US2970811A

US2970811A - Self protecting air heater

Info

Publication number: US2970811A
Application number: US707280A
Authority: US
Inventors: Ruch Alan; Paul H Vaughan
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1958-01-06
Filing date: 1958-01-06
Publication date: 1961-02-07
Anticipated expiration: 1978-02-07

Description

Feb. 7, 1961 A. RUCH m1 2,970,811

SELF PROTETING AIR HEATER Filed Jan. 6, 1958 Gas In Fla! w OuT iat iiiiiiiinFkza /0\ fill? AIL In a ZZZZZZZZZZEL \44 .;=/z 40 46 INVENTORS. L Ian Ruck 2 Gas OuT BY Paul H. Vaughan United 2,970,811 SELF PROTECTING AIR HEATER Filed Jan. 6, 1958, Ser. No. 707,280 2 Claims. (Cl. 257-224) This invention relates to heat exchangers and has particular relation to heat exchangers wherein the heating medium is a moisture containing gas the condensate of which contains corrosive constituents that attack the surfaces of a heat exchanger with heat exchangers employing combustion gases as the heating medium being in this category and with air heaters for steam generators and the like being particularly concerned.

The problem of preventing the formation of condensation in and the resulting corrosion of heat exchange surfaces has long been a troublesome one in the steam generator field as well as other fields where combustion gases are employed in a heat exchanger to heat a relatively cold fluid medium and where it is desired to abstract as much heat as possible from the combustion gases in order to have a high operating efficiency. In the air heaters for steam generators, it often happens that when the steam generator is operating at a relatively low load condensation of the combustion gases will occur at the cold end of the air heater unless specific precautions are taken to prevent the temperature of the surfaces of the heat exchanger from being lowered to a point below the dew point of the combustion gases.

Several arrangements have heretofore been proposed and are presently used to overcome this difficulty the most widely used of which, as applied to the air heater of a steam generator, are the recirculation of air, i.e., the introduction of a portion of the hot air leaving the air .heater into the cold air entering the air heater, and the vemploying of a steam heater operating on steam generated by the steam generator and positioned at the air inlet of the air heater. Both of these arrangements raise the temperature of the air entering the air heater and thereby prevent or at least tend to prevent the temperature of the surfaces of the heat exchanger from being lowered below the dew point of the combustion gases. Howeverfthe first of these arrangements requires that the fan which forces the air through the air heater handle an additional quantity of air, i.e., the recirculated air, resulting in a larger fan as well as an increase in the power consumption to run the fan, while the second of these arrangements utilizes a substantial quantity of steam which is bled from the system of the steam generator at some location, generally in the turbine, resulting in a reduction in efficiency and this arrangement also requires the use of condensate piping to carry the condensate back to the system of the steam generator and relatively extensive controls. In certain installations these specific arrangements as well as other arrangements heretofore provided have not proved to be entirely satisfactory solutions to the problems of preventing the formation of condensation on and corrosion of the surfaces of the heat exchanger. This invention is concerned with an improved heat exchange organization and method for overcoming or diminishing the problem relating to the corrosion of the heat exchanger due to condensation of the heating medium.

in accordance with the invention there is provided an rates Patent C indirect heat exchanger comprised of metal members that are heated by a heating medium, such as combustion gases from a furnace, the condensate of which will contain corrosive constituents. Air or other fluid to be heated is passed in contact with or over these metal members and a portion of the heat imparted to these members from the heating gas is absorbed by this air; This heat exchanger may be of the so-called recuperative or regenerative type and the direction of flow of the heating medium and the air or other fluid that is heated are preferably opposite to each other. In order to raise the temperature of the air before it contacts the metal heat exchange surface and thereby prevent or tend to prevent reduction of the temperature of the metal surface below the dew point of the heating gas and accordingly eliminate the formation of condensate from this gas on this surface there is provided a system for transferring heat from the combustion gases after they have passed over the metal members of the air heater to the air immediately prior to its contacting this surface. This system comprises a small vapor generator or boiler that receives the combustion gases exiting from the air heater as its heating source. These gases are passed in indirect heat exchange relation with a vaporizable fluid, preferably water, that is contained in the boiler. The heat from these gases vaporizes this fluid and the vapor is conducted through tubular members that are disposed in the path of the cold air entering the air heater and at a location upstream of where this air contacts the metal heat transfer members of the air heater. Heat is thus transferred from this vapor to the entering air stream of the air heater raising the temperature of this entering air and accordingly tending to prevent lowering of the metal temperature of the air heater below the dew point of the combustion gases. The tubular members thus act as condensers for the vapor that passes through them and these members are constructed so that condensate formed within them will drain by gravity back to the body or pool of vaporizable fluid.

It is an object of this invention to provide an improved heat exchanger organization and method for reducing or eliminating the possibility of deterioration of the heat exchanger due to the condensing of the heating medium on the surfaces of the heat exchanger.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

Fig. 1 is a vertical sectional view of a preferred embodiment of the invention which takes the form of a tubular air heater such as employed with steam generators and the like.

Fig. 2 is a partial sectional view taken along line 2 of Fig. 1.

Referring now to the drawing wherein like reference characters are used throughout to designate like elements, the illustrative embodiment of the invention depicted therein is a so-called tubular air heater which comprises a tube bank 10 formed of parallel spaced tubes 12 and which is positioned within housing 14 with the ends of the tubes being received in complementary openings provided in the end faces 16 and 18 of this housing.

Air is admitted to housing 14 through inlet 20 and passes from the housing through outlet 22. This air in passing through housing 14 travels a sinuous or tortuous path back and further across tube bundle 10 with this p'ath being defined by longitudinally spaced

baffles

24, 26', and 28, with

baifies

24 and 28 extending from the side wall 30 of housing 14 across tube bundle and terminating in spaced relation with the opposite wall32 while baffle 26 is oppositely disposed extending from wall 32 across tube bundle 10 and terminating in spaced relation with wall 30.

I A heating gas, such as combustion gas from the furnace of a steam generator or the like, enters the upper end of tubes 12 from duct 34- and passed downwardly through the tubes while the air passes back and forth over the exterior of the tubes in the manner indicated by the arrows and in general counterflow relation with the heating gas.

The section or" tube bundle 10 located between baffle 24 and end face 16 of housing 14 is the so-called cold end of the heat exchanger with the metal temperature of this portion of tubes 12 being the lowest and accordingly it is this portion of the tube bundle where condensation is likely to occur. This is particuarly so in certain installations and under certain operating conditions. For example, in an air heater organization for a steam generator it often happens that when the steam generator is operated at low loads, unless special precautions are taken the temperature of the metal at the cold end of air heater will fall below the dew point of the combustion gases causing condensation to form on the surface of the air heater with this condensation being highly corrosive and rapidly deteriorating the metal of the heat exchanger.

In order to raise the temperature of the air entering the heat exchanger and initially contacting tubes 12 there is provided an auxiliary steam generating system for absorbing heat from the combustion gases after they are passed through the air heater and transferring this heat to the air entering the air heater through inlet 20. As embodied, this auxiliary system comprises a boilerlocated at the combustion gas outlet of the air heater and which is effective to generate steam with this being conveyed in indirect heat exchange relation with the air entering inlet 20. This boiler includes housing 40 secured to the lower end of the air heater housing and through which extends tubes 12a, with these tubes being extensions of tubes 12 of the bundle 1i) and with the lower end of tubes 1251 being received in complementary openings provided in the bottom 42 of housing 40 in a fluid tight manner. Extending upwardly from housing 40 and transversely of the air inlet 20 are a number of inverted U-shaped tubular members 44 with these members, as illustrated, being provided with fins which are parallel to the direction of air flow through inlet 20. Within housing 40 there is contained a vaporizable fluid, which may be water, and which surrounds the tubes 12a that extend down through the housing. The combustion gases that pass down through tubes 12a and to outlet duct 46 heat this body of fluid and vaporize a portion of it with the vapor passing into the tubular elements 44 and accordingly heating the. air entering the air heater and prior to its contact with the tube bundle 10.

The tube members 44 act in effect as a condenser for the vapor generated in the boiler and these members are disposed so that the condensate that is formed in them drains by gravity into housing 40 and into the body of fluid therein.

The amount of heat initially imparted to the 'air entering the air heater through the medium of this small auxiliary boiler and the heat exchange surface (tubular members 44) associated therewith, can be regulated, within predetermined limits, to assure that the metal temperature of the tubes 12 at the cold end of the air heater does not fall below the dew point of the combustion gases passing down through these tubes. This regulation is effected by regulating the height of thefluid in housing 40 which will of course regulate the amount of heat transferred from the combustion gases through the walls of tubes 12a to the body of fluid. The regulation of fluid height may be had by the valved conduit 48 through whichfluid may be added to or removed from housing 40..

With this improved organization a self-protecting air heater is provided which is extremely simple in construction and operation, requiring no pumps or complicated and expensive regulating equipment and which does not lower the eficiency of operation of the system in which the air heater is incorporated but rather employs what would otherwise be waste heat. With this organization the heat input to the cold air entering the air heater may be regulated as required to insure that condensation of combustion gases on the metal at the cold end of the air heater does not occur with this regulation being extremely simple and effective and once the desired regulation for a particular installation is eifected the organization will continue to give the required protection without any further attention to the air heater being necessary.

.While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention. What we claim is:

1. A combustion gas to air air heater comprising a housing within which is disposed metallic heat exchange surface and in contact with which air and hot combustion gas are directed for the indirect transfer of heat from the gas to-the air, means for directing gas downwardly through the housing,'said'ho using having the outlet for combustion gases at the lower region thereof and the inlet for air above and adjacentto said outlet another housing at the outlet for said gases from the first mentioned housing and secured directly to and forming an extension of the first mentioned housing, said other housing extending beneath the air inlet for the first mentioned housing and containing a vaporizable fluid, means operative to convey said gases directly from said outlet in indirect heat exchange relation with said fluid to vaporize the same, and conduit means for the vapor thus generated extending upward from said other housing in the inlet of the first mentioned housing in the path of the air entering said first mentioned housing to initially heat said air, said conduit means being disposed so that condensate formed therein drains back to the body of vaporizable fluid.

2. An air heater comprising a vertically disposed tube bundle adapted to have hot combustion gases passed downwardly therethrough, a housing disposed about the upper portion of said bundle and including means to conduct air over said bundle from the lower to the upper portion in serpentine fashion, another housing disposed about the lower portion of the tubes and extending from the first mentioned housing, said other housing having a body of water disposed therein through which the lower portion of said bundle extends so that the gases passing through said tubes will vaporize said water, and conduits communicating with and extending up from the upper region of said lower housing into heat exchange relation with the air to be heated prior to its contacting said tube bundle for imparting heat from said vapor to said air.

References Cited in the file of this patent UNITED STATES PATENTS 1,123,392 Schmidt Ian. 5, 1915 1,941,365 Patterson et a1 Dec. 26, 1933 2,735,660 Craig Feb. 21. 1956 2,854,220 Vaughan Sept. 30, 1958 FOREIGN PATENTS -12,699 Great Britain Nov. 14, 1912