US3580237A

US3580237A - Air heater

Info

Publication number: US3580237A
Application number: US796532A
Authority: US
Inventors: Alan Barsby
Original assignee: Marshall Sons and Co Ltd
Current assignee: Marshall Sons and Co Ltd
Priority date: 1968-02-08
Filing date: 1969-02-04
Publication date: 1971-05-25
Anticipated expiration: 1988-05-25
Also published as: DE1905689A1; GB1184777A; BE728123A

Abstract

This invention relates to an air heater comprising a combustion chamber, means to supply air and a combustible fuel to the combustion chamber, an exhaust for the products of combustion from the combustion chamber, a first duct connecting the combustion chamber to the exhaust, an inlet and an outlet for air to be heated, a second duct interconnecting the inlet and outlet, said second duct surrounding the combustion chamber for part of its length and for a different part of its length being parallel to, and sharing a common boundary with, said first duct, to define a thermal regenerator, the thermal regenerator comprising a plurality of heat transfer discs extending through said common boundary into both ducts, and means to rotate each disc about its axis of symmetry.

Description

6 United States Patent 1 3,580,237

[72] Inventor AlanBarsby 3,189,017 6/1965 Hahn 126/110X A I N m g g Gamsborough, England FOREIGN PATENTS pp O. [221 Filed M51969 51132 11352 iiliinai 12212 [45] Patented May 1971 250172 5/1927 G tB 'ta' 165/5 [73] Assign Marsha Cmpmy Limited 255 418 7/1927 oiz tsiimi 165/6 Gainsborough LincolnshhmEngland i [32] Priority Feb. 8, 1968 Primary Examiner-Albert W. Davis, Jr. [33] Great Britain Attorney-Michael S. Striker [31] 6194 ABSTRACT: This invention relates to an air heater compris- [54] m HEATER ing a combustion chamber, means to supply air and a com- 9 Claims 7 Drawing Figs bustible fuel to the combustion chamber, an exhaust for the products of combustion from the combustion chamber, a first LS- Cl. duct onnecting the combustion hamber to the exhaust an 126/ 1 17, 55/5, 35/7, 165/9, 165/10 inlet and an outlet for air to be heated a second duct intercon- [51] Int. Cl F2411 3/12 acting the inlet and outlet Said second duct Surrounding the [50] Field of Search 165/6, 7, 9, combustion chamber f g f its length and f a diff t 5, 10; 263/20; 126/110, 108 art of its length being parallel to, and sharing a common [561 References CM 12222132? l;l"1fi$?f IZZZZEFJZZEJ; UNITED STATES PATENTS discs extending through said common boundary into both 1,601,355 9/1926 Esbran 165/9 ducts, and means to rotate each disc about its axis of sym- 3,l64,145 1/1965 Tolson 126/110 metry.

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Patented May 25, 1971 3 Sheets-Sheet 3 r up I I INVENTUR MM mm W/al/f "i %w'(/. AT OPNE Y AIR HEATER This invention relates to an improved air heater employing a fuel-fired combustion chamber to generate a hot gas stream and a thermal regenerator to transfer thermal energy from the hot gas stream (the heating system) to the air to be heated (the heated stream).

Thermal regenerators currently in vogue employ a permeable heat transfer member in the form of a drum-shaped rotor containing a suitable heat-transferring material exhibiting a very large surface area to the heated and heating streams. In order to keep the rotor dimensions to a reasonable size, it is a consequence of the provision of a large surface area that the fluid passages provided through the heat-transferring material are narrow and thus prone to blocking by solid particles entrained in either, or both, of the fluid streams.

This invention relates to an improved air heater which utilizes a form of thermal regenerator (generally held to be of outmoded design) which utilizes discs as the heat transfer members, each disc rotating about its axis of symmetry, thus providing an air heater of simple construction which, since the fluid passages in the thermal regenerator need not be narrow, can be operated without danger of the thermal regenerator being blocked by particles (e.g. of soot) swept along by the hot gas stream.

According to the invention an air heater comprises a combustion chamber, means to supply air and a combustible fuel to the combustion chamber, an exhaust for the products of combustion from the combustionchamber and a first duct connecting the combustion chamber to the exhaust, an inlet and an outlet for air to be heated, a second duct interconnecting the inlet and outlet, said second duct surrounding the combustion chamber for part of its length and for a different part of its length being parallel to, and sharing a common boundary with, said first duct to define a thermal regenerator, the thermal regenerator comprising a plurality of heat transfer discs extending through said common boundary into both ducts and means to rotate each disc about its axis of symmetry.

