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 pile goes, it is more economical to use exhaust steam even at a high back pressure.

As heating systems are now designed, one which requires a pressure of 5 pounds to ensure a good circulation is defective in design, and 2 pounds is more than ought to be required in most cases. A back pressure of this amount on an engine running at 50 pounds mean effective pressure would increase the coal consumption but a fraction of 1 per cent., while taking the heating power that is available in the exhaust steam directly from the boiler would increase the coal consumption over 60 per cent.

Another consideration enters, however, into the question of circulation in a steam-heating apparatus. Besides merely forcing the steam and water through the radiators and piping, it is necessary to force out the air which accumulates, and to do this the system must carry a pressure somewhat above that of the atmosphere, unless a vacuum system, which will be described later, be used.

Theoretically it would be possible to operate a simple gravity system below the atmospheric pressure if the whole system was perfectly air tight and the air was all boiled out of the water and forced out of the system in the first place. In such a case if the fires were put out and the system allowed to become cold, the condensation of steam would leave a perfect vacuum, and on starting up the fire, steam could be carried at any pressure below or above the atmospheric, according to the intensity of the fire.

But if it be attempted to run much below atmospheric pressure the slightest leak anywhere in the system will rapidly break the vacuum and allow air to accumulate. It is, however, impossible to make a system theoretically air tight, and steam invariably contains some air from the feed water, as water will absorb several times its own volume. Air in the radiators and piping is, therefore, an evil that cannot be avoided, and it rapidly accumulates in the radiators or ends of pipes where the flow of steam is slowest. Consequently an air valve is almost a necessity on every radiator, and those which are now almost universally used are automatic; that is, they close as soon as the hot steam comes in contact with them, and open if air accumulates and they become cold. To some extent these automatic air valves enhance the air problem, inasmuch as when the radiator is cold it entirely fills with air at atmospheric pressure. In any case the result of the presence of air is that the pressure of steam in the system must be sufficient