261660-YTG-C-0707
14
Unitary Products Group
CFM, STATIC PRESSURE, AND POWER - ALTI-
TUDE AND TEMPERATURE CORRECTIONS
The information below should be used to assist in application
of product when being applied at altitudes at or exceeding
1000 feet above sea level.
The air flow rates listed in the standard blower performance
tables are based on standard air at sea level. As the altitude
or temperature increases, the density of air decreases. In
order to use the indoor blower tables for high altitude applica-
tions, certain corrections are necessary.
A centrifugal fan is a "constant volume" device. This means
that, if the rpm remains constant, the CFM delivered is the
same regardless of the density of the air. However, since the
air at high altitude is less dense, less static pressure will be
generated and less power will be required than a similar
application at sea level. Air density correction factors are
shown in Table 8 and Figure 2.
The examples below will assist in determining the airflow per-
formance of the product at altitude.
Example 1
: What are the corrected CFM, static pressure,
and BHP at an elevation of 5,000 ft. if the blower performance
data is 6,000 CFM, 1.5 IWC and 4.0 BHP?
Solution: At an elevation of 5,000 ft the indoor blower will still
deliver 6,000 CFM if the rpm is unchanged. However, Table
8 must be used to determine the static pressure and BHP.
Since no temperature data is given, we will assume an air
temperature of 70°F. Table 8 shows the correction factor to
be 0.832.
Corrected static pressure = 1.5 x 0.832 = 1.248 IWC
Corrected BHP = 4.0 x 0.832 = 3.328
Example 2
: A system, located at 5,000 feet of elevation, is to
deliver 6,000 CFM at a static pressure of 1.5". Use the unit
blower tables to select the blower speed and the BHP
requirement.
Solution
: As in the example above, no temperature informa-
tion is given so 70°F is assumed.
The 1.5" static pressure given is at an elevation of 5,000 ft.
The first step is to convert this static pressure to equivalent
sea level conditions.
Sea level static pressure = 1.5 / .832 = 1.80"
Enter the blower table at 6000 sCFM and static pressure of
1.8". The rpm listed will be the same rpm needed at 5,000 ft.
Suppose that the corresponding BHP listed in the table is 3.2.
This value must be corrected for elevation.
BHP at 5,000 ft = 3.2 x .832 = 2.66
TABLE 8: ALTITUDE CORRECTION FACTORS
AIR TEMP
ALTITUDE (FEET)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
40
1.060
1.022
0.986
0.950
0.916
0.882
0.849
0.818
0.788
0.758
0.729
50
1.039
1.002
0.966
0.931
0.898
0.864
0.832
0.802
0.772
0.743
0.715
60
1.019
0.982
0.948
0.913
0.880
0.848
0.816
0.787
0.757
0.729
0.701
70
1.000
0.964
0.930
0.896
0.864
0.832
0.801
0.772
0.743
0.715
0.688
80
0.982
0.947
0.913
0.880
0.848
0.817
0.787
0.758
0.730
0.702
0.676
90
0.964
0.929
0.897
0.864
0.833
0.802
0.772
0.744
0.716
0.689
0.663
100
0.946
0.912
0.880
0.848
0.817
0.787
0.758
0.730
0.703
0.676
0.651
