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 exhaust-steam feed-water heater on its way to the boiler. The pump is also usually operated automatically, as will be discussed later.

The feed-water heater is an essential in all steam plants, and its purpose is to utilize as much as possible of the heat in the exhaust steam in heating the water fed to the boiler. As, however, not more than 18½ per cent. of the exhaust steam can in any case be required to heat the coldest feed water to the full temperature of the exhaust steam, 212 degrees Fahr., there is always a considerable quantity left, which can be utilized in heating the building; and furthermore, as the hot return water is always in one way or another fed back to the boiler, the more steam that is required for the building the more return water there is and the less steam is needed to heat cold feed water. If the heat in the exhaust steam is not thus used in heating the feed water or heating the building, or both, it would be wasted, and its equivalent in coal would have to be used under the boiler to replace it.

There are two distinct classes of exhaust-steam feed-water heaters; the closed or pressure heaters, and the open heaters. In the former the feed water is pumped through the heater against the boiler pressure; the exhaust steam passing into an inlet chamber, and generally through a series of tubes into the outlet chamber, the tubes being set in wrought-iron plates, which divide the inlet and outlet chambers from the water space around the outside of the tubes. In a water-tube heater the position of the steam and water is reversed. In the open heater the water and steam are practically together in the same chamber, the water flowing in from some source against only the pressure of the exhaust steam. The suction of the feed pump is connected to the heater and the delivery direct to the boilers. With the closed heater cold water is pumped through the heater to the boiler, while with the open heater hot water from the heater is pumped direct to the boiler.

The scheme of steam supply described is represented in Figure 6, and is, with various modifications of detail, almost universally employed in heating systems in which exhaust steam is used. It will be noticed that in the figure the pipe to the heating system is provided with a live-steam connection. This is necessary in a great many plants where, in extremely cold weather, the exhaust steam is not sufficient to heat the building. In modern practice such a connection is always provided with a reducing-pressure