High Efficiency Heat Transit System

Abstract

A closed tube in substantially horizontal position and approximately half filled with vaporizable liquid under very low absolute pressure has one exterior portion of its length exposed to hot fluid from which heat is extracted to vaporize liquid in the tube and another exterior portion of its length exposed to a relatively cool fluid which extracts heat from the vapor in the tube and condenses it. Vapor formed in the portion of the tube exposed to the hot fluid migrates to the portion of the tube exposed to the cool fluid and the condensate resulting from such cooling flows back to the tube portion exposed to hot fluid. The major portion of the internal periphery of that portion of the tube exposed to hot fluid is lined with wicking, but a gap is left between ends of the arcuate liner below the surface of the liquid in the tube for transmission of heat from the fluid externally of the tube directly to liquid in the tube to expedite its vaporization at that location.

Parent Case Text

This application is a continuation-in-part of U.S. Pat. application Ser. No. 817,483, filed Apr. 18, 1969 for Method of and Apparatus for Heat Transit, now abandoned.

Description

A principal object of the present invention is to provide a device for transferring heat from one fluid medium to another fluid medium with high efficiency and which has large capacity.

A further object is to provide such a device which can effectively transfer heat between such fluid mediums under widely different temperature conditions.

Another object is to provide a heat transfer device which has one portion exposed to hot fluid and such entire portion is maintained at substantially uniform temperature to prevent local failure and to provide maximum efficiency of operation.

In such a heat transfer device, it is an object to be able to utilize any of various vaporizable liquids.

An additional object is to operate such a device under low pressure conditions so that, if a failure should occur, minimal or no damage will result.

FIG. 1 is a somewhat diagrammatic vertical section through a representative heat transit device of the present invention.

FIG. 2 is a side elevation of a somewhat modified form of heat transit device, parts being broken away, and FIG. 3 is a transverse section through such device taken on line 3--3 of FIG. 2.

FIG. 4 and FIG. 5 are sections through heat transfer devices corresponding to FIG. 3, but showing devices of the same type as in FIG. 1, although of different size.

FIG. 6 is a top perspective of a section of a device shown in FIG. 1 with a portion broken away.

FIG. 7 and FIG. 8 are vertical sections through the heat transit device corresponding to FIG. 1 but showing the device in different attitudes, and FIG. 9 is a diagram comparing the relationship of the attitudes of the device shown in FIGS. 1, 7 and 8.

Vaporizable liquid vapor has been used heretofore as a heat transfer medium employing the latent heat of vaporization to increase the heat transfer capacity. Where the liquid has been evaporated from a wick at one location by absorption of heat and such vapor has been condensed at a different location by extraction of heat and the resulting liquid returned to the heat absorption location by the capillary action of a wick, the apparatus performing such operation has been designated a "heat pipe." Such a device is disclosed in U.S. Pat. No. 3,229,759 and is discussed in an article entitled, The Heat Pipe, by G. Yale Eastman in the Scientific American for May, 1968, Volume 218, page 38. It is pointed out in that article near the top of page 44 that the upper limit to the power handling capacity of a given heat pipe is determined by the ultimate pumping capacity of the wick.

The apparatus of the present invention also relies on the latent heat of vaporization of a vaporizable liquid to transfer heat, but it has advantages over a heat pipe and does not have some of the disadvantages of the heat pipe. A principal difference of the apparatus of the present invention over the heat pipe is that it does not rely on the capillary action of a wick to return condensed heat transfer liquid medium from a heat-extracting location to a heat-absorbing location but utilizes a body of liquid for that purpose.

As shown in FIG. 1, the heat transit device of the present invention includes a tube 1 closed at both ends and having a wicking liner 2 held in place on the inner periphery of the tube 1 by a perforated or reticulated wick retainer sheet 3. Such wick retainer is sufficiently apertured to enable vapor to escape readily through it from the wick into the hollow interior of the tube 1 and to allow atomized liquid to pass readily from the interior hollow of the tube through the wick retainer into the wicking material.

The wicking material can be any of various types of more or less rigid fibrous or foam material. If the wicking is of foam material, it should be of the interconnected pore type. The wick retainer sheet can be of perforated metal or plastic sheet or of metal or plastic screen provided that it is strong enough to support the material of wick 2 to provide an unobstructed hollow extending axially of the tube 1. Also, the radial thickness of the wick is not critical, although it should not be so thick as to restrict unduly the hollow space extending axially of the tube.

