US3294160A

US3294160A - Heat transfer apparatus

Info

Publication number: US3294160A
Application number: US414217A
Authority: US
Inventors: Richard P Siegfried; Frank E Garrison
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1964-11-27
Filing date: 1964-11-27
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27
Also published as: GB1088869A

Abstract

1,088,869. Heating air by waste gases. BABCOCK & WILCOX CO. Nov. 26, 1965 [Nov. 27, 1964], No. 50311/65. Heading F4S. In an air heater generally as described in Specification 988,353 in which a combustion air stream for a steam generator is heated as it flows serially over banks (61, 62, 63, Fig. 1) of flue tubes, the tubes extend between upper and lower tube plates and the upper tube plates 58A, 58B of the tube banks 61, 62 are carried in a common supporting structure which is cooled by air bled from the main combustion air stream. This cooling air reduces thermal stresses in the supporting structure caused by the temperature differential between the tubes of the respective banks. As shown, the supporting structure includes ducts 102, 102A for the cooling air. These ducts extend around the perimeter of each of the upper tube plates 58A, 58B. The tube plates are freely movable axially of the tubes and are connected to the structure by flexible bellows 90A, 90B which are protected from ash &e. in the flue gases by shields 92A, 92B. Air tapped from the main stream passing over the tubes, flows through a narrow gap provided between each tube plate edge and adjacent walls 88A, 88B of the supporting structure, to the space behind each bellows member, whereby the latter is kept cool and may therefore be made of a relatively cheap steel. The cooling air passes thence to the ducts 102, 102A by way of apertures 98 in the walls 88A, 88B; and is discharged from the ducts through an outlet 110.

Description

Dec. 27, 1966 R. P. SIEGFRIED ETAL 3,294,160

HEAT TRANSFER APPARATUS Filed Nov. 27, 1964 I 2 Sheets-Sheet 1 FIG.1

INVENTORS Rlchard P S|egf ned BY Frank E.Garr|son ATTORNEY United States Patent Ofiice Patented Dec. 27, 1966 3,294,160 HEAT TRANSFER APPARATUS Richard P. Siegfried, Bath Township, near Akron, and Frank E. Garrison, Clinton, Ohio, assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Nov. 27, 1964, Ser. No. 414,217 2 Claims. (Cl. 165-83) This invention relates generally to heat transfer apparatus, and more particularly to tubular air heaters wherein air for supporting combustion in a furnace is indirectly heated by the gaseous products of combustion from the furnace.

A tubular air heater has a passageway through which air flows into contact with several banks of tubes within the passageway so as to be heated thereby. The tubes serve as conduits for the hot gases exhausted from the steam generating unit. Conventional air heaters of this type direct the hot gases serially first through the tubes of the tube bank adjacent the air outlet of the passageway and then the gases, at reduced temperature, flow through the tubes of the bank adjacent the air inlet. In order to overcome the problem of cold-end corrosion associated with the dew point-temperature relationship of the gaseous constituents it has been proposed in the copending application of Arthur M. Frendberg, Serial No. 268,985, filed March 29, 1963, now US. Patent No. 3,194,214, granted July 13, 1965, to route a small portion of the comparatively hot gases as received from the generating unit gas pass to a relatively small tube bank located at the air inlet, immediately upstream of the main air heater, for the purpose of preheating the entering air to a temperature sufiicient to minimize formation of corrosive products within the tubes at the air inlet to the air heater.

In apparatus constructed according to the cited Fre-ndberg patent, the preheater section is thus located adjacent the main air heater gas outlet section which, during normal operation, will be handling gases which are at a very much lower temperature than the gases in the neighboring preheater. This temperature difference between the adjacent sections may exceed 500 F. and it will be readily understood that this differential would produce severe thermal Stresses in the structural components unless adequate provision is made to counteract their effect. It is therefore the principal object of the present invention to minimize such thermally induced stresses in the apparatus by minimizing the temperature differential between the aforementioned air heater sections.

The problem as set forth is solved, according to the present invention, by providing a duct structure which extends about the periphery of the upper tube sheet of the preheater tube section with means for directing air from the air heater inlet duct into the aforementioned duct and across the expansion joint associated with the preheater upper tube sheet. This air cools the expansion joint and the surrounding structure, and thereafter may be put to further use, such as for creating an air seal or as elutriating air of a shot cleaning system (not shown).

