Steam after being used in an engine contains the greater part of its heat, and if not condensed or used for other purposes it can usually be employed for heating without affecting to any great extent the power of the engine.

The systems of steam heating which have been described are those in which the water of condensation flows back into the boiler by gravity; where exhaust steam is used the pressure is much below that of the boiler and it must be returned either by a pump or return trap. The exhaust steam is often insufficient to supply the entire heating system and must be supplemented by live steam taken directly from the boiler. This must first pass through a reducing valve in order to reduce the pressure to correspond with that carried in the heating system.

The exhaust steam discharged from non-condensing engines contains from 20 to 30 per cent of water, and considerable oil or greasy matter which has been employed for lubrication. When the engine is exhausting into the air, the pressure in the exhaust pipe is but slightly above that due to the atmosphere. The effect of passing exhaust steam through the pipes and radiators of a heating system is likely to increase the back pressure on the engine and reduce its effective work; this must be offset by raising the boiler pressure or increasing the cut-off of the engine.

An engine does not deliver steam continuously but at regular intervals at the end of each stroke and the amount is likely to vary with the work done since the governor is adjusted to admit steam in such a quantity as is required to maintain a uniform speed. If the work is light, very little steam will be admitted to thee engine and for this reason the supply available for heating may vary somewhat depending upon the use made of the power delivered by the engine. In mills the amount of exhaust steam is practically constant; in office buildings where power is used for lighting, the variation is greater, especially if power is also required for the running of elevators.

Exhaust Steam Heating 1000133

Fig. 52.

The general requirements for a successful system of exhaust steam heating include a system of piping of such proportions that only a slight increase in back pressure will be thrown upon the engine; a connection which shall automatically supply live steam at a reduced pressure as needed; provision for removing the oil from the exhaust steam; a relief or back pressure valve arranged to prevent any sudden increase in back pressure on the engine, and a return system of some kind for returning the water of condensation back to the boiler against a higher pressure. These requirements may be met in various ways depending upon actual conditions found in different cases.

Exhaust Steam Heating 1000134

Fig. 53.

To prevent sudden changes in the back pressure due to irregular supply of steam, the exhaust pipe from the engine is often carried to a closed tank having a capacity from 30 to 40 times that of the engine cylinder. This tank may be provided with baffle plates or other arrangements and serve as a separator for removing the oil from the steam as it passes through.

Any system of piping may be used but great care should be taken that as little resistance as possible is introduced at bends and fittings; and the mains and branches should be of ample size. Usually the best results are obtained from the system in which the main steam pipe is carried directly to the top of the building, the distributing pipes run from that point, and the radiating surfaces supplied by a down-flowing current of steam.

Before taking up the matter of piping in detail a few of the more important pieces of apparatus will be described in a brief way.