Lab exhaust fan control 1

[SAK9]

Guys
I am designing a lab ventilation system which uses active pressure control by modulating exhaust volume from each rooms.The exhaust is discharged through a high plume exhaust fan to maintain a discharge velocity of 3000 fpm.There is a bypass damper at the fan inlet to maintain the discharge velocity constant when the room exhaust volume varies due to operational requirements and night setbacks.HEPA filters are provided in the room exhaust duct.I am not sure how to control the fan as the HEPA filters load.I think the bypass damper air flow will go on increasing as the HEPA filters load thereby reducing the room exhaust flow rate.What is the best way to control this system ?Please note that the fan exhaust volume needs to be kept constant to maintain the discharge velocity 3000 fpm.

 

[imok2]

I would install a velocity pressure sensor in the exhaust duct to control the fan motor and when the velociity becomes too low install an alarm and light if you feel that's its critical

 

[SilFox]

Dear SAC9!
That I've caught.
In connection with a condition that you want to ensure
constant flow of exhaust air by means of baypass you will take a part of air passing HERA filters.
In such case it will result in incorrect work of these filters.
Concerning control of the damper I can offer the following
variant. On the side of an output fan duct to cut the
sensor of pressure. At downturn of pressure, PID-controller
develops a signal on opening of baypass' damper.
Regards,
fs

 

[lilliput1]

Is the exhaust fan VFD or constant speed. Is there only one exhaust fan? Does the bypass damper admit in outdoor air to mix with the exhaust air inlet to the fan? If you have lots of pressurized rooms I recommend controlling by fixed CFM differential (controlled by DDC supply, hood exhaust & general room exhaust air terminal boxes)instead of active pressure control. When doors are open, the room may call for more exhaust that can be delivered without affecting the pressurization of other rooms. The VFD fan then would be controled to maintain a minimum static pressure at static pressure sensors located about 2/3 the duct run. You can have multiple sensor points & have a lowest signal selector control the fans. The OA bypass damper control should be sequence to open when the net exhaust has reached a design minimum CFM corresponding to 3000 FPM stack velocity. DP sensors at prefilters & HEPA filters should alarm when dirty condition is reached. Owner should set up a service contract w. HEPA manufacturer to DOP test them & certify every 6 months.

 

[SAK9]

Lilliput
Many thanksfor your post.I have been advised that the most simple way to control this system is by static pressure control in the fan suction plenum by modulating the bypass damper.Any comments on this suggestion?

 

[lilliput1]

If your fans are constant speed, you can locate the pressure sensor in the plenum. If the fans are VFD & you want the fan speed to reduce first before you sequence the OA bypass damper, then the sensor is better of located remote.