US2056241A - Desuperheater - Google Patents

Description

0a. 6, 1936. WO'OD DESUPERHEATER Filed April 4, 1965 lEiiaEaesE 9 INVENTOR doH/v EDWIN W000.-

ATTORNEY Patented Dot. 6, 1936' PATENT Fries DESUPERHEATER John Edwin Wood, Eltham, England, assignor to The superheater Company,'NewYork, N. Y.

Application April 4, 1935, Serial No. 14,553 In Great Britain April 19, 1934 4 Claims. (Cl; 122-479) This invention relates to improved means for regulating the temperature at which steam will be delivered from a steam superheater incorporated in a boiler. Generally a convection type 5 superheater incorporated in a boiler has a characteristic which causes the steam temperature to increase as the load increases, i. e. as the quantity of steam passing through the superheater increases and the steam temperature normally 10 reaches a maximum at the maximum load. Certain factors other than the load cause some variations, in the final steam temperature, but these variations as compared with those due to changes in load are. relatively small. There are distinct l5 thermal economic advantages to be gained by maintaining superheat constantly as high as possible over a wide range of loads up to the maximum. The maximum steam temperature which can be used safely is governed by the materials 20 used in, and the mechanical construction of, the turbine. Any reduction of steam temperature .below the predetermined safe maximum involves a loss of thermal efficiency. Due to the characteristic rise of steam temperature with increase 25 in load in a convention superheater it is not possible to operate at the permissible maximum temperature at, say, half load, without risk of damage to the turbines at higher loads unless compensating means are employed to maintain superheat 30 atthe required figure at all loads higher than half load and thus prevent increases of temperature.

Various means have been proposed heretofore for regulating the temperature of steam delivered 5 by convection type steam superheaters which involve the use of a desuperheater arranged between the initial and final portions of the superheater.

It is the principal object of the present invention to provide a simple and efiicient means for controlling the ultimate temperature of steam delivered by an installation comprising a convection type superheater incorporated in a boiler, the means being such as to eifect automatically 45 the required regulation of the temperature of the steam leaving the installation and maintain such temperature constant or approximately constant regardless of the load on the boiler, Without necessarily employing thermostatically operated,

to manually-operable, float-controlled, or other valves, or of any other moving parts.

It is an object of the present invention to provide in addition to the apparatus for compensating for variations. in steam temperature due to 55 changes in load on the boiler, means whereby the regulation of the desuperheating efiected will be obtained in a mannerwhich will compensate for the variations in superheat temperature referred to as arising from causes other than variations in the load on the boiler. 5

It is another object of the present invention to provide a steam temperature control apparatus such that the. size of the desuperheater apparatus required may be reduced to the smallest practical limits, a further object being to provide means to ensure circulation of the water in the desuperheater so as to prevent a rise in the concentration of solids in such water and the consequent increase in the liability of incrustation on the tubes of the desuperheater apparatus.

It is well known that the pressure drop in a steam superheater is a function of the load on the boiler and the present invention takes advantage of this fact.

The means for regulating steam temperature according to this invention may be employed to regulate the temperature of steam after it is delivered by the superheater, or to control its final temperature by regulating the temperature of the steam after leaving the primary section .25 of the superheater and before entering the final section thereof. The desuperheater employed is of the type comprising a casing containing tubes open at their upper ends and having the water in the desuperheater, the level of which water is variable to vary the desuperheating effect, sub-- ject on its surface to the pressure obtaining at the outlet of the superheater, or of the primary section of the superheater. The improved apparatus for effecting steam temperature regulation according to the invention is defined in the appended claims and will be described hereinafter with reference to the accompanying drawing, in which Fig; 1 illustrates diagrammatically steam tem- 4.0 perature regulating means, according to one embodiment of the invention, applied to regulate the temperature at which steam from the primary section of a convection type superheater, incorporated in a boiler, is delivered to the seco-ndary-or final section of the superheater, and

Figs. 2 and 3 illustrate diagrammatically other embodiments of the invention.

1 Referring to the drawing, while the invention is not restricted to superheaters incorporated in any particular type of boiler it may be assumed for the purpose of this specification that the superheater installation is incorporated in a water tube boiler of the vertically inclined type; the desuperheater I would be disposed above the wa- 5 ter level in the upper drums and fed with water through a connecting pipe 2 from one of such drums, preferably that one into which the fresh feed water is fed and which is marked 3. As already stated, the desuperheater I is of the type in which the tubes 4 are open at their upper ends to the steam space above the water in the desuperheater, which water has its upper surface exposed to the incoming steam to be desuperheated. In the embodiment of the invention shown the outlet header 5 of the primary,

superheater 6 is connected to the top of the desuperheater I by a pipe 1 and the desuperheated steam is led from the bottom of the desuperheater by a pipe 8 to the inlet header 9 of the secondary section IQ of the superheater. It will be understood that the steam generated in the desuperheater mixes with incoming superheated steam from pipe I and passes therewith through the tubes 4.

