CA1116961A

CA1116961A - Boiler

Info

Publication number: CA1116961A
Application number: CA000326660A
Authority: CA
Inventors: James R. Hurley; Edward F. Searight
Original assignee: Thermo Electron Corp
Current assignee: Thermo Fisher Scientific Inc
Priority date: 1978-05-01
Filing date: 1979-04-30
Publication date: 1982-01-26
Also published as: JPS54146046A; CH644199A5; DE2917759C2; DE2917759A1; JPS6240626B2; IT7922203A0; GB2019986A; IT1112842B; GB2019986B; FR2425035B1; FR2425035A1

Abstract

BOILER

ABSTRACT

A boiler for heating liquid which combines a source of hot gases directed first along the inner wall of an annular tank in a tortuous path formed by fins disposed upon the inner wall and then along the outer wall of the tank through a jet-impingement heat transfer system.

Description

11~

_ACKGROUND OF THE INVEI~TIO~

Conservation of energy has become an objective of increasing concern in recent years and one obvious approach to that goal is to increase the efficiency of utilization of energy. Boilers and other heaters of liquid are so commonly used both in industry and residentially that any improvement in their efficiency would of course result in vast savings of energy, It has long been recognized that greater efficiency in liguid heating can be had by increasing the area of heat-exchan&ing surfaces between the source of heat and the liquid to be heated. Various designs of liquid tanks have been proposed and one which has shown promise is an annular tank.
Generally, in such designs, hot products of combustion in gaseous form are int~oduced into the cylinder formed by the interior wall of the annular tank. The hot gases pass over the interior wall which serves as a heat-exchange surface to heat the liquid in the tank. It has even been proposed that the ~ gases then be redirected over the exterior wall of the annular tank, in which case, both walls serve as heat transfer surfaces.
In some instances, fins, convoluted surfaces, or other area-increasing devices have been used to attempt to improve efficiency of heat transfer but such devices have been self-defeating to some extent because of their interference with the flow of hot gases, because of fabrication costs, and because of complexity of s~ructure, Hence, it is the primary

-2-1, , 111696i object of the present invention to maximize the benefits and the gains in efficiency of heat transfer which can be realized by combining with annular tanks efficient sources of hot gases and heat transfer enhancers which are relatively simple and inexpensive and which operate to avoid the effects of boundary-layer interference with heat transfer.

SUMMARY OF THE INVENTION

The invention is concerned generally with the heatin~
of liquids but relates more particularly to a highly efficient and compact boiler of simple and relatively inexpensive design.
The boiler includes as basic elements a source of hot gases such as a burner, an annular water tank and heat transfer enhancers to maximize the transfer of heat from the hot gases through the inner and outer walls of the annular tank to the water within the tank.
An interior chamber is formed at least in part by the inner wall of the tank and gases from the combustion taking place in the burner are forced along a tortuous path through that chamber by stacked groups of radial fins attached in heat conductive relationship to the inner wall. rlormally existing boundary layers along the fins and inner wall are disrupted by the turbulence imparted to the gases by the tortuous path set up by rotationally offsetting adjacent groups of fins.
After passing through the interior chamber the gases are redirected along the outer wall of the annular tank throuch jet impingement heat transfer devices in which elements havinr 1~69~:1 perforated walls form the gases into jets directed against adjacent unper-forated walls. Again, boundary layers are disrupted, heat transfer is en-hanced, and water heating is efficiently achieved.
According to a broad aspect of the present invention, there is provided a boiler comprising: an annular tank for holding liquid to be heated, said annular tank having concentric inner and outer walls serving as heat-exchange surfaces, an inner chamber being formed within said inner wall; an outer shell spaced from and surrounding said outer wall~ an outer chamber being formed between said outer shell and said outer wall; means including a burner for directing hot gases first through said inner chamber and then through said outer chamber; a plurality of heat-exchange enhancing fins disposed in said inner chamber in heat-conductive relationship to said inner wall; means disposed in said outer chamber for forming said hot gases into jets directed upon surfaces transferring heat to said liquid.
The invention will now be described in greater detail with refer-ence to the accompanying drawings, in which:
Figure 1 is a sectional elevation of a liquid heating system embodying the present invention;
Figure 2 is an end view of the system of Figure 1 with portions broken away to show certain interna] features o:f the system; and Figures 3 and 4 are fragmentary views showing construction methods and details for one of the heat transfer mechanisms useful in the embodiment of Figures 1 and 2.

