Kiln wall heat transfer

Although these relations have no direct application to determining flow in the turbulent regime of the rotary kiln, they can be of help in estimating dust loss and wall heat transfer coefficients. 

The time constants characterizing heat transfer in convection or radiation dominated rotary kilns are readily developed using less general heat-transfer models than that presented herein. These time constants define simple scaling laws which can be used to estimate the effects of fill fraction, kiln diameter, moisture, and rotation rate on the temperatures of the soHds. Criteria can also be estabHshed for estimating the relative importance of radiation and convection. In the following analysis, the kiln wall temperature, and the kiln gas temperature, T, are considered constant. Separate analyses are conducted for dry and wet conditions. 

Operabihty (ie, pellet formation and avoidance of agglomeration and adhesion) during kiln pyrolysis of urea can be improved by low heat rates and peripheral speeds (105), sufficiently high wall temperatures (105,106), radiant heating (107), multiple urea injection ports (106), use of heat transfer fluids (106), recycling 60—90% of the cmde CA to the urea feed to the kilns (105), and prior formation of urea cyanurate (108). 

Equation (12-57) does not account for gas radiation at high temperature when the kiln charge can see the burner flame hence, the method will yield a conservative design. When a kiln is fired internally, the major source of heat transfer is radiation from the flame and hot gases. This occurs directly to both the sohds surface and the wall, and from the latter to the product by reradiation (with some conduction). 

Design Methods for Rotary Kilns In rotary kilns, the material is not showered through the air stream but is retained in the lower part of the cylinder. Gas-solids contacting is much less efficient than in flighted units. Heat transfer is by radiation and convection from the flowing gas to the kiln brick and exposed bed surface and by radiation from die brick to the bed. For units employing separate combustion chambers, it can be assumed that at high temperatures the wall-film... 

A 5-m-vvide, 4-m-high, and 40-ra-long kiln used to cure concrete pipes is made of 20-cm-thick concrete walls and ceiling (k 0.9 W/m - °C). The kiln is maintained at 40 C by injecting hot steam into it. The two ends of the kiln, 4 in X 5 m in size, are made of a 3-tnm-thick sheet melal covered with 2-cm-thick Styrofoam k = 0.033 W/ra - °C). The convection heat transfer coefficients on the inner and the outer surface.s of the kiln arc 3000 W/m C and 25 W/m °C, respectively. Dl.srcgaiding any heat loss through llie floor, determine the rate of heat loss from the kiln when the ambient air is at — 4°C. 

Reconsider Prob. 3-61, Using EES (or other) software, investigate the effects of the thickness of the wall and the convection heal transfer coefficient on the outer surface of the rate of heal loss from the kiln. Let the thickness vary from 10 cm to 30 cm and the convection heat transfer coefficient frotn 5 W/m °C to 50 W/m "C. Plot the rate of heat transfer as functions of wall thickness and the convection heat transfer coefficient, and discuss the results. 

Figure 7.15 Predicted radiation heat transfer coefficients from freeboard gas to the exposed wall for a 41 cm diameter pilot kiln (Barr et al., 1989).

These equations formed the basis for the various one-dimensional kiln models that have appeared in the literature (Brimacombe and Watkinson, 1978 Wes et al., 1976 and others). In these models an energy balance on the wall must be included, as well as the kinetic expressions for any reactions. The latter led to a set of mass conservation expressions that must be solved along with the energy equation. For example, if the evaporation of free moisture is controlled by heat transfer, rather than mass transfer, an additional thermal balance on the moisture can be included as... 

Modified Penetration Model for Rotary Kiln Wall-to-Bed Heat Transfer... 

Figure 8.1 presupposes that the kiln radius is large compared to the penetration depth, d(0), that develops during contact of the heated wall coming from the exposed wall with the bed. We can set the initial conditions and boundary conditions for the solution of a two-dimensional transient heat transfer equation, similar to Equation (8.5), using the time variable in terms of the rotation as f = (0 + g/2)/2ir( ), which is zero at the instant of initial wall-to-bed contact. The final time is t = /2ttco when the wall exits the covered bed. Between the time of initial contact and the maximum time, y = R-r where R is the kiln radius and r is the radius at the penetration depth, that is. 

Other relationships based on conventional penetration theory for packed bed have deduced that Nu = 2 i -JPe and for low Peclet numbers to the order of 10, Tscheng and Watkinson (1979) deduced an empirical correlation, where Nu = 11.6 x7e°. Any of these would suffice in estimating the wall-to-bed heat transfer coefficient as functions of the kiln s rotational speed, w, and the dynamic angle of repose, The calculated values of Nusselt numbers using Perron and Singh s... 

The system of equations. Equations (8.30a,b,c), can be solved for successive axial positions by any of a variety of techniques (e.g., Runge Kutta) provided that the various heat transfer terms are characterized in terms of the local gas, bed, and wall temperatures. Thus, by starting at either end of the kiln, a complete solution of the thermal problem can be developed. It is chiefly the methodology employed in evaluating the heat transfer terms that distinguishes the various one-dimensional models. 

A description of the heat-transfer processes taking place between solids, gas and walls, including heat losses from the outside walls of the kiln to the surroundings ... 

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