Where the resistance to flow of the water through the connecting pipe 2 between the boiler and desuperheater is very small the level of the water in the desuperheater above that in the boiler drum 3 will depend on the pressure drop across the superheater 6. As the said pressure drop will vary approximately with the square of the load on the boiler, there will be a corresponding change in the water .level in the desuperheater and a consequent alteration in desuperheating eifect and in the rate of evaporation in the desuperheater due to changes in the amount of water immersed heat exchange surface.

Thus the height of the bottom of the desuperheater above the level of the water in the boiler drum 3 will determine the boiler load at which reduction in steam temperature by desuperheating commences, after which the amount of reduction in steam temperature will increase as the load increases, which is the operation required to correct the rising characteristic of a convection superheater.

If the water space of the desuperheater i be connected freely to the water space of the boiler drum 3 a small variation in the pressure drop across the primary superheater 6 will cause a relatively large variation in the water level in the desuperheater, and to reduce the extent of such variation in water level in the desuperheater and thereby enable the height of the desuperheater to be reduced to more convenient dimensions, a fixed resistance or choke II is inserted in the pipe 2 feeding water from the boiler drum 3 to the desuperheater.

The fall in pressure across this resistance II will be proportional to the square of the rate of water evaporation in the desuperheater, and will be added to the head of water in the desuperheater and water feed pipe 2 to balance the difference in pressure between the boiler and the desuperheater.

Boilers apparently identical in construction and similarly fired are frequently found in practice to have different characteristics, and the characteristic of any particular boiler must be brought into consideration in determining the value of the resistance or choke II to be placed in the water feed pipe 2 to the desuperheater. Consequently it is desirable that the construction and arrangement shall be such that should it be found on test that the predetermined resistance included in the water feed pipe 2 to the desuperheater requires to be modified, the necessary adjustment can be made without difficulty. For

example, at an appropriate position in its length the said pipe 2 may be divided and between opposing fianges on the respective pipe sections, there may be clamped a disc, block, or other member having through it an aperture or passage of the desired cross-sectional area for the flow of water, whereby adjustment of the resistance may be effected by removing the disc or equivalent and substituting therefor one having an aperture or passage of a different cross area.

In order to minimize the concentration of solids in the desuperheater water, the water feed pipe 2 to the desuperheater is shown as being supplied with water from the feed water trough l2 in the boiler drum 3 so that the water in the desuperheater will be almost pure feed water. The desuperheater may be blown down at the usual times according to boilerhouse routine, or otherwise emptied of water when necessary, in order that the water containing the concentration of solids will be discharged and replaced by fresh water from the feed trough.

While the temperature regulating means according to the embodiment of the invention shown in Fig. 1 will be effective to maintain constant or approximately constant the final steam temperature notwithstanding variations in the load on the boiler, those variations in temperature which are due to changing fire, feed water and other conditions may not be wholly compensated, but fluctuations in steam temperature at any given load will be damped owing to the variations in heat transfer in the desuperheater which will accompany these fluctuations.

Reference will now be made to Fig. 2 of the drawing, in which are shown certain modifications of the apparatus shown in Fig. 1; the parts which are identical with those shown in Fig. 1 are given the same reference numbers as in that figure and will not be further described.

According to the embodiment of the invention illustrated by Fig. 2, means are provided for adjusting the pressure drop between the steam space of the boiler and the steam inlet to the desuperheater to compensate for the effect on the steam temperature of certain factors amongst which is the presence of ash on the superheater elements or boiler tubes. Conveniently the pipe 7 conveying steam from the superheater 6 to the desuperheater I of the 'apparatus according to Fig. 1 is divided o-r bifurcated at l3, l4 for an appropriate distance, one leg [3 of the bifurcated section constituting a fixed resistance, and there being provided in the other leg M a variable resistance IS. The fixed resistance I3 would be of such a value as to obtain a predetermined pressure drop with the variable resistance l5 at its maximum opening, and thereby determine the load at which under normal conditions the desuperheater I will commence to operate. The variable resistance I 5 may be a valve of any appropriate type adapted to be adjusted from time to time as necessary, either by the boilerhouse engineer, or automatically by thermostatically controlled means, the thermostat being located at the outlet from the desuperheater. Opening or closing the variable resistance IE will reduce or increase the pressure drop between the steam space of the boiler and the desuperheater, and thus determine the water level in the desuperheater. Preferably the apparatus would be such that with all conditions normal, at a predetermined boiler load, the desuperheater would be free from water or would assent:

