GB2094460A - Refrigerated food display apparatus - Google Patents

Abstract

A food display apparatus comprises a food display support (20) for receiving unwrapped food to be displayed whilst maintained in a chilled condition. A recirculating fan (28) is arranged to circulate air around a closed path (30) within the cabinet of the apparatus, a heat-exchanger (22) of a refrigerating system being located within the path of the airstream for cooling thereof. An ice sensor (34) is directly associated with the heat-exchanger, and the arrangement is such that when the sensor (34) responds to ice build-up, the refrigeration system compressor is de-activated to enable defrosting of the heat-exchanger. During the defrosting cycle, the recirculation of air around the closed path (30) is maintained, whereby the air within the display cabinet is rehumidified during the defrosting cycle. <IMAGE>

Description

SPECIFICATION Improvements in and relating to food display apparatus This invention relates to refrigerated food display apparatus incorporating a defrosting system.

Food display equipment incorporating a chilled air recirculation system is becoming increasingly popular as it enables food to be attractively and hygienically displayed to customers while being maintained in good condition for an extended period. Such display equipment normally comprises a cabinet providing a closed path around which air is driven by a fan, the path including a portion lying between the fins of a heat-exchanger through which the air passes so as to be cooled before flowing over the food on display.Condensation occurs on the fins of the heat-exchanger producing a build-up of ice which reduced the efficiency of thermal transfer and thus causes a progressive increase in the temperature of the circulated air; in cases of prolonged build-up the ice completely blocks the spaces between the fins, radically reducing the area of the heattransfer surface and attenuating the flow path. If the food on display is packed or sealed the buildup of ice poses no great problem as there is no source of moisture around the path and the ice which does build up over a prolonged period can be removed by occasional defrosting.Where, however, the food on display is not wrapped or packed, which is often desirable to maximise its attractiveness to customers, the conventional system has two disadvantages: firstly the flow of chilled air over the exposed food on display dehydrates it rapidly impairing its appearance and flavour; secondly, the icing up of the heatexchanger fins occurs far more rapidly due to the moisture removed from the food necessitating frequent defrosting cycles in order to regain efficient cooling of the circulated air.

The present invention provides food display apparatus comprising a cabinet including a support for presenting food to be displayed, means for recirculating air within said cabinet around a closed path at least a portion of which extends across said support so that food when placed thereon becomes located within or adjacent the airstream, refrigeration means including a heat-exchanger located within said airstream for cooling the recirculated air, temperature sensing means for controlling the operation of a compressor of the refrigeration means in order to maintain the recirculated air at a predetermined temperature, and frost detector means located immediately adjacent a portion of said heat exchanger and arranged to sense the build-up of a predetermined amount of frost or ice upon said heat-exchanger, the arrangement being such that upon response of the frost detector means the compressor of the refrigeration means is de-activated to permit the ambient temperature in the region of the heat-exchanger to rise and to allow the frost and/or ice thereon to melt, and that during de-activation of the said compressor the said air recirculating means is operated to continue recirculation of air within the cabinet, whereby moisture from the melting frost or ice is re-absorbed into the airstream to maintain its humidity.

The arrangement in accordance with the invention has that advantage that since the recirculation of air through the heat-exchanger is maintained during a defrosting cycle controlled by a frost detector directly associated with the heatexchanger, not only is the build up of frost or ice upon the heat-exchanger more effectively controlled than in the case where defrosting is initiated in response to a timer or thermal sensing means, but the fact that the air within the cabinet itself is recirculated as a heat-exchange medium to effect defrosting ensures that the air becomes rehumidified during the defrosting cycle, which alleviates the problem of conventional refrigerated display cabinets as regards the tendency of refrigerated air to cause drying of unwrapped food on display.

The frost detector means may take any convenient form, for example a capacitance effect or ultra sound may be employed. However, it is preferred according to a further aspect of the present invention that the detection means are constituted by an ice sensor comprising a light source for emitting a beam of light, which is preferably in the infra-red range, along a path which is in operation affected by the accumulation of ice on a heat-exchange surface, and receiving means disposed in the path of said beam for generating a signal when the beam is effected by accumulation of ice above a predetermined thickness. The light path may comprise an incident length which terminates at a reflecting surface by which the beam is reflected onto the receiving means along a reflected length provided that the reflecting surface is not coated with ice.

