US1841361A - Air heater and method of making the same - Google Patents

Description

Jan. 19, 1932. c BULKELEY 1,841,361

AI'R HEATER AND METHOD OF MAKING THE SAME Filed Nov. 14, 1928 4 sheat s-Sheet 1 yer Jan. 19, 1932. c. A. BULKELEY AIR HEATER AND METHOD OF MAKING THE SAME Filed Nov. 14, 1928 4 Sheets-Sheet 2 gmxmcoi C61 4 afiwang awommq S Jan. 19, 1932. Q BULKELEY 1,841,361

AIR HEATER AND METHOD OF MAKING THE SAME Filed Nov. 14. 1928 4 Sheets-Sheet 4 D far D D l D c xi f?) B 17 p 3 a' Q A y 4 I 60 Y Y s. Z% .7; 67 v 7 j x J H I 6' gwmntoz J as * H H *r r T C/wd A My 2 \g 1 zz I 0W1 010% aflozncqs Patented Jan. 19, 1932 UNITED STATES PATENT OFFICE CLAUDE A BULl iELEY, OF KENMORE, NEW YORK, ASSIGNOR TO NIAGARA BLOWER COMPANY, OF BUFFALO, NEW YORK, A CORPORATION OF NEW YORK AIR HEATER AND METHOD OF MAKING THE SAME Application filed November 14, 1928. Serial No. 319,434.

This invention relates to an air heater and more particularly to a heater embodied in a unitary structure in which the air to be heated is drawn over heated surfaces by a motor driven fan and subsequently discharged into the room, although the principles of the present invention can also be embodied in other analogous devices where heat transfer occurs, such as in air conditioners or the like.

One of the objects of this invention is to provide a heater assembly composed of tubes and headers in which a materially increased heat exchange is effected as compared with similar structures now in use, thereby permitting of employing more compact assemblies for given requirements and economizing in the size of the complete heater and the space occupied by the same.

Another important object of this invention is to provide a heater composed of a plurality of pipes around which the air to be heated is drawn .by a motor driven fan in which a. greater rise in temperature of the same amount of air passing through the coil at the same velocity is effected without increasing the friction tothe flow of air passing through the coil and without increasing the face area or face dimensions of the coil. By this means, with a demand for heaters of varying capacity the manufacturer can employ the identical shell, motor and fan in each heater and by varying the heating coil in accordance with the present invention,can provide heaters of the different capacities desired. Such heaters are of identical dimensions and consequently are received interchangeably in the same heater casings and since they have the same resistance to the flow of air, the same fans and motors can be used for each. It is also obvious that should the owner desire a heater of greater capacity, it is only necessary to replace the heating coil to effect this result.

A further aim is to provide such a coil which is composed of hair pin tubes connecting the two headers in which the tubes are so formed as to provide a proper gradient for each tube regardless of whether the tube bundle or assembly is arranged horizontally or vertically, thereby permitting of the manufacture of one coil to meet both conditions.

A further aim is to provide a hair-pin tube bundle or assembly in which the ends of the tubes are connected to a single tube plate and separate headers are provided. which are separately welded to the tube plate. By this means a heater is provided which has the manufacturing and structural advantages of a single tube plate, and all portions of the contacting edges of the headers are readily accessible for properly welding the headers to the tube plate.

Another object is to provide a plate which receives the outer bends of the hair-pin pipes and holds them in fixed relation to each other and also serves as a support for a cover or box which covers these bends and prevents the flow of air past the same, the heating effect of these bends being relatively negligible. By this means the proper spatial relation of the pipes is maintained to secure the advantages of an increased heat transfer for a coil of given face dimensions and the tube bundle as a whole is strengthened. The covering or box also forms a support for that end of the bundle for slidingly supporting the coil on rails or tracks in the heater shell so that the tube bundle can be readily slid out of the same when repairs on the same are necessary.

A further aim is to provide an enclosed heater of the character described in which the tube bundle or assembly is supported on angle bars along the sides of the shell so that upon removing the pipe connections, the bundle can be slid out as a unit from the shell for the purpose of repairing either the tube assem ly or the shell.

