GB2134236A - Improvements in or relating to evaporative heat exchangers - Google Patents

Abstract

An evaporative heat exchanger comprises a tube 2 which is closed at both ends and disposed vertically in use. A spray device 5, fed by conduit 3, is disposed adjacent the upper end of the tube for spraying refrigerant onto the inner surface of the tube such that the refrigerant will pass over the inner surface of the tube down the tube. Heat is transferred from a fluid to be cooled through the tube to evaporate the refrigerant and the evaporated refrigerant exits the tube at an outlet 25 and conduit 4 therefor also provided adjacent the upper end of the tube. The level to which the refrigerant coats the tube alters according to the operating conditions. If the temperature of the fluid to be cooled falls, the level will fall; if the temperature of the fluid to be cooled rises, the level will rise. <IMAGE>

Description

SPECIFICATION Improvements in or relating to evaporative heat exchangers This invention concerns evaporative heat exchangers.

Conventional evaporative heat exchangers comprise a vessel containing refrigerant through or around which by means tubes or a casing a fluid to be cooled is passed. Heat is exchanged from the fluid to the refrigerant through the walls of the tubes or casing by virtue of the temperature difference between the fluid and the refrigerant. The transfer of heat is dependent on the thermal conductivity of the walls themselves, the cleanliness of the wall surfaces and the thickness of the insulating boundary layer present within the fluid and refrigerant and adjacent to the wall surface. The refrigerant within the heat exchanger may be charged into the vessel either in liquid form or sprayed into the vessel in the form of a wet vapour.In either form the refrigerant changes state into initially a saturated vapour after which by further contact with the walls of the vessel or the tubes becomes superheated. It is then drawn from the heat exchanger to a condensing unit where the heat is removed and the refrigerant returned to the liquid state for re-charging into the evaporative heat exchanger. As the load or amount of heat to be removed from the fluid being cooled by the heat exchanger changes, the flow rate of refrigerant must be controlled to ensure that it will change state into vapour form before passing to the condensing unit. In the case of the liquid filled heat exchanger the level of the refrigerant is controlled to a preset level by means of level sensor actuating a flow control valve in the charging line to the heat exchanger.

In the case of a spray charged heat exchanger, the change in degree of superheat in the vapour leaving the heat exchanger is sensed and used to control a flow control expansion valve in the charging line. In the latter method of control, the change in flow rate produces a corresponding change in differential pressure across the valve which in turn changes the temperature of the wet vapour leaving the valve.

Both types of evaporative heat exchanger utilising either method of control have a limited range of operation to deal with wide variations in the temperature of the fluid load (that is the fluid to be cooled). Too high a temperature in the fluid load will produce excessive evaporation in the heat exchanger which in turn will produce excessive superheat in the vapour which will create unacceptably high temperatures in the compressor of the condensing unit leading to overloading of the compressor and the degradation of the refrigerant (which typically would include an oil) in the system. Too low a temperature in the fluid load can produce incomplete evaporation of the refrigerant leading to liquid refrigerant entering the compressor and causing mechanical damage.Variations in temperature of the fluid load by virtue of the method of control of the liquid charge into the heat exchanger create cyclic loading changes in the condensing unit which reduce the overall efficiency of the system.

An object of this invention is to provide an evaporative heat exchanger which enables the above-mentioned disadvantages to be overcome or at least reduced.

The invention provides an evaporaive heat exchanger comprising a tube closed at both ends, inlet means disposed adjacent one end of the tube, which is disposed above the other end thereof in use, for directing refrigerant onto the inner surface of the tube such that the refrigerant will pass over the inner surface of the tube down the tube towards the other end thereof, and outlet means adjacent said one end portion for the refrigerant after it has been evaporated on heat transfer through the tube from a medium exterior thereof.

Between predetermined limits of fluid load a heat exchanger in accordance with the invention is able to operate without the need for a thermostatically controlled expansion valve and without cyclic ioading. Within the predetermined limits the refrigerant will provide a layer of liquid over the inner surface of the tube to a level between the ends thereof dependent upon the fluid load. A decrease in the temperature of the fluid load will cause this level to move downwards away from the inlet and outlet means and an increase in the temperature of the fluid load will cause this level to move upwards towards the inlet and outlet means.

In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic axial cross-section of an evaporative heat exchanger; Figure 2 is a section taken along the line A A in Fig. 1; and Figure 3 is an enlarged detail of the heat exchanger.