A portion of the length of the tube 1 is exposed to heat-supplying means such as extending through a portion of a duct 4 through which heated fluid flows. Such duct, for example, may be a stack conveying gaseous products of combustion. Another portion 5 of the length of the tube 1 is exposed to heat-extracting means shown in FIG. 1 in the form of a casing 6 surrounding the portion 5 of tube 1 and containing heat-absorbing fluid 7. Such fluid preferably is liquid and can flow through the casing 6 by supplying cool liquid through the supply pipe 8 and discharging heated liquid through the discharge pipe 9. A baffle 10 projects from a wall of the casing 6 toward the heat transfer pipe end 5 between the cool-liquid supply pipe 8 and the hot-liquid discharge pipe 9 to deter flow of liquid from such supply pipe to such discharge pipe along a direct path instead of flowing around the heat transfer pipe end 5.

The heat transit tube 1 contains free vaporizable liquid 11 of substantial depth. Such liquid may be any of various types but should have a reasonably high specific heat, quite a high latent heat of vaporization and condensation and a boiling point which is not excessively high or low. A further desirable characteristic of such liquid is that it be economical and that it provide substantial agitation under the condition of nucleate boiling. For most installations, water has characteristics satisfactory for use as the heat transfer medium.

In operation it is desirable for the portion of the wick 2 in the portion of the tube 1 exposed to the source of heat supply to be completely saturated with vaporizable liquid. Such saturation will prevent the occurrence of localized areas of higher temperature or hot spots which could deteriorate the wicking material of some types and in any case would reduce the differential in temperature between that of the wicking and the temperature of the heat-supplying fluid. Such reduction in temperature differential results in a reduction of heat transfer efficiency between the heat-supplying fluid and the vaporizable liquid. Also, complete saturation of the wick 2 will insure that there is always adequate heat transfer liquid available for evaporation from the wicking by heat absorbed from the heat-supplying fluid in duct 4.

That portion of the heat transfer liquid 11 which is evaporated both from the wicking and directly from the liquid body in the portion of the tube 1 exposed to the heat-supplying fluid will flow into the interior hollow of the tube 1 and fill the portion 5 of such tube which is exposed to the heat-extracting means. Heat extracted from such vaporized heat transfer fluid by the liquid 7 will result in condensation of the heat transfer fluid in the portion 5 of the tube 1 so that such liquid joins the body of liquid in the tube and can flow to the left as seen in FIG. 1 from the condenser section of tube 1 into the vaporizer section of such tube.

An expedient for insuring that the wick liner 2 is always completely saturated with vaporizable heat transfer liquid is to provide a construction which will produce nucleate boiling of the liquid body of the heat transfer fluid in the fluid-vaporizing section of the tube 1. Such nucleate boiling is accomplished by interrupting the continuity of the wick 2 over portions of the vaporizing section of the tube 1.

In FIGS. 3, 4, 5 and 6, the wick liner is shown as having arcuate ends 12 and 13 spaced apart circumferentially of tube 1 to leave a gap 14 in which the heat transfer fluid 11 in liquid form is in direct contact with the wall of the tube. In this gap the heat transfer liquid is not insulated from the heat-conducting wall of the tube by the wick liner 2 so that at this location, heat will be absorbed from the heat-supplying fluid more readily by the heat transfer liquid than at the locations where the tube wall is covered by the wick.

The result of such concentrated heat absorption by the heat transfer liquid in the gap 14 is to cause localized vaporization of the liquid below the surface of the liquid to produce vapor bubbles 15 within the body of the liquid which rise to the surface. During such nucleate boiling of the liquid, the liquid body surface will be greatly agitated and bubbles will rise from the surface a greater or lesser distance and burst so as to provide both vapor and unevaporated atomized liquid in the space above the surface of the liquid body. Such atomized liquid will pass through the apertured wick retainer sheet 3 to supply liquid directly for absorption by the upper portion of the wicking above the liquid body instead of relying solely on the capillary action of the wicking to maintain such wicking in fully-saturated condition by osmosis from the portion of the wicking submerged in the liquid to the upper portion of the wicking above the liquid body.

