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, Vt,gal (m ), can be calculated by the following formula,
recomm
ended by ASHRAE Handbook 1996, HVAC Systems and Equipment:
3
= Vs (
V2
V1
- 1 - 3 α
(
T2 - T1
)
1
-
P1
P2
)
T1 =lower temperature, ⁰ F (⁰ C)
T2 =higher temperature, ⁰ F (⁰ C)
Vs=volume of water in system, gal(m
3
)
P1 =absolute pressure at lower temperature, psia(kPa abs.)
P2 =absolute pressure at higher temperature, psia(kPa abs.)
V1, V2 =specific volume of water at lower and higher temperature, respectively, ft3/lb(m
3
/kg)
α =linear coefficient of thermal expansion: for steel. α =6.5x10 -6 in./in · ⁰ F (1.2x10-5 per ⁰ C);
for copper, α =9 .5x10 -6 in./in · ⁰ F (1.7x10 -5 per ⁰ C)
In a chilled water system, the higher temperature T2 is the highest anticipated ambient temperature when the chilled water system
shu
ts down during summer. The lower temperature in a heating system is often the ambient temperature at fill conditions(for example,
50
⁰ F or 10 ⁰ C).
• Q
s
is the heat energy (in kcal) required for room heating during the defrosting operation, which may be obtained from:
• Q
s
= Q
h
x 860 x ( t
f
/ 60)
• Where:
• t
f
is the duration of defrosting operation (in mins)
• Δ T
t
is the permitted water system temperature drop during defrosng operaon (in ⁰C )
Equation 7
39
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