102777-YIM-E-0206
Unitary Products Group 65
SUPPLY AIR DRIVE ADJUSTMENT
At unit start-up the measured CFM based on Figures 61 and
62 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 966 RPM drive, but the unit is only
delivering 11,000 CFM per Table 33.
• Use the equation to determine the required PD for the
new motor sheave (12,000 CFM/11,000 CFM X 6.9” =
7.53 inch).
• Use the 30 ton Table 37.To select a Browning 2B5V74
which will increase the speed of the unit’s drive and its
supply air CFM 108.7%.
• New drive speed = 1.087 X 966 = 1,050 RPM
• New supply air = 1.087 X 11,000 = 11,957 CFM
• Re-use the existing belts and blower sheave.
.
New motor BHP = (speed increase) 3 X estimated motor BHP
to original start-up with 11,000 CFM and 966 RPM = (1.087)
3
X 13.5 BHP = 1.284 X 13.5 BHP = 17.3 BHP New motor
amps = (speed increase)
3
X measured motor amps at original
start-up with 11,000 CFM and 966 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 Table 52. All parameters affecting the unit with that spe-
cific set of options and for that specific application must be
reviewed. In many cases, the default settings 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 35) by the cube ratio of the
blower speed. Static pressure will increase by the
square ratio of the blower speed. Tables 36, 37
and 38 are for reference only. All blower speed
changes must be made by qualified personnel with
strict adherence to the fan laws.
TABLE 35: 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
To Next Page
Loading...
Need help?
General
