OPTIMIZATION OF HEAT AND MASS TRANSFERS IN COUNTERFLOW CORRUGATED-PLATE LIQUID-GAS EXCHANGERS USED IN A GREENHOUSE DEHUMIDIFIER

Heat and mass transfers occuring in a counterflow direct contact liqui d-gas exchanger determine the performance of a new greenhouse air dehu midifier 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 co ntact with combustion gas from a high efficiency boiler equipped with a condenser. The heat and mass transfers between the thin film of dilu ted TEG and the hot gas were simulated by a model which uses correlati on formula from the literature specifically relevant to the present cr oss-corrugated plates geometry. A simple set of analytical solutions i s first derived, which explains why some possible processes can clearl y 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 suitab le conditions were defined for the process, which lead to a new design of the apparatus. In this second prototype, a gas-gas exchanger provi des dryer and cooler gas to the basis of the regenerators, while a war mer TEG is fed on the top. A whole range of operating conditions was e xperimented and measured parameters were compared with numerical simul ations of this new configuration: recondensation did not occur any mor e. As a consequence, this second prototype was able to concentrate the desiccant fluid at the desired rate of 20 kg H2O/hour, under temperat ure and humidity conditions which correspond to the dehumidification o f a 1000 m(2) greenhouse heated at night during the winter season.