US2080800A - Heating apparatus - Google Patents

Description

May 18, 1937'. E. L. WIEGAND 2,030,800

HEATING APPARATUS Filed Sept. 20, 1955 2 Sheets-Sheet 1 I E v c- UFTJVENTOR.

ATTORNEY.

May 18, 1937. E. 1.. WIEGAND HEATING APPARATUS Filed Sept. 20, 1935 2 Sheets-Sheet 2 m {KY m VQ Q t m m l Q R INVENTOR. w g. W1:

ATTORNEY.

Patented May l8, 1937 UNITED STATES PATENT. OFFICE HEATING APPARATUS Application September 20, 1935, Serial Pia-41,438

3 Claims.

This invention relates to electric resistance heating units and apparatus embodying the same, and one of the objects of the invention is to provide a heating unit or apparatus made up of a plurality of such heating units, each heating unit including a plurality of heating elements of the 'strip type, having fins thereon so disposed and arranged that the energy produced in the heating element will be emitted or difiused more quickly and easily than with previous devices, thus enabling the heating element proper to operate at a lower temperature than otherwise with a given energy input, thereby greatly increasing the life of the heating element and contributing to a more efilcient conversion of the electrical energy into heat.

Another object of the invention is to provide a heating apparatus which is made up of a plurality of separate and independent heating units secured together contiguous to each other, each unit comprising a plurality of fiat, thin, elongated strip heaters arranged side by side in spaced relation and having a plurality of spaced fins thereon, the fins on each such strip heater being so disposed and arranged as to provide a plurality of transverse ducts extending parallel to the length of the fins and across the longest side of the strip heaters, such fins being of such size and shape and so disposed that the temperature of the fins immediately adjacent the strip shall not greatly exceed 300 F. above room temperature and the temperature at the edges of the fins shall not be substantially less than 150 F. above room temperature when the heating apparatus is in normal operation whereby the fins will be maintained at a temperature below a point where the amount of heat dissipated by radiation becomes critical.

Another object of the invention is to provide a heating element and fin construction therefor in which the diffusion of energy is accomplished as largely as possible from a practical standpoint by an electrical potential drop and with a minimum thermal gradient.

A further object of the invention is to provide a heating unit which will convert the maximum amount of energy in a minimum cubical space and within a minimum projected area of the device with respect to its operating face. Another object of the invention is to provide a heating unit of the character described which will materially advance that phase of the heating art which has to do.with the transmission of energy by a fluid convecting medium, such as molten solids, liquids or gases.

A more specific object of the invention is to provide a heating unit of the character described which will limit and balance the maximum temperature gradient between the heat source and the convecting medium to the end that the limit of capacity may be increased with a given heat source temperature or the heat source temperature may be kept at a minimum at a given capacity thus avoiding the limitation of the electrical heating art as imposed by the internal temperature on the one hand and by permissible external temperature on the other hand.

' A further object of the invention is to provide a I heating unit of the character described which makes use of these principles for the purpose of obtaining maximum economy in respect to the materials required for transmitting energy from the resistor to the convecting medium, as well as in the cubical space, surface area or both.

My heating unit is particularly adaptable and useful in heating gases and fluids, by convection where a forced draft is employed to move the convecting medium. My improved heating unit also enables me to deliver a maximum amount of heat, that is, wattage or B. t. u. in a given space or area, and at the same time enables me to materially reduce the weight and area of metal required for a given capacity heating unit.

- I have found that a rather critical relation exists between the size of the heating element proper and the radiating fins in order to obtain the maximum diffusion of heat with a minimum difference in temperature gradient at difierent points on the fin. In other words, there exists a rather critical relation between the size, shape and spacing of the fins and the size and shape of the heating element proper in order to obtain an efficacious diffusion of heat by convection. With most metals economically usable in heating elements of the character described, it is not economical or efflcient to raise the fin temperature more than say 280-300 F. above room temperature for the purpose of heating by convection, for the reason that when the temperature of the fin greatly exceeds this point, too large a proportion of heat is trans-' mitted by radiation instead of by convection which is undesirable in a convection heater. It is therefore desirable to maintain the temperature of the fin such that the maximum amount of heat is diffused by convection, as the amount of heat which a moving stream of fiuid or gas will absorb under convection conditions is limited.

