The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.
Although the noise and increased air velocity make it seem like the fan is actually moving more air, the truth is that it is moving less. The air that it IS moving is traveling at a very high velocity, or speed. This is what makes the increased noise. But since you are effectively making the hole that the air travels through smaller, less air is able to get through, even at higher velocity. This is more difficult to show. To get accurate readings, you really need the air to be moving through some ductwork.
Another point to discuss is the difference between a blower with a traditional PSC motor and one with an EVM blower. The behavior we have been discussing is typical of an AC induction motor, like the PSC motors that come standard on most blowers. However, an ECM blower motor senses the change in work and increases its speed enough to actually overcome the resistance, so that the fan moves the same amount of air even against increased resistance. Since this requires more electrical energy, the amp draw for an ECM blower will increase when the fan is restricted. The ECM technology solves one problem: losing airflow due to increased resistance. But is creates a new one: increased electricl use to overcome the resistance.
To read more, check out Unit 41 Fundamentals of Psychrometrics and Airflow and Unit 75 Fans and Air Handling Units in Fundamentals of HVACR 2nd edition. You can find the blower labs in the new Lab Manual for Fundamentals of HVACR, 2nd edition. They are labs 75.3 AC Induction Motor Blower Properties and lab 75.4 ECM Blower Properties.