f。 If the planner's forecasts change in the processes to occur within the next couple of years, which would require an increase in the amount of replacement or exhaust air, then consider purchasing a larger capacity fan and oversized wiring。

2-4。2。2        Location。  Locate the exhaust fan after the air pollution control equipment to protect fan blades from contaminated air-stream。 Provide access for maintenance to all fans, including ladders and guardrails where necessary。 Refer to NFPA 70, National

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。

2-4。3 Exhaust Stacks

2-4。3。1 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 housing。

Figure 2-1。  Exhaust stack designs。

2-4。3。2 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。

2-4。4 Air Pollution Control Equipment。  Requirements for air pollution equipment vary by process and geographical region in the United States。 Contact the

local activity environmental manager to determine the pollution control requirements for the process。

2-4。5 Replacement Air。  Replacement air is as important as exhaust air in controlling industrial process contaminants。 Properly designed replacement air will (1) ensure that exhaust hoods have enough air to operate properly, (2) help to eliminate cross-drafts through window and doors, (3) ensure proper operation of natural draft stacks, (4) eliminate cold drafts on workers, and (5) eliminate excessive differential pressure on doors and adjoining spaces。 The method of distributing replacement air and the quantity of replacement air are critical with respect to exhaust air。 Design the replacement air system in accordance with the decision tree shown in Figure 2-2。

Figure 2-2。  Decision tree for replacement air design。

2-4。5。1 Space Pressure Modulation。 Control the ventilated space pressure by modulating the quantity of replacement air。 Use a variable frequency drive (VFD) motor to control the fan speed (see MIL-HDBK-1003/3, Heating, Ventilating, Air Conditioning, and Dehumidifying Systems for information of VFD motors)。 Using barometric dampers to control replacement air quantity is inefficient and unreliable。  Sensor controlled transfer grilles are acceptable provided there will not be a problem with contaminated migration。

2-4。5。2