaporator.

On many room air conditioners now in use. various thermal sensing systems are used to detect the presence of frost on the evaporator and to initiate a defrost cycle by deenergizing the compressor while continuing to draw room air into the unit for passage over the evaporator and discharge back into the room. Such room air conditioners utilize in their defrost initiation systems a temperature-responsive, fluid-expansion bulb (e.g., a capillary tube) spaced in front of the evaporator a predetermined distance by frost clips or by other mounting means so as to sense the temperature of the room air as it is drawn into the unit and to sense the build-up of frost on the evaporator. Upon the capillary tube being cooled to a predetermined temperature (either by room air impinging on the capillary tube or by increased heat transfer from the capillary tube to the evaporator via the frost clips as occasioned by the build-up of frost on the evaporator), a bellows-type switch is actuated by the condensation of fluid in the capillary tube. This bellows switch deenergizes the compressor until the capillary tube warms to its abovementioned predetermined temperature and the bellows switch is closed by the expansion ofthc fluid in the capillary tube. While the compressor is off, the air circulating fan continues to draw room air over the evaporator and to thus melt any frost build-up thereon.

Generally, capillary-tube sensors work well in most situations, however, these capillary tubes are expensive and may be easily displaced relative to the evaporator during repair or cleaning of the air conditioner. upstream and down stream If the capillary tube contacts the evaporator, the compressor will run only a few seconds before the fluid in the capillary tube chills below its condensation temperature and cuts off the compressor. This is referred to as short cycling and. in essence. makes the air conditioner ineffective. On the other hand, if the capillary tube is moved away from the evaporator, it may not sense the build-up of frost.

In US. Pat. No. 3,640,087 a bimetal thermostat is used to control both the temperature of the room and to initiate a defrost cycle of the unit. However, before this bimetal thermostat can sense the temperature of the room air, the room air is partially chilled by the evaporator so that the thermostat only indirectly senses room temperature. If this system is subjected to drastic changes in ambient relative humidity and temperature, or if the air conditioners filter becomes partially blocked, the temperature and flow rate of the air passing over the thermostat may be affected so that the room temperature cannot accurately be controlled.

In refrigerators, freezers and other refrigerated units equipped with an automatic defrost system, the defrost system is typically controlled by a timer which initiates operation of the defrost system at certain times of the day or after the compressor has run a upstream and downstream predetermined length of time. The rate at which frost builds up on the evaporator is a function of the amount of water vapor in the air passing over the evaporator, the greater the water vapor content the faster the frost accumulates. For example, in a refrigerator the amount of water vapor in the air to be cooled depends on the ambient conditions (i.e., room temperature and relative humidity) outside the refrigerator due to ambient air being introduced into the refrigerator each time the door is opened, the frequency with which the door is opened, and water vapor sources (e.g.,' wet produce and open containers of liquids) within the refrigerator. With time-controlled defrost cycle initiation systems and with a slow build up of frost on the evaporator, 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 which accelerates spoilage. On the other hand, under heavy frost conditions, excessive frost may build upon the evaporator between the timed defrost cycles, thus reducing the efficiency of the refrigerator and increasing the power consumed thereby.

UM ARY of THEJNYE IIQN Among the several objects of this invention may be noted the provision of an automatic defrost cycle initiation system for refrigeration apparatus (e. g., an air conditioner, a refrigerator, a freezer, or other refrigerated unit) which initiates a defrost cycle upon sensing a build-up of frost on the evaporator of the refrigeration apparatus; the provision of such a defrost initiation system which eliminates unneeded defrosting cycles which are inherent in the operation of conventional defrost systems designed to handle worst case frost conditions;

the provision of such a defrost cycle initiation 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 cycle initiation system which uses only thermal sensing elements to initiate a defrost cycle; the provision of such a defrost initiation system in which the means for sensing the build-up of frost on the evaporator is a bimetal thermostat which may also be used to control the temperature of the zone to be refrigerated or cooled by the refrigeration apparatus; the provision of such a defrost cycle initiation system in which the bimetal thermostat is protected so as to prevent electric shock to persons cleaning or repairing the refrigeration apparatus; the provision of such a defrost initiation system which when applied to a room air conditioner prevents short cycling (i.e., relatively short periods of compressor run time) which may be caused by erroneously sensing that frost has accumulated on the evaporator or that the room has been cooled to a desired temperature; the provision of such a defrost initiation system which will permit the refrigeration apparatus to run continuously when free of frost thereby to provide maximum cooling efficiency; and the provision of such a defrost cycle initiation system which is relatively simple, which is of reliable operation, which is of economical construction and which will reliably operate regardless of the ambient climatic conditions. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, a defrost cycle initiation system of this invention is intended for use on refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, the cooling means being subject to frost build-up. The refrigeration apparatus has a main flow path for the intake of air from the refrigerated zone and for passage of the air over the cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing air through the main flow path. The defrost cycle initiation system comprises an auxiliary flow path providing communication between the main flow path and the refrigerated zone, air normally flowing in the auxiliary flow path from the refrigerated zone to the main flow path when the refrigeration apparatus is in its normal mode of operation. Means is provided for sensing the temperature of air flowing through the auxiliary flow path, this means initiating a defrost cycle upon sensing a predetermined initiation temperature of air. Means is also provided for effecting a reversal of the flow of air in the auxiliary path in response to build-up of frost on the cooling means thereby to direct air of a temperature less than the predetermined depressed temperature through the auxiliary flow path to lower the temperature of the air sensed by the sensing means below the predetermined initiation temperature and for reestablishing the flow of air in the auxiliary flow path from the refrigerated zone to the main flow path upon clearing the cooling means of frost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a semi-diagrammatic view of refrigeration apparatus (e.g., a portion of a refrigerator-freezer unit) incorporating a first embodiment of an automatic defrost cycle initiation system of this invention with arrows indicating the direction of the flow of air through the refrigeration apparatus and through the system of this invention when the evaporator is substantially free of frost build-up;

