Fan Law - Belt Drive vs Direct Drive

[gooutandgetem]

Can someone help on the question below?

If I change from a Belt Drive Roof Top Centrif Exhaust Fan to a direct drive will I save energy? See details

Old Fan: Motor RPM 1800, 1/4 hp and Fan RPM 1000
New Fan: Motor RMP 1000, 1/4 hp and Fan RPM 1000


The new fan has an ECM type motor that allows me to adjust the speed down to 1000 RPM, can I calculate power saved from the motor speed reductiion? horsepower2=(Speed1/Speed2)cubed * horsepower1. Or since the fan speed never really changes does horsepower savings not happen?

 

[MintJulep]

Same fan, same speed, same work done --> no energy savings.

Possibly some trivial difference associated with motor efficiency.

 

[gooutandgetem]

Makes sense Thank you!

 

[Shophound]

There may be some savings in the direct drive since you no longer have losses via the belts. The belt drive assembly must have enough force to spin the motor sheave, belts, and fan sheave, along with attaining the desired speed of the fan or blower wheel to deliver the desired CFM. There is no doubt losses due to friction and belt tension in that arrangement. Direct drive achieving the same CFM...no transfer losses through a belt drive train.

 

[Wilbur55]

How long is the payback on making this change to reduce the energy used by a 1/4 hp motor?

What is the annual operating cost of the 1/4 hp motor at its current load?

How much will it cost to make the proposed change?

 

[walkes]

The only savings you should see are due to belt/drive losses.

 

[Artisi]

Go for direct drive- less maintenance over the next 10 years. Forget about the difference in efficiencies - not worth you time and effort calculating for a 1/4 HP motor.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[gooutandgetem]

Thanks all for your posts. I ended up putting direct drive fans with ECM motors. I was able to slow them down 30% because the exhaust from the toilet rooms was above ASHRAE 62.1-2010 requiments. I was also able to change out the pulley on the 100 OA Unit to slow it down about 30%. The system runs 24/7. The payback was less then 3 years,this was a great Energy Conservation Measure! Thanks again.

 

[ScottyUK]

One factor to consider from a practical viewpoint is that the full load current (FLC) of a slow speed motor is generally higher than for a higher speed machine. The typical FLC values published are for a 4-pole motor, but 6-pole types (which you have) tend to draw a bit more current for the same power output and 8-pole types are higher still. It's unusual to see motors with more than 8 poles these days, although they are still available as 'specials' and there are more than a few slow speed machines from long ago still in service.

The upshot is that you may need to adjust or replace the overcurrent protection relay if the motor is running somewhere near full output.