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There are three types of expansion tanks generally used.
1. for closed tanks with air / water interface
Vt=Vs (( V2/ V1) -1))-3 @ DELTA t / (Pa/P1)-(Pa/P2)
2. For Open tanks with air water interface
Vt=Vs(V2/V1 -1)-3 @ delta t)
3. For Diaphragm tanks
2.Vt=Vs (( V2/ V1) -1))-3 @ DELTA t / 1-(Pa/P2)
@=Linear Coefficient of thermal expansion(in/in.ºF)=6.5E- for steel
Vt=Volume of expansion tank
Vs=Volume of water in system
t1=Lower temperature
t2=Higher Temperature
Pa=Atmospheric Pressure
Pi=Initial Fill or minimum Pressure at tank
P1=Absolute Pressure at lower temperature (Pi + Pa) i.e generally the head loss of pump calculated
Pmax=Maximum System Allowable Pressure
P2=Absolute Pressure at higher temperature (Pmax + Pa) generally1.5 to2 times P1
v1=Specific volume of water at lower temperature
v2=Specific volume of water at higher temperature
* If any one need the calculation sheet send me mail
Sizing is based on ASHRAE200 HVAC Systems & Equipment, Chap12, Hydronic Heating & Cooling System Design
Vt = Vs { (v2/v1) -3 * X * delta T } / {1 - (P1 / P2)}
where:
x=Linear Coefficient of thermal expansion(in/in.ºF)=6.5E-06 for steel
Vt=Volume of expansion tank
Vs=Volume of water in system
t1=Lower temperature
t2=Higher Temperature
Pa=Atmospheric Pressure
Pi=Initial Fill or minimum Pressure at tank
P1=Absolute Pressure at lower temperature (Pi + Pa)
Pmax=Maximum System Allowable Pressure
P2=Absolute Pressure at higher temperature (Pmax + Pa)
v1=Specific volume of water at lower temperature
v2=Specific volume of water at higher temperature
