18

| ISSUE 94Qpedia | ISSUE 94Qpedia

TECHNOLOGY REVIEW:

HEAT PIPES IN HEAT EXCHANGERS

Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications. We are specifically focusing on patented technologies to show the breadth of development in thermal management product sectors. Please note that there are many patents within these areas. Limited by article space, we are presenting a small number to offer a representation of the entire field. You are encouraged to do your own patent investigation. Further, if you have been awarded a patent and would like to have it included in these reviews, please send us your patent number or patent application. In this issue our spotlight is on application of heat pipes in heat exchangers. There is much discussion about its deployment in the electronics industry, and these patents show some of the salient features that are the focus of different inventors. Table 1 below lists the patents that will be reviewed in this issue.

PATENT NUMBER TITLE INVENTORS DATE OF AWARD

4,537,247 Heat Pipe Heat Exchanger Okamoto, E., et al. Aug 27, 1985

5,329,993Integral Heat Pipe, Heat Exchanger

and Clamping PlateEttehadieh, Ehsan Jul 19, 1994

5,924,479 Heat Exchanger with Heat-Pipe Amplifier Egbert, Mark Jul 20, 1999

Table 1. Patents Reviewed Featuring the Application of Heat Pipes in Heat Exchangers

HEAT PIPE HEAT EXCHANGER4,537,247, OKAMOTO, E., ET AL.

Heretofore, various types of heat exchangers utilizing heat pipes which transfer heat from a high temperature fluid to a low temperature fluid have been developed.

As a typical one thereof, a fixed-type heat pipe heat exchanger is known wherein a group of heat pipes are arranged in box form and the central part thereof is partitioned, a high temperature fluid being let to flow into one and a low temperature fluid into the other one, respectively, so that, by the specific properties of the heat pipes, the heat given from the high temperature fluid is transferred to the low temperature fluid through the sealed-in fluid in the heat pipes.

However, if this kind of fixed-type heat pipe heat exchanger is used to recover heat effectively from the combustion gases containing dust, sulfur oxide (SOx) and nitrogen oxide (NOx) at high concentration exhausted from

| ISSUE 94Qpedia

TECHNOLOGY REVIEW

large-sized boilers or industrial furnaces for steam-power plant. These dust and the like adhere to the gaps of a number of fins 2 attached to the outside of heat pipes 1 on the high temperature side to blockade the passage for the high temperature gas, and, also, the surface temperature of the heat pipes on the high temperature fluid side drops below the acid dew point temperature of the exhaust gas, so that the sulfuric acid content in the exhaust gas condenses to adhere to the surface of the heat pipes, this having been the cause of corroding the heat pipes.

Accordingly, the present invention, being one accomplished in view of the above-mentioned circumstances, has for its object the providing of a fixed-type heat pipe heat exchanger which can be used to recover heat from the combustion gases containing dust, sulfur oxide (SOx) and the like at high concentration exhausted from a large-sized boiler or the like of a steam-power plant and utilize the heat effectively, and wherein the gas passages are not blocked up by dust and the like and yet the heat pipes are not corroded by the sulfuric acid content in the exhaust gas.

INTEGRAL HEAT PIPE, HEAT EXCHANGER AND CLAMPING PLATE5,329,993, ETTEHADIEH, EHSAN

Methods and apparatus for an integrated heat pipe heat exchanger and clamping plate for use with electronic instrumentation and computation electronics are disclosed. In the following detailed description of the preferred embodiment, for purposes of explanation, specific numbers, powerdensities, heat transfer coefficients, etc., may be set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known devices are shown in block diagram form in order not to obscure the present invention unnecessarily.

A base plate 25 has connected to it a number of condenser tubes 30 extending away from base plate 25 in a substantially perpendicular direction. Base plate 25 has a number of channels 27 extending horizontally across the width of base plate 25 and located generally at the center point of the vertical thickness of base plate 25. Channels 27 have a sintered wick sprayed or otherwise applied to the interior surfaces of channels 27. Similarly, base plate 25

20

| ISSUE 94Qpedia

introduced into the interior volume of heat pipe assembly 20 as defined by channels 27, 28, and 29 and condenser tubes 30.

In operation, it is anticipated that heat pipe assembly 20 will be clamped or other intimately attached to some heat.

further has a number of channels 28 extending horizontally across the length of base plate 25 and located centrally about the vertical median point of the vertical dimension of base plate 25.

Condenser tubes 30 are located and attached to base plate 25 at intersection points of channels 27 and 28. The interior volume of each condenser tube 30 will be in fluid communication with channels 27 and 28 through vertically extending channels 29. Channels 29 extend vertically between exterior surfaces of base plate 20 at the intersection points of channels 27 and 28, vertical channels 29 having an opening 29a into which condenser tubes 30 are inserted. Condenser tubes 30may be brazed or otherwise suitably attached to base plate 25, effecting a secure mechanical (i.e., leak tight) and thermal bond. A number of horizontally extending fins 32 extending between all condenser tubes 30 are attached as in the prior art. Spacing between fins 32 may be chosen in accordance with the particular heat transfer application encountered. Structural strengtheners 35 are structurally part of base plate 25 and are delineated by regions of base plate 25 not containing channels 27, 28, or 29. Thus, although structural strengtheners 35 may be thought of as an `absence of structure`, the material of base plate 25 remaining in the regions where strengtheners 35 are located results in added strength and torsional rigidity or stiffness. The dimensions of structural strengtheners 35 may be specified in accordance with the bending and torsional stiffness required for the particular application. The entire heat pipe assembly 20 is subsequently evacuated and thereafter a small amount of working fluid, for example water,

HEAT EXCHANGER WITH HEAT-PIPE AMPLIFIER5,924,479, EGBERT, MARK

PRIOR ART, shows a typical refrigeration coil 20 in isometric view. Such a coil may be employed either as an evaporator, or as a condenser. In refrigeration systems, evaporators function as cooling coils and condensers as heating coils. Coil 20 has two end plates 22, (header sheets) one at each end of the coil, for supporting the coil tubing and for providing mounting means. Traversing the end plates are a matrix of tubes 32, 36, 38, 40 for carrying the evaporating (cooling) or condensing (heating) refrigerant. Each run of tube 32 is connected to another tube run 34 by a U-bend or return bend 30. While each return bend in FIG. 1 is shown connecting adjoining tube runs, other coil designs have return bends connecting tubes which are not adjacent.

