Handbook of air conditioning and refrigeration / Shan K

Closed Expansion Tank
float valve, and an internal overflow drain is always installed. A float valve is a globe or ball valve
connected with a float ball to regulate the makeup water flow according to the liquid level in the
tank. An open expansion tank is often connected to the suction side of the water pump to prevent
the water pressure in the system from dropping below the atmospheric pressure. The pressure of the
liquid level in the open tank is equal to the atmospheric pressure, which thus provides a reference
point of known pressure to determine the water pressure at any point in the water system. The minimum
tank volume should be at least 6 percent of the volume of water in the system V s,ft 3 (m 3 ). An
open expansion tank is simple, more stable in terms of system pressure characteristics, and low in
cost. If it is installed indoors, it often needs a high ceiling. If it is installed outdoors, water must be
prevented from freezing in the tank, air vent, or pipes connected to the tank when the outdoor temperature
is below 32°F (0°C). Because the water surface in the tank is exposed to the atmosphere,
oxygen is more easily absorbed into the water, which makes the tank less resistant to corrosion than
a diaphragm tank (to be described later). Because of these disadvantages, an open expansion tank
has only limited applications.
A closed expansion tank is an airtight tank filled with air or other gases, as shown in Fig. 7.8. When
the temperature of the water increases, the water volume expands. Excess water then enters the
tank. The air in the tank is compressed, which raises the system pressure. When the water temperature
drops, the water volume contracts, resulting in a reduction of the system pressure.
To reduce the amount of air dissolved in the water so as to prevent corrosion and prevent air
noise, a diaphragm, or a bladder, is often installed in the closed expansion tank to separate the filled
air and the water permanently. Such an expansion tank is called a diaphragm, or bladder, expansion
tank. Thus, a closed expansion tank is either a plain closed expansion tank, which does not have a
diaphragm to separate air and water, or a diaphragm tank.
For a water system with only one air-filled space, the junction between the closed expansion
tank and the water system is a point of fixed pressure. At this point, water pressure remains
constant whether or not the pump is operating because the filled air pressure depends on only the
volume of water in the system. The pressure at this point can be determined according to the ideal
gas law, as given by Eq. (2.1): pv � RT R. The pressure in a closed expansion tank during the initial
filling process or at the minimum operating pressure is called the fill pressure p fil, psia. The fill
pressure is often used as the reference pressure to determine the pressure characteristics of a water
system.
Size of Diaphragm Expansion Tank
If a closed expansion tank with its filled volume of air is too small, the system pressure will easily
exceed the maximum allowable pressure and cause water to discharge from the pressure relief
valve, thus wasting water. If the closed tank is too large, when the water temperature drops, the system
pressure may decrease to a level below the minimum allowable value and cause trouble in the
air vent. Therefore, accurate sizing of a closed expansion tank is essential.
For diaphragm expansion tanks, the minimum volume of the water tank, V t, gal (m 3 ), can be calculated
by the following formula, recommended by ASHRAE Handbook 1996, HVAC Systems and
Equipment:
v2 / v1 � 1 � 3�(T2 � T1) Vt � Vs � 1 � p1 / p �
2
where T 1 � lower temperature, °F (°C)
T 2 � higher temperature, °F (°C)
V s � volume of water in system, gal (m 3 )
p 1 � absolute pressure at lower temperature, psia (kPa abs.)
WATER SYSTEMS 7.21
(7.3)