Air Flow shear in Pipes

[Barry P]

I am working on a project that feed air flow and pressure to Air Pumps to supply fresh water and air to fish in a Fish Farm.
I need to operate three Blowers (two operating at one time and alernate with the third)
My issue is when two blowers are operating at the same time one of the blowers will dead head, either due to the shear or air pressure in the pipe.

The System:
Operating pressure is 55 inWC and 800 CFM each Blower.
The air travels through a common pipe.
The Blowers are operating with a VFD and a common pressure Transducer.
I use 8" electric actuated valves on the discharge of each Blower.
Sequence of operation.
- Blower #1 and #2 operate Producing 800 CFM @ 55 inWC eac. The Valve for Blower #3 is closed.
- After a set time (1 week) Blower #1 will stop and Blower #3 will start. All three Blowers Close during the Transition and Valve #1 Closes while Valve #3 opens.
This is done to stop any of the air from travelling in reverse through the Blowers.

How can I get two Blowers to operate at the same time through a common pipe without one dominting the other?

Thank you
Barry P.

 

[LittleInch]

You can always try running the motors at a bit more than 20hp? If you're running at a higher speed than 50 or 60 Htz then the cooling fan would be OK so just tweak the limits a bit and run at say 22 hp?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Barry P]

These are inside a Sea Can so there may be issues during a hot day so I rather oversize the motors.

 

[FacEngrPE]

You need to try to limit the temperature in the shelter box. Either leave the doors open, or better, install some louvers and a exhaust fan. This becomes more important, if you need to use up your service factor in operation, as other things being equal cooler motors last longer.

If your VFD trips and the VFD indicates the trip was the I²T protection (calculated motor temperature) that trip can be adjusted upward a bit, with the understanding that the motor service life may be shortened.

In a system like this you need to be careful with using bull headed "T" arrangement when the flow velocity is high as it sometimes generates undesirable acoustic problems (the system can resonate at low frequencies). If the problem occurs, replacing some of the fittings where "T"s are used to combine flows, with Y fittings can fix the problem. This is an occasional problem in HVAC ductwork.

 

[Barry P]

Thank you
FacEngrPE