The value of fuel is estimated by the number of heat units generated by its combustion. The fuels generally used in heating are composed of carbon, hydrogen and ash, with sometimes small quantities of other substances not materially affecting its value. Anthracite coal, when not freely mixed with ash, produces 14.70 heat units; semi-bituminous coal, 13.70 heat units, and soft coal, from 12 to 13 heat units. Slack, the screenings from coal, burned by means of a grate glower adapted for that purpose, is nearly equal in value of combustion to regular coal, but its percentage of refuse is greater. In reckoning the cost of fuel, it may safely be assumed that the intensity of the fire will be nearly the same for all kinds of combustibles under like conditions.
A single return pipe system, utilizing the same pipe for fresh steam and its return to the boiler after condensation, may be cheaper to install, but it has the disadvantage of liability to sudden Stoppage because of the steam and water constantly flowing through the same pipe in opposite directions. Substituting for the one-pipe return system a two-pipe gravity system will assure more satisfactory results. In the latter system, steam flows from the boiler through risers and is conveyed to the radiators by suitable steam branches, and the water formed by condensed steam travels back to the boiler by means of a small condensation tube.
The large branches conveying steam to the radiator are placed in a horizontal position, except where the radiator is a considerable distance from the riser. In this case the branch is so inclined that the condensation water within it will flow to the radiator, at which point it is emptied by a small relief pipe into the return branch, to prevent water from accumulating in the radiator. The return pipe, through which all condensation-water is returned to the boiler, is so inclined that all water will flow back by gravity to the boiler, to be again converted into steam. Widely advertised exhaust and vacuum systems, utilizing the waste steam, are usually costly, as they entail the expense of installing high-priced patented apparatus.
The gravity system just described will require pipe coils or radiators with piping, and piped connections for the steam mains and returns. Genuine wrought-iron pipe endures longest and is by far the best material to employ. There are scores of good connections on the market, but valveless ones are the most desirable. The radiators in common use are made of cast-iron. Double column radiators offer the most exposed heating surface and therefore give the most heat. A light-weight pressed-metal radiator that is easily attached to the wall, and, because of its smooth surface, easily cleaned, has recently come to notice, but sufficient time, however, has not elapsed to test thoroughly its durability.