We prefer to have the plane of each disc parallel to the direction of gas flow in the ducts (since this facilitates the division of either or both ducts into two or more passes), but an angle of inclination (say up to 30) may assist in generating turbulence in the ductsand result in improved heat transfer rates between the gases and the discs.

In one convenient embodiment of the invention the first and second ducts are rectangular in cross section and the discs are arranged in sets spaced apart along the ducts, each disc in one set being substantially coplanar with a corresponding disc in the other set or each other set, whereby a baffle may be located in either duct between two discs of one set and, extending in a direction parallel with the plane of one of said discs, pass between the corresponding two discs of every other set.

Conveniently discs are arranged in sets, each set comprising a plurality of parallel discs with their axes of symmetry collinear, the common boundary in the vicinity of each set then being formed by an array of spacers located one between each adjacent pair of discs. Preferably the spacers are located to one side of the axes and are urged into frictional contact with the surfaces of the discs to serve as scrapers. The discs and the spacers may be slidably mounted for movement in the axial direction of the drive shaft employed for rotating the discs and are resiliently urged in that direction whereby wear occurring in the discs or spacers is always compensated for.

Preferably the combustion chamber and thermal regenerator lie sideby-side in a common casing with the air inlet adjacent to the combustion chamber, and with the two ducts bent back on themselves through l80 at the same end of the casing. This arrangement leads to a compact self-contained air heater.

Conveniently one or both ducts in the thermal regenerator are adapted to contain baffles to force the gas to flow in a plurality of passes past the discs. Where spacers are used to define the boundary between adjacent discs, these spacers can be adapted to support the baffles.

One embodiment of air heater in accordance with the invention will now be described by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a side elevation of a thermal regenerator and an associated furnace which together form the air heater,

FIG. 2 is an end elevation of the air heater of FIG. I seen from the right,

FIG. 3 is a section on the line AA of FIG. 2 with a part of the regenerator removed for reasons of clarity,

FIG. 4 is an end elevation of part of a set of heat transfer discs used in the regenerator of FIGS. 1 to 3,

FIG. 5 is a perspective view of a scraper bar as used in the set of discs shown in FIG. 4,

FIG. 6 shows part of a disc guide as employed in the regenerator of FIGS. 1 to 3, and

FIG. 7 is a plan from above showing how the disc guides of FIG. 6 are supported.

Referring to the drawings, the air heater comprises a furnace (generally designated 1) and a thermal regenerator (generally designated 2). The furnace 1 comprises a combustion chamber 3, into which an oil burner assembly 3a projects, and a fire tube 4 through which the hot gaseous combustion products are conveyed to an end chamber 5.

The thermal regenerator 2 comprises an upper duct 6 through which air to be heated is conveyed (i.e. the second duct in the terminology hereinbefore employed) and a lower duct 7 through which the hot gases from the end chamber 5 are led, The two ducts 6 and 7 are both of rectangular cross section and the duct 7 is divided into four passes by partitions 8, 9 and 10, the cross-sectional area of the successive passes getting smaller to compensate for the volume reduction in the combustion gases as the latter cool during passage through the duct 7. The outlet end of the duct 7 leads to an exhaust 12.

The air supply for the duct 6 and the combustion chamber 3 is provided by an air blower 11. Part of the air from the blower l I is fed into the chamber 3 while the rest passes along an annular passage 13 surrounding the fire tube 4. The outlet end of the passage I3 terminates in an annular chamber 14 from whence the air is fed to the inlet end of the duct 6. The outlet end of the duct 6 would, in practice, be a connecting pipe which leads the heated air to its point of use (for example a drying chamber).

Mounted within the thermal regenerator 2 are three sets of metal discs 16 (e.g. of mild steel). The sets are designated as a, b, and c in FIGS. 1 and 3. Each set comprises a number of discs 16 mounted in parallel spaced relationship on a

shaft

17a, 17b, 170, respectively, the discs being arranged so that each disc in one set is substantially coplanar with a corresponding disc in each of the other two sets. The shafts 17a to lie slightly above the medium plane of a boundary wall 18 (see FIG. 4), which is common to the two ducts 6 and 7, so that slightly over half of each disc is in the upper duct 6 and slightly under half in the lower duct 7.

