66389-BIM-C-1208
Johnson Controls Unitary Products
31
SUPPLY AIR DRIVE ADJUSTMENT
At unit start-up, the measured CFM may be higher or lower
than the required CFM. To achieve the required CFM, the
speed of the drive may have adjusted by changing the datum
diameter (DD) of the variable pitch motor sheave as
described below:
Required CFM
Measured CFM
Use the following tables and the DD calculated per the above
equation to adjust the motor variable pitch sheave.
EXAMPLE
A 12.5 ton unit was selected to deliver 4,000 CFM with a 3
HP motor, but the unit is delivering 3,800 CFM. The variable
pitch motor sheave is set at 2 turns open.
Use the equation to determine the required DD for the new
motor sheave,
4,000 CFM
3,800 CFM
Use Table 22 to locate the DD nearest to 4.21 in. Close the
sheave to 1 turn open.
New BHP
3
= (Speed increase) • BHP at 3,800 CFM
3
= (Speed increase) • Original BHP
= New BHP
New motor Amps
3
= (Speed increase) • Amps at 3,800 CFM
3
= (Speed increase) • Original Amps
= New Amps
FIGURE 25 - DRY COIL DELTA P
DW-12 Units
Indoor Coil Pressure Drop vs. Airflow
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
3500
4000
4500
5000
5500
6000
6500
Airflow (CFM)
Dry Coil Delta P (IWG
)
Before making any blower speed changes review
the installation for any installation errors, leaks or
undesirable systems effects that can result in loss of
airflow.
Even small changes in blower speed can result in
substantial changes in static pressure and BHP.
BHP and AMP draw of the blower motor will
increase by the cube of the blower speed. Static
pressure will increase by the square of the blower
speed. Only qualified personnel should make blower
speed changes, strictly adhering to the fan laws.
(
)
• Existing DD = New DD
Belt drive blower systems MUST be adjusted to the
specific static and CFM requirements for the appli-
cation. The Belt drive blowers are NOT set at the
factory for any specific static or CFM. Adjustments of
the blower speed and belt tension are REQUIRED.
Tighten blower pulley and motor sheave set screws
after these adjustments. Re-checking set screws
after 10-12 hrs. run time is recommended.
(
)
• 4.0 in. = 4.21 in.
