AU659502B2

AU659502B2 - Improvements in and relating to heat pipes

Info

Publication number: AU659502B2
Application number: AU27438/92A
Authority: AU
Inventors: Aliakbar Akbarzadeh; Peter William Johnson; Tomek Wadowski
Original assignee: Technisearch Ltd
Current assignee: Technisearch Ltd
Priority date: 1991-10-29
Filing date: 1992-10-29
Publication date: 1995-05-18
Anticipated expiration: 2012-10-29
Also published as: AU2743892A

Description

ill 659502 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE USE Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art: o o 0 0 0 o o o o 0 00 o 0, *o 0 t t t L I I Name of Applicant: Address of Applicant: Actual Inventor(s): Address for Service: TECHNISEARCH LIMITED 449 Swanston Street, Melbourne, 3000 Victoria, Australia Aliakbar AKBARZADEH Tomek WADOWSKI Peter William JOHNSON DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, Victoria, 3000 I t S ti Complete specification for the invention entitled: IMPROVEMENTS IN AND RELATING TO HEAT PIPES The following statement is a full description of this invention including the best method of performing it known to me: i! 1 ii :a i i: 1 ;1 B at ii j iiu j q21029,q:\oper\niab,heat,1 p- -2- IMPROVEMENTS IN AND RELATING TO HEAT PIPES The present invention relates to heat pipes.

A heat pipe usually comprises an elongated sealed cylinder containing a vaporisable material, such as water, refrigerant or alcohol, having liquid and vapour phases at the temperatures of operation of the heat pipe. Heat applied to one end portion of the cylinder, known as the evaporator section, vaporises the liquid. The vapour then flows in a high speed stream to the other end portion of the cylinder, known as the condenser section, which is maintained at a lower temperature than the evaporator section causing the vapour to condense whereby heat is given off. The heat pipe provides very efficient heat o transfer between a higher temperature zone surrounding the evaporator section and a lower temperature zone surrounding the condenser section. The temperature difference between the two sections need not be substantial and may be as low as a few degrees Celsius. The vapour condenses in the condenser section and is returned to the evaporator section by means of the capillary action of a wick structure affixed to the inner surface of the cylinder.

Z The evaporation rate, and consequently the heat transfer rate, is determined, t, generally, by the structure of the heat pipe and the material of which it is formed, the temperature difference and the amount of liquid available for I evaporation in the evaporator section. The rate of return of the condensate is thus a limiting factor in the operation of the heat pipe and, in some instances, a fluid pump is required to enhance this return rate. A thermosyphon is a simple form of heat pipe where the return of the condensate from the condenser section to the evaporator section is achieved by gravity. Therefore i thermosyphons are also called gravity assisted heat pin. Heat pipes generally have very high thermal conductance, or rate of heat transfer, compared with solid metal conductors of similar overall dimensions. This characteristic has resulted in heat pipes being sometimes 921029,p:\oper\mab,heat,2 I c I -3referred to as thermal superconductors. When a heat pipe is started from a state of no temperature difference between the evaporating and condensing ends, initially the effective thermal conductance of the heat pipe is low and a threshold temperature difference is required to cause the heat pipe to reach its full conductivity. This threshold difference will depend on the particular working liquid. However, after the full conductance has been reached, reduction of the temperature difference to below the starting threshold level can result in the full conductance being retained to a significantly lower threshold level than was needed to obtain high conductance initially. This phenomenon causes the heat pipe to exhibit an hysteresis effect. The relatively poor conductance at temperature differences below the starting threshold level, referred to above, has rendered heat pipes unsuitable for some applications requiring heat to be transferred between low temperature differences. The present invention seeks to provide a means whereby a heat pipe can be caused to exhibit high thermal conductance at temperature differences lower than the normal starting threshold level, thereby extending •a u the ability of heat pipes to operate with high conductance under low temperature differences.

q r According to the present invention there is provided a heat pipe incorporating an evaporator section and a condenser section interconnected to allow passage of vapour from the evaporator to the condenser and to allow return of liquid from the condenser to the evaporator, and low-powered heating means for heating the liquid in the evaporator section whereby to initiate operation of the heat pipe, said low-powered heating means being additional to the primary heat source from which it is the main function of the heat pipe to transfer heat, and said low powered heating means being inoperative for at least a part of the period during which heat is transferred from the primary heat source by the heat pipe.

Advantageously, the low-powered heating means is provided by an electrical heating wire in close thermal contact with, but electrically insulated from, the part of the evaporator normally containing liquid at the starting condition of the heat pipe. Advantageously, the power rating of the heating o

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-4wire is high enough to initiate high thermal conductance operation of the heat pipe, but low in relation to the heat transfer rate through the heat pipe.

Preferably a controlling and switching means is provided which limits the timing and duration of operation of the heating wire to fulfil the purposes of the present invention.

Advantageously, the low-powered heating means is activated under low temperature difference conditions in which the conductance of the heat pipe would otherwise be low, and is de-activated under conditions in which the heat pipe has attained relatively high thermal conductance.