Another purpose is to provide a coil assembly, composed of a tube plate, hair-pin tubes welded to the tube plate, headers welded to the tube plate and boxes surrounding the headers and the bends of the hair-pin tubes in which each of these elements is made of aluminum. By this means not only is a lighter and substantially non-corroding assembly provided, but it is also materially less expensive both in cost of materials and in assembly and fabrication.

In the accompanying drawings:

Figure 1 is a vertical transverse section through the shell of a heater embodying my invention.

Figure 2 is a side elevation thereof showing a part of the shell broken away and disclosing the heater bundle or assembly.

Figure 3 is a top plan view of the tube bundle or heater assembly, showing the same removed from the shell.

Figure 4 is a vertical longitudinal section through a heater shell showing the present invention applied to a ceiling type of heater in which the heater shell is supported horizontally from the ceiling.

Figure 5 is a View similar to Fig. 3, showing indetail the manner in which the bends of the tube and the headers are enclosed to prevent the flow of air past the same, and to support the bundle and maintain the proper spatial relation of the tubes.

Figure 6 is a side elevation thereof.

Figure 7 is a vertical section, taken on line 77, Fig. 5.

Figure 8 is a view similar to Fig. 7 but showing the invention applied to a bundle having a single row of hair-pin tubes.

Figure 9 is a vertical section through the tube plate and hairpin tubes of a tube bundle or assembly embodying my invention, a single series of hair-pin tubes being shown and the section taken being parallel to the directional flow of air through the bundle, this form of the coil being designated as coil 1.

Figure 10 is a horizontal section of the structure shown in Fig. 9, the section being at right angles to the directional flow of air through the bundle.

Figure 11 is a diagrammatic end view of the structure shown in Figs. 9 and 10.

Figures 12, 13 and 14 are views similar to Figs. 9, 10 and 11, respectively, and showing the invention applied to a tube bundle having two staggered series of hair-pin tubes, this form of the coil designated as coil 2.

Fig. 15 is a view similar to Fig. 11 showing a simplified form of heating surface in which the hairpin tubes are substituted by straight tubes.

Fig. 16 is a view similar to Fig. 14 showing a simplified form of heating surface in which the hairpin tubes are substituted by straight tubes.

Similar reference characters refer to similar parts in each of the several views.

In its general organization the form of heater shown in the accompanying drawings comprises a shell or casing which is open at its lower end, a slidingly removable heater assembly composed of hairpin tubes which may or may not be provided with fin surfaces and which are welded to a tube plate on the opposite sides of which separate headers with pipe connections are welded, a fan housing in the upper part of the shell, a motor driven fan in the fan housing and an outlet through which the air is discharged into the room. The invention is shown as a plied to a single fan unit, although it is obvious that it may also be embodied in multi-unit heaters and may also be embodied in analogous air heating or cooling devices, such as air conditioners or the like.

The shellor casing which carries the fan and heater assembly is composed of side panels 15, end panels 16, and a top plate 17, these panels and plate being secured to angle frame bars 18, the vertical corner bars of whlch are extended downwardly to form legs 19. The l( wer end of the shell is open to provide an air inlet 20.

In one end wall 16, a rectangular opening is provided which is covered by a removable cover plate 21, and on the inner sides of the side panels 15, angle bars 22 are secured. These angle bars extend the full length of the shell and incline upwardly from the lower edge of the opening covered by the cover plate 21 and form parallel rails for removably supporting the heater assembly indicated generally at 23.

Suitably mounted in the upper part of the shell is a fan housing 24 having two or more eyes 25 through which air is drawn from the suction chamber 26 of the shell and discharged through an outlet 27 by a fan 28. This fan is mounted on a fan shaft 29 which is journaled in suitable bearings -30 on the shell and is driven by an electric motor 31 mounted on a suitable bracket.

The heater assembly, indicated generally at 23 is generally constructed as follows:

The numeral 32 represents an aluminum tube plate which is of rectangular form and extends substantially from one side panel 15 to the opposite side panel and rests at its lower corners on the side rails or bars 22. In the upper and lower sides of these tube plates parallel series of holes 33 are provided which holes receive the opposite ends of aluminum hair-pin tubes 34.

