Direct Drive, Evolved

Previous articles on this site have discussed the advantages of direct-drive plug fans and the technical tricks required to apply them correctly. However, despite their many advantages, there are times that direct-drive fans just haven't made sense.

In large part, this is because direct-drive fans have been applied as if they were belt-drive fans. It turns out, however, that there is a better way to apply these fans.











See, the problem with direct-drive is that due to the peculiarities of motor performance (discussed in the links above) you usually want to select your fan at a design speed very close to a synchronous motor speed (900, 1200, 1800 rpm, nominally). This limitation can be made up for by varying the width of the fan wheel, but this can cause an unacceptable decrease in static efficiency, or an unacceptable increase in fan noise. Or it can lead to the use of an oversized, less readily available low-rpm motor.

Another strategy is to consider selections of multiple fans, which opens up more design possibilities. However, in standard HVAC designs, this option has practical limits in the number of fans that can be arranged in a cabinet. In the traditional belt-drive paradigm, one or two large fans are mounted on the air handler floor. In unusual situations, three or more fans can be arranged this way, but this requires unusual cabinet geometries that are not often appropriate. This limits the number of direct-drive solutions that can be brought to bear, limitations that are not present with the infinitely-variable fan speeds that are available with belt-drive equipment.

But with a deceptively simple re-thinking of a traditional fan mounting, it becomes possible to stack fans one above another in an air handler cabinet--and suddenly a whole new universe of design solutions present themselves.

It is this evolved fan mounting that is the basis of the ClimateCraft Matrix system.

Matrix is an array of direct-drive plug fans designed to allow maximum flexibility in the selection of fan performance to maximize the benefits of direct drive without the traditional tradeoffs that used to be required. Five fan wheels between 16 and 27 inches are available, with motor sizes between 3 and 30 hp. The fan wheels are AMCA-certified welded aluminum wheels. The wheels are ‘modified class II’ to cover up to 11” static, or class III for higher pressures. The motors are premium efficiency, VFD compatible, 1600V insulation, ODP or TEFC—off the shelf replaceable.

An obvious temptation when mounting multiple, small fans is to avoid the costs associated with isolation and to mount them rigidly to the air handler itself. This simplistic approach, however, can result in repercussions downstream. In fact, the ASHRAE Applications handbook, chapter 47 recommends spring isolators on fans operating above 500 rpm with brake HP below 40. The issue isn’t so much transmitted vibrations, although that certainly can be a problem, but instead bearing life. Strong vibrations can kill bearings, and when the fan bearing is also a motor bearing as is the case in direct-drive plug fans, a bearing failure can be awfully expensive. Climate Craft avoids this problem by isolating every fan from the air handler structure with a unique three-point, seismically-restrained spring isolation system to prevent developing harmonics and creating damaging vibrations. But they have taken the effort even further and used a finite-element analysis to ensure that no harmonic frequencies exist in their fan base anywhere in the operating RPM of their fan systems. This step essentially converts every individual fan base into an inertia base.

This measure ensures that no VFD frequency lock-outs are needed to prevent violent vibration at the fan—a step that is often overlooked in commissioning and can cause unacceptable rates of motor failure. The unique isolator design has the added benefit of preventing the fan base from contacting the seismic restraints and causing a short-circuiting of the fan vibration directly to the frame of the unit and thus the building. This sort of grounding out of the seismic isolation is common on variable airflow systems where the fan thrust changes depending on the fan speed required at any given service point.

The Matrix allows fans to be stacked in towers 1, 2 or three high inside an air handler cabinet. Multiple towers can be installed across the air handler air tunnel. Since the fan wheels are much smaller than typical for the air handler size, many such towers can fit horizontally where only one or two typically sized fans could fit before. These fan towers are designed so that only two different designs are required to support all five different fan wheels and any required motor frame. This greatly aids in construction, making this approach a very cost competitive approach to fan mounting. The towers also serve as a rigid support truss for the fan inlet wall to support the interior of the unit at a location where the pressure differences are often quite high.

Operator Benefits

This sort of a change in fan concept represents a significant advantage for the operator of these systems. This system enhances the air handler’s reliability and serviceability. Redundancy is almost total, since in multiple-fan arrays, the loss of a single fan can often be overcome by the remaining fans simply by ramping up the RPM slightly. Replacement of a failed motor is also much easier. First of all, Matrix uses off-the shelf motor sizes that are easily obtainable on short notice. So simply getting a replacement is easier. Additionally, a typical Matrix motor and wheel assembly might weigh 150 lbs and be easily maneuvered into place by a couple of men, while a typical large fan motor may weigh 1500 lbs or more, and require special rigging to get into place. This may require a significant facility shutdown or even crane work in some cases.


Additionally, the multiple fan array allows shorter air handlers, making a more efficient use of valuable facility square footage. The smaller, faster fans also shift the acoustical signature of the system into higher octave bands, making sound attenuation easier and less expensive.

And a maintenance person will never have to tighten or align a belt on a Matrix system.

Matrix is the next evolution in fan system design.