0 0k •0 Alternatively, low-powered heating means other than electrical heating a wires may be used to achieve the initiation of the high thermal conductance mode of operation of the heat pipe.

The invention will now be further described by way of example only with reference to the accompanying drawing, the sole figure of which is a cross sectional view of a thermosyphon in accordance with a preferred embodiment c of the invention. The application of the present invention to a heat pipe is closely similar to the example shown here for a thermosyphon.

Referring to the Figure, the thermosyphon 1 of the preferred 1 embodiment comprises a substantially vertical elongate enclosure 2, preferably S of cylindrical form, and comprises an evaporator section A containing a S working liquid reservoir 3 at its lower end 4, a condenser section C at its upper end 5, and an adiabatic section B disposed between the evaporator section A and the condenser section C. To minimise potential heat losses in the adiabatic section thermal insulation 6 is disposed on the outer surface of the adiabatic section B.

An electrically insulated electrical heating wire 7 is wound in close 921029,p\oper\mab,heat4 thermal contact with the lower part 8 of the evaporator section A of the thermosyphon 1 and is connected to a low-powered power supply 9 which may be AC or DC.

Under conditions when it is desired to transfer heat by the thermosyphon 1 and the temperature difference between the evaporator section A and the condenser section C are low, and the effective thermal conductance of the thermosyphon 1 is relatively low, electrical power is supplied to the heating wire 7 from the low-powered power supply 9 by way of a switch, or controller, 10 for a short period of time, but sufficient to cause the thermal conductance of the thermosyphon 1 to attain a relatively high level.

The electrical power supply 9 is then cut off until such time that, for any Sreason, the effective thermal conductance of the thermosyphon 1 has again become low at which time the electrical power is again supplied to initiate high thermal conductance opera:ion.

The power supplied to the electrical heating wire 7 may be manually controlled whereby the power may be switched on or off as required or the S, level of power supplied to the electrical heating wire 7 may be varied.

Further, the switching operation may be initiated by the sensing of a temperature differential, as measured by temperature sensors 11 and 12 SC .t t adapted to be connected to the evaporator section A and to the condenser section B respectively. Alternatively, the power to the electrical heating wire 7 may be provided as a timed pulse whereby the power is switched on and off for a fixed, or predetermined, period of time in a continuous cycle. Heating ,j means other than electricity may also be used to initiate high conductance of heat pipes.

It is found that the application of heat pipes and thermosyphons to low temperature difference heat transfer situations can be significantly enhanced by the use of the present invention, with the result that a heat pipe or 921O29,p:\oper\=ab.heat,5 -6thermosyphon has its potential field of applications widened. There are particular benefits to be obtained for heat pipes and thermosyphons used in waste heat recovery applications.

The embodiment has been described by way of example only and modifications are possible within the scope of the invention.

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Claims

1. A heat pipe incorporating an evaporator section and a condenser section interconnected to allow passage of vapour from the evaporator to the condenser and to allow return of liquid from the condenser to the evaporator, and low-powered heating means for heating the liquid in the evaporator section whereby to initiate operation of the heat pipe, said low-powered heating means being additional to the primary heat source from which it is the main function of the heat pipe to transfer heat, and said low powered heating means being inoperative for at least a part of the period during which heat is transferred from the primary heat source by the heat pipe.

2. A heat pipe according to claim 1, wherein the low-powered heating means is provided by an electrical heating wire in close thermal contact with, c, but electrically insulated from, the part of the evaporator normally containing liquid at the starting condition of the heat pipe. 4

3. A heat pipe according to claim 2, wherein the power rating of the condieating wire is high enough to initiate high thermal conductance operation ofw, the heat pipe, but low in relation to the heat transfer rate through the heattained relatively high pipe.thermal conductance.

4. A heat pipe according to clain 2 or claim 3, wherein a controlling and ^switching means is provided which limits the timing and duration of operation of the heating wire.

A heat pipe accord'sg to any one of the preceding claims, wherein the low-powered heating means is activated under low temperature difference conditions in which the conductance of the heat pipe would otherwise be low, and is de-activated under conditions in which the heat pipe has attained relatively high thermal conductance. 950303,p:\opermab,eat7 -8-

6. A heat pipe substantially as hereinbefore described with reference to the accompanying drawings. Dated this 3rd day of March, 1995 TECHNISEARCH LIMITED By its Patent Attorneys DAVIES COLLISON CAVE 04 4* 0.4. *0* it C ~0I C *00 t C. iiZ :1 tc C, It C CC C~ C C .t C C C II. I 9S0303,p:\oper\flLb,heat,8 -9- ABSTRACT A heat pipe incorporating an evaporator section and a condenser section interconnected to allow passage of vapour from the evaporator section to the condenser section and to allow the return of liquid from the condenser section to the evaporator section and low-powered heating means for heating the liquid in the evaporator section whereby to initiate operation of the heat pipe, said low-powered heating means (7) being additional to the primary heat source from which it is the main function o of the heat pipe to transfer heat. o c e S e02,p per bha