18.7 Pressure drop of the solar thermal system
■ The specific flow rate for the collectors is determined by the type of
collector and the intended method of operation of the collector array.
The way the collectors are linked determines the pressure drop of
the collector array.
■ The overall flow rate for the solar thermal system results from mul-
tiplying the specific flow rate by the absorber area. Assuming a
required flow velocity of between 0.4 and 0.7 m/s (see page 148),
the pipework dimension is then determined.
■ Once the pipework dimension has been determined, the pressure
drop for the pipework (in mbar/m) is then calculated.
■ External heat exchangers must be calculated as well and should not
exceed a pressure drop of 100 mbar/10 kP. For smooth tube internal
indirect coils, the pressure drop is much lower and can be ignored
in solar thermal systems with a collector area of up to 20 m
2
.
■ The pressure drop of further solar circuit components can be seen
from the technical documentation and is included in the overall cal-
culation.
■ When calculating the pressure drop, take into account the fact that
the heat transfer medium has a different viscosity to pure water. The
hydraulic characteristics become more similar as the temperature of
the media increases. At low temperatures around freezing, the high
viscosity of the heat transfer medium may result in a pump rate some
50 % higher than for pure water. With a medium temperature above
approx. 50 °C (controlled operation of solar thermal systems), the
difference in viscosity is only minor.
Pressure drop of the solar flow and return lines
Per m pipe length, corrugated stainless steel pipe DN 16, relative to
water, corresponds to Tyfocor LS at approx. 60 °C
3
5 6
10
20 30 40
Flow rate
in l/min.
Pressure drop
3
5
10
20
30
50
70
100
200
mbar
kPa
0.3
0.5
1
2
3
5
7
10
20
Information regarding design and operation
(cont.)
VITOSOL
VIESMANN
145
5822 440 GB
18
