×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Lab exhaust fan control

Lab exhaust fan control

Lab exhaust fan control

(OP)

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.

RE: Lab exhaust fan control

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

RE: Lab exhaust fan control

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

RE: Lab exhaust fan control

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.

RE: Lab exhaust fan control

(OP)
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?

RE: Lab exhaust fan control

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources