US2854220A

US2854220A - Air heater

Info

Publication number: US2854220A
Application number: US621271A
Authority: US
Inventors: Paul H Vaughan
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1956-11-09
Filing date: 1956-11-09
Publication date: 1958-09-30
Anticipated expiration: 1975-09-30

Description

Gas

P. H. VAUGHAN 2354,2263) AIR HEATER F iled Nov. 9, 1956 Fig. H.

Air Oui INVENTOR Paul H. Vaughan ATTORNEY United States Patent Am HEATER Paul H. Vaughan, Pound Ridge, N. Y., assignor to Combustion Engineering, Inc., New York, N. Y., a corporation of Delaware Application November 9, 1656, Serial No. 621,271

6 Claims. (Cl. 257-1) 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 efliciency. 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 difliculty, 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 positioning of a steam heater at the air inlet of the air heater. Both of these arrangements raise the tem perature of the air entering the air heater and thereby prevent the temperature of the surfaces of the heat exchanger from being lowered below the dew point of the combustion gases. However, the 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 and requires the use of condensate piping and relatively expensive controls. In certain installations these specific arrangements as well as other arrangements heretofore provided have not proved to be entirely satisfactory solutions to the problem 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 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 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 mem- Patented Sept. 30, 1958 hers and a portion of the heat imparted to these mem bers 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 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 separate heat exchange system for transferring heat from the air after it has passed over a portion of the heat exchange surface to the air immediately prior to contacting this surface. This heat exchange system may comprise a finned tubular coil or panel which has a portion thereof disposed in the path of the air flow downstream of at least a portion of the heat exchange surface of the heat exchanger and a portion disposed at the air inlet of this heat exchanger. Within this coil there is provided a heat exchange medium, such as a high boiling point fluid, which is continuously circulated through the system or panel.

It is an object of this invention to provide an improved heat exchanger organization 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 2-2 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 forth across tube bundle 10 with this path being defined by longitudinally spaced

bafiles

24, 26, 28,

with baffles 24 and 23 extending from the side wall 30 of housing 14 across tube bundle 10 and terminating in spaced relation with the opposite Wall while bafile 26 is oppositely disposed extending from said opposite wall across tube bundle 10 and terminating in spaced relation with wall 30.

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 passes downwardly through the tubes and out of the bottom thereof into duct 36 while the air passes back and forth over the exterior of the tubes in the manner indicated by the arrows and in general couuterflow relation with the heating gas.

The section of tube bundle located between bafile 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 hundle'where condensation is likely to occur. This is particularly 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 the 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 and separate heat exchange system which is illustratively disclosed as comprising conduit 38 sinuously bent as shown in Fig. 2 to form panel 40 which extends across the interior of housing 14, is in parallel relation with tube bundle 10 and extends through baffie 24 so that the upper portion 42 of the panels is swept by air that has twice traversed the tube bundle while the lower portion 44 is contacted by the air prior to its initial contact with the tube bundle it A heat exchange fluid, which is preferably a high boiling point fluid such as diphenyl oxide, is continuously circulated through conduit 38 by pump 46 and the heat transfer capacity of the panel is increased by providing the coils thereof with fins 43 which are parallel with the air flow. With this organization heat is transferred from the air after it has been partially heated by the air heater to the cold air entering the air heater thereby preventing this cold air from lowering the metal temperature of the heat exchanger to the extent that it ordinarily would and accordingly reducing the possibility of this temperature falling below the dew point of the heating gases.

With this improved organization the heat exchanger is entirely self-contained and no external connections such as steam lines or condensate drains are required as was the case when a steam heater was employed to accomplish the result here achieved and no special duct work or large fan power loss is required as has been the case when air was recirculated or was passed in indirect heat exchange relation with the incoming air. Furthermore because of the compactness of the auxiliary heat exchange system it may be conveniently provided in almost any existing air heater or other heat exchange organization.

