Air Balance; Pressure Drop Across A Dry Indoor Coil Vs Supply Air Cfm For All Unit - York SUNLINE MagnaDRY DR240 Installation Manual

AIR BALANCE

On VAV units be certain that the VFD drive is
set to maximum output, exhaust dampers are
closed and individual space damper boxes are
full open.
VFD units with bypass must not be in bypass
mode ('LINE' position) unless all individual
space dampers are full open.
Start the supply air blower motor. Adjust the resis-
tances in both the supply and the return air duct sys-
tems 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 Fig-
ures 12 and 13.
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 posi-
tion perpendicular to the air flow so that veloc-
ity pressure will not affect the static pressure
readings.
3. Using an inclined manometer, determine the pres-
sure drop across a dry evaporator coil. Since the
moisture on an evaporator coil may vary greatly,
measuring the pressure drop across a wet coil
under field conditions would be inaccurate. To
assure a dry coil, the compressors should be deac-
tivated while the test is being run.
Johnson Controls Unitary Products
PRESSURE DROP ACROSS A DRY COIL VS SUPPLY AIR CFM
0.65
0.55
0.45
0.35
0.25
0.15
0.05
4000 5000 6000 7000
NOMINAL AIR FLOW (CFM)
FIGURE 20 - PRESSURE DROP ACROSS A DRY
INDOOR COIL VS SUPPLY AIR CFM
FOR ALL UNIT TONNAGES
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 20.
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 BE-
FORE TAKING ANY TEST MEASUREMENTS
TO ASSURE A DRY INDOOR COIL.
284811-YIM-C-1008
DR180
DR300
DR240
8000 9000 10000 11000 12000
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