Viessmann VITODENS 242-F Technical Manual page 71

Design information
The length of pipe that holds steam during stagnation (steam spread)
is calculated from the balance between the steam production capacity
of the solar 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 follows:
Dimensions Heat loss in W/m
12 x 1/15 x 1/18 x 1
22 x 1/28 x 1.5
Expansion vessel and heat sink in the return
The steam can spread in the flow and return.
A
B
P
C
D
E
A Solar collector
B Safety valve
C Vitodens
D heat sink
E Expansion vessel
The required residual cooling capacity is calculated from the differen-
tial between the steam production capacity of the solar collector array
and the heat loss of the pipework to the point where the diaphragm
expansion vessel is connected to the heat sink.
Note
The SOLSEC program is available at www.viessmann.com for calcu-
lating the residual cooling capacity and sizing the heat sink.
This program offers three suggestions for implementation:
■ Sufficiently long, uninsulated pipework branching to the expansion
vessel
■ A sufficiently large pre-cooling vessel, in relation to the cooling
capacity
■ A correctly sized heat sink
Specification for heat sinks
Output at 75/65 °C in W
Stagnation heat sink
– Type 21
Pre-cooling vessel
VITODENS
(cont.)
482
—
■ Steam spread is less than the pipe run in the solar circuit (flow and
return) between solar collector and expansion vessel:
The steam cannot reach the expansion vessel in the event of stag-
nation. The displaced volume (solar collector array and pipework fil-
led with steam) must be taken into account when sizing the expan-
sion vessel.
■ Steam spread is greater than the pipe run in the solar circuit (flow
and return) between solar collector and expansion vessel:
Plan in a cooling line (heat sink) to protect the expansion vessel dia-
25
phragms against thermal overload (see figures below). Steam con-
30
denses in this cooling line and reduces the temperature of the liq-
uefied heat transfer medium to below 70 °C.
Expansion vessel and heat sink in the flow
The steam can only spread in the flow.
D
E
Conventional radiators, the capacity of which is determined as 115 K,
are assumed as the heat sink. For greater clarity, the program indi-
cates the heating output as 75/65 °C.
Note
Due to the expected high surface temperatures, Viessmann stagnation
heat sinks (see technical guide Vitosol) are equipped with a plate that
receives no flow, for protection against accidental contact. If commer-
cially available radiators are used, protection against accidental con-
tact must be provided. In addition, the connections must be diffusion-
proof.
Cooling capacity during stagnation
in W
A
B
P
C
Liquid content in l
964
450
VIESMANN
6
12
71