GB2194324A

GB2194324A - Heat pipes

Info

Publication number: GB2194324A
Application number: GB08619353A
Authority: GB
Inventors: Peter Ian Patrickson
Original assignee: ISOTERIX Ltd
Current assignee: ISOTERIX Ltd
Priority date: 1986-08-08
Filing date: 1986-08-08
Publication date: 1988-03-02
Also published as: GB8619353D0

Abstract

A heat pipe of wicked or thermosyphon type is provided with a heat exchanger arrangement for its condensor in the form of a "T" fitting 3 through whose interior volume extends at least one conduit 5 for conveying a working fluid which is to receive heat from the heat pipe so that the outer surface of the conduit(s) is surrounded by the condensing vapour phase of the heat pipe working medium. <IMAGE>

Description

SPECIFICATION Heat exchanger for heat pipe The present invention relates to a heat exchanger for a heat pipe, more particularly for extracting heat from the condensor end of such a heat pipe.

Heat pipes, both of the wicked and thermosyphon are well known and often comprise an elongate tube of heat conductive material such as copper containing a quantity of a working fluid or heat exchange liquid. This fluid is vaporised by heat input to the evaporator end and is driven along to the condensor end where it condenses, giving up its latent heat of vaporisation; it is then returned to the evaporator end by either capillary forces or gravitational forces. Such heat pipes are used in heat transfer applications where high power and/or fast response is required.

At the condensor end, it is frequently desired to transfer the heat rejected from the condensor end to a cooling medium such as a fluid flowing through a conduit. Conventionally, this has been done, for example, by placing the condensor end in heat exchange relation with a manifold block through which the medium to be heated passes. The use of a manifold block, however, slows down the speed of response due to its thermal inertia and detracts from the overall efficiency of the heat transfer. Alternatively the jacket through which the fluid flows may be fitted around the condensor section. This invariably results in excessive back pressure of the fluid flow.

The present invention relates to the idea of a heat exchanger arrangement for the condensor of a heat pipe, whether of the wicked or thermosyphon type, in which one or more conduits for conveying the fluid medium to which the heat pipe is to transfer heat extends through the interior volume of the condensor end of the heat pipe so that in use it can be surrounded by the vapour phase of the heat pipe working medium.

As will become apparent from the following description of an illustrated embodiment of the invention, the invention can be put into practise by providing the heat pipe condensor end with a fitting of approximately "T'' shape, the leg of the "T" being constituted by the main body of the heat pipe and the crossbar of the "T" forming an annular chamber to which the evaporated heat pipe working liquid is driven in use and through which extends a conduit, such as a cylindrical pipe, through which the medium to which the heat exchange is to occur flows.

One particular application of the present invention is to solar collectors. In such an application, a plurality of heat pipes may be provided as part of a solar collector arrangement with the condensor ends of the heat pipes being in heat exchange relationship with a working medium, such as water, which is to be heated.

The invention will be further described by way of non limitative example with reference to the accompanying drawings in which: Figure 1 is a somewhat schematic crosssectional view of one embodiment of the present invention; Figure 2 is a cross-section on ll-ll in Figure 1; Figure 3 is a cross-section on Ill-Ill in Figure 4; Figure 4 is an end elevation of part of the end fitting of Figure 1.

Figure 1 shows the condensor end of a thermo-siphon type heat pipe 1 which is provided with a fitting 3 by means of which the condensing heat pipe working fluid heats cooling water passing through a cooling water pipe 5 coaxial with the fitting 3. Although not shown in the Figures, the heat pipe 1 may be one of a number incorporated in a solar collector arrangement with the water pipes 5 being in any desired parallel, serial or parallel/serial flow arrangement. Each such heat pipe 1 has the major, lower part of its length including the evaporator section mounted on a laterally extending fin, the outer surface of the heat pipe and the fin being darkened to maximise the absorbtion of radiant energy and being located within an evacuated transparent sleeve of glass or other suitable material so as to minimise heat loss from the collector part of the arrangement.

As will be seen from Figure 1, the condensor section comprising the fitting 3 of the heat pipe 1 is generally of a "T" configuration with the limb of the T being constituted by the inclined main body la of the heat pipe 1 and the crossbar being constituted by the hollow fitting 3 which defines an annular space 6 surrounding the pipe 5 intp which the vapour phase of the heat pipe working medium is driven by boiling occurring in the lower, evaporator end of the heat pipe.

