The present invention relates to a heating system in a cooling device such as a flooded cooler and in particular to a heating system for preventing water in the cooler from freezing.

Prior art air-conditioning systems, such as air-cooled chillers for example, contain a cooler, such as a flooded cooler. Often these chillers are installed outside the building to which they supply air-conditioning and during cold periods the ambient temperature may be around or below the freezing point of the medium flooding the cooler. In such situations, if the cooled medium is water and if the ambient temperature is about or below 0° C, then it is necessary to prevent the water from freezing. This is particularly important when the cooler is not operating, which may occur for example when the chiller is not required to provide cooling or when a fault occurs in the chiller.

When a chiller stops due to an unexpected event or emergency, such as a power shortage or a safety cut-out, and if the ambient temperature is low, then just after the compressor of the system stops, the condenser cools down very quickly to reach the ambient temperature level. Consequently the pressure inside the condenser also drops very quickly to reach a saturated pressure corresponding to the ambient air temperature. The refrigerant, which just prior to the chiller stopping was located in the cooler, is subject to a reduction in pressure and therefore boils at a temperature level corresponding with the ambient air temperature whilst migrating to the coldest chiller system point, i.e. the condenser. Before all the refrigerant in the cooler evaporates, a significant amount of cooling is provided to the water in the cooler and,

particularly if that water is not circulating (for example if the power shortage or safety cut-out stops the entire chiller and/or the water pumps etc.), it will rapidly freeze, forming ice. Even an operating cooler can be susceptible to freezing if the ambient temperature is low and particularly if it remains low for a prolonged period. There are typically two types of cooler used in chillers, direct expansion evaporators in which refrigerant evaporates inside a plurality of tubes and water circulates around the outside of the tubes and flooded coolers in which water circulates inside the tubes and refrigerant surrounds the tubes. The above described situation in which water freezes inside a chiller is particularly detrimental to the flooded type of cooler since, as water freezes to form ice, the volume occupied by the fluid increases by approximately 10% (compared with the volume occupied by the water alone). This volume increase significantly increases the pressure within the tubes causing high stresses in the tube body and thus splitting or bursting the tubes and allowing refrigerant and water to mix. Prior art solutions to overcome the above problem have been suggested. For example, one common solution is to provide dedicated electrical heaters, which deliver energy to portions of the cooler when the ambient temperature is sufficiently low, particularly if the chiller or the cooler is not operating. The heaters increase the temperature of the portions of the cooler that are heated and thus increase the temperature of the water in the cooler (or whatever coolant is employed) to a temperature above its freezing point. The heaters can comprise resistive wires applied to the cooler housing or shell, and/or immersion heaters directly immersed in the water supply at a suitable location, such as at the water inlet and/or the water outlet manifolds. However, the above heating solutions are expensive and complicated to install,

operate and maintain. In particular, in the resistive wire type heating systems, it is typically the shell that is heated by the wires and the heat is delivered to the pipes in the cooler, which carry the water, only by conduction and convection of heat from the shell. Therefore it is necessary to provide sufficient heat to the shell to ensure enough heat reaches all regions of the tubes, otherwise frozen sections could occur, say, in the centre of the pipes which are furthest from the shell. Furthermore, a good thermal connection must be made between the resistive wires and the shell in order to conduct sufficient heat to the shell and therefore installation of these systems is complicated, time-consuming and subject to failure if the connections weaken with time.

Whilst immersion heating systems apply heat direcdy to the water, these systems are typically installed in the water inlet or outlet, since they cannot reside in the pipes without interfering with the flow of the water and thus reducing the efficiency of the chiller. Furthermore, immersion systems are complicated to install, since they must be immersed in the water requiring inlets to be cut into the shell, which must be water-tight once closed. Immersion heater systems are also expensive to operate due to their high power requirements, particularly over prolonged periods of time.

Therefore it is desirable to provide a heating system for a cooler, particularly although not exclusively a flooded type cooler, for conditions when the medium in the cooler is susceptible to freezing and that is cost effective, easy to install and does not unduly affect the cooler efficiency or performance.

It is an object of the present invention to provide a system and method for protecting a cooler against freeze damage. In accordance with a first aspect of the present invention, there is provided a

cooler for an air-conditioning system, the cooler comprising a housing or shell, at least one tube within the housing or shell which is capable of carrying a first medium, and heating means comprising means for providing an electrical current to at least one of the housing or shell and the tube and returning the electrical current from said housing or shell or said tube such that flow of the electrical current through said housing or shell or said tube generates heat in said housing or shell or said tube.

