PH26269A

PH26269A - An air heater for corrosive atmosphere

Info

Publication number: PH26269A
Application number: PH38716A
Authority: PH
Inventors: Yves Braud
Original assignee: Yves Braud
Priority date: 1989-05-30
Filing date: 1989-05-30
Publication date: 1992-04-01

Description

. symm t! ei ’ a : 26 9 g! 2 0 G1 JN 14 P1:00

FIELD OF THE INVENTION

The present invention cree BRI for boilers, ovens and other similar thermal equip= ment, in which the products of the hot and corrosive combustion are used for heating cold aire

It is known that the use in such thermal eqguipe ment of fossile fuels, particularly liquid, generates fumes which include more or less large quantities of polluting and corrosive agents, notably sulfur oxidized components and sodium and vanadium salts. The result is that air heaters formed of nests of tubes placed in the path of travel of the corrosive combustion gases prior to their discharge in the atmosphere are sub jected to corrosion hazards at high temperatures due to the sodium/vanadium salts, and to corrosion hazards at low temperatures due to the acid condensations caused by the presence of sulfur oxides. : BACKGROUND OF THE INVENTION

Air heaters with tubular nests resisting the corrosion caused by the condensation of the fume : acid vapours are known (see notably FR-A=2, 454,598-2, 2,545,197) in which the incoming air is subjected in the first tubes of the nest to a preheating at a high temperature, in such manner that the temperature of “a. BAD ORIGINAL 9

- the tubes is maintained at a value which is higher than that of the acid dew point of the fumes. How ever, these known solutions do not prevent corrosion caused by the sodium and vanadium salts since pre heating the air flowing in the first tubes of the tubular nest subjected to the action of the corrosive fumes maintains these first tubes at a temperature which is above the corrosion critical temperature of sodium/vanadium, ‘

One will understand that in order to avoid, or at least reduce, simultaneously the corrosion hazards due to the sulfur oxidized components and the sodium and vanadium salts, it is imperative to main~ tain the skin of the heater tubes in contact with the fumes, on the one hand at a temperature which is be. oo low the corrosion critical temperature of sodium/= vanadium, and on the other hand at a temperature which is above the acid dew point of the fumes. ‘ In an attempt to solve this problem; air heaters

Tan br 20 Pi tubular nests are provided including a nest made of corrosion resisting tubes and fed with air at a temperature and at a flow rate such‘that the tempera- - ture of the skin of these tubes remains below the cor- rosion critical temperature of the sodium and vanae~ dium salts. On the other hand, in order to reduce

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Cog ane . } “3 - aap a . iu the corrosioh hazards due to the acid condensations caused by the sulfur oxides, the final passes, in the direction of the flow of the fumes, can be effected in a cocurrent way (meaning that the two | fluids: fumes and air to be heated, flow in the same direction), thereby avoiding a too low tempera- ture on the tube skins.

According to this solution, one will under- . stand that the design of the air heaters is fixed from the very start and that it is not possible to thereafter adapt the design to the various operating conditions usually encountered in the equipment mounted upstream. This fixed design is therefore a . B na jor disadvantage since it not only causes possible throughout drops but it does not allow total supresm - . BE sion of corrosion hazards in the extreme cases of : maximum and minimum operation. ! oo | OBJECTS AND SUMMARY OF THE INVENTION

Lo . : An object of present invention is therefore to mitigate the disadvantages of the hereabove dese . cribed prior art solutions by providing an air heater "which includes means allowing controlling permanently. : the temperature of the tubular nest tubes which are most subjected to corrosion hazardsa :

Cu BAD ORIGINAL 9

Consequently, the invention relates an air heater resisting to corrosion, using combustion products from thermal equipment containing pollute : ing and corrosive agents such as sulfur oxidized components and sodium and vanadium salts, which in- cludes at least one tube nest through which flows the air to be heated, thich are placed in the path of travel of the corrosive fumes, prior to their disw charge in the atmosphere, said corrosive fumes floww ing countercurrent to the air to the heated and flow~ ing through the tubes, the heater includes a pro- tective tubular nest; placed upstream of said tube nest, this protective nest being fed with a constant flow of cold air circulating in the same direction, as the fumes. The cold air flow and the exchange To surface of said protective nest are chosen in such manner that the temperature of the first tubes of the heater tubular nest is at all times below the sodium and vanadium corrosion critical temperature, and the rere 220 7 dofigtant air flow exiting from said protective nest

