Heat convective drying

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... 

Heat loss, dry The heat exchange that fakes place from the human body to the surroundings by convection, radiation, and conduction but not by evaporation. 

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],... 

Fig. 4.2. Waste heat boiler for a copper smelting flash furnace (Peippo et al, 1999). Note, left to right (i) flash furnace gas offtake (ii) boiler radiation section with water tubes in walls (iii) suspended water tube baffles in radiation section to evenly distribute gas flow (iv) convection section with hanging water tubes. Steam from the boiler is used to generate electricity, to power the acid plant s main blower and for general heating and drying.

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. 

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. 

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. 

Convection is possibly the most common mode of drying particulate or sheet-form or pasty solids. Heat is supplied by heated air or gas flowing over the surface of the solid. Heat for evaporation is supplied by convection to the exposed surface of the material and the evaporated moisture carried away by the drying medium. Air (most common), inert gas (such as N2 for drying solids wet with organic solvent), direct combustion gases, or superheated steam (or solvent vapor) can be used in convective drying systems. 

The determination of transport coefficients by a drying experiment is achieved as follows. During constant rate drying, the temperature of the drying surface is, in a purely convective drying, the so-called wet bulb temperature. With this value, the steady-state heat flux is... 

Since impingement yields very high heat or mass transfer rates, it is a popular system for convective drying when rapid... 

One of the most critical components on the conveyor dryer is the product feeder or spreader. As in any convective drying system, it is imperative that airflow through the product be uniform. Convective heat and mass transfer is proportional to airflow velocity past the product. 

Dostie, M., 1992. Optimization of a drying process using infrared radio frequency and convection heating. In Drying 92f A.S. Muiumdar (Ed.). Elsevier, Amsterdam, the Netherlands, 679pp. 

Dostie, M., J.-N. Seguin, D. Maure, Q.-A. Ton-That, and R. Chatigny, 1989. Preliminary measurements on the drying of thick porous materials hy comhination of intermittent infrared and continuous convection heating. In Drying 89, A.S. Mujumdar and M.A. Roques (Eds.), Hemisphere, New York. 

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