Various other objects, features and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawings forming a part of the present invention, and in which:

FIG. 1 is a partially diagrammatic, sectional view of a vapor generating and superheating unit having an air heater embodying the present invention;

FIG. 2 is a fragmentary plan view of the air heater of FIG. 1, on an enlarged scale;

FIG. 3 is a sectional side view of the air heater, taken along line 33 of FIG. 2; and

FIG. 4 is an enlarged vertical sectional view of a portion of the air heater shown in FIG. 3.

Although it is not so limited, the invention, as illustrated in the drawings, has been applied to an air heater used in conjunction with a forced-flow, once-through vapor generating and superheating unit.

Referring to FIG. 1, the illustrated unit includes wall structure 10 defining a vertically elongated furnace chamber 12 having a gas outlet 14 at the top of the furnace chamber opening to a horizontally extending gas pass 16, the latter communicating at its rear end with the upper end of an upright gas passage 18. At the lower portion of the furnace chamber 12 is a fuel firing section consisting of horizontally extending cyclone furnaces 20 which burn fuel and discharge high temperature gaseous products of combustion into the lower portion of the furnace chamber 12. The gas pass 16 contains secondary superheater platen sections 22 and reheater platen sections 24 arranged in series with respect to gas flow. The gas passage 18 contains primary superheater sections 26 and economizer sections 28, also arranged in series with respect to gas flow. A cavity 30 is provided between the economizer sections 28 and the primary superheater sections 26 to facilitate withdrawal of flue gases from the main gas stream. If desired, flue gases can be withdrawn from between the economizer sections 28. The lower end of the gas passage 18 may incorporate an ash hopper 32 with a gas outlet 34 which leads to the air heater 4%, and although not shown, the unit may have a suitable fan and ductwork between the gas passage 18 and tie furnace chamber 12 to provide gas recirculation.

The air heater 46, as shown, includes a housing or casing 42, at the sides of the air heater, defining an air passageway 41 having an associated inlet 44 connected with an inlet duct 46 to a forced draft fan 48. The air heater 40 is of the bottom-supported type, being supported on girders 43 and supporting columns 45, as shown in FIG. 1. The fan 48 supplies pressurized atmospheric air via inlet 44, which is heated in the passageway 41, then discharged through the outlet 50 and conveyed to the furnace windboxes 52 by hot air duct 54. The air flowing in the passageway 41 is indirectly heated by contact with spaced successive parallel rows of upright

gas conducting tubes

56A, 56B, and 56C which are secured at opposite ends thereof, as by rolling, to upper

horizontal tube sheets

58A, 58B, and 58C and lower

horizontal tube sheets

60A, 69B, and 60C, respectively. The tube rows in passageway 41 are grouped within the housing 42 in three separate,

successive tube banks

61, 62, and 63, with the relatively small preheater tube bank 61 comprising tubes 56A positioned adjacent the air inlet 44, the tube bank 63 comprising tubes 56C positioned adjacent the air outlet 50, and the tube bank 62 comprising tubes 56B disposed between the

tube banks

61 and 63. There is one cavity 64 between the first and

second tube banks

61 and 62, and another cavity 66 between the second and

third tube banks

62 and 63.

The combustion products from the furnaces 20, after traveling first upwardly through the furnace chamber 12 and then horizontally through the gas pass 16, next turn and proceed downwardly through the gas passage 18. The main stream of flue gas leaves the gas passage 18 through the gas outlet 34 and thereafter is turned downwardly within duct 68 for passage through the tubes 56C of the third tube bank 63, after which it enters an ash hopper 70. The latter is defined by downwardly and inwardly sloping bottom walls 72 of the air heater housing 42. The gas leaving the tubes 56C is deflected by a baffle 74 toward the bottom of the ash hopper 70.

