Resheave to save electrical costs
Resheave to save electrical costs
(OP)
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
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






RE: Resheave to save electrical costs
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.
RE: Resheave to save electrical costs
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Resheave to save electrical costs
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.
RE: Resheave to save electrical costs
Good on ya,
Goober Dave
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RE: Resheave to save electrical costs
RE: Resheave to save electrical costs
RE: Resheave to save electrical costs
RE: Resheave to save electrical costs
43 percent less power at those levels means a lot of money. Just do it!
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Resheave to save electrical costs
Bill
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"Why not the best?"
Jimmy Carter
RE: Resheave to save electrical costs
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."
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RE: Resheave to save electrical costs
So, you don't have to wait even three days to start saving money!
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Resheave to save electrical costs
"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: Eng-Tips.com Forum Policies