Steam power plants, their design and construction . dinary conditions, dependingupon the size of the plant. Advantages of mechanical draft.—Draft produced by fanspossesses many advantages over chimneys as ordinarily propor-tioned. Probably the greatest of these is its flexibility, it beingpossible to regulate the speed of the fan so that the proper rateof combustion for the amount of steam required is maintainedentirely independent of the weather conditions; another importantadvantage is the ability of the fans to create a much greater draftthan is possible with a chimney. Steam engines for dr
Steam power plants, their design and construction . dinary conditions, dependingupon the size of the plant. Advantages of mechanical draft.—Draft produced by fanspossesses many advantages over chimneys as ordinarily propor-tioned. Probably the greatest of these is its flexibility, it beingpossible to regulate the speed of the fan so that the proper rateof combustion for the amount of steam required is maintainedentirely independent of the weather conditions; another importantadvantage is the ability of the fans to create a much greater draftthan is possible with a chimney. Steam engines for driving fansare frequently fitted with valves arranged to govern the speed of STEAM POWER PLANTS. 123 the engine according as the boiler pressure varies, increasing itas the pressure falls and reducing it as it rises above the draft enables economizers to be placed in the flue andreduce the temperature of the escaping gases by heating the feed-water far below, the temperature that is necessary in a chimney 9xmiM>i„jjmm*7!B7,. ngrfn . ENaiiiciniHO RECORD Figure 50.—Power House, Olympia Mills, Columbia, S. C W. B. SMITII-WHALEY, ENGINEER. to create a draft. The reduction in draft due to the use of econ-omizers is a much greater percentage of the available draft witha chimney than it is of the draft where fans are employed. Again,the greater draft of fans enables cheap low-grade fuels to beburned that could not easily be used with the chimney draft, and 124 STEAM POWER PLANTS. the saving that these fuels brings about in some locaHties is a veryconsiderable sum of money. Still another point in favor of me-chanical draft lies in the portability of the fans in case a change oflocation is desired. Theory of fans.—Before explaining the method of selectinga fan for a given service it is necessary to consider briefly theirtheory of operation. Fans for mechanical draft are invariablyof the centrifugal type and consist of a paddle wheel revolvingin a sheet iron c
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