Reworking the example by using the lower temperature as 0°C and the higher
temperature as 80°C, we will get:
[ ]
(0.001028/0.001) - 1] - (3*11.7*10
-6
)(80-0)
1 - (80/300)
V
t
= 125
= 4.29 litres
(5.1 litres with 20% safety factor)
Thus, a 6 litres tank is needed.
Therefore, the higher the water temperature, the larger the volume of the tank that
is required.
Note: If the tank is located at a location with lower pressure, it can be seen that the
volume required will be smaller. Therefore, the expansion tank should be located
at the position with the a low pressure, e.g. on the water return suction line or at the
highest point of a vertical pipe system.
To calculate the expansion tank size for cooling only systems, use the lower temperature as
the design chilled water temperature and the higher temperature as the ambient
temperature (e.g. 35°C).
Generally, the size of the expansion tank for chilled water system is much smaller compared
with the hot water applications.
Using a larger expansion tank than the calculate requirement is OK. The larger volume
will not affect the system, rather it will help to cater for a larger water volume than the value
used in the calculation.
An expansion tank is not required for an open piping system as the reservoir tank itself
will allow for the expansion.
There should only be ONE expansion tank in the close piping system.
Section 6 Page 92
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