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GUIDE TO THE SIZING OF THE EXPANSION TANK
As a consequence of the variation in temperature of the water between intake and output, the volume
occupied by the water changes, this change increasing with increase in the temperature variation. The buffer
tank on the hydraulic circuit compensates for this change in volume.
If this buffer tank were not fitted, the water, which already fills the available internal volume of the system,
would increase in pressure to potentially dangerous levels and could cause joints to break.
The project elements to consider when selecting the dimensions of the buffer tank for a system are:
C
The quantity of water in the system in litres
e
the expansion coefficient of the water, calculated as the maximum temperature difference between
when the system is off and when the system is running (the values are given in the table below)
p
The absolute initial pressure, equivalent to the pre-charge pressure of the buffer tank (normally 2.5
i
bar, i.e. 1.5 bar-r);
p
The absolute tolerated pressure, which must be less than the pressure at which the safety valve is
f
set, taking account of any difference in height between the valve and the tank.
The total capacity of the buffer tank is expressed as:
⋅
C
e
=
V
t
p
−
i
1
p
f
using the expansion coefficient values in the following table.
Water temp.
[°C]
10
20
30
40
50
Water expansion coefficient
It is also possible to calculate the average value of 'e' between the initial water temperature (generally
assumed to be 10°C) and the operating temperature, using:
−
=
⋅
6
e
,
7
5
10
IMPORTANT: On units with the optional pump group the water circuit is fitted with an expansion tank with
safety valve set at 3 bar. In applications which require water side pressure higher than 3 bar the unit must not
be fitted with the pump group. In this case all enquiries must be addressed to UNIFLAIR ITALIA before the
order is placed.
Density
Value of "e"
3
[kg/m
]
at 10°C
999.6
-
997.9
0.0017
995.6
0.0040
992.2
0.0075
988.1
0.0116
(
)
2
⋅
−
T
4
Water temp.
[°C]
60
70
80
90
100
T [°C]
21
Density
Value of "e"
3
[kg/m
]
at 10°C
983.2
0.0167
977.8
0.0223
971.8
0.0286
965.3
0.0355
958.4
0.0430
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