All About Engines

CHAPTER IV Raising Steam A BOILER is a vessel for converting water into steam, and steam is wanted because, pound for pound, it contains more heat than water, from which it was formed. Whether the steam is to be used for warming buildings, or for certain factory processes, or for producing power in engines, it is the heat it contains that is valued. And as the heat is produced by burning coal the boiler is a device in which the energy of burning coal is absorbed by water and conveyed by means of steam to wherever it is required. Every pound of fairly good coal yields 14,000 units of heat on burning. Some of this is lost by warming bodies in the neighbourhood of the boiler, and some goes up the chimney in the waste gases, and the best boiler is that in which these losses are reduced to the least possible quantity. Some boilers are constructed to give steam regu- larly over long periods ; others have to be capable of producing it at short notice. From some boilers the heat is conveyed by steam at low temperature, and from others at high temperature ; and since the pressure exerted by steam is entirely dependent upon its temperature, except where a special appli- ance, called a superheater, is employed, there is 57