`" 111~

DESCRIPTION OF A PREFERRED EMBODIMEIIT
_ In Figure 1, a preferred liquid heater is shown in the form of a water boiler enclosed in a double-walled insulated jacket 11, Basic working elements include an annular water tank 12l a burner 14 arranged to fire hot gases first through an i.nner chamber 16 defined in part by the inner wall 20 of the annular tank from which the gases proceed through an exterior chamber 18, defined in part by the outside yte?~l ol' c,h an~r ~ wall 22 of the annular tank 12. In the o~ there are disposed jet impingement heat transfer enhancing elements described in greater detail below.
The annular water tank 12 is formed into a closed container by concentric inner and outer cylindrical walls 20 \ and 22, an end plate 24 and an end cap 26. ~later~to be heatéd is introduced into the annular tank through an inlet~line 28 which is sealed through the end plate 24. Hot water passes, from the annular tank through an outlet line 30 séaled in similar fashion through the end cap 26, To prevent the ~low of water directly fro~ the inlet line 28 to the outlet line 30, a deflection plate 32 is disposed within the annular tank adjacent the point of entry of the inlet line 23. The deflection plate 32 is curved to conform to and is attached to the inner cylindrical wall 20 and extends radially toward the outer cylindrical wall 22. It has the shape of a section of a disc subtending an angle of about 90 and havin~ its central area disposed rouehly oPposite the inlet line 28.

The burner 14 may be of conventional design but is ~ preferably a screen burner to which fuel such as gas through an inlet 29 and air through a blower 31 may be fed for combustion. The outlet of the blower is sealed centrally through the end plate 24 adjacent a spark ignitor 40 which is also sealed through the end plate 24 and has electrodes 41 disposed adjacent the screen of the burner 14.
The inner chamber which is bounded by the inner wall 20 of the tank may be left with an open center, but it has proven effective to force hot gases toward the wall 20 by inserting at the center a baffle rod or bar of high-temperature insulating material 34. The insulating bar 34 is preferably tapered at its end which confronts the burner and the tapered end may serve to support and center the screen of the burner 14.
Upon the inner wall 20 are stacked arrays or groups of fins 36, 37, 38 and 39 which extend radially inwardly from that wall. Each group of fins may be eighteen in number, symmetrically arranged and spaced 20 apart around the periphery of the inner surface of the wall 20. The fins are composed of material of high heat conductivity and may be formed integr211y with, or welded or brazed in good heat conducting relationship with the inner wall 20.
The disposition of fin groups 36 and 37 is best seen in Fig. 2, and in the syste~ shown in Figs. 1 and 2 four such groups are employed. The fins of each group are rotationally offset from the adjacent group or groups. In Fig. 2, which ~ 1116961 illustrates the case of fins having 20~ separations, a desira~le offset of the fins of group 36 to those of group 37 is approxl-mately 10 as shown, The fins of the succeeding groups 38 and 39 are similarly offset by 10 from adjacent groups, The slots between the fins are in planes passing through the axis of the inner wall 20 but the staggered arrangement of each group or array relative to its neighboring group imposes a tortuous path upon the flow of hot gases and gives rise to turbulence which disrupts the boundary layers normally encount~red in the use of heat transfer surfaces. In other words, the fins are so spaced and arranged that resistan~e to the flow of gases axially through the chamber 16 is not significantly increased as would be the case with transverse fins extending radially inwardly from the inner wall 20. On the other hand, if there were no rotational offset and the fins of all grcups were aligned, the straight passage of gases would not disrupt the boundary layers sufficiently to promote good heat transfer.
~t the end of the annular tank 12 opposite the burne-, hot gases emerge from the inner chamber into a plenum 43 and are diverted outwardly and turned back by a deflector 45 into the external annular chamber 1~ formed by the inner wall of thL
insulating jacket 11 and the outer wall 22 of the tank 12.
Disposed within the external chamber 18 and extendin~
substantially along its full length are a series of jet imping -ment members, The precise design of these members may be vari-~
but the key design consideration is the formation of jets of hot gases which impinge upon heat transfer surfaces in such a shion as to disrupt boundary layers, as explained in greater I
~ I

detail below. In the specific structure shown in Fig. 2, however, walled channels are formed from a single length of sheet material which is perfor-ated and folded as illustrated in the fragmentary views of Figs. 3 and 4.
Specifically, in Fig. 3 there is shown a portion of a length of sheet meta] in which rows of regularly spaced holes are punched and fold lines are formed. The holes 51 of the first row are symmetrically offset from the holes 52 of the second row and the arrangement is repeated for succeeding rows. The length of material is then folded along the lines 53 and similar fold lines as shown in Fig. 4. The resulting corrugated or convoluted sheet is then wrapped about the outer tank wall 22 to form the rectangular channels mentioned above, the sheet being fastened or held tightly against the wall 22 to assure good heat-conducting relationship between sheet and wall. The channels may run substantially the full length of the wall 22 or several groups may be formed by sheets stacked one on another. For example, in the embodiment shown in Fig. 1, four sheets 54a, 54b, 54c and 54d are shown stacked in series. The desired jet-forming apertures and confronting unperforated heat-transfer surfaces may be con-structed in various ways. Applicable theoretical considerations and com-parable fabrication techniques are to be found in U.S. Patent No. 3,804,159, owned by the assignee of the present applicat:ion.
The important point is that the staggered array of holes must be so arranged that each hole in any given channel wall confronts an unperfor-ated wall in order that gases will be jetted upon the unperforated wall to disrupt boundary layers and increase the efficiency of heat transfer.
Reverting to Fig. 2, and referring to the typical channels 57 and 58, the channel 57 opens into the plenum 43 and the channel 58 is closed.
The sequence of open and closed channels is repeated about the periphery of the structure.
Adjacent the plate 24 another plenum 49 is formed. For those 3Q embodiments of the invention in which a single corrugated sheet is employed and the rectangular channels run substantially along the full length of the B~

