"MEDIUM TEMPERATURE SUPERMARKET REFRIGERATOR EVAPORATOR UNIT"

BACKGROUND TO THE INVENTION

THIS invention relates to medium temperature supermarket refrigerators, i.e. refrigerators suitable for use in supermarkets, and in particular to an evaporator unit for use in such refrigerators.

So-called "medium temperature" supermarket refrigerators are used to keep non-frozen food items cold and to display such food items for sale to customers. The food items in question might, for example, be dairy products such as milk or yoghurt, or fresh meat, fruit or vegetables.

In a conventional compressor type refrigeration circuit air which is to be used for cooling food products is passed, before it encounters the food products, through an evaporator unit which removes heat from the air and cools it to a temperature appropriate for the cooling work which is to be done. In the coils of the evaporator unit itself, compressed and cooled refrigerant is expanded to a vapour state such that the latent heat of vaporisation is withdrawn from cooling fins which are attached to the evaporator coils and through which the air circulates.

Problems common to most known supermarket refrigerators is a tendency for the cooling fins to ice up. Apart from the fact that this reduces the cooling efficiency of the evaporator, it necessitates regular defrosting of the evaporator unit. This in turn means that the refrigerator must be switched off to allow defrosting to take place.

SUMMARY OF THE INVENTION

The invention provides an evaporator unit for a medium temperature refrigerator suitable for supermarket use, the refrigerator including a refrigeration space for goods which are to be refrigerated, a compressor- driven refrigerant circuit in which refrigerant is circulated and at least one fan for causing air to pass through the evaporator unit and into the refrigeration space, wherein the evaporator unit is arranged to receive refrigerant from the refrigerant circuit and comprises:

first and second stages arranged in series such that the air passes sequentially through the first and second stages, both stages being arranged to receive refrigerant circulating in the refrigeration circuit, and

- a pressure regulating means for controlling the pressure of the refrigerant in the first stage such that water vapour in the air passing through the first stage is cooled to liquid form but is not frozen, whereby the first stage runs wet and air which passes through the second stage after passage through the first stage is dried by its passage through the first stage.

The second stage is preferably arranged to cool air passing through it to a temperature below the freezing point of water. The pressure regulating means typically comprises a pressure regulator located downstream of the first stage in the refrigerant circuit of the refrigerator.

A compact refrigerator structure is achieved if the evaporator unit is arranged to be located at least partially at the rear of the refrigeration space, preferably with the second stage positioned on top of the first stage.

According to another aspect of the invention there is provided a medium temperature refrigerator suitable for supermarket use, the refrigerator including a refrigeration space for goods which are to be refrigerated, a

compressor-driven refrigerant circuit in which refrigerant is circulated, front and rear compartments beneath the refrigeration space, an evaporator unit as summarised above and at least one fan for causing air from the refrigeration space to pass through the rear compartment and then through upwardly through the evaporator unit, the first stage of the evaporator unit being positioned at the rear of the rear compartment.

Yet another aspect of the invention provides a method of operating a supermarket refrigerator which includes an evaporator unit for cooling air, a refrigeration space for goods which are to be refrigerated and a compressor-driven refrigerant circuit for supplying refrigerant to the evaporator unit, the method comprising the steps of causing air to pass sequentially through respective first and second stages of the evaporator unit and controlling the pressure of refrigerant in the first stage of the evaporator unit such that water vapour in the air passing through the first stage is cooled to liquid form but is not frozen, whereby the first stage runs wet and air which then passes through the second stage after passage through the first stage is dried by its passage through the first stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a diagrammatic side view of a medium temperature supermarket refrigerator according to the present invention; and

Figure 2 shows a circuit diagram illustrating the operation of the two- stage evaporator unit forming part of the refrigeration circuit of the refrigerator seen in Figure 1.

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DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Figure 1 shows an open front supermarket refrigerator 10 having a refrigeration space or cabinet 12 in which shelves 14 are arranged one above the other to support refrigerated food items for display and sale to customers. The refrigerator is a co-called "medium temperature" refrigerator suitable for refrigeration and display of non-frozen food items such as fruit and vegetables and dairy and fresh meat products.

