Fans in series and interlocks

[remp]

Im Upgrading an existing HVAC system. Have all the equipment and overlooked one small thing....

Existing plantroom has 4 air handling unit sucking air from an outsdie air louvre. The biggest unit is 20 m3/s and the other 3 are about 2 m3/s each. All unit run 24/7.

The louvre is being removed to cater for buiding expansion so the air has to be ducted down from the roof. A new supply air axial fan will bring the air down from the roof and into a plenum where the louvre was located.

I have a presure sensor in the plenum and the new supply air axial fan ( 26m3/s) will maintain a constant pressure in this plenum through frequency invertor (will be determined during commissioning but I would say it will be about (0-10Pa).

My question is do I need interlocks between the 4 air handling units and the large supply air axial fan. It seems very complex with so many fans. I was thinking of ignoring the interlocks. If the main supply air axial is switched off due to a break down then the air handling units just keep sucking (ok we will have less duty due to the static pressure increase). If any or all the air handling units are off due to breakdown or maintenance the static pressue in the plenum will go up so the fan will just ramp down. Perhaps I need a max perssure set point in the plenum to switch off the fan if it goes 50pa above setpoint when it is running at minimum speed. Could this lead to hunting??

What do you reckon? Am I on the right track

Thanks

 

[STYMIEDPIPER]

Not fully knowing your existing parameters can the large axial fan on the roof be replaced with a supply louvered penthouse??

 

[fredt]

remp

I think that you are on the right track and I can't think of any problems. If the axial fan had been fixed speed there would have been an issue with stall but not if you regulate speed to maintain a low positive pressure in the plenum. You probably won't need the 50Pa shut down (it may be impossible for the axial fan to generater 50Pa at minimum speed against a low resistance). I presume that you have calculated that you need the axial flow fan to overcome the resistance of the extra ducting and that the air handlers cannot handle this extra resistance with minimal flow reduction.