A further expedient is used to increase the boiling action and to promote vaporization of the heat transfer liquid. Such expedient is to provide a very low absolute pressure within the tube 1 instead of such pressure being equal to or greater than atmospheric pressure. Such low pressure is produced in the pipe 1 without a pumping operation simply by heating the tube to promote vigorous boiling of the liquid in it when it is sealed. Such boiling will insure that the space above the liquid body is filled with vapor of the heat transfer liquid instead of air. When the tube is cooled, whatever portion of the vaporized liquid is condensed will reduce the pressure within the sealed tube 1 correspondingly.

Absorption of heat from the heat-supplying fluid by the heat transfer fluid absorbed by the wick 2 to vaporize such fluid from the wick can be increased to some extent by increasing the effective heat transfer area of the surface of pipe 1. Such increase in heat transfer surface area may be accomplished by providing annular fins 16 in axial spaced relationship on the external periphery of the tube as shown in FIGS. 2 and 3. Except for such fins, the construction of the heat transit tube of FIGS. 2 and 3 is the same as that shown in FIGS. 1, 4, 5 and 6. Also, the tube 1 of FIGS. 2 and 3 is shown as being of the same size as the tube of FIGS. 1 and 6 which may, for example, be 21/2 inches or 3 inches in outside diameter.

The heat transit tube 1 of FIGS. 1, 2 and 3 is shown as being approximately half filled with heat transfer fluid in liquid form. If a heat transit tube 1 of a considerably larger diameter, such as shown in FIG. 4, is used, which may have an outside diameter of 4 or 5 inches, the depth of heat transfer fluid in liquid form 11a should be greater than the depth of such liquid in the smaller tube so that the space above the surface of the liquid into which the bubbles 15a rise and in which they burst will be approximately the same distance from the upper portion of the wick liner 2a held in place by the wick retainer sheet 3a.

On the other hand, if the heat transit pipe is of a diameter much smaller than that shown in FIGS. 1, 2 and 3, such as shown in FIG. 5, in which the tube 1b may have an outer diameter of 1 to 11/2 inches, the level of the heat transfer fluid in liquid form 11b will be lower than that shown in FIGS. 1, 2 and 3. Again, the height of the space between the surface of the liquid in the tube and the upper portion of the wick layer 2b, held in place by the wick retainer 3b, should be about the same as in the tubes of FIGS. 1, 2, 3 and 4. Under these circumstances the bubbles 15b will have approximately the same distance to travel between the surface of the liquid and the wick liner as in the other instances.

Particularly if the heat-supplying fluid is flowing through the duct 4 and one side of that portion of the tube 1 exposed to the heat-supplying fluid faces upstream, it is desirable for the gap 14 in the wick liner to be on the upstream side of the heat transit tube. Thus, the gap 14 between the arcuate ends 12 and 13 of the wick liner, as shown in FIG. 3, the gap 14a between the arcuate ends 12a and 13a of the wick liner 2a shown in FIG. 4 and the gap 14b between the arcuate ends 12b and 13b of the wick liner 2b shown in FIG. 5 should be on the upstream side of the heat transit tube while still being well below the surface of the liquid body of heat transfer fluid in the heat transit tube.

The heat transit tube 1 should be disposed substantially horizontal in order to accomplish the desired operation, and, in any case, should not be tilted more than five degrees in either direction as indicated by the lines in FIG. 9. The line S1--C1 designating the heat source and the condenser is horizontal corresponding to the position of FIG. 1. The line S7--C7 of FIG. 9 indicates the situation shown in FIG. 7 in which the condenser end of the heat transit tube is lower than the heat source end and the tube is inclined downward at an angle of 5.degree.. The line S8--C8 of FIG. 9 indicates the situation of the heat transit tube in FIG. 8 in which the condenser end is higher than the heat source end and the inclination of the tube is 5.degree..

When the heat transit tube is in the horizontal position shown in FIGS. 1 and 2, the heating of the liquid in the gap 14 will produce vigorous bubbling of the nucleate boiling liquid, which bubbling will be substantially uniform throughout the length of the portion of the heat transit tube exposed to the heat source 4 if the temperature of such heat source is approximately the same across the width of the duct.

When the heat transit pipe slopes downward toward the condenser, as shown in FIG. 7, however, the liquid will not be sufficiently deep at the opposite end to provide bubbling which will be effective to insure that the wick in the upper part of this lengthwise portion of the tube will be completely saturated at all times. Also, it will be observed at the right end of FIG. 7 that the passage for vapor from the heated portion of the heat transit tube to the condenser end of the tube is greatly constricted, if not completely blocked, so that the vapor of the heat transfer liquid cannot pass readily to the condenser. Also, the volume of the condenser occupied by vapor is extremely small so that the condenser cannot operate effectively. As vapor is condensed in the condenser, however, additional vapor may pass to the condenser in a surge causing undesirable violent periodic agitation in the transit tube.

