92
MODERN GASWORKS PRACTICE
the temperature of which is 145° F., while a smaller boiler is provided with water at 90° F., taken from the cooling tanks of a 60 b.h.p. gas engine.. Thus, by judicious selection of sources of waste heat, other than that from retorts, it is possible to do a good deal of work which. would otherwise be appropriated by complete ■cooling of the waste gases.
Incidentally it may be noted that the difference in temperature between the steam generated and the exit gases varies normally between 100° and 180° F., -according to th.e type of waste-heat boiler employed. If, of course, complete recovery
of sensible heat is desired, then the power required to drive the fan must
Fig. 38.—The Bonecourt Waste Heat Boilek •coupled up to Vehtical Rbtort-Bench Chimney.
heat'ng surface is so small that a boiler when large volumes of
■ever, is so constructed the rate found with the
be set off as a contra-account against the work done by the boiler. The power absorbed by the fan should not, in an example such as has been con-sidered above, be more tb.an 10 or 12 electrical units an hour, the usual drive being by variable-speed direct-current motor. There can be no question, however, that induced draught presents a disadvantage, in that some consider-able leakage of cold air through, the brickwork of the flues usually occurs when a fan is employed.
Losses through Brickwork.—It is in this direction that the Bonecourt method of recovering waste heat differs from the more common system of building a boiler into a brickwork chamber. The Bonecourt waste-heat boiler (Fig. 38) is of the multi-tubulär type, the hot gases passing through the tubes, and the water around them. When the ordinary type of multitu-bular boiler is employed for the pur-pose, it is usually found that the evaporation obtained per unit of water
of unwieldy dimensions has to be installed
The Bonecourt boiler, how-at approximately four times The in-
waste gases are to be dealt with. that evaporation takes place more common forms of multitubular boilers.
•creased speed of evaporation entails a moderately high velocity of hot gases over the heating surfaces ; consequently the best results are obtained when induced draught by fan is arranged for. It may be noted here that whereas an efficiency of 65 per •cent, is comparatively easy to obtain when steam is generated at 100 1b. per square