Therefore there is provided a cooler in which electricity, preferably very low voltage, high current electricity is applied across at least a part of the cooler, which generates heat in the part or parts of the cooler in which the current flows thereby maintaining or raising the temperature of the medium in the tubes. Heat is preferably generated in the part or parts of the cooler through which the current flows by means of impedance heating, which is known in the art and generally speaking is a combination of heat generated by the resistance of the parts of the cooler through which the current flows and also the various effects of the magnetic fields generated in the region of the cooler. The impedance heating directly heats the parts of the cooler through which the current flows and conduction and convection of the generated heat also provides dissipated heating to other regions of the cooler which are thermally connected to the heated parts of the cooler, thereby providing heating over most of or substantially all of the cooler and, thus providing an effective and uniform heating system.

In a preferred embodiment of the present invention, the cooler comprises a plurality of tubes. The medium in the tubes is dierefore distributed across the cooler providing a large surface area for contacting the medium with the tubes and a large surface area that can be subjected to the impedance heating mechanism. Preferably the medium in the cooler comprises water. Water is well known in

the art for its suitability in conventional coolers and is particularly suitable for the preferred cooling systems of the present invention, such as air-cooled chillers.

As mentioned above, whilst heat can be generated by impedance heating in parts of the cooler, it is desirable for that heat to be at least partially transferable to any or all regions of the cooler in which the current does not flow. Therefore it is preferable for the shell and the tube or tubes to be thermally coupled such that heat generated in one is transferable to the other, typically by conduction and/or convection. Therefore in a preferred embodiment of the present invention the tubes comprise an electrically and thermally conductive material and the shell also preferably comprises an electrically and thermally conductive material. Conduction of the heat from one region of the cooler, such as the shell, readily occurs to other regions of the cooler such as each of the tubes. In a particularly preferred embodiment, the material of the shell is less electrically and less thermally conductive than the material of the tubes. This is particularly preferred because this arrangement allows more heat to be generated and/or transferred to the more conductive material

(i.e. the tube material) than to the less conductive material (i.e. the shell) and it is desirable to heat the tubes rather than the shell because the medium is more susceptible to freezing in the tubes rather than in the shell. In a particularly preferred embodiment, the tubes comprises copper or a copper based alloy. Further, it is preferable for the shell to comprise steel or a steel based alloy.

The electrical current provided to the cooler may comprise direct current (DC). However, supplying DC electricity to the cooler may cause corrosion of the components over a prolonged period of use. Preferably therefore, the electrical current provided to the cooler comprises an alternating current (AC). Furthermore, it is preferred that the electrical current has a relatively high current and a relatively

low voltage (compared, for example with the conventional mains supply in the US and/or Europe etc.) to provide effective impedance healing. Preferably the electrical current is from 50 to 1000 Amperes. Preferably the electrical voltage corresponding with the electrical current is from about 0.1 to about 24 Volts. Therefore it is preferable for the heating system for the cooler to further comprise a transformer, preferably a step down transformer that is suitable for connecting to a building mains supply or to a dedicated electrical supply for the system, said electrical current provided by a secondary side of the transformer.

As discussed above, it is desirable to regulate the temperature of the medium within the cooler, and particularly within the tub es to prevent the medium freezing in low ambient temperatures. The heating system is therefore preferably controllable such that the medium temperature is monitored and/or the ambient temperature is monitored and one or both of these temperatures is used to control activation and deactivation of the impedance heating system of the present invention. The above-mentioned and other features of the various embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a cross-section of a prior art flooded cooler for a chiller having a plurality of tubes for carrying water and a refrigerant in the cooler surrounding the tubes;

Figure 2 shows a cross-section of a prior art flooded cooler for a chiller similar to the cooler of Figure 1 but having conventional water heating systems; and

Figure 3 shows a cross-section of flooded cooler for a chiller in accordance with an embodiment of the present invention having an impedance heating system. With reference to Figure 1, a prior art cooler 10 for a chiller, such as an air-

cooled chiller, is shown. The cooler 10 comprises a shell 12 which holds a liquid 30, which is typically a refrigerant as is known in the art. A refrigerant inlet 32 and a refrigerant outlet 34 are provided in the shell 12. The cooler also contains a plurality of tubes 20 which are immersed in the refrigerant 30 and are arranged in this embodiment such that the refrigerant completely surrounds each of the tubes 20.