My heating unit is especially useful and desirable for occasional or auxiliary heating where a forced draft is employed to diffuse the convection medium and I have provided a unit which empioys a plurality of so-called strip heaters having fins thereon, there being a definite relation between the shape and size of the fins and the shape and size of the strip heaters proper. The shape and contour of the strip heater proper is also of importance in a unit of this character and I preferably make use of a plurality of thin narrow heating elements which may be disposed in a group in such positionthat the convecting 1 medium is caused to pass over the thin or fiat side of the strip which enables me to position a plurality of heaters one behind the other or in staggered. relation in very close proximity to each other without unduly obstructing the fiow of the convecting medium through the unit.

According to my invention, the heating element, whatever may be its detailed construction, is surrounded by a plurality of metal fins spaced apart and projecting laterally and afiording an extended area for the diffusion of heat generated inside the element. When the insulation is surrounded by a metallic sheath, these fins preferably have metal-to-metal contact with the sheath and each fin preferably has integral therewith a laterally extending portion disposed at one edge thereof which at least engages and preferably interlocks with a similar portion on an adjacent fin so as to intercept all heat passing outwardly from the resistor and so as to secure the whole fin structure in place. Preferably the fins are transverse to the major axis of the element although I do not limit myself to such a construction. Owing to the relatively large diffusion surface provided by the fins combined with the close and firm contact of the metallic parts with each other, the heat generated in the resistor is dissipated so rapidly and easily that the resistor is maintained at a comparatively low temperature for a given energy input, thus reducing the danger of the heater burning out and greatly improving the emciency of the unit due to the lack of need for any overpotential or excessive temperature gradient in order to produce the required dissipation of heat. Likewise the large dissipating surface enables a transfer of heat from the metal to the surrounding fluid with a smaller temperature difference than otherwise which is important in case of air-heating as tending to avoid the production of any unpleasant odor, and is also important in connection with the heating of other fluids in that the tendency to decomposition of the fluid is reduced.

I have shown my improvements as applied to a sheathed strip heater of the type shown in my 55 Patents Nos. 1,614,330, issued January 11, 1927,

and 1,614,938, issued January 18, 1927, although my invention is not limited to this particular form of heating element. However, I prefer to employ an electrical resistor which is close- 0 ly embedded in a refractory insulating material having good heat conductivity and the whole being covered by a metal sheath or casing of an appropriate contour, although my invention is not limited to any particular form of sheath or to 65 any particular insulating material.

In the drawings accompanying and forming a part of this specification, Fig. 1 is a view partly in section and partly in elevation showing one form of heating unit with parts broken away to 7 disclose the interior construction of the heating element proper; Fig. 2 is a vertical sectional view on the line 22 of Fig. 1; Fig. 3 is a view in front elevation of a heating unit made up of a plurality of strip heaters arranged within a single casing;

7 Fig. 4 is a vertical sectional view on the line 4-4 of Fig. 3; Fig. 5 is a fragmentary view in front elevation showing a plurality of units such as shown in Fig. 3 secured together contiguous to each other; and Fig. 6 is a fragmentary top plan view of the apparatus shown in Fig. 5.

Referring now to the drawings, each heating element preferably consists of an elongated sheet metal sheath i which may be formed of iron, steel, copper, brass or any other suitable material and which is relatively thin with respect to its length and in which is arranged a resistance wire or ribbon 2 which is preferably embedded in a mass of granular refractory insulating heat conducting material '3 which is tightly compacted about the resistor. The resistor may be in the form of a ribbon or a helically wound coil and preferably has a pair of terminals 4' and 5 disposed at one end of the sheath as shown most clearly in Fig. 1, although variations in the particular form of the resistor and the arrangement of the terminals may readily be made without departing from the spirit of my invention. The top of the metal sheath is preferably closed by means of a cover 8 which is held in place by the bentover side edges of the lower half of the sheath. Surrounding the metal sheath and arranged in spaced relation thereon are a plurality of metal fins I shown in detail in Fig. 2. Each fin is preferably shaped to provide a radially disposed plate portion and a laterally extending somewhat flattened tubular flanged portion which has extended metal contact with the shell or sheath and with a similarly shaped portion of an adjacent fin. The flanged portion of each fin is shouldered and overlaps a similar flanged portion of an adjacent fin, as shown most clearly in Fig. 3. Each fin engages and preferably interlocks with an adjacent fin so as to intercept all heat passing outwardly from the resistor and so as to secure the whole fin structure in place. The fins are preferably arranged transverse to the major axis of the element. The end fins are preferably rigidly secured to the sheath by being welded thereto. The thickness ,of the sheath is designated by the character a. Each fin is ofsuch size and so proportioned with respect to the sheath that the distance from the surface of the longer side of the sheath to the adjacent outer edge of the fin, which is designated b, is the same as the distance from the opposite side of the sheath to the opposite edge of the fin. The distance b is preferably from one to two times the distance a. The distance from the surface of the shorter side or narrow edge of the sheath to the outer edges of the fin is designated by the reference character b'. However, the distance D is not as important as the distance b. The maximum dimension of b should be such that the temperature of the fin immediately adjacent the sheath, which we will call T, never exceeds twice the temperature at the outer edges of the fin. which we will call T, or in other words T should never be less than The maximum for the distance b is also such that T should not greatly exceed 2T. These dis tanoes b and b are preferably, though not necessarily, the same but the ratio between the distance b and the thickness of the sheath remains constant. In most cases I have found that better results are obtained when the relationship between the sheath and fin is as shownin Fig. 2, though it is quite possible to obtain quite efilcient diffusion of heat if the sheath and fin have such as shown in Figs. 1 to 3 inclusive.

