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5188235-YIM-E-0518
Checking Supply Air CFM
The RPM of the supply air blower will depend on the required
CFM, the unit accessories or options and the static resistances
of both the supply and the return air duct systems. With this
information, the motor speed tap (direct drive) or the motor
pulley number of turns open (belt drive) can be determined from
the Blower Performance Data Tables.
Note the following:
1.
The supply air CFM must be within the limitations shown in
the Unit Physical Data Table .
2.
Pulleys can be adjusted in half turn increments.
3.
The tension on the belt should be adjusted as shown in the
Belt Adjustment Figure 20.
4.
Tighten blower pulley and motor sheave set screws after
any adjustments. Re-check set screws after 10-12 hrs run
time is recommended.
Start the supply air blower motor. Adjust the resistances in both
the supply and the return air duct systems to balance the air
distribution throughout the conditioned space. The job
specifications may require that this balancing be done by
someone other than the equipment installer.
To check the supply air CFM after the initial balancing has been
completed:
1.
Remove the two 5/16" dot plugs from the blower motor and
the filter access panels shown in Figure 8.
2.
Insert at least 8" of 1/4 inch tubing into each of these holes
for sufficient penetration into the air flow on both sides of
the indoor coil.
NOTE: The tubes must be inserted and held in a position
perpendicular to the air flow so that velocity pressure
will not affect the static pressure readings.
3.
Using an inclined manometer, determine the pressure drop
across a dry evaporator coil. Since the moisture on an
evaporator coil may vary greatly, measuring the pressure
Table 19: Additional Static Resistance
Size
CFM
(Tons)
1000
1200
1400
1600
036 (3.0)
1800
048 (4.0)
2000
060 (5.0)
2200
2400
2600
2800
3000
1. Add these values to the available static resistance in the respective Blower Performance Tables.
2. Deduct these values from the available external static pressure shown in the respective Blower Performance Tables.
3. The pressure drop through the economizer is greater for 100% outdoor air than for 100% return air. If the resistance
of the return air duct is less than 0.25 IWG, the unit will deliver less CFM during full economizer operation.
48
1
Cooling Only
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.23
0.26
0.29
0.32
drop across a wet coil under field conditions would be
inaccurate. To assure a dry coil, the compressors should
be deactivated while the test is being run.
4.
Knowing the pressure drop across a dry coil, the actual
CFM through the unit can be determined from the curve in
Pressure Drop vs. Supply Air CFM (Figure 22).
Failure to properly adjust the total system air quantity and
static pressure can result in extensive system damage.
After readings have been obtained, remove the tubes and
reinstall the two 5/16" dot plugs that were removed in Step 1.
NOTE: De-energize the compressors before taking any test
measurements to assure a dry indoor coil.
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
500
Figure 22: Pressure Drop Across Coil
2 3
Economizer
0.07
0.08
0.09
0.11
0.13
0.15
0.17
0.20
0.23
0.26
0.30
1000 1500 2000 2500 3000 3500
NOMINAL CFM
2
Electric Heat kW
7 - 15
20 - 30
0.04
0.06
0.05
0.07
0.06
0.08
0.07
0.09
0.08
0.11
0.10
0.13
0.12
0.15
0.14
0.17
0.16
0.20
0.19
0.23
0.22
0.26
Johnson Controls Ducted Systems
060
036
048
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