FIG. 2 is a view similar to FIG. 1 illustrating the flow of air through the refrigeration apparatus and through the system of this invention when the evaporation is at least partially blocked by frost;

FIG. 3 is a partial perspective of a refrigeratorfreezer with its front doors removed and with a horizontal partition dividing it into a food compartment and a freezer compartment, a second embodiment of the defrost cycle initiation system of this invention being incorporated in the partition;

FIG. 4 is a plan view of the partition shown in FIG. 3 further illustrating the defrost cycle initiation system of this invention with the arrows indicating the flow of air through the partition when the evaporator of the refrigerator-freezer is substantially free of frost;

FIG. 5 is a vertical section taken on line 5-5 of FIG.

FIG. 6 is a horizontal section taken on line 6-6 of FIG. 5;

FIG. 7 is a vertical sectional view taken on line 77 of FIG. 4;

FIG. 8 is a view similar to FIG. 4 illustrating the flow of air through the partition when the evaporator is at least partially blocked by frost build-up;

FIG. 9 is a vertical sectional view of a refrigeratorfreezer illustrating still another embodiment of the defrost cycle initiation system of this invention incorporated in the partition;

FIG. 10 is a first schematic of an electrical circuit for controlling the refrigeration apparatus and the defrost cycle initiation system of this invention; I

FIG. 11 is a schematic of an alternative electrical circuit for controlling the refrigeration apparatus and the defrost cycle initiation system of this invention;

FIG. 12 is a perspective view of a room air conditioner unit with its cover removed, this room air conditioner incorporating a defrost cycle initiation system of this invention;

FIG. 13 is a plan view of the room air conditioner illustrated in FIG. 12 with parts borken away to further illustrate various refrigeration components and the defrost cycle initiation system of this invention;

FIG. 14 is an enlarged view of a portion of FIG. 13 illustrating the flow of air (as indicated by the arrows) through the evaporator of the room air conditioner and the flow of air through the defrost initiation system of this invention when the evaporator is substantially free of frost; and

FIG. 15 is a view similar to FIG. 14 illustrating the flow of air through the evaporator and through the defrost cycle initiation system when the evaporator is at least partially blocked by frost build-up thereon.

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

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings and particularly to FIGS. 1 and 2, a defrost cycle initiation system 1 for refrigeration apparatus 3, for example, a refrigeratorfreezer 5a (as shown in FIGS. 1 and 2), 5b (as shown in FIGS. 3-8) or (as shown in FIG. 9), or a room air conditioner 7 (shown in FIGS. 12-15), having cooling means, as indicated generally at 9, for absorbing heat from a refrigerated zone 11. The cooling means is subject to the build-up of frost F thereon. The refrigeration apparatus (whether it be a refrigerator-freezer or an air conditioner) has a main flow path 13 for the intake of air from the refrigerated zone, for the passage of the air .over the cooling means to chill the air, and for the discharge of the chilled air into the refrigerated zone. A blower or fan is provided for forcing the air through the main flow path.

The defrost cycle initiation system 1 of this invention includes an auxiliary flow path 170 providing communication between main flow path 13 and refrigerated zone 11. With cooling means 9 substantially free of frost build-up and with the refrigeration apparatus in its normal (or cooling) mode of operation, air flows in the auxiliary flow path from the refrigerated zone to the main flow path. Means 19, such as a thermostatic switch, for sensing air temperature is provided within the auxiliary flow path to sense the temperature of air within the auxiliary flow path and to initiate a defrost cycle upon sensing a predetermined initiation temperature of air flowing therethrough. The pressures within the main and the auxiliary flow path is such that it constitutes means for effecting a reversal of flow of air through the auxiliary flow path (see FIG. 2) in response to build-up of frost F on cooling means 9 thereby to direct air having a temperature lower than the abovementioned predetermined initiation temperature through the auxiliary flow path to lower the temperature of the air sensed by means 19 below the initiation temperature so as to cold-bias the sensing means. Upon cooling means 9 becoming clear of frost, the pressures within the main and the auxiliary flow path is such that the flow of air from refrigerated zone 11 to main flow path 13 is once again reestablished. While FIGS. 1 and 2 illustrate the defrost cycle initiation system of this invention installed in a refrigerator-freezer, it will be understood that this system may, with suitable modifications be applied to a variety of refrigerated units and evaporator locations.

More particularly, a refrigerator-freezer 5b, as shown in FIGS. 3-8, having a defrost cycle initiation system 1 of this invention installed therein is shown to have a cabinet 21 including a top wall 23, side walls 25, a back wall 27 and a horizontal partition 29 dividing the interior of the cabinet into a freezer compartment 31 and a food compartment 33, these compartments constituting refrigerated zones 11. It will be understood that the fronts of the food and freezer compartments are normally closed by doors (not shown). In operation, the freezer compartment is maintained at a temperature (e.g., 0 F.) well below the maximum temperature of the, food compartment (e.g., 40-42 F.) The refrigeratorincludes a refrigeration system including a compressor and motor 35 (see FIGS. 14) and 11), a condenser (not shown), an evaporator 37 constituting cooling means 9, and various refrigerant lines (not shown) interconnecting the various refrigeration components. The evaporator is shown mounted within a channel 39 (see FIG. 5) in partition 29, this channel constituting main flow path 13 and being defined by a lower panel 411 constitutingthe top of the "food compartment and an upper panel 43 spaced above lower panel 11 and constituting the floor of the freezer compartment. Lower panel 41 has a seriesofopenings 45 therein adjacent its front edge and upper panel 43 has a series of openings 47 therein generally above openings 45, these openings constituting inlet openings for the intake of air (return air) into the main flow path 13 from both the food and