The fitting 3 is composed of two pressings 5a and 5b which are substantially identical to one another. Each comprises a frustro-conical main part 7, 7a, a cylindrical extension 9a, 9b into which the pipe 5 passes and, at its larger diameter end, a radially outwardly extending flange 11. Seals between the flanges 11 and between the cylindrical extensions 9 an the pipes 5 may be achieved by welding, soldering or any other suitable means. Because of the shape of the fitting 3, the chamber 5 defined around the pipe 5 widens and deepens towards the central region of the pipes.

Part of the periphery of the wider end of each of the parts 7a and 7b is shaped so as to conform with a cylinder whose axis extends chordally with respect to the axis of the fitting 3 (see Figure 2). These two semi-cylindrical parts together form a cylindrical aperture 12 into which the heat pipe 1 extends and to which it is sealed. It will be appreciated that the use of two substantially identical parts 7a and 7b to form the fitting 3 and the manner in which they are assembled together and with the heat pipe 1 and the cooling water pipe 5 makes for a very simple and inexpen sive construction.

Within the condensing chamber 6, substan tially the entire peripheral surface of the cool ing water pipe is surrounded by the volume of the chamber 6 to maximise the area available for condensation of the heat pipe working me dium. The peripheral surface of the pipe 5 within the chamber may be provided with a series of circumferential scores or other sur face treatment so as to increase the conden sation coefficient of the surface of the pipe 5 and to assist in returning the condensed working medium back to the main limb la of the heat pipe. The inner surface of the upper end of the heat pipe may similarly be scored or surface treated to promote the return of the condensed working medium towards the condensor end of the heat pipe.

As will be seen from Figure 2, the main limb 1 of the heat pipe joins the fitting 3 at its lowest point. The flaring side walls of the fittings 5a and 5b below the water pipe 5 assist in returning the condensed heat pipe working medium to the main limb 1, this join ing the chamber 5 at its lowest point.

Typically, the main limb 1 of the heat pipe will be between 1 and 3 metres long and, for European climates, be inclined at an angle of about 45" to the horizontal. In conventional heat pipe design, approximately one third of the length of the heat pipe (at room or filling temperature) is filled with the liquid phase of the working medium. For applications such as solar collection, it has been found to be be neficial if only a minimal quantity, say corre sponding to about 5. cm of the length of the heat pipe of liquid working mediums used.

This assists in reducing the static pressure head at the bottom of the heat pipe, so mak ing it easier for the liquid to vaporise, and aiso improves the response time of the heat pipe.

Although in the above the present invention has been described with reference to its appli cation to solar collectors; it is not, of course, limited to that use and it may be used in a wide variety of other applications of heat pipes.

Claims

-CLAIMS

1. In combination, a heat pipe of the wicked or thermsyphon type having a condensor end with an interior volume, the heat pipe contain ing a working medium, and a heat exchanger arrangement for the condensor end of the heat pipe, in which at least one conduit for conveying a fluid medium to which the heat pipe is to transfer heat extends through the interior volume of the condensor so that in use the conduit can be surrounded by the vapour phase of the heat pipe working medium.

2. A combination according to Claim 1 wherein the condensor end of the heat pipe comprises a fitting of approximately "T"shape, the leg of the "T" being constituted by a main body of the heat pipe and the crossbar of the "T" forming an annular chamber into which the evaporated heat pipe working medium is driven in use and through which extends the at least on conduit.

3. A combination according to Claim 2 wherein the fitting flares in cross-section in the direction from the evaporator end of the heat pipe towards the location of the at least one conduit.

4. A combination according to Claim 3 wherein the fitting comprises two frustro-conical hollow portions disposed in base-to-base relation with one another, the main body of the heat pipe being connected to the fitting at the junction of the bases of the frustro-conical portions.

5. A combination according to Claim 3 or 4 wherein the axis of the main body of the heat pipe extends chordally with respect to the axis of the fitting.

6. A combination according to Claim 4 or 5 wherein the frustro-conical portions are provided at their outer ends with cylindrical extensions sealed to the outer surface of the conduit.

7. A combination according to any one of the preceding claims wherein the outer surface of the conduit is provided with circumferential scoring or other surface treatment to increase the condensation coefficient of the surface on the conduit.

8. A combination according to any one of the preceding claims wherein the heat pipe is filled with an amount of liquid working medium corresponding to about 5cm of the length of the heat pipe.

9. The combination of a heat pipe and a heat exchanger arrangement for the condensor end thereof, the heat pipe and heat exchanger arrangement being constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

10. A solar collector incorporating at least one combination according to any one of the preceding claims.

11. A solar collector according to claim 10 wherein there are a number of such combinations whose conduits are in a serial, parallel or serial/parallel flow arrangement.