These hairpin tubes are welded to the tube plates as indicated at 35 and extend substantially to the opposite end panel 16 as indicated in Fig. 2 and each leg 36 and 37 thereof may or may not be provided with an aluminum fin surface, such as the aluminum spiral fins 38. The hairpin tubes may be arranged to provide a single series as indicated in Figs. 8-11, inclusive, or they may be arranged to form two series 60 and 61 in staggered arrangement relative to one another, as indicated in Figs. 2, 5, 6, 7, and 12-14, inclusive, in the latter form the tubes of one series being arranged between the tubes of the other series and the two series being ofi'set relative to one another. This latter form provides an increased rise in temperature of the same quantity of air passing through the coil without increase in the face area or friction of the coil as hereinafter described.

On the side of the tube plate 32 opposite to that from which the hairpin tubes extend, an upper header 39 and a lower header 40 are welded. These headers are made of cast or pressed aluminum and the upper header encloses the ends of the upper legs 36 of the hairpin tubes and is welded at the edges of its open side to the face of the tube sheet 32, while the lower header 4O similarly encloses the ends of the lower legs 37 of the hairpin tubes. The upper header 39 is formed to provide a steam inlet nipple 41 which extends through an opening provided in the cover plate 21 and the lower header 40 is formed to provide an outlet 42 which also extends through the cover plate 21. The 1nlet-41 and the outlet 42 are adapted to be connected so that steam enters the upper header 39, traverses the upper legs 36 of the hairpin tubes, passes around the bends 43 thereof, through the lower legs 37 and out through the lower header 40 and outlet 42.

It will be noted that by providing a slngle tube plate, the ends of the tubes will be held in fixed proper relation to one another and also provide a rigid and strong support for the tubes and by the provision of separate headers, the edges of the same can be easily welded to the tube plate and a strong and tight joint effected.

In order to hold the outer ends of the hampin tubes in a definite relation to one another, an aluminum plate 44 is provlded which has a plurality of vertical slots 45 conforming in spacing and number to the tubes. The bend'43 in each tube is unprovided with fins and upon completing the assembly of the header, tube sheet and tubes. the slotted plate 44 is slipped over the bends of the tubes so that each bend is arranged in one of the slots. Each leg of each tube is then welded at its outer side to the slotted plate as indicated at 46 in Figs. 5 and 6, and the tubes at their bends are thereby held in rigid relation to one another. JVhen a slotted plate 44 is made for a heater having a single series of hairpin tubes, such as shown in Figs. 911, only one series of spaced slots 45 is provided in the slotted plate, as shown in Fig. 8. When, a slotted plate is made for a heater assembly in which two series of offset or relatively staggered hairpin tubes are employed, the slotted plate is provided with two series of slots 45 which are offset or staggered relatively to each other and each series of slots receiving the bends of the corresponding, series of tubes. .By this means, it is apparent that the same relative spatial relation of the legs of the hairpin tubes can be maintained throughout the length of the tube, this spatial relation being of prime importance in the present invention as hereinafter more fully explained.

The extensions of the heater assembly beyond the tube plate 32 and the slotted plate 44 have little heating effect on the air and in order to secure the maximum transfer of heat to the air passing through the coils, it is necessary to prevent the passage of the air past the headers and tube bends by the provision of suitable boxes or filler-s. As best shown in Figs. 5 and 6, the boxing oil of the space around the headers is effected by suitably welding an aluminum box 47 to the rear side and at themargin of the tube plate 32. This box extends rearwardly from the tube sheet and in the installed position of the heater assembly lies within the frame formed by the angle irons 48 around the heater assembly removal opening which is covered by the plate 21. It therefore follows that none of the air passing up through the shell can pass between the tube sheet 32 and the adjacent end wall of the shell and that the air on this side is compelled to pass through the finned legs of the tubes.

In a similar manner an aluminum box 49 is welded at its edges to the rear face of the slotted plate 44, as indicated in Figs. 5-8, and in the installed position of the assembly, the rear wall of this box lays against the panel 16, as shown in Fig. 2. It will be observed that the boxes formed by the tube plate 32 and the box 47 and the box formed by the slotted plate 44 and the box 49, besides operating to enclose the relatively non-heating parts of the assembly, also rest on the angle iron rails 22 and slidingly support the other parts of the assembly thereon. These boxes also make the assembly adequately rigid so that upon removing the pipe connections of the heater and the cover plate. 21, the entire assembly can he slid out on the side rails 22 and removed from the casing without danger of distorting or otherwise injuring the same. These boxes also permit the assembly to be supported on the floor after being removed without danger of injuring the fin surfaces on the tubes.

