US3383877A - Defrost control means for refrigerating systems - Google Patents

Description

J. LJEBERMANN ETAL 3,383,877

5 Sheets-Sheet 1 DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS Filed May 10, 1966 May 21, 1968 H m NZ. 0 N N N T R N M R 0 R T W 1B- L A UE M O HR OE IUJ Y B J. LIEBERMANN ETAL 3,383,877

May 21, 1968 DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS 5 Sheets-Sheet Filed May 10, 1966 Fig. 6

INVENTORS JOHN LIEBERMANN JEROME L LORENZ W WW4! m y 1968 J. LIEBERMANN ETAL 3,383,877

DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS 5 Sheets-Sheet 5 Filed May 10, 1966 W and A77 R/VEYS United States Patent corporation of Ohio Filed May 10, 1966, Ser. No. 548,959 4 Claims. (Cl. 62140) The present invention relates to control apparatus for automatically defrosting the cooling unit of forced air refrigerating systems to maintain the cooling unit relatively free of frost.

Refrigerating systems for the storage and self-serve display of frozen foods are commonly used in stores for convenience in the sale of foods, and the foods are maintained well below the freezing temperature by circulating air thereover which has been chilled by the cooling unit of the refrigerating system to about l0 F. During cooling of the air, moisture condenses and freezes on the heat exchange surfaces of the cooling unit, thereby insulating these surfaces from the air and reducing the space through which the air moves across the cooling unit, all of which results in considerable loss in efficiency of the cooling unit. It is desirable that the cooling unit be cleared of frost before the frost coats the surfaces of the unit to a depth which prevents adequate chilling of the air. A principal object of the present invention, therefore, is the provision of a new and improved control means for initiating and terminating defrosting cycles in the cooling unit of refrigerating systems of the type mentioned.

Another object of the invention is the provision of a new and improved control means for defrosting the cooling units of refrigeration systems of the type described which includes relatively simple and reliable circuitry in which electrical resistance elements are employed as sensing means which can be conveniently located in an air duct and at the air cooling unit to sense frosting conditions and to terminate the defrost cycles, and which circuitry can be readily adjusted to provide proper defrost cycles according to the operating characteristics encountered in refrigerating systems of different design details.

Still another object of the invention is the provision of a control means of the character referred to which includes two temperature responsive self-heating electrical resistance members, the resistances of which vary with the temperature thereof and which can be conveniently located in the path of the air flowing in a duct of the refrigerating system, one of the resistance members being more or less shielded from the air flow so that when the velocity of air in the duct changes due to collection of frost on the cooling unit the resistance of the unshielded resistance member changes relative to the other member to effect a defrost cycle, and a third thermally responsive resistance member being responsive to the temperature of the cooling unit to sense a frost melting temperature to effect termination of the defrost cycles, the resistance member being connected in circuit means in such a manner that the air velocity resistances are ineffective to initiate a defrost cycle during the time the temperature of the cooling unit is being reduced from a frost melting temperature to the normally maintained sub-freezing temperatures so that the effect of the velocity sensing resistance members in the control circuit is cancelled until the normal temperature differentials of these members has been reestablished.

Other objects and advantages of the present invention will become apparent from the following detailed descrip tion thereof and the accompanying drawings which form a part of the specification and in which:

FIG. 1 is a fragmentary perspective view of a frozen food display and dispensing cabinet embodying the present invention;

FIG. 2 is a schematic transverse sectional view of the cabinet shown in FIG. 1;

FIG. 3 is a schematic showing of the refrigerating system employed in the food cabinet;

FIG. 4 is a schematic fragmentary view of portions of the refrigeration system and the defrost control means, applied thereto of the invention;

FIG. 5 is a wiring diagram of control means for the refrigeration apparatus shown in FIG. 3; and

FIG. 6 is a sectional view taken substantially along a plane indicated by line 66 of FIG. 4.

As representing a prefcrr d embodiment of the present invention, a refrigeration apparatus 10 is shown in FIG. 1 which is particularly designed for the display and selfserve dispensing of frozen food stuffs, although the invention could be embodied in many other forms of refrigerating apparatus. The apparatus 10 comprises a typical food storage and self-serve dispensing type cabinet 11 having an air chilling heat exchange unit 12 and a food storage space 13 which is cooled by air forcefully drawn from the space, directed across the cooling unit and returned to the space. The food stored in the space 13 is maintained at about 0 F. or lower, for example, and is accessible through an opening 14 across the front of the cabinet.

