A lower system pressure is beneficial where stagnation characteristics
are concerned:
1 bar/0.1 MPa
positive pressure (during filling and at
a heat transfer medium temperature of approx. 20 °C) at the collector
is adequate. A definitive parameter when designing pressure mainte-
nance and safety equipment is the
steam production capacity
. This
indicates the output of the collector array, which during stagnation is
transferred to the pipework in the form of steam. The maximum steam
production capacity is influenced by the draining characteristics of the
collectors and the array. Subject to collector type and hydraulic con-
nection, different steam production capacities can occur (see figure
below).
A
B
A
Flat-plate collector without liquid pocket
Steam production capacity = 60 W/m
2
B
Flat-plate collector with liquid pocket
Steam production capacity = 100 W/m
2
Note
For vacuum tube collectors based on the heat pipe principle, a steam
production capacity of 100 W/m
2
can be expected, no matter where
the collectors are installed.
The length of pipe that holds steam during stagnation (steam spread)
is calculated from the balance between the steam production capacity
of the collector array and the heat loss from the pipework. The actual
values assumed for the loss from a solar circuit pipe made from copper
and 100 % insulated with commercially available material are as fol-
lows:
Dimensions
Heat loss in W/m
12 x 1/15 x 1/18 x 1
25
22 x 1/28 x 1.5
30
■ Steam spread
less
than the pipe run in the solar circuit (flow and
return) between collector and expansion vessel:
The steam cannot reach the expansion vessel in the event of stag-
nation. The displaced volume (collector array and pipework filled
with steam) must be taken into account when sizing the expansion
vessel.
■ Steam spread is
greater
than the pipe run in the solar circuit (flow
and return) between collector and expansion vessel:
Plan in a cooling line (heat sink) to protect the expansion vessel dia-
phragms against thermal overload (see figures below). Steam con-
denses again in this cooling line and reduces the liquefied heat
transfer medium to a temperature below 70 °C.
Expansion vessel and heat sink in the return
Expansion vessel and heat sink in the flow
The steam can spread in the flow and return.
P
B
C
D
E
A
The steam can only spread in the flow.
P
E
D
B
C
A
A
Collector
B
Safety valve
C
Solar-Divicon
D
Heat sink
E
Expansion vessel
The necessary residual cooling capacity is determined from the differ-
ential between the steam production capacity of the collector array and
the heat dissipation of the pipework up to the connection point for the
expansion vessel and the heat sink.
Note
The "SOLSEC" program is available at www.viessmann.com for cal-
culating the residual cooling capacity and sizing the heat sink.
The program offers three options:
■ Sufficiently long, uninsulated pipework branching to the expansion
vessel
■ A sufficiently large pre-cooling vessel, in relation to the cooling
capacity
■ A correctly sized stagnation cooler
For the heat sink, standard radiators with an output calculated at
115 K are assumed. For greater clarity, the program indicates the
heating output as 75/65 °C.
Information regarding design and operation
(cont.)
152
VIESMANN
VITOSOL
18
5822 440 GB
