25 October 2004
Electrical Code for motor controller and disconnect location requirements. In all cases,
install exhaust fans outside the building that they serve. Installing the fan outside the
building envelope will isolate the working space from contaminants during fan
maintenance, minimize noise inside the building, and ensure that ductwork within the
building envelope is under negative pressure.
Design Considerations. Refer to the ACGIH IV Manual for exhaust
stack design criteria. The best designs are cylindrical, vertical discharge stacks as
shown in Figure 2-1. The best protection from rain, when the ventilation system is not
running, is the "offset stack" design C, as shown in Figure 2-1. Water may still enter the
system with straight stack design A. Provide a means to drain water from the fan
Figure 2-1. Exhaust stack designs.
Location and Structural Considerations. Refer to ASHRAE Handbook,
Fundamentals for information on airflow around buildings. Do not select stack locations
based on prevailing winds. A stack must provide effluent dispersion under all wind
conditions. Refer to UFC 1-200-01, Design: General Requirements for exhaust stack
structural design considerations. Some structural considerations are wind load,
lightning protection, and stack support. Refer to MIL-HDBK-1004/6, Lightning (and
Cathodic) Protection and SMACNA GSSDC, Guide for Steel Stack Design and
Construction for additional information.
Air Pollution Control Equipment. Requirements for air pollution
equipment vary by process and geographical region in the United States. Contact the