VFDs for Exhaust Fans

what is your role big boy?

If these fans can be run at part speed for sizable fractions of time, VFDs can save energy on both the supply and exhaust fans.

On the other hand, if the fans are typically used at full speed, or cycled off under light loads, VFDs will not save any (or much) energy.

Agreed, though you are going to install VFDs on the supply fans which will change the building pressures, it may be possible to just cycle off the relief / exhaust fans since there is probably other sources of relief in the building, bathroom exhaust, elevator shafts. Which will save even more money.

Most importantly, is the design constant volume or VAV? If CV you will need to take a look at the refrigeration system to be sure it can operate properly on low load. You mentioned humidity if it is a DX system the humidity will be pulled out of the air and into the DX coil and down the drain.

Reading the OP though it sounds like big centrifugal "swamp coolers" so no you do not want to stop relief / exhaust if that is the case. Install the VFDs and run the relief / exhaust fans based on a 0-10 - 4-20 signal out put from a humidity transducer to the Exhaust fan VFDs to regulate speed. This will assure proper humidity control so the people or items in the building do not end up soaking wet due to slower speed delivery with higher water concentration and no way to remove the moist air.

DX system is not applicable here. The fresh air passes through water sprinkler than passes through pre- filters and than after passing through bag filters the air is now supplied to the plant area.The idea is to reduce fresh air temp by 5 to 6 degree C by water sprinkler.

Due to the dissipation of heat from motors as well as heat generated from the oil heaters install ,in the plant , it is necessary to exhaust the hot air into the atmosphere .

Is it necessary to replace the exhaust centrifugal fans when the management is keen to replace the Supply centrifugal fans

"Any type of VSD will induce further harmonics into your electrical system, and that alone will add to your monthly power bill."

How will it increase your monthly power bill?

Harmonics add to your power bill by becasue of the additional loss of power, disapated in heat. (VERY short answer)

So you are paying to warm up the atmosphere rather than run you applicance or whatever you are powering, on our web site www.thccontrols.com the powerpoint presentation explains this in detail.

As far as the VFDs in the market place today most come with the proper filters or chokes in them to assure a clean supply to the end device.

The problem with harmonics usually comes with a bad install or improper end device mating to the drive. The power feed wires and the supply to the end device (load) must be in separate conduit or noise on line may occur making it back the the MCC or main panels. Same with the motors. Be sure they are rated for VFDs, if not install Ceramic bearings to keep harmonic noise from getting to ground.

Costing more on energy consumption for a building after installing a drive, never seen that yet.

Dear,

Normally, Exhaust fans are running continously. If you run the fans at full speed, then , investment will be a waste.

If you reduce the speed by say, 5%, you may save consumption in the cube root proportionate, say 8 - 13 % of the previous load. Life of equipment will also increase.

But, you have to make a break even graph between

Capital Cost V/s Power reduction / cost saving by load reduction.

Absolutely. And for best results they should be controlled to maintain a very slight positive pressure in the building, in the order of .05" to 0.10.

If you are replacing the motors as you mentioned this would be the time to go to high efficiency motors. The pay back on this plus the VFDs will be very attractive and the on-going energy savings significant. Also check with the local utility for potential rebates. Good luck. Lou Bindner

thanks for your encouraging reply with special reference to building positive pressure.yes the plant management wants to go for high efficiency motors.I am not HAVC engineer will highly appreciate if you pass me the link to know more about positive building pressure.

Thanks