The shafts 17a to 170 are connected to a motor 19 via a gearbox 20, and chain drives 21 and 22 so that the sets of discs are rotated at slow speed (in a typical case at a speed of 2 rpm.) As the discs rotate they are heated by the combustion products in the lower duct 7 and give up heat to the air in the upper duct 6. Referring to FIG. 4 it will be seen that the boundary wall 18 in the region through which the discs 16 pass is made up of rigid metal scraper bars 23 (e.g. of cast iron), and these are provided with holes 23a by which the bars are supported between the discs 16 on spaced-apart rods 24 (only one of which is shown in FIG. 4). The discs 16 are slidably (but nonrotatably) supported on the shaft 170 and the scraper bars 23 are urged into contact with the discs 23 by springs 25. The scraper bars 23 serve to keep the discs 16 clean and, in particular free from any soot deposits which may form on them while they are in the lower duct 7. Any deposits removed from the faces of the discs by the bars 23 drop into the lower duct 7 from whence they may be periodically removed.

If the air in the duct 6 must be completely free from any contamination by the products of combustion flowing in the duct 7, this may be achieved by ensuring that the air pressure in the duct 6 is greater than the gas pressure in the duct 7, so that any leakage through the boundary wall 18 will simply result in a small loss of air, and will not cause contamination of the air by the products of combustion.

in the example shown in the drawings, the combustion gas makes four passes along the duct 7 and this, inter alia, compensates for the difference in volume flow between the air and the gas and helps to achieve a more nearly equal flow rate in the two ducts. Some scraper bars may be provided with spaced-apart downwardly extending projections (23b see FIG. 4) between which pass-defining partitions may be slotted. This arrangement has the advantage of ensuring that as the axial length of a set of discs 16 reduces (due to wear of the scraper bars), the partitions remain located between two adjacent discs.

' A consequence of the arrangement of passes shown in the drawings, is that the discs on the right-hand side of the regenerator 2 (as seen in FIG. 2) are those included in the first pass of the combustion gas and are thus hotter than the discs on the left-hand side, so that by dividing the upper duct 6 with a partition (e.g. as shown dotted at 26 in FIG. 2) air at two different temperatures can be obtained at the outlet end of the duct 6. These different temperature airstreams can be kept separate (by suitably dividing the connecting pipe) and used separately. Thus, for example, if the air heater shown is used for a tea dryer employing a heated feeder conveyorthe higher temperature air can be fed to the heated feeder conveyor and the lower temperature air can be fed to the drying chamber.

The rate of heat transfer between a fluid and a surface in contact therewith is, inter alia, proportional to the relative velocity between the surface and the fluid. Although the disc velocity must be the same in the two ducts, the velocity of fluid flow can be increased by utilizing a number of passes (as has already been described) or by reducing the volume of the duct available to the fluid. This reduction in volume can be achieved by placing slabs or blocks in the duct (eg between some of the discs) or by contouring the wall of the duct so that projections, which extend between the discs, are provided. By locating the rotational axis of the discs in one of the ducts, a central portion of each disc adjacent to the axis is always in said one duct whereby the area of the disc available for direct heat transfer between the fluid streams is limited to the annular region surrounding said central portion. However, since the surface area of an annular strip varies as the square of the radius of the strip, the small extent by which the boundary wall 18 is offset from the axis produces a negligible loss of effieiency.

The projections extending between the discs may be employed to stabilize the peripheries of the discs against flexure caused by air pressures or thermal distortions. HO. 6 shows an arcuate disc guide 27 (conveniently designed to extend over at least one-quarter of the circumference of the disc) which can be slidably mounted on bars 28 (see FIG. 7) with intercalated springs 29. The springs 29 ensure appropriate spacing of the peripheries of the discs and the slidable disposition of the guides 27 on the bars 28 allows the guides to approach each other as the axial length of the set of discs reduces due to wear of the scraper bars.

We prefer to use discs of metal (e.g. copper, mild steel or stainless steel) but discs of other materials (eg ceramic material) can also be employed. The discs may be textured or contoured on their faces to increase turbulence between the disc and the fluid stream (and thus aid heat transfer) provided that the surface can still be kept clean by the abrading action of the scraper bars. Thus, for example the discs can be corrugated in concentric rings coaxial with the axis of the disc. This feature will also increase the rigidity of the disc and increase its surface area.

In the air heater shown in the drawings, the discs 16 are made of mild steel, are some 2 feet in radius and some l/loinch thick. The temperatures of the gasses in the combustion chamber 3, end chamber and exhaust 12 are some 2,700 F,

All.

