EXHAUST. The exhaust steam from an engine is disposed of in various ways: (1) It escapes direct to the atmosphere, or by way of an exhaust-head which acts as a silencer, and also pre vents condensed steam and oil from being thrown down on to adjacent buildings. In a portable engine or locomotive it creates draught to help the fire by being directed up the chimney. (2) It operates an injector (see INJECTOR) or a feed-water heater, as illustrated, thus assisting the boiler by so much heat already raised and avoiding strain on the plates by feeding a cold flow against the hot surfaces. Or some other kind of heating apparatus is worked by exhaust steam, such as an air-heater for warming buildings, workshops, or drying clothes in laundries, also wheat, cloth, timber, yarn, etc. (3) In condensing engines and turbines it passes into a partial vacuum created in a condenser (q.v.).
Exhaust steam from a big engine, such as a colliery winder or a rolling-mill engine, is made to do further work in low-pressure tur bines. As winders run inter mittently the supply of exhaust steam fluctuates and a steam accumulator, a large boiler-like object, containing a mass of hot water, is employed to pass steam into and furnish a re serve for short carry-over pe riods when the winder stops.
Another practice which is economical of fuel is concerned with factories requiring large quantities of low-pressure heat ing steam for processes; these include flour-mills, paper-mills, dye-works, bleach-works, tan neries, wool-combing shed s, b r e w e r i e s and others. This process steam can be generated in low-pressure boilers, but it is cheaper to produce high-pres sure steam, pass it through the engine, and take the exhaust for the process work. If the demand for heating steam is about equal to that given out by the engine, a single-cylinder back-pressure engine is employed, giving up all its exhaust, but with fluctuating requirements a compound cylinder heat-extraction engine is selected. The whole of the steam required for driving and heating passes through the high-pressure cylinder to an intermediate receiver. From this latter the desired quantity of process steam is drawn, and any excess goes in to the low pressure cylinder and thence to the condenser.
The chief feature in respect to the exhaust from an internal combustion engine is to let it escape as freely as possible, so as not to hamper the engine; but the necessity for silencing in so many cases renders this ideal difficult of realisation. Exhaust from large units may be led into a pit, or into a box providing for expansion of the gases, baffles or small stones assisting to break up the flow. On motor-cars baffles or perforated compartments per form a similar duty. Extra service may be obtained from an exhaust, such as to blow a whistle, or warm a car.