York Y33 User Manual download pdf (Page 28)

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.

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York Y33 User Manual Page 28