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 to impose a pressure cushion. The elevation of the tank above the circulation
system pump provides the required head to prevent flashing of the supply high
temperature water.
5.2 Inert Gas Pressurized System. I n this system an inert gas, usually
nitrogen, is used in the expansion tank to impose a pressure higher than the
maximum saturated pressure of the high temperature water. The expansion tank
c a n be located at floor level. When an inert gas is used to pressurize the
system, the expansion tank is generally connected to the main return line and
n o circulation takes place in the tank. Compressed air is never used for
pressurization because absorption of oxygen by water would occur with
unacceptable corrosion of the metallic elements of the system.
6 PUMPING SYSTEMS. Two main pumping systems that are in use as a result of
the different pump arrangements are the combined pumping system and the
separate pumping system.
6.1 Combined Pumping System. In this system the same pumps are used to
circulate water through both the HTW generators and the system. I n general,
t h e s e systems are used where the circulation rate in a single distribution
s y s t e m is fairly constant, and heat load capacities do not exceed 31,500,000
Btu per hour. Figure 4-1 illustrates a steam pressurized HTW combined pumping
system. Combined pumping systems are generally provided with the following
equipment.
FIGURE 4-1.
Steam Pressurized HTW Combined Pumping System
4-5
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