Boiler Accessories Feed Apparatus

Evaporators. No engine can be run without a certain loss of water, due either to a slight continuous leakage or to blowing off. In stationary practice, this loss can be readily made up by the application of fresh water; hut at sea it is seldom possible to carry a sufficient amount of fresh water, and the make-up must be had either from sea water, or from fresh water provided by the use of evaporator. Theevaporator is really a small boiler, the water in which is heated by a steam coil supplied from the main lx)iler. The evaporated water—called the evaporation—passes into the condenser and then becomes a part of the regular feed water.

In a single evaporator, if the evaporation passes directly to the condenser, its heat is lost to useful work. To provide a more economical arrangement, multiple evaporators are installed, which consist of a series, the evaporation from the first passing into a coil in the bottom of the second ; the water in the second condenses the evaporation from the first, while at the same time the evaporation from the first helps to heat the water of the second. The steam and water pass through the series of heaters in opposite directions.

It is a rule in the French Navy, to provide 380 lbs of fresh water per hour for each 1,000 indicated horsepower; this provides for a loss of about 2 per cent without , drawing on the reserve supply, which is 4,500 lbs. for the same amount of power.

The evaporator may be arranged to communicate with a low-press u r e valve-chest, in Which case the evaporation may be made to do work in a lowpressure cylinder of a triple-expansion engine before entering the condenser, or it may be connected with the feed-water heater if the exhaust steam is inadequate.

Feed-Water Heaters. The introduction of feedwater at a high temperature increases the economy and tends to prolong the life of the boiler. The injurious effects from unequal expansion are diminished; and when the feed is warmed by exhaust steam or by the waste gases in the uptake, the saving of fuel is considerable.

If this gain comes from waste gases or exhaust steam, which would otherwise make no return for their heat, the gain .is clear; but

there is no gain in thermal economy by heating feed-water with live steam directly from the boiler.

There are. several ways of heating the feed-water. In condensing engines, the feed-pump discharges from the condenser into the hot well, and the water is drawn from the hot well at a temperature of 100° to 140° F. This, however, if the pressure is over 100 lbs., is entirely inadequate; and for the best economy, feed-water at this temperature should be passed through some form of feed-water heater. In the non-condensing engines, it is absolutely necessary that in some way the feed-water should be heated by the exhaust steam or by waste gases from the chimney, the apparatus in the first case being called a feed-water heater, and in the second an 'economizer.

The feed-water heater may be arranged so that it will not only heat the water, but will at the same time purify it, precipitating the calcium and magnesia salts, which collect on suitably prepared plates, and gathering, at the bottom of the heater, dirt and other sediment that would injure the boiler.

There are two types of feed-water heater—the open, which is frequently used in land work; and the closed, which may be used either on land or at sea. In the open heater, the steam raises the temperature of the water by mingling with it in direct contact. The closed type of heater resembles in its action a surface condenser; the steam used for heating purposes surrounds tubes which contain the feed-water, or the water circulates about tubes through which the heating steam passes.

Fig. 56 shows a feed-water heater of the closed type, the exhaust steam heating the feed-water within the tubes. The heater shown in Fig. 57 is of the open type, the feed-water becoming heated and depositing sediment while flowing from one tray to another.

The "Cochrane" heater, Fig. 58, is a combined heater and purifier of the open-heater type, the water entering at the top and flowing in a thin sheet over a series of trays. The exhaust steam enters through the oil separator, and rising among the trays, heats the water to about 210°F, the action being similar to that of a jet condenser.