By making the entire heater assembly composed of the hairpin tubes and their fin surfacing, the tube plate, the headers, the slotted plate and the boxes attached to the tube and slotted plates of aluminum, it has been found that not only is a lighter assembly provided which is substantially non=corroding, but also that the heater is less expensive both in point of the actual cost of material used and the fabrication thereof than when combinations of iron, steel. or copper, are employed in making an assembly as heretofore employed. The aluminum is very ductile which permits it to be readily formed into the desired shapes, and it is easily welded together and provides a strong joint. Moreover, by the provision of a heater made throughout of the same material, all parts of the heater have the same .coeflicient of expansion and hence internal expansion strains on different parts of the heater assembly are eliminated.

In Fig. 4 is illustrated a heater of the ceiling type embodying the present invention.

In this form the shell is provided with a bottom panel 50, side panels 51, a top panel 52 and an end panel 53. The shell is supported from theceiling by suitable hangers 54 and is left open at one end to provide an air inlet opening 55. A fan housing 56 is arranged in one end of the shell and a motor driven fan 57 draws the air through the shell and discharges it through an outlet 58 in the end wall of the shell. A heater assembly 23, similar in all respects to the heater assembly in the fioor type of heater is employed except that the headers, tube plate and box are in a vertical position and the tubes are arranged in vertical alinement. In this form converging angle bars 59 can be provided for confining the heater assembly in its proper position in the shell. In the ceiling form, the heater assembly is removed through an opening (not shown) provided in the side panel 51 instead of through the end wall as in the floor type of heater.

The present invention also proposes a construction of header and hairpin tubes in which the maximum drainage gradient is provided regardless of whether the two legs of the hairpin tubes are arranged in a generally vertical plane (as indicated in Figs. 9 and 12) or in a generally horizontal plane (as indicated in Figs. 10 and 13). To accomplish this the legs 36 and 37 are spread apart so that they converge toward the bend 43 of the tube. this spreading apart being indicated by the distance in Figs. 9, 11, 12 and 14, and thereafter the bend 43 of the tube is put in a jig, the leg 36 is bent laterally, (or at right angles to the plane of the bend 43) in one direction and the leg 37 bent in the opposite direction. This last bending offsets each of the two legs, relative to the plane of the bend, the distance of and offsets the outer or free ends of the two legs the distance of M, as indicated in Figs. 10, 11, 13 and 15.

When the hairpin tubes are arranged in a vertical plane, as in Figs. 9 and 12, the drainage of condensate presents no problem. The legs are spread apart a distance J-H which is sufiicient to insure that the water condensing in the upper leg 36 flows freely down that leg. around the vertically disposed bend 43 and down the lower leg 37. Should the gradients not drain properly, the legs of the tubes can obviously be spread further apart.

When, however, the tube sheet is placed on end and the hair-pin tubes arranged in generally horizontal planes, it is obvious that drainage gradients become a distinct problem. Obviously, if the legs 36 and 37- were not ofiset laterally at all, there would be no drainage at all since both-legs and the bend 43 of each hairpin tube would be arranged in a horizontal plane. If the legs 36 and 37 are merely oilset laterally in opposite directions, some drainage gradient will result, but this drainage gradient will be very small, the greater part of the distance M available for this gradient being consumed by the bend 43 which represents but a fraction of the length of the tube. Upon now, however, twisting the bend 43 back to its original plane, in accordance with the present invention and as shown in the drawings, it is apparent that the relatively short bend 43 is arranged entirely horizontal and has no drainage gradient and the full distance M is employed as the drainage gradient for the two legs 36 and 37 Thus by holding the bend 43 fast in a jig and bending the legs 36 and 37 laterally outward relative to the plane of the bend, the whole distance M is available for the drainage of condensate from the two legs. This would not be true if the bend 43 were not so held as then the bend 43 would also twist and consume the greater part of the gradient distance M. This feature of the invention is, of course, only important when the tubes and header are so placed that the tubes are in generally horizontal planes and has no application whatsoever when the tubes and header are placed so that the tubes are in generally vertical planes. 1