EE is thereafter redistributed at the inlet or at the outlet of the heater tubular nest, as a function of the operating conditions, so that the temperature of the skin of the tubes of this tubular nest is al- ways slightly above the acid dew point temperature ng for oo B ER. _ 5. BAD ORIGINAL og of the fumes. 9 6 26 9 ; . Other features and advantages of the pre~ sent invention will become morc apparent from the , following description made with reference to the accompanying drawing, the single FIGURE of which _ being a schematic view showing a non-limiting embodiw ment of the air heater according to the invention.

DETAILED DESCRIPTION QF THE INVENTION

: 10 with reference to the drawing, one sces that ) in this embodiment, the air heater designated as a whole by reference 10 includes two tubular nests 164 : 18 of countercurrent type, meaning that the air to ) ’ be heated flows in their tubes countercurrent to the corrosive combustion gases forming the heat~conveyor means. These tubular nests 16, 18 are placed in the flue 12 where the corrosive combustion products flow, perpendicularly to the tubes of nests 16, 18,

The tubular nests 16, 18 are fed with air at a témperature Sg via one or several ventilators such as 1h, 147, the air flowing from the first nest 16 go the second nest 18 by passing through a lower disge tribution box 20, prior to being discharged in the / "e- |3AD ORIGINAL p), . bem heat installation oven (not shown) via a collector 226

According to the invention, there is placed in flue 12, upstream of the tubular nests 16, 18 of " heater 10, a protective nest made here of two tubes 2h, 24, fed with air at a temperature 9, via a ven- : tilator 26, This ventilator 26 can either suck the air directly from the atmosphere or be placed in a bypass (line 28 shown in dashed lines) on the heater 10 circuit fed by ventilators 1h, 14!; The delivery pressure of ventilator 26 is clearly superior to the pressure prevailing at the inlet of heater 10,

Due to the small relative flow rate of pro- " tective nest 2h, 2h, a loss of charge cn occur, cConw sequently causing a speed higher than the usual speed so as to favour the tubes/air exchanges and to better } set the skin temperature of the nest tubes of heater

To 10 which are more mxposed to the hot fumes, by setw ‘ ting a notable preference in the convective air/tubes . exchange with respect to the tubes/fumes exchange,

WL COSA As may be seen in the figure, protective nest 2k, 24 is placed upstream of the last zone of counw tercurrent heater 10, that is in front of the first tubes of nest 18 of this heater, and is calculated . as regards flow rate and surface to reduce the tem=

Ch nn wept |p Or - . “7 Ne perature of the fumes So that during the maximum operation conditions of the heat equipment situated upstream (not shown in the figure), the temperature 0x of the first heater tubes of nest 18 is always be- low the corrosion critical temperature of sodium and vanadium, this critical temperature being a function of the metallurgical quality of the tubes and of the composition of the fumes.

Under normal operating conditions, the hot pir exits from protective nest 24, 2h' at a tempera- ‘ture Og and is directed toward collector 22 through duct 30 on which is mounted a valve 30! which therew after feeds an oven, the heater operating with an optimum throughputs when the thermal equipment situated upstream of the heater is operating at a low speed, to a cons- tant exchange surface corresponds to a lowering of . the fumes temperature, which is such that the tems perature ©, of the last tube of heater nest 16 runs the risk of falling below the acid dew point teme perature, which would subject the tubes of heater 10 to a corrosion hazard by the condensation of the sul- fur oxidized components contained in the fumes, this being what the invention aims precisely to avoids

Consequently , the hot air flow is deflected in totality ~8- avo UruGINAL P —— ;

26269 + or in part at temperature Og from protective nest 2h, 24" to the downstream zone of collector 3k, “bringing the cold air at temperature 9, to heater 10. According to the invention, there is therefore, provided a bypass duct 32, and a regulation valve 32¢ : + for bringing the hot air at temperature oo to col- lector 34, Thus, air delivered to tubular nest 16 of heater 10 is heated at the temperature 0,7 95s said temperature being such that the temperature °, of the skin of the tubes of heater 10 is always above the acid dew point temperature of the fumes.

