Handbook of air conditioning and refrigeration / Shan K

Pump Location
Air in Water Systems
WATER SYSTEMS 7.23
p 2 � absolute pressure at higher temperature, psia (kPa abs.)
v 1, v 2 � specific volume of water at lower and higher temperature, respectively, ft 3 /lb (m 3 /kg)
� � linear coefficient of thermal expansion; for steel, � � 6.5 � 10 �6 in./in�°F (1.2 �
10 �5 per °C); for copper, � � 9.5 � 10 �6 in./in.�°F (1.7 � 10 �5 per °C)
In a chilled water system, the higher temperature T 2 is the highest anticipated ambient temperature
when the chilled water system shuts down during summer. The lower temperature in a heating
system is often the ambient temperature at fill conditions (for example, 50°F or 10°C).
The location of the pump in a water system that uses a diaphragm expansion tank should be
arranged so that the pressure at any point in the water system is greater than the atmospheric pressure.
In such an arrangement, air does not leak into the system, and the required net positive suction
head (NPSH) can be maintained at the suction inlet of the water pump. NPSH is discussed in
detail in Sec. 7.7.
A water pump location commonly used for hot water systems with diaphragm expansion tanks
is just after the expansion tank and the boiler, as shown in Fig. 7.8b. In this arrangement, the pressure
at the pump suction is the sum of the water pressure and the fill pressure. In another often-used
arrangement, the diaphragm expansion tank is moved to the highest point of the water system, and
the pump is still located after the boiler. In a chilled water system, the location of the chilled water
pump is usually before the water chiller, and the diaphragm expansion tank is usually connected to
the suction side of the water pump.
In a closed recirculated water system, air and nitrogen are present in the following forms: dissolved
in water, free air or gas bubbles, or pockets of air or gas. The behavior of air or gas dissolved in liquids
is governed and described by Henry’s equation. Henry’s equation states that the amount of gas
dissolved in a liquid at constant temperature is directly proportional to the absolute pressure of that
gas acting on the liquid, or
x � (7.4)
where x � amount of dissolved gas in solution, percent by volume
p � partial pressure of that gas, psia
H � Henry’s constant; changes with temperature
p
H
The lower the water temperature and the higher the total pressure of the water and dissolved gas,
the greater the maximum amount of dissolved gas at that pressure and temperature.
When the dissolved air or gas in water reaches its maximum amount at that pressure and temperature,
the water becomes saturated. Any excess air or gas, as well as the coexisting water vapor,
can exist only in the form of free bubbles or air pockets. A water velocity greater than 1.5 ft/s
(0.45 m/s) can carry air bubbles along with water. When water is in contact with air at an air-water
interface, such as the filled airspace in a plain closed expansion tank, the concentration gradient
causes air to diffuse into the water until the water is saturated at that pressure and temperature. An
equilibrium forms between air and water within a certain time. At specific conditions, 24 h may be
required to reach equilibrium.
The oxygen in air that is dissolved in water is unstable. It reacts with steel pipes to form oxides
and corrosion. Therefore, after air has been dissolved in water for a long enough time, only nitrogen
remains as a dissolved gas circulating with the water.