freezer compartments

31 and 33. This return air is drawn by blower 15 over evaporator 37 which absorbs heat from the return air and chills it. The blower then forces the chilled air to the back of the cabinet and discharges it into the freezer and food compartments via vents 49 in the freezer compartment and outlet 51 in the food compartment (see FIGS. 7 and 9). A defrost heater H 1 is provided in the channel 39 below evaporator 37 to heat up the evaporator so as to clear the evaporator of frost upon initiation of a defrost cycle, the heater being energized and deenergized in response to the defrost cycle initiation system 1 of this invention. Evaporator 37 may, for example, be a finned refrigerant line formed to constitute a helical coil. Heater H1 may either be a Radiant heater, such as shown, or a conduction heating element carried on the exterior of the evaporator coil for melting frost from the evaporator by conduction. It will be noted that the return air in main flow path 13 upstream of evaporator 37 is a mixed air flow from both the freezer and

food compartments

31 and 33, respectively, and the temperature of this mixed flow of air (e.g., +8 F.) is substantially below the temperature of the return air in the food compartment (e.g., 35 F. to 42 F.) and below the initiation temperature (e.g., 23 F.)

In FIGS. 3-8, auxiliary flow path 17b, is shown to'be formed in partition 29 at one side (the left side as viewed in FIG. 3) of channel 39. The auxiliary flow path includes an auxiliary inlet 53 in partition 29 in communication with food compartment 33 for the intake of air from the food compartment, and an inletoutlet opening 55 also in partition 29 in communication with main flow path 13 on the upstream side of evaporator 37. A duct 57 is formed in partition 29 and provides communication between auxiliary inlet 53 inlet/outlet opening 55, and an outlet 59 downstream of the evaporator in main flow path 13. A defrost initiation thermostat TSla is mounted in the auxiliary flow path adjacent auxiliary inlet 53 in inlet/outlet opening 55, this defrost initiation thermostat constituting means 19 for sensing the temperature of the air flowing through the auxiliary flow path and initiating a defrost cycle (in a manner as will appear) when the temperature of the air in the auxiliary flow path drops below the above-mentioned predetermined initiation temperature.

More particularly, thermostat TSla is a normal differential, bimetal thermostat. A temperature control thermostat (cold control) TS3a is positioned in food compartment 33 in a selected location (see FIG. 7) to sense and to regulate the temperature of the air in the food compartment. A defrost termination thermostat T52 is mounted within channel 39 adjacent heater H1. These components are electrically interconnected as illustrated in FIG. 10.

Referring now to FIG. 10, compressor motor 35 is connected in a series circuit across electrical power supply lines L1 and L2, this series circuit including defrost initiation thermostat TS 1a and cold control thermostat TS3a. Fan 15 is shunt-connected across compressor motor 35 so that these two units are concurrently energized in response to the contacts of thermostats TSla and TS3a being in their positions as shown in FIG. 10. The defrost initiation thermostat TSla, when sufficiently cooled (i.e., when cooled to its predetermined initiation temperature which, for example, may be approximately 23 F.), is actuated to the alternate position of its contacts to disconnect the fan and compressor and to supply power (when thermostat TS3a is closed) to defrost heater H1 and to defrost termination thermostat TS2. A booster heater Rla (e.g., a small resistor-type heater) is optionally enclosed within thermostat TSla for reheating the thermostat for reset purposes, this booster heater being shuntconnected across the contacts of thermostat TSla for resetting the contacts thereof by heating the thermostatic element therein after the thermostat has been cold-biased below its predetermined initiation temperature. Both thermostatic switches T510 and T82 are of the wide differential type, thermostat TSla being actuated from its FIG. 10 position to complete the defrost heater H1 circuit in response to sensing its predetermined depressed temperature and remaining in that position until thermostat TSla warms to a substantially higher temperature. The switch contacts of defrost termination thermostat TS2 will remain closed, as shown in FIG. 10, until the temperature within channel 39 rises to about 85 F., these contacts remaining open until thermostat TS2 recools to a substantially lower temperature upon restart of compressor 35 (e.g.,

With refrigerator 5b operating in its normal refrigeration mode, the contacts of cold control thermostat TS3a and those of defrost termination thermostat TS2 are closed and the contacts of defrost initiation thermostat TSla are positioned as shown in FIG. so as to energize compressor motor 35 and blower and to maintain heater H1 deenergized. With blower 15 operating and with evaporator 37 substantially free of frost F, the blower draws in air from the refrigerated zones (e.g., from the freezer and

food compartments

31 and 33, respectively) into main flow path 13 (i.e., channel 39) by means of

openings

45 and 47. This air passes over evaporator 37 which absorbs heat from the air and chills the air. Upon contact with the evaporator, excess water vapor in the air condenses on the evaporator and forms frost F. The rate at which frost builds up on the evaporator depends upon the moisture content in the air to be chilled and the relative length of the on cycle of compressor unit 35.

