Many industrial environments have processes which require either ventilation or local exhaust systems to provide a safe working environment from industrial contaminates. Safe concentration levels have been set for many contaminates which will prevent occupational illness, disability or death to the majority of those exposed. These levels are defined as threshold limit values (T~Vs), and the purpose of the exhaust system is to continuously provide a concentration level in the working environment below the TLV level (3). This result is often referred to as the air quality.

The passage of the Occupational Safety and Health Act (OSHA) in 1970 required strict adherence to TLVs for all industrial contaminates at all places of work, regardless of the number of employees (11). This requires all workplaces with hazardous contaminations to have reliabe industrial exhaust systems to insure a healthful environment. In addition to OSHA requirements, the recent increases in energy costs and prospects of energy shortages have placed an additional constraint on industrial exhaust systems to use a minimum of energy for operation and heating or cooling make-up air.

To meet the required safe environment needs, we can use a multiplying (or safety) factor and design the exhaust system much larger than required by the maximum TLV. However, this increases the need for costly energy. Therefore, to meet the twin objectives of safety and energy conservation, we need to design exhaust systems which will remove the least amount of air and still provide the minimum environment needs. These objectives require that a determination of the reliability of the exhaust system be made before adequate system design analysis and component selection are made.