The cabinet 11 includes a bottom wall 15, vertically extending front and rear walls 16, 17, and end walls 29, 21. The rear wall 17 is integral with a horizontally extending top wall 22 having a downwardly directed front portion 23. A partition 24 is located in spaced relation to the bottom, rear and top walls 15, 17, and 22, respectively, and cooperates with these walls and the end walls 20, 21 to form an air passageway or duct 25 which extends upwardly along the rear of the cabinet and forwardly across the top thereof, terminating in a downwardly facing discharge opening 26. The width of the duct or passage 25 extends substantially the length of the cabinet and the air discharged from the opening 26 across the top of the forward side of the cabinet sweeps downwardly over the food in the space 13 to maintain the food at about 0 F. and is returned to the passage 25 towards an opening 27, across the lower portion of the cabinet. The cooling unit 12 extends transversely of the duct or passage 25 and across the bottom of the cabinet in the path of air returned to the passage through opening 27. The unit 12 is suitably supported above the bottom wall 15, and baffles 30, 31 are arranged to direct all of the air moving into the passage through the cooling unit and into the vertical section of the duct 25.

Air is cooled as it is drawn through the cooling unit 12 and is forced upwardly through the duct 25 and into the space 13 by a plurality of constant speed electric motor driven fans 36, only one of which is shown, located in openings formed at intervals along the baflie 3! over the top of the cooling unit.

Forwardly extending shelves 32, 33 are supported on the partition 24 and, with the lower forwardly extending portion 34 of the partition, provide food storage racks in the path of the chilled air descending from the opening 26 to the inlet opening 27, as is indicated by the arrow 35.

The unit 12 comprises the evaporator of a conventional compressor-condenser-expander type refrigerating system consisting of an electric motor driven compressor connected through suitable conduit-s with :a condenser 41 and the evaporator 12, which connections include a reversing valve 42 and a refrigerant expansion device 43, all of which are well-known in the art. The reversing valve 42 is controlled by a solenoid 44 so that when the solenoid is deenergized the valve effects a flow of refrigerant from the compressor to the condenser, thence to the evaporator and return to the compressor to provide normal refrigeration and when the solenoid is energized, the flow of refrigerant is from the compressor to the evaporator and thence to the condenser, which flow is utilized to provide a defrost cycle which heats the evaporator 12 and melts ice or frost therefrom, all of which is well-known practice.

The cooling unit 12 is normally maintained well below F. by conventional thermostatic control means for the compressor which means is not shown as such are well-known.

The cooling unit 12 comprises a serpentine formed pipe as extending through openings in closely spaced vertical fins 46 which form heat transfer surfaces from the pipe for air passing therebetween, which construction is typical.

When the apparatus is in normal operation, air is drawn through the cooling unit 12 by the fans 36, forced upwardly through the passage and is discharged downwardly from the outlet of the duct onto the food stored on the shelves and returned to the cooling unit. Inasmuch as the chilled air is exposed to atmospheric air at the opening 14, it entrains moisture laden air as it passes to the inlet of the air duct, and the moisture condenses and freezes on the fins and coils of the cooling unit, thereby forming an insulation covering thereof and reducing the dimensions of the air passages through the unit. The rate of frost accumulation varies, depending on the humidity.

To prevent excessive accumulation of frost on the fins of the cooling unit 12, the evaporator is heated in response to an undesirable accumulation of frost thereon by reversing the how of refrigerant through the system by the valve 42 as described hereinbefore.

In accordance with the present invention, the solenoid 44 of the reversing valve 42 is energized and deenergized by control means 50 which senses a reduction in the velocity of air in the passage 25, resulting from a build up of frost on pipe 45 and fins 46 of the unit 12, and initiates a defrost cycle until the frost is melted, after which normal refrigeration is reinstated.

The control means 50 comprises a circuit 53, shown in FIG. 5, which includes a solenoid winding 54 of a double-throw relay switch 55 which controls energization of the reversing valve solenoid 44 and the fans 36. The fans 36 are energized during the time the valve solenoid 44 is deenergized and vice versa, so that during defrost cycles no air is forced through the cooling unit and passage 25.

