Case study: Calculating an exhaust system
Referencing code and using the correct calculations will allow the design to exhaust a battery room properly
A new battery storage room required a new exhaust system. The design and size of any battery room will vary depending on the types and number of batteries, but some design considerations will always apply. Battery rooms should be enclosed in a separate room with walls extending up to the roof with minimum openings. The room should be made to contain any hazardous gases, prevent exposure of the hazardous gases to any operational areas next to the room that might contain an ignition source for a fire or explosion and be maintained under negative pressure throughout with an exhaust system.
The fan and duct material might also require protection. Fans, ductwork and all components should be selected to meet all performance and code requirements. For example the fan might need to be AMCA spark resistant Type B to reduce the risk of a fan-induced explosion.
The initial ventilation was determined by the International Mechanical Code for stationary storage battery systems, which states that a continuous ventilation rate shall be provided at not less than 1 cubic feet per minute per square foot (cfm/sf) of floor area of the room. In discussions of the room’s requirements, the overall exhaust cfm was considered to increase to even 2.5 cfm/sf or higher, but it was decided to use about 1.5 cfm/sf. With coordination of conduit and structural conflicts, the designer determined the exhaust ductwork layout. Note that the exhaust system provides some ventilation from both a high-level and a low-level location that assists in overall mixing of the air in the room. Also, all the exhaust air is exhausted directly outside. Both are good design practices.
Once complete, the calculations indicated a total flowrate and static pressure requirement of 400 cfm and 0.25 inches w.g. and a spark-resistant fan was selected. A specific fan performance table and fan curve for the particular operation point was generated to illustrate and understand the selection chosen (see Table 2). In this design, makeup air was provided from the adjacent space passing through louvers in both panels of a double door entrance to the room.
In this example, the operating point for the system curve intersects the fan curve in a slightly sloped or steeper area of the curve but may have been a little better if chosen farther to the right on the curve. This is a constant volume system, with no filters so this is not a bad selection if the pressure drop in the system is correct. Note the dashed line where the fan should not operate.