US1661106A - Method of controlling flow through superheaters - Google Patents

Description

Feb. 28, 1928. Y B. BROIDO METHOD 0 CONTROLLING FLOW THROUGH SUPERHEATERS Filed Oct. 30. 1924 3 m a l INVENTOR A TTORNE Y Patented Feb. 2a, 1928.

UNITED STATES 1,661,106 PATENT OFFICE.

BENJAMIN BBOIDO, OF NEW YORK, N. Y., ASSIGNOB TO THE SUPEBHEATEB C OMP ANY, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE METHOD OI CONTROLLING FLOW THROUGH SUPERHEATERS.

Application filed October 30, 1924. Serial No. 748,800.

My invention relates to steam superheaters, and particularly as applied to large power plant boilers.

In the. design and operation of modernv steam plants, there. has been a strong ten:

dency for some time past toward the use of very high boiler ratings in the production of steam, The most recent development, involving the use of water-cooled furnace and combustion walls, has made possible the operation of boilers at ratings as high as 500%, ormore, and this high rate of steam production, together with other factors aflected by the inclusion of water-cooled walls, has materially changed the conditions under which the superheater must operate. Because of the abstraction of large quantities of radiant heat from the combustion ases while they are still in the combustion chamber, the temgo perature of the gases entering the tube banks of the boiler is considerably lower than in boilers where the waterscreens are'not employed. As a consequence, a comparatively shallow bank of tubes is suflicient to abstract a the remaining useful heat from the combusthe necessary length of travel for the steam 5 through the superheater. At the same time the superheater elements must be small in cross-section, to provide the necessar steam velocity 'at low boiler ratings, so t at the elements may be sufliciently cooled at all 40 times by the steam passing therethrough to prevent their injury through overheating;

and in boilers operating at the extreme ratingsmentioned, this latter factor becomes of great importance because of the very w1de limits between which the amount of steam passing through the superheater varies.

Superheaters placed. above the tube bank of "the boiler in a low temperature gas zone and formed of a numberof long, small diameter superheating units, are satisfactory in so far as they produce the uired degree of superheat and afford a satis actory minimum steam velocity at low boiler rating? but this form of superheater, because of t e long; restricted steam path, produces a pressure drop which at ordinary ratings is obcritical velocity, above which the flow of the steam becomes turbulent, any increase in steam velocity above this critical point servng only to increase the pressure drop through the superheater without a corresponding increase in the rate of heat transfer. It is apparent, therefore, that no advantage can be derived from increasing the steam velocity above the critical velocity, and by my invention I. propose to provide a superheater'of the type described, in which the objectionable drop in pressure is at all times limited toan amount below a predetermined figure and in which the steam flow may be held below the "critical velocity.

I attain these ends by the provision of a pressure controlled bypass across the supereater, and, as a secondary consideration, I may obtain by this-construction a certain amount of superheat temperature regulation, as will appear more fully hereinafter in the following detailed description of aspecific embodiment of my invention, taken in coninlwhich: I v

Fig. 1, is a fragmentary side elevation of a conventional cross-drum, water tube boil er and superheater embodying my invention, a portion of the boiler casing being broken I away to indicate the location of the su erheater with reference to the boiler tu es; Fig. 2 is an elevation of a portion of the boiler taken at right angles to Fig. 1; and

Fig. 3 is a large scale section showing in detail one form of means for controlling the superheater by-pass. p

In the embodiment of my invention illustrated, I have shown my inventiomas ap- 1W plied to a superheater installed in a eonventional, crossdrum, water tube boiler, comprising a bank of horizontally inclined steam generating tubes receiving water from the steam and water drum 2 and discharg ing steam thereto through the usual upflowand downflow headers and horizontal circulators 3. Boilers of this type are usually baflled as shown inFig. 1, so that the combustioh ases pass upwardly through the u junction with the accompanying drawings bank of tubes 1 to the right of bafile 4, then downwardly through the tubes 1 between baflles 4 and 5, and finally upwardly through the tubes 1 on the left of baffle 5, to the outlet 6. The boiler construction, which is subject to considerable variation, forms no part of the present invention and it is not considered necessary to describe the boiler structure in detail. The triangular space between the inclined generating tubes 1 and the horizontal circulators 3 forms the location for the superheater and, as previously noted, in this type of boiler this area is a comparatively low temperature zone, inasmuch as the combustion gases before reaching it have had their temperature lowered by the abstraction of heat by the Water-cooled combustion chamber walls, and the first pass of I the tube bank 1. In this space I have shown a superheater comprising an inlet header 7 connected to the steam and water drum 2 by a suitable steam supply conduit 8; an outlet header 9, discharging into the conduit 10 carrying the steam to the point of use, and a plurality'of tubular return bend superheater units 11 connecting the inlet with the outlet header. As may readily be seen from Fig. 1, the steam path through the superheater is of considerable length, the units 11 in the embodiment shown consisting of four loops each.

