Effect of Air Flow on Heat Transfer and Water Evaporation in Jet-Drying Systems

Air-impingement utilizing a pattern of air jets whose air stream impinges at a 90-degree angle to the drying surface appears to be a very effective means of creating desirable boundary-layer conditions for rapid veneer drying. The boundary layer or film of slow moving air next to the drying surface has been shown to act as an insulator, decreasing heat transfer and retarding water evaporation. Air-impingement systems can produce the thin boundary layer and high turbulence required for rapid heat transfer and moisture removal. This study investigated heat transfer rates to a dry surface and evaporation rates from a wet surface using air-impingement systems. Three variables of such systems were empirically evaluated: air velocity, total orifice area and vertical distance of orifice from the drying surface. Orifice area greater than 2.2 percent of the surface area will achieve maximum power efficiency. Highest drying rates are obtained when the distance between the orifice and the material being dried is no greater than six orifice diameters. At air velocities of 4,800 to 20,000 feet per minute heat transfer rates of 14 to 78 Btu/hr/sq.ft./?F were generated from two orifice plate designs, one having spacing and orifice diameter equalling 1.227 percent of the surface area and the other 2.182 percent. Compared with parallel air flow at 1,000 feet per minute, air-impingement systems operating at the same air velocity would double the rate of heat transfer, and at jet-air velocities of 10,000 and 20,000 feet per minute the heat transfer rate would be 10 to 20 times greater than parallel flow systems.

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