A smaller but hotter stream of flue gas is withdrawn from the main stream of flue gas at the cavity 30, next conducted by bypass conduit 76 to the first tube bank 61 for downward flow through the tubes 56A, and upon leaving the tubes is deflected by baffle 78 toward the bottom of the ash hopper 70. The gas from the tubes 56A combines with the main stream of flue gas which has passed through the tubes 56C. The resultant combined gas stream flows upwardly from the ash hopper 70, between the bafiies 74 and 78, into and through the tubes of the intermediate tube bank 62. It then is conducted by an exhaust duct 80 through a dust collector 82 and subsequently discharged to the atmosphere.

The arrangement, as described, is intended to initially preheat the relatively cool atmospheric air supply entering the main air heater in order to minimize the possibility of cold-end corrosion. The corrosion attack on air heater tubing results from the condensation of corrosive gases, such as sulfur compounds and water vapor, constitutents present in the gases, as the relatively hotter gases come into contact with the tube metal which is being cooled by the colder air entering the air heater inlet. In addition, the moisture present on the interior surface of the tubing will also tend to collect corrosive dust or ash particles entrained in the flue g-as flowing through the tubes, with the result that in addition to corrosion effects the accumulation of dust particles restricts the flow area within the tubes and decreases the heat transfer efficiency of the tubing.

In order to facilitate the identification of similar parts associated with the

several tube banks

61, 62, and 63, the letter A will follow the reference numeral designating each component and associated part relating to the preheater tube bank 61; and, likewise, the letters B and C will be employed following correspondingly similar parts designated by the same basic reference numeral of associated and component parts of the main air

heater tube banks

62 and 63, respectively.

Air entering air heater passageway 41, on its way to the cyclone furnaces 20, is at relatively low temperature, say 100 F. This relatively cool air is preheated by the high temperature flue gas taken from the cavity 30 between the primary superheater sections 26 and the economizer sections 28, and it is conveyed through by-pass conduit 76 to the tubes 56A adjacent the air heater inlet 44. This by-pass gas may be at a temperature in excess of 850 F. entering the tubes 56A to provide tube metal temperatures well above the corrosion limit of 230 F. or so, while at the same time quickly raising the temperature of the air above the temperature range conducive to promoting corrosion. The next tube bank 62 in the flow path of the air will have gas flowing therethrough at a temperature in the range of between 480 F. and 300 F. so as to further elevate the temperature of the air. Finally, the temperature of the gas flowing through the tube bank 63 will be in the range of between 500 F. and 680 F. for raising the temperature of the air still further.

It can be seen from the foregoing that with the temperatures of the

upper tube sheets

58A and 58B tending to approach the temperature of the gases they are contacting, the tube sheet temperatures will be of the order of 850 F. and 300 F. respectively, with a resultant temperature differential of the order of 550 F. As will be explained hereinafter, supporting structure associated with these tube sheets may have corresponding temperatures, and it is imperative to reduce this temperature differential in order to prevent distortion of the supporting frame structure.

Supporting structure, as shown in FIGS. 2, 3, and 4 of the drawings, comprises horizontal channel members, including

channel members

86, 88A, and 88B. Longitudinal channel members 86 extend in parallel relationship lengthwise of the air heater passageway 41 substantially for the length of the air heater 40. Transverse channel members 88A are disposed on opposite sides of the tube sheet 58A; and transverse channel members 883 are disposed on opposite sides of the upper tube sheet 53B. These

transverse channel members

88A and 88B are connected at their extremities to the respective webs of the longitudinal channel members 86.

As shown in the drawings, the upper tube sheet 58A of the preheater tube bank 61 is bounded on all four sides by channel members, on two opposite sides by transverse channel members 88A, and on the two other opposite sides by extended portions of the channel members 86 so as to define a region which provides an extension conduit 89 of the by-pass conduit 76. Heating gas flows downwardly through

conduits

76 and 89 into the upper, open ends of the tubes 56A which are secured to and project through the tube sheet 58A.

Disposed within the extension conduit 89 is an expansion joint A which serves to effect a gas-tight seal between the by-pass conduit 76 and the tube sheet 58A.

