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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. 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.
Balancing the Kitchen Exhaust
System
A. To determine the proper dining room air balance:
1. Refer to engineering drawings to determine total
exhaust CFM from dining areas. (exhaust fans,
heating and air conditioning units, rest rooms, etc.)
2. Determine the total CFM of make-up air supplied
to dining area.
3. Subtract #1 from #2 above. If the result is a
negative number, a negative pressure is present in
the dining area. In this case, kitchen exhaust odors
could be drawn from the kitchen to the dining
area. Therefore, exhaust or supply air should be
adjusted to provide a slight positive pressure in
the dining area.
12
Kitchen Hoods • Type I and Type II
B. To determine proper kitchen air balance:
1. Refer to engineering drawings to determine total
exhaust from the kitchen area. (exhaust hoods,
dishwasher hoods, etc.)
2. Determine total CFM of make-up air supplied to
kitchen area. (make-up air hoods, heating and air
conditioning units, etc.)
3. Subtract #1 from #2 above. The result should be a
negative number. If the result is a positive number,
a positive pressure is present in the kitchen area.
Kitchen odors could be forced into the dining
area. Also, a positively balanced kitchen area can
adversely affect the performance of the exhaust
hood.
CAUTION
According to NFPA 96, Ch. 8-3 Replacement Air:
Replacement air quantity shall be adequate to prevent
negative pressures in the commercial cooking area(s)
from exceeding 4.98 kPa (0.02 in. wg).
NOTE
The airflow rates were established under controlled
laboratory conditions.
NOTE
Greater exhaust and/or lesser supply air may be
required for complete vapor and smoke control in
specific installations.
Testing Hood Air Volume - Rotating Vane
Method
Baffle Filter Style Hoods
A. Exhaust:
With all the filters in place, determine the total hood
exhaust volume with a rotating vane anemometer as
follows:
1. All cooking equipment should be on.
2. Measure the velocities. Velocity measurements
should be taken at five locations per filter. These
must be over a filter slot as in Fig. 6.
Fig. 6
X
X
X
L
Nominal Filter Size
H/4
X
H
X
H/2
H/4
®
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