vanEE 90 H - 3.3 Calculating Duct Size

6
3.0 INSTALLATION (CONT’D)
VI0003
end
branches
main branch
5ӯ
70 cfm
6ӯ 140 cfm
140 cfm
figure 7
figure 8
3.3 Calculating Duct Size
Use the table below to ensure that the ducts you intend to install will be
carrying air flows at or under
the recommended values. Avoid installing
ducts that will have to carry air flows near the maximum values and
never install a duct if its air flow exceeds the maximum value.
Duct Diameter Recommended Air flow Maximum Air flow
4ӯ (102 mm) 40 cfm (19 l/s or 68 m
3
/h) 60 cfm (28 l/s or 102 m
3
/h)
5ӯ (127 mm) 75 cfm (35 l/s or 127 m
3
/h) 110 cfm (52 l/s or 187 m
3
/h)
6ӯ (152 mm) 120 cfm (57 l/s or 204 m
3
/h) 180 cfm (85 l/s or 306 m
3
/h)
7ӯ (178 mm) 185 cfm (87 l/s or 314 m
3
/h 270 cfm (127 l/s or 459 m
3
/h)
8ӯ (203 mm) 260 cfm (123 l/s or 442 m
3
/h) 380 cfm (179 l/s or 645 m
3
/h)
NOTE: Examples 3.3.1 and 3.3.2 use imperial measures. The same calculation
applies to metric measures.
3.3.1 Example of calculation
Problem: My installation requires two exhaust registers (one for the
kitchen, one for the bathroom). I will connect these registers to a main
duct which will connect to the unit (high speed performance value
of 140 cfm). What size of duct should I use for the main exhaust duct
and for the two end branches leading to the registers? (See figure 7).
Solution: Simplified method.
(For a more detailed method of
calculating duct size refer to the ASHRAE or HRAI HANDBOOK).
Main duct:
Table indicates for a 6ӯ duct: recommended air flow:
120 cfm; maximum air flow: 180 cfm. The high speed air flow of 140 cfm
is close enough to the recommended value (120) and far
enough away from the maximum value (180). Therefore a 6ӯ duct
or larger is an appropriate choice for the main exhaust duct.
End branches: Each end branch will have to transport an air flow of
70 cfm (140 divided by 2). Table indicates for a 5ӯ duct:
recommended air flow: 75 cfm; maximum air flow: 110 cfm. The high
speed air flow of 70 cfm is close enough to the recommended
value (75) and far enough away from the maximum value
(110). Therefore a 5ӯ duct or larger is an appropriate choice
for the 2 end branches
.
N.B.: A 4ӯ duct would have been too small because the maximum
acceptable value for a 4ӯ duct is 60 cfm.
3.3.2 Example of a design for a fully ducted system for a
unit having a high speed performance of 222 cfm
VI0004
4ӯ 42 cfm
6ӯ 129 cfm
5ӯ 65 cfm
5ӯ 64 cfm
6ӯ 93 cfm
5”
6”
7”
7”
6”
6”
6”
4”
4”
4”
4”
7ӯ 222 cfm
7ӯ 222 cfm
4ӯ 42 cfm
6ӯ 84 cfm
6ӯ 96 cfm
6ӯ 138 cfm