WATER GAS
713
is an enclosed pressure-raising device which can be regulated to give almost any desired discharge pressure up to the safe limit. In the case of the open. fan, if the main delivery pipe is suddenly closed the pressure increase will aniount to no more than 1 oi 2 inches w.g. With. a positive blower, however, if the main outlet was closed the pressure in the casing and pipe would go on building up until something gave way, or the engine was pulled up. The most notable example of these blowers is the Roots type. Dellwik plants invariably employ a positive biast, and in this way it is said that rekindling can be done more xapidly, and, therefore, the duration of the “ blow ” may be curtailed. When such a blower is iised, however, it is essential to fit a relief-valve on the main biast pipe.
Fig. 442.—Tvebo-driven Water-Gas Fan.
In general, the high-speed fan blower is usually preferred, mainly owing to the faet that it is cheaper and occupies less room than the positive type. The discharge pressure of the fan varies in accordance with the tangential velocity of the blades ; accordingly, increase of pressure is obtained only by increase in. speed. Its economy of operation is decidedly impaired by the faet that it is invariably kept Tunning while the gasmaking run is proceeding. Thus, for flve minu tes out of every ■cycle of eight minutes the fan is consuming, to no useful end, from 40 to 50 per cent, of the power taken when operating at füll load. As against this the positive blower may be automatically governed so that it is running at a reduced speed during gasmaking. When, however, two or more water-gas plants are being operated from the same blower it becomes almost imperative to employ a fan, in view of the faet thai the positive machine will not give sufficient uniformity of pressure.