In large, manifolded exhaust systems of 20 or more fume hoods, the range of exhaust air flows that must be handled by the system can be wide, with five to one turn-down ratios possible. Therefore, when a variable volume exhaust system is used to compensate for turn-down ratios, fan speed or volume reductions must be limited to insure that a minimum stack exit velocity is maintained under all operating conditions. [Laboratory Control and Safety..., 1994; Neuman, 1990]
