ESTIMATING SAVINGS TO JUSTIFY A VSD 4

[pumpedup]

Is there a "rule of thumb" or a formula used to estimate energy savings (by implementing a VSD) based on motor loading?

I was given the following formula to calculate the predicted energy use with a VSD but it makes no sense to me (I have no idea where it came from)

Energy used = (full load power / VSD rated eff) * (% rated speed)^3 * (# operation hours)

* I'm not 100% sure what the (% rated speed) is......or where to get the (VSD rated eff).

I'm looking at several pumping systems inorder to determine if implementing VSD technology is feasilbe and economical. I am lacking most of the necessary system and pump information in order to make accurate energy saving calculations.

I'm a recent graduate (way over his head) and would greatly appreciate your help.

 

[buzzp]

I addressed your post in the pump engineering forum. Never seen this formula before.

 

[Rick99]

VFD efficiency (in percent) varies as a function of HP rating and percent full operating speed. A discussion and chart of VFD efficiency ratings can be found at the bottom of page 7 in the Energy Matters, Winter 2002 issue. Available at:

http://www.oit.doe.gov/bestpractices/energymatters/pdfs/emwint02.pdf

Another resource: Adjustable Speed Drives Applications Guide, TR-101140, pg 106, Electric Power Research Institute, c. 1992

 

[dpc]

The drive efficiency is secondary concern. If you can't show energy savings by increased efficieny of the driven equipment, the VFD efficieny isn't going to help you. The VFD losses are something you have to overcome with the increase efficiency of the driven equipment.

For typical pump or fan applications, the energy savings results from operating the punmp at a reduced speed and not throwing away energy across a throttling valve or damper.

The motor will also be less efficient when operating from a VFD than operating across the line.

I haven't looked lately, but many of the drive manufacturers have some "tools" that will provide a rough estimate of possible energy savings for pump and fan applications. Actual savings will probably be somewhat less than these programs indicate.

 

[ChiefMark]

I agree with dpc. Most of the manufacturer "tools" that I've seen are too generous when calculating the potential savings. The formula that you have is just a simplification of the affinity law for a centrifugal fan or pump that the power varies as the cube of the speed. In order to get to the "% Rated Speed," you need to use the other afinity laws to determine what speed will provide you with the necessary flow or pressure required.

One precaution: I've seen people estimate the new speed based soley on the required flow using the affinity law. Many processes have a minumum pressure requirement for adequate damper/valve control or system distribution requirements. If so, this needs to be considered dring these calculations. It's possible that the drive speed that's adequate for your flow requirement won't meet your minimum pressure requirement.

 

[GaneshJ]

The simplest method of estimating the potential benefit is to measure the pressure drop across the valve, which is being used currently to control the pump. Express the pressure drop as a percentage of the total pressure rise of the pump. This will be the maximum potential.

There are lot of case studies on VSD implementation in

http://www.greenbusinesscentre.com.

Please go thro.