The expansion joint 90A is suitably shielded to minimize accumulations of ash entrained in the flue gases by telescoping parts comprising a short upstanding collar 91A disposed about the periphery of the tube sheet 58A, and a cooperating sleeve 92A closely spaced therefrom and connected to the back face of the webs of the

channel members

86 and 88A. Thus the collar 91A and the sleeve 92A are in vertically overlapping relationship, with a sliding fit between them which permits the tube sheet 58A to move in a vertical direction as the tubes 56A undergo expansion and contraction due to changes in the gas and air temperatures, while at the same time shielding the expansion joint 90A from ash particles entrained in the flue gas stream. Furthermore, sleeve 92A serves as an additional pressure barrier between the passageway 41 and the conduit 76; and the collar 91A reinforces the sleeve 92A.

The upper extension of the peripheral expansion joint 90A is secured to the back face of the web of

channel members

86 and 88A, and the lower extended portion thereof is secured to an angle iron frame 96A which extends about the periphery of the tube sheet 58A. The expansion joint 90A is suitably provided with spaced folds or leaves disposed between the upper and lower extended portions to permit vertical expansion and contraction of the joint while maintaining the aforementioned seal. Thus, the tube sheet 58A is freely movable in vertical direction while the expansion joint 90A effectively prevents the corningling of air and flue gas.

In order to cool the expansion joint 90A and the

transverse channel members

88A and 88B, provision is made for purposely leaking relatively cool inlet air from the passageway 41 .past the edges of the upper tube sheet 58A into the region between the expansion joint 90A and the back surfaces of the

channel members

86 and 88A, after which the cooling air passes through apertures 98 in the webs of these channel members, as shown.

In order to keep the tube sheet 58A in horizontally spaced relationship with the

channel members

86 and 88A, vertically extending spacer bars are arranged about the edges of the upper tube sheet 58A and secured to the back surfaces of the mentioned channel members. The relationship of spacer bars 93 and the edge of the upper tube sheet 58A is shown in FIG. 4.

By reducing the environmental temperature at the expansion joint 90A, this part can be made of low cost carbon steel material, rather than of alloy steel material. Furthermore, the same cooling air which cools the expansion joint also cools the

transverse channel members

88A and 88B of

adjacent tube banks

61 and 62, thereby maintaining them at substantially the same temperature and preventing unequal thermal expansion in the supporting structure of the air heater. Without this provision for cooling the neighboring transverse channel members, there would be a temperature difference upwards of 550 F. existing between the two adjacent tube banks. It will therefore be appreciated that distortion due to unequal thermal expansion is avoided by the present arrangement.

Referring now especially to FIG. 3, the specific transverse spaced channel members 88A and 8813 which are

adjacent tube banks

61 and 62, respectively, have their flanges extending toward one another. Upper and lower plates, 100 and 101, respectively, secured to the extending legs of the channels are employed to close the space between the flanges of these

channel members

88A and 88B to thereby define a duct 102. These plates 1'00 and 101 are preferably secured to the flanges by welding in order to make the duct 102 air-tight. The duct 102 is continued about the entire periphery of the expansion joint 90A by means of a skirt 104 of U-shaped cross section having its free edge portions joined, as by welding, in abutting relationship with the flanges of the

channel members

86 and 88A. It is noted that the extension of the duct 102 provided by the skirt 104, is on the outer sides of the

channel members

86 and 88A; and, in order to provide communication with the first mentioned portions of the duct 102, a rectangular opening 106 is formed in the web of the longitudinal channel member 86, as shown in FIGS. 3 and 4.

With this arrangement, air can flow from the air passageway 41 through the regions between the spacer bars 93, then into contact with the outer surface of the expansion joint 90A, and through the apertures 98 into the duct 102 for cooling the

channel members

86, 88A, and 88B, after which the air exists through an exhaust pipe 110.

Theair which is exhausted through pipe 110 may be beneficially used, such as for sealing air, or as elutriating air for a shot cleaning system, with the pressure differential existing between the pressure in air passageway 41 and that at the point of use producing the air flow.

From the foregoing, it can be seen that the air heater 40 includes a preheating tube bank 61 ahead of the main air

heater tube banks

62 and 63. Further, the air heater 40 has been provided with means for cooling the expansion joint 90A conducting by-pass gas to the preheater tube bank 61, and which cools supporting members so as to prevent a temperature differential between adjacent supporting members. This is accomplished by utilizing inlet air flowing through duct structure formed in part by the supporting members. It is an advantage of the present invention that the air heater casing can be secured directly to the supporting structure without the necessity of expansion joints therein. Furthermore, as mentioned previously, the expansion joint itself may be made from less expansive materials, even when by-pass flue gases of relatively high temperature are directed to the preheater tube bank 61.

