PRINCIPLES OF NAVAL ENGINEERING 



condenser. To avoid this difficulty, provision is 

 made for returning some of the condensate to the 

 main condenser when the condensate reaches a 

 certain temperature. As a rule, the recirculat- 

 ing line branches off the condensate line just after 

 the gland exhaust condenser. In most installa- 

 tions, the recirculating valve in this recirculat- 

 ing line is thermostatically operated. 



VENT CONDENSER 



The vent condenser is actually a part of the 

 deaerating feed tank, being installed in the tank 

 near the top. It is described separately here 

 because it is functionally quite separate from the 

 deaerating feed tank. 



In the vent condenser, as in the air ejector 

 condensers and the gland exhaust condenser, 

 condensate on its way from the main condenser 

 to the deaerating feed tank is used to cool and 

 condense the steam from a steam-air mixture. 

 The vent condenser receives steam and air from 

 the deaerating feed tank. The steam condenses 

 into water, which falls toward the bottom of the 

 tank. The air goes to the gland exhaust condenser 

 and is vented to atmosphere. The condensate 

 which is used as the cooling medium in the vent 

 condenser is sprayed out into the deaerating feed 

 tank and is deaerated before beingused as boiler 

 feed. 



DEAERATING FEED TANK 



The deaerating feed tank serves to heat, de- 

 aerate, and store feed water. The water is heated 

 by direct contact with auxiliary exhaust steam 

 which enters the tank at a pressure just slightly 

 greater than the pressure in the tank. The 

 deaerating feed tank is usually designed to oper- 

 ate at a pressure of about ISpsigand to heat the 

 water to between 240° and 250° F. 



One type of deaerating feed tank is shown in 

 figure 13-5. Condensate enters the tank through 

 the tubes of the vent condenser and is forced out 

 through a number of spray valves in a spray head. 

 The spray valves discharge the condensate in a 

 fine spray throughout the steam-filled upper sec- 

 tion of the deaerating feed tank. The very small 

 droplets of water are heated, scrubbed, and par- 

 tially deaerated by the relatively air-free steam. 

 As the steam gives up its heat to the water, much 

 of the steam is condensed into water. The 

 droplets of water (including both the entering 

 condensate sprayed out from the vent condenser 

 and the steam condensed in the deaerating feed 



tank) are collected in a cone-shaped baffle which 

 leads them through a central port, to the 

 deaerating unit. 



Steam enters the deaerating unit, picks up the 

 partially deaerated water, and throws ittangen- 

 tially outward through the curving baffles of the 

 deaerating unit. In this process, the water is even 

 more finely divided and is throughly scrubbed by 

 the incoming steam. Thus the last traces of dis- 

 solved oxygen areremovedfromthe water. Since 

 the water enters the deaerating unit at saturation 

 temperature, having already been heated by the 

 steam in the upper part of the deaerating feed 

 tank, the incoming steam does not condense to 

 any marked degree in the deaerating unit. 

 Therefore all (or practically all) of the incoming 

 steam is available for breaking up, scrubbing, 

 and deaerating the water. 



The thoroughly deaerated water falls into the 

 storage space at the bottom of the tank, where it 

 remains under a blanket of air-free steam until 

 it is pumped to the boilers. Meanwhile, the mix- 

 ture of steam plus air and other noncondensable 

 gases travels over the spray head (where much 

 of the steam is condensed as it heats the incom- 

 ing condensate) and over the tubes of the vent 

 condenser (where more steam is condensed into 

 water which then goes into the deaerating unit). 

 The air and other noncondensable gases, together 

 with a little remaining steam, goto the gland ex- 

 haust condenser. 



As shown in figure 13-5, the deaerating feed 

 tank has a recirculating connection that allows 

 water to be sent back to the condenser from the 

 deaerating feed tank. The recirculating line is 

 used to provide a high enough condensate level 

 in the condenser so that the condensate pump can 

 take suction. The recirculating line is also used 

 at slow speeds and when the plant is first started 

 up to ensure a sufficient supply of cooling con- 

 densate to the air ejector condensers and to the 

 gland exhaust condenser and to keep the deaerat- 

 ing feed tank at the prescribed temperature. 



The deaerated feed water from the deaerat- 

 ing feed tank is pumped to the boiler by the feed 

 booster pump and the main feed pump. The feed 

 booster pump takes suction from the bottom of 

 the deaerating feed tank and discharges to the 

 suction side of the main feed pump. The 

 feed booster pump provides a positive suction 

 pressure for the main feed pump and thus pre- 

 vents the hot water from flashing into steam at 

 the main feed pump suction. The main feed 

 pump operates at variable speed in order to 

 maintain a constant discharge pressure under all 



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