Heat Exchangers

As stated above all power and refrigeration plants contain equipment which has as its major function the transfer of heat from one fluid to another. This equipment includes boilers, super heaters, economizers, heaters, coolers, condensers, and evaporators and. is called a heat exchanger. The same laws of heat transfer, fluid flow, and economics apply to all heat exchangers. Heat exchangers differ in design characteristics only because of the different functions which they perform and conditions under which they operate.

Two heat exchangers commonly found in stationary power plants are the steam condenser and feed-water heater. They are distinct and separate pieces of equipment, and they differ in their relative positions and primary functions in the cycle. The purpose of the feed-water heater is to increase the overall efficiency of the cycle. This is accomplished by heating the boiler water before it enters the boiler with either waste steam or steam extracted from the turbine. With the feed water entering the boiler at high temperatures, the boiler is relieved of a part of its toad and temperature stresses within the boiler are reduced. Feed-water heaters are designed as direct-contact heaters or surface heaters.

Direct-Contact Feed-Water Heaters

The direct-contact heater is often called an open heater, although it may operate at pressures above atmospheric pressure. A typical direct-contact heater consists mainly of an outer shell in which trays or pans are placed. Water enters at the top of the shell. It feeds by gravity over rows of staggered trays which break up the solid stream of water. Steam entering near the center of the shell intimately mingles with the water and condenses.

When condensing, the steam gives up heat to the water. The heated water and condensate mixture is collected at the bottom of the shell and is removed by a boiler feed pump. Afloat control operating the inlet water valve maintains a constant level in the feed-water tank. A vent at the top removes the excess steam and the non-condensable gases. In the larger heaters where the vented steam is appreciable, a vent condenser may be employed. Water, before it enters the tray section of the feed-water heater, is passed through coils in the vent condenser. Heat is transferred from the vented steam to the water as the steam is condensed. The condensate from the vent condenser is returned to the heater. Non-condensable gases are expelled to the atmosphere.

Because of the stress limitations of the heater shell, the steam pressure is limited to a few pounds per square inch above atmospheric pressure, although pressures to 70 psi have been used. Consequently, the feed water is rarely heated above 220° F. If direct-contact heaters are used in series, a feed-water pump must be installed ahead of each heater. The advantages of the direct-contact feed-water heater are:1) complete conversion of the steam to water is accomplished; 2) non-condensable corrosive gases are removed from the feed water; 3) the removal of impurities In the water is possible; 4) the water is brought to the temperature of the steam; 5) the heater acts as a small reservoir.