GB2177787A - Improvements relating to defrosting of refrigerated cabinets - Google Patents

Abstract

The invention provides that a plurality of refrigerated display cabinets 10, 12, 14, 16 are defrosted in turn by supplying the electrical power for the defrosting heaters 24 by means of a single cable 18 by using a solid state control system which by electrical pulses ensures that only one heater is switched on at any one time, the cabinets being therefore defrosted in turn. <IMAGE>

Description

SPECIFICATION Improvements relating to defrosting of refrigerated cabinets This invention relates to defrosting of refrigerated cabinets, such as refrigerated display cabinets for containing consumables such as foodstuffs and beverages. The refrigeration systems of such cabinets which maintain the contents in refrigerated condition are required to operate for twenty four hours a day, and it is necessary that the cases be defrosted periodically, for example four or five times a day to ensure that the frost deposits in the cases do not become excessive.

The present invention is concerned with the defrosting on a cyclic basis of a plurality of refrigerated display cases or the like.

In the defrosting of a refrigerated display case, a heater in the display case is supplied with electric current in order to heat the atmosphere inside the case, and such heaters are substantial electrical power consumers, drawing typically of the order of 15 amps.

The conventional method of refrigerating a plurality of display cases is to provide a cable capable of carrying at least 1 5 amps for each display case. In actual fact, each display case will probably include other current consuming devices such as lights and fans, taking in aggregate say 2 amps, and therefore each supply cable to each case must be rated at at ieast 17 amps. The disadvantage of this arrangement is that the provision of separate high rated cables for the separate cases means that the installation is expensive. If a single supply cable were provided for all cases, then it would have to have an amperage rating equal to 17 amps times the number of cases supplied, which would be completely prohibitive.

Another disadvantage of an arrangement wherein separate supply cables for the defrosting power are provided is that it is possible that defrosting of two or more cases make take place simultaneously, which means high power consumption.

In accordance with the present invention, the defrosting of a plurality of refrigerated display cabinets comprises using the same supply cable for the current to the defrosting heaters, the cable being rated upon the heater current for the heaters of one of the cases, and wherein a control arrangement includes switching devices operative to switch on the defrosting heaters of the respective cases in sequence but not simultaneously. An activation unit is provided for the heater in each case, such activation unit being controlled by two signals namely a main signal and a firing signal, the arrangement being that the main signal is applied for a defrost period which is followed by a dwell period, the firing signals being applied at intervals equalto the cycle time made up of the defrost period and the dwell period.

Of each firing device, it is activated to switch on the associated heater only when the main signal is present and the activating signal is applied, and the heater is switched off at the end of the defrost period by vitue of the removal of the main signal.

The current supplied to the heaters is supplied through a single cable whose rating will in fact be equal to the rating of a heater of one case plus the aggregate of the ratings for the ancillary devices such as lamps and fans for all other cases. Thus, if each heater is rated at 15 amps, and the ancillary devices of each case are rated in aggregate at 2 amps each, and there are four cases, then the supply cable has to be rated at 15 amps plus 4 X 2 amps (equal to 8 amps) and therefore the total rating is 15 plus 8 equals 23 amps.

The control system may be arranged so that there is no commencement of defrost cycling until an initial period enabling the establishment of the refrigeration of all cases, has elapsed.

The main signal may have an activation time of the order of 30 minutes, followed by a dwell time of 30 minuites, whilst the firing signals may be of the order of 10 seconds.

The accompanying diagrammatic drawings illustrate one embodiment of the invention, and in the drawings: Fig. 1 is a circuit diagram; and Fig. 2 is a signal diagram.

Referring to Fig. 1, this figure illustrates four refrigerated display cases 10, 12, 14 and 16 which are supplied with electrical current through a power line 18, the line 18 branching as indicated at 18A, 18B and 18C to the respective display cases 10 to 16 so as to supply power thereto.

Each of the cases 10 to 16 is shown as being provided with current consuming devices and fans, and a heater 24 for the defrosting of the display case. Under normal running conditions i.e. when na defrosting is taking place, line 18 supplies the electrical power for the running of the lamps 20 and fans 22 of all cases, and typically the lights and fans of each case may draw of the order of 2 amps, and therefore the cable 18 will supply 8 amps for the lamps and fans. If each heater 24 is rated at 1 5 amps, then if defrosting of all cases took place simultaneously the maximum current demand on line 18 would be 68 amps.Such a cable would be prohibitively expensive, and therefore in accordance with this embodiment of the invention, the heaters 24 of the respective cases are controlled so as to be operative one at a time only, and by so arranging the operation of the heaters, then the maximum rating of cable 18 at any one time would be only 23 amps during a defrost mode.

