US2676238A - Heat transfer unit - Google Patents

Description

April 20, 1954 s. N. COATES HEAT. TRANSFER UNIT Filed June 30, 1952 INVENTOR. svo/vgv M 00A 755 Patented Apr. 20, 1954 UNITED STATES PATENT OFFICE HEAT TRANSFER UNIT Sydney N. Coates, Seattle, Wash.

Application June 30, 1952, Serial No. 296,409

4 Claims.

The present invention concerns a heat transfer unit by which heat generated by an electric resistance element of the rod type is transferred to air in an ambient space. That air may be moved in a current through a duct, as by a fan, or it may simply be air in the general space surrounding the heat transfer unit.

Primarily it is an object to provide a heat transfer unit of the general nature indicated, which has a number of important advantages over known heat transfer units which employ rod-type electric resistance elements, in that the newer type of'this invention is explosion-proof, is cheaper to manufacture and more durable, is electrically and thermally more efficient, operates at lower sheath temperatures, and can operate at the usual or even at higher voltages without danger of electrically charging the unit, permitting more than one such element to be connected in series.

More specifically, it is an object of the invention to utilize a rod-type electric resistance element as the heat source, without modification or addition in any way, but to transmit the heat so generated across a closed air space to a much larger heat-radiating, finned housing manufactured as a separate component, for transfer in turn to the air, and to electrically insulate the rod-type element, including its sheath, from the housing in such way that electrical discharge from the rod-type element to the housing, even at voltages so high that the sheaths of the rodtype elements might under previously expected conditions become charged, is virtually impossible. Thereby it becomes possible to achieve many advantageous objectives, as will appear hereinafter.

It is also an object of the invention to provide a heat transfer unit of the general type indicated, and a method of making the same, in which the fins normally used as the primary heat transfer surfaces may be mounted and held upon the tubular unit which surrounds, encloses, and picks up the heat from the rod-type resistance element, yet needs no means to mount them, such as welding, brazing or the like, and can be readily and permanently fixed and held in position, and also in such manner that high temperatures, and the expansion accompanying the same, will operate to hold the fins even more tightly in place, rather than, as heretofore when they were brazed to the rod-type elements, to break them loose.

The present invention consists of a tubular metallic housing surrounding and spaced by an appreciable distance from a normal rod-type electric resistance element which extends through the tubular housing, the two being mutually supported. by end closures or caps of dielectric and refractory material, so that the enclosed space within the tubular housing is a closed air space,

across which heat is transmitted from the rodtype electric resistance element to the tubular housing, and the tubular housing is provided with encircling fins, mounted upon it, and capable of radiating the heat which they pick up from the tubular housing. A heat transfer unit of this nature is of appreciably larger diameter than the known type in which the fins contact directly the rod-type electric resistance element, and thereby the radiating surface is greatly increased, the sheath and the housing temperatures can both be kept low, and a small number of units, operating at high efficiency and yet at lower actual housing temperatures, will produce heating the equivalent of a much larger number of the old type heat transfer units.

The invention comprises the novel heat transfer unit in its entirety, the special finned tubular housing as an element or component thereof, and the process by which the fins are assembled and held in place on that tubular housing.

The invention is disclosed in the accompanying drawings, and will be hereinafter described and claimed in the form which presently is preferred by me, but it will be understood that various changes may be made in the form, character and arrangement of the parts without departing from the spirit of the invention, and that no restriction is to be implied other than is incorporated within the claims.

Figure 1 is a longitudinal axial sectional view through such a heat transfer unit in its completed form, one end only being shown and the unit being of indeterminate or indefinite length, to suit conditions, and Figure 2 is a transverse half-sectional view through this unit, the viewpoint being indica ed by the line 22 in Figure 1.

Figure 3 is an isometric view of several such units, with some partly broken away, installed in a typical form in a duct, also broken away to show the interior.

Figure 4 is an axial sectional view through such a housing and fins in the process of mounting and fixing the fins on the tubular housing.

Rod-type electric resistance elements are known, and it is the intention to take such known units and to utilize them in a different combination and relationship, but without change in the rod-type electric resistance elements themselves. However, for purposes of clarification such an element is shown in section in Figure l, and consists of a coiled wire 9 of material possessing a high electrical resistance, mounted upon and in electrical contact at its ends with pins Bil, the whole closely enclosed within a metallic sheath 3!, which is filled with a granular dielectric mate rial indicated at 92, magnesium dioxide being usually employed. ihe pin projects from the end of the sheath 9| and is threaded at 93 for the rod-type the reception of binding nuts 94, by which the terminal 80 of an electrical conductor 3 is electrically connected.

Heretofore such rod type electric resistance elements have had fins mounted directly upon them, and held in place by brazing the fins to the elements sheath. The normal high temperature of the sheath is lowered by conduction into the fins, and so long as the heat can be removed, by convection or radiation or both, such prior units could operate reasonably satisfactorily. However, if the finned unit of the known type is not adequately cooled-to do which usually requires the blowing of air past it continually with a fan-its temperature may rise so high that the fins become detached, and the element, which is not then adequately cooled, burns out. Such prior units, therefore, are usu ally employed only in installations that include a circulating fan. This adds to the total initial cost, weight, complexity, and operating complications of the installation as a whole.