Alternatively the light path may be arranged so that light falls on the receiving means only when ice is present, for example by arranging for light to fall on the receiving means only when reflected by accumulated ice. Preferably the light path is arranged so as to be obstructed by the accumulation of ice; for example the light beam may be arranged to pass through an orifice in the fin of a heat-exchanger so that ice which forms in the orifice obstructs the beam.

In order to ensure that during each defrosting cycle the heat-exchanger surface is fully cleared of accumulated frost or ice, which may occur some time after the removal of frost or ice from the immediate area sensed by the frost detector means, a timing means is preferably arranged to deactivate the compressor for a predetermined period of time required for a complete defrosting cycle, upon each response of the frost detector means to the presence of frost or ice upon the heat-exchanger.

Preferably, in order to ensure uniform defrosting of the heat-exchanger, a plurality of frost detector means are associated with said heat-exchanger in spaced apart relationship, and the arrangement is such that a defrosting cycle is initiated when any one of said frost detector means responds to the presence of frost or ice.

The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a section through a refrigerated food display stand employing a defrosting system according to the present invention; Figure 2 is a perspective view of an ice sensor employed in the system of Figure 1; Figure 3 is a block diagram of the refrigeration circuit and associated defrosting system; and Figure 4 is a circuit diagram of a control circuit incorporating a plurality of ice sensors as shown in Figure 2.

Figure 1 shows a food display stand 10 having a front wall 12, floor 14, rear wall 16, rear cover 18 and food display tray 20. Housed below the rear cover 18 and against the rear wall 16 is a heat-exchanger 22 cooled by the evaporation of a conventional refrigeration system of which the compressor 24 (and associated condenser) and connecting pipes 26, 27 are shown diagrammatically in Figure 3. A fan 28 is mounted below the heat-exchanger 22 and drives air around a recirculatory path indicated by the arrows 30, that is through the heat-exchanger 22 where it is cooled, over food displayed on the tray 20 and returning through the duct formed between the tray 20 and floor 14.

A temperature sensor 32 is mounted at the edge of the tray 20 near the front wall 12 and an ice sensor 34 is mounted on a bracket (not shown) projecting from the rear wall 1 6. The ice sensor 34 comprises a base 36 on which are mounted an infra-red (IR) light source 38 and a detector 40.

The sensor 34 is mounted such that the light source 38 and detector 40 lie one on each side of a fin of the heat-exchanger 22 which has an orifice so as to allow a light beam emitted by the source 38 to fall on the detector 40. As shown in Figure 3 a lead 42 from the temperature sensor 32 and a lead 44 from the ice sensor 34 are connected to a switch device 42 controlling one mains power supply lead 44 to the compressor 24, the other lead being indicated by the reference 46.

In operation, air is circulated around the path indicated by the arrows 30 by the action of the fan 28, the air being chilled to a temperature of 5-60C by passage through the heat-exchanger 22 and subsequently cooling the food over which it passes. A drop in air temperature below a predetermined value is sensed by the temperature sensor 32 causing actuation of the switch device 42 to shut off the compressor 24 until the air temperature again rises to the desired level.

Moisture absorbed from the food on display deposits on the fins of the heat-exchanger 22 in the form of ice. As ice forms in the orifice in the heat-exchanger fin the beam of IR light falling on the detector 40 is interrupted, causing the compressor 24 to be shut off through actuation of the switch device 42, any signal from the temperature sensor 32 being overridden. Ice formed on the heat exchanger 24 consequently melts and the resultant water is absorbed by the air which is being continuously re-circulated by the fan 28. The consequent high water vapour content of the air reduces dehydration of the food not only in this defrosting phase of the cycle but also, because the higher water vapour content is maintained to some extent, during the normal cooling phases.Viewed from a different aspect it can be seen that dehydration must be reduced as no water is being removed from the system by being drained away from the heat-exchanger, but is instead being re-absorbed into the recirculating air stream.

The intensity of the beam emitted by the source 38 is variable so that the detector 40 may be triggered when ice forms in the orifice to predetermined thickness. In high humidity, high temperature conditions, the system may become too warm with no ice formed on the heatexchanger fins and in such a case the ice sensor 32 is set to allow the formation of a little ice.

Because of the sensitivity of the abovedescribed defrosting system the defrosting cycles are more frequent and little ice is therefore aliowed to accumulate on the heat-exchanger fins during the intervals between them. This leads to more efficient heat transfer between the circulating air and the fins and a consequent saving in energy demanded by the compressor 24, as well as maintaining a minimum level of humidity of air within the cabinet.