With evaporator 37 substantially free of frost F, the pressure onthe upstream side of the evaporator is substantially the same as the pressure on the downstream side of the evaporator, and consequently blower 15 draws air from the food compartment 33 via auxiliary inlet 53 and discharges this relatively warm air (e.g., 35 42 F.) into main flow path 13 via inlet/outlet opening 55 on the upstream side of the evaporator and outlet opening 59 on the downstream side of the evaporator. This relatively warm air from the food compartment in the auxiliary flow path passes over defrost initiation thermostat TSla and maintains the contacts thereof as shown in FIG. 10. Upon frost building up on the evaporator to such a degree that the flow of air therethrough is at least partially blocked or restricted, the pressure in the main flow path on the downstream side of the evaporator drops below the pressure on the upstream side of the evaporator and thus effects a pressure differential across the evaporator. This pressure differential causes a reversal of the flow of air in auxiliary flow path 17b so that air from main flow path 13 on the upstream side of evaporator 37 enters the auxiliary flow path via inlet/outlet opening 55 and flows over thermostat TSla. This air from the main flow path is colder (e.g., 8 F.) than the predetermined initiation temperature (e.g., 23 F.) of thermostat TSla and thus cold-biases thermostat TSla so as to move its contacts from the position shown in FIG. 10 and to connect defrost heater H1 to lines L1 and L2. With the contacts of cold control thermostat TS3a and defrost termination thermostat TS2 closed, defrost heater H1 is energized and thus heats evaporator 37 so as to clear it of frost. Upon the temperature within channel 39 rising to a predetermined elevated temperature (e.g., F.), the contacts of termination thermostat TS2 open and deenergize defrost heater H1. Defrost heater H1 may, for example, be in its defrost mode for about 10 minutes. Booster heater Rla is shunt-connected across the contacts of thermostat TSla and is energized whenever the contacts of cold control thermostat TS3a are closed. Thus, heater Rla is energized concurrently with the energization of defrost heater H1 so that the heat from the defrost heater and the heat generated by the booster heater reset the contacts of thermostat TSla. Normally, the time required to reset the contacts of thermostat TSla is appreciably longer than the time required for termination thermostat TS2 to deenergize defrost heater Hl. For example, it may require 20-25 minutes to reset the contacts of thermostat TSla while it may require only 11 minutes for thermostat TS2 to deenergize heater Hl. This time differential (e.g., 9-14 minutes) is referred to as the drip or soak timeduring which the temperature within channel 39 remains elevated so as to insure that all frost and ice within channel 39 is melted and that any water may drain from the channel. Upon resetting of the contacts of the defrost initiation thermostat TSla and upon the temperature of the food compartment 33 risingto a preestablished temperature, cold control TS3a is actuated thereby to energize compressor motor 35 and fan 15 to again place the refrigerator in its normal mode of operation. It will be noted in certain applications termination thermostat TS2 in FIG. 10 may be replaced by a thermal fuse.

In FIG. 9, illustrating a third embodiment 5c of a refrigerator-freezer in which auxiliary flow path is formed in partition 29 below channel 39 thereby to show that the routing and configuration of the auxiliary flow path may be varied, but so long as it interconnects refrigerated zone 11 and main flow path 13 on both the upstream and downstream sides of evaporator 37, the reversal of air in the auxiliary flow path will be effected by the build-up of frost on the evaporator. FIG. 9 depicts a cold control thermostat TS3b located in a selected location in food compartment 33 and a defrost heater initiation thermostat TSlb (also referred to as a thermostatic switch) located in auxiliary flow path 17c. Initiation thermostat TSlb has an optional booster heater Rlb enclosed therewithin, this booster heater heating the contacts of the thermostat for reset purposes in a preselected time. It will be noted that thermostat TSlb may be reset in a preselected time by the air within auxiliary flow path 17c which has been heated by heater H1. In FIG. 11, booster R112 is shown to be shunt-connected across heater H1 for concurrent energization therewith, operation of the circuit shown in FIG. 11 will be more particularly described hereinafter.

Referring now to FIG. 11, compressor motor 35 is shown connected in a series circuit somewhat different from the circuit shown in FIG. 10, this new circuit including the normally open contacts of defrost cycle initiation thermostat TSlb and the contacts of temperature control thermostat (cold control) T8312 across electrical power supply lines LI. and L2. Blower I is shunt-connected across the compressor motor 35 so that these two units are concurrently energized and deenergized. Cold control thermostat TS3b is positioned within food compartment 33 in a selected location (see FIG. 9) to sense and to regulate the temperature of the air in food compartment 33. The defrost initiation thermostat TSlb, when cooled (i.e., cold-biased) below its initiation temperature (e.g., 23 F.) is actuated to close its contacts and to connect heater Hi to line L2. Subsequent to thermostat TSlb closing and upon cold control thermostat TS3b sensing that the food compartment has attained a desired lower temperature, the contacts of thermostat TS3b are reversed from the position shown in FIG. 11 so as to disconnect the compressor fan and to connect heater H]. to line Ll thereby initiating the defrost cycle. Booster heater Rlb is shunt-connected across defrost heater HI and energized concurrently with the defrost heater.

Operation of the defrost cycle initiation system I of this invention, wired in accordance with FIG. 11, as included in a refrigerator-freezer 5c, is as follows:

With refrigerator 50 operating in its normal refrigeration mode, the contacts of cold control thermostat TS3b are closed and those of defrost initiation thermostat TSlb are open, as shown in FIG. 11, thereby energizing compressor motor 35 and blower l5 and maintaining heater Hll deenergized. With blower l5 operating and with evaporator 37 substantially free of frost, the blower draws in air from the refrigerated zones 11 (e.g., from freezer compartment 31 and from food compartment 33) via

inlets

45 and 47 into channel 39. This air passes over evaporator 37 which absorbs heat from the air and chills the air.

