VFD fan control vs fan outlet vane control

Amps times voltage = watts, watts = power, power is power, so which ever way you reduce amps you reduce power used.

(a different anonymous coward) Vanes aren't a very efficient way of reducing airflow; there's a lot of turbulence and you can get fan caviation. Also, most motors are designed for lowest first cost, which means they're right on the hairy edge of saturation at regular voltage and speed - in other words, not designed for max efficiency. If you can ease off on the motor drive instead of using vanes, most of the motor heating should disappear and that alone should save enough energy to make a VFD worth it. (I assume we're talking fairly big fans.)

VFD's can save you money not just on electric bill but also on maintenance of vanes. You may have to still keep the vanes if they are required to shut off the duct at any time - eg emergency shutdown. Our experience has shown VFD's to be efficient when running airflows above 25%.

Using a variable torque VFD for this application instead of a constant torque VFD will maximize your savings.

The only time watts = volts times amps is in DC circuits, or when the AC circuit is powering a resistive load.

Otherwise, in AC circuits that contain inductors and / or capacitors (such as motors, Power factor correction caps, etc), watts = volts times amps times power factor. You are billed for the watts used, with penalties for low power factor.

Yes, VFDs are a good investment for energy savings for fan and pump loads.

unless you reduce amps without reducing watts. Then you only improve power factor and you donl;t save metered kW.

I am the chief engineer of a dairy, and was previously an engineer in the navy. I have had extensive experience in both methods of airflow control and power consumption. It has always been my experience that VFD's are usually the best option (if you can afford the initial cost). We even use them on small 5hp motors for the simple fact that as conditions change over time, the VFD allows you to alter machinery states more readily, and with little or no extra cost. Variable inlet guide vanes are useful in areas where you require a specific type of airflow, however they can pose problems to equipment with cavitation as well as "surge and stall" (this is when the airflow is directed in such a manner that it supplies either excessive or insufficient amounts of air to the fans/compressor). This is more prevalent in a turbine, but is theoretically applicable in any case. The problem this poses is that insufficient air causes excessive strain on equipment to meet the demand, and excessive air can cause vibration (in minor cases) or explosive forces (in the most extreme cases). No matter which way I look at it, I am almost always in favor of VFD's, unless the particular application absolutely requires variable inlet guide vanes. Hope this helps.

Go Navy!

I generally agree with most of the previous posts, generally VFD fan control will save more electricity than inlet vane control when you have a wide volume range to work with. My recollection is inlet vanes can save 10-15% while VFDs can save 25-35% (too lazy to look it up). One caution from a bitter past experience, make sure the VFDs can handle the facility's environment (dust, humidity, temperature) or have a clean, dry, comfortable location to install the units.

Yes. VFD,s will save you money. We have around 50 Power Flex Allen Bradley drives. They are the best drives I have found so far for the money and are very user friendly.

Another consideration is motor cooling. Many motors have cooling fans mounted directly on the motor shaft (TEFC). If you plan to reduce the speed of such a motor below about 40% or 50% of the rated 60 Hertz speed, you may wish to consider providing an external, constant speed cooling fan to avoid overheating the motor.

VFD's are the most efficient way to turn down fan performance especially at very low turndowns, but you must make sure that the motors are correctly specified for the VFD's ie cooling and an insulated NDE bearing.

Regards

Jakes

Alstom Mechanical Equipment

Engineered Fans Division