I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg
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COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106
Tel: 817-624-0820
After you’ve measured and adjusted the diameter to the required size, re-install the sheave onto the
shaft. You can use an air flow hood to verify that the air flow you expected is being achieved.
Fan Laws
Fan laws are equations or formulas that allow you to calculate belt and sheave changes prior to making
them. This saves you time and the hassle of having to make a minor adjustment and measuring again. If
you are able to do the math right halve.
Using the first fan law, you are able to calculate the change in sheave diameter you would need in order
to increase or decrease fan speed to make the fan deliver your desired, or required, air flow.
1) Start by measuring the air flow at the fan.
Fan air flow can also be plotted by measuring the fan speed in RPM and the fan operating total
external static pressure, and by then plotting the fan air flow in the manufacturer’s fan
performance tables.
The two pieces of an adjustable sheave side by side.
2) Measure the outside diameter of the belt riding on the sheave.
Also measure the physical outside diameter of the sheave just to be sure it is large enough to
adjust to the required diameter.
Let’s use this example.
You have a 3″ adjustable motor sheave and the fan delivers 2300 cfm on a system that demands
3000 cfm. Here is the formula with our data:
First, divide 3000 cfm (desired) by 2300 cfm (current) to find the ratio of air flow increase.
3000/2300 = 1.3
This shows us that we are 30% “off” when comparing our required air flow vs. what the fan is actually
delivering. So, the air flow needs to increase by 30% for the fan to deliver the required air flow.
Now we multiply the original sheave belt diameter of 3″ times 1.3 to find our new sheave belt diameter
of 3.9″.
Now you can adjust the motor sheave pitch to 3.9″ to raise the air flow to out desired set point of 3000
cfm. This fan law will only work if the sheave diameter is at least 3.9″ or so.
Keep in mind that you can’t just go around your building or property and starting increasing the air flow.
In our example, we would want to make certain that both the fan and motor have the capacity or rating
to now handle 3000 cfm.
