882108-YIM-B-1012
Johnson Controls Unitary Products 61
SUPPLY AIR DRIVE ADJUSTMENT
At unit start-up the measured CFM based on Figures 52 and
53 may be higher or lower than the specified CFM. 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:
• (Specified CFM/Measured CFM) X PD of standard
sheave = PD of new sheave.
Use the following tables and the PD calculated per the above
equation to select a new motor sheave.
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 20.
• Use the equation to determine the required PD for the
new motor sheave (12,000 CFM/11,000 CFM X 6.7” =
7.3091 inch).
• 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 to 111.9%. Thus select the 7.5” PD at 112%
increase over standard.
• New drive speed = 1.1194 X 856 = 958.2 RPM
• New supply air = 1.1194 X 11,000 = 12,313 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 =
(1.119)3 x 11 BHP = 1.4012 x 11 BHP = 15.41 BHP New
motor amps = (speed increase)3 x measured motor amps
at original start-up with 11,000 CFM and 856 RPM.
.
SYSTEM SETPOINTS
Constant Volume and Variable Air Volume:
Thermostat and space sensor offsets must be made external
to the unit. For internal settings, refer to the Settable Parame-
ters in the Unit Control document. All parameters affecting
the unit with that specific set of options and for that specific
application must be reviewed. In many cases, the default set-
tings will be fine - but verify and fine tune where necessary.
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 39) by the cube ratio of the
blower speed. Static pressure will increase by the
square ratio of the blower speed. Tables 40, 41
and 42 are for reference only. All blower speed
changes must be made by qualified personnel with
strict adherence to the fan laws.
TABLE 39: BLOWER SPEED RATE OF CHANGE
CHANGE IN RPM
CHANGE IN
CFM TSP BHP AND
MOTOR
AMPS
.90 .90 .81 .73
.93 .93 .86 .79
.95 .95 .90 .86
.98 .98 .95 .93
1.00 1.00 1.00 1.00
1.03 1.03 1.05 1.08
1.05 1.05 1.10 1.16
1.08 1.08 1.16 1.24
1.10 1.10 1.21 1.33
1.13 1.13 1.27 1.42
1.15 1.15 1.32 1.52
1.18 1.18 1.38 1.62
1.20 1.20 1.44 1.73