contain a very small quantity of water. Onan increase in steam temperature occurring, dueto a rise in the load, the water level in the desuperheater will rise,- on account of the increase the pressure drop as between the steam space the boiler and the desuperheater. Should deposits on the superheater or boiler tubes, fall infieed water temperature, or other conditions, cause the temperature or the steam delivered by the superheater to vary, such variation willbe compensated for by variation in the pressurev drop effected by adjusting the variable resistance I5, either by hand or automatically, as already referred to.

It is also in accordance with the presentinvention to supply water to the desuperheater I from the feed water line I-B instead of from a boiler drum as in the embodiment of the invention shown in Fig. 1. By taking the water supply for the desuperheater fromthe feed line greater purity of the water in the desuperheater is obtained and the necessity for blowing down the desuperheater water is reduced to a minimum. The deposit of impurities on the watertouched surfaces of the tubes 4 of the desuperheater apparatus is minimized by the use of the pure, or relatively pure, feed water. In carrying this. part of the present invention into practice; the pipe 2 connected to the bottom of the water space of the desuperheater is: a branch pipe taken from the boiler feed. line Iii between the-teed valve I-I-- and theboiler 3* and thus 'theswat'er space of the desuperheater isinconstantlyopen communication with the boiler drum. As in the apparatusaccordingto Fig. l

a: resistance or choke? H of predetermined fixed value. is-.i'nserted.irrthe pipe 2: feeding water to the; desuperheater in order to reduce the extent of variation of water level in the desuperheater due: to variation in the. pressure drop: across the superheater and thus permit. the height of the desuperheater apparatus to. be reduced. Convenientlythe water feed pipe. 2 to;. the: desuperheater instead of being connected: tattle wa'ter space of the desuperheater through the side wall of the. latter, may bev connected to a passage through the bottom wall I8 of the water space of the desuperheater constituting the tube plate thereof, which wall or plate I8 would have a thickness appropriate for the purpose.

In order to compensate for the variation in superheat which accompanies a variation in rate of water feeding, a resistance or choke I9 is provided in the portion 20 of the feed pipe I6 between the branch pipe 2 to the desuperheater and the boiler drum 3. The value of the resistance I9 will be such that the increase in the drop in pressure of the water in passing through such resistance will cause a rise of the water level in the desuperheater I sufficient to compensate for the increase in superheat produced by the increase in the rate of water feeding. A reduction in the rate of water feeding likewise produces a fall of water level.

By supplying the desuperheater with water from the feed water line the eflect on superheat of variation in feed water temperature is wholly or partly compensated for, since a fall in feed temperature which will cause the superheat to increase also increases the rate of heat transfer in the desuperheater.

It is within the ambit of the invention to include in the apparatus certain by-passes equipped with controlling valves which may be actuated automatically or manually as desired.

desuperheater.

connection 2Iz', equipped with av valve 22,- bypassing the superheater 6 may be provided between thepipe 23 leading to the inlet header 24 of the superheater 6 and the pipe I conveying steam to; the desuperheater I-, in which case the bifurcated section I3, I4 andvalve I5 may be omitted, the pipe I remaining as in Fig. 1. and as shown in dotted lines in Fig. 2. If for any reason a low superheat temperature persists an appropriate quantity of saturated. or low/temperature steam may be taken from the inlet header and mixed with the steam entering the Thus a reduced pressure drop between the boiler drum and the desuperheater is effected. the water level falls, and a reduction.- in the amount of desuperheating results.

A. pipe 25by-passing the desuperheater I may be provided in order that, if desired, an appropriate quantityof steam may pass to the boilerhouse range or the secondary superheater without passing through the desuperheater, the said quantity of steam being mixed with the desuperheated steam.

Theprovisionof this by-pass 25 enables the diameter of the desuperheater I to be reduced and by appropriately increasing the length of the tubes 4 the required amount of desuperheating can be effected without resorting to excessive steam velocity through the desuperheater. This by-pass 25. may be permanently open, or may have a valve 26 which may be thermostatically controlled or manually operable, or it may have both a thermostatically controlled valve and a manually operable valve.

