GB2191309A - De-frosting system for refrigerated cabinets, freezers or the like - Google Patents

Abstract

In a refrigerated cabinet, a de-frosting system is activated at intervals determined by an electronic timer (10). The timer may simply be arranged to operate the de-frosting system at regular intervals, but may alternatively be arranged to increase de-frosting frequency with demand, for example by measuring the passage of time only during periods when the compression of the refrigeration system is running. <IMAGE>

Description

SPECIFICATION De-frosting system for refrigerated cabinets, freezers or the like THIS INVENTION relates to a de-frosting system for a refrigerated cabinet, refrigerator, freezer or the like, all of which are herein referred to, for convenience, as refrigerated cabinets, whether these are of the type having a vertical door, or of the chest type having a top closed by a lid which can be lifted, or of any other kind. The term "refrigerated cabinet" is intended to include so called "freezers" intended for relatively long term storage of frozen foods as well as refrigerators mainly intended for short-term food storage as well as for units combining both functions.

It is well known that a refrigerated cabinet, as herein defined, requires to be de-frosted at intervals if it is to remain efficient. That is to say, the surfaces of the active cooling element within the cabinet and which, for example, are in direct contact with the refrigerating medium utilised in the refrigeration system, tend to be substantially cooler than the mean temperature within the cabinet and consequently frost, due to moisture in the atmosphere, forms preferentially on such surfaces and, unless removed by a de-frosting operation, will eventually become thick enough to form an insulating coating on the cooling surfaces of the active cooling unit and will impair the ability of the latter to extract heat from the surroundings.In the defrosting operation, the cooling of the cooling surfaces of the active cooling element is temporarily suspended to allow these surfaces to rise to a temperature sufficient to melt the frost formed so that the resultant liquid water can run off these surfaces for appropriate collection. Whilst, in very simple arrangements, the whole of the interior of the refrigerated cabinet, including the cooling surfaces of the active cooling device, may be allowed to rise gradually to above freezing point by the penetration of heat from the surroundings, it is generally preferred to supply heat to the cooling surfaces directly, during the de-frosting operation, so that these can be raised rapidly to an appropriate de-frosting temperature without allowing the mean temperature within the cabinet, at points relatively far from the active cooling element, to alter substantially.

It is known to provide, in connection with refrigerated cabinets and the like, a mechanically operated (e.g. electrically driven clockwork) timer which operates at pre-set intervals to disconnect the refrigeration system from the mains power supply and to activate heaters for the purpose of de-frosting. However, such mechanical timing devices have various disadvantages. Thus, such mechanical timing devices are relatively noisy in operation and are subject to mechanical wear and thus are relatively unreliable from the point of view of long term operation. Furthermore, because of the safety regulations which attend the operation, in this context, of control switching means which use mechanical switches to control the connection of the mains supply to the refrigeration system and de-frosting system, the scope of operation of such mechanical devices is limited.

It is an object of the present invention to provide an improved defrosting system for a refrigerated cabinet which avoids such disadvantages.

According to one aspect of the invention there is provided a de-frosting system for a refrigerated cabinet, including electronic timing means arranged to interrupt the cooling of the cabinet, or the cooling of a selected part or parts thereof, to defrost the same at intervals and to allow such cooling to proceed again at the end of a relatively short period after each such interruption.

According to another aspect of the invention there is provided a refrigerated cabinet incorporating a refrigerating system operable normally to maintain the interior of the cabinet at a pre-determined temperature below ambient temperature, and a defrosting system arranged to interrupt the normal operation of the refrigerating system at intervals to allow the temperature of at least the cooling surfaces within the cabinet to rise in temperature to effect de-frosting, for relatively short periods, wherein said de-frosting system is electronically operated and, in particular, incorporates an electronic timer.

An embodiment of the invention is described below with reference to the accompanying drawings which is a schematic circuit diagram illustrating an electronic de-frosting system embodying the present invention.

Referring to the drawing, a low-voltage timing circuit indicated generally at 10 is powered from the neutral and live mains supply lines, indicated at N and L respectively, by way of a step-down transformer 12 and a rectifier 14.

