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Physical Background
Any drying process is based on the fact that water molecules change from liquid into gas, called vapour.
This requires energy. Our option is freely available solar energy.
The driving force of drying is the difference between the partial vapour
pressure inside the sludge and the ambient air. In order to avoid an equilibrium between the vapour pressure inside and outside the sludge,
the air has to be evacuated. This is naturally helped by the fact that water vapour is lighter than dry air.
The warmer the air is the more water vapour can be transported. However the partial vapour pressure in the air
rises with the amount of water dissolved in the air. For more detail see the h/x-diagram of Mollier.
Except for very damp days, the drying process still occurs even in low sunshine. This is the same process
that allows a wet road to dry at night.
Open Air Drying Beds - Natural Ventilation
They are naturally ventilated by the wind. The higher the wind speed blowing over the sludge surface,
the faster the drying speed. However at sites with regular rainfall, an open air drying bed is not as
satisfactory.
For this reason in Middle Europe open air drying beds are no longer in use. Solar drying is done in
greenhouses which require controlled ventilation systems.
Greenhouse Drying - Controlled Ventilation
An efficient ventilation system has been developed and monitored in the different plants and sites where
WendeWolf® turning machines are installed.
Our ventilation system has three components:
- Roof Ventilation Flaps
Motor driven roof ventilation flaps are installed over the entire length of the drying hall.
These are similar to those used in many greenhouses for ventilation.
- Axial Fans
Fans are placed inside the hall in such a way that air turbulence is created above the entire surface of
the drying bed, destroying the moist boundary layer above the sludge surface. This artificial wind is
important for the drying process as it avoids any stratification of temperature or humidity.
- Aperture Slits
An aperture slit is positioned between the greenhouse and the walls on which the WendeWolf® is travelling.
Whenever the roof flaps are opened, a natural draft occurs and fresh, dry air enters the greenhouse.
Picture:
Solar drying halls showing roof flaps
Bilten (Switzerland)
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Control Strategy - Solar Energy Only
The temperature and the relative humidity is monitored outside and inside each hall. The PLC (Process Logical Control)
calculates the temperature difference and the absolute water content inside and outside for each hall separately.
The respective thresholds can be altered.
- Axial fans are switched on when the temperature inside is five degrees higher than outside
- Roof flaps are opened when the calculated value of the absolute humidity inside exceeds a threshold of 1.5 g/kg Air
Control Strategy with Auxiliary Heating
When auxiliary heat is available, a heating system may be installed in the hall. Different systems are possible
and have been used in different sites:
- Floor Heating
- Hot Air Blowers
- Infrared Radiators
The axial fans are controlled by the temperature difference inside/outside blowing the warm air downwards and
breaking up the moist layer of air at the sludge surface. The hall must have doors and the roof flaps are able
to be closed to limit thermal energy loss. The flaps are only opened when the absolute water content of the air
inside has reached a certain ratio to the outside ambient air. As warm and moist air rises, the exchange is
rapidly achieved and the flaps close again.
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