Viessmann Vitosol 100-FM Technical Manual page 133

Information regarding design and operation
Pressure in solar thermal systems with Vitosol-FM and Vitosol
300-TM
The set pressure for switching collectors prevents the formation of
steam, and in extreme cases, its spread into the solar thermal sys-
tem. Safety devices for the expansion vessels (stagnation cooler or
pre-cooling vessel) are not required. For calculating the required
pressure, see page 135. If the pressure is set too low, a small
amount of steam can form which will normally remain in the collec-
tors and not be pushed into the system. Switching collectors can
therefore be used in systems where the collector array is below the
DHW cylinder.
Pressure in solar thermal systems with Vitosol-F and Vitosol
200-TM
The pressure set ensures controlled evaporation of the heat transfer
medium. Depending on collector type/hydraulics or connection ver-
sion, the collector has a higher or lower steam-producing power.
This influences the selection and positioning of various technical
components of the solar thermal system. In conventional solar ther-
mal systems, where the steam produced can spread as far as the
expansion vessel, a stagnation cooler or a pre-cooling vessel are
installed to protect the diaphragm.
Do not position the collector array below the DHW cylinder. Other-
wise the steam produced during a system standstill can rise towards
the DHW cylinder in an uncontrolled manner. In the DHW cylinder,
the heat is transferred and the steam condenses and flows back
towards the collectors. The result is an uncontrollable system state.
Steam-producing power, pressure maintenance and safety
equipment
In the collectors, temperatures are reached that exceed the boiling
point of the heat transfer medium. For this reason, solar thermal sys-
tems must be designed to be fail-safe in accordance with the rele-
vant regulations.
With regard to stagnation behaviour, with the exception of switching
collectors Vitosol-FM and Vitosol 300-TM, a low system pressure is
advantageous: 1 bar/0.1 MPa (during filling and at a heat transfer
medium temperature of approx. 20 °C) on the collector is sufficient.
A definitive parameter when designing pressure maintenance and
safety equipment is the steam-producing power. This indicates the
power of the collector array, which during stagnation is transferred to
the pipework in the form of steam. The maximum steam-producing
power is influenced by the draining characteristics of the collectors
and the array. Subject to collector type and hydraulic connection, dif-
ferent steam-producing power levels can occur (see diagram below).
VITOSOL
(cont.)
A B
A Flat-plate collector without liquid pocket
Steam-producing power = 60 W/m
B Flat-plate collector with liquid pocket
Steam-producing power = 100 W/m
Note
Steam-producing power
■ Vitosol 300-TM: 0 W/m 2
■ Vitosol 200-TM: 60 W/m 2
The length of pipe that holds steam during stagnation (steam
spread) is calculated from the balance between the steam-producing
power 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 mate-
rial 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
■ Steam spread is 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:
Include a cooling line (heat sink) in the design, to protect the
expansion vessel diaphragms against thermal overload (see dia-
grams below). Steam condenses in this cooling line and reduces
the temperature of the liquefied heat transfer medium to below
70 °C.
2
2
25
30
VIESMANN
133
13