CA1168225A - Heat pipe - Google Patents

Abstract

HEAT PIPE

ABSTRACT
A heat pipe comprising an iron container having enclosed therein an aqueous solution of a vanadate as a working liquid.

Description

2;~5 HEAT PIPE

The present invention relates to a heat pipe for use in waste heat recovering apparatus and the like, and more particularly to an iron heat pipe which is usable for a prolongediperiod of time without entailing a reduction in performance.
Heat pipes made of iron are usable over a wide range of operating temperatures including low temperatures (70 to 130 C) and high temperatures (200 to 400 C), less costly than those made of copper and fully satisfactory also in respect of strength. Accordingiy they have found wide use. The working liquid most suitable for such he~t pipes is water because water is usable over a wide range of operating temperatures and has high latent heat of evaPoration (540 cal/g) and limiting heat flov~ (109 ~I/cm ).
However, when water is used as the working liquid for an iron container serving as a heat pipe, the water reacts with iron to evolve hydrogen gas. ~he hydrogen gas diffuses in the form of atoms into the wall of the iron container and is partly released from t~e container at a constant rate, but a major portion of the hydrogen gas remains in the condensing portion of the container therein, consequently impairing the performance of the heat pipe.
In view of this problem, it has been practice to use alkaline aqueous solutions~such as NaOH aqueous solution, as working liquids and containers formed with a corrosion preventing coating of ~e304 on the inner surface.
However, the coating is not fully satisfactory, permitting corrosion of the container inner surface and allowing the evolution of hydrogen gas to some extent. It is therefore impos~ible to limit the rate of evolution of hydrogen gas to a level not higher than the rate of release of the gas from the container and to prevent hydrogen gas from remainining within the container.

The object of the present invention, which has been accomplished in view of the above situation, is to provide a heat pipe which is free of the likelihood that hydrogen gas will remain in its interior to result in a reduced efficiency.
~ he heat pipe of this invention comprises an iron container having enclosed therein an aqueous solution 2;~

of a vanadate as a working liquid.

The drawing is a view in vertical section show-ing a heat pipe.

Examples of iron containers useful for providing heat pipes according to the present invention are contain-ers made of stainless steel, carbon steel or the like and such containers having an aluminized coating on the outer surface.
Examples of useful vanadate are ammonium meta-vanadate, sodium metavanadate, potassium metavanadate, lithium metavanadate, etc., to which this invention is not lim~ted. The aqueous solution of such a vanadate, when enclosed in the container, forms on the inner surface of the container a compact coating having high resistance to corro~ion. The concentration of the vanadate is prefer-ably 0.1% to the saturation concentration. When having a concentration of less than 0.1%, the solution fails to form a corrosion-resistant coating of sufficient thick-ness.
The aqueous solution of vanadate may be formed first by dissolving the vanadate in ~/ater treated by ion exchange and then placed into the container, or m~y be prepared within the container by placing the vanadate 2;~5 into the container first and then pouring such water into the container. When the container having the working liquid enclosed therein is heat-treated at a high temper-ature of 160 to 200 C, the formation of the corrosion-resistant coating can be accelerated.
With the heat pipe of this invention, slight corrosion is likely to occur on the inner surface of the container to evolve a ~mall amount of hydrogen gas. The rate of e~olution of the gas is ~evertheless lower than the rate at which the hydrogen gas diffuses through the wall of the container and flows out from the container.
The amount of the gas evolved is therefore not so large as to impair the performance of the heat pipe. To release the small amount of hydrogen thus evolved, it is desired that the heat pipe container 1 be provided,at one end 1_ thereof forming the condensing portion, ~ith a linear hydrogen-permeable member 2 of palladium or an alloy thereof extending through the end and brazed thereto with silver as illustrated in the drawing. The provision of the hydrogen-permeable member 2 is effective especially when the iron container is externally aluminized for the purpose of rendering the container resistant to corrosion and attaching aluminum fins having a brazin~ ]ayer to the container by vacuum brazing, because the hydrogen gas diffusing through the container wall will then be 32;~S

prevented from flowing out by the aluminized coating.
Further since the hydrogen-permeable member is linear and extends from the interior of the container to the outside thereof through its wall, the corrosion of the container that would result from the action of electric cell between the member and the container wall can be inhibited to the greatest possible extent.
Example A pipe of ST~ 35 steel was prepared which had a diameter of 27.2 mm, a length of 1000 mm and a wall thickness of 1.6 mm. An end wall having a pa-lladium wire extending therethrough was welded to one end of the pipe to cloae the end. The pipe wa~ then aluminized only over the outer surface. The interior of the pipe was cleaned with nitric acid and then with acetone. An end wall having a working liquid injecting nozzle was welded to the other end of the pipe. Aluminum fins having a brazing layer were attached by vacuum brazing to the heat pipe container thus made. About 0.6 ~ of ammonium metavanadate was placed into the container, 120 ml of water subjected to ion exchange was placed into the container, and the nozzle was closed, whereby a finned heat pipe was prepared. Three finned heat pipes were further prepared in the same manner as above except that aqueous solutions of ammonium metavanadate 11t;~2;~5 havin~ concentrations of 1~, 3% and saturation were used as working liquids. The four heat pipes thus obtained were tested for a prolonged period of time to find that the temperature difference ~T between the evaporating portion and the condensin~ portion of each pipe remained 2ero, hence no reduction whatsoever in efficiency. In contrast, when heat pipes were tested with use of an aqueous solution of sodium hydroxide and ion-exchanged water, the pipes exhibited greatly increasing ~T with time, i.e. a markedly reduced efficiency.

Claims (5)

1. A heat pipe comprising an iron container having enclosed therein an aqueous solution of a vanadate as a working liquid.

2. A heat pipe as defined in claim 1 wherein the vanadate is a salt selected from the group consisting of ammonium metavanadate, sodium metavanadate, potassium metavanadate and lithium metavanadate.

3. A heat pipe as defined in claim 1 wherein the aqueous vanadate solution has a concentration of 0.1%
to saturation concentration.

4. A heat pipe as defined in claim 1 wherein a hydrogen-permeable member of palladium or an alloy thereof extends through its condensing portion.

5. A heat pipe as defined in claim 1 wherein the iron container has an aluminized coating over the outer surface thereof, aluminum fins, the aqueous vanadate solution enclosed in its interior and a hydrogen-permeable member extending through the condensing portion of the pipe.