GB2312955A - Monitoring temperatures of surfaces of heat exchangers - Google Patents

Abstract

A method of monitoring the temperature of a visible surface of a heat exchanger e.g. the freezer plate in a refrigerating apparatus, comprises the applying to the surface of a thermochromic composition which changes colour over a desired temperature range, and monitoring the colour of the thermochromic composition, visually or spectroscopically. The composition may be applied directly or as an adhesive patch, by printing or coating. A liquid crystal composition may be used, producing a continuous change of colour over the chosen temperature range, or a change of colour at a critical temperature, reversibly or irreversibly.

Description

TITLE: METHOD AND DEVICE The present application relates to a method for monitoring temperature in a heat exchanger, and particularly a heat exchanger plate for a refrigeration apparatus. The invention also provides a heat exchanger for use in the method, and refrigeration apparatus in which is mounted such a heat exchanger.

BACKGROUND TO THE INVENTION A heat exchanger provides an interface between fluids so that if the fluids are at different temperatures one fluid warms the other by thermal conductivity through the heat exchanger.

In a refrigerator or freezer apparatus, cold fluid is pumped through a heat exchanger plate and the air in the apparatus is cooled to a preferred temperature. The efficiency of the heat exchanger plate at conducting heat affects the performance of the apparatus. The temperature of the fluid which surrounds the heat exchanger in a refrigerator, freezer, or other apparatus may be monitored by conventional means. However if this temperature is found not to be within preferred or expected temperature limits it is not known whether the difference is caused by defective performance of the heat exchanger, or by faulty seals or other reasons.

It is therefore desirable to be able to monitor the surface temperature of a heat exchanger within a desired temperature range in order to determine the efficiency of the heat exchanger.

Summary of the Invention: According to the present invention there is provided a method of monitoring the temperature of a visible surface of a heat exchanger, which method comprises the steps of: a) applying to the surface a thermochromic composition which changes colour over a desired temperature range; and b) monitoring the colour of the thermochromic composition.

A change in colour of the thermochromic composition indicates that it has passed through a given temperature level.

The method allows a temperature change of one or more surfaces of the heat exchanger to be readily monitored.

If the temperatures of the fluids inside and outside the heat exchanger are both know, measurement of the temperature of an external surface of the heat exchanger will provide an indication of the efficiency of the heat exchanger. If the surface temperature is close to that of the internal fluid temperature, this is an indication that the heat exchanger is working efficiently. However if the surface temperature is close to that of the fluid outside the heat exchanger, and the temperature of the internal fluid is different from that of the external fluid, this indicates that the heat exchanger is operating less efficiently, ie it is acting as an insulator rather than a conductor.

The thermochromic composition may be applied to a surface of the heat exchanger prior to its being used with a piece of apparatus, and the invention therefore provides a heat exchanger having a surface to which has been applied a thermochromic composition which changes colour over a desired temperature range, for use in the above method.

Preferably the heat exchanger is a cooling or freezing element in a refrigeration system, notably a freezer plate, and the thermochromic composition is selected to change colour over a temperature range around the freezing point. For example a thermochromic composition may be selected which changes from red to blue in the temperature range -20 C to +50C.

In a preferred embodiment the heat exchanger is mounted in a refrigeration apparatus, and the invention therefore also provides a refrigeration apparatus in which is mounted a heat exchanger having a surface to which a thermochromic composition has been applied, which composition changes colour over a desired temperature range.

If it is required to know only whether the surface of the heat exchanger is above or below a certain critical temperature or temperature range, the thermochromic composition may be chosen so that it changes colour at that temperature or over that temperature range.

However if it is preferred to know the surface temperature within a preselected temperature range, a thermochromic composition may be used which changes through a range of colours within the temperature range. The surface temperature may then be monitored by observing the colour of the composition and relating this to the temperature which corresponds to that colour.

The colour of the thermochromic composition may be monitored by direct visual observation. Alternatively or additionally the colour may be monitored by spectroscopic means which may optionally include a computer program to monitor and control the operation of the heat exchanger.

Where the change in colour is monitored spectroscopically, the colour spectrum may include infrared and ultraviolet radiation provided that a spectrophotometer or other measuring device is being used which is sensitive to infrared or ultraviolet wavelengths.

Thermochromic materials for use in the composition may change colour over a relatively narrow temperature range, or over a broad temperature range. The thermochromic composition may therefore be selected so as to indicate temperature change over virtually any preferred range.

The term "composition" may include a single chemical species of thermochromic material, or a plurality of chemical species which may comprise more than one state of matter; for example solid and liquid, or solid and liquid crystal.

The thermochromic material may be inorganic or organic, and the colour change may be effected by absorption of light, for example by Basic Green 4, Solvent Yellow 56, or thermochromic polyacetylenes such as those disclosed in European Patent Application 0 036 899, or by selective reflection of a particular wavelength range of light, for example by a conventional chiral nematic liquid crystal or a liquid crystalline chiral nematic polymer. Preferably the thermochromic composition changes colour reversibly with temperature, however it would also be possible to use a thermochromic composition which changes irreversibly above a given temperature range.

A thermochromic composition which changes colour irreversibly above a given temperature may be of particular use where the heat exchanger is a freezer plate, to provide a warning that the temperature of the freezer has exceeded a predetermined minimum value. For example if food or other perishable materials stored in the freezer will deteriorate unacceptably if exposed to a temperature in excess of 50C, an irreversible thermochromic material may be selected which permanently changes colour when exposed to a temperature above, say, 30C.

