GB2241776A

GB2241776A - Heat transfer fluid

Info

Publication number: GB2241776A
Application number: GB9005179A
Authority: GB
Inventors: Stephen Forbes Pearson
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 1990-03-08
Filing date: 1990-03-08
Publication date: 1991-09-11
Also published as: GB9005179D0

Abstract

A heat transfer fluid of enhanced heat transfer capacity comprises a non-freezing liquid (such as water-glycol mixture) having suspended therein a multiplicity of capsules (21) containing a freezable liquid (such as water). The capsules are usually disc or platelet shaped, and may be hollow or formed of a skinned or unskinned hydrogel. A refrigeration system using the heat transfer fluid comprises a central recirculating circuit(s) from which non-freezing fluid is drawn off via strainers to hold back the capsules into subsidiary cooling circuits before being returned to the central circuit. <IMAGE>

Description

HEAT TRANSFER FLUID The present invention relates to a heat transfer fluid for use in a refrigeration system, and which has enhanced heat transfer capacity.

The circulation of a liquid for the transfer of heat in applications which require cooling or heating is a well established method. The heat absorbed or rejected by the circulating liquid equals the mass flow of liquid, multiplied by the temperature rise (or fall), multiplied by the specific heat of the liquid. Water is an almost ideal heat transfer liquid because it is cheap, non-toxic and has a high specific heat. However, even with water there are severe limitations if the quantities of heat to be transferred are large. Also, there may be problems if the distance between the cold source and the location to be cooled is great, since heat leakage into the circulating water within pipelines raises the water temperature and reduces the effectiveness of heat transfer.

It is an object of the present invention to increase the effective heat capacity of the circulating heat transfer fluid, and also to alleviate the problem of heat leakage.

One approach to the problem would be to use fluids of higher specific heat. However, the choice of suitable fluids is severely limited. The temperature of the fluid may be reduced, but this exacerbates the problem of heat leakage.

Another system capable of high rates of heat transfer is the so called heat pipe. This is a sealed pipe where liquid is evaporated at one end and condensed at the other end. Due to the relatively large heats of vaporisation, high rates of heat transfer are possible.

However, the system is inflexible and not suitable for heat transfer over relatively long distances.

The present invention is based on a heat transfer regime which utilises the latent heats of solidification for at least some of the heat transfer load. In particular, the invention envisages trapping a freezable liquid in small capsules capable of being entrained in the recirculating heat transfer liquid flow.

Thus, a first aspect of the present invention provides a heat transfer fluid for a refrigeration system which comprises.

- a non-freezing liquid having suspended therein a multiplicity of capsules, each capsule containing a freezable liquid; - the freezing point of the non-freezing liquid being lower than that of the freezable liquid, such that in use of the heat transfer fluid, the freezable liquid is at least partially frozen whilst the non-freezing liquid remains in the liquid state.

The invention also extends to a corresponding method of transferring heat using the heat transfer fluid.

The two liquids are preferably chosen so that any leakage from the capsules will not cause a problem. For example, the freezable liquid within the capsule could also be a component of the non-freezing liquid. In particular, the freezable liquid within the capsules might be pure water (having a freezing point of 0 C) whilst the non-freezing liquid could be an aqueous solution containing a solute whose presence reduces the freezing point below 0 C.

The pairs of liquids could be weaker and more concentrated solutions of the same components, such as solutions or liquid mixtures (e.g. water and glycol); in fact, any pair of compatible liquids having suitable freezing points known in the art could be employed.

It is also preferable that the freezable liquid within the capsule and the non-freezing liquid should be miscible. Thus, should leakage from the capsules occur, large deposits of solidified material will not build up within the system.

The purpose of encapsulating the freezable liquid is to ensure that when the liquid freezes it does so in discrete managable particle sizes, rather than as a single agglomerated mass which could lead to blockage in the system.

The capsule itself will normally be hollow and formed of a plastics material of sufficient physical strength to withstand stresses due to freezing and thawing, and also to withstand the shocks and accelerations due to passage through circulation pumps.

Polyethylene, polypropylene polyamides and polytetrafluorethylene are preferred materials.

The capsules might also be solid and formed of a hydrophilic polymeric substance capable of taking up water to form a hydrogel. Such materials may be capable of absorbing several times their own weight of water.

