ENERGY EFFICIENCY OF ROTARY DRYERS IN MANUFACTURING PLANTS

摘要

This paper documents two preliminary studies conducted by the Industrial Assessment Center at North Carolina State University to estimate the potential for energy savings and the implementation costs associated with retrofit measures to improve the efficiency of rotary dryers operating in industrial settings. The first study deals with shell heat loss recovery from two large direct-fired rotary kilns at a paper mill. Previous works have shown that heat recovery in the form of pre-heated air for combustion can be realized if hoods are installed over hot sections of the kilns. Combustion air is preheated as it is drawn through the gap between the hood and the kiln. Economic analyses presented herein show that relatively large heat transfer areas can be justified. The heat recovery system must be designed such that the local kiln wall temperature does not exceed safe operating limits at the point where the hot air leaves the heat exchanger. The second study addresses the minimization of stack losses in three low temperature rotary dryers at a mining facility. The parameters that limit energy savings made possible by adjusting the excess air in the burners are the allowable wall temperatures of the unlined dryers and the temperatures of the exhaust gases, which could damage downstream filters. It is difficult to estimate the wall temperatures accurately because the walls are in contact with combustion gases and wet solids. Also, different heat transfer correlations from the literature predict different exhaust gas temperatures. Significant energy savings can be realized by designing the burners and induced draft fans based on conservative calculations, but experiments will be needed to establish the true optimum flow of combustion air at this facility.