Convection dryer

Fig. 3. Humidity chart illustrating changes in air temperature and humidity in adiabatic direct-heat (convection) dryers. AB is an adiabatic saturation line.

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. 

Direct (convective) dryers The general operating characteristics of direct dryers are these ... 

The mass balance is usually performed) on the principal solvent and gives the evaporation rate E (kg/s). In a contact or vacuum dryer, this is approximately equal to the exhaust vapor flow, apart from any noncondensibles. In a convective dryer, this gives the increased outlet humidity of the exhaust. Eor a continuous dryer at steady-state operating conditions. 

Here Cg is the humid heat Cpc + YCpy. In convective dryers, the left-hand side is dominated by the sensible heat of the hot inlet gas GCsiTci in contact dryers, the heat input from the jacket pin is dominant. In both cases, the largest single term on the right-hand side is the latent heat of the vapor GXYq- Other terms are normally below 10 percent. This shows why the operating line of a convective dryer on a psychrometric chart is roughly parallel to a constant-enthalpy line. 

Here p is the rate of heat transfer from the heated wall to the solids, and ATtj/s is the temperature driving force. The latent heat of evaporation Xev should allow for bound moisture and heating of solids and vapor to the final temperature. A typical wall-to-solids heat-transfer coefficient h s for the given dryer type should be used. The calculation is less accurate than the one for convective dryers. Again, the heat-transfer rate is assumed to be the overall limiting factor. 

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 ... 

Convective dryers may be used as well in papermaking. In the Yankee dryer (Fig. 12-85), high-velocity hot airstreams impinging on the... 

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. 

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