The increase of drainage gradients for the legs of hairpin tubes arrangedin a generally horizontal position, as just described, can be clearly observed by manipulating an ordinary hairpin. Take a hairpin and arrange it horizontally and it will be observed that there is no gradient to either of the legs, all parts of the hairpin being arranged in a horizontal plane. Then, without holding the bend of the hairpin, move one leg up and the other leg down, and it will be observed that both legs and the bend are given a gradient but that by far the greater part of this gradient is in the bend. Now, holding the legs, twist the bend only of the hairpin back to its original horizontal position and while so twisting the bend, observe that the gradient of the legs is greatly increased, the full gradient formerly consumed by the bend being transferred to the legs. Thus the present inven-. tion, when the hairpin is arranged in a generally horizontal plane, increases the gradient of the legs at the expense of the bend.

Since, in its application to heating tubing, not only does the bend form only a small part of the length of the tube but also forms no part of the heating surface (the bends being encased, see Fig. 5) it is apparent that the gradient of the legs is paramount and that the existence or non-existence of a gradient in the bends is immaterial.

The invention also comprehends the production of a tube and header heater which can be constructed to provide, within limits, any desired heat transfer while still maintaining constant face dimensions and a constant resistance of the tubes to the passage of air.

In unit heaters standardization is desired wherever possible in order to obtain the advantages of mass production. Thus for the full line of heaters, it is desirable to have but one form and size heater shell or casing equipped with one form and size of fan and motor. At the same time the trade demands heaters of different capacity. To meet the demands of the trade and still obtain standardization in the construction of the heater casings, motors and fans, it is apparent that provision must be made to supply heating coils of constant face dimensions (each have ing the same dimensions P and C, since otherwise the dimensions of the casing would have to be altered) and the heating coils must also each have the same resistance to the flow of air (since otherwise fans and motors of different capacities would have to be provided) and at the same time each of the coils must be so designed as to provide the desired heat transfer required by the particular customer.

The accomplishment of this involves two principles of heating as applied to air heaters of the present type. The amount of heat transferred varies directly as the increase in the number of coils or pipes and the friction of the pipes varies inversely as the square of the velocity of the air. In other words the greater the number of pipes, the greater, proportionally, the heat transfer, and by decreasing the velocity of the air, the decrease in the surface friction of the pipes is squared.

For clarity Figs. 15 and 16 have been included, these figures corresponding to Figs. 11 and 14, respectively, but being simplified to eliminate the complications of hairpin tubing in considering this phase of the invention. In

the simplified form shown in Figs. 15 and 16,-

the tubes 34a are assumed to be straight tubes running between tube sheets 32-a, the size of the tubes 34a and the face dimensions of the heater (the distances P and C, Fi s. 10 and 13) being identical.

onsidering Fig. 15 which has a single row of pipes 34a, and assuming that it is desired to increase its heating capacity, it is obvious that this could be done by adding another pipe 34w. Assume that this is done by moving all of the pipes 34a closer together and adding the new pipe at one end of the series. We obtain the desired increase in heating effect in the same face dimensions of the heater (the dimensions P and C, Figs. 10 and 13) but the spaces between the pipes 34a, or free area of the heater is decreased so that the resistance of the coil is increased and a fan of greater power and capacity must be employed to draw the same amount of air through.

Assume now that instead of doing that, We lengthen the tube sheet, add the new tube 34-a at the end and at the same time space all of the tubes 34a a definite distance further apart. The desired heat increase is now obtained by the addition of the new pipe 34.a and the resistance of the pipes is maintained constant since we have spaced the pipes far enough apart to reduce the velocity of the air passing between them just enough to reduce the friction of the surfaces of these pipes to a point where the friction of the new number of pipes is equal to the friction of the old number of pipes. In other words, by spacing the pipes further apart we square the reduction in their friction to the flow of air to exactly counteract the added friction of the single pipe we have added.

