Heat Energy Transfer under Vacuum Conditions

Heat Energy Transfer under Vacuum Conditions 

Unlike the influence of the fluidization behaviour the heat and mass transfer between the process air and the product is highly dependent upon the solvent used. Certain physical properties are the density of the gas at particular pressures and specific heat capacity as well as the essential gas volume. The behaviours of the individual solvent vapours do not differ from one another until below 100 mbar. It is therefore barely possible for acetone vapours to transfer heat energy into the particles at 40 mbar system pressure, no matter which temperatures are obtained in the air heater. Even the radiation of the inlet air duct can hardly be compensated. Then again xylene vapours can be heated under similar circumstances without problems. 

This is also valid for spraying purposes, particularly spray drying, in which the considerably high content of solvent has to be removed out of the spraying liquid fast enough to avoid over-moistening. 

Vacuum fluidized bed reactor (With permission from Glatt Ingenieurtechnik 

Furthermore coating processes for applying protective or film coatings (for pharmaceutical purposes, e.g.) to fluidized particles are possible [3, 5, 6]. 

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Heat Energy Transfer under Vacuum Conditions... 

Heat transfer may occur by conduction, convection, or radiation. We can reduce conduction with good thermal insulation at the boundary, we can eliminate conduction and convection with a vacuum gap, and we can minimize radiation with highly reflective surfaces at both sides of the vacuum gap. The only way to completely prevent heat during a process is to arrange conditions in the surroundings so there is no temperature gradient at any part of the boundary. Under these conditions the process is adiabatic, and any energy transfer in a closed system is then solely by means of work. 

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