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 in which, consequently, exhaust steam is not readily available for heating, steam for this purpose is taken direct from the boilers through a reducing pressure valve and used in the heating system at a pressure of 5 to 20 pounds per square inch. In such systems the water is generally returned to the boilers by an automatic pump and receiver, or by one of the special styles of traps referred to, which for operating at such pressures can be made much simpler than when used for the extremely low pressure of the ordinary exhaust systems.

Vacuum systems.—As a refinement of exhaust-steam heating there has been developed within the last decade what is known as vacuum systems of steam heating, the object of these being to exhaust the air from the system by artificial means so that circulation may be effected at atmospheric pressures with absolutely no back pressure on the exhaust pipes from the engines. There are two distinct forms, one known as the Paul system, the other as the Webster. The former system provides each radiator with an automatic air valve of special construction and connects a very small pipe, usually ¼ inch, to each of these valves, bringing them together in pipes of proper size in the basement of the building, and connecting to a special exhauster, which maintains a constant suction on the entire system of air piping. The steam and return pipes for this system are entirely independent of the air pipe and it may be installed on any of the systems previously mentioned.

The Webster system operates on an entirely different principle, in that it employs an automatic air-and-water valve at the return outlet of the radiator. This thermostatic valve, as it is called, is constructed on a principle much like the automatic air valve, but is of larger proportions. It is adjusted so that it closes automatically when it comes in contact with the steam temperature, and opens when water or air collects about it, and the temperature is reduced. The system is necessarily a two-pipe system, the returns being connected to these thermostatic valves, but no other air valves or air piping are used. The return pipes are connected in the basement to a vacuum pump which puts a strong suction on the returns, and by means of which both air and water are drawn through the thermostatic valves, the water being delivered by the vacuum pump to an open heater or receiving tank, while the air is separated by an automatic device. The return pipes of this system are very small, just sufficient to take care of the water, no