Where the thermochromic material is a chiral nematic liquid crystal (also known as a cholesteric liquid crystal), the liquid crystal material may be chosen or prepared so as to give selective reflection of light over virtually any temperature range which is of interest, with continuous change of colour over that temperature range. This allows the visible observation of variations in temperature, and hence performance of the heat exchanger, over a large area of the heat exchanger with the use of only a single thermochromic composition.

The thermochromic composition may be formulated in a carrier for application to the surface of the heat exchanger. The invention therefore also provides a carrier composition for application to a visible area of a heat exchanger, which carrier composition contains a thermochromic composition which changes colour over a desired temperature range.

The carrier composition may be fluid, solid, or semisolid, for example an ink, paint, lacquer, paste, film or patch. The carrier composition may comprise the thermochromic composition in solution in a solvent, or it may comprise a suspension of particles or droplets of the thermochromic composition in a liquid carrier medium.

Other materials may also optionally be included in the carrier composition, for example surfactants, viscosity modifying agents, or curing agents. Where the thermochromic composition is a chiral nematic liquid crystal it is preferred that the liquid crystal be microencapsulated, coacervated, emulsified, or otherwise contained in a suitable carrier medium so that it forms a film which is dry to the touch. For example the liquid crystal may be emulsified with an aqueous solution of poly(vinyl alcohol) to give an emulsion which dries to a coherent film in which the liquid crystal is dispersed.

Where the carrier composition is a fluid, it may be applied to the surface of the heat exchanger by any suitable coating or printing technique. For example the carrier composition may be applied by screen printing, curtain coating, spray coating, flexo printing, ink jet printing, or by means of a Meier bar or other metering device.

Alternatively, the carrier composition may comprise a thermoplastic or adhesively coated film or patch which is applied to the heat exchanger by means of heat and/or pressure. The film or patch may carry the thermochromic composition on its surface, or the thermochromic composition may be contained beneath the surface. The film or patch may be formed, for example by mixing an inorganic thermochromic material or a microencapsulated thermochromic liquid crystal with a monomeric resin, for example methyl methacrylate, casting a liquid film and polymerising the film by means of heat and/or catalysis.

Alternatively the thermochromic material may be mixed with a suitable film-forming solution or suspension, for example a natural or synthetic latex or a solution of polystyrene in toluene. The mixture may then be cast as a film and allowed to dry by water or solvent evaporation.

The use of an adhesively coated film or patch allows the thermochromic composition to be applied to a surface of the heat exchanger without the release of significant amounts of solvent or other liquid carrier medium.

The composition may be applied directly to the surface of the heat exchanger, or the composition may be applied on top of one or more other coatings. For example a composition which includes a chiral nematic liquid crystal may be coated on top of a black coating on the substrate so that the black coating absorbs the wavelengths of light which are not selectively reflected by the thermochromic material, and hence enhancing the contrast between the reflected colour and the background. One or more transparent or translucent coatings may optionally be applied on top of the thermochromic composition. For example a tough lacquer may be applied to provide a scratch-resistant finish.

The composition may be applied in the form of numbers, letters, a pattern, or other indicia. In a preferred embodiment the composition is applied in the shape of one or more letters or numbers onto a surface which is closely matched in colour to the colour of the composition at a preset temperature. When the colour of the thermochromic composition changes at a different temperature the different colour of the letters or numbers contrasts with the background colour. For example a thermochromic composition which changes from blue to yellow at 80C C may be applied in the form of the numerals "80" to a blue coating on a heat exchanger plate. When the temperature of the surface reaches 80 C this is indicated by the number 80 appearing as yellow numerals on a blue background.

Although the invention has been described for convenience in terms of the use of a chiral nematic liquid crystal, it is not limited to this embodiment. Chiral smectic liquid crystals, for example chiral smectic C materials, may also be used, provided that they are thermochromic within the desired temperature range.

The invention provides a convenient method of measuring the temperature of a visible surface of a heat exchanger.

Claims (12)

1. A method of monitoring the temperature of a visible surface of a heat exchanger, which method comprises the steps of: a) applying to the surface a thermochromic composition which changes colour over a desired temperature range; and b) monitoring the colour of the thermochromic composition.

2. A heat exchanger which has a surface to which has been applied a thermochromic composition which changes colour over a predetermined temperature range, for use in the method of Claim 1.

3. A heat exchanger as claimed in Claim 2, wherein the thermochromic composition is applied in a pattern which represents a number or letter.

4. A heat exchanger as claimed in Claim 2 or Claim 3, wherein the thermochromic composition is applied by means of an ink jet printer.

5. A heat exchanger as claimed in Claim 2 or Claim 3, wherein the thermochromic composition is coated on a label which is adhered to a surface of the heat exchanger.

6. A heat exchanger as claimed in any one of Claims 2 to 5, which is a freezer plate.

7. A heat exchanger as claimed in any one of Claims 2 to 6, wherein the coated thermochromic composition is covered by a protective layer of a translucent or transparent coating.

8. A heat exchanger as claimed in any one of Claims 2 to 7, wherein the thermochromic composition contains a liquid crystal material.

9. A heat exchanger as claimed in any one of Claims 2 to 8, wherein the thermochromic composition is selected to change colour permanently when exposed to a temperature which exceeds a predetermined minimum value.

10. Refrigeration apparatus in which is mounted a heat exchanger as claimed in any one of Claims 2 to 9.

11. A method of monitoring a visible surface of a heat exchanger substantially as herein described.

12. A heat exchanger substantially as herein described.