Question About Air Volumes...

I may be talking out the top of my head, but why have 2 fans? One exhaust fan and a slightly restricted inlet should give the required negative pressure in the chamber while allowing the required air flow.

The two fans are required by the NFPA standard.

What happens when one fan fails?

They must be interlocked with the alarm system for the wet well system. In Canada, they must also be interlocked with all of the electrical equipment in the wet well.

OK have two exhaust fans. One in one out doesn't make sense. If the outlet fan fails you've got a pressurised chamber.

Dear North, I believe you can achieve what your driving towards with Two equal fans for a balanced draft ..... ADJUSTED.... to maintain a negative pressure. I do not have access to NFPA 820 /STANDARD FOR FIRE PROTECTION IN WASTEWATER TREATMENT AND COLLECTION FACILITIES. You will be able to locate numerous Blower companies on Globalspecs.

So whether you over drive the exhaust or restrict the supply, my premise is pretty much the same as what you mentioned. I can see where properly adjusted dampers on the inlet side could do what I need to see happen.

Thanks for the input.

Dampers should not be used in wet well ventialtion. Installing a VFD on your makeup air unit, or oversizeing your exhaust fan are your two best options.

Your fan capacity is decided by 12 times air changes per hour. Install two exhaust fans of same capacity. Only one should run, in case of failure, other fan should start automatically.

Slight vacuum of 0.1 in can be achieved easily by adjusting the openings for air inlet. Inlet lovers can be provided at appropriate places ensuring proper circulation of air before getting exhausted by the fan.

Along the same line as Klearzen.

You could use a single double shafted blower with one end, drawing or exhausting more than the other. This could be done by dampers.

With only one motor you may be able to included a second unit as back up, or a lead lag situation..

What type of fan it is . Is it axial type or propeller type or blower. If it is blower is it forward or backward or normal curved ones. As the systems is designed as per the NFPA standards it is presumed required interlocking between the fans have been incorporated in the system design.

My thanks to all who offered comment... enjoy your day.

YOU FANS CAN BE CONFIGURED TO TO MAINTAIN THE NEGATIVE WITH VFD'S CONTROLLED BY SIGNAL FROM DIFFERENTIAL PRESSURE TRANSMITTER WITH HIGH PRESSURE SIDE PIPED TO ATMOSPHERE AND LOW SIDE IN TANK A CHEAP CONFIGURABLE CONTROLLER CAN ALSO RECIEVE THE SIGNAL FROM THE DPT AND OUTPUT THE PROPER SIGNAL TO THE VFD'S AND SHUTDOWN THE FANS IF ONE DROPS AND THE CONDITION BECOMES A PROBLEM. iT COULD ALSO OUTPUT TO ANY FORMS OF ALARM YOU PREFER. vERY SIMILAR OPERATING SEQUENCE TO PHARMACY ROOMS, WE DO IT ALL THE TIME. PLEASE EXCUSE THE CAPS, I CAN'T TYPE AND I WON'T RETYPE.

Your logic is right on! Equip the exhasut fan with a variable speed drive (VFD) and a controller with a differential pressure sensor sensing both the well and atmospheric pressures and you're set to go. Using the VFD instead of a controlled damper in the exhasut fan discharge will pay for itself with the savings in energy consumed.