Suitable hydrophilic substances include cross-linked carboxymethyl cellulose, treated polyamide, polyhydroxyethyl methacrylate, and methyl methacrylate vinyl pyrrolidone copolymers. The substance may be hydrated and sealed within an impervious membrane, or may be sealed within a semipermeable membrane which will allow water to permeate but not other components (such as salts) of the aqueous non-freezing liquid.

Alternatively, there may be no membrane at all, the capsules being formed of a hydrophilic polymeric substance which is dimensionally stable and which selectively absorbs water from aqueous non-freezing liquid.

It is expected that there will be a certain loss of capsules over time so that some periodical topping up may be required. The dimensions of the capsule depend on the size of the pipes and clearances through which the capsules have to pass. Moreover, small capsules have improved heat transfer properties in view of their higher surface area to volume ratio. Thus, capsule sizes in the region 0.1 to 5mm, particularly 1-3mm are preferred for most applications.

In principle, the capsules can be of any shape robust enough to be transported around the refrigeration system, and could be for example, spherical, ovoid, cylindrical, pellet-shaped, toroidal, platelet-shaped, disc-shaped etc. The shape should be chosen such that there is no danger of blockages being formed in the refrigeration system. In order to provide enhanced heat transfer properties, it is preferable that the surface area to volume ratio should be as high as practical, and for this reason disc or platelet shapes are preferred.

A particularly preferred form of capsule is in the form of a toroid with a web extending across the diameter of the ring. This form is robust enough to withstand pumping but has an extended surface in the form of the web which promotes heat transfer.

The capsules are entrained within the non-freezing liquid for circulation around the system. In order to minimise sedimentation of the Capsules, it is preferred that the density of the capsules should closely approximate the density of the non-freezing liquid.

A second aspect of the invention provides a refrigeration apparatus which comprises cooling means, and duct means cooled by the cooling means and containing the heat transfer fluid. The cooling means can be any type of refrigerator.

A third aspect of the invention provides a refrigeration system which comprises; - cooling means; - duct means cooled by the cooling means and containing the heat transfer fluid; - heat exchanger means for delivering cooling effect; and - pipe means connected for recirculating the heat transfer fluid between the duct means and the heat exchanger means.

The invention also extends to a corresponding method of operating a refrigeration system.

The heat exchanger means are provided for cooling a desired location, and may be of any suitable known construction. The heat transfer fluid may pass directly into the heat exchanger means. Alternatively, the heat transfer fluid may be arranged to flow through a central recirculating circuit from which non-freezing liquid is withdrawn to feed individual heat exchanger means before being returned to the central circuit. Strainer means are preferably provided at the upstream end of each subsidiary circuit to hold back the capsules and allow only the non-freezing liquid to flow to the heat exchanger means. In this way, a number of subsidiary circuits and their associated heat exchangers may be fed from a single central circuit. This arrangement also reduces the danger of blockage of the heat exchanger means by capsules.

A fourth aspect of the present invention provides a sealed heat transfer capsule of a size and shape capable of being transported around a refrigeration system suspended within a non-freezing liquid; the capsule containing a freezable liquid.

An embodiment of the present invention will now be described by way of example only with reference to the attached drawings wherein; Figure 1 is a schematic view of a refrigeration system; and Figure 2 is a cross-sectional view of a toroidal-type capsule.

The refrigeration system comprises a cooler 1 enclosing a duct 3 cooled thereby. The duct is connected into a central circuit 5 containing heat transfer fluid which is circulated by means of a pump 7.

The heat transfer fluid typically comprises a mixture of water and ethylene glycol having a freezing point several degrees below zero, and containing toroid-type capsules of the type shown in Figure 2 filled with pure water.

Two subsidiary circuits 7 and 9 withdraw non-freezing liquid (i.e. ethylene glycol/water) from the central circuit via respective strainers 11, 13 which hold back the capsules and allow only the non-freezing liquid to flow into the subsidiary circuits. Each subsidiary circuit includes a respective subsidiary pump 15, 17 and a heat exchanger 19,21.

Alternatively, the subsidiary circuits could be connected in parallel to the central circuit.

Figure 2 shows a toroidal-type capsule 21 which comprises an outer shell 23 formed of polypropylene.