The movable contact 56 of the switch 55 is connected to one line of a conventional 115 v. AC power source L1, L2 and the fixed contact 57 is connected to one terminal of the fan motors 36, the other terminal of which are connected to L2. The fixed contact 60 of switch 55 is connected to one terminal of the solenoid 44, the other terminal of which is connected to L2. When relay solenoid 54 is energized, the armature 61 .of the relay moves con tact 56 to contact 60 to energize the valve solenoid 44- and open the fan circuit, and when the relay solenoid is deenergized the armature drops to a position in which contact 56 engages contact 57 and breaks contact with the contact 60.

The relay solenoid 54 is energized by a 12 v. unfiltered DC power source which is provided by a center tap stepdown transformer 62, the primary winding of which is connected between lines L1, L2 and the terminals of the secondary winding of which are connected to a conductor 63 through diodes 64, 65 and the center tap of the winding is connected to a conductor 66. The relay solenoid 54 is connected between the conductors 63, 66 in series with the emitter and collector of a PNP transistor 67, and a resistor 70, so that the solenoid 54 forms part of an output circuit of an amplifier, described fully hereinafter. When the transistor 67 is turned on and off the solenoid S4 is energized and deenergized. Preferably, a diode 68 is connected in parallel with the solenoid coil 54 to permit stable coil operation on the unifiltered DC supply and to provide a discharge path for the coil energy stored during the on cycle.

The input of the amplifier for controlling the solenoid 54 comprises an NPN transistor 71, the collector of which is connected with a junction 72 between the base of the transistor 67 and a resistor 73 connected to conductor 63. The emitter of transistor 71 is connected to the sliding contact of a potentiometer 74, the resistance member 75 of which is connected in series with a resistance 76 connected to the junction of the resistance 7 8 with the emitter of the transistor 67, and a resistor 77 connected to conductor 66.

The base of the transistor 71 is biased by alternative voltage divider networks, one of which is responsive to a decrease in air velocity in the passage 25 and includes a junction 80 between two wire wound resistors 81, 82 connected in series between the conductors 63, 66. The second network is responsive to the temperature of the cooling unit 12 and includes a junction 83 between two resistors 84, 85 connected in parallel between conductor 63 and junction 83, and a third voltage divider circuit comprising a junction 86 between resistor 87, rheostat 90, and a thermistor 91, which are series connected between conductors 63, 66.

The base of transistor 71 is connected with junction 80 through a movable contact 92 of a double throw switch having fixed contacts 93, 94. The movable contact 92 is actuated by the armature 61 and when the relay solenoid 54 is deencrgized contact 92 engages contact 93 to connect the base of the transistor 71 with the air velocity sensing network at junction 80 and removes resistor 84 from circuit. A diode 95 is interposed between the contact 93 and junction 86 and prevents current flow to the base of transistor '71 and provides voltage and temperature compensation for the base emitter voltage drop of transistor 71.

The base of the transistor 71 is disconnected from junction 80 when the solenoid 54 is energized by the shifting of contact 92 from contact 93 to contact 94, so that the network including thermistor 91 is then effective to control the transistor 71 rather than the nework comprised of the resistors 81, 82. A diode 96 is interposed between the junctions 83, 86 and prevents flow of current from junction '86 to the base of transistor 71.

With'the exception of the resistances 81, 82 and the thermistor 91, the circuit 53 is mounted in a suitable casing 97, which may be conveniently located on the apparatus 10.

The resistances 81, 82 are of the wire wound self heating type and are positioned within cylindrical passages 108, 181 respectively formed through a block 102 of insulating material, such as a plastic, which is suitably attached to the upper forwardly extending portion of the partition 24 of the air duct 25 so that the passages extend in the direction of air movement through the duct. The passage 101 is blocked by a disc 103 positioned in one end thereof so that no air is circulated in contact with resistor 82 while an appreciable volume of air flows in contact with the resistor 81 and tends to reduce its temperature according to the rate of air flow. The leads 105 for the resistors 81, 82 and for the junction 80 extend from the block 102 to a conventional plug-in type connector 106 which is plugged into a suitable plug receptacle in the wall of the casing 97 but not shown in detail.

The electrical resistance of resistors 81, 82 increase with increase in temperature thereof and are adapted to become heated above atmospheric temperatures when the transformer 62 is energized. Both resistors 81, 82 are identical and during normal operation of the refrigerating system, the chilled air flowing through the passage of block 102 cools the resistor 81 to a degree well below that of resistor 82 which has no air flowing thereabout and accordingly retains more of the heat generated by the current flowing therethrough. As frost and ice accumulate on the fins and coils of the cooling unit 12, the volume of air flowing therethrough is reduced and the corresponding reduction in the velocity of m'r passing through passage 100 of block 102 results in a rise in temperature of the resistor 81 through a greater increment than that of resistor 82 in the blocked passage 101.