Referring now to Figure 4, there is shown in more detail an ice sensor logic circuit incorporating a plurality of sensors of the kind shown in Figure 2 arranged in association with the heat-exchanger 22 and located in spaced relation to one another. The logic circuit serves to control the duration of each defrosting cycle, so that each cycle has a predetermined minimum duration regardless of the state of the ice sensors, and also enables the defrosting cycle to be triggered by response of any one of the sensors to the presence of ice.

As shown, the circuit comprises three lightemitting diodes LEDI to LED3, each of which corresponds to the source 38 of one of the light sensors, and is connected in series a corresponding load resistor R1 to R3 and potentiometer VR 1 to VR3 to power supply terminals 50,51. PhototransistorsTR1 toTR3, each of which corresponds to the detector 40 of the ice sensor shown in Figure 2, are each coupled as a Darlington pair with corresponding switching transistors TR4 to TR6, each pair of transistors being connected in series with a corresponding load resistor R4 to R6 between the power supply terminals 50, 51. The tapping between each pair of transistors and the corresponding load resistor is connected to one input of a logic circuit formed from NOR gates 52, 53, 54, 55. The output from the logic circuit provided by NOR gate 55 is coupled via an RC section R7, C1 to the base of a further switching transistor TR7. The collectoremitter current path of the transistor TR7 is connected in series with a load resistor R8 to the power supply terminals 51, 52, and the tapping between transistor TR7 and resistor R8 is connected to the input of a timing circuit of conventional type and indicated generally at 56.

The output 57 of the timing circuit is connected to the lead 44 of the control circuit illustrated in Figure 3.

The operation of the circuit illustrated in Figure 4 is as follows. Each of the variable resistors VR 1 to VR3 is initially adjusted to calibrate the ice sensor provided by the light emitting diode and phototransistor so that when the sensor is at its working temperature the collectors of the coupled phototransistor/switching transistor pair are at a predetermined standing voltage. When ice forms and bridges the gap between the light emitting diode and phototransistor, the light received by the phototransistor is gradually reduced, as a result of which the collector voltage thereof correspondingly increases to a point at which the connected NOR gate is triggered. When any or all of the NOR gates 52, 53 and 54 are triggered in this way, the output of NOR gate 55 goes high thus turning on transistor TR7 whereby its collector voltage suddenly falls and triggers the timing circuit 56. The timing circuit 56 accordingly provides at its output 57 a corresoonding control signal which, applied on lead 44, initiates a defrosting cycle as already described. The timing circuit 56 is set to run for a predetermined period of time, for example 12 minutes, which has previously been ascertained to be sufficient to effect complete defrosting of the heat exchanger 22. During the period of time set by the timer 56, the removal of frost or ice from the frost detectors will have caused the output voltages thereof to revert to their standing values, and thus upon expiry of the period set by the timer the circuit will revert to its idle condition until one of the frost detectors again responds to the presence of ice.

Claims (6)

1. Food display apparatus comprising a cabinet including a support for presenting food to be displayed, means for recirculating air within said cabinet around a closed path at least a portion of which extends across said support so that food when placed thereon becomes located within or adjacent the airstream, refrigeration means including a heat-exchanger located within said airstream for cooling the recirculated air, temperature sensing means for controlling the operation of a compressor of the refrigeration means in order to maintain the recirculated air at a predetermined temperature, and frost detector means located immediately adjacent a portion of said heat-exchanger and arranged to sense the build-up of a predetermined amount of frost or ice upon said heat-exchanger, the arrangement being such that upon response of the frost detector means the compressor of the refrigeration means is de-activated to permit the ambient temperature in the region of the heat exchanger to rise and to allow the frost and/or ice thereon to melt, and that during deactivation of the said compressor the said air recirculating means is operated to continue recirculation of air within the cabinet, whereby moisture from the melting frost or ice is re-absorbed into the airstream to maintain its humidity.

2. A food display apparatus as claimed in Claim 1, including timing means forming maintaining said compressor de-activated for a predetermined period of time sufficient to enable complete defrosting of said heat-exchanger following said response of the frost detector means.

3. A food display apparatus as claimed in Claim 1 or 2, comprising a plurality of said frost detector means located in spaced relation adjacent said heat-exchanger, the arrangement being such that the said compressor is de-activated upon response of any one of said frost detector means to the presence of ice.

4. A food display apparatus substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

5. A food display apparatus substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings as modified by Figure 4.

6. The features as herein described, or their equivalents, in any novel selection.