We still have not achieved our desired object, however, as by carrying out the latter plan we have increased the face dimensions of the heater both by adding another pipe 84a and by spacing it and the original pipes further apart. Our remaining problem, therefore is to add another pipe 34a, space all of the pipes further apart to square the reduction in their friction to the flow of air to exactly counteract the added friction of the single pipe we have added and to do this within the same face dimensions of the original heater. The obvious solution to this final problem is to stagger the pipes 34a as indicated in Fig. 16.

The heater in Fig. 16 has the same dimensions P and G as that shown in Fig. 15 and the frictional resistance of the pipes 34-a to the air flow in Fig. 16 is exactly the same as that in Fig. 15. To make the heating surface shown in Fig. 16, we have added four p'pes 34-a to the eight pipes 34-a shown in Fig. 15.

In doing so, we have increased the spaces D and the spaces Y so that the increase in the sum of the spaces Y squares the reduction in the friction to the flow of air to exactly counteract the added friction of the four pipes 34a which we have added. At the same time we have not increased the face dimensions (G and P, Figs. 10 and 13) by the simple expedient of staggering the pipes 34ca to secure both an increase in their number and an increase in their zig-zag spacing. We therefore have achieved our desired object, namely, an increase in heat transfer without increase in the resistance of the pipes and without increase in the face dimensions of the heater.

Obviously with the new surface, the sum of the distances Y must be greater than the sum of the distances D since the distances Y must determine the resistance of the surface and not the distances D. It is also apparent that the invention can be carried out with hairpin tubes as illustrated by Figs. 11 and 14 which correspond to Figs. 15 and 16.

As a whole this invention provides a simple and inexpensive heater in which a rise in temperature of the air delivered can be secured by providing a coil made as described without altering the face area or dimensions of the same or without increasing the resistance of the coil to the flow of air. The coil also is arranged to provide an adequate gradient for the condensate in either a vertical or horizontal position, its non-heating surfaces are adequately boxed in, the coils are securely held in fixed relation to each other and form a rigid structure, and the coil is so supported in the shell as to be readily removable therefrom. Furthermore, by making the entire assembly of aluminum, not only is a light and non-corroding structure provided, but the same is more easily fabricated and can be produced at a lower cost.

I claim as my invention:

1. A hairpin tube of the character described wherein the legs of the tube are offset laterally in opposite directions relative to the plane of the bend, thereby to provide a relatively large gradient in said legs when said bend is horizontally disposed.

2. A hairpin tube of the character described comprising a bend and divergent legls,

- said legs being laterally offset substantia perpendicular relative to the plane of said bend, thereby to provide a relatively large adient in said legs when said bend is horizontally disposed.

3. A coil assembly comprising headers, means connecting the legs of a series of hairpin tubes with said headers, the centers of the bends of said tubes lying in the same straight line and the legs of each of said tubes being divergent from the plane of said bend, the corresponding legs of the tubes being uniformly laterally offset in one direction relative to said planes and the other legs being uniformly laterally ofiset relative to said planes and in the opposite direction relative to said first named legs whereby when said bends are horizontally disposed a ielatively large gradient is provided in said egs.

4. A coil assembly comprising a tube plate,

hairpin tubes extending laterally from oneface of said tube plate, the legs of said tubes extending through said tube plate, headers secured to the opposite side of said tube plate and enclosing the ends of said legs, the centers of the bends of said tubes lying in the same straight line and the legs of each of said tubes being divergent from the plane of said bend, the corresponding legs of each tube being uniformly laterally offset in one direction relative to said planes and the other legs bein uniformly laterally ofi- I:

a second independent'header welded at its edges to said tube plate and enclosing the ends of the other legs of said tubes, and an inlet and outlet for said headers.

6. A coil assembly including a series of hair pin tubes, headers connected with the legs of said tubes, and a plate at the outer ends of said tubes and holding said tube ends in fixed spaced relation.

7. A coil assembly including a series of hairpin tubes, headers connected with the legs of said tubes, and a slotted plate engaging the outer ends of said tubes and holding said tube ends in fixed spaced relation, the bends of said tubes extending through said slots and being welded to said plate.