The maximum pressure drop through the I steam supply line is controlled by a by-pass which may connect the steam drum of the boiler directly with the steam consumer, or which may connect any two intermediate points in the steam line. In the specific embodiment illustrated, I have shownthe bypass 12 connecting the su erheater inlet header 7 with the outlet header 9, this posi tionbeing the one which will probably be most commonly used in practice, as the portion of the line wherein the greater portion of the pressure drop takes place is by-passed bya conduit of minimum length. When the boiler is operated at a rating within its normal operating range the by-pass 12 is closed by the valve 13 and the entire steam supply from the boiler passes from the inlet header 7 through the units 11, header 9 and conduit 10 to the point ofconsumption. \Vhen, however, the boiler is forced to a rating beyond its normal operating range and the consequent pressure drop through the superheater reaches a predetermined maximum limit, the difference in pressure between the inlet header 7 and the outlet header 9, causes the valve 13 .to open andallows steam to pass directly to the outlet header by way of the by-pass 12. The combined cross sectional area of the superheater and the by-pass is such that further in creases in the rating at which the boiler is operated produce only a very small increase inthe pressure dropfover that existing at the time the by-pass valve opens, and, the point at which the valve opens may be so regulated that this further small increase in pressure drop is not sufficient to carry the total drop above the permissible maximum,

even when the boiler is operated at its highest possible rating. As long as extreme load conditions continue, and the boiler is operated at a rating above its normal operating range, the valve 13 will remain open and by-pass a certain portion of the steam delivered from the boiler, and when the load diminishes so that the boiler is operated at a rating within its normal operatlng range, with a consequent decrease in the difference between the pressure in the inlet and outlet headers, the valve 12 will close and thereafter the entire steam supply will pass through the superheater. I have shown valve 13' as an ordi- .nar globe relief valve with inlet at 14 and out et at 15, and carrying in the main body thereof valve seat 16, upon which is seated the valve 17 Threaded into the valve cover plate 18 are two standards 19, which carry at their upper end yoke 20 through which passes in sliding engagement the valve stem 21, to which is attached the valve 17'. Intermediate its ends the stem 21 passes in threaded engagement through a second yoke 22 19. A spring 23 between the yokes 20 and 22 serves to seat the valve 17 against the differential pressure existing between inlet 14 and outlet 15, and tending to open the valve. 1 A

As may readily be seen, the differential pressure with which the Valve is held in engagement on its seat by spring 23 can be adjusted by varying the compression on spring 23 which is slidably mounted on thestandard through rotation of the stem 21, by means of .hand wheel 24. While I have described. in

detail oneform of pressure relief valve, it' will be obvious that the specific structure of the valve forms no part of my invention and that any equivalent structure in the nature of a relief valve may be used Without departing from the spirit of my invention.

In many superheater installations of the general type described, a characteristic feature of the superheater performance is rising superheat temperature with inceasing boiler loads. At the higher ratings the superheat temperature may rise to an undesirable degree, and it has been proposed to remedy this condition by by-passing saturated steam to the superheat header when the temperature in the latter exceeds a certain degree. As may readily be seen, my invention is particularly useful in installation of this character as the superheater elements may be so de- I By the application of my invention, with the by-pass relief valve so adjusted as to open just before this critical point is reached, not only is the pressure drop through the superheater held to a certain definite maximum amount and turbulent flow through the units prevented, but the superheat temperature is prevented from rising to an undesirable degree with further increases in load by the saturated steam which is by-passed directly to the outlet header; I am thus able to effeet, by my invention, a regulation of the maximum superheat delivered in addition to the regulation of the pressure drop.

In the type of superheater just mentioned, wherein the superheat rises with the load, the tendency of the by-passed saturated steam to lower the final superheat temperature to an undesirable extent is counteracted by the fact that in the load range where the bypass is open, the saturated steam mingles in the superheat header with steam delivered from the units 10 at a temperature higher than is finally desired. The net result is that when the by-pass is open, steam which would otherwise be too highly superheated is reduced in temperature to the desired degree.

In fact, as amatter of practice, in cases where the superheater would not ordinarily have the abovementioned rising temperature characteristic, and my invention is employed, the superheater can be specifically so designed that at the ratings above which the by-pass is open, the superheater units will deliver steam to the outlet header at a temperature in excess of that desired as a final temperature, and this temperature will be reduced to that desired by the saturated steam entering the outlet header from the Icy-pass;

In the above cases theonly loss of superheat due to the use of the by-pass will arise from the fact that the steam passing through the units 10 must be heated to a temperature higher than would otherwise be the case, the consequence being a smaller difference in temperature between the combustion gases passing over the units and the steam flowing through them. Because of the lower temperature difference, there will of'course be a slightly lower rate of heat transfer to the superheater when operating under these conditions. 1 1

As will be obvious, the cross-sectional area high ratings,

of the by-pass may be selected so that the desired result in any particular installation may be attained. If there would be an undesirably high superheat temperature at as well as excessive pressure drop, the by-pass may be made large enough to efi'ect the proper temperature reduction, and while such a by-pass may reduce the pressure drop more than is necessary, such reduction is in the nature of an added advantage. In cases where reduction in pres sure drop is the sole consideration, a comparatively small by-pass will sutfice, and no material reduction in final steam temperature will result, as the quantity of steam flowing through the by-pass will at no time exceed a minor fraction of that passing through the superheater. 7

What I claim is:

l. The method of operating a boiler and superheater which consists 1n generating steam in said boiler at varying rates, establishing a lower and an upper range of rates of steam generation, passing all of the steam generated in the lower range through the superheater to a consumer, and by-passing a portion of the steam generated in the upper range directly from the boiler to the consumer.

- 2. The method of operating a boiler and superheater which consists in generating steam in said boiler at varying rates, establishing a lower and an upper range of rates of steam generation; said lower range including the normal generating capacity of the boiler, passing all of the steam generated in the lower range through the superheater to a consumer, and tion of the steam generated in the upper range directly from the boiler to the consumer.

3. The method of superheater which consists in generating steam in said boiler at varying rates, establishing a lower and an upper range of rates of steam generation, said upper range ineluding all rates at which turbulent flow occurs in the superheater, passing all of the. steam generated in the lower range through the superheater to a consumer, and by-passing a portion of the steam generated in the upper range directly from the boiler to the consumer.

BENJAMIN BROIDO.

operating a boiler and by-passmg, a por-

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