Ventilation Blowers in Series 2

[duke962]

I have a three blower ventilation system. Blower 1 and 2 are in parallel. Blower 3 is a booster fan connected in series with the other two. Blower 2 currently has a VFD and I would like to put a VFD on blower 3 to account for changes from blower 2. How will blower 1 be effected if the speed of blower 3 is lowered. Will the SP or BHP change? How about the Volume moved by blower 1, will that change? Any information or websites where I can find information will be of great use.

 

[malz]

It really depends on exactly how the fans are set up:
Is the booster fan before the other 2 or after, and what are the respective sizes? Does blower 3 have any means of varying air volume (ie. axial vane pitch mechanism, etc.), or is it a straight shot centrifugal unit? Any changes to speeds of fans connected in parallel will affect the other if it is discharging into a space, where SP is monitored and the controls would have the fan(s) respond accordingly. Or does the system spill to atmosphere,where the SP is monitored only on the inside? Also remember centrifugal fans work within a "curve" and will not move air properly (or at all) if it operates outside of the design criteria.
PS- I hate booster fans added to already existing systems. These items (that put my fan systems out of spec.) have caused me more problems than I care to list. If the designed fan system cannot operate to capacity needs, then it should be upgraded or replaced.(my humble opinion only!!!!!)

Malz