While some heat transfer coils employed in refrigeration have no fins, most employ fins 23 to provide extended heat transfer surface, thereby increasing the amount of heat that a given group of tubes can transfer. The fins 23 are securely mounted to the refrigerant tubes 32, etc. In most cases the secure thermal connection between tubes and fins is made by expanding the tubes after they have been inserted into the group of adjacent fins (fin pack). Tube expansion is routinely performed by filling the coil with water and increasing the pressure to several thousand

| ISSUE 94Qpedia

TECHNOLOGY REVIEW

pounds per square inch (psi). In other applications the tubes are expanded by pushing or pulling an oversize mandril through the tube. In still other cases, the thermal bond is secured by dipping the coil into molten zinc or solder or even by furnace brazing in those applications where the coil is expected to be exposed to very high temperatures, such as those existing in furnace exhaust gasses.

Once all the tubes have been expanded into the fins, the fin-tube-header sheet becomes a rigid structure, in most cases capable of supporting itself from the two header sheets 22, 25 without sagging.

A heat pipe is a sealed tubular device containing a volatile liquid. In a typical application a straight heat pipe is tilted slightly, thereby providing a higher end and a lower end. By design the lower end will be exposed to a higher temperature and the higher end will be exposed to a cooler temperature. Under these conditions, the volatile liquid, residing by gravity in the lower end, vaporizes and the vapor flows toward the higher end. When the vapor reaches the cooler condition at the higher end it condenses. The liquid, resulting from the condensation, flows by gravity toward the lower end where the cycle is endlessly repeated, so long as the temperature difference between the higher end and the lower end is maintained. Some heat pipe designs include a porous material lining the interior of the heat pipe or, in some cases, even filling the heat pipe interior. The porous material acts like a wick and carries the volatile liquid from the cooler portion toward

the warmer portion, even when the warmer portion is at the same level as, or even slightly higher than the cooler part. The vapor of the volatile liquid is not influenced by gravity and freely flows from the warmer end where it had been vaporized to the cooler end where it condenses.

Tests and experience have shown that heat pipes provide an even more effective way to conduct heat from a warmer to a cooler location than a bar of highly heat conductive metal such as a copper or silver having the same dimensions as the heat pipe.

The invention employs heat pipes (secondary tubes) to allow fewer refrigerant carrying tubes (primary tubes) in a heat exchanger while simultaneously providing high or substantially undiminished heat transfer capacity. In a refrigeration system a heat exchanger employing fewer primary tubes needs less refrigerant charge for that heat exchanger and therefore for the refrigeration systems as a whole. While each heat pipe does contain a small amount of volatile liquid, which may itself be a refrigerant, the risk of significant refrigerant loss is sharply reduced since it is unlikely that the heat pipe tubes will have leaks simultaneously with the occurrence of leaks in the main system. Further, in addition to the invention allowing substantially fewer primary refrigerant carrying tubes for a coil of given heat transfer capacity, each primary tube employed requires substantially less refrigerant charge than the standard coil by virtue of the fact that in this invention substantially every primary tube has a large fraction of its volume occupied by the active portion of a heat pipe assembly.

22

| ISSUE 94Qpedia

The intended function of the coil is as an evaporator or cooling coil. The coil has end plates 22 and 25 and fins 23 positioned substantially parallel to the end plates. Traversing the end plates and the fins are three rows of tubes (2A). Each row has two runs of primary larger tubes directly embedded in the fins 23 plus two runs of smaller secondary tubes 52 embedded in the fins 23. Primary tube 44 has an inlet end 43. Primary tube 46 has an outlet 45. The larger primary tubes 44 and 46 are joined at their distal ends by a U-connection 48. The U-connection 48 is also known as a U-bend or return bend. The inlets and outlets of the primary tubes may be interchanged according to the preferences of the design engineer.

Within each primary tube is embedded an active portion 50 of a heat pipe. A fin-embedded slave portion 52 of each heat pipe is connected to its active portion by a U-bend 51. Each heat pipe assembly, comprising an active run, a slave run and the U-bend, is charged with a volatile refrigerant such as propane, CFC 12 or some other volatile material. The charging procedure requires evacuation of the interior of the heat pipe to a high vacuum, then charging the interior with the correct amount of volatile liquid. The ends are then sealed by crimping and soldering or brazing.

Each slave portion 52 of each heat pipe assembly is positioned below its active portion 50. When cold refrigerant is introduced into an inlet 43 of the primary tube 44 it simultaneously cools both the fins 23 surrounding it and the active portion 50 of the heat pipe assembly. The cooling effect of the refrigerant on vapor of the volatile liquid within the heat pipe assembly causes the vapor to condense to a liquid. The liquid then flows out of the active section 50 of the heat pipe assembly and into the slave portion 52 of the heat pipe assembly.

LEARN MORE AT WWW.QATS.COM