US2594471A

US2594471A - Heat exchange apparatus

Info

Publication number: US2594471A
Application number: US702860A
Authority: US
Inventors: Leonard J Marshall
Original assignee: Combustion Engineering Superheater Inc
Current assignee: Combustion Engineering Inc
Priority date: 1946-10-11
Filing date: 1946-10-11
Publication date: 1952-04-29
Anticipated expiration: 1969-04-29

Description

April 29 1952 L. J. MARSHALL 2,594,471

HEAT EXCHANGE APPARATUS Filed oct. 11, 194e Patented Apr. 29, 1952 HEAT EXCHANGE APPARATUS Leonard J. Marshall, Tenay, N. J., assigner to Combustion Engineering-Superheater, Inc., a

corporation of Delaware Application October 11, 1946, Serial No. 702,860

This invention relates to heat exchangers and particularly to the arrangements of economizers and air heaters for large steam generating units such as are used in the power stations of public utilities.

The usual arrangement of the large steam generating units comprises a furnace which is generally provided with steam generating tubes on its walls for absorbing radiant heat from the burning fuel, followed, with respect to the flow of the gases from the furnace, by convection heat absorbing surfaces including steam generating boiler tubes placed either ahead of and/or following a superheater in which the steam is heated, an economizer for heating the feed water to the generator, and an air heater for heating the air for combustion.

The air heater is generally selected of the largest practical size since it is a relatively inexpensive form of heat absorbing surface. It provides an air temperature leaving the heater which approaches the entering gas temperature and often the temperature difference between the gas entering and the air leaving is less than 100 deg. F. As this difference becomes less, the heat absorption per unit area of heating surface diminishes and consequently the required amount of heating surface increases and approaches a practical limit. The temperature of the gas leaving the heater is generally determined by the desired overall eiiiciency of the steam generating unit. By selecting a maximum practical sized air heater, the permissible entering gas temperature is established.

The economizer is selected to raise the temperature of the feed water to below that at which the water boils within the steam generator, which is the saturation temperature of the steam. Customarily this water leaving temperature is about 40 to 50 deg. F. below the saturated steam ternperature at the normal operating loads of the steam generator. In this manner assurance is provided against generating steam in the economizer under the normal operating loads.

Having established the gas temperature entering the air heater and calculating the heat absorbed by the water owing through the economizer when heated up to the desired temperature, the temperature of the gas entering the economizer is determined. From the calculations of the heat absorbed from the burning fuel by the steam generating tubes in the furnace and that absorbed from the gases leaving the furnace vby the boiler tubes ahead of the superheater and by the superheater tubes, the temperature of 5 Claims. (Cl. 257-220) gases leaving the superheater is established. The difference in the temperatures between the gases leaving the superheater and that of the gases entering the econornizer is a measure of the amount of heat to be absorbed from the gases by additional steam generating boiler tubes.

'It is costly to provide a large number of such tubes since the number and spacing of these tubes affects the diameter and thickness of the drums `into which they connect and the drums constitute relatively large percentages of the cost of high pressure steam generators. It is therefore desirable to reduce or even eliminate the steam generating boiler tubes located between the superheater and the economizer in steam generating units, where such tubes are now required.

It is accordingly an object of this invention to provide improved apparatus for reducing or even eliminating said steam; generating tubesV and therewith the, cost of the steam generating unit.

How the foregoing, together with other objects and advantages as may hereinafter appear or are incident to my invention are realized, is illustrated in preferred form in the accompanying drawings wherein: I

Figure l is a vertical sectional view of a steam generating unit including heat recovery apparatus embodying the invention;

Figure 2 is a section taken on line 2-2 of Figure 1; and

Figure 3 is an end view of a damper control quadrant.

The embodiment of the steam generating unit illustrated includes thefurnace A in which fuel is burned and from which the products of combustion or gases pass successively through a superheater B, a boiler C, a first vair heater D, an economizer E and thence through a second air heater F. The furnace A is of well known construction which may have steam generating tubes lll and ll (here indicated schematically) mounted on the side and end walls respectively. Burners l2 may be located adjacent the corners or" the furnace A through which the fuel may be projected into the furnace in such manner as to cause a turbulent vortical flame.

The boiler C comprises a bank of vertical tubes I3 (also shown single line) which precede the superheater B with respect to the direction of gas flow and a bank of tubes I4 which follow the superheater B. Both banks of tubes are connected to top and bottom drums I5 and l5.

Bafiles Il and I8 direct the gas now downwardlythrough superheater B andl upwardly through bank M. A conduit I9 conducts the gases from the boiler C to the respective gas inlets and 2| of air heater D and economizer E. Conduit 22 receives the gases from the respective gas outlets 23 and 24 of air heater D and economizer E and conveys them to the gas inlet 25 of air heater F. Conduit 26 conveys the cooled gases from the gas outlet of air heater F. Within conduit 22 may be provided dampers 21 and 28 which may control the eillux from the gas outlets 23 and 24 of the respective air heater D and economizer E.

