246653-YTG-E-0108
28
Johnson Controls Unitary Products
SUPPLY AIR DRIVE ADJUSTMENT
At unit start-up, the measured CFM may be higher or lower
than the specified CFM shown in Figures 22, 23, and 24. To
achieve the specified CFM, the speed of the drive may have
to be decreased or increased by changing the pitch diameter
(PD) of the motor sheave as outlined below:
Example
:
A 30-ton unit was selected to deliver 12,000 CFM with a 20
HP motor and a 856 RPM drive, but the unit is only delivering
11,000 CFM per Figure 23.
Use the equation to determine the required PD for the new
motor sheave (12,000 CFM / 11,000 CFM) x 6.7” = 7.31”.
Use the 30-ton table to select a Browning 2B5V74 which will
increase the speed of the unit's drive and its supply air CFM
109%.
New drive speed = 1.091 x 856 = 933 RPM
New supply air = 1.091 x 11,000 = 12,000 CFM
Re-use the existing belts and blower sheave.
New motor BHP = (speed increase)3 x estimated motor
BHP at original start-up with 11,000 CFM and 856 RPM =
(1.091)3 x 11.0 BHP = 1.2986 x 11.0 BHP = 14.3 BHP
New motor amps = (speed increase)3 x measured motor
amps at original start-up with 11,000 CFM and 856 RPM.
Failure to properly adjust the total system air quan-
tity can result in extensive blower damage.
Before making any blower speed changes review
the installation for any installation errors, leaks or
undesirable systems effects that can result in loss
of air flow.
Even small changes in blower speed can result in
substantial changes in static pressure and bhp.
Bhp or amp draw of the blower motor will increase
(see table) by the cube ratio of the blower speed.
Static pressure will increase by the square ratio of
the blower speed. Tables 26, 27 and 28 are for
reference only. All blower speed changes must be
made by qualified personnel with strict adherence
to the fan laws.