On the other hand, if the condenser end of the heat transit tube is substantially higher than the heated end portion of the tube, the wick may not be wetted adequately adjacent to the condenser and insufficient space may be provided for effective vaporization of liquid from the wick adjacent to the opposite end of the tube so that the operating efficiency of the evaporator section of the tube is decreased. Also, the boiling liquid may block a portion of the tube spaced from the left end to trap vapor in the left end which will escape periodically through the liquid block to the condenser, thus causing periodic surging more violent than when the tube is sloped toward the condenser as shown in FIG. 7. It is therefore important that the heat transit tube be disposed substantially horizontally in order to obtain the most efficient heat transfer operation.

Because the heat transit tube 1 is sealed when the heat transfer liquid is heated to boiling temperature, at least most of the air in the upper portion of the heat transit tube 1 will be displaced by vapor of the heat transfer fluid at the time of sealing. Consequently, during normal operation, the absolute gas pressure in the space above the liquid will be less than atmospheric pressure. Even if the heat transit tube is heated excessively, the pressure in the space above the liquid will not be increased greatly over atmospheric pressure, and, consequently, such gas pressure will not have any great tendency to rupture the tube. Even if severly overheated, therefore, the tendency of the sealed heat transit tube 1 to explode is negligible providing an important safety feature. The reduction in the latent heat of vaporization of the heat transfer liquid resulting from this operation under low pressure, however, is quite small.

As has been mentioned above, water is a satisfactory heat transfer fluid for most operations. Some heat transfer fluids have the disadvantage that during nucleate boiling the bubbles liberated do not produce agitation in the body of liquid as vigorous as is desirable.

Claims

I claim:

1. In a heat transit device including a container having one portion exposed to heat-supplying means and a different portion exposed to heat-extracting means, the improvement comprising the combination of wick liner means on the inner periphery of the container portion exposed to the heat supply means, and the container containing a body of free vaporizable heat transfer liquid of substantial depth in the lower part of the container portion exposed to the heat-supplying means, at least a portion of the lower part of the inner periphery of the container below the surface of said liquid body not being covered by said wick liner means, and said liquid body also extending to the lower part of the container portion exposed to the heat-extracting means whereby bubbles resulting from the boiling of the liquid will saturate with liquid a portion of said wick liner above said liquid body.

2. In the heat transit device defined in claim 1, the wick liner means being interrupted for a portion of the peripheral extent of the container wall beneath the surface of the body of heat transfer liquid.

3. In the heat transit device defined in claim 2, the wick liner means being interrupted along a generally horizontal band.

4. In the heat transit device defined in claim 3, the band along which the wick liner means is interrupted being offset from the central portion of the container bottom.

5. In the heat transit device defined in claim 1, the wick liner means covering a portion of the inner periphery of the container portion beneath the surface of the heat transfer liquid for direct absorption of liquid by the wick liner means from the body of liquid.

6. In the heat transit device defined in claim 1, the surface of the body of heat transfer liquid in the container being substantially midway between the top and bottom of the container.

7. In the heat transit device defined in claim 1, the absolute pressure in the space above the surface of the body of liquid being less than atmospheric pressure.

8. The method of transferring heat from a heat-supplying location to a heat-extracting location by a heat transfer fluid in a container having a wick liner lining an upper portion of the container and only part of the lower portion of the container, which comprises placing in the container sufficient vaporizable heat transfer fluid so that a substantial portion of the heat transfer fluid is in the form of a free liquid body beneath the portion of the wick liner lining the upper portion of the container and above a portion of the wick in the lower portion of the container and a portion of the free liquid body is located in the heat-extracting portion of the container, heating a portion of the container to effect nucleate boiling in the free liquid body for promoting evaporation of the heat transfer fluid from its liquid state to its vapor state and for moistening of the portion of wick liner above the liquid body, also heating a portion of the container upper portion lined with the wick liner for evaporating heat transfer fluid from the wick liner, and cooling the container at the heat-extracting location for condensing heat transfer fluid vapor back into its liquid state to rejoin the free liquid body.