By using water from the feed line as the cooling fluid. in the, desuperheater I the latter may be reduced in size to a certain extent as compared with the apparatus shown in Fig. 1 as the feed water is at a lower temperature than the water in the boiler drum. The feed water, as

already indicated, is more nearly pure than is the boiler water, and less frequent blowing down of the water in the desuperheater will be necessary, but if desired,-.in order to avoid the necessity of blowing down, means may be provided 1 whereby water from the feed line will flow through the desuperheater into the boiler so that a stagnant body of water does not obtain therein, but the water is constantly being changed. To effect this the apparatus shown in Fig. 2 may be modified'as shown in Fig. 3, there being provided according to this embodiment of the invention an ejector-like apparatus 21 in the section 20 of the feed pipe I6 between the boiler drum 3 and the pipe 2 leading to the desuperheater I; the ejector-like apparatus 21 may comprise a Venturi tube 28, and the Venturi tube make take the place of the resistance or choke I9 in the said pipe 20 or the two may be used together as shown. To the restricted portion or throat of the Venturi tube 28 there is connected a pipe 29 which communicates with the water space of the desuperheater I at 30 close to the bottom thereof, and at one or more places 3I in its height. The throat of the Venturi tube 28 also is connected with the interior of the desuperheater casing at a point 32 above the intended highest water level of the desuperheater. If desired, instead of a common connection from the Venturi tube to the water and steam spaces of the desuperheater the steam space may be connected to the Venturi tube by an independent duct or pipe 33 as shown. The connection at 32 has for its object to prevent water flowing over into the open ends of the tubes 4 of the desuperheater, the said connection being made at a point below the upper ends of said tubes. As water is fed to the boiler by the pipe [6 an ejector action takes place at the Venturi tube 28 which draws water from the Water space of the desuperheater through pipe 29, fresh feed water flowing in through pipe 2 to maintain the water level. By this means concentration of solids in the water in the desuperheater is avoided.

It is to be understood that, although some constructional details are shown or have been referred to herein, the invention is not restricted to such constructional details. Further it is to be understood that the desuperheater controlled in the manner herein described may be disposed between the superheater and the boilerhouse range to moderate as may be necessary the temperature of the steam flowing from the superheater before it is delivered to the range, or it may be interposedbetween the primary and final sections of the steam superheater so as to regulate the temperature of steam fed to the final section of the superheater, and thus maintain constant, or substantially constant, the temperature of steam delivered by such final section of the superheater.

What I claim is:

1. In apparatus of the class described, the combination of a boiler having a water and a steam space, a superheater, a desuperheater extending above the water space of the boiler and comprising a shell, a tube sheet at its lower end and tubes fixed in the tube sheet and extending to a point adjacent to the upper end of the shell, connections from the boiler to the superheater, from the superheater to the upper end of the desuperheater and from the lower end of the desuperheater to the point of use of the steam, and a connection from the water space of the boil-er to the shell at a point adjacent to but above the tube sheet, the last named connection having a choke opening in it, the combination further including a boiler feed line opening into said last named connection at a point between combination of a boiler having a Water and a steam space, a superheater, a desuperheater comprising a vertically placed cylindrical shell with a tubesheet at its lower end and tubes fixed in the tube sheetand extending to a .point adjacent to the upper end of the shell, the tube sheet being above the water level of the boiler, connectionsirom the, boiler to the superheater, from the superheater to the upper end of the desuperheater, and from the lower end of the desuperheater to the, point of use of the steam, a feed water supply to the boiler with a branch to the desuperheater, a choke opening in each branch, an ejector in the branch leading to the boiler with the feed water on its way to the boiler acting as ejecting fiuid, and connections from, the ejector to different levels of the water and steam spaces of the desuperheater, whereby feed water on its way to the boiler draws water from the water space of the desuperheater and delivers it to the boiler.

3. Apparatus in accordance with claim 2, the connection from the ejector to the steam space of the desuperheater opening into the latter at a point below the upper ends of the tubes but above the intended highest water level in the desuperheater.

4. In apparatus of the class described, the combination with a boiler having a water space and a steam space, and a superheater, of a desuperheater disposed above the water space of the boiler and comprising a shell, a tube sheet at its lower end, and tubes fixed in the tube sheet and extending to a point adjacent to the upper end of the shell; a connection from the boiler to the superheater; a connection from the Water space of the boiler. to the shell at a point adjacent to but above the tube sheet for supplying water to the desuperheater; a connection from the superheater to the upper end of the desuperheater subjecting the Water therein to the pressure of steam entering the latter from the superheater; and a connection from the lower end of the desuperheater to the point of use of the steam. j 7

JOHN EDWIN WOOD.

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