Air heaters D and F may comprise abundle of

tubes

29 and 30 respectively throughA which the gases flow while the air passes over the outside of said tubes. Air enters the inlet 3| ofair heater F through conduit 32. By means of partitions 33. 34 and 35

tube sheets

36 and 31 and

conduits

38, 39 and 40, the air is caused to flow in a sinuous path as shown by the arrows, across the tubes of air heater F. A conduit 4l connects air outlet 42 of air heater F with the air inlet 43 of air heater D. By means of partition 44 and

tube sheets

45 and 46 and conduit 41 the air is caused to flow through air heater D in a sinuous path as shown by the arrows. Conduit 48 is connected to the air outlet 49 of air heater D and conveys the heated air to the point of use.

Economizer E may comprise a multiplicity of.

tube coils 50 placed side by side to forma bundle of tubes across which the gases ow and through which feed water is conducted from` inlet header 5| thence through said coils through outle header 52.

In Figure 3 is shown a typical quadrant having lever 54 for adjusting dampers 21, the lever being connected to the shaft 55 of damper 21. The dampers may be constructed to form a group of louvres extending transversely of the conduit 22, the louvres being interconnected by the usual connecting rods, not shown. Like damper 21, damper 28 is constructed in similar manner and adjusted by a similar quadrant.

From the above it will be seen that the iiow of gases leaving the boiler is divided-into two portions, one portion passing through the economizer E and the other portion passing through the air heater D. Thereafter both portions join and pass through air heater F. It will be noted that all of the air that passes through air heater F also passes through air heater D so that the ratio of air to gases is relatively increased within air heater D. This apportionment of the air quantity will cause the metal or .tube temperatures of air heater D to be substantially lower than they would be if all of .the-gases passed therethrough and I have found that forfv this reason I can deliver relatively hotter gases to air heater D and yet maintain safe metal temperatures.

By delivering hotter gases to air heater D, which then absorbs relatively more heattherefrom, it becomes possible to remove a substantial part of and in certain circumstances all of heat absorbing tube surface I4 from the boiler C, which surface is now no longer needed to absorb said heat. The combined air heaters D and F will then absorb a greater amount of lheat than will a single air heater like Fif used alone in the usual manner and yet the increase in surface of heaters D land F over a single heater will not be proportional absorption. l

The economizer E, like air heater D,'receives but a portion of the gases leavingthe boiler so to the increased f heat that the ratio of water to gas is relatively high, and it therefore also may receive hotter gases without danger of adding so much heat as to cause steaming. Because the economizer is now located in a hotter gas zone, its size will be smaller and its cost less. As in the case of air heater D, by delivering hotter gases to the economizer E, it is now possible to remove a substantial part of and in certain circumstances all of heat absorbing tube surface I4 from the boiler C. The lower drum. I6 will then reduce to the size of a header, accommodating but a relatively few rows of boiler tubes.

Obviously the portions of the gases delivered to the air heater D and the economizer E may be distributed to suit desired temperatures of air and water leaving these elements. Under certain conditions. economizer E may be omitted, whereupon a portion of the gases may ow through by-pass 56 around air heater D, or the economizer E may be replaced by other heat exchanger.

In cases where low outlet gas temperatures from the air heater F can be justified from the standpoint of increased over-all efficiency, it is sometimes impossible to obtain suihcient air temperatures for mill drying of high moisture coals when using the single air heater. The suggested arrangement of two-stage air heater provides higher air temperatures which can be then used for successful mill drying.

By-pass 56 controlled by damper 51, which may be adjusted like dampers 21 and 28, may .be provided between the air heater D and economizer E. Such a by-pass may be advantageously used at low loads of the boiler when there is a possibility of the lower end of air heater F becoming too cold and thereby subject to corrosion due to condensation of the moisture in the gases. Damper 51 is then opened suiciently to permit some of the hot gases to pass uncocled into air heater F and thereby raise the temperature of the entire heater and particularly the cold end thereof so as to prevent said condensation and incident corrosion.

The by-pass 5E and control damper 51, together with dampers 21 and 28 may be used to advantage during the starting up of the steam generating unit, by stopping all gas ow through the economizer E, so as to avoid steaming therein and causing the gases to flow throughair heater D and by-pass 56. By increasing the normal flow of gas through the air heater D at starting, relatively hotter air may be delivered from air heater D to pulverizing mills for the purpose of drying the fuel.

While the preferred embodiment of my invention has been shown and described, it will bc understood that minor changes in construction, combination and arrangement of parts may vbe made without departing from the spirit and scope of the invention as claimed.

