Heat and mass transfers occuring in a counterflow direct contact
liquid-gas exchanger determine the performance of a new greenhouse
air dehumidifier designed at INRA. This prototype uses triethylene
glycol (TEG) as the desiccant fluid which extracts water vapor
from the air. The regeneration of the TEG desiccant fluid is
then performed by direct contact with combustion gas from a high
efficiency boiler equipped with a condensor. The heat and mass
transfers between the thin film of diluted TEG and the hot gas
were simulated by a model which uses correlation formula from
the literature specifically relevant to the present cross-corrugated
plates geometry. A simple set of analytical solutions is first
derived, which explains why some possible processes can clearly
be far from optimal. Then, more exact numerical calculations
confirm that some undesirable water recondensations on the upper
part of the exchanger were limiting the performance of this prototype.
More suitable conditions were defined for the process, which
lead to a new design of the apparatus. In this second prototype,
a gas-gas exchanger provides dryer and cooler gas to the basis
of the regenerators, while a warmer TEG is fed on the top. A whole
range of operating conditions was experimented and measured parameters
were compared with numerical simulations of this new configuration:
recondensation did not occur any more. As a consequence, this
second prototype was able to concentrate the desiccant fluid
at the desired rate of 20 kg H2O/hour, under temperature and
humidity conditions which correspond to the dehumidification
of a 1000 m2 greenhouse heated at night during the winter season.