Referring to the drawings, the illustrated heat exchanger comprises a tube 2 which is closed at both ends. One of the ends of the tube is disposed above the other in use, and preferably the tube is disposed substantially vertically as illustrated. A tubular jacket 1 is removably mounted concentrically about the tube 2, a flow path for the fluid to be cooled being defined between the tube 2 and the jacket. A coarse mesh of expanded metal 8 or similar material is interposed between the tube and jacket in the flow path for causing turbulent flow of fluid over the outer surface of the tube. The lower end portion 20 of the jacket is enlarged as a conical widening and provided with a conduit 21 which forms an inlet for the fluid to be cooled. The upper end 22 portion of the jacket has a dished closed end from which extends a conduit 23, providing an outlet for fluid which has been cooled.

The tube 2 is closed at its lower end by a flange plate 6, and the lower end portion 20 of the jacket is releasably fastened to the periphery of the plate 6 which extends radially outwardly of the tube 2 by a plurality of bolt fastenings 7. Packing 24 is provided between the plate 6 and jacket 1 to provide a seal therebetween. As will be appreciated the arrangement for mounting the tube and jacket together enables easy removal of the jacket for cleaning of the flow path between the jacket and the tube.

Two concentric conduits 3 and 4 extend through the flange plate 6 at th'e lower end of the tube 2 to locations adjacent the upper end thereof. The inner conduit 3 provides a flow path for refrigerant to an inlet means formed as a spray device 5 disposed adjacent the upper end of the tube and arranged to spray refrigerant onto the inner surface down the tube under gravity towards the lower end of the tube. The outer conduit 4 is provided adjacent the upper end of the tube with outlet openings 25 for the refrigerant after it has been evaporated on heat transfer through the tube from a medium exterior thereof - i.e. in the illustrated example, from the fluid to be cooled flowing along the flow path between the tube and the jacket. Such evaporated refrigerant flows along a flow path defined between the conduits 3 and 4 downwardly through the heat exchanger.Thus the conduits 3 and 4 define concentric flow paths.

The spray device 5 is provided with a conical deflecting member 11 which is arranged to deflect sprayed refrigerant towards the inner surface of the tube.

The inner surface of the tube is covered with a layer of fine mesh material 9 to control the flow of refrigerant down the inside surface of the tube.

A tubular, perforate deflector 10 is disposed coaxially within the tube to prevent liquid refrigerant being thrown by the action of violent evaporation away from the mesh material 9. The apertures 26 in the deflector although shown in a regular pattern, are preferably disposed irregularly, and allow the passage of evaporated refrigerant inwardiy of the deflector for passage to the outlet openings 25. To allow passage of vapour to the deflector 10 from the fine mesh material 9, a course mesh material 27 (see Fig. 2), for example expanded metal, is interposed between the tube and the deflector 1 0. The mesh material 27 also assists in returning deflected liquid refrigerant to the fine mesh material 9.

Fig. 3 indicates a simple syphoning device for ensuring the return of lubricating oil into systems utilising non or partially miscible refrigerants. This device comprises a dip tube 1 2 of small bore, one end of which is situated adjacent the low end of the tube 2 and the other end of which is disposed in the space between conduits 3 and 4 above the maximum possible liquid level in the heat exchanger such that the pressure difference and venturi effect will draw oil, or any liquid, from the bottom of the tube 2 in a controlled manner into the space between conduits 3 and 4 for return to a condensing unit coupled to the heat exchanger.

In use, liquid refrigerant is supplied to the spray device 5 through inner conduit 3 and sprayed thereby onto the inner surface of the tube at the upper end thereof to pass down the tube under gravity to provide a substantially even coating of liquid refrigerant on the surface. Fluid, either liquid or gas, to be cooled is passed upwardly between the tube and jacket from the inlet conduit 21 to the outlet conduit 23. Heat is exchanged from the fluid to the refrigerant through the tube causing the refrigerant to evaporate and rise up the tube to the outlet openings 25 through which it flows out of the heat exchanger via the flow path defined between the conduits 3 and 4.

For a given inlet flow of refrigerant the level to which the refrigerant coating, or film, on the inner surface of the tube descends, depends on the fluid load. Thus for a given flow rate of fluid to be cooled, an increase in the temperature of the fluid will cause this level to rise and a decrease will cause it to fall. At a maximum temperature of the fluid load limit all of the refrigerant is evaporated on contact with the inner surface of the tube and passes out of the heat exchanger with a small degree of superheat. At a minimum temperature of the fluid load all of the refrigerant will pass over the inner surface of the tube without being evaporated and collect in the bottom of the tube, from which it cannot exit as the outlet openings 25 are disposed at the upper end of the tube. Control of refrigerant flow and temperature at and beyond these limits can be achieved if required using a conventional thermostatic expansion/flow control valve. However between the limits refrigerant flow into the heat exchanger is constant and the load on a condensing unit coupled to the heat exchanger is practically constant, variations in the fluid load being taken up by a rise or fall in the level to which the refrigerant coating extends in the tube. In this connection it has been found that the area of the inner surfce coated with refrigerant changes approximately in inverse proportion to the temperature difference between the fluid and refrigerant.

