These people discussing partially blocking air flow in squirrel cage fans in central air handlers (HVAC), another forum. Look at "If you want to regulate..."



And look at ... "I always used..." below:

 

blocking the path for airflow results in less air moved. with that, there is less to do for the fan so it speeds up. same happens also with vacuum cleaner and hair drier...

 

Ask the experts.
https://forums.mikeholt.com/threads/vfd-controlling-hvac-blower.65258/

 

why not just test it yourself? seeing is believing.... and this is easy to confirm - just need a fan and multimeter.

 

I had a project where we installed a large gas fired package boiler. I designed an induced draft system for the boiler using 400 HP, 480V, 3 Phase, motor driven fan and an inlet air control shutter for draft control. I was sitting in my office when the entire office building had a serious brown out and all the UPSs installed on the main computer room and all throughout the 2 floor office building started alarming. That and with the boiler house next door to the office building something started emitting this horrendous sound. Someone had started the 400hp induced draft fan without the shutter controls and the inlet shutter was completely closed causing the fan to try and pull a vacuum inside the boiler before the motor eventually tripped its breaker ending the horrific noise and brownout. Variable draft fans must have fan controls to closely control without overloading the fan motor. Better to have a variable speed fan. A manual variable shutter does not work well with a fixed speed fan without causing the fan motor to become overloaded.

 

was just playing with some circuits when one of older automotive fans showed up. this fan needs PWM to run. so i used opportunity to compare current draw when blocking the fan inlet and outlet. voltage and PWM are kept constant. the fan speed went up and current draw decreased when either inlet or outlet are blocked. video was too large so here is a picture...