Exhaust Hood VFD Operation

[mech5]

I have a room with 20 canopy hoods (10 per branch) mani-folded to a common exhaust fan. The owner is concerned with unnecessary conditioned air leaving the building and is interested in a VFD drive for the exhaust fan. Where would the best location be for a static pressure sensor to be installed? How can I insure that each hood maintains the required CFM exhaust rate using manual dampers at each hood location? I've determined that the static pressure in the system with all hoods operating is 1.625 wg. What kind of control sequence could I notate to have the HVAC contractor install the system properly?
Thanks

 

[ChrisConley]

Rule of thumb location for a static pressure sensor is ~2/3 of the way into the system.

Are the hoods shut-off/turned-on automatically or manually?

How critical is it that each hood move an exact amount of air; labratory critical, kitchen critical or production line critical?

 

[mech5]

The hoods will have manual on/off dampers. The hood exhuast rate is not too critical, but should be relatively consistent throughtout the entire system. Will this be possible?

 

[boaterbill]

As each of the manaual dampers open/close, the static setpoint will change. installing a static pressure sensor 2/3 of length of the duct from the fan as in put to the controls will allow the VFD to ramp up/down to maintain the static pressure needed.

BTW, the real installation will be lucky to maintain 1.625 in WG. if you just have a setpoint of 1.6 it will operate.

 

[bunyala]

This may be thinking outside the box...
Putting the pressure sensor on the farthest hood and eliminate a damper on this hood may be an option. If the pressure setpoint is set to say 0.4 in.wc (best to get the setpoint after the test & balancing has been completed), when the other dampers are closed, flow in this critical hood will increase thus increasing the pressure. The VFD will ramp down to maintain the pressure setpoint.
I am skeptical placing the sensor 2/3 in the airstream, for an exhaust system, will control the VFD.