Expansion Tanks

[homerphish]

Is there any type of code or formula to follow when sizing an expansion tank for a secondary coolant? We are doubling the amount of R-23 in one of our systems and I'm pretty sure the current expansion tank will be large enough.

 

[supaman79]

homerphish,

Different types of expansion tanks have different methods to sizing them properly. Is it Diaphragm, Open/Closed, Bladder-type? I'm assuming it's a bladder-type expansion tank, if it is here's the equation:

Vt=Vs x [(v2/v1)-1]-3a(t2-t1)/1-(P1/P2)

Vt=volume of expansion tank, gal.
Vs=Volume of Water in system, gal.
t1=lower temperature, F
t2=higher temperature, F
P1=pressure at lower temperature, psia
P2=pressure at higher temperature, psia
v1=specific volume of water at lower temperature, ft^3/lb
v2=specific volume of water at higher temperature, ft^3/lb
a=linear coefficient of thermal expansion, in./in. F
=6.5x10^-6 in/in F for steel
=9.5x10^-6 in/in F for copper

good luck

 

[supaman79]

homer,

let me correct myself, the equation I gave for you is for diaphragm tanks. For more help go to ASHRAE's Systems and Equipment Chapter 12

 

[pennpoint]

homerphish
Actually what you are describing is the Low Stage refrigerant expansion tank for a cascade system.The expansion tank is sized to hold the major volume of low stage charge at ambient temperature (when the system is off).
It's sizing is critical to the performance and safety of the low stage system (as this is the primary cooling system).
In order to properly size the expansion tank you need to determine the volume of refrigerant in the system as a liquid and a vapour.
This volume is derived from the main liquid line, suction cool line, filter drier volume, cascade receivers volume, 10% of the cascade. In addition, the low side (suction) and high side (discharge) of the piping must also be considered. The high and low side volume can be subtracted from the required expansion tank volume.
The system you have may state the balance pressure at 70 deg. F. This does not tell you how much R-23 by weight the charge really is.
How do you know your doubling the charge?
By what method did you arrive at your conclusion "that your pretty sure it's big enough".
Why do you wish to add more refrigerant?

Regards

pennpoint

 

[homerphish]

The system originally started with 900 lbs of R23 in it, but after two failures of two poorly constructed shell and plate heat exchangers, we decided to go with a shell and tube hx instead. As a result, the system calls for an extra 800 lbs of R23 due to the difference in size between the shell and tube and the shell and plate design. The expansion tank was originally sized for a system with only 900 lbs of R23. Now I need to figure out if it will still be OK or if I will need a larger one.