With evaporator 37, substantially free of frost, the pressure in channel 39 on the upstream side of the evaporator is substantially the same as the pressure on the downstream side of the evaporator and consequently blower l5 draws air from food compartment 33 via auxiliary inlet 53 and then discharges this relatively warm air into channel 39 via inlet/outlet opening 55 and outlet 59 on both the upstream and downstream sides of the evaporator. This relatively warm air passes over defrost initiation thermostat TSlb in auxiliary flow path 170 and maintains the contacts thereof open. Upon frost build-up on the evaporator to such a degree that the flow of air over the evaporator is at least partially blocked or restricted, the pressure in the main flow path on the downstream side of the evaporator is caused to drop below the pressure on the upstream side of the evaporator thereby to create a pressure differential across the evaporator. This pressure differential causes a reversal of the flow of air in the auxiliary flow path so that air from the main flow path on the upstream side of the evaporator enters the auxiliary flow 7 path via inlet/outlet opening 55 and flows over thermomostat TS3b sensing that the food compartment has been chilled to a desired temperatureteg, 35 F), it deenergizes compressor motor 35 and blower and completes the circuit to heater H1 thereby to initiate the defrost cycle. Booster heater Rlb is energized concurrently with heater H l. Booster heater Rlb requires approximately 1 1 minutes to heat thermostat TSlb to a temperature sufficient to reset (open) its contacts and to deenergize heater H1. Thus heater H1 remains energized for about I 1 minutes. During the time heater H1 is energized and for some time thereafter, the temperature in food compartment 33 rises to a predetermined temperature (e.g., 40-42 F.) to reset the contacts of cold control TS3b. Typically, cold control TS3b will reset in about 25 minutes. This difference be- 7 tween the reset times of the cold control thermostat stat TSlb. This air from the main flow path is colder (e.g., 8 F.) than the predetermined initiation temperature (e.g., 23 F.) of thermostat TSlb and thus coldbiasesthermostat TSlb so as to close its contacts and connect heater H1 to line L2. Upon cold control therand the defrost initiation thermostat (e.g., 14 minutes) is referred to as the soak or drip time during which the temperature within channel 39 remains elevated to complete defrosting of the frost on the evaporator, to

melt any ice which may have accumulated in the main flow path and to permit any water to drain from channel 39. Thus, it can be seen that the circuits shown in FIGS. 10 and 11 essentially serve the same function, but the thermostat TSlb in FIG. 11 serves a dual function of defrost cycle initiation and defrost heater termination so that the defrost heater termination thermostat in FIG. 10 may be omitted.

It will be particularly noted that the defrost cycle initiation system 1 of this invention, as applied to a combination refrigerator-

freezer

5a, 5b or 50 initiates a defrost cycle only in response to a build-up of a predetermined amount of frost on evaporator 37, and accordingly the frequency of defrost cycles and the exposure of refrigerated or frozen items to unnecessary defrost cycles is minimized. Also, the power consumed by a refrigerator-freezer incorporating the demand defrost cycle initiation system of this invention is reduced by elimination of unnecessary defrost cycles and by keeping the evaporator free of excessive frost build-up, thereby to maintain a high heat transfer efficiency of the evaporator. Use of the defrost cycle initiation system of this invention also eliminates the use of timers to initiate the defrost cycle.

Referring to FIGS. 12 -15 which illustrate the defrost cycle initiation system 1 of this invention as installed in a room air conditioner unit (RAC) 7, the RAC is shown in FIG. 12 with its cover removed. The RAC includes a base 101 and a front face plate 103 having an air inlet grill 105 through which room air is inducted into main flow path 13 for passage over cooling means 9, an air outlet grill 107 for directing chilled air back into the room, and an escutcheon plate 109 mounting the manual controls for the RAC. In FIG. 13, the refrigeration system for the RAC is shown to include a compressormotor unit 111, a condenser 113, a condenser fan for blowing outside air over the condenser, and an evaporator 117, this evaporator constituting cooling means 9. It will be understood that refrigerant lines interconnect the compressor, condenser and evaporator in the conventional manner so that the compressor may circulate refrigerant through these components for cooling purposes. The evaporator is separated from the condenser by means of a wall or partition 119 so that air cooled by the evaporator is not mixed with the relatively warm outside air circulated by the condenser fan. The evaporator is a conventional expansion coil refrigerant line having fins 121 and end turns 123 for the various turns of the coil. An air filter 125 is shown positioned behind inlet grill 105, but it will be understood that the filter may be placed immediately in front of evaporator 117. A centrifugal blower 127 is provided behind evaporator 117 to draw room air into the room air conditioner via grill 105, to draw this room air over the evaporator to be cooled and to exhaust the cooled air back into the room (i.e., the refrigerated zone 11) via outlet grill 107. A motor 128 drives both condenser fan 115 and evaporator blower 127. A horizontal panel 129 constituting an air divider is installed behind the evaporator to isolate the blower inlet from the blower outlet. This air divider has an opening 131 constituting an outlet for blower 127 and provides a passage for the chilled air to be directed to outlet grill 107. It will be understood that inlet grill 105, base 101, the cover (not shown), partition 119, air divider 129, and outlet grill 107 together constitute main flow path 13.