The thermistor 91 is positioned in a bracket 107 suitably attached to the cooling unit 12 preferably in a location which is last to lose frost during a defrost cycle and the leads 108 thereof are attached to the plug connector 106. The thermistor 91 is the type which decreases in resistance as the temperature thereof increases.

In the form of the invention disclosed herein the values of the various components and type designations are as follows:

The operation of the defrost control means is as follows: The compressor controlled by the conventional control means mentioned, maintains the temperature of the cooling unit 12 at about l0 F., and the fans 36 operate to circulate air over the unit and into the compartment 13. Switch contact 92 engages contact 93 so that the junction 80 is connected with the base of transistor 71. When the unit 12 is substantially free of frost, the air passing about resistor 81 cools the resistor to a temperature considerably below that at which resistor 82 is cooled and consequently the resistance of the latter is substantially greater than that of resistor 81. Accordingly, the base of transistor 71 is negatively biased so that it is held off, as is transistor 6-7, and current flows through resistor 85, switch contacts 92, 93, diode 95, junction 80, resistor 81 to conductor 66. At the normally low temperature of the cooling unit 12, the resistance of the thermistor 91 is relatively high and in conjunction with the resistances of rheostat 90 and resistance 87 biases the cathode of diode 96 more positive than its anode so that no current flows to the base of transistor 71 frornhe voltage divider network including the thermistor 91.

As frost builds up on the surfaces of cooling unit 12, the rate of air flow through the passage decreases and the air velocity sensing resistor 81 is not cooled as eifectively as previously so that its temperature and resistance increase. This change in resistance is reflected at the junction 80 which becomes more positively biased and at a pre-set point in this shift in bias, the base of transistor 71 becomes slightly positive causing it to start turning on. This results in transistor 67 turning on and the increased current in the resistor 70 causes the emitter bias of transistor 71 to become more negative thereby biasing transistor 71 and subsequently transistor 67 fully on. Current through the collector of transistor 67 energizes the relay solenoid 54, causing the armature to move contact 56 from contact 57 to contact 60, thereby shutting down the fans 36 and energizing the solenoid 44 of the reversing valve 42. Also, contact 92 is moved by the armature from contact 93 to contact 94 which disconnects the voltage divider network comprising resistances -81, 82 from the circuit and throws resistance 84 in parallel with resistance 85, resulting in appreciably increased forward bias on the base of transistor 71.

Energization of the reversing valve solenoid 44 results in hot refrigerant being directed into the cooling unit and warming its heat exchange surfaces and melting the frost therefrom. As the temperature of the cooling unit increases, the thermistor 91 is warmed, thereby causing a drop in its resistance. At a given temperature at which all frost should be melted from the cooling unit, determined by adjusting the resistance of the potentiometer 74, the decrease in resistance of the thermistor 91 results in a potential at junction 83 which is negative relative to the emitter of the transistor 71, which shuts off transistor 71, and consequently transistor 67 is turned off, thereby deenergizing the relay coil 54. Armature 61 drops, closing contact 56 onto contact 57, deenergizing valve solenoid 44 to terminate the defrosting cycle of the cooling unit and restoring normal refrigeration. At the same time, contact 92 drops onto contact 93, reconnecting the network including resistors 81, 82 with the base of the transistor 71; however, resistor 84 is disconnected from parallel circuit with resistors 85 and 87 so that the relatively low resistance of the thermistor 91 now draws sufficient current to maintain the base of transistor 71 biased off because the resistance of the parallel network comprising the resistors 81, 82 is relatively high. When the temperature of the cooling unit and the thermistor 91 is reduced to about 0 F., the increased resistance thereof with the reduction in resistance of the resistors 81, 82 as they are cooled by the fiow of air through the passage 100 and about the block 102 causes the control of the base of transistor 71 to be shifted to the resistors 81, 82. Thus, on initial operation of the cooling unit 12 from room temperature or upon resumption of a refrigeration cycle following a defrost cycle, the unstable thermal condition of the resistors 81, 82 during that time cannot effect a false defrost cycle.

The diode 96 provides voltage and temperature compensation for the base emitter voltage drop of transistor 71 and effectively removes the air velocity sensor resistors 81, 82 from the circuit during the time the temperature of the cooling unit is reduced from above freezing to near 0 F.

