17
Canopy Hood
®
This is a guide to assist in determining if multiple hoods on one fan can be balanced to have equal
static pressure. For multiple hoods on one fan to achieve their designed exhaust flow, all of the hoods
must have equal static pressure at their designed exhaust flow.
The laws of physics force the static pressure for each branch of a duct system on one fan to always be
equal. This will happen by the flow rate increasing in low static branches and decreasing in high static
branches until the static pressure is equal in all branches.
Checking for Balance
Every hood with Exhaust Air Balancing Baffles (EABB) has a range for its static pressure. The low
number in this range is given by the standard calculation for hood static. (Static that is printed with
the CAPS submittal). The maximum increase above the low number can be calculated from the duct
velocity at the low static, (also given on CAPS submittal). This is then added to the low number to get
the highest static pressure possible with an EABB.
The maximum potential increase in static is given in the graph, or can be calculated from:
Max. Increase = 0.00000036 x (Duct velocity)2
After the range for each hood is calculated,
it should be compared to the hood with the
highest static pressure. If the highest hood
falls inside of the range, then the hoods can be
balanced with the EABB. If it is higher than the
range, the hoods cannot be balanced.
Example 1:
Hood 1:
Ps = 0.58 in. wg
Duct Velocity = 1900 FPM
Hood 2:
Ps = 0.44 in. wg
Duct Velocity = 1800 FPM
Hood 2 has the lower Ps, at 1800 FPM the maximum increase in Ps is 1.17. The range for Hood 2 is
0.44 to 1.61. Hood 1 is less than 1.61 so these hoods can be balanced.
Example 2:
Hood 3:
Ps = 2.00 in. wg
Duct Velocity = 2000 FPM
Hood 4:
Ps = 0.44 in. wg
Duct Velocity = 1500 FPM
Hood 4 has the lower Ps, at 1500 FPM the maximum increase in Ps is .81. The range for Hood 4 is 0.44
to 1.25. Hood 3 is higher than 1.25 so these hoods cannot be balanced.
Note 1: For many systems, an EABB may not be needed on the hood that has the highest static pressure. The
exception to this is if the individual ductwork has uneven static pressures.
Note 2: When sizing the fan, use the static pressure from the highest hood and sum the CFM from all the hoods.
Maximum Increase in Static Pressure for Exhaust Air Balancing Baffle
(Fully Closed)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
500
1000
1500
2000
2500
3000
3500
Duct Velocity FPM
In
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Exhaust Air Balancing Baffles (EABB)