The circuit 10 has a negative ground rail 16 and a positive rail 18. The output from the rectifier 14 is connected between the ground rail 15 and input line 20 which is connected to rail 18 via a voltage regulator 25 which provides a stabilised d.c. potential on the rail 18. The timing circuit comprises two integrated circuits, IC1 and IC2. These are commercially available devices (Ferranti ZN1034) incorporating internal circuitry which, in combination with an external timing resistor and capacitor forms a clock which will run at a rate determined by the values of the timing resistor and capacitor. For IC1, the timing resistor is alternatively R1 or R2, (as explained below) whilst the timing capacitor is C3.For IC1, the timing resistor is R3 whilst the timing capacitor is C5. such internal circuitry also includes a series of counters arranged to count the pulses in the clock signal produced by such clock and further includes switching and control circuitry whereby a signal applied to one pin, indicated at 28, will set the internal counters to zero and whereby, when the preset count within the internal counters has been reached, an output signal will be delivered to a further pin indicated at 38.As will be noted, the pin 28 of IC1 is connected with the pin 38 of IC2 and the pin 28 of IC2 is connected with the pin 38 of IC1. Consequently, when the internal counting circuitry in IC1 has reached its pre-set count, a signal appearing at pin 38 of IC1 is applied to pin 28 of IC2 to set the counting circuitry within IC2 to zero, and when the count in IC2 has reached its pre-set count, a signal appearing at pin 38 of IC2 is applied to pin 28 of IC1 to set the counting circuitry in IC1 to zero. When the pre-set count within either 1C1 or IC2 is reached, an output signal is further provided on a further pin, indicated at 40 of IC1 or IC2 respectively.In operation, when IC1 is set to begin timing its pre-set period, the signal applied to input 28 of IC2 is such as to maintain IC2 in its set condition in which IC2 does not operate to time out its period and in which the signal applied to input 28 of IC1 by output 38 of IC2 does not inhibit operation of IC1. In this condition, pin 40 of IC1 is held substantially at the voltage of rail 16 whilst pin 40 of IC2 is substantially open-circuited.

When IC1 times out its set period IC2 is set running and at the same time the signal on pin 38 of IC2 changes to a value such as to inhibit running of IC1. In this condition, pin 40 of IC2 is held substantially at the voltage of rail 16 whilst pin 40 of IC1 is substantially open-circuited. IC1 and IC2 thus operate alternately, and in this way the circuit operates rather like a multi-vibrator with an extremely slow period producing output signals alternately at the output 40 of the circuits IC1 and IC2 with respective durations, alternating with one another, of, for example, eight hours for IC1 and 12-1/2 minutes for IC2.

Connected between positive rail 18 and pin 40 of IC1 is the energising coil of a first reed relay Ray 1. Likewise, connected between the pin 40 of IC2 and the positive supply line 18 is the energising coil of a second reed relay RY2. The reed of relay RY1 forms a normally open switch connecting the live supply line L with the gate of a triac TR1 and likewise the reed of relay RY2 forms a normally open switch connecting the live supply line L with the gate of a triac TR2.

The circuit shown in the figure is intended for a freezer having a refrigerating system of conventional type incorporating a compressor driven by an electric motor and incorporating electrical heaters for de-frosting of the active cooling surfaces of the active cooling device, which may, for example, take the form of metal walls of the cabinet compartment which are in heat-conductive connection with metal tubing through which fluid refrigerant is circulated in known manner by a compressor performing the refrigerating heat cycle, and driven by an electric motor.Also in known manner, a thermostat, sensitive to the temperature within the cabinet, acts to close a switch which (subject to the state of the de-frost control system) connects the motor of the compressor with the mains supply when the set temperature rises above an upper threshold and disconnects the compressor motor from the mains supply when the said temperature falls below a lower threshold, whereby the interior of the cabinet is maintained approximately at a pre-determined low temperature by intermittent energisation of the compressor motor.

The triac TR1 is connected in series with the thermostat (not shown) and the compressor motor whilst the triac TR2 is connected in series with the de-frost heaters. Thus, in operation, so long as IC1 is in the course of timing out its set period, the compressor motor is effectively connected, through the open or closed thermostat switch, with the mains supply so that the compressor operates, as required, under the control of the thermostat in the normal manner and the de-frost heaters are energised. When IC1 reaches the end of its pre-set period, the relay RY1 is de-energised and the triac TR1 rendered non-conductive thus ensuring that the compressor motor is deenergised or is kept de-energised, whatever the state of the thermostat switch.At the same time, the timer IC2 is set to begin timing out its relatively short period (around 12-1/2 minutes) during which time the relay switch of relay RY2 is kept closed and triac TR2 rendered conductive thereby energising the defrost heaters. At the end of the 12-1/2 minute period of IC2, the relay RY2 is deenergised, terminating energisation of the defrost heaters and triggering lC1 once again so that relay RY1 is closed once more and triac TR1 rendered conductive allowing the compressor to operate as and when required.

