US2699759A - Feed water heating - Google Patents

Description

Jan. 18, 1955 M. H. KUHNER FEED WATER HEATING Filed NOV. 17, 1951 INVENTOR MA); H KUHNER ATTORNEY United States Patent FEED WATER HEATING Max H. Kuhner, Worcester, Mass., assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts Application November .17, 1951, Serial No. 256,865

2 Claims. (Cl. 122-421) One of the problems which face the manufacturers of steam generating equipment "is that of economizer corr o sion. This is particularly true for industrial and central station installations located in districts where the fuel to be burned is high in sulphur and where at the same time, because of station heat balance, the feed water temperature is very low. It has been suggested that corrosion is due not so much to the temperature of the combustion gas as it is the temperature of the water passing through the tubes which produces condensation of sulphur dioxide with resultant corrosion. This is because the outer skin temperature of the economizer tubes. only a few degrees higher than the temperature of the water within the tubes regardless of what the gas temperature may be. This contention is confirmed by the fact that external economizer tube corrosion is being experienced with installations where the economizer is placed ahead of an airheater and where the economizer exit gas temperature is over 500 F., but where at the same time the temperature of the feedwater entering the economizer is in the neighborhood of 200 F. The temperature of condensation of flue gas moisture (dew point) rises with the increase of sulphur in the fuel. The higher the sulphur content the higher is the temperature at which condensation takes place. It is therefore important to keep the temperature of the water entering economizer tubes so high that it is above the condensation temperature of gas to prevent condensation and corrosion. The invention of the applicant obviates the difiiculties described above and experienced in prior art installations.

It is therefore an outstanding object of the invention to provide means for preventing economizer corrosion.

Furthermore, it is an object of the present invention to provide a novel means of heating feedwater before its en trance into the economizer of a steam generating unit.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to certain of its structural forms, as illustrated by the accompanying drawings in which:

Figure 1 is a sectional, somewhat schematic view of a steam generating unit embodying the invention.

Generally speaking, the present invention consists in apparatus for passing the feedwater through a heat exchanger submerged under the boiler water and installed in one of the boiler drums. Referring to Figure 1 wherein is shown the preferred form of the invention, the steam generating unit, generally designated by the reference character 10, comprises a boiler 11 situated in a setting 12. The boiler 11 has an upper steam drum 13 and a lower submerged or water drum 14 joined in a well-known manner by downcomers 15 and risers 16. A spreader stoker 17 and a traveling grate 18 provide for fuel burning. A header 19 situated immediately above the stoker 17 is joined to the steam drum 13 by water wall tubes 20. A vertical wall 21 rises above the rear end of the grate 18 and serves to define a combustion chamber 22. At least one row of risers 16 is situated forwardly of the wall 21 to constitute further water wall tubes. A downwardlyextending vertical wall 23 and an upwardly-extending vertical wall 24 serve to define the convection passes of the boiler and baflles 25 serve to cause the hot products of combustion to travel in a sinuous path from the combustion chamber 22 to a flue passage 26.

I feedwater temperature rise is approximately 60 "ice In the flue passage 26 resides an economizer 27 of the return-bend type having tube loops 28 lying transversely of the path of the gases in the flue passage and having headers 29 lying externally of the flue passage to which the tube loops are connected in a well-known manner.

A conduit 30 is connected with the source of feedwater and to the entrance of a dividing valve 31, which valve may be automatically controlled, as will be described hereinafter. One exit of the valve 31 is connected by a conduit 32 to a heat-exchanger 33 situated in the submerged drum 14. The heat-exchanger 33 may consist of a number of coils of extended-surface tubing as is well-known in the art. The exit end of the heat exchanger 33 is joined to a conduit 34. The other exit of the valve 31 is connected to a two way, hand-operated shut-off valve 35 by a conduit 36. The other side of the valve 35 is connected to a conduit 37 which joins with the conduit 34 to merge with a conduit 38.

The valve 31 is connected mechanically to apower relay 39 which in turn is connected electrically to a temperature measuring device such as a thermocouple 40 which is situated adjacent one of the last loops of the economizer.

The conduit 38 is connected to the first header of the economizer. A header next subjacent the first header is connected by a conduit 41 to the last header. The header just below the header from which conduit 41 emerges is connected to the steam drum 13 by means of a conduit 42.

