Drying convection

Depending on the way to apply heat to the product the drying is differentiated in several different modes. The main drying modes are the Convection Drying and the Contact Drying but also Radiation Drying is becoming more popular. 

Convection drying is typically carried out under atmospheric pressure with air or nitrogen as carrier gas in flash dryer, fluid bed dryer, tray dryer or belt dryer. 

1) Product Presentation Heinkel Process Technology GmbH. Ferdinand Porschestr. 8, D-74354 Besigheim. 

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Convection Drying Modeling. Models of the drying process have been developed to estimate whether a particular coating can dry under the conditions of an available dryer. These models can be mn on desktop personal computers (see Computer technology). 

To model convection drying both the heat transfer to the coated web and the mass transfer (qv) from the coatiag must be considered. The heat-transfer coefficient can be taken as proportional to the 0.78 power of the air velocity or to the 0.39 power of the pressure difference between the air in the plenum and the ambient pressure at the coatiag. The improvement in heat-transfer coefficients in dryers since the 1900s is shown in Figure 20. The mass-transfer coefficient for solvent to the air stream is proportional to the heat-transfer coefficient and is related to it by the Clulton-Colbum analogy... 

Dry-heat processes kill microorganisms primarily through oxidation. The amount of moisture available to assist sterilization in dry-heat units varies considerably at different locations within the chamber and at different time intervals within the cycle. Also, the amount of heat available, its diffusion, and the environment at the spore/air interface all influence the microorganism kill rate. Consequently, cycles tend to be longer and hotter than would be expected from calculations to ensure that varying conditions do not invalidate a run. In general, convection dry-heat sterilization cycles are run above 160°C [37],... 

Microwave-assisted (0.1 or 0.2 Wg-1) convection drying was also applied to osmotically dehydrated blueberries, leading to dried berries that were comparable to freeze-dried ones in much shorter time (Venkatachalapathy and Raghavan, 1998). Frozen blueberries were also dried in a microwave and spouted bed combined dryer (MWSB) after a pretreatment using ethyl oleate and a NaOH dipping solution followed by sucrose osmotic treatment (Feng et al., 1999). Osmotic dehydration prevented the blueberries from... 

Lenart, A. and Lewicki, P.P. 1988. Energy consumption during osmotic and convective drying of plant tissue. Acta Alimentaria Polonica 1, 65-72. 

Lewicki, P.P., Le, H.V., and Pomaranska-Lazuka, W. 2002. Effect of pre-treatment on convective drying of tomatoes. J. Food Engineer. 54, 141-146. 

Lewicki, P.P. and Lukaszuk, A. 2000. Effect of osmotic dewatering on rheological properties of apple subjected to convective drying. J. Food Engineer. 45, 119-126. 

Ramaswamy, H.S. and van Nieuwenhuijzen, N.H. 2002. Evaluation and modelling of two-stage osmo-convective drying of apple slices. Dry. Technol 20, 651-667. 

Szymczak, J.A., Plocharski, W.J., and Konopacka, D. 1998. The influence of repeated use of sucrose syrup on the quality of osmo-convectively dried sour cherries. In Drying 98 (C.B. Akritis, D. Marinos-Kouris, and G.D. Saravacos, eds), Vol. A, pp. 895-902. Ziti Editions, Thessaloniki, Greece. 

The evidence of a correlation between the drying induced stresses and the AE is illustrated on the cylindrical kaolin sample under convective drying. Although the AE method enables us to point out the period in which the stresses reach maximal values, we are not able yet to assess the magnitude of these stresses. 

Figures 2 show a comparison of convective drying with or without microwaves [23], Whilst for convective drying there are definite eonstant rate and falling rate periods, when microwaves are added the form of the eurves ehanges [24-25],...

Ilic, M. and Turner, I.W. Convective drying of a consolidated slab of wet porous material, Int. JHeat Mass Transfer, 32 (12),[1989). 

Convection is the transfer of heat from one point to another within a fluid by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is caused by gradients of temperature and gravity. In forced convection, the motion is caused by mechanical means that enhance the rate of heat transfer over natural convection. An example of convection drying would include the use of hot air in tray dryers and fluid bed dryers. 

The Bowen chart is a plot of enthalpy (abscissa) against humidity (ordinate). It is convenient to be able to read enthalpy directly, especially for near-adiabatic convective drying where the operating line approximately follows a line of constant enthalpy. However, it is very difficult to read accurately because the key information is compressed in a narrow band near the saturation line. See Cook and DuMont, Process Drying Practice, McGraw-Hill, New York, 1991, chap. 6. 

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