the relationship shown in Figs. 4 to 'l inclusive. The distance between adjacent fins is preferably about one-half the thickness of the sheath and one-eighth the width of the fins. 4 The length of the fins is not of extreme importance. v

While I do not limit myself to the exact proportions herein mentioned, I have found that the best results are obtained if these proportions and relationships are substantially adhered to. It is also desirable that the fins have a close and somewhat extended metai-to-metal contact with the sheath, substantially as shown in the drawings, although I do not intend to limit my invention to these exact proportions.

In Figs. 4 and 5, I have disclosed a heating element made up of a plurality of strip heaters,

This element comprises an upper header member 8 and a lower header member 9 which are preferably formed of sheet metal and shaped in section as shown most clearly in Fig. 4. The adjacent faces of the header members are provided with oppositely disposed slots 8 and 9 which are disposed in alignment with each other and adapted to receive therein the strip heaters which are designated by the reference character I. Closing the open sides of the upper and lower headers are a pair of closure members i0 and H which may be held in place in any suitable manner and are readily removable to permit insertion of the strip heaters in the casing or removal therefrom. I prefer to provide a single unit or casing and disposed therein is an electrically operated fan which is adapted to blow air across the heating elements. The upper and lower headers 8 and 9 are connected together and held in spaced relation by means of a pair of oppositely disposed plates or supports I2 and i3 which are secured in place in any suitable manner.

As shown in Figs. 3 and 4, one of the fins I provided on each strip heater I, in this instance the lowermost fin I, rests on the top of the lower header member 9. The uppermost one of the fins I is spaced from and below the bottom of the upper header member 8 as shown in Figs. 3 and 4. It is obvious that the construction illustrated will permit the strip heaters to expand longitudinally when heated.

The fins are so disposed and arranged as to provide a plurality of transverse ducts which extend parallel to the length of the fins and across the longer sides of the strip heaters and through which air is delivered by the fan ll. Suitable thermostatic controls (not shown) may be provided for cutting off the supply of current to the fan or heaters under predetermined conditions. A heating unit of this character is particularly well adapted for heating isolated buildings or rooms in place of auxiliary double steam unit heaters, or for heating rooms periodically to eliminate the need of maintaining steam or for night-time operation on off-peak power, permitting steam boiler fires to be banked.

These heaters I prefer to term unit heaters as each unit is separately and independently operable, as desired and is especially adapted for use for industraial process work and similar applications and can he installed, if desired, in air circulating ducts for heating air under forced circulation.

In Figs. 5 and 6, I have disclosed a plurality of such heating units which are indicated by the reference character A arranged side by side and immediately adjacent each other. It is of course obvious that these heaters may be arranged in thus making it possible and practical to make use of any number of these units in combination to fit any size opening. These units are of standard size and may be assembled at the place of installation. The strip heaters which make up the individual units are also readily removable and replaceable so that strip heateqs of difi'ei'ent ratings both. as to wattage and voltage, may be used.

