US2438851A

US2438851A - Plate arrangement for preheaters

Info

Publication number: US2438851A
Application number: US508452A
Authority: US
Inventors: Robert M Gates
Original assignee: Air Preheater Co Inc
Current assignee: Alstom Power Inc
Priority date: 1943-11-01
Filing date: 1943-11-01
Publication date: 1948-03-30
Anticipated expiration: 1965-03-30

Description

March 30, 1948. R, M, GATES 2,438,851

PLATE vARRANGILMENT FOR PREHEATERS Filed Nov. l, 1943 Paiemed Mar. 3o, 194s PLATE ARRANGEMENT FOR PREHEA'IERS Robert M. Gates, Scarsdale, N. Y., assignor to The Air Preheater Corporation, New York,

Continuation of application Serial No. 424,562, December 27, 1941. This application November 1, 1943, Serial No. 508,452

4 claims. (c1. 2574-6) The present invention relates to improvements in regenerative heat exchangers and particularly to minimizing clogging of the gas and air passages through the heat transfer plates in an air preheater of the rotary type.

In order to obtain the desired heat transfer,

most preheaters have the heat transfer plates packed in so closely that the channels through the heat absorbing surface are very narrow. When air preheaters are operatedV under such conditions that the temperature of the metallic heat transfer plates is at or below the dew point of the gases entering the preheater, moisture is condensed from the gases and causes corrosion of the plates. Fly ash and other material may adhere to the plates thereby reducing the free area of the gas and air channels. This may -resuit in clogging of the channels, particularly at the.cold end of the'preheater where condensation may occur on the surfaces of the plates due to moisture in the entering air or in cooled gases leaving the preheater. The present invention contemplates minimizing such clogging and itsV effects by increasing the net free area for gas flow between the heat transfer surfaces at the cold end of the preheater while maintaining' at other points an arrangement of heat transfer surface adapted to effect the desired heat transfer. This is accomplished by utilizing several types or arrangements of heat transfer plates so that the free gas areas at the cold end'of the preheater :may be different than at the hot end.

The draft and air pressure losses in an air preheater are dependent upon the Velocity of the gases and air, and upon the friction factor.` If

the spaces at the cold end are made wider, there y is a small reduction in' velocity, thus tending to reduce the draft loss.'. It is then necessary to increase the length of the gas passages so that enough heating surface is installed to reduce the gases to the proper final temperature. The net result of these two factors is practically no change in the total resistance.

The friction factor increases rather rapidly with decrease of the spacing or mean hydraulic If the spacing between plates 2 such time as the mean hydraulic radius is d creased by the clogging to the point where it is equal to the hydraulic radius of the closer spaced plates. Since deposits of material causing clogging are usually the same in thickness, Whether the plates are spaced on wide or narrow centers.' it is obvious that the resistance of the preheater is always less with the plates on wide centers at the cold end. In the above discussion it has been assumed that regardless of the spacing of the plates at the cold end, the same overall heat recovery is desired. The -metal temperature in the plates at the cold end is, therefore, the same, regardless of the spacing, and the possibility of clogging is the same inboth cases. The effect of wider spacing cn the plates is chiefly to reduce the effect of clogging on the resistance of the preheater.

In the drawings:

Figure 1 is an elevational View, partly broken away, of a Ljungstrom type regenerative air preheater embodying the present invention;

Figures 2, 3 and 4 are sectional views on correspondingly designated section lines in Figure 1 and illustrate three different types of heat absorbing surface in the rotor.

Figures 5, 6 and 7 are' views corresponding to Figs. 2 to 4 but showing a modified arrangement of the heat transfer surface utilizing flat rather than corrugated or undulated plates.

In the drawings the numeral I0 designates the housing of an air preheater-of the Ljungstrom type having a cylindrical rotor il divided by radial partitions or diaphragms I2 into a plurality of sector-shaped compartments. -Ezach rotor compartment contains regenerative heat transfer surfaces in the form of undulated or corrugated plates |3A, I3B, 13C which first absorb heat from the h ot gases when passing through the gas side of the preheater and then impart the heat to air passing through the air side of the heater,.as the rotor is turned slowly about its axis I4 by a m0- tor l5 operating through suitable reduction gearing. The inlet and outlet ducts for gas are designated 20 and 2l in Figure l and those for air are numbered 22 and 22 respectively. The preheater construction as described above is conventional.

In carrying out the invention undulated or corrugated plates are employed as usual for the heat transfer elements but they are mounted in two or more superimposed layers in the rotor il.`

As shown there are three layers, the elements ISA in the uppermost layer (Fig. 2) near the cold end of the preheater having deeper corruga- 3 tions than the elements ISB in the intermediate layer (Fig. 3) or in the lowermostlayer (Fig. 4). y

spacing plates 23B with notches or

corrugations

25, 26 which make theradial distance between elements I3B or between them andthe adjacent spacers 23B less than between elements I3A in the uppermost and greater than between elements I3C in the lower layer, the arrangement being substantially the same as in Figs. 2 and-4 except that the corrugations determining the radial width of the channels for the iiow of gas and air are in the spacers 23B rather than in the elements I3B themselves.

Here, however, the aggregate gas free area is greater than in the channels of Fig. 4 while the gas contacted area of the plates I3B (and spacers 23B) is lass.

With the arrangement described it is possible to pack the heat transfer plates in the lowermost layer or layers as closely together as required to obtain the desired heating effect while at the same time by utilizing plates having deeper corrugations in the cooler zones where corrosion vis apt to occur clogging of the preheater is minimized. Furthermore, the heat transfer plates in the intermediate or lowermost layers may be replaced, if required, by others more closely packed in order to increase the amount of heat transfer surface and the heat recovery. Conversely if less surface is required because the temperature of the entering gases is not as high as was anticipated in designing the preheater, the plates in the lowermost or intermediate area may be replaced by others having deeper corrugations.