In boiler or power plant organizations it often happens that at low loads both steam heating and recirculation of air to prevent condensation at the cold end of the air heater are ineffective. This is so in the former instance because the steam is generally taken from a low pressure bleed point of a turbine and below a certain load the pressure at this point is too low to supply any steam for heating air while in the latter case, at low loads on the boiler such a high recirculation quantity of air is required that this method becomes totally impracticable.

While I have illustrated and described a preferred embodiment of my 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. 1 therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. Heat exchange apparatus for heating air by hot combustion gases from a boiler or the like comprising means providing'heat exchange surface over which said air and said hot combustion gases are conveyed in a manner so that heat is imparted indirectly fi'om said gases to said air, a closed heat exchange system having a.

fluid circulated therethrough that is entirely separate from said gas and air, said system having a portion disposed in heat exchange relation with the air after it has passed over a substantial part of said heat exchange surface to impart heat from said air to said fluid and a portion in heat exchange relation with the air immediately prior to its passage over said heat exchange surface to impart to this entering air some of said heat imparted to said fluid.

2. In combination, an air heater wherein air is heated by hot combustion gases with these gases transferring heat to and the air absorbing heat from metallic memhers, an auxiliary heat exchange system for indirectly transferring heat from the air after contact with said metallic members to the air prior to its contact with said metallic members, said system comprising tubular members through which a fluid independent of said gas and air is circulated and which have portions disposed inheat exchange relation with the air after it has been heated by passage through at least a portion of the air heater and other portions in heat exchange relation with 1 said air prior to its entrance into and passage through the heat exchanger.

3. A recuperativeair heater employing metallic heat an indirect transfer of heat from the gases to the air, a

closed heat exchanger through which a separate heat transfer fluid is conducted and which has a portion disposed so the air flows in contact therewith after it has been heated by the combustion gases to transfer heat to said heat transfer fluid and another portion disposed so the air flows in contact therewith immediately prior to entrance to and passage over said metallic heat transfer surface to thereby absorb a portion of this heat transferred to this medium to heat the air prior to its contacting said metallic heat transfer surface.

4. A heat exchanger comprising a bundle of parallel,-

spaced tubes, a housing disposed about said tubes and forming therewith a chamber separated from the interior of said tubes, said housing having an inlet and an outlet at opposite ends relative to the disposition of the tube bundle, baflie means in said housing generally transversely of said tubes forming a sinuous passageway in said chamber traversing said tubes for the conveyance of a fluid to be heated over said tubes with the tubes being adapted to have a heating fluid conveyed therethrough, a tubular heat exchanger through which a fluid separate from those flowing over and through the tubes is circulated and having a portion disposed in the inlet of the housing and a portion in said sinuous passage at a downstream location relative to the flow of the fluid to be heated and where this latter fluid has passed over said tubes so that this fluid to be heated serially passes over said portions of said system.

5. A heat exchanger comprising a pair of end walls, an elongated casing extending between said end walls, a tube bundle disposed in and longitudinally of said casing, said bundle being comprised of spaced parallel tubes the ends of which are received within complementary apertures provided in said end Walls, a' plurality of baffles alternately extending from a pair of opposed walls of said casing transversely of the tubes of the tube bundle and terminating in spaced relation with the opposite wall thereby forming a sinuous passageway in the casing which winds across the tube bundle, said casing having apertures therein at the extremities of said passageway for the ingress and egress of air, a heat exchanger in the form of a panel of sinuously bent tubular members disposed;

in the casing so as to extend across the two legs of the initial loop of said sinuous passageway, traversing the passageway adjacent its inlet and upstream of the tube bundle and also downstream ,of the tube bundle where the passageway has traversed the tube bundle twice, and

5 6 pump means for circulating a separate heat exchange References Cited in the file of this patent medium through said tubular members. UNITED STATES PATENTS 6. The organization of claim 5 wherein said tubular members are provided with fins the plane of Which is 2735660 Cralg 1956 parallel with the air flow. 5 FOREIGN PATENTS 530,792 Great Britain Apr. 5, 1940 929,503 France July 15, 1947