During use between the fluid load limits some vapour will fall to the bottom of the tube and become superheated by the dry portion of the inner surface of the tube below the level to which the refrigerant coating extends. This superheated vapour will rise by buoyancy and small quantities will mix with the main flow of vapour, which is upwards from the level to which the refrigerant coating extends, to create a small degree of superheat in the resulting mixture. Of course, the lower the level to which the refrigerant coating extends, the less the area of the dry portion of the inner surface for heating the vapour which falls to the bottom of the tube and the less the degree of superheat in the mixture.

The length of the tube is chosen so that for given flow rates the liquid refrigerant coating will extend substantially to the bottom of the tube when the fluid to be cooled is at the lowest temperature at which the heat exchanger is desired to operate.

The tube 2 and jacket 1 are constructed of materials compatible with the nature and pressure of the refrigerant and fluid to be cooled.

In this connection, at least the jacket has a degree of flexibility which allows for expansion of the space therebetween in the event that the fluid being cooled is caused to freeze.

Whilst the arrangement shown in Fig. 1 with the flange plate 6 at a lower end of the heat exchanger and conduits extending therethrough to the upper end of the tube is preferred, it is to be appreciated that the arrangement may be modified to work the other way up by repositioning the spray device 5 and outlet openings 25 adjacent the plate 6 and by shortening and modifying the conduits 3 and 4 accordingly.

In either configuration the jacket 1 can be dispensed with and the tube 2 simply immersed in fluid to be cooled and for example contained in a tank. External fins in any form may be fixed to the outside of tube 2 to increase the area of heat exchange.

Claims (14)

1. An evaporative heat exchanger comprising a tube closed at both ends, inlet means disposed adjacent one end of the tube, which is disposed above the other end thereof in use, for directing refrigerant onto the inner surface of the tube such that the refrigerant will pass over the inner surface of the tube down the tube towards the other end thereof, and outlet means adjacent said one end portion for the refrigerant after it has been evaporated on heat transfer through the tube from a medium exterior thereof.

2. A heat exchanger as claimed in claim 1, wherein said inlet means comprises a spray device for spraying refrigerant onto the inner surface of the tube.

3. A heat exchanger as claimed in claim 2, wherein the spray device is provided with a conical deflecting member for deflecting sprayed refrigerant towards said inner surface.

4. A heat exchanger as claimed in any one of the preceding claims, wherein a tubular, perforate deflector is disposed coaxially within I said tube.

5. A heat exchanger as claimed in claim 4, wherein a mesh material is interposed between said tube and said deflector.

6. A heat exchanger as claimed in any one of the preceding claims, wherein said inner surface is covered with a layer of mesh ma terial.

7. A heat exchanger as claimed in any one of the preceding claims, wherein the tube is provided with external fins.

8. A heat exchanger as claimed in any one of claims 1 to 6, wherein said is mounted within a tubular jacket, a flow path for fluid to be cooled by said heat exchanger being de fined between said tube and said jacket and having an inlet adjacent said other end of the tube and an outlet adjacent said one end thereof.

9. A heat exchanger as claimed in claim 8, wherein a mesh material is interposed between said tube and said jacket for causing turbulent flow of said fluid over the outer surface of said tube.

10. A heat exchanger as claimed in claim 8 8 or 9, wherein two cocentric conduits extend through said other end of the tube to said inlet and outlet means for providing concen tric flow paths thereto and therefrom respec tively.

11. A heat exchanger as claimed in claim 10, wherein siphoning means are provided adjacent said other end of the tube between the interior of the tube and the flow path from said outlet means.

1 2. A heat exchanger as claimed in any one of claims 8 to 11, wherein said jacket is removably mounted about said tube.

1 3. A heat exchanger as claimed in claim 12, wherein said jacket is releasably fastened to to the periphery of a flange plate closing said other end of said tube and extending radially outwardly of said tube.

14. An evaporative heat exchanger sub stantially as hereinbefore described with refer ence pence to the accompanying drawings.