Taking in account the dynamic pressure availw- able in circuit 32, it is possible to give preference to the last layer or layers of tubes of heater 10 by making collector 34 in such manner that it ine ~ ‘ cludes nozzles 36' facing the last layers 16' of the tubes of nest 16 of the heater fed by collector 34.

The pressure difference between duct 32 and the delivery pressure of ventilators 1h, 14t is re= iy 320 fn £188ked by an impulse of air jets, incoming from cole - SE Jector 34k, sufficient to force this air through the aforementioned tubes of heater 10. It is possible to bbtain a similar effect by subdividing the inlet col~ lector 34 of heater into two or more subcollectorsy corresponding to the last layers of tubes 16! being

Co a Ap OR J o 1G cy. ERO gh then fed in a preferential manner by duct 32 as a function of the respective temperatures °, at the inlet of heater 10, and °, which is the discharge temperature in the atmosphere of the fumes which have flowed through the heater.

From the preceding description, one will understand that the invention allows: . on the one hand providing a temperature % for all operation conditions such that the skin tempera- , lo ture ©, of the tubes of nest 16 of heater 10 is always slightly above the acid dew point temperas ture of the fumes, and ’ on the other hand maintaining the temperature ©, of . the first layers of tubes of nest 18 of heater 10 at a value which is always below the corrosion - critical temperature of sodium/vanadium,

The invention therefore provides an air heater : eliminating the corrosion hazards at high temperatures of the sodium/vanadium salts and at temperatures bew

Sry © low tne acid condencations due to the presence of sulfur oxidized components in the fumes, of course, the present invention is not limited to the embodiments hereabove described and it encom= . passes all the variants thereof.

Claims

Se 26269 WHAT I CLAIM IS: 9 JN 14 Pi . - : i Ad l. An air heater estat MERRY: y using - Ce ET combustion products from thermal equipment, said combustion products being corrosive fumes containing polluting and corrosive agents such as sulfur oxidized components and sodium and vanadium salts, said air heater including at least one heater tubular nest through which flows the air to be heated, which is placed in the path of travel of the corrosive fumes prior to their discharge to the atmosphere, said corm rosive fumes flowing countercurrent to the air to be heated and flowing through the tubes, said air heater further comprising a protective tubular-nest placed upstream of said heater tubular nest, this protective nest being fed with a constant flow of cold air cire ’ culating in the same direction as the fumes, this cold air flow and the exchange surface of said protective nest bding chosen in such manner that the temperature of the first tubes of the heater tubular nest is at all timee below the sodium and vanadium corrosion : 20 critical temperature, and means are provided wherein the constant air flow exiting from said protective nest is thereafter redistributed at the inlet or at the outlet of the heater tubular nest, as a function of the operating conditions, so that the temperature er ve (BAD ORIGINAL A -1l "

‘ 26269 of the skin of the tubes of the heater tubular nest : is always slightly above the acid dew point tempera ture of the fumes.

5. An air heater according to Claim 1, where= in the protective nest is fed with air at a pressure which is superior to the pressure prevailing at the inlet of the tubular nest of said heatere 3, An air heater according to Claim 1, where in said protective nest is fed with air by a ventila- tor sucking the aire : 4, An air heater according to Claim 1, wherew in the protective nest jg fed by a ventilator sucking air directly from the atmospheres

5. An air heater according to Claim 1, where=- in the protective nest is fed by a ventilator sucking the air from a by-pass duct on a feeding circuit of oo the heatero Ca i Br Ee 6, An air heater according to Claim 1, where- wl "in as a function of the desired operating condi tion, the hot air incoming from the protective nest is either sent back to a hot air collector for feed- . } ing an oven, the whole heater operating with an op= Ce Co. timum output, or bypassed, in totality or in party . ' ’ . = , ‘ aAD ORIGINAL gi ww 1l2 = _

toward a downstream zone of a cold air inlet col- lector in the heater. .

: 7. An air heater according to Claim 4, wherein a collector feeding the heater with cold air is provided with nozzles facing the last layers of heater tubes which are fed by this collectore YVES BRAUD : inventor 4 Eras oy . To ee ee ’ ! wee gf —