Design features of the refrigerator 10 as a whole are described in detail in the specification of a co-pending international patent application, filed simultaneously with the present application by the same applicant and entitled "Supermarket Refrigerator". Reference should be made to that specification for such details. The refrigerator cabinet has a moulded and insulated bottom panel 16 having front and rear portions 16.1 and 16.2 respectively, an intermediate, inclined wall portion 16.3 joining the front and rear portions, and a vertical rear wall portion 16.4. Mechanical refrigeration circuit components, indicated generally by the numeral 18, are housed in a front compartment 20 located beneath the front portion 16.1, forwardly of the wall portion 16.3. Such components include a compressor, heat exchanger and receiver as well as associated pipework and valves. One or more air circulation fans 22 are located in a rear compartment 24 above the rear portion 16.2 of the bottom panel 16, beneath a removable fan support panel 26.

The numeral 28 indicates the two-stage evaporator unit forming the subject matter of the present invention. The lower part of this unit is located in the rear compartment as illustrated. Each of the two stages includes finned refrigerant coils.

In use, air is circulated by the fan(s) 22 along the path indicated by the arrows 30. In the rear compartment 24 the air is caused to flow upwardly through the unit 28 in which it is cooled. Some of the cooled air thereafter flows forwardly, through openings in an upright rear panel 31 , between the

shelves 14 to cool food items thereon while remaining air is caused to form an air curtain 30.1 in front of the shelves.

As indicated above, the unit 28 is of two stage design. The first and second stages are indicated respectively by the numerals 28.1 and 28.2 and it will be understood that the air flows upwardly and sequentially through the finned coils of the first and second stages. In other words, the upwardly flowing air initially passes between and around the cooling fins of the first stage 28.1 and thereafter between and around the cooling fins of the second stage 28.2.

Figure 2 diagrammatically illustrates the manner in which refrigerant circulating in the refrigerant circuit is routed in the first and second stages 28.1 and 28.2 of the unit 28. The numeral 32 indicates a line from the receiver of the refrigerant circuit. Referring firstly to the first stage 28.1, the refrigerant enters the coils of the first stage at a valved inlet 34. After passage through the coils of the first stage, the refrigerant leaves the first stage through an outlet 36 and flows back to the compressor side of the refrigerant circuit along a line 38.

A pressure regulator 39 in the line 38 is used to control the pressure of the refrigerant circulating through the first stage such that the temperature of air flowing upwardly through the first stage, as indicated by the arrows 30, is reduced to a low temperature above the freezing point of water. Thus water vapour present in the air is condensed to liquid water but is not frozen. The first stage accordingly runs "wet".

The refrigerator 10 also includes a trap and drain system which is described in detail in the specification of a copending South African patent application, filed simultaneously with the present application by the same applicant, entitled "Refrigerator Trap and Drain System" and claiming priority from ZA2006/05614. Reference should be made to this specification for technical details of the system, which includes one or more drain outlets at a low point in the rear portion 16.2 of the bottom wall 16. The

arrangement is such that condensate precipitates from the fins of the first stage onto the upper surface of the rear portion 16.2 and is drained through the drain outlet.

The pressure regulator may for instance be such as to cool the air flowing through the first stage to a temperature of about 1 ⁰ C. Water vapour in the air stream is condensed to liquid form, with the result that the air stream which flows upwardly through the second stage is substantially drier than the initial air stream.

Refrigerant from the receiver enters the coils of the second stage 28.2 along a line 40 and through a valved inlet 42. Refrigerant leaves the second stage through an outlet 44 and returns to the compressor side of the refrigeration circuit along a line 46. The design of the second stage is such that the temperature of the air stream flowing upwardly through the second is reduced to a temperature typically in the range -3 ⁰ C to -6 ⁰ C. Thus air which passes through the second stage is cooled further to a temperature suitable for refrigerating the food items accommodated in the refrigerator.

An important advantage of the described embodiment is the fact that the first stage runs wet, so there is little or no icing of the coils and fins in this stage, and the second stage receives dried air which is less likely to cause icing of the coils and fins in this stage.

Thus, viewed overall, it is envisaged that there will, compared to conventional evaporator units, be a reduced requirement for defrosting of the coils and fins.

An added benefit is that the reduced likelihood of icing of the fins means that the fins can be placed closer together, thereby allowing for a greater heat transfer contact area and more efficient transfer of heat from the air to the refrigerant and, accordingly, a more compact evaporator unit which can be accommodated in the illustrated position at the rear of the compartment 24.