how to justify replacing blower FCV to a VFD system 1

[sspence]

I have a set of Blowers that provide air to a Waste Water plant for aeration, discharge at ~9psig.
The blower air flow is controlled with Flow control valve on the inlet of the blower, the blower motor runs a full speed but the motor load is typically about 55% of capacity (in amps, via restricting the inlet air).
How do I calculate the anticipated cost savings in replacing the control valve with a VFD unit?

Also is there any other reason to justify the conversion to VFD besides direct energy savings?

I have the motor HP & Flow curves

 

[DickDV]

The major drive players in the HVAC field have computer simulation software that, given your system data, can project a remarkably accurate prediction of energy usage and related changes.

Other than energy savings, there are significant reductions in maintenance with respect to belts, bearings and louver linkage.

There are disadvantages too. The drive may cause electrical noise issues in your power network that need to be addressed and your motor will run slightly more noisy. You also have to provide space for the VFD and deal with the waste heat it generates.

Bottom line is almost always heavily in favor of changing to the VFD, moreso as your normal operating speed goes down.

 

[ozmosis]

Just a word of warning, often 'blowers' on waste water treatment plants are sometimes positive displacement type blowers (I usually refer to them as Roots blowers). These are typically constant torque and require a high starting torque. The software DickDV mentions would typically be designed for variable torque centrifugal loads where the energy savings will be far more apparant than on blowers.
However, the advantages DickDV mentions are still valid for fitting a VFD on a blower.

 

[DickDV]

sed2developer is certainly correct concerning PD or Roots blower systems requiring a different analysis than centrifugal systems.

Thanks, sed2developer

 

[sspence]

This system is using Hoffman Centrifual blowers.
One site has motors rated at 100HP (3 units).
Another similar site with same design has motors rated at 900HP (6 units).

I agree whole heartedly with both of your comments.
Any recommendations on where to get the software?

 

[dpc]

Google: "VFD Energy Saving Calculator"

That should get you started. It's good to make sure they've accounted for the losses in the VFD, filters, reactors, etc. These sometimes get lost in the song and dance.

 

[quark]

One more, in the never ending list of VFD applications(pun intended).

Like sed2 observed, all the aerobic lagoons for waste water treatment, I maintained, have roots blowers. The advantage with them is that you will have constant air flowrate irrespective of back pressure. (For ex. variable fluid level in the lagoon). Your case may be of higher flowrates and that is why centrifugal fans.

All the energy saving calculators of variable speed applications are based upon affinity laws. The ratio flowrates is proportional to ratio of speeds, that of static heat is proportional to square of speeds and power to the cube of speeds.

In your case, when you try to reduce the speed by half to get half the flowrate, the static pressure reduces by 75% and this defeats the actual purpose of the fan. You can reduce static pressure only to the extent that was wasted at the flow control valve.

Before attempting to slow down the fan, I would first know the minimum static pressure required. Then I will check at what speed the fan should run. If the margin is not wide, I would rather continue controlling the flow by a FCV or change the pulley size (if fans can't be replaced). It will be of help if you can post the performance curve with design duty point marked.

Danfos has a good energy savings calculator at,

http://www.namc.danfoss.com/hvac/energysavings.html

 

[itsmoked]

quark; An excellent point!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ozmosis]

yes, good information quark.