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FORM 100.50-EG5 (108)
17
JOHNSON CONTROLS
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 airfl ow rates listed in the standard blower perfor-
mance 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 applications, certain corrections
are necessary.
The examples below will assist in determining the airfl ow
performance 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 7 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 7 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
Altitude and Temperature Corrections
A centrifugal fan is a “constant-volume” device. This
means that if the RPM remains constant, the CFM de-
livered 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 7 and
Figure 2.
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
information is given so 70°F is assumed.
The 1.5" static pressure given is at an elevation of
5,000 ft. The fi rst step is to convert this static pressure
to equivalent sea-level conditions.
Sea-level static pressure = 1.5 / 0.832 = 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 blower
table is 3.2.
This value must be corrected for elevation.
BHP at 5,000 ft = 3.2 x .832 = 2.66
TABLE 7 – 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 0.95 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 0.73 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 0.88 0.848 0.817 0.787 0.758 0.730 0.703 0.676 0.651