Choice and sizing

It is necessary to properly size the expansion vessel getting the volume.

For example, the water heating up from 0°C to 100°C increases its volume of about 4,5%. This means that there should be a space inside the system that can keep the exceeding volume of water. This space is the expansion tank.

The increase of water volume is absorbed by the tank. This means that the volume of the tank must be higher than the total possible expansion of the heating system.

The volume can be calculated using the following formula:

Where:
e =expansion coefficient of the water; this is the difference between the expansion of the water at its maximum temperature
and the expansion of the water at its minimum temperature when the system is not working
(usually Tmax=90°C and Tmin=10° therefore e=0,0359; see table below).
C =total capacity of the system (usually between 10 and 20 litres for each 1000Kcal/h of boiler power).

To calculate the exact size of the tank to be installed use the following formula:

In where:
η=internal volume of the tank
Pi=pre-charge pressure of the tank (bar)
Pf=maximum pressure set on the safety valve considering the difference in height between the valve and the tank (bar).

Example

System data: e= 0,0359 C= 400LitersPi= 1,5 bar Pf= 3 bar

In any case we will adopt the closest measure to the calculated value

Water temperature(°C)Coefficient of expansion
00,00013
100,00025
200,00174
300,00426
400,00782
500,01207
550,01450
600,01704
650,01980
700,02269
750,02580
800,02899
850,03240
900,03590
950,03960
1000,04343