What I claim is:

1. In heat exchange apparatus for transferring heat from high temperature gases obtained from a given hot gas source; the combination of a first and second air heater, each air heater having an air inlet and an air outlet, a gas inlet and a gas outlet; a heat exchanger having a gas inlet and a gas outlet; conduit means for delivering said high temperature gas to both the gas inlet of said first air heater and the gas inlet of said heat exchanger; conduit means for delivering said high temperature gas to the gas inlet of said second air heater thereby by-passing said rst air heater and said heat exchanger; conduit means connecting the gas outlet of both the rst air heater and the said heat exchanger to the gas inlet of the second air heater; conduit means for delivering the air to be heated to the air inlet of the second air heater; conduit means connecting the air outlet of the second air heater to the air inlet of the rst air heater; conduit means for removing the heated air from the rst air heater; and means for conducting a fluid through said heat exchanger for extracting heat from the portion of the said high temperature gases passing therethrough.

2. In apparatus for usefully reclaiming heat from the high temperature gases obtained from a given hot gas source; the combination of an economizer for heating water; rst and second air heaters organized to accomplish successive heating of air serially passed through both heaters; means for flowing one portion of said high temperature gases through said economizer whereby to reduce their temperature to an intermediate value Iby transfer of heat therefrom to said water; means for owing another `portion of said high temperature gases through said first air heater whereby also to reduce their temperature to an intermediate value by transfer of heat therefrom to air coming into that first heater from said second heater; means for flowing through said second air heater said intermediatetemperature gases from the economizer plus said intermediate-temperature gases from the first air heater whereby to bring those combined oneportion and other portion combustion gases to a still lower temperature by transferring further heat therefrom to the unheated air entering that second heater; and means for forming a gas bypass around both the said economizer and the said first air heater whereby to flow all the portions of said high temperature gases over said second air heater at substantially the same temperature at which said combustion gases would otherwise enter said economizer and said rst air heater.

3. In heat exchange apparatus for transferring heat from high temperature gases obtained from a given hot gas source; the combination of a rst and second air heater each air heater having an air inlet and an air outlet, a gas inlet and a gas outlet; a heat exchanger having a gas inlet and a gas outlet; `conduit means for delivering said high temperature gas to both the gas inlet of said first air heater and the gas inlet of said heat exchanger; conduit means for delivering said vhigh temperature gas to the gas inlet of said second air heater thereby icy-passing said rst air heater and said heat exchanger; conduit means connecting the gas outlet of both the rst air heater and the heat exchanger to the gas inlet of the second air heater; conduit means connecting the air inlet of the second air heater to the air inlet of the first air heater; conduit means for conducting a ilow of air through the rst and second air heaters to be heated therein; and means for conducting a fluid through said heat exchanger for extracting heat from the portion of the gases passing therethrough.

4. In a heat exchange apparatus for transferring heat from high temperature gases obtained from a given source; the combination of a rst air heater and a second air heater each heater having means forming a gas space for flow of gas therethrough and means forming an air space for flow of air therethrough; a heat exchanger having means forming a gas space for the flow of gas therethrough; means for conducting a predetermined portion of said high temperature gas through the gas space of said first air heater; means for conducting a predetermined portion of said high temperature gas through the gas space of said heat exchanger; means for conducting a predetermined portion of said high temperature gas through the gas space of said second air heater thereby by-passing said first air heater and said heat exchanger; conduit means connecting the gas space of the rst air heater to the gas space of the second air heater, conduit means connecting the gas space of the heat exchanger to the gas space of the second air heater; means for conducting a flow of air through the air space of the rst and second air heaters to be heated therein; and means for conducting a uid through said heat exchanger for extracting heat from the portion of said high temperature gases passing therethrough.

5. In a heat exchange apparatus for transferring heat from high temperature gases; the combination of a rst air heater and a second air heater each heater having a gas inlet and a gas outlet; a heat exchanger having a gas inlet and a gas outlet; means for conducting a predetermined portion of said high temperature gases to the gas inlet of said first air heater; means for conducting a predetermined portion of said high temperature gases to the gas inlet of said heat exchanger; means for conducting a predetermined portion of said high temperature gases to the gas inlet of said second air heater thereby by-passing said rst air heater and said heat exchanger; conduit means connecting the gas outlet of the first airrheater to the gas inlet of the second air heater; conduit means connecting the gas outlet of the heat exchanger to the gas inlet of the second air heater; conduit means for conducting a ow of air through the rst and secondair heaters to be heated therein; and means for conducting a 'uid through said heat exchanger for extracting heat from the portion of the gases passing therethrough.

LEONARD J. MARSHALL.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,075,044 Lucke Mar. 30, 1937 2,217,512 Donley Oct. 8, 1940 2,386,188 Artsay Oct. 9, 1945 2,392,325 Kuhner Jan. 8, 1946 2,418,815 Baver Apr. 15, 1947 FOREIGN PATENTS Number Country Date 429.797 Great Britain June 6. 1935