I have observed that when a heater of the type described is subjected to a forced draft of air, there is an abrupt rise in the quantity of heat dissipated by radiation as distinguished from heat dissipated by convection at a fin temperature more than 300 F. above room temperature. While the reason for this is not certain, it seems likely that it may be due to a flattening of the radiation curve from zero to the critical point by reason of radiant heat being transferred to air particles which are rapidly carried away so that there is always relatively cold air presented to absorb the radiant heat from the heater. This effect appears to reach a maximum at air velocities commonly used with heaters of this character and at about the critical temperature mentioned above. In other words, when the temperature of the fins greatly exceeds 300 F., the amount of heat d ssipated by radiation rapidly increases while the amount of heat diffused by convection does not increase so rapidly with the rise in temperature. To obtain the most efilcient results, it is d- :irable to have such a fin width that the temperature difference on each fin immediately adjacent the sheath and at the outer side edges thereof shall not greatly exceed one-fifth the absolute temperature of the fin immediately adjacent the sheath when the heater is in normal operation. If the width of the fin is materially increased, it will not materially affect the amount of heat diffused but will not be economical and will result in a waste of material as well as space. For practical purposes, I propose to regulate the width of the fin such that the temperature on the outer side edges thereof shall not greatly exceed 150 F. above room temperature and the temperature on the inner edge thereof immediately adjacent the sheath shall not greatly exceed 300 F. above room temperature. In other words, I propose to maintain a maximum fin temperature below an elevation above room temperature where a critical increase in heat dissipation by radiation occurs.

Such construction and arrangement as I have described enables me to convert the maximum amount of energy within a minimum cubical space and with a minimum temperature gradient between the heat source and the convecting medium which enables me to diffuse a maximum amount of heat with a minimum resistor temperature. I am also able to materially reduce the weight and area of metal required for a given capacity heating unit. By such an arrangement I am able to reduce the weight of metal to 1 pounds per kilowatt or even lower.

By the above construction and arrangement and with the relationship between the sheath and the fins as herein described, it is possible to convert and transfer as much as 25 kilowatts electrical energy per cubic foot of space occupied by the heating element without exceeding permissible limitations as to resistor and sheath temperature and air temperature and velocity, and without exceeding permissible overall apparatus temperature; and this is without exceeding two rows or layers oi heating elements in any one direction. -withsuch a construction I am able to difluse as much as 10 kilowatts per square foot of projected area oi the heat emitting iac oi the heating apparatus. 7

It will now be clear that I have provided an electrical heating unit which will accomplish the objects of the invention as hereinbeiore stated. It will of course be understood that the embodiments oi the invention herein disclosed are merely illustrative and are not to be considered in a limiting sense as various changes may be made in the details of construction as well as in the arrangement oi parts without departing from the spirit of my invention as my invention is limited only in accordance with the scope of the appended claims.

Having thus described by invention, what I claim is:

1. An electric unit heater, comprising: a pinrality of electric heating elements; and mounting means for said elements, including a casing in which said elements are disposed; each of said elements having an elongated body portion, each body portion having transverse fin means disposed along the length of the body portion and extending laterally from the .body portion; said mounting means being constructed and arranged to mount said elements in spaced relation, free to expand longitudinally, said mounting means comprising'spaced supporting members each provided with a series of recesses, the recesses of at least one series being in the form of open-end slots, the two series together forming a series pairs of recesses each pair receiving the body portion of one of said elements beyond the longitudinal endmost portions of said fins.

2. An electric unit heater, comprising: a'. pluaoa aoo rality of electrical heating elements; and mounting means ior said elements, including a casing in which said elements are disposed; each of said elements having a fiat, elongatedymetal-sheath body portion of the strip-heater type, each body portion having transverse fin means disposed along the length of the body portion and extending laterally from the body portion; said mounting means being constructed and arranged to mount said elements in laterally spaced relation, tree to expand longitudinally, said mounting means comprising spaced supporting members each provided with a series of recesses, the recesses 0! at least one series being in the form 0! open-end slots, the two series together forming a series of pairs oi recesses each .pair receiving the body portion of one of said elements beyond the longitudinal endmost portions of said fin means.

3. An electric unit heater, comprising: a plurality of electric heating elements; and mount-- ing means for said elements, including a casing in which said elements are disposed; each oi said elements having an elongatedbody portion,

each body portion having transverse fin meansdisposed along the length of the body portion and extending laterally from the body portion; said mounting means being constructed and arranged to mount said elements substantially vertically in spaced relation, tree to expand longitudinally, said mounting means comprising ver tically spaced supporting members each provided with a series of apertures, the two series together forming a series 01 pairs of apertures each pair receiving the body portion of one of said elements; each of said elements having said fin means constructed and arranged to engage the lower one of said supporting members to support the element longitudinally at the lower end only.

EDWIN L WIEGAND.

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