1,900 F. and 500 F. respectively. Air taken in at a temperature of 70 F. has acquired a temperature of F. in the annular space 14 and exits from duct 6 in a hotter stream at 250 F. and a cooler stream at 210 F. The above figures are of course purely exemplary and are not to be considered as imposing limitations on the scope of the invention described.

Although a rotational speed of only 2 r.p.m. is employed for the discs in the air heater described, it is important to point out that rotational speeds either smaller, or greatly in excess of this can be used if circumstances make this desirable. For example rotational speeds of a few hundred r.p.m. are contemplated.

The rotational speed is normally chosen as the minimum speed that will cause the temperature of any point on the disc to fluctuate only a few degrees (up to, say, 25 F.) during a complete revolution. It will therefore be understood that the temperature of the disc never exceeds very greatly the average temperature of the two fluids, and as a consequence a disc material is required to withstand only this average temperature. However, if such a material is employed, rather than one which could continuously withstand the temperature of the hotter fluid, means should be provided to automatically shut down the oil burner 3a if there is a change in the speed of rotation of the discs 16. Similar provision could be made in the event of failure of the air blower 11.

The air heater shown in the drawings is a compact packaged unit which merely requires to be supplied with power for the blower 11 and the motor 19 and fuel for the burner 3a. The composite assembly of furnace l and regenerator 2 are enclosed in a casing 30.

The arrangement of discs in sets so that there is a coplanar relationship between corresponding discs in the three sets enables, as has already been explained, the easy insertion of baffles into both ducts, and this in turn enables the air heater to be operated in a variety of different modes. Thus, for example a plurality of baffles could be used in each duct so that, for example, air in duct 6 makes its initial pass on the left-hand side and exits on the right-hand side while combustion gas in duct 7 makes its initial pass on the right-hand side and exits on the left-hand side. With this arrangement it will be appreciated that the air in duct 6 can be made to contact successively hotter discs as it flows from its inlet to its outlet.

lclaim:

1. An air heater, comprising a combustion chamber; supply means for supplying a combustible fuel to said combustion chamber; exhaust means for exhausting combustion gases from said combustion chamber; first duct means connecting said combustion chamber with said exhaust means; second duct means for air to be heated and comprising one duct section surrounding said combustion chamber and another duct section parallel to and sharing a common boundary with, said first duct means; a plurality of sets of heat transfer discs extending through said common boundary into both of said duct means, said sets of discs being spaced apart lengthwise of said duct means; rotating means for rotating each disc about the axis of symmetry of the same; and baffle means located in at least one of said duct means and extending in substantial parallelism with the plane of at least one of two discs of each of said sets so as to force said combustion gases to flow in multiple passes past said discs.

2. An air heater as defined in claim 1, said first duct means and at least said other duct section of said second duct means being of rectangular cross section.

3. An air heater as defined in claim 1, said baffle means comprising at least one baffle located in said first duct means.

4. An air heater as defined in claim 3, said multiple passes including at least one initial pass and at least one terminal pass each including contact of said gases with different ones of said discs; and further comprising an additional baffle mounted in said other duct section and defining therein two parallel additional passes one of which includes the discs contacted in said initial pass and the other of which includes the discs contacted in said terminal pass.

5. An air heater as defined in claim 4, wherein the discs contacted in said other pass of said second duct means are the discs contacted in said initial pass of said first duct means.

6. An air heater as defined in claim 1, said rotating means comprising a rotary shaft on which said discs are mounted; and further comprising a plurality of spacers constituting said common boundary and being respectively mounted between adjacent ones of said discs slidable axially of said shaft; and biasing means urging said spacers for sliding movement axially of said shaft.

7. An air heater as defined in claim 6, said spacers being mounted to one side of the axes of said discs and having surface portions abutting against the major surfaces of respective ones of said discs under the influence of said biassing means so as to scrape said major surfaces in response to rotation of said discs.

8. An air heater as defined in claim 6, and mounting means mounting said baffle means on said spacers.

9. An air heater as defined in claim 1, in which the combustion chamber and said other duct section lie side-by-side in a common easing with the air inlet adjacent to the combustion chamber, both duct means being bent back on themselves through at the same end of the casing.

Claims

9. An air heater as defined in claim 1, in which the combustion chamber and said other duct section lie side-by-side in a common casing with the air inlet adjacent to the combustion chamber, both duct means being bent back on themselves through 180* at the same end of the casing.