All About Engines

176 All About Engines and condenser a turbine will give a good deal of power when fed with steam at or about atmo- spheric pressure. Either the impulse or reaction type may be used for this purpose, and the steam is usually obtained from the exhaust of reciprocat- ing engines. Mixed pressure turbines (Fig. 106) have two sets of wheels and blades in the same casing, buf with different inlets for exhaust or live steam. Thus, in the power station of the Mersey Railway, which connects Liver- pool with Birkenhead and other places in the Wirral peninsula, the main engines are 2,ooo-horse-power vertical reciprocating, and there is a 1,200-horse- power mixed pressure turbine which usually works on exhaust steam from the main engines. A governor prevents the speed rising above 1,800 revolutions a minute, but if for any reason the supply of exhaust steam becomes insufficient to maintain this speed, the governor opens a valve and admits live steam just to the extent which is necessary to maintain the speed. Such an arrangement is extremely valu- able where there is a variable load, as is the case in winding engines, rolling mills, etc. Another type, intermediate between the high- pressure and exhaust steam turbines, is known as a reducing turbine. It is used in paper mills and chemical works where a large quantity of low-pressure steam is required for heating purposes, and elec- tricity for lighting and driving machinery. For vari- ous reasons boilers are most economical when pro- ducing high-pressure steam, and advantage is taken