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FAN FUNDAMENTAlS
131
Exhaust fan to be sized
8 ft.
40 ft.
30 ft.
Louvers to supply
Makeup air
DETERMINING CFM
After the model is known, the cfm must be determined.
Consult local code requirements or the table below for
suggested air changes for proper ventilation.
The ranges specified will adequately ventilate the
corresponding areas in most cases. However,
extreme conditions may require “Minutes per
Change” outside of the specified range. To
determine the actual number needed within a range,
consider the geographic location and average duty
level
of the area. For hot climates and heavier than
normal area usage, select a lower number in the
range to change the air more quickly. For moderate
climates with lighter usages, select a higher number
in the range.
To determine the cfm required to adequately
ventilate an area, divide the room volume by the
appropriate “Minutes per Change” value.
SAMPLE PROBLEM
A building requires an exhaust fan to ventilate a
general office (see diagram below) which measures
30 ft. x 40 ft. x 8 ft. The office is often crowded.
SOLUTION:
The total room volume is 30 ft. x 40 ft. x 8 ft. = 9600
cubic feet. From the chart, the range for general
offices is 2-8 minutes per change. Since the office
has heavier than normal usage, 4 minutes per
change is recommended. Therefore, the required
exhaust is:
9600 ft
3
= 2400 cfm
4 min.
Since the air to be exhausted is relatively clean, this
is an ideal application for a model GB fan.
NOTE: In this example, make-up air was provided
through a set of louvers at the wall farthest from
the exhaust fan. If there were no provisions for
make-up air in this room, a supply fan would also
have to be sized. The supply cfm should equal the
exhaust cfm. Supply fan location should be as far
as possible from the exhaust fan.
SUGGESTED AIR CHANGES FOR PROPER VENTILATION
Room Volume
cfm =
Room Volume = L x W x H (of room)
Min./Chg.
Introduction to FAN SELECTION
Area Min./Chg. Area Min./Chg. Area Min./Chg.
Assembly Hall 3-10 Dance Hall 3-7 Machine Shop 3-6
Attic 2-4 Dining Room 4-8 Mill 3-8
Auditorium 3-10 Dry Cleaner 2-5 Office 2-8
Bakery 2-3 Engine Room 1-3 Packing House 2-5
Bar 2-4 Factory 2-7 Projection Room 1-2
Barn 12-18 Foundry 1-5 Recreation Room 2-8
Boiler Room 1-3 Garage 2-10 Residence 2-6
Bowling Alley 3-7 Generator Room 2-5 Restaurant 5-10
Cafeteria 3-5 Gymnasium 3-8 Restroom 5-7
Church 4-10 Kitchen 1-5 Store 3-7
Classroom 4-6 Laboratory 2-5 Transfer Room 1-5
Club Room 3-7 Laundry 2-4 Warehouse 3-10