Fan Sheave Change for a Target CFM
The drive sheave diameter that hits a target airflow.
Example
You enter
- Current airflow (cfm, measured) 8000
- Target airflow (cfm) 9600
- Current fan speed (rpm) 700
- Motor speed (rpm) 1750
- Current drive (motor) sheave pitch dia (in) 4
- Current brake horsepower (0 = skip) 3
- Motor nameplate hp (0 = skip) 5
- Current static pressure (in w.c., 0 = skip) 1.5
You get
- Ratio 1.2
- Required fan (RPM) 840
- New drive sheave (pitch dia) 4.8
- Driven sheave (in) 10
- Power and static pressure 5.184
- Bhp increase (%) 72.8
- New sp inwg 2.16
Checked against published worked examples from: AMCA / ASHRAE fan affinity laws.
Details, formula, and sources
ScopeTarget airflow sets the ratio, and with the fan sheave fixed the required DRIVE sheave scales the same way. The point is the cube law landing on the motor: 8,000 to 9,600 cfm is a 4.0-in sheave going to 4.8 in, 700 rpm to 840 - and 3.0 bhp to 5.18, which OVERLOADS a 5 hp motor at only 20% more air. That is the most common way a well-meant airflow fix cooks a motor.
Formula or tableratio = target CFM / current CFM; required fan rpm = current rpm x ratio; on a belt drive fan rpm = motor rpm x (drive sheave / driven sheave), so with the fan sheave fixed the new drive sheave = current drive sheave x ratio; SP scales with ratio^2 and BHP with ratio^3.
Source and editionFan affinity laws (AMCA / ASHRAE Handbook - Fundamentals) solved for the drive sheave rather than for a known speed, combined with the belt-drive speed ratio.
Free accessThe affinity laws and the belt-drive ratio are public engineering relations published in every fan and drive catalog.
What governsEstimate. AHJ and licensed professional govern.
API field namescurrent_cfm, target_cfm, current_fan_rpm, motor_rpm, drive_sheave_in, current_bhp, motor_hp, current_sp_inwg, ratio, required_fan_rpm, new_drive_sheave_in, driven_sheave_in, new_bhp, bhp_increase_pct, new_sp_inwg
- Affinity laws Q ~ N, SP ~ N^2, BHP ~ N^3 for the same fan on the same system curveAMCA / ASHRAE Handbook - Fundamentals
- Belt-drive ratio fan rpm = motor rpm x drive sheave / driven sheave; the fan sheave is held fixedbelt-drive geometry
- Motor check new BHP compared against the nameplate horsepower; the cube law is what fails itcomputed in the tile