Can 2 centrifugal blowers work in parallel if they're operating at different speeds?
I'm a commercial HVAC service technician and have recently acquired a blower assembly out of the dumpster for a laminar flow hood I want to build. It's a double shafted, 3/4HP, 1075RPM motor with 2 separate scroll cages and wheels. Unfortunately the units data plate was worn off or otherwise lost in the dumpster so I don't know what model it was to get the manual and fan curves.
I know the flow hood will be operating around 1" wc static pressure, but I don't know what speed I'll need to get the correct CFM for the 100fpm velocity for laminar flow. While disassembling and cleaning the 2 halves I had a thought.. What if, instead of buying a direct replacement double shafted motor, I buy (x2) 3 or 4 speed motors and run the airflow in parallel. I would wire them so I could run them at the +- 1 speed difference (i.e. Lowest speed: 1 fan low speed 2nd fan off, 2nd lowest speed: both fans low speed, 3rd lowest speed: 1 fan low speed and 2nd fan med speed, etc.).
My question is how well would running 2 blowers in parallel work if they're not operating at the same speed? Would the pressure difference at the different outlets cause air to flow backwards through the slower fan, even if they're just 1 speed different? Should I just stick with a double shafted motor so they're running the same speed? Am I just needlessly overcomplicating this design as I am want to do?
Anyway, thanks in advance for any insight or advice you can give me.