Greenheck Canopy Hoods (452413 IOM) - Manual (Page 24)

Kitchen Hoods • Type I and Type II 24
®
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
cr
ease i
n
Co
ll
a
r St
ai
tc
Pr
essur
e
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 static
pressure number (when EABB is open) in this range is
given by the standard calculation for hood static and is
printed on the CAPS submittal page for that hood. The
high static pressure number (when EABB is closed) in
this range can be found by calculating the maximum
potential increase of static and adding that value to the
low static pressure number.
High static pressure number = low static pressure
number + maximum increase
The maximum potential increase in static is given in the
graph, or can be calculated from the following formula:
Maximum Increase = 0.00000036 x (Duct velocity)
2
Refer to the examples.
Example 1:
Hood 1: Ps = 0.58 in. wg
Duct Velocity = 1900 ft/min.
Hood 2: Ps = 0.44 in. wg
Duct Velocity = 1800 ft/min.
Hood 2 has the lower Ps, at 1800 ft/min. 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 ft/min.
Hood 4: Ps = 0.44 in. wg
Duct Velocity = 1500 ft/min.
Hood 4 has the lower Ps, at 1500 ft/min. 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
For many systems, exhaust air balancing baffles 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
When sizing the fan, use the static pressure from the
highest hood and sum the CFM from all the hoods.
Step 1: Find the hood with the lowest static pressure
as shown on the CAPS submittal pages.
Step 2: Calculate the high static pressure number for
this hood as instructed above.
Step 3: Compare this high static pressure number to
the low static pressure number of the remaining hoods.
If the remaining hood’s low static pressure number is
lower than the high static pressure number calculated,
then the system can be balanced; if the low static
pressure number is higher than the calculated high
static pressure number, then the system cannot be
balanced.
Exhaust Air Balancing Baffles