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ST0133 – A 02/09
SOLAR
4.4 Dimplex solar expansion vessels SOLEV
4.4.1 General description
Dimplex solar offer an extensive range of expansion vessels from 18 to 80 litres. All
expansion vessels are particularly suited for the application in solar thermal systems
with a high temperature resistant membrane which has also excellent diffusion barrier
characteristics in conjunction with the heat transfer medium used in solar thermal sys-
tems.
In solar thermal systems the expansion vessels has a number of functions to ensure the
safe and reliable operation of the system:
- absorption of additional system volume when system is heating up during normal op-
eration
- absorption of steam volume of system when installation enters stagnation
- holding of medium reserve when system temperature drops below fill temperature
4.4.2 Expansion vessel sizing
A number of parameters have to be considered when sizing the expansion vessel. The
parameters to be considered are:
- total system volume
- static height of system
- expansion coefficient of heat transfer medium
- opening pressure of pressure relief valve
- required pressure in highest system point
- number of collectors installed
- fluid content of each collector
Applying the above system parameters, Equation 5 can be used to calculate the expan-
sion vessel size V
EV
, the cold fill pressure of the system and the membrane pre-charge
pressure of the expansion vessel.
(
)
(
)
6.0)(
1
⋅−
+⋅⋅++
=
pcrv
rvcoleppc
EV
pp
pVnVV
V
[5]
Where: V
EV
[l] minimum expansion vessel volume
V
pc
[l] pre-charge volume
V
ep
[l] system expansion volume
n [-] number of collectors connected to expansion vessel
V
col
[l] collector volume (SOLC220 = 1.7 litre)
p
rv
[bar] relief valve pressure
p
pc
[bar] cold fill pressure
Equation 6 is to be applied to determine the expansion vessel pre-charge volume V
pc
.
litreVV
syspc
3005.0 ≥⋅= [6]
Where: V
pc
[l] pre-charge volume
V
sys
[l] total system volume