The heaters 24 of the respective cases are controlled for defrosting by means of firing devices 26 in the cases 10 to 16. The firing devices 26 are logic devices each driven by two inputs, the first input of each device being connected to a common main signal line 28, and the second input of the respective devices 26 being connected to respective firing lines 30, 32, 34, 36. The main line 28 and the lines 30 to 36 are controlled via a programmable logic controller (not shown) such that, as illustrated in Fig. 2, the lines 28 to 36 carry signals in the time pattern illustrated in Fig. 2.

Referring now to Fig. 2, the graphs 1 to 9 illustrate the signals and conditions indicated, and graph 1 which illustrates the main signal shows that after an initial period to which ensures refrigeration of all cases the main signal is applied for a defrost cycle time t, and is followed by a cut off or dwell time t2, and then the cycle made up of times t1 and t2 repeats continuously. The first case start signal is illustrated by graph 2, and it will be seen that the start signal is a short pulse of duration ts. The first case start signal is applied on line 30 simultaneously with the first defrost signal t applied on line 28 which causes actuation of the heater 24 in the first case and as shown in graph 3 the heater 24 of the first case is operative during the period t, of the main signal.When the main signal stops at the end of the first cycle, heater 24 of the first case is cut-off and defrost of the first case is terminated.

No further defrosting takes place during the dwell period t2 of the first cycle, and at the end of-that period the second case start signal is applied on line 32 for the duration t3 which establishes defrosting of the second case for the period t of the second cycle.

The cycling repeats, the start signals being applied in turn on the lines 30, 32, 34 and 36 and then back to line 30 so that defrosting repeats at intervals equal to the number of cases multiplied by the cycle time. This arrangement prevents the simultaneous defrosting of two cases at the same time.

Because the controller is a programmable controller, then the periods to, ti, t2 and t3 can be varied as desired, but typically, the periods tT and t2 may be 30 minutes each, whilst the period t3 may be 10 seconds. The start signals applied on lines 30 to 36 must be of sufficient duration to switch on the heater 24, and the firing device 26 will contain appropriate logic to ensure that the heater locks on after it has been switched by the appropriate start signal, but will not terminate when the start signal terminates. The operation of the heater will terminate either at the end of a defrost period t by termination of the main signal, or it may be terminated by means of an overriding defrost termination control for each case or a group of cases. The defrost termination control operates under the control of the case defrost condition and if the heater or heaters is or are switched off by the defrost control during a main period t1, the microprocessor emitting the main signals is not affected but the heater or heaters will not be re-activated during the same main period.

It will be understood that the invention is not limited to the control of four display cases as the number can vary, and in addition the invention is not to be considered as being limited to the controlling of the defrosting of refrigerated display cases. The system can be used in any application involving the defrost of a plurality of spaces or locations.

Claims (11)

1. A method of defrosting a plurality of refrigerated locations each having electrical defrosting heating means, wherein acommon cable is used for the supply of the electrical power to the creative means of all locations, and controlling the supply of electrical power so that only one heating means is operative at any one time and the power supply is switched so that respective heating means are operated in turn to effect defrosting of the locations in turn but avoiding that the supply cable carries, in respect of the defrosting, more than sufficient current for the operation of one heating means.

2. A method according to Claim 1, wherein each heating means is provided with a firing device, conditioned by two signals a first which when present permits supply of electrical power to the heating means, and a second which fires the firing device commencing the supply of power to the heating means.

3. The method according to Claim 2, wherein a sequence of first signals at fixed cycle times are applied over a common first signal line to all firing devices and second signals are applied at the commencement of said signals but sequentially to the firing devices of the respective heating means in turn.

4. A method according to any preceding Claim wherein the defrosting of the locations does not take place until all the locations have been refrigerated for a predetermined period.

5. A method according to any preceding Claim, wherein said locations comprise the interiors of refrigerated display cases.

6. A method according to any preceding Claim, wherein each heating means is a resistance heater.

7. A method of defrosting a plurality of locations substantially as hereinbefore described with reference to the accompanying drawings.

8. A system for defrosting a plurality of refrigerated locations comprising a plurality of electric heating means, one for each location, a common electrical supply cable for supplying electrical power to said heating means, a plurality of gate devices, one of each heating means for controlling the supply of electrical power to the heating means respectively, and control means controlling the operation of said gate devices in turn so that only one gate device is operative at any one time so that the heating means are operated one at a time and in sequence.

9. The system according to Claim 8, wherein the control means comprises a first signal line common to all gate devices on which are supplied first signals applied at frequent intervals and for defrost periods, and individual firing signal lines connected to the respective gate devices on which are applied firing signals in turn and at said frequent intervals at the beginning of each defrost signal, each gate device being adapted to switch on the associated heating means when a defrost signal and a firing signal are applied to the gate device and to maintain the heating means on as long as the defrost signal lasts, unless the heating means is otherwise switched off.

10. The system according to Claim 9, wherein said locations are the interiors of refrigerated display cases.

11. A system for defrosting a plurality of refrigerated locations substantially as hereinbefore described.