The invention permits the use of an optimum spacing between the plates forming the heating surface and each layer thereof in order to minimize the eiect of deposits on the surface on the fluid vresistances. Where deposits form on the heating surface of an air preheater, the thickness of these deposits increases as the temperature of the surface decreases. At the cold end where greatest deposits of solids on the surface take place, the optimum spacing of the plates would be that at which the deposits are prevented from bridging over the space between the plates and closing the passages. As the spacing between the plates or the hydraulic diameter increases, less force is required to prevent the deposits from bridging over-and closing the spaces between the plates than whensmaller spaces or hydraulic diameters are used, the force available being inl the velocity of the fluids passing over the surface. It is possible to so select a spacing for any given the portion of the surface where there is a deposit of .22 `mm. thickness a height of notch'of 5 mm. instead of 2.5 mm. were-used, the pressure drop increase for this portion of the surface would be zgiegcreased from approximately '15% to rough- Y Thus another aim of the invention is to provide the spacing between the plates in each individual layer of surface lo as to obtain a mini-V mum increase in. iiuid resistance over the surface due to deposits thereon. the spacing in each rate of flow and any given type of deposit that L bridging over by the deposit of the spaces between the plates is prevented and this spacing would be the optimum. By lselecting an optimum spacing of heating surface in each layer of a preheater as covered by the invention, it is possible to select a larger height of notch for the portion of the surface where the heaviest deposit occurs or to graduate the depth of notch or spacing as operating conditions demand. Assuming that for layer corresponding to this condition being what we term the optimum spacing.

It will be apparent that in order to vary the hydraulic radius there are ways other than changing the depth of plate corrugations oi heat absorbing surface. For example, in a given gas passage even flat plates 30 may be utilized with spacers 3l having notches of appropriate depths to maintain them in spaced positions as appears in Figs. 5 to 7, there being fewerplates 3,0 sepa, rated by spacers 3l wiht deeper corrugation-like notches in the section at the cold end with the result that less of the passage is occupied by the plates 30 themselves and their spacers 3| with consequent increase in the hydraulic radius. A similar effect may be attained when using corrugated plates if instead of changing the radial depth of the corrugations their pitch is altered and the corrugations are thereby formed further apart so that the overall length or surface of each plate is decreased. v, In addition it may be mentioned that for a.

given standard height of rotor, the desired heat recovery can be obtained by varying the radial depth of the corrugations or the height of the heating elements. In other words, it would not be necessary to have so many dierent rotor heights as now being used but instead vary the radial depth of the corrugations or the height of the heating surface in order to obtain the required heat recovery.

This application is a continuation passages as to provide for counterow of the hot and cold gases through said preheater; regenerative heat exchange material comprising a plurality of superimposed layers of substantially parallel plates mounted in the rotor compartments and formed with corrugations spacing the plates and disposed generally parallel tothe direction of fluid ilow to form a multiplicity of channels parallel to the direction of uid flow with the plates in the layer nearest the air inlet and gas outlet end of the rotor having corrugations of greater depth than thoseinthe other layers so as to provide channels of greater cross-sectional area through the rst mentioned layer.

2. In a regenerative air preheater or the like having a rotor comprising a cylindrical shell divided by radial partitions into a plurality of sector-shaped compartments for carrying regenerative heat exchange material through air and gas passages formed in the heater housing parallel to the rotor axis, and inlet and outlet of that ined in my name on December 27, 1941, under Serial ducts for hot gas' and relatively cool air so connected to opposite ends oi said passages as to provide for counteriiow of the hot and cold gasesv through said preheater; regenerative heat exchange material comprising a plurality of superimposed layers of substantially parallel plates mounted in the rotor compartments and formed with corrugations spacing the plates and -disposed generally parallel to the direction of fluid ilow to form a multiplicity of channels parallel `to the rotor axis with the plates in the layer v they ends oi said passages as to provide for counteriiow of the hot and cold gases through said exchanger, and a rotor comprising a cylindrical shell divided by radial partitions into a plurality of sector-shaped compartments regenerative material to be carried successively through saidpassages: said materialbeing divided into a plurality of contiguous sections spaced axially of the rotor and each substantially iilled with a plurality o! undulated plates separated by intervening spaces with-the plate undulations shaped so that adjacent surfaces thereof form a multiplicity oi'channels for iiow of gastherethrough, the plate undulations being'proportioned dinerently in diiierent sections with the heat exchange surface presented thereby for heat exchange being least and the total area'oi' the channels ior gas 4D 517,600

iiow lbeing greatest in the section at the cold end of the rotor.

4. In a rotary regenerative heat exchanger for gaseous media having a casing providing es therethrough for hot and cold gases, respectively,

inlet and outlet ducts so connected to the ends of said passages as -to provide for counterow of the hot and cold gases through said exchanger, and a rotor comprising a cylindrical shell divided by yradial partitions into a plurality of sector-shaped compartments regenerative material to be carried successively through said passages; said material being divided into a plurality of contiguous sections spaced axially of the hydraulic radius oi' the individual channels dev lined by the plate undulations and the total area for gas iiow therethrough being greatest in,the section at the cold end of the rotor.

noemer M. Gams.

assumons' crrnn The following references are o! record in the` file of this patent:

Number Name 4 Date 35 1,461,357 Kling- July 10, 1923A I y FOREIGN PATENTS l Number Country d l Date 291,402 Great Britain Dec. 13, 1928 Umm STATES PATENTS Great Britain Nb. 2, 1040