The tubes 20 are connected to an associated inlet 22 for a supply for providing a medium to be cooled 40 through the tubes 20 via a manifold 14 at a first side of the shell 12. The medium 40 passes through the tubes 20 to a manifold 16 at the other side of the shell 12 where it leaves the cooler via an outlet 24. The medium in an air- cooled chiller is typically water. Heat transfer between the water 40 flowing dirough the tubes 20 and the refrigerant 30 surrounding the tubes 20 occurs during normal operation of the cooler 10 to cool or heat the water 40. However, when the ambient temperature of the environment in which the chiller is situated falls below the freezing point of water, and particularly if the water 40 is not circulating through the tubes 20 due, for example, to a power failure, then the water 40 begins to freeze thus forming ice which begins to build in the tubes 20 and which can cause damage to the tubes due to the increased volume frozen water compared to unfrozen water.

The prior art cooler 10 of Figure 2 shows a conventional arrangement for compensating for low ambient temperatures of a water filled chiller. Like reference numerals are used for like components in this arrangement. The prior art cooler 10 of

Figure 2 is identical to the cooler 10 of Figure 1 except that several heaters 50, 52 are provided in and around the cooler 10. In particular, immersion heaters 50 are inserted into manifolds 14, 16 in order to warm the water 40 entering from the inlet 22 and passing to the outlet 24 respectively. These heaters 50 each require an electrical power source 54 which typically would consume high levels of power when

operational. Installing the immersion heaters 50 in the water 40 inlet 22 and outlet 24 regions of the shell requires holes to be cut in the manifolds 14, 16 and water tight seals to preventing leaking. Furthermore, these heaters 50 are not effective at preventing water 40 in other parts of the cooler 10 from freezing, particularly in regions such as the central portion 26 of the tubes 20. Therefore further heating is required and this is provided by resistive wire heating systems 52 in which wires are applied to the cooler shell 12, for example as wires or sheets wrapped around the shell. Electrical power sources 54 provide the high levels of power required to warm the shell via the wires, which heat not only the shell 12, but also the refrigerant 30. It is still difficult to provide sufficient heat to tubes 20a in die centre of the cooler 10 since they are furthest spaced from the heat sources 50, 52. Since die tubes 20 themselves are not heated, there may be regions of the cooler 10 that are still susceptible to freezing, thereby having an increased risk of damage or requiring increased power consumption to reduce the damage risk. Figure 3 shows a preferred embodiment of the present invention. A conventional cooler 10 is shown, like die coolers of Figures 1 and 2. However, in this embodiment of die present invention, heating is provided to the cooler 10 by an impedance heating system 60, 62, 63, 64, 65. A conventional power source 54, such as alternating current mains voltage or a dedicated electricity supply is connected to a step-down transformer 60 on the primary side 66, which reduces the voltage and increases die current of me alternating current supply. The transformer secondary 68 is connected via cable 62 to terminal 63 which makes electrical contact with the manifold 14 on a first side of the shell 12. The transformer secondary 68 is also connected via cable 64 to terminal 65 which makes electrical contact witih the manifold 16 on the second side 16 of the shell 12. This completes an electrical loop

or circuit and therefore alternating is passed through the circuit, generating heat in all of the metallic components of the cooler 10 that are electrically connected to the terminals 63, 65 via the manifolds 14,16 and shell 12 which is electrically connected to the manifolds. In this embodiment, the shell 12 comprises steel and each tube 20 comprises copper and therefore current can flow through each of these connected components, thus generating heat in each of them. This is a particularly advantageous arrangement because the tubes 20 memselves are heated thus leaving no regions, such as the centre of the tubes as shown at 26 in Figure 2, that do not receive heat.

Although described above in the context of an air-cooled chiller system, the principles of the present invention can be incorporated into any system in which water or another medium passes through tubing or the like and which may be caused to freeze. Furthermore, although not discussed in detail above, it is contemplated that the heating system need not be connected to the shell, but could be connected to the tubes or to other metallic parts of the cooler. Therefore it will be appreciated that details of the foregoing embodiments, given for purposes of illustration only, are not to be construed as limiting the scope of this invention and those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the scope of the invention as defined in the following claims.