The rheostat 90 may be used to adjust the network including thermistor 91 to set the defrost termination temperature. The resistors 84, 85, and 87 are effective to prevent forward bias of the diode 95 and a reinstatement of the control of the resistances 81, 82 on the base of transistor 71 until the temperature of the thermistor 91 at the cooling unit has dropped to about 0 F. By this time the temperatures of the sensing members 81, 82 will be stabilized at their normal differential by the high velocity air flow so that they are in condition to retain transistor 71 off until a drop in air velocity is sensed to initiate a defrost cycle as described hereinbefore.

It will be appreciated that the control circuit 53 is relatively simple and may be formed of durable and relatively inexpensive components. The air velocity sensing members and the defrost cycle terminating members are compact and can readily be installed in the refrigerating system. The potentiometer 74 and rheostat 90 are of conventional types which can be located in the case 95 so as to be accessible for adjustment by a tool, such as a screw driver, inserted in openings 109, in the easing 97 whereby the sensitivity of the air velocity network and the defrost cycle terminating network can be conveniently and accurately calibrated.

Although but one form of the invention has been shown and described, other forms, modifications, and adaptations thereof may be made, all falling within the scope of the claims which follow.

We claim:

1. In a refrigeration system for cooling a storage space and comprising an air cooling heat exchange unit having spaced heat exchange surfaces, blower means operable at a given rate to force air across said surfaces and into the space to be cooled, defrosting means for heating said surfaces to remove frost therefrom, control means for said defrosting means comprising an electrically actuated control element, circuit means for controlling energizetion of said control element and including a source of DC potential, said element being connected across said source of DC potential, a pair of self heating resistance members connected in series across said DC source and comprising a first voltage divider network, means responsive to the temperature of said cooling unit and connected across said DC source and in parallel with said resistance members, means for varying the relative resistance of said resistance members in response to a change in the velocity of the air moved through said cooling unit by said blower means, resistance means connected in series with said temperature responsive means and comprising with said temperature responsive means a second voltage divider network, amplifying means for controlling the flow of current through said control element and including a transistor, means for alternatively connecting the base of said transitor to the junction between said resistance members of said first network and a junction in said second network, and means responsive to said control element for actuating said switch means.

2. In a refrigeration system of the character defined in claim 1 in which said temperature responsive means includes a thermistor and said resistance means connected in series with said thermistor includes a rheostat.

3. In a refrigeration system of the character defined in claim 1 in which said means for varying the relative resistance of said resistance members comprises a body having two air passageways therethrough, a self heating resistor in each of mid-air passageways, and means restricting the passage of air through one of said passageways.

4. In a refrigeration system of the character defined in claim 3 in which the last named means comprises a disc-like member having an opening therethrough.

References Cited UNITED STATES PATENTS 3,220,208 11/1965 Oram 62140 3,222,882 12/1965 Sutton 62-140 XR 3,248,892 5/1966 Sutton 62l56 3,335,576 8/1967 Phillips 62-156 MEYER PERLIN, Primary Examiner.

Claims (1)

1. IN A REFRIGERATION SYSTEM FOR COOLING A STORAGE SPACE AND COMPRISING AN AIR COOLING HEAT EXCHANGE UNIT HAVING SPACED HEAT EXCHANGE SURFACES, BLOWER MEANS OPERABLE AT A GIVEN RATE TO FORCE AIR ACROSS SAID SURFACES AND INTO THE SPACE TO BE COOLED, DEFROSTING MEANS FOR HEATING SAID SURFACES TO REMOVE FROST THEREFROM, CONTROL MEANS FOR SAID DEFROSTING MEANS COMPRISING AN ELECTRICALLY ACTUATED CONTROL ELEMENT, CIRCUIT MEANS FOR CONTROLLING ENERGIZATION OF SAID CONTROL ELEMENT AND INCLUDING A SOURCE OF DC POTENTIAL, SAID ELEMENT BEING CONNECTED ACROSS SAID SOURCE OF DC POTENTIAL, A PAIR OF SELF HEATING RESISTANCE MEMBERS CONNECTED IN SERIES ACROSS SAID DC SOURCE AND COMPRISING A FIRST VOLTAGE DIVIDER NETWORK, MEANS REPSONSIVE TO THE TEMPERATURE OF SAID COOLING UNIT AND CONNECTED ACROSS SAID DC SOURCE AND IN PARALLEL WITH

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