Steam, Its Generation and Use

by Babcock & Wilcox Company

[Pg 110]

FEED WATER HEATING AND METHODS OF FEEDING

Before water fed into a boiler can be converted into steam, it must be first heated to a temperature corresponding to the pressure within the boiler. Steam at 160 pounds gauge pressure has a temperature of approximately 371 degrees Fahrenheit. If water is fed to the boiler at 60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to increase its temperature 371 degrees, which increase must take place before the water can be converted into steam. As it requires 1167.8 B. t. u. to raise one pound of water from 60 to 371 degrees and to convert it into steam at 160 pounds gauge pressure, the 311 degrees required simply to raise the temperature of the water from 60 to 371 degrees will be approximately 27 per cent of the total. If, therefore, the temperature of the water can be increased from 60 to 371 degrees before it is introduced into a boiler by the utilization of heat from some source that would otherwise be wasted, there will be a saving in the fuel required of 311 ÷ 1167.8 = 27 per cent, and there will be a net saving, provided the cost of maintaining and operating the apparatus for securing this saving is less than the value of the heat thus saved.

The saving in the fuel due to the heating of feed water by means of heat that would otherwise be wasted may be computed from the formula:

Fuel saving per cent

=

100 (t − ti) –––––––––––––––––– H + 32 − ti

(1)

where, t = temperature of feed water after heating, t_i = temperature of feed water before heating, and H = total heat above 32 degrees per pound of steam at the boiler pressure. Values of H may be found in [Table 23]. [Table 17] has been computed from this formula to show the fuel saving under the conditions assumed with the boiler operating at 180 pounds gauge pressure.

[TABLE 17] SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER GAUGE PRESSURE 180 POUNDS Init’l Temp. ° Fahr. Final Temperature—Degrees Fahrenheit Init’l Temp. ° Fahr. Final Temperature—Degrees Fahrenheit 120 140 160 180 200 250 300 120 140 160 180 200 250 300 32 7.35 9.02 10.69 12.36 14.04 18.20 22.38 95 2.20 3.97 5.73 7.49 9.25 13.66 18.07 35 7.12 8.79 10.46 12.14 13.82 18.00 22.18 100 1.77 3.54 5.31 7.08 8.85 13.28 17.70 40 6.72 8.41 10.09 11.77 13.45 17.65 21.86 110 .89 2.68 4.47 6.25 8.04 12.50 16.97 45 6.33 8.02 9.71 11.40 13.08 17.30 21.52 120 .00 1.80 3.61 5.41 7.21 11.71 16.22 50 5.93 7.63 9.32 11.02 12.72 16.95 21.19 130 .91 2.73 4.55 6.37 10.91 15.46 55 5.53 7.24 8.94 10.64 12.34 16.60 20.86 140 .00 1.84 3.67 5.51 10.09 14.68 60 5.13 6.84 8.55 10.27 11.97 16.24 20.52 150 .93 2.78 4.63 9.26 13.89 65 4.72 6.44 8.16 9.87 11.59 15.88 20.18 160 .00 1.87 3.74 8.41 13.09 70 4.31 6.04 7.77 9.48 11.21 15.52 19.83 170 .94 2.83 7.55 12.27 75 3.90 5.64 7.36 9.09 10.82 15.16 19.48 180 .00 1.91 6.67 11.43 80 3.48 5.22 6.96 8.70 10.44 14.79 19.13 190 .96 5.77 10.58 85 3.06 4.80 6.55 8.30 10.05 14.41 18.78 200 .00 4.86 9.71 90 2.63 4.39 6.14 7.89 9.65 14.04 18.43 210 3.92 8.82

[Pg 111]

Besides the saving in fuel effected by the use of feed water heaters, other advantages are secured. The time required for the conversion of water into steam is diminished and the steam capacity of the boiler thereby increased. Further, the feeding of cold water into a boiler has a tendency toward the setting up of temperature strains, which are diminished in proportion as the temperature of the feed approaches that of the steam. An important additional advantage of heating feed water is that in certain types of heaters a large portion of the scale forming ingredients are precipitated before entering the boiler, with a consequent saving in cleaning and losses through decreased efficiency and capacity.

In general, feed water heaters may be divided into closed heaters, open heaters and economizers; the first two depend for their heat upon exhaust, or in some cases live steam, while the last class utilizes the heat of the waste flue gases to secure the same result. The question of the type of apparatus to be installed is dependent upon the conditions attached to each individual case.