End turns 123 at one end of evaporator 117 (i.e., the left end as viewed in FIG. 13) are enclosed in an endturn enclosure or compartment 133. A thermostat enclosure 135 is provided adjacent end-turn enclosure 133 and a bimetal thermostat 137 is mounted within the thermostat enclosure, this thermostat constituting temperature sensing means 19. Thermostat 137 is a relatively narrow differential (e.g., having a differential of 3 6 F adjustable thermostat. It will be noted that with thermostat 137 installed in enclosure 135, the thermostat is protected so as to prevent electrical shock during ordinary use and maintenance of the RAC. An auxiliary air inlet 139 is provided in front face plate 103, this auxiliary inlet opening being in communication with thermostat enclosure 135. As shown in FIGS. 1315, end-turn enclosure 133 has an opening 141 providing communication between main flow path 13 on the upstream side of evaporator 117 and the endturn enclosure, and an opening 143 providing communication between the end-turn enclosure and the thermostat enclosure, the latter having an opening 145 providing communication between enclosure 135 and the main flow path on the downstream side of the evaporator. Thus, auxiliary inlet 139,

enclosures

133 and 135, and

openings

141 and 143 and 145 together constitute auxiliary air flow path 17d.

Operarion of the defrost initiation system of this invention as installed on a room air conditioner will be further described hereinafter. With evaporator 117 substantially free of frost build-up, blower 127 draws room air into main flow path 13 via inlet grill 105 and into auxiliary flow path 17d via auxiliary inlet 139. This room air passes over thermostat 137 thereby to permit the thermostat to accurately sense the temperature of the room air (thus permitting the thermostat to accurately regulate the room temperature) and exits enclosure 135 via

openings

143 and 145. A portion of the room air in the auxiliary flow path enters end-turn compartment 133 and is drawn into main flow path 13 on the upstream side of evaporator 117. Room air passing through the evaporator and being cooled thereby causes water vapor to condense on the evaporator. Since the temperature of the evaporator may be below 32 F frost F may begin to build-up on the evaporator. Upon a quantity of frost accumulating on evaporator 117, the flow of room air therethrough is partially restricted thereby causing a pressure differential across the evaporator. This difference in pressure causes air from main flow path 13 on the upstream side of evaporator 117 to enter

enclosures

133 and 135 via

openings

141 and 143, respectively. As this air passes through the end-turn compartment it is cooled by evaporator end-turns 123 to a temperature well below the room temperature (e.g., below F.) which cold biases thermostat 137 and thus stops compressor 111. Blower 127 continues to blow relatively warm room air (e.g., 72 F.) over the evaporator to thus clear it of frost. Upon the air flowing through auxiliary flow path 17 via end-turn compartment 133 warming above the abovementioned compressor start temperature (e.g. 72-75 F) thermostat 137 again energizes the compressor and the RAC may be again operated in its normal mode free of frost F.

Thermostat 137 is wired in a series circuit (not shown) with the motor of compressor unit 111 so that the compressor is energized and deenergized by the closing and opening of the thermostat contacts. For example, these contacts may open to terminate operation of the compressor when the thermostat senses that the air temperature in auxiliary flow path 17 is about 68 71 F. and may close to initiate operation of the compressor when the thermostat senses that the air temperature in the auxiliary flow path rises to about 72 75 F. Blower motor 128 is preferably continuously energized so that blower 127 draws room air over evaporator 117 at all times, regardless of the operation of compressor unit 111. When the compressor unit is deenergized, the flow of room air (e.g., 72 F. air) over the evaporator clears the evaporator of any frost F which may have built up thereon. In some RAC units, motor 128 may be energized concurrently with the compressor, but it will be understood that the defrost initiation system of this invention will function regardless of whether the evaporator blower operates continuously or intermittently.

In accordance with this invention, sensing means 19 (especially in a room air conditioning application) may serve the dual function of a room temperature regulator (cold control) and a defrost cycle initiation system. As seen in FIGS. 11 and 12, when evaporator 117 is free of frost, room air in sufficient quantity is drawn into thermostat enclosure 135 so that thermostat 137 may directly and accurately sense the temperature of the room air thereby to provide accurate control of the room temperature by terminating operation of the compressor when the thermostat senses a first predetermined temperature (e.g., 68-7 1 F.) and by starting operation of the compressor when the thermostat senses a second predetermined temperature (e.g., 7275 F. When the evaporator becomes at least partially blocked by frost F (see FIG. 15), a reversal of the flow of air from thermostat enclosure 135 and from end-turn enclosure or compartment 133 is effected so that air from main flow path 13 on the upstream side of the evaporator 117 flows into the end-turn enclosure thereby to be chilled by end turns 123 well below the room temperature (e.g., less than 60 F.) so as to coldbias thermostat 137 and to thus terminate operation of compressor unit 111. Blower 131 will continue to draw in relatively warm room air and to pass it over the evaporator thereby to melt any accumulation of frost thereon.

It will be understood that if filter becomes clogged with dirt thereby restricting the flow of air through main flow path 13, the defrost cycle initiation system of this invention will continue to operate satisfactorily but its sensitivity may be affected. It will further be understood that the defrost cycle initiation system of this invention may be operated satisfactorily if the outlet 145 from thermostat enclosure 135 is in communication with a low pressure region behind partition 119 so as to place enclosure 135 in communica tion with condenser fan 115 rather than with evaporator blower E27,.

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.

What is claimed is:

l. A defrost cycle initiation system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost build-up, said refrigeration apparatus having a main flow path for the intake of air from said refrigerated zone, for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said main flow path; said system comprising an auxiliary flowpath providing communication between said main flow path and said refrigerated zone, air normally flowing, in said auxiliary flow path from said refrigerated zone to said main flow path when said refrigeration apparatus is in its normal mode of operation, means for sensing the temperature of air flowing through said auxiliary flow path adapted to initiate a defrost cycle upon sensing a predetermined initiation temperature of air, and means for effecting a reversal of flow of air in said auxiliary path in response to build-up of frost on said cooling means there-by to direct air having a temperature less than said predetermined initiation temperature through said auxiliary flow path to lower the temperature of the air sensed by the sensing means below said predetermined initiation temperature and for reestablishing the flow of air in said auxiliary flow path from said refrigerated zone to said main flow path upon clearing the cooling means of frost.

2. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus is a refrigerated unit, such as a refrigerator or the like, said refrigerated zone is a refrigerated compartment and said cooling means is an evaporator, said main flow path being in communication with said refrigerated zone and said evaporator being located in said main flow path so that air from said refrigerated compartment enters said main flow path, passes over said evaporator for being chilled, and is discharged into the refrigerated compartment, said auxiliary flow path being in communication with the refrigerated compartment for the intake of air from the refrigerated compartment and for the discharge of compartment air into the main flow path on the upstream and downstream side of the evaporator when the evaporator is substantially free of frost, said auxiliary flow path providing communication between the main flow path on the upstream side of the evaporator and the main flow path on the downstream side of the evaporator for the intake of air from the main flow path-on the upstream side of the evaporator and for the discharge of said air from the main flow path back into the main flow path on the downstream side of the evaporator when the evaporator is at least partially blocked by frost, the temperature of said air from the main flow pathon the upstream side of the evaporator being belowsaid predetermined initiation temperature when said evaporator is at least partially blocked by frost, thereby to actuate said sensing means upon said air from the upstream side of said evaporator flowing through the auxiliary flow path and to initiate a defrost cycle.

3. A defrost cycle initiation system as set forth in claim 2 wherein said refrigerated unit is a refrigeratorfreezer having a food compartment and a freezer compartment, said food and freezer compartments constituting said refrigerated zone, said freezer compartment being maintained at a. colder temperature than said food compartment, said main flow path being in communication with the food compartment and the freezer compartment for the intake of air from both said compartments, said auxiliary flow path being in communication with said food compartment so that with the evaporator substantially free of frost said blower means draws in relatively warm air from said food compartment into the auxiliary flow path for discharge of this food compartment air into the main jflow path on the upstream and downstream side of the evaporator, and upon build-up of frost on the evaporator said blower draws air from the mainflow path on the upstream side of the evaporator into said auxiliary flow path for flowing over said sensing means thereby to cold-bias the latter below said predetermined initiation temperature and initiate a defrost cycle.

4,. A defrost cycle initiation system as set forth in claim 3 wherein said refrigerator-freezer has a defrost heater located adjacent said evaporator, said heater, upon energization thereof, clearing said evaporator of frost, said heater being energized by said sensing means thereby to clear said evaporator of frost.

5. A defrost cycle initiation system as set forth in claim 4 wherein said refrigerator-freezer has a compressor and a cold control thermostat for energization and deenergization of said compressor there-by to regulate the temperature of the refrigerated zone, said sensing means comprising a thermostatic switch, said cold control thermostat and said thermostatic switch adapted to energize said heater upon said thermostatic switch sensing said predetermined initiation temperature and said cold control thermostat sensing a desired temperature in said food compartment, said thermostatic switch adapted to reset in a preselected time to deenergize said heater.

6. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus is an air conditioner, said refrigerated zone is a room which is to be cooled, and said cooling means is an evaporator, said auxiliary flow path being in communication with the room for the intake of room air and for the discharge of this room air into the main flow path on the upstream and downstream side of the evaporator when the evaporator is substantially free of frost build-up and for the intake of air from said main flow path on the up stream side of the evaporator and for discharge of said air from the main flow path back into the main flow path on the downstream of the evaporator when the evaporator is at least partially blocked by frost buildup, the temperature of said air from the main flow path on the upstream side of the evaporator being cooled by the at least partially blocked evaporator to a temperature below room temperature whereby said flow of air from the upstream side of the evaporator through said auxiliary flow path actuating said sensing means and initiating said defrost cycle.

7. A defrost cycle initiation system as set forth in claim 6 wherein said air conditioner has a main inlet for intake of room air into said main flow path and an auxiliary air inlet for intake of room air into the auxiliary flow path, an enclosure housing said sensing means, said enclosure constituting a portion of said auxiliary flow path, a first opening in said enclosure providing communication between said housing and said main flow path on the upstream side of said evaporator and a second opening in said enclosure in communication with said main flow path on the downstream side of the evaporator, whereby with said evaporator substantially free of frost, room air enters said enclosure via said auxiliary inlet and at least part of said room air entering said enclosure is discharged into said main flow path via said first opening and whereby upon frost accumulating on said evaporator, air is drawn into said enclosure from said main flow path via said first opening, said air from said main flow path drawn into the first opening being chilled below said room temperature.

8. A defrost cycle initiation system as set forth in claim 7 wherein said first opening is adjacent a portion of said evaporator whereby upon said flow of air in said main flow path through the evaporator becoming at least partially restricted by the build-up of frost on the evaporator, said flow of air from the main flow path on the upstream side of the evaporator into the enclosure via said first opening is chilled by said evaporator thereby to cold-bias said sensing means to initiate a defrost cycle.

9. A defrost cycle initiation system as set forth in claim 8 wherein said evaporator is a coiled refrigerant line having a plurality of end turns, said apparatus including a compartment enclosing said end turns of the evaporator, said end turn compartment being in communication with said main flow path on the upstream side of the evaporator and being in communication with the enclosure via said first opening whereby with said evaporator substantially free of frost build-up at least part of said air flows from said auxiliary air inlet into said main flow path on the upstream side of the evaporator via said enclosure and said end turn compartment, and with said evaporator at least partially blocked by frost, the flow of air in said auxiliary flow path is reversed so that air from the main flow path on the upstream side of the evaporator enters the end turn compartment thereby to be chilled by the evaporator end turns and then enters said enclosure thereby to cold-bias said sensing means and initiate a defrost cycle.

