This can be quite simple if a balancing report, design drawings, and calculations exist for the system, or quite time consuming if no data is available. In general, a pressure loss calculation must be updated or done from “scratch” starting with a duct layout that includes flow in every “critical path between the furthest point of the system and the stack. With the calculated branch velocities and fitting loss coefficients (from ASHRAE or SMACNA guides), pressure losses can be calculated by branch and system including the scrubber. The total pressure loss is then compared to the fan flow and pressure rating leading to a decision that may only require increasing the fan speed but could require a larger motor, or replacing the fan or replacing overloaded sections of duct.
The scrubber capacity must also be evaluated, considering the through velocity, worst case gas concentration, scrubbing efficiency, and limits on effluent concentration.
Whether one is adding a gas cabinet, exhausted manufacturing equipment, or rearranging the air conditioning duct system for an office, the process is essentially the same.
Ralph Cohen, P.E., who provided this information, is an engineering consultant to customers for gas systems components and HVAC, specializing in semi-conductor and other industrial manufacturing facilities. He has over 30 years in the field, and continues problem solving for clients through his consultancy. More information on Ralph’s project work, publications, etc. can be found here RalphCohenConsultancy or call 971-227-8989.