Cooling method

Description

The present invention relates to a method and equipment for keeping articles, particularly foods, cold.

When food is displayed which has to be kept cool or cold it can be displayed in a refrigerated cabinet which has an open or glass or transparent plastic front. In conventional cabinets the food is displayed on shelves or decks and is cooled by cold air supplied from within the cabinet. The air is circulated through the cabinet using fans and the bulk of the air is directed into the display area as a jet which forms an air curtain across the cabinet opening. The air is then recirculated within the cabinet where it is cooled by a plate fin and tube heat exchanger containing a low temperature refrigerant with a saturated refrigerant temperature of approximately −7° C. The evaporator is normally in counterflow with the air, which enters the evaporator at approximately 5° C. and leaves at approximately −5° C, the exact temperatures will vary with the make and type of cabinet the temperature variations may be for example ±3K.

It has been found that, in practice it is difficult to maintain display cabinets at low temperatures e.g. below 5° C. and as result food is stored on display at higher temperatures. This is because convection is, however not an effective cooling mechanism. This is due in part to the low heat transfer coefficient caused by low velocity air, but it is also because the air close to the cabinet front is warm due to air ingress. As a result convective cooling systems maintain a large spread of temperatures within the cabinet, with the higher temperature food near to the front opening. Lower food temperatures are achievable with conventional systems, however, this requires the use of lower temperature air, and because of the large temperature spread with the cabinet, low temperature air will lead to localised freezing of some food products and frost damage to food.

Providing more powerful cooling systems, as well as being expensive can lead to local freezing and ice formation so that defrosting is required.

Although the food is normally delivered to the cabinet at the required food storage temperature, cooling is required to offset gains to the food which are purely a result of radiant interchange between the food and the external supermarket/shop environment. The cooling load necessary to offset these gains is only a fraction of that delivered to the cabinet. At best this low heat transfer rate results in excessive energy use, high equipment costs and reduces food shelf life. At worst, the low efficiency can result in failure to meet food hygiene temperatures which can present food safety issues.

It has been proposed to provide significant additional cooling to the food by positioning the food directly in contact with a cold base. This was achieved by siting the food on a base in contact with the evaporator. Significantly increasing heat transfer to the food in this way enables the cabinet to operate more efficiently and results in significant reductions in energy consumption, as well as improvements in food quality/safety. However as most chilled cabinets utilise adjustable shelving, producing conduction by siting the food shelves onto a fixed evaporator surface is not a practical option.

We have now devised a method of providing conductive cooling to the food but still maintaining adjustable shelving.

According to the invention there is provided a method of cooling articles in a cabinet in which method the articles are positioned on shelf or deck in the cabinet which shelf or deck incorporates a heat pipe at least a part of which heat pipe is cooled.

The invention also provides equipment for keeping articles cool which equipment comprises a cabinet incorporating at least one shelf or deck which shelf or deck incorporates a heat pipe there being a means to cool at least part of the heat pipe.

The heat pipe is used because it is a passive, highly efficient heat transfer mechanism with a reported conductivity equivalent to approximately 500 times that of copper. Heat pipes are robust, simple and maintenance free devices that operate isothermally, require no pumping power and can transfer heat over a long distance. Heat pipes are described in an Article by Dunn P. D. and Reay D. A. (1994) in Heat Pipes 4^th Edition pp 1-3, 100, Elsevier Science Ltd. Oxford UK.

In use the food is placed on the shelf which incorporates the heat pipe and preferably the end of the heat pipe at the rear of the shelf is cooled e.g. by blowing cold air over the pipe from the cabinet's cooling system. The heat pipe cools the shelf and keeps the food cool by conduction from the shelf to the food.

The shelf can comprise one flat heat pipe or a series of connected heat pipes.

Preferably the cooling means is a conventional cooling system in which air is recirculated within the cabinet where it is cooled by a plate fin and tube heat exchanger containing a low temperature refrigerant e.g. with a saturated refrigerant temperature of approximately −7° C. The evaporator is in counterflow with the air, which enters the evaporator at approximately 5° C. and leaves at approximately −5° C.

In use in a cabinet for keeping food cool there can be conventional convective cooling which is supplemented by conductive heat transfer provided by a shelf cooling mechanism incorporating a heat pipe so that the shelf is used to display the food and also to transfer heat from it to the cabinet supply air duct.

Preferably the shelf is constructed as a heat pipe and positioning through the display area back plate into the supply air duct, which is at approximately −5° C.

There can be phase change materials incorporated in the shelves or adjacent the shelves so as to provide extra cooling capacitance in case of power or other failure so that the change of phase provides a cooling effect in the case of such failure.

The invention can be applied to conventional cabinets by replacing the existing shelves with shelves incorporating heat pipe or pipes and it is a feature of the invention that it can contribute to lower core food temperatures e.g. of up to approximately 2.5 to 3K which would lead to significant reductions in energy and capital costs as well as improvements in food quality and shelf life. Furthermore using heat pipe shelving could result in avoiding electric defrost, which is energy intensive.

The invention can be used with any type of cabinet e.g. open fronted, closed cabinets and all types of frozen food cabinets.

The invention is described in the accompanying drawings in which:

FIG. 1 shows the principle of operation of the simple heat pipe

FIG. 2 shows a conventional cool cabinet

FIG. 3 shows a cabinet incorporating the invention and

FIG. 4 shows a shelf in detail

Referring to FIG. 1 a heat pipe comprises a sealed tube (1) containing a wick (2) and a working fluid. When heat is applied, the working fluid evaporates to form vapour (5) at heat source (4) and moves to the heat sink (3) where it condenses. The wick then returns the condensed liquid to the heat source (4) by capillary action. Referring to FIG. 2 a typical cool cabinet comprises an insulated cabinet (15) with a transparent or open front. At the bottom of the cabinet are an evaporator (10) and a fan (11). Food is stored on shelves (12). In use the fan (11) blows air over evaporator (10) which cools the air and cold air e.g. at approximately −5° C. passes over the back of the shelves in a duct (14) and over the food as shown to cool the food. The food is warmed by radiant heat, which is then transferred to the cooling air, which is recirculated over the evaporator to cool it.

Referring to FIG. 3 the cabinet (21), evaporator (22) and fan (23) are as in FIG. 2, but the shelves are replaced by shelves (20) incorporating a heat pipe, shown in detail in FIG. 4. The ends of the heat pipes (20a) protrude through the back of the display section into duct through which cooled air from the evaporator passes. As well as the cooling effect of the air, the heat pipe removes heat from the food on shelves (20) and so aids the convective cooling effect. The air is recirculated as in FIG. 2. Referring to FIG. 4 the shelves (20) incorporating a heat pipe (23) have food (21) placed on them. Heat from the food is conducted by the heat pipe to the back of the heat pipe (20a), which is in the cold airflow from evaporator (22). The heat pipe rapidly conducts heat from the food to the cold part of the heat pipe (20a) and thus aids the convective cooling of the food.