Electrician Talk banner
1 - 9 of 9 Posts

·
Registered
Joined
·
23 Posts
Discussion Starter · #1 ·
Here at my work, we still have 3 air handlers that do not have VFD's. Return fans are 25 H.P. and supply fans are 70 H.P. respectively.

Does anyone know of a free online cost savings calculator for adding VFD's? They want me to justify expense and while I can do so with pen and paper, it would be better to give something official that can be printed out.

Thanks so much.
 

·
RIP 1959-2015
Joined
·
39,618 Posts
Here at my work, we still have 3 air handlers that do not have VFD's. Return fans are 25 H.P. and supply fans are 70 H.P. respectively.

Does anyone know of a free online cost savings calculator for adding VFD's? They want me to justify expense and while I can do so with pen and paper, it would be better to give something official that can be printed out.

Thanks so much.
How about this.

http://www.cerusind.com/VFDEstSavingsCalc.asp
 

·
Registered
Joined
·
3,838 Posts
Unless you actually reduce the amount of work that the fans are doing, you don't get a lot of savings by switching to a VFD. Reducing the speed of the motor by using the VFD does decrease the work, but also decreases the air flow as the air flow is the work.

There is little difference in the reduction in the amount of energy used when you reduce the air flow by using a VFD as compared to reducing the air flow using dampers in the duct work.
 

·
Registered
Joined
·
5,563 Posts
http://rockwellenergycalc.com/

Click on Fan calculator. If you don't know a lot of the stuff you are going to be asked for, you can enter the motor HP, then click the button that says "Load Sample Data" and it will plug typical numbers gathered on similar fans of that size from years of other people using this calculator. The result will not be gnat's a$$ perfect, but it will be close.

There is little difference in the reduction in the amount of energy used when you reduce the air flow by using a VFD as compared to reducing the air flow using dampers in the duct work.
That is of course 100% true if you are not VARYING the speed of the fan and just running at a constant speed all the time. But if you are varying the flow with dampers, there is a very significant reduction in the energy used by the fan COMPARED to using damper control, as long as you remain within the fan operating curve. Damper control reduces energy in a relatively linear fashion, speed control reduces it as a cube (Affinity law) function.

From the ones I have done for example, at 50% speed, you get a reduction in energy consumed to around 1/8th of full power, whereas for an equivalent flow using damper control, the energy is dropped to about 60% of full. At >95% they are about the same and at full speed, you actually lose about 2-3% more in VFD losses, so you don't want to spend the money on a VFD if you are not going to be spending much run time at lower flows. But if you are, the VFD usually pays for itself in under 18 months in my experience.
 

·
Registered
Joined
·
3,838 Posts
If you can save energy by reducing the air flow and still make the system work correctly, then the system was not correctly designed in the first place.
 

·
Registered
Scada Supervisor
Joined
·
3,918 Posts
more than just elect cost

I just did one for a bag house, and besides saving on elect it increased the bag life by 100% or more, (by not pushing dust into bag material). Also in NY we got an energy saving credit that paid for 25% of the drive cost.
 

·
Registered
Joined
·
5,563 Posts
If you can save energy by reducing the air flow and still make the system work correctly, then the system was not correctly designed in the first place.
Right. Unless it was always intended to be a variable air volume system. Reducing is not the same as varying. So yes, if all you want is to permanently reduce, then don't use a VFD. But if the demand for air volume varies all the time, then the VFD will save energy COMPARED TO other means of varying the air volume.
 
1 - 9 of 9 Posts
Top