The Hollisterian Effect, Explained

One of the best things about working with seasoned experts is that you get to benefit from their previous, um, experiences. You don't always have to learn the hard way yourself.

Sometimes, these not-quite-the-way-I-planned it episodes are actually elegant illustrations of physical principles, and deserve something more fitting than being remembered as that one time someone screwed something up. Two particular examples certainly fit this bill, and, as it turns out, they both have to do with applying direct drive in custom air handlers. The principles that they illustrate have been christened the "Hollisterian" and "Florentine" effects by our own Jake Marley, in honor of certain colleagues who shall remain unidentified for the purposes of this post. I will discuss the Hollisterian effect here, and the Florentine effect in a future post.

And, instead of dredging up the actual events that gave rise to the discovery of these principles, the gist of which I am sure most readers could figure out, I will instead focus on the principles themselves.

The Hollisterian Effect

Direct Drive fans offer some great advantages to a system designer. There are no belts to maintain, no belt dust to foul the discharge air, no inefficiencies from the belt drive and far less vibration than a belted system. But they also do carry some design limitations that must be dealt with appropriately.

The first limitation? Direct drive fans are direct drive. In other words, they are directly coupled to the motor shaft, and therefore turn at the speed of the motor. Which is great, if you have a design condition where the fan needs to turn at 1800 or 1200 or 900 rpm. If you have a design condition that requires a fan selection at, say, 1500 RPM, then you need to do pick a motor/fan system at an 'asynchronous' design condition.

No big deal, right? We've got VFD's today, so this is a piece of cake.

This is exactly where the Hollisterian effect can get you. See, VFD's are not constant horsepower devices. They are, up to 60 Hz, constant torque devices.

Let's look at the equation for motor power:

hp = (Torque x Speed)/5250

If you have a constant-torque motor, this equation simplifies to"

hp=C x Speed

Where C is a constant equal to the torque constant divided by 5250.

So, what you have got is something like this:

(click on image for larger view)

Where the HP available (the blue line) increases linearly up to 60 Hz, at which point the HP then remains constant and the available torque drops away.

So what does this mean to a designer?

Well, let's say you selected a direct-drive fan to meet your design criteria at a 1500 rpm design condition. Let's say the brake horsepower of that fan selection is 13.5 HP. You select a 15 HP, 1800 RPM motor driven by a VFD. You're good, right?

Well, let's look back at our HP equation--Applying the math, you now have only 1500/1800 (or 5/6th) of the motor hp available at 1500 RPM, or, in this case, 11.7 HP. You really needed a 20 HP motor!

If you are working with low speed fans, and you are selecting in the 400-500 RPM range, you can see that you are going to be robbing about half of the nameplate HP from the selected motor, assuming you are going to select a reasonably available standard motor speed. In the above example, that would turn the 20 HP motor into a 30 HP motor!

So what do you do? Well, one way to attack this problem is to select the bigger motor (and VFD) and call it good. Other than some additional first costs, this might be the right solution. A more elegant solution might be to see if you can't select a fan wheel with slightly shorter blades to bring your design condition in closer to a synchronous speed. Energy Labs provides direct drive systems regularly, and thus will allow you to select plug fans from 50% to 105% of the standard AMCA wheel width to address these sorts of issues. Aaon's fan selection routine in their Ecat32 software allows for variable width wheels and actually takes into account any Hollisterian or Florentine effects (discussed later) in the sizing of their motors!

Variable-width wheel selections allow you to shift the whole fan curve leftwards on the page without losing height--reducing CFM to match your needs, but preserving peak static pressure. This means you could select a fan wheel at 1800 RPM, but reduce the total air delivered by providing a 80% wheel width so that you don't exceed your design flow at the faster speed.

Or, lastly, you could instead chose a 1200 RPM motor, and just select it for 72 Hz service. As long as the motor and drive manufacturer are happy with this selection, there is nothing preventing you from over-speeding your motor.