Although the invention has been shown in but one form, it will be obvious to those skilled in the art that it is not so limited, but it is susceptible of various changes and modifications without departing from the spirit thereof as covered by the claims.

What is claimed is:

1. An air heater comprising:

an air passageway,

first, second and third tube banks each including:

a multiplicity of tubes and a tube sheet supporting said tubes, means for moving relatively low temperature air through said passageway into successive contact with the outer surfaces of the tubes of said first, second and third tube banks,

means for directing relatively high temperature heating gas through tubes of said first and second tube banks and through tubes of said third and second tube banks, whereby heat is transferred to the air contacting said tubes, supporting means connected to the tube sheets of at least two neighboring tube banks and extending along the adjacent edges thereof, means defining with said supporting means a duct open to said passageway for conducting at least a portion of the air flowing through said passageway into heat transfer relationship with said supporting means, and an expansion joint between said supporting means and the tube sheet of said first tube bank providing a flexible seal between the air flowing through said duct and the heating gas flowing through said directing means. 2. An air heater comprising: a horizontally elongated air passageway having an inlet and an outlet, first, second and third tube banks each including:

a multiplicty of upright tubes and upper and lower horizontal tube sheets supporting said tubes, said first tube bank being positioned adjacent to said inlet and said third tube bank being positioned adjacent to said outlet, means for moving relatively low temperature air through said passageway into successive contact with the outer surfaces of the tubes of said first, second and third tube banks, means for directing one stream of relatively high temperature heating gas downwardly through tubes of said first tube bank and upwardly through tubes of said second tube bank and for directing another stream of relatively high temperature heating gas downwardly through tubes of said third tube bank and upwardly through tubes of said second tube bank, supporting means including horizontal members connected to the tube sheet of said first tube bank and extending about the edges thereof, means defining with said members a duct open to said passageway for conducting at least a portion of the air from said inlet therethrough into heat transfer relationship with said supporting means, and an expansion joint flexibly connecting said members and edge portions of the tube sheet of said first tube bank and providing a seal between said one stream of heating gas and the air flowing between said passageway and said duct.

References Cited by the Examiner UNITED STATES PATENTS 1,780,294 11/1930 Davis -82 2,828,946 4/1958 Smith 165-83 2,965,358 12/1960 Behrens et a1 l65-82 FOREIGN PATENTS 1,211,918 10/ 1959 France.

734,008 7/1955 Great Britain.

ROBERT A. OLEARY, Primary Examiner. I, W. STREULE, JR., Assistant Examiner,

Claims

1. AN AIR HEATER COMPRISING: AN AIR PASSAGEWAY, FIRST, SECOND AND THIRD TUBE BANKS EACH INCLUDING: A MULTIPLICITY OF TUBES AND A TUBE SHEET SUPPORTING SAID TUBES, MEANS FOR MOVING RELATIVELY LOW TEMPERATURE AIR THROUGH SAID PASSAGEWAY INTO SUCCESSIVE CONTACT WITH THE OUTER SURFACES OF THE TUBES OF SAID FIRST, SECOND AND THIRD TUBE BANKS, MEANS FOR DIRECTING RELATIVELY HIGH TEMPERATURE HEATING GAS THROUGH TUBES OF SAID FIRST AND SECOND TUBE BANKS AND THROUGH TUBES OF SAID THIRD AND SECOND TUBE BANKS, WHEREBY HEAT IS TRANSFERRED TO THE AIR CONTACTING SAID TUBES, SUPPORTING MEANS CONNECTED TO THE TUBE SHEETS OF AT LEAST TWO NEIGHBORING TUBE BANKS AND EXTENDING ALONG THE ADJACENT EDGES THEREOF, MEANS DEFINING WITH SAID SUPPORTING MEANS A DUCT OPEN TO SAID PASSAGEWAY FOR CONDUCTING AT LEAST A PORTION OF THE AIR FLOWING THROUGH SAID PASSAGEWAY INTO