The shell includes a toroidal portion 25 and a central web portion 27 which enhances its surface to volume ratio. Within the capsule is contained the freezable liquid (water). The shape of the capsule is also effective to accommodate stresses set up by the expansion of the water on freezing.

The refrigeration system may be operated as follows. The heat transfer fluid in the duct 3 within the refrigerator/cooler 1 is cooled to a temperature just below O"C so as to freeze the water inside the capsules, but not to freeze the surrounding liquid (ethylene glycol/water). In freezing the capsules, the latent heat of freezing of the water is withdrawn, so that the capsules act as a cold reservoir. The heat transfer fluid is then pumped around the central circuit 5 by means of the main pump 7. Ethylene glycol/water is withdrawn by means of pump 15 into subsidiary circuit 7 via strainer 11 which retains the capsules in the central circuit. The ethylene glycol/water then passes through the heat exchanger where a cooling effect is required (and where the liquid becomes warmed) before returning to the central circuit once again.On returning to the central circuit, the ethylene glycol/water is very quickly cooled down again by contact with the capsules, which begin to thaw a little. Thus, the temperature of the heat transfer fluid in the main circuit 5 is maintained almost constant, and the cooling effect in the heat exchanger 19 is provided largely by latent heat of freezing of the water in the capsules.

The process is repeated for subsidiary circuit 9.

In this way, the heat transfer fluid has a substantial capacity to transfer heat, without the temperature of the heat transfer fluid being unduly low. Low temperatures lead to substantial heat losses in long pipelines due to the fact that the rate of heat loss into the pipeline from the surroundings depends on the temperature difference between the heat transfer fluid and the surroundings.

Also, for the same flow of heat transfer fluid, very much larger quantities of heat can be transferred.

Typically, liquid flows might be reduced by a factor of five to ten using the heat transfer fluid of the present invention. This would lead to benefits in terms of reduced costs of piping, pumps and pumping.

Claims

1. A heat transfer fluid for a refrigeration system which comprises: - a non-freezing liquid having suspended therein a multiplicity of capsules, each capsule containing a freezable liquid; - the freezing point of the non-freezing liquid being lower than that of the freezable liquid, such that in use of the heat transfer fluid, the freezable liquid is at least partially frozen whilst the non-freezing liquid remains in the liquid state.

2. A fluid according to claim 1 wherein the non-freezing liquid and the freezable liquid are miscible.

3. A fluid according to any preceding claim wherein a component of the freezable liquid is also a component of the non-freezing liquid.

4. A fluid according to any preceding claim wherein the freezable liquid is water, and the non-freezing liquid is an aqueous solution or aqueous liquid mixture.

5. A fluid according to any preceding claim wherein the capsule is a solid body formed of a hydrophilic polymeric substance capable of forming a hydrogel.

6. A fluid according to claim 5 wherein the capsule has an impervious outer membrane, or has a semipermeable outer membrane permeable to water but not to other components of the non-freezing liquid.

7. A fluid according to any preceding claim wherein the capsules have a size in the region 1 - 3mm.

8. A fluid according to any preceding claim wherein the capsules are of a disc or platelet shaped.

9. A fluid according to any preceding claim wherein the capsules are each in the form of a toroid having a diametrically extending web.

10. A fluid according to any preceding claim wherein the density of the capsules and of the non-freezing liquid are substantially equal such as to minimise sedimentation.

11. A refrigeration system which comprises: - cooling means; - duct means cooled by the cooling means and containing the heat transfer fluid of any preceding claim; - heat exchanger means for delivery cooling effect; and - pipe means connected for recirculating the heat transfer fluid between the duct means and the heat exchanger means.

12. A refrigeration system according to claim 11 wherein the pipe means is arranged such that the heat transfer fluid is arranged to flow through a central recirculating circuit from which non-freezing liquid is withdrawn via strainer means and fed to the heat exchange means prior to being returned to the central circuit; the strainer means retaining the capsules in the central circuit but permitting passage of the non-freezing liquid.

13. A sealed heat transfer capsule containing a freezable liquid, as defined in any of claims 1 to 10 and of a size and shape capable of being transported around a refrigeration system suspended within a non-freezing liquid.