Blowers, Props and How They Behave (or Misbehave)

Fan laws and fan curves are a deep subject with a lot of nuance and variation. Just to get our heads around the subject let's focus on two different types of fans that we see all the time in HVAC, the prop/axial type and the radial/centrifugal type. Centrifugal / Blower With a typical PSC blower motor (non-variable/ECM) as we increase the pressure differential across it due to any variety of factors (small ducts, dirty filter, dirty coils) the blower moves less air and it uses less power to do it. The easiest way to test this is to measure the amperage of a PSC blower with a blower door off and then measure again with it on. The current will be higher with the panel off because the static pressure is LOWER and the blower is moving more air. Take a look at this chart which shows the huge impact static pressure (and input voltage) have on airflow If you do the same test (door on then off) on an ECM constant torque or constant airflow motor the amperage will go DOWN with the door off but this is because of the motor characteristics ramping the RPM down not because of the blower wheel properties. Take a look at this chart for an air handler that uses an ECM motor. The lines of airflow to static pressure are pretty constant until the static gets above the 0.5″wc If you were to check amperage on an ECM blower you will notice it draws higher amperage the higher the static pressure across it gets due to the motor ramping up to maintain the designed flow or torque. The PSC motor is the opposite, if we increase static the airflow, amperage and wattage all drop due to the characteristics of the centrifugal blower. To Summarize –

A blower wheel decreases in power used as static pressure increases UNLESS there is a ECM motor changing the RPM to compensate

Axial / Prop A prop fan performs in an opposite way in relation to pressure. As pressure differential across it increases the power used INCREASES even as the airflow it produces decreases. This means that if you block a condenser coil the fan will move less air and draw higher amps… illustrating again why keeping condenser coils clean has a big impact on performance. To Summarize –

A prop fan “loads” more based on pressure while a centrifugal fan “loads” based on mass flow

Again… this is a simplification but for a technician understanding these relationships can help you diagnose and understand system issues. Read specific system fan charts and curves for a better look at how a particular fan performs. — Bryan

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