In the first case, the heat to be supplied by the furnace is the total heat, less that which the feed water contains, or 1185 - 28 = 1157 heat units. In the second case it is 1185 - 168 = 1017 heat units, the difference being 1157 - 1017 = 140, which represents a saving of 140⁄1157 or about 12 per cent.
Where feed water is heated no more than 20 degrees above its normal temperature the gain effected cannot amount to more than 2%, not sufficient to pay for the introduction and maintenance of a feed water heating device, no matter how simple, but if the temperature of the water can be increased 60 degrees the gain will be in the neighborhood of 5%. To make feed water heating practical and economical it would be necessary to increase the temperature of the water about 180 degrees at least, and to do this, using the exhaust from a non-condensing engine without back pressure, would require such a capacity of heater as would give fully 10 square feet of heating surface to each horse power of work developed, and to raise the temperature above this would require a certain amount of back pressure or an increased capacity of heater, so that the subject resolves itself into a question of large capacity of heater, or a higher temperature of the exhaust steam, which could only be obtained through a given amount of back pressure.
In the same way has been calculated the following table, showing percentages of saving of fuel by heating feed-water to various temperatures by exhaust steam, otherwise waste:
Percentage of saving. ( Steam at 60 pounds gauge pressure.)
| Final Temp. Fahr. | Initial Temperature of Water (Fahrenheit). | ||||||
|---|---|---|---|---|---|---|---|
| 32 Deg. | 40 Deg. | 50 Deg. | 60 Deg. | 70 Deg. | 80 Deg. | 90 Deg. | |
| 60 | 2.39 | 1.71 | 9.86 | … | … | … | … |
| 80 | 4.09 | 3.43 | 2.59 | 1.74 | 0.88 | … | … |
| 100 | 5.79 | 5.14 | 4.32 | 3.49 | 2.64 | 1.77 | .90 |
| 120 | 7.50 | 6.85 | 6.05 | 5.23 | 4.40 | 3.55 | 2.68 |
| 140 | 9.20 | 8.57 | 7.77 | 6.97 | 6.15 | 5.32 | 4.47 |
| 160 | 10.90 | 10.28 | 9.50 | 8.72 | 7.91 | 7.09 | 6.26 |
| 180 | 12.60 | 12.00 | 11.23 | 10.46 | 9.68 | 8.87 | 8.06 |
| 200 | 14.36 | 13.71 | 13.00 | 12.20 | 11.43 | 10.65 | 9.85 |
| 220 | 16.00 | 15.42 | 14.70 | 14.00 | 13.19 | 12.33 | 11.64 |
| 100 Deg. | 120 Deg. | 140 Deg. | 160 Deg. | 180 Deg. | 200 Deg. | ||
| 60 | … | … | … | … | … | … | |
| 80 | … | … | … | … | … | … | |
| 100 | … | … | … | … | … | … | |
| 120 | 1.80 | … | … | … | … | … | |
| 140 | 3.61 | 1.84 | … | … | … | … | |
| 160 | 5.42 | 3.67 | 1.87 | … | … | … | |
| 180 | 7.23 | 5.52 | 3.75 | 1.91 | … | … | |
| 200 | 9.03 | 7.36 | 5.62 | 3.82 | 1.96 | … | |
| 220 | 10.84 | 9.20 | 7.50 | 5.73 | 3.93 | 1.98 | |
A good feed-water heater of adequate proportions should readily raise the temperature of feed-water up to 200° Fahr., and, as is seen by inspection of the table, thus effect a saving of fuel, ranging from 14.3 per cent. to 9.03 per cent., according as the atmospheric or normal temperature of the water varies from 32° Fahr. in the height of winter, to 100° Fahr. in the height of summer.
The percentage of saving which may be obtained from the use of exhaust steam for heating the feed water, with which the boiler is supplied, will depend upon the temperature to which the water is raised, and this, in turn, will depend upon the length of time that the water remains under the influence of the exhaust steam. This should be as long as possible, and unless a sufficient amount of heating surface is employed in the heater best results cannot be expected.
It does not necessarily require all the exhaust steam—or the whole volume of waste steam passing from the engine to bring the feed water up to the temperature desired, and the larger the heating appliance the smaller proportion is needed—hence heaters are best made with two exits nicely proportioned to avoid back pressure and at the same time utilize enough of the exhaust to heat the feed water.
An impression prevails among many who are running a condenser on their engine that a feed water heater can not be used in connection with it; large numbers of heaters running on condensing engines with results as follows: the feed water is delivered to the boiler at a temperature of 150° to 160° Fahr., depending on the vacuum: the higher the vacuum the less the heat in the feed water.