8. A heater including a shell, a coil assembly in said shell, and means for forcin air through said shell and over said coi, said coil assembly including a tube plate, hairpin tubes extending outwardlyfrorn one side of said tube plate, headers arranged on the opposite side of said tube plate, and enclosing the ends of said tubes, an inlet and outletfor said headers, a slotted plate at the outer ends of said hairpin tubes, the bends of said tubes extending through said slots and being welded to said slotted plate,

a wall projecting outwardly from said tube plate and preventing the flow of air past said headers, and a second wall projecting outwardly from said slotted plate and preventing the flow of air past said bends.

9. A heater including a shell, a coil assembly in said shell, and means for forcing air through said shell and'over said coil, said coil assembly including a tube plate, hairpin tubes extending outwardly from one side of said tube plate, headers arranged on the opposite side of said tube plate, and enclosing the ends of said tubes, an inlet and outlet for said headers, a slotted plate at the outer ends of said hairpin tubes, the bends of said tubes extending through said slots and being welded to said slotted plate, a wall projecting outwardly from said tube plate and preventing the flow of air past said headers, a

tube plate and slotted plate and the walls connected thereto to removably support said coil assembly in said shell.

10. A heater including a shell, a coil assembly in said shell, and means for forcing air through said shell and over said coil, said coil assembly including a tube plate, hairpin tubes extending outwardly from one side of said tube plate, headers arranged on the opposite side of said tube plate, and enclosing the ends of said tubes, an inlet and outlet for said headers, a slotted plate at the outer ends of said hairpin tubes, the bends of said tubes extending through said slots and being welded to said slotted plate, a wall projecting outwardly from said tube plate and preventing the flow of air past said headers, and a second'wall projecting outwardly from said slotted plate and preventing the flow of air past said bends, and means for removably supporting said coil assembly in said shell comprising an opening provided in one wall of said shell, and angle bars secured to opposite sides of said shell and terminating at said opening, said angle bars being adapted to support the said tube plate and slotted plate together with their associated walls at their lower corners to permit said assembly to he slid out through said opening on said angle bars.

11. A coil assembly including a series of hairpin tubes, tube plate means welded to the ends of said tubes, headers welded to the tube plate means and enclosing the ends of said tubes, an inlet and outlet for said headers, and a plate welded to the other ends of said tubes, said elements being composed of aluminum.

12. A coil assembly including a series of hairpin tubes, tube plate means welded to the ends of said tubes, headers welded to the tube plate means and enclosing the ends of said tubes, an inlet and outlet for said headers, a wall welded to said tube plate means and projecting laterally therefrom to prevent the flow of air past said headers, a slotted plate at the outer ends of said tubes, the bends of said tubes being received in said slots and being welded to said slotted plate, and a wall welded to said slotted plate and projecting laterally therefrom to prevent the flow of air past said bends, said elements being composed of aluminum.

13. A heater composed of two adjacent series of spaced heating members through which series the fluid to be heated successively passes, the members of one series being staggered with reference to the members of the other series and the ratio between the number of heating elements and the staggered free area of the/members being such that the amount of staggered free area is increased substantially as the square root of the increase in the number of heating members and vice versa.

14. A heater composed of a series of spaced heating members through which the fluid to be heated passes, some of said members being arranged in advance of the others with reference to the fluid flow, and the ratio between the number of heating elements and said effective free area being such that the amount of effective free area is varied uniformly and directly substantially as the square root of the variation in the number of heating elements.

15. A heater composed of at least two series of spaced heating members, the members of each series being arranged substantially in the same plane and the two series being arranged substantially parallel and in proximity to one another with the members of one series staggered wit-h reference to the other series so that a planar free area exists between the members of each series and a zig-zag free area exists between the members of the two series, said elements being also so arranged that the planar free area exceeds the zig-zag free area, whereby the zig-zag free area forms substantially the only area affecting the resistance of the heater to the flow of the fluid to be heated therethrough and said zig-zag free area being accurately determined to provide the exact resistance desired.

16. A method of making heaters for air or the like which consists of mounting two series of heating elements in spaced relation with the members of one series staggered with relation to the members of the other series and in such manner that the ratio between the number of heating elements and the staggered free area between the members is such that the amount of said staggered free area varies substantially as the square root of the variation in the number of heating members.

In testimony whereof I hereby aflix my signature.

CLAUDE A. BULKELEY.

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