9f;~

wall 22, the sequence of open and closed channels adjacent to the plenum 49 is reversed from that adjacent to the plenum 43. In other words, typical channel 57 which opens into the plenum 43 is closed off from the plenum 49 and typical channel 58 which is closed off from the plenum 43 opens into the plenum 49. For arrangements such as that shown in Fig. 1 in which two or more sheets are stacked to form a jet impingement structure with multiple stages, each sheet has al*ernate channels closed off at one end thereof and a reverse sequence of open and closed channels at the opposite end.
An exhaust line or flue 61 is sealed through the insulating shell 11 and communicates with the plenum 49 to carry off waste gases.
The heat exchange members in the outer or external chamber 18 may be as simple as a perforated cylinder surrounding the outer wall 22, jets being formed by the apertures of the perforated cylinder and directed against the outer wall to break up boundary layers existing on that wall or they may be in any of several designs which include perforated or slotted fins to assure the impingement of hot gases upon unbroken confronting surfaces of adjacent fins again to break up boundary layers.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A boiler comprising: an annular tank for holding liquid to be heated, said annular tank having concentric inner and outer walls serving as heat-exchange surfaces, an inner chamber being formed within said inner wall; an outer shell spaced from and surrounding said outer wall, an outer chamber being formed between said outer shell and said outer wall; means including a burner for directing hot gases first through said inner chamber and then through said outer chamber; a plurality of heat-exchange enhancing fins disposed in said inner chamber in heat-conductive relationship to said inner wall; means disposed in said outer chamber for forming said hot gases into jets directed upon surfaces transferring heat to said liquid.

2. A boiler as defined in claim 1 and further including members dis-posed in said outer chamber in heat-conductive relationship with said outer wall and in the path of said jets.

3. A boiler as defined in claim 1 wherein said means disposed in said outer chamber includes a plurality of members formed into channels having confronting walls, said walls having openings formed therein through which said hot gases are jetted, said openings in said confronting walls being staggered and so disposed that gases jetting therefrom impinge upon unper-forated areas of confronting walls whereby boundary layers on said unperfor-ated areas are disrupted and heat transfer to said liquid is enhanced.

4. A boiler as defined in claim 1 wherein said means for directing hot gases further includes a baffle of high-temperature insulating material centrally disposed in said inner chamber to aid in directing said hot gases toward said inner wall.

5. A boiler as defined in claim 1 wherein said plurality of fins dis-posed in said inner chamber comprise: a plurality of stacked arrays of fins extending radially inwardly from said inner wall, each said array being rotationally offset from an adjacent array.

6. A boiler as defined in claim 5 wherein each said array of fins comprises a second plurality of fins equiangularly spaced from one another, the angle of spacing between said fins being approximately twice the angle by which each array is off-set from an adjacent array.

7. A boiler as defined in claim 1 wherein the plurality of fins disposed in said inner chamber form a tortuous path along said inner wall for the flow of said hot gases, whereby turbulence is imparted to said gases and boundary layers on said fins are disrupted.

8. A boiler comprising an insulated outer shell, an annular water tank having inner and outer concentric walls dis-posed within said shell, an inlet for introducing water into said tank, an outlet for removing water from said tank, said inlet and said outlet being sealed through said outer shell, a water baffle disposed adjacent said inlet within said tank to inhibit direct flow of water to said outlet, a burner sealed through said shell and disposed of fire hot gases within said inner wall, a plurality of arrays of radial fins disposed upon said inner wall, said fins extending radially inwardly there-from in equally spaced relationship, each said array being rotationally offset from an adjacent array whereby a tortuous path for said gases is formed, a deflector disposed within said shell at the end thereof opposite said burner for redirecting said hot gases along said outer wall, at least one jet impingement member disposed between said outer wall and said shell and in heat conductive relationship with said outer wall, said member having surface portions with jet-forming apertures therein and unperforated surface portions opposite said apertures so that hot gases directed through said apertures form jets which impinge upon said unperforated portions, and a flue sealed through said shell for outward passage of said gases.