The water and air that are drawn from the receiver by the vacuum pump are discharged into an open tank, or feedwater heater, from which the air readily escapes. The water is then pumped back into the boiler by an ordinary feed-pump.

In some cases, the additional cold water required to feed the boilers is injected into the receiver in a series of fine streams, the object being to condense as much as possible of the steam that may be present and thus improve the vacuum. When the exhaust steam from an engine is turned into the ordinary low-pressure heating system, the back pressure is increased, and the capacity of some engines is correspondingly diminished, sometimes to such an extent as to become detrimental.

One of the principal advantages of the vacuum system is that a great part of the back pressure is taken off the engines, and the capacity of the engines to do useful work is thereby increased.

Webster System.-A conventional arrangement of the Webster vacuum system is illustrated in Fig. 1, which shows methods of connecting different appliances, the arrangement given being modified in practice to suit the conditions imposed. The pipe a supplies steam to the main engine only, while the pipe b supplies steam for the pumps and fan engine, not shown. A separator c is placed in the pipe a near the engine. The drip from this separator passes through a steam trap d to the reedwater heater e. The exhaust steam from the engines and pumps passes through the pipes ƒ and g into the closed feedwater heater, where the water is warmed. The heater is connected to the main vacuum return pipe through which the air is extracted, and the pressure in the heater is reduced to less than that of the atmosphere. The partial vacuum in the feedwater heater causes steam to flow freely into the heater, where it is condensed and heats the make-up water required to supplement the water of condensation returned from the heating system. The exhaust pipe ƒ from the engine rises to the ceiling, where the exhaust pipe g from the pumps connects into it. A grease extractor h is

inserted in the exhaust pipe before connection is made to the heating apparatus, to which a live-steam-by-pass connection is provided. The escape pipe is provided with a back-pressure valvej and is carried upwards from a point near the grease extractor to and above the roof of the building, where it terminates in an exhaust head from which the condensation is dripped back to the feedwater heater through k', the oil and grease having been extracted. Live steam may be employed for heating when necessary, the steam passing from the main b through the reducing valve l to the heating main or riser m. The return pipe n is carried downwards to a point below the level of the feedwater heater, and is thus made a sealed return, although the pipe could be carried above the heater if required. This return pipe does not connect with the feedwater heater directly, but with a vacuum pump o, in the connection to which a strainer o' is placed; a jet of cold water is introduced to cool the return water and thereby assist the pump in maintaining the vacuum. The water and air drawn from the return pipe are forced by the pump into a receiver p placed at an elevation above the feedwater heater, so that the water in the receiver will flow, by gravity, into the feedwater heater. The receiving tank p has a vent pipe q to the atmosphere through which the air in the system is discharged. In the pipe r from the receiver p a water trap or loop seal s is provided, so that in case of a pressure in the system, the water or the steam in the feedwater heater will not escape should the vacuum pump be stopped. The seal also prevents the atmosphere from rushing into the heater when the pressure in the heater is below that of the atmosphere. The working pressure in the heating mains is about that of the atmosphere. The water in the feedwater heater is pumped into the boiler by a feed-pump t. The vacuum pump o is fitted with an automatic controlling device u connected to the steam and vacuum pipes, so that the pump will slow down when the vacuum has reached a certain point, and speed up again as the vacuum is lost. The waste drips are connected into a pipe v that discharges into the

sewer. The pressures in the heating main and return main are shown by the gauges at w.

The connections to the radiators and other fixtures are somewhat different from those commonly employed with gravity systems. The steam-supply radiator connection is usually provided with an angle valve, but the return pipe has a thermostatic valve in the connection at the radiator. When the radiator is cold the expansion stalk in the valve will be contracted, so that when the steam is admitted to the radiator the partial vacuum in the return pipe will cause the steam to fill the radiator quickly. The water of condensation and air is drained into the return pipe. The steam, when it comes in contact with the stalk, expands it, and the valve is closed. As the stalk cools again, the valve opens.

The sizes of pipe to be used in the Webster vacuum system are given in the accompanying table.

SIZES OF SUPPLY AND RETURN PIPES FOR WEBSTER VACUUM SYSTEM.