Drying convection dryer

Beard(Z.) has developed a useful mathematical model of a convection dryer for studying the use of energy in a tenter frame. The model is based on a set of simultaneous differential equations which can be solved numerically to obtain fabric temperature and macroscopic moisture contents along the length of the dryer. The model considers the fabric as a moist layer of fabric sandwiched between two dry layers of fabric. Thermal energy is convected from the dryer to the external surface of the dry layer and then from the exterior of the fabric to the interface between the wet and dry layers. At the interface, the water is evaporated and diffuses as vapor through the dry layer to the surrounding hot make-up air. Assumptions in the model include ... 

If conduction or radiation is the predominant mode of heat transfer, the surface (and possibly the interior) moisture may literally boil regardless of the temperature or the humidity of the environment. This may be readily demonstrated by microwave drying. Thus, if control of granulation temperature is important, direct heat (convection) dryers usually offer greater control and product safety since the material s surface does not exceed the wet-bulb temperature during the steady state period. However, it will be shown later in this chapter that properly controlled dielectric drying may also be used to dry heat sensitive materials. 

Example 19 Drying of Particles A convective dryer is to be used to dry 720 kg/h (0.2 kg/s) of particulate material from 0.2 to 0.02 kg/kg moisture content (all flows and moistures on dry basis), using air at 180°C and 0.005 k kg humidity. Estimate the required air flow rate and dryer size for a fluidized-bed dryer (0.5 m/s inlet velocity) and a pneumatic conveying dryer (20 m/s inlet velocity). Assume outlet RH is approximately 20 percent. What is the effect of 10 percent heat losses ... 

Convection dryers are also used to heat and dry substrates. Typically, high velocity heated air is blown at the substrate from both sides so that the substrate is elevated between the nozzles. In many cases, the heated air is used for both heat and mass transfer, to volatilize any liquids on or in the substrate such as water, and then carry the vapor away from the substrate. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

Reay, D., Modelling continuous convection dryers for particulate solids— Progress and problems, Drying 85, R. Toei and A.S. Mujumdar, eds.. Hemisphere, New York, 1985, pp. 67—74. 

Another advantage of pulse combustion drying is the very short contact time of drying agent and drying material. Table 23.5 compares the residence times of the dried material in the pulse combustion dryer with selected convectional dryers. The typical residence time is 0.01-1 s for pulse combustion drying. For this reason, this technique can be applied even to extremely sensitive materials (Swientek, 1989). 

Natural convectional dryer has been used in Philippines to produce dried sweet potato. These dryers utilize heat from burning the wastes. About 50 kg of sweet potato chips of high quality are dried for 7-8 h. Another form in which sweet potato is dried and eaten as a dessert or snack is as an edible leather also known as fruit roll or crush. In this preparation, the flesh of sweet potato is cooked, mashed, and sieved, mixed with 0.5% (w/w) carboxymethyl cellulose (a binder), 200 ppm sodium bisulfite, and 7% (w/w) sugar and formed into a sheet 1-mm thick, which is oven-dried at 55°C-75°C and I0%-17% moisture level. Samples dried at 75°C were chewier than those dried at lower temperatures. Samples were crisper and crunchier when dried at 55°C or 65°C. A laboratory-scale high-temperature short-time (HTST)... 

Osmotic pretreatment before microwave-assisted air drying increase the final overall quality of the product [187]. Fruits and vegetables treated by osmosis can be further dehydrated in a convection dryer to lower the water activity to the level of 0.65-0.90. At those water activities, water content in the material is still high and the product presents such organoleptic attributes as chewiness, softness, elasticity, and plasticity [76,188-190], The product has a natural color, well-preserved flavor, and high retention of vitamins [132], Its shrinkage is much smaller when compared with that observed in convection-dried products at the same water activity. 

Thermal drying Steam can Convection dryer Radiant predryer... 

Convection dryers are the most common type of dryers used for drying textiles and carpets. The drying medium is usually... 

---