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:
100 (t - t_{ i}) Fuel saving per cent = --------------- (1) H + 32 - t_{ i}
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.
SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER GAUGE PRESSURE 180 POUNDS
+-----------+-----------------------------------------+ | Initial | Final Temperature--Degrees Fahrenheit | |Temperature|-----+-----+-----+-----+-----+-----+-----| | Fahrenheit| 120 | 140 | 160 | 180 | 200 | 250 | 300 | +-----------+-----+-----+-----+-----+-----+-----+-----+ | 32 | 7.35| 9.02|10.69|12.36|14.04|18.20|22.38| | 35 | 7.12| 8.79|10.46|12.14|13.82|18.00|22.18| | 40 | 6.72| 8.41|10.09|11.77|13.45|17.65|21.86| | 45 | 6.33| 8.02| 9.71|11.40|13.08|17.30|21.52| | 50 | 5.93| 7.63| 9.32|11.02|12.72|16.95|21.19| | 55 | 5.53| 7.24| 8.94|10.64|12.34|16.60|20.86| | 60 | 5.13| 6.84| 8.55|10.27|11.97|16.24|20.52| | 65 | 4.72| 6.44| 8.16| 9.87|11.59|15.88|20.18| | 70 | 4.31| 6.04| 7.77| 9.48|11.21|15.52|19.83| | 75 | 3.90| 5.64| 7.36| 9.09|10.82|15.16|19.48| | 80 | 3.48| 5.22| 6.96| 8.70|10.44|14.79|19.13| | 85 | 3.06| 4.80| 6.55| 8.30|10.05|14.41|18.78| | 90 | 2.63| 4.39| 6.14| 7.89| 9.65|14.04|18.43| | 95 | 2.20| 3.97| 5.73| 7.49| 9.25|13.66|18.07| | 100 | 1.77| 3.54| 5.31| 7.08| 8.85|13.28|17.70| | 110 | .89| 2.68| 4.47| 6.25| 8.04|12.50|16.97| | 120 | .00| 1.80| 3.61| 5.41| 7.21|11.71|16.22| | 130 | | .91| 2.73| 4.55| 6.37|10.91|15.46| | 140 | | .00| 1.84| 3.67| 5.51|10.09|14.68| | 150 | | | .93| 2.78| 4.63| 9.26|13.89| | 160 | | | .00| 1.87| 3.74| 8.41|13.09| | 170 | | | | .94| 2.83| 7.55|12.27| | 180 | | | | .00| 1.91| 6.67|11.43| | 190 | | | | | .96| 5.77|10.58| | 200 | | | | | .00| 4.86| 9.71| | 210 | | | | | | 3.92| 8.82| +-----------+-----+-----+-----+-----+-----+-----+-----+
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.