AT-84-14-2 -- Emission Control Equipment Fractional Efficiency Considerations for Recirculated Exhaust Systems PDF

Process exhaust recirculation is an often considered, simple method for heat recovery. However, since most process exhaust streams contain some type of contaminant, the air must be cleaned prior to recirculation. In many cases, air-cleaning equipment has already been installed under the impetus of air pollution control regulations. Although adequate for compliance with these regulations, this same control equipment may not be efficient enough to permit recirculation.

The mass collection efficiency basis inherent to air pollution control regulations is not necessarily relevant to the evaluation of a potential exhaust recirculation situation. The fractional efficiency, or the control equipment's ability to collect particles of specific size, is far more relevant. All control equipment exhibits varying degrees of reduced efficiency in the respirable particulate size range. Knowledge of the extent of this reduction for the actual system under consideration is very important, since it can result in increased hazard and preclude recirculation as a heat recovery option.