Industrial facilities love a good checklist: swap in premium motors, add variable frequency drives, call it efficiency. But when it comes to motors and drives, that plug-and-play mindset can backfire. In many cases, the difference between real savings and wasted capital comes down to one thing—understanding what the system is actually trying to do.
Last week, I kicked off a look at the typical top ten industrial efficiency measures, starting with lighting and HVAC – the usual low-disruption suspects. I unearthed and dusted off opportunities for major savings with HVAC. This week, I’ll continue down the list and dig into where deep savings exist. Here is the list:
- Lighting upgrades (LEDs, controls)
- HVAC optimization (high-efficiency units, economizers, controls)
- High-efficiency electric motors
- Variable frequency drives (VFDs)
- Compressed air system optimization (leak repair, pressure reduction, controls)
- Pumping and fan system optimization (right-sizing, system redesign)
- Process heat improvements (high-efficiency burners, furnaces, insulation)
- Waste heat recovery systems (recuperators, heat exchangers, ORC systems)
- Industrial heat pumps / thermal integration
- Advanced process controls and real-time optimization (automation, digital twins, AI)
- The Easy Win: Premium Efficiency Motors
Install NEMA (National Electrical Manufacturers Association) premium efficiency motors when replacing burnouts—the end.
VFDs Are Not a Universal Solution
Variable frequency drives (VFDs) are not automatic savers to bolt onto systems or equipment. For example, I recently read a misguided tip to install VFDs on refrigeration compressors. Not so fast. Although it may be fine, a refrigeration system expert needs to analyze whether the system can accommodate that. For example, there needs to be enough refrigerant flow to carry oil for compressor lubrication, or the compressor may burn up. That could kill any consideration of deep energy efficiency in the facility for years.
Systems with Flow Objectives
Variable frequency drives are often controlled by static pressure in fan and pump systems. The objective is to minimize unnecessary work and energy use caused by overpressurizing ductwork and piping systems, then throttling flow with dampers and control valves to achieve desired outcomes. Pressure setpoints don’t need to be static and shouldn’t be (i.e., ‘set it and forget it’). Pressure can be reset by monitoring damper and valve positions to keep them as open as possible and minimize pressure loss.
Systems with Pressure Objectives
In other cases, VFDs may not save squat. Applications include those with high or fixed static pressure requirements, where energy may be wasted. Think of it this way: in the previous paragraph, the objective described is flow = moving volumes of air or water. In high static-pressure applications, the objective is pressure. These applications include municipal water and high-rise commercial buildings that require high elevation changes. The target municipal water pressure is 60 psig (pounds per square inch gage) pressure (static pressure), which equates to roughly 140 feet. There is nothing to throttle in a high-pressure application, so there is typically no need for a VFD to reduce power or energy consumption.
The Trap of Constant Torque Systems
Torque is a rotational force. Think of the force applied to your bicycle pedals if you are one of the few who still use your legs rather than an explosive battery as the prime mover. The application described in the previous paragraph is actually close to a constant-torque scenario. Constant pressure requires a high minimum speed and associated torque to maintain it. Other constant-torque loads include conveyors, augers, elevators, and water towers. The objective of these applications is largely vertical gain, but there is also no loss proportional to the square of the flow, which dictates the pressure drop in flow channels like ductwork and pipes. (we won’t go into that here). Many of these constant-torque scenarios have work and energy that are directly proportional to speed. Reducing speed reduces demand, but it does not reduce total energy consumed. Obviously, there is a cost to reducing speed in terms of time.
Why Slowing Down Doesn’t Always Save Energy
Batch processes include mixers, presses, and stamping equipment. Like constant-torque applications, slowing the equipment saves no energy because it merely extends operating time and decreases productivity. It’s like low-flow showerheads and aerators. All they do is make for a frustrating and slow washing and bathing experience.
Positive Displacement: The Exception That Proves the Rule
Positive displacement equipment includes reciprocating, scroll, and screw compressors, as well as positive displacement pumps. My AI companion said these are not good applications for VFDs, but that is not always the case. For instance, VFDs save energy on reciprocating refrigeration compressors compared to the typical cylinder unloading. Cylinder unloading leaves the suction-port valves on the cylinder head open during the compression stroke, allowing gas to flow back to the suction line at low pressure and minimal work. However, friction losses from the piston and gaseous refrigerant movement are still considerable. Twenty years ago, I remember chatting with an industrial refrigeration expert who doubted the savings potential of VFDs applied to reciprocating refrigeration compressors. The drive rep gave him the drives at zero cost. The savings, he said, were 20% compared to cylinder unloading. Danfoss conservatively estimates 10–25% savings, depending on compressor type.
Next up: compressed air. There is too much material to tackle on that topic for this post. I will dive into that one soon.