The operation of the apparatus is as follows: the feed water enters the conduit 30 and passes into the dividing valve 31. Some of the feedwater leaves the valve 31 through the conduit 32, passes through the heat exchanger 33 into the conduit 34. While passing through the heat exchanger 33, the feedwater receives heat from the warm water in the submerged drum 14. Some of the feedwater is bypassed by flowing from the valve 31 into the conduit 36, through the shut-off valve 35 into the conduit 37. The warmed feedwater in the conduit 34 and the by-passed feedwater in the conduit 37 mingle in the conduit 38 to an intermediate temperature which should be higher than the dew point of the flue gases. The temperature of the feedwater in the conduit 38 is dependent on the setting of the dividing valve 31, which in turn is determined by the power relay 39 responsive to the thermocouple 40. The thermocouple is associated with the last loop of the economizer since this is the portion where corrosion takes place most readily due to the fact that the gas is coolest and the feedwater is coolest in a counter-flow economizer.

In the preferred embodiment the feedwater passes through the conduit 38 into the first loops of the economizer to temper the feedwater even further before it passes through the conduit 41 into the last loop of the economizer. The feedwater then proceeds in counterfiow to the gases in the usual manner before passing through the conduit 42 into the drum 13 of the boiler.

The feedwater temperature rise is about 30 F. in a small installation where the feedwater temperature is 210 F. and the operating pressure is 200 p. s. i. g. at full load of the boiler. In other words, the temperature of the water entering the economizer will be approximately 240 F. with the system of the invention instead of 210 F. Invariably, this is sufficient to prevent condensation. Naturally, a higher water temperature rise can be obtained with higher operating pressures when the temperature of the water contained in the drum is higher.

One of the objections to the system of the invention may be based on the assumption that the heat recovery in the economizer will be impaired by the higher feedwater temperature entering the economizer. While this may be true to a certain extent, for the average industrial installation a loss of perhaps one quarter of one percent in overall etficiency will be compensated for by the absence of, or at least greatly reduced, expense for economizer maintenance and extended boiler outages.

The performance of the heat exchanger is such that a lower feedwater temperature rise is obtained with the highest rate of feedwater flow corresponding to the highest rate of boiler steam output. For example, the F. at half load and 40 F. at full load. This characteristic is desirable because it is usually necessary to maintain a higher feedwater temperature at part load to prevent condensation of flue gas moisture than at maximum load when the temperature of the flue gas is naturally higher. Itcan also be seen that the heat recovery in the economizer is least impaired by this scheme during high loads when high efliciency is important.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desided to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

l. A steam generating unit comprising: a boiler having a steam drum and a water drum, an economizer for heating feedwater before its introduction into said boiler, a heat exchanger situated in said Water drum, means responsive to the temperature of the section of the economizer over which the coolest gases flow for causing a portion of the feedwater to flow throughthe heat exchanger before entering the economizer to maintain the temperatureof the said section of the economizer above the dew point of the gases passing thereover.

2. A steam generating unit having an external source of feedwater, comprising: a boiler having a steam drum and a water drum, an economizer for heating feedwater before its introduction into said boiler, a heat exchanger situated in said water drum, a three-way dividing valve, a temperature measuring device associated with the section of the economizer over which the coolest gases flow, a power relay connected to said device and responsive thereto, said power relay being also connected to said dividing valve to change its setting in response to changes in economizer temperature, said dividing valve being con nected to the source of feedwater, the heat exchanger, and the economizer for causing a portion of the feedwater to flow through the heat exchanger before entering the economizer to maintain the temperature of the said section of the economizer above the dew point of the gases passing thereover.

References Cited in the file of this patent UNITED STATES PATENTS 565,986 Hogan Aug. 18, 1896 856,900 Nichols June 11, 1907 877,355 Milne Jan..21, 1908 1,453,703 Cederblom May 1, 1923 1,472,474 Jacobus Oct. 30, 1923 1,612,854 Broido Jan. 4, 1927 1,753,363 Coghlan et a1 Apr. 8,1930 1,883,293 Jacobus Oct. 18, 1932 1,938,072 Karlsteen Dec. 5, 1933 2,070,075 Bessler Feb. 9, 1937 2,623,506 Dalin et al. Dec. 30, 1952 2,635,587 Dalin et al. Apr. 21, 1953 FOREIGN PATENTS 690,582 Germany Apr. 29, 1940

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