Aahh, the V-F-D! Once a bone of contention, with its bulky footprint and less-than-reliable functionality, now a ubiquitous element in our everyday HVAC world. Since their inception, VFDs have come a long way, in terms of price, size, and reliability. So far have they come, you wonder why all motor-driven equipment isn’t required to have one. In fact, we might not be so far off from that, and with so many reasons to equip a motor with a VFD, it may soon become the norm rather than the exception, even with equipment traditionally not thought of as ever needing one. Consider this:

• Only about 1 in 10 “HVAC duty” electric motors are on VFDs
• Most pump and fan systems are oversized to account for maximum loads
• Capacity usually exceeds demand
• Power is proportional to the change in speed cubed (reduced speed = big savings).

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Throw in the fact that, depending on where you live and do business, there are incentive and rebate programs offering “money back” for installing VFDs, and you have yourself all kinds of good reasons for utilizing VFDs on existing motors. Following are a few of the more common strategies that we find in our biz.

VFD to Replace Inlet Guide Vanes

This is the classic opportunity for installing a VFD. Inlet guide vanes (IGVs) are commonly found on older, larger air handling equipment installed prior to the development of reliable VFDs. IGVs are actuator-driven, either electrically or pneumatically, to vary the volume of air that a fan can deliver. Without getting into a discussion on the physical attributes of IGVs, suffice it to say that the use of IGVs isn’t extremely energy-efficient.

A VFD retrofit in this application entails either complete removal of the IGVs, or more typically, removing the actuator and pinning the guide vanes wide open. The VFD is installed on the motor, and the control signal that drove the IGV actuator is fed to the VFD as the “speed reference signal”. Pretty much all there is to it. The VFD also needs a run command, in the form of a contact closure, and in turn can give a status or alarm point, also in the form of a contact closure. Your I/O device will need at least one extra output, for the run command, and if it has any extra inputs, it can accommodate the status and/or alarm points.

VFD on Exhaust Fan

Any constant speed exhaust fan can be put on a VFD, however not every opportunity makes sense to do so. A larger fan serving multiple spaces by means of a ductwork distribution system may be a good candidate. Or even a roof mounted “general exhaust” fan serving, for example, an open warehouse or factory space. In this opportunity, typically these fans are constant volume and run continuously, with no regard for what’s happening in the space. Yet there may be ample opportunity to “dial down” the amount of air being exhausted from the space, either manually via operator intervention, or automatically to maintain some temperature and/or pressure setpoint. See it now? Good, now go ahead and make it happen!

VFD on Constant Volume Air Handling Unit

Just because an air handling system is designed to serve a single zone, doesn’t necessarily mean that the fan needs to operate at “constant volume”. There are opportunities for running this type of air handler at less than full capacity, for sure. Like the exhaust fan opportunities discussed above, there may be times when the fan simply does not need to run at full speed. These opportunities need to be identified, so that the justification can be made to install a VFD, and the control sequences can be generated and implemented in order to take advantage of these opportunities.

VFD on Constant Volume Packaged RTU

Same concept as above, however maybe not as “cleanly” done. With a packaged rooftop unit, one that sits outdoors and is virtually self-contained, the concept of equipping the supply fan with a VFD requires careful consideration. Yet it can be done. Things to consider: can the VFD fit inside an electrical compartment of the unit? If so, the VFD may need to be rated for ambient temperatures. If not, the VFD can be mounted to the side of the unit, however it then also needs to be rated for the elements. As a last resort, the VFD can be located indoors, remotely from the rooftop unit, but of course this has its drawbacks as well. Operationally, unlike with the air handling unit discussed previously, which is more typically equipped with hot and chilled water coils, a packaged rooftop unit will come standard with gas-fired heating and DX cooling, requiring that a minimum volume of air be delivered through the unit for all heating/cooling cycles. That minimum may be high compared to the turndown that the VFD can offer, but it still beats running the fan at full bore for all periods of operation.

VFDs on Cooling Tower Fans

This is a very popular opportunity, especially with older chilled water systems, whose cooling tower fans were originally installed without VFDs, and were controlled as “on-off” in order to maintain condenser water temperature setpoint. Install the VFD indoors as possible, and intercept the power wiring out to the fan motor. Control the VFD in typical PID fashion to maintain a leaving condenser water temperature setpoint. If there is more than one fan, then sequence the control of the fan motor VFDs, so that the first one ramps up to full speed before the second one begins its ascent. It is such a simple concept, and one so easily implemented, that we’ll be seeing this scheme of control applied not only with cooling towers, but with air cooled condensers and condensing units as well, wherein typically numerous fans complete the package.

[an error occurred while processing this directive] VFDs on HW/CHW pumps

In many older “single loop” hydronic systems, three-way valves were used at the “appliance level” (unitary heaters, fan-coils, VAVs, etc), ensuring that the main system pump(s) would constantly deliver full flow and not ever be subject to a “deadheading” situation. Times have changed. If you see a new construction project designed in this manner, call the engineer and demand a redesign! Seriously though, the opportunities for these existing systems are out there. The easiest thing to do is to physically (and permanently) close off the bypass port at each and every three-way valve, save for the last one or two on any branch line, so as to provide for a minimum amount of flow at all times throughout the entire piping system. Exercise special caution with chilled water systems, in regard to variable pumping through the chiller. There is a minimum flow rate that the chiller evaporator is rated at, and it must be maintained. Often with new construction projects, VFDs will be specified for the chilled water pumps, along with a bypass valve that modulates open to maintain the minimum required flow.

VFDs on CW Pumps

So here I am reviewing a large plan/spec project, and I notice that there are VFDs specified for the condenser water pumps. I do believe that that’s the first time I’ve seen this, yet I’m pretty darn certain that it won’t be the last. Kind of the “last bastion” of constant speed / constant volume operation in HVAC, the water flow through a water cooled chiller’s condenser bundle has historically been specified to be such, and with good reason, seeing that your chiller manufacturers are insisting that you maintain constant condenser water flow through their machines. And I have to admit that I agree with the requirement, but here I am reading a specification and there it is in black and white: VFDs for the CW pumps! I will say that there is no specified type of control with regard to varying the speed of the pumps, so maybe the VFDs are here simply for the “soft start” functionality of the pump motors. And maybe, just maybe, there’s a little “wiggle room” here, in which the pump speed, and therefore the condenser water flow, can be turned down just a touch. Hey, any little bit helps, and seeing that pumps (and fans for that matter) are typically oversized to begin with, there may be some true justification for equipping these pumps with VFDs. Just exercise caution, or you’ll find yourself tripping the unit out on its internal safeties (or worse!).

Other VFD Opportunities

Now that you’re in tune with the concept, let you’re imagination take over! Installing VFDs is certainly by no means restricted to HVAC equipment. Other non-HVAC equipment (domestic water pumps come to mind) can be operated from a variable frequency drive. You need to weigh the options and see if it makes sense, of course. Smaller motors are typically not real good candidates, as are motors that are “maxxed” out, meaning that there is no real opportunity to turn down the operation and therefore no real payback for installing a VFD. But still, there are many opportunities out there, so let your mind wander, and go out there and find them!

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