035-17311-002-A-0803
32
Unitary Products Group
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 31 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 23 - DRY COIL DELTA P
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.
(
)
• 4.0 in. = 4.21 in.
(
)
• Existing DD = New DD