The effective value of the timing resistor of the device lC1 is made rapidly switchable between two values whereby, for testing purposes, the set period of IC1 can be changed from eight hours to, for example, 2-1/2 minutes. To this end, the respective pin of IC1 is connected to one terminal each of two resistors R1, R2 of different values and the other terminals of these two resistors R1 and R2 are connectable, alternatively, by means of a simple tap or switch PP, with the remainder of the timer network.

In a development of the circuit shown in the drawings (not illustrated) the timing operation of IC1 is arrested, without losing count of the time already elapsed, during periods when the compressor motor is not energised. This may be achieved, it has been found, by holding the voltage of a line 50 which connects the timing capacitor C3 with the negative supply rail, at a predetermined voltage between that of lines 16 and 18 or by effectively open circuiting the line 50. To this end, in this development, the line 50 is connected, not with the base line directly, as shown in the drawing, but to the output of further circuitry (not shown) which holds the voltage on line 50 at said intermediate voltage when the compressor motor is deenergised but holds line 50 at the potential of line 16 when the compressor motor is energised.This further circuitry may operate, for example, by detecting the position of the thermostat switch. One preferred mode of achieving this is to connect a neon lamp across the contacts of the thermostat switch and to sense the state of illumination or otherwise of the neon lamp by appropriate opto-electronic sensing moans, such as a photo diode, controlling the state of the further control circuitry associated with the line 50.

As a result, in operation of this variant the overall frequency of defrosting is dependent upon the proportion of the total time that the compressor is in operation, which is dependent on the overall demand on the cooling system. As a major factor affecting such demand is the frequency with which the cabinet is left open, and the overall time for which the cabinet is kept open, this factor also substantially determines the frequency at which defrosting is needed.

It will be appreciated that the circuit described may readily be adapted by adding further circuitry, to control various additional functions, for example to maintain operation of a cooling system associated with another part of the cabinet, for example in so-calied fridgefreezers.

Claims (1)

1. A de-frosting system for a refrigerated cabinet, as herein defined, including electronic timing means arranged to interrupt the cooling of the refrigerator, freezer, or the like, or the cooling of a selected part or parts thereof, to defrost the same at intervals and to allow such cooling to proceed again at the end of a relatively short period after each such interruption.

2. A de-frosting system according to claim 1 wherein the timing means is arranged to interrupt such cooling at regular intervals.

3. A de-frosting system according to claim 1 wherein the timing means is arranged only to take account of the passage of time in periods when a cooling system is actually in operation, but is arranged to interrupt such cooling each time that the sum of the length of such periods, timed from the preceding interruption, reaches a predetermined value, whereby the overall frequency of defrosting is dependent on the overall demand on the cooling system.

4. A refrigerated cabinet incorporating a refrigerating system operable normally to maintain the interior of the cabinet at a pre-determined temperature below ambient temperature, and a defrosting system arranged to interrupt the normal operation of the refrigerating system at intervals to allow the temperature of at least the cooling surfaces within the cabinet to rise in temperature to effect defrosting, for relatively short periods, wherein said de-frosting system is electronically operated and, in particular, incorporates an electronic timer.

5. A refrigerated cabinet according to claim 4 wherein said refrigerating system includes an active cooling device having a cooling capacity substantially greater than that required in normal ambient temperatures to maintain the interior of the cabinet at said predetermined temperature when the cabinet is closed and further includes a thermostat arranged to energise the active cooling device when the cabinet temperature reaches an upper threshold and to de-energise the active cooling device when the cabinet temperature reaches a lower threshold, and wherein said electronic timer is arranged only to register the passage of time during which said active cooling device is energised and to initiate a defrosting operation each time that the cumulative measure of the intervals during which the active cooling device has been energised since the previous de-frosting operation reaches a predetermined value, whereby frequency of de-frosting is dependent on the demand on the refrigerating system.

6. A refrigerated cabinet according to claim 4 or claim 5 including one or more energisable facilities in addition to said refrigerating system, such as a further refrigerating system for a particular compartment within a cabinet, or an air-circulating fan, and wherein said defrosting system allows said air-circulating fan to remain energised during said de-energising of the said cooling device.

7. A refrigerated cabinet according to claim 4, 5, or 6 wherein said defrosting system incorporates heaters to raise said cooling surfaces to a temperature to effect de-frosting, the arrangement being such that said heaters are energised during said defrosting periods.

9. A de-frosting system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

10. A refrigerated cabinet incorporating an electronic de-frosting control system, substantially as hereinbefore described with reference to the accompanying drawings.

11. Any novel feature or combination of features described herein.