Moreover, the temperature in such prior units was high at best, and higher in the event of failure of the fan or blockage of air ci ulation, and always created an explosion hazard, both from high temperature and from the danger of an electrical spark.

Electrically, the small gap between the sheathed coil 9 and its sheath ill, even though reinforced by dielectric granular material could withstand safely only a relatively low volt-- age; also, the small gap between the pin and the end of the sheath ill, in open air, peli itted discharge to the sheath. if the voltage is only reasonably high, and these factors 1. ted the voltage permissible, wherefore such L its could not be connected in series, using a high volta e, and even at low voltage there was danger of electrical discharge from the coil or the terminal to the sheath, and of consequent charging of the sheath and everything in conductive relation therewith.

Mechanically, the fins in the prior units had to be mounted upon and brazed to the sheath, which is small and subject to relatively great ranges of expansion and contraction, due to the wide range of temperature to which it is subjected. This was a difiicult and somewhat oi:-

pensive job, and not infrequently the hard solder holding the fins melted and the fins came loose, and hence lost much of their value to remove heat from the sheath. This mechanical breakdown occurred usually when the units becarne overheated from lack of an adequate supply of. cooling air passing over the units although sometimes in other circumstances-and the danger of such a breakdown required that fans and ducts be used with such prior units, and by so much limited their usefulness and increased their initial cost, weight, complications, and operating expense.

According to the present invention these known rod-type, electric resistance elements are employed in somewhat the same way, but now elements are taken as they are completed by the manufacturer and are incorporated without addition or change in the heat transfer unit as a whole. Now, according to this invention, such a unit is supported within a tubular housing 1, preferably one made of metal of good thermal conductivity, which housing surrounds, extends lengthwise of, and is spaced appreciably atv all points from the sheath 9!. The fins 2 are mounted, according to the present invention, not

upon the sheath 9!, but upon the housing I, which they encircle. These fins 2 may be in the form of individual disks, of whatever marginal contour desired, or they may be in the form of a continuous helix. Separate circular disks are il lustrated herein, but the principle is the same in any case; they constitute metal fins which are mounted on and in heat-transfer engagement with the housing, and they project outwardly from the housing. The manner of mounting them, and of retaining them, will be explained hereinafter.

The tubular housing I, with its fins 2, is closed at its ends by a centrally axially bored dielectric closure member 3, preferably shouldered at 30 to receive the end of the housing i, its central aperture or bore constituting a support for the rodtype electric resistance element and its sheath 9| Preferably the sheath terminates within the dielectric closure 3, insulating washers e1 complots the filling of the bore in the closure to prevent discharge from the protruding pin iii} to the sheath iii, and a washer of insulating material overlies the end of the bore and is held in place by a cap 99. This cap as serves to anchor the rod-type element in place, in conjunction with the binding nuts 94.. The dielectric closure cap 3 likewise be shouldered at 35 for mounting within an aperture of a duct l. The rodtype resistance elements may operate more nearly at their normal temperatures, without fluctuation, and so their electrical and thermal efficiency is greater, yet being enclosed within a completely enclosed dead air space, any electrical break:-

' down that could charge the sheath til (now extrenieiv unlikely because of the thorough dielec insulation at all points) cannot possibly create a hazard to persons contacting in-- stallation, and the absence of the possibility of electrical discharge, coupled with low housing temperature, eliminates any explosion or fire hazard. Instead of a gap of a small fraction of an inch between the resistance coil e and the sheath 9!, and between the pin iii} and the end of the sheath, which precludes he use of high voltage when the sheath is in contact with grounded parts, the sheath is now mounted in the large dielectric closure 3, the pin all is thoroughly insulated electrically from sheath, and the sheath is spaced by a large distance from the housing i, so that there is little or no danger of discharge across to the housing, even should the sheath become charged, which, as has been indicated, is now extremelyunlihely. Now several such units may be conne ted in series, with high voltage current supply, without danger of the housings. becoming charged electrically.

It would be difficult to express. any formula for determining the spacing between the sheath 9| and the tubular housing I, since various factors, diii'ering in different installations, may govern. In general, there should be a rather large air space, radially, within the housing. The larger the air space the greater the. surface area of the housing and the lower its surface. temperature, in general. Of course, this spacing must not be so great that the transfer of heat from the rod-type element to the housing is impeded to the extent that the element becomes overheated, and is in danger of burning out, or to the extent that the unit as a whole becomes excessively bulky. The optimum air cap i mall enou h that heat is rcmoved from. the. rod typcelcment fast enou h o keep it reasonably 0001. yet lar e. enough to avoid the development in the housin of an excessivel 5. high temperature. Purely by way of illustration, a unit wherein the air gap is -7 inch all around produced a housing temperature of about 1 F. when the sheath temperature of the rod-type element was 750 approximately.

