Resheave to save electrical costs 4

[Cybernalt]

I have several fans that are operating about 17% too fast. The applicator is Air Turnover Units which are too large for the load and also too noisy (exceeding the specified dB) . The contractor who installed them will resheave for free, but he has suggested we run these on VFD' - which will coast $100,000 (less incentive from Duke Power so my final cost will be $53,000).

He says the VFD's will save enough energy to pay for themselves in a year - which I am sure is true (but I need $53,000). If we sheave, the noise problem goes away (there is no concern for the load) - but will I save that much power?

I ask because the applicable affinity law is apparently based on "speed". The flow is reduced by fan rpm and the power to turn that fan is reduced by old-Power x (new speed/old speed)3 ... but resheave does not impacting the speed of the MOTOR.

Make sense?

I'm unclear how to estimate the savings. The contractor (a reputable design-engineer-build company) is dragging his foot coming back with the answer to cost saving based on resheave.

Thanks

 

[ScottyUK]

The motor will deliver the power demanded by the load, within the capability of the motor. If you re-sheave then you reduce the speed of the driven load, and therefore reduce the power required. The motor will deliver less power. If you don't need variable speed then this is a great way to reduce energy costs. A change of sheaves introduces no reliability problems or maintenance demands which you didn't already have, and doesn't suffer the losses incurred by running a VFD.

Don't forget that the motor will still behave the same way during a start (i.e. a 200HP motor with a 100HP load still starts like a 200HP motor, and requires controlgear suitable for that duty) and the reactive power consumed by the larger motor will be higher than with a more appropriately sized motor. You can of course offset the reactive load using power factor correction capacitors if you need to.

 

[Skogsgurra]

The motor speed remains the same as before. The fan's speed will be reduced by 17 percent and the fan will thus need less torque - according to the known affinity laws.
A 17 percent lower fan speed means .83^3=.57 power. That means 43 percent less power and I would not hesitate a minute to have the resheaving done.

A positive side effect is that your belts will also last longer due to less force transmitted by them. Belt slippage will also be reduced and that adds to the positive side of the equation.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[camurphy]

If you run the fan at a constant speed the VFD will increase your energy use compared to a softstart or across the line starter with a resheave.

The VFD will only save you money if there are times you need to turn down the fan speed.

The VFD will increase your energy usage because it is actively converting the energy for the load. A starter will just have the electricity running through metal with very little impedance. The losses in the VFD may be somewhere between the low 90% range to the high 90% range. I'm looking at a Eaton drive that claims >96%. So that 4% is the additional energy lost by the VFD.

It is also advisable to connect the VFD to motors that are inverter duty rated, which your existing motors may not be. This rating increases the motors ability to withstand the reduction in power quality from the VFD. It should result in greater motor longevity.

As ScottyUK pointed out the VFD will also introduce additional reliability/maintenance issues.

 

[DRWeig]

I agree with all answers give so far. To estimate savings (from re-sheave or VFD), we need to know the operating kW or horsepower of the motors at the high speed (you can compute new kW at the slower speed), number of hours per week that they run, and electric rate for kWh and demand kW.



Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[TheBlacksmith]

I've got to agree, the fan speed reduction (for free) will yeild the largest savings be reducing the motor load and power demand. VFD's "save" energy by allowing you to slow a "widget" during low demand, like a cooling fan at night. If the speed reduction is constant and permanant, the resheaving is the way to go.

 

[Compositepro]

All the responses are correct. The big question is what fan speed do you really need? Is this need variable?

 

[LionelHutz]

Yes, the VFD running the motors and fans at 17% reduced speed will save energy compared to the fans running at their current speed. But, they will use more energy than the fans resheaved to run 17% slower. The VFDs will be energy wasters unless you can run the fans at a speed slower than this 17% reduction for significant periods of time. In those cases, running 17% slower will use more energy but when running >17% slower you save energy. Make the savings more than the extra used and you save at the end of the year.

 

[Skogsgurra]

All data were given in the OP. I don't think there's anything to discuss. As I said before: "A 17 percent lower fan speed means .83^3=.57 power. That means 43 percent less power and I would not hesitate a minute to have the resheaving done"

43 percent less power at those levels means a lot of money. Just do it!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

What Gunnar said!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

How about this angle?

Changing the sheave ratio will save the same amount of energy as putting in the VFD (not really, but work with me here*). The VFD will end up with a net implementation cost of $53,000, so your energy savings $$ don't kick in until AFTER that is paid back, which from my experience is typically in 18-24 months. Your implementation cost for sheaves will be what, $200 maybe? So your energy $$ savings begin after maybe 3 DAYS. So by the time you pay off the VFD, let alone start getting paid back, you sheave implementation will have put over %52,000 in your pocket already.

* Previously identified above as not true because of energy wasted in the VFD itself.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

OP again: "The contractor who installed them will resheave for free"

So, you don't have to wait even three days to start saving money!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

LOL, missed that detail!

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376