The proper dimensioning of an internal heating system seems to be more difficult because of
the different currents due to pumping, agitation, thermo-convection and the inflow of bio-
mass.
Under-floor heating systems have been very popular, as they have no disturbing parts in the
digester itself. Due to sedimentation and the resulting worsening of heat transportation into
the digester, under-floor heating is no longer recommended. With the growth of digester
volumes and the need of bigger heating systems, it is also more difficult to build under-floor
heating big enough to provide the necessary heat.
Heating coils installed at the inner wall of the digester are a rather new practice. Heating coils
made out of steel are much more expensive than heating coils out of plastic material (PE).
Materials developed during the last years make such a system more stable while not
increasing the costs of the heating system.
Another option is to construct two digesters connected in series, the first heated, the second
unheated. The first digester can be used as sedimentation tank , in which the substrate is
heated up. The second digester is well isolated to reduce loss of heat.
54