Page 270 - HVAC Pump Handbook

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Configuring an HVAC Water System

Configuring an HVAC Water System 267

and fall. All the systems discussed in this book will be of the four-
pipe variety, two pipes for cooling and two pipes for heating.

9.7 Location of Expansion Tanks
Expansion tanks should be located at the point where the system is to
be stabilized, not necessarily at the suction of the pumping system.
With the advent of variable-speed pumps and their digital control, the
operation of a pump is not dependent on its suction pressure being
constant. In the past, the expansion tank was always located at the
pump suction and took its air from the air separator. This expansion
tank was of the plain type without an internal bladder that separates
the air from the water. The result was absorption of air by the water
and waterlogging of the tank.
This nuisance was eliminated by the bladder-type expansion tank.
This expansion tank is located away from the air separator, and this
enables the designer to locate the expansion tank at the desired point
of pressure regulation in the water system, not just at the point of
installation of the air removal equipment.
On low-rise buildings, the expansion tank for both hot and chilled
water systems can be located at the suction of the pumps. On high-
rise buildings, the expansion tank and water makeup equipment
should be located at the top of the building to ensure continuous pres-
sure at that point in the system.

9.8 Elimination of Air in HVAC Systems
In the past, air elimination from HVAC water systems depended on
mechanical devices such as air vents and mechanical separators. The
development of better chemical treatment for these systems has
resulted in the use of chemicals for complete removal of air. The
mechanical separator did not remove any of the dissolved air from a
water system. The so-called pot feeder is an economical chemical
feeder that can accept a passivating chemical such as sodium sulfite
for removing all the oxygen from an HVAC water system. It is much
less costly than a mechanical separator for many sizes of pipe.
On chilled water systems, mechanical separators do very little air
separation except at the initial fill of the water into the system. The
reason for this is the fact that the temperature of the makeup water
is equal to or greater than the chilled water operating temperature.
For example, the makeup water may be at 60°F and then reduced to
40 to 50°F. The solubility of air in the water is actually increased.
Table 2.5 demonstrates this.

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