As a result of the enlargement of the heat transfer unit as a whole, not only is the surface area of the housing increased greatly over the surface area of the sheath 9!, but so also is the area of the fins 2 increased greatly over the area of the fins as mounted heretofore directly on rodtype resistance element. The much larger area can therefore dissipate the same amount of heat in a given period of time at a much lower temperature, and a few units will accomplish what before required many units. No heat is lost; as much as is generated by a given current is of necessity transmitted to the housing and thence to the surrounding air, and the omission of fans is not harmful.

Further according to the present invention, each housing I, being made of sheet material such as aluminum, may have a relatively high coeflicient of thermal expansion, and yet be soft enough to be deformable with reasonable readiness. The fins 2, however, are preferably of somewhat harder material, but whether harder or not, they should be of material having a lower coeflicient of thermal expansion. These relationships make possible a special form of assembly, and assure the retention of the fins regardless of thermal changes. I

Initially, then, the housing I is of slightly smaller external diameter than the interior margins or openings in the fins 2, as shown in Figure 4. The fins are disposed in properly spaced relationship, encircling but not necessarily contacting the exterior of the smaller housing I. The ends of the housing are closed as by a closure member 4, and by means of a conduit at a pressure fiuid, whether pneumatic or hydraulic is unimportant, is admitted to the interior of the housing I, and the pressure is such that the tubular housing I is expanded. When it expands until it contacts with the interior margins of the fins 2, that portion in contact cannot expand farther, but continuation of or addition to the pressure within the housing expands the tube in the portions intermediate the fins until it bows outwardly, as shown in Figure 1. Upon removal of the pressure fluid this expansion or distortion of the housing I persists, and now, without brazing, welding or any permanent connection, the fins are mounted and held securely in proper spaced relationship upon and in good thermal contact with the exterior of the housing I.

Moreover, when the housing and the fins are heated in use, the greater coeificient of thermal expansion of the housing I with relation to that coefficient of the fins 2 will cause a tendency for the housing I to expand more greatly than the fins expand, and this serves further to grip the fins by the expanding housing, and so they are retained firmly in place. When the housing and fins cool down the fins are still held in place by the initial expansion-produced distortion of the housing, that is, by the bowing of the housing intermediate the fins.

I claim as my invention:

1. A heat transfer unit comprising a straightthrough rod-type electric resistance element, two axially bored caps of insulating material, one for each end of the element, within the bore whereof the respective ends of the elements sheath are received, whereby to support said element, insulation filling the bore, outwardly of the sheaths ends, to prevent discharge from the elements projecting terminals to its sheath, and a finned tubular heat transfer housing surrounding and spaced at all points from the element, and mounting said caps as closures for its opposite ends, to complete a dead air space within said housing.

2. A heat transfer unit comprising a tubular housing, a rod-type electric resistance element axially centered within and extending from end to end of and spaced from said tubular housing to leave an air space therebetween, the opposite terminals of the resistance element projecting from the opposite ends of the housing, said housing being formed of metal which possesses a given coefficient of thermal expansion, dielectric closure means for the ends of said housing, and supporting the ends of said resistance element, and fins mounted on and in heat transfer engagement with, and projecting outwardly from, said housing, said fins being formed of metal which possesses a lower coefficient of thermal expansion than said housing.

3. A heat transfer unit comprising a fin-encircled tubular housing, a rod-type electric resistance element axially centered within and extending through said tubular housing, with its opposite ends projecting from the opposite ends of the housing, and spaced at all points from said resistance element to leave an air space therebetween, said housing being formed of sheet metal soft enough to have been expanded under the infiuence of internal pressure in the spaces between fins, and the fins which encircle said housing being formed of metal hard enough to resist expansion of the tube and so to be held in place and in heat exchange relation by the intervening expansion of the tube, the metal of the housing having also a coefiicient of thermal expansion which exceeds that coefiicient of the metal of the fins, whereby their relative expansion when heated tends to tighten the housing within the fins, and dielectric closure means mounted on the ends of the tube and supporting the ends of said resistance element.

4. A finned metal heat-transfer tubing which comprises a tube formed of a metal of a given coefiicient of thermal expansion, and which has a softness such that it has been expanded by internal pressure, and hence is bowed outwardly intermediate successive fins, a plurality of separate fins of a metal having a lesser coefiicient of thermal expansion, encircling said tube at spaced intervals, and held in place by such expansion and bowing of the tube.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,475,162 Abbott Nov. 27, 1923 1,668,508 Kettering May 1, 1928 1,741,217 Winslow Dec. 31, 1929 1,821,702 Freeman Sept. 1, 1931 1,946,547 Russell et a1 Feb. 13, 1934 1,960,955 Becker May 29, 1934 2,372,150 Whittaker Mar. 20, 1945- 2,458,189 Morgan Jan. 4, 1949 2,473,783 Brown June 21, 1949 2,594,465 Lovfald Apr. 2 1952 FOREIGN PATENTS Number Country Date 406,498 Great Britain Mar. 1934

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