10. A defrost cycle system as set forth in claim 7 wherein said air conditioner includes a compressor and wherein said sensing means is a bimetal thermostat, said thermostat being operable to initiate operation of said compressor upon sensing air at a first predetermined temperature and to terminate operation of the compressor upon sensing air at a second and lower predetermined temperature, whereby with said evaporator substantially free of frost and with said thermostat sensing the room air temperature said thermostat initiating operation of the compressor upon the temperature of the room air rising to said first predetermined temperature and said thermostat terminating operation of said compressor upon the temperature of the room air dropping to said second predetermined temperature.

11. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus includes a refrigeration system comprising a compressor and an evaporator, the latter constituting said cooling means, said sensing means being operable to initiate operation of the compressor upon sensing a first predetermined temperature and to terminate operation of the compressor upon sensing a second and lower predetermined temperature, whereby with said evaporator substantially clear of frost the temperature sensing means senses the temperature of air from the refrigerated zone so that upon the temperature of the air in the refrigerated zone rising to said first predetermined temperature said sensing means initiates operation of the compressor, and upon the temperature of the air in the refrigerated zone falling to said second predetermined temperature said sensing means terminates operation of the compressor and thus maintains the temperature of the air in the refrigerated zone between said compressor start and compressor stop temperatures.

12. A defrost cycle initiation system as set forth in claim 11 wherein said refrigeration apparatus is a room air conditioner and said temperature sensing means is a bimetal thermostat.

13. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus is a room air conditioner and said temperature sensing means is a bimetal thermostat.

Claims

1. A defrost cycle initiation system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost build-up, said refrigeration apparatus having a main flow path for the intake of air from said refrigerated zone, for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said main flow path; said system comprising an auxiliary flow path providing communication between said main flow path and said refrigerated zone, air normally flowing in said auxiliary flow path from said refrigerated zone to said main flow path when said refrigeration apparatus is in its normal mode of operation, means for sensing the temperature of air flowing through said auxiliary flow path adapted to initiate a defrost cycle upon sensing a predetermined initiation temperature of air, and means for effecting a reversal of flow of air in said auxiliary path in response to build-up of frost on said cooling means there-by to direct air having a temperature less than said predetermined initiation temperature through said auxiliary flow path to lower the temperature of the air sensed by the sensing means below said predetermined initiation temperature and for reestablishing the flow of air in said auxiliary flow path from said refrigerated zone to said main flow path upon clearing the cooling means of frost.

2. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus is a refrigerated unit, such as a refrigerator or the like, said refrigerated zone is a refrigerated compartment and said cooling means is an evaporator, said main flow path being in communication with said refrigerated zone and said evaporator being located in said main flow path so that air from said refrigerated compartment enters said main flow path, passes over said evaporator for being chilled, and is discharged into the refrigerated compartment, said auxiliary flow path being in communication with the refrigerated compartment for the intake of air from the refrigerated compartment and for the discharge of compartment air into the main flow path on the upstream and downstream side of the evaporator when the evaporator is substantially free of frost, said auxiliary flow path providing communication between the main flow path on the upstream side of the evaporator and the main flow path on the downstream side of the evaporator for the intake of air from the main flow path on the upstream side of the evaporator and for the discharge of said air from the main flow path back into the main flow path on the downstream side of the evaporator when the evaporator is at least partially blocked by frost, the temperature of said air from the main flow path on the upstream side of the evaporator being below said predetermined initiation temperature when said evaporator is at least partially blocked by frost, thereby to actuate said sensing means upon said air from the upstream side of said evaporator flowing through the auxiliary flow path and to initiate a defrost cycle.

3. A defrost cycle initiation system as set forth in claim 2 wherein said refrigerated unit is a refrigerator-freezer having a food compartment and a freezer compartment, said food and freezer compartments constituting said refrigerated zone, said freezer compartment being maintained at a colder temperature than said food compartment, said main flow path being in communication with the food compartment and the freezer compartment for the intake of air from both said compartments, said auxiliary flow path being in communication with said food compartment so that with the evaporator substantially free of frost said blower means draws in relatively warm air from said food compartment into the auxiliary flow path for discharge of this food compartment air into the main flow path on the upstream and downstream side of the evaporator, and upon build-up of frost on the evaporator said blower draws air from the main flow path on the upstream side of the evaporator into said auxiliary flow path for flowing over said sensing means thereby to cold-bias the latter below said predetermined initiation temperature and initiate a defrost cycle.

4. A defrost cycle initiation system as set forth in claim 3 wherein said refrigerator-freezer has a defrost heater located adjacent said evaporator, said heater, upon energization thereof, clearing said evaporator of frost, said heater being energized by said sensing means thereby to clear said evaporator of frost.

6. A defrost cycle initiation system as set forth in claim 1 wherein said refrigeration apparatus is an air conditioner, said refrigerated zone is a room which is to be cooled, and said cooling means is an evaporator, said auxiliary flow path being in communication with the room for the intake of room air and for the discharge of this room air into the main flow path on the upstream and downstream side of the evaporator when the evaporator is substantially frEe of frost build-up and for the intake of air from said main flow path on the upstream side of the evaporator and for discharge of said air from the main flow path back into the main flow path on the downstream of the evaporator when the evaporator is at least partially blocked by frost build-up, the temperature of said air from the main flow path on the upstream side of the evaporator being cooled by the at least partially blocked evaporator to a temperature below room temperature whereby said flow of air from the upstream side of the evaporator through said auxiliary flow path actuating said sensing means and initiating said defrost cycle.