036-21120-002 Rev. A (1102)
Unitary Products Group
5
APPLYING FILTER PRESSURE DROP TO
DETERMINE SYSTEM AIRFLOW
To determine the approximate airflow of the unit with a filter in
place, follow the steps below:
1.
Select the filter type.
2.
Select the number of return air openings or calculate the
return opening size in square inches to determine the
proper filter pressure drop.
3.
Determine the External System Static Pressure (ESP)
without the filter.
4.
Select a filter pressure drop from the table based upon
the number of return air openings or return air opening
size and add to the ESP from Step 3 to determine the
total system static.
5.
If total system static matches a ESP value in the airflow
table (i.e. 0.20, 0.60, etc.,) the system airflow corre-
sponds to the intersection of the ESP column and Model/
Blower Speed row.
6.
If the total system static falls between ESP values in the
table (i.e. 0.58, 0.75, etc.), the static pressure may be
rounded to the nearest value in the table determining the
airflow using Step 5 or calculate the airflow by using the
following example.
Example:
For a 100,000 BTUH furnace with 2 return open-
ings and operating on high speed blower, it is found that total
system static is 0.58" w.c. To determine the system airflow,
complete the following steps:
1.
Obtain the airflow values at 0.50" & 0.60" ESP.
Airflow @ 0.50": 1717 CFM
Airflow @ 0.60": 1590 CFM
2.
Subtract the airflow @ 0.50" from the airflow @ 0.60" to
obtain airflow difference.
1590 - 1717 = -127 CFM
3.
Subtract 0.50” from the total system static, and divide
this difference by the difference in ESP values in the
table, 0.60" - 0.50", to obtain a percentage.
(0.58 - 0.50) / (0.60 - 0.50) = 0.8
4.
Multiply percentage by airflow difference to obtain airflow
reduction.
(0.8)x(-127) = -102
5.
Subtract airflow reduction value from airflow @ 0.50" to
obtain actual airflow @ 0.58" ESP.
1717 - 102 = 1615
TABLE 4: FILTER PERFORMANCE - PRESSURE DROP INCHES W.C.
Airflow Range
Minimum Opening Size
(in.
2
)
Filter Type
Disposable
Hogs Hair
Pleated
1 Opening
2 Openings
1 Opening
2 Openings
1 Opening
2 Openings
1 Opening
2 Openings
0 - 750
230
0.01
0.01
0.15
751 - 1000
330
0.05
0.05
0.20
1001 - 1250
330
0.10
0.10
0.20
1251 - 1500
330
0.10
0.10
0.25
1501 - 1750
380
658
0.15
0.09
0.14
0.08
0.30
0.17
1751 - 2000
380
658
0.19
0.11
0.18
0.10
0.30
0.17
2001 & Above
463
658
0.19
0.11
0.18
0.10
0.30
0.17
