The full benefits of VSD compressors - Part II

It’s time to eliminate the spin and point out both the limitations and engineering required if you are to derive the benefits of a VSD compressor.

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By Chris E. Beals

PlantServices.com

View Part 1

Much of the VSD compressor’s popularity is a result of marketing spin. Often, free audits exaggerate paybacks by including savings the end user can achieve without purchasing a VSD compressor. The marketers also fail to provide the necessary engineering details required to maximize the purported benefits. It’s time to eliminate the spin and point out both the limitations and engineering required if you are to derive the benefits of a VSD compressor.

Power factor

The spinners claim there are no penalties for spikes or low power factor. In the limited context of the compressor, this statement is true and refers to the fact that the VFD provides a soft start similar to a Wye-Delta or electronic starter and that the power factor of a motor on a VSD compressor is close to unity over the complete speed range. A unity power factor is a benefit and it can reduce energy costs if the VSD motor constitutes a significant portion of the plant’s motor load. On the other hand, if the local utility doesn’t have a power factor penalty or the VSD compressor constitutes only an insignificant portion of the motor load, then a unity power factor offers little cost benefit.

Free energy analysis

The spinners offer free energy analyses or audits. The old adage, you get what you pay for applies here. Many compressor manufacturers gear these free audits to get their sales personnel into a plant to sell equipment. Free air audits don’t address pressure drop, piping, air quality, risk of an outage or demand-side issues. Nor do they offer any non-equipment solutions.

The whole process is automated so it produces an equipment quote for the sales personnel regardless of expertise or experience in the industry. When clients ask us to review these free audit reports, we generally find that the salesperson has misinterpreted the data, the proposals are incomplete, and plant personnel can achieve the majority of the projected savings without purchasing any compressor.

Turndown

At a 100-psi operating pressure, many lubricated VSD compressors can regulate capacity from 100% down to 15% before they must operate in either start-stop or load-unload mode. Spinner literature suggests that all VSD compressors have the same turndown capability, but the fact is that at 100 psi, some can only regulate output capacity from 100% down to 55%.

In addition, turndown range decreases as discharge pressure increases. For example, at 125 psi, the turndown can be as little as 40%, from 100% capacity to 60% capacity.
Turndown range is important to VSD compressor selection because a smaller turndown decreases the purported savings, increases the size and frequency of pressure swings, and can result in compressors short cycling. Or, it can increase the cost to integrate the VSD compressor into the system properly. In summary, turndown varies significantly among compressor manufacturers, models and pressure settings.

Nameplate horsepower

The actual horsepower size of a VSD compressor can be confusing because spinners often play games with the motor nameplate data. For example, a motor might be a 125-hp unit with a 1.15 service factor or a 100-hp motor with a 1.4 service factor. The motor manufacturer can nameplate the motor either way. This practice has resulted in end users installing undersized electrical circuit breakers. In summary, it’s important to determine the actual power applied at full load at the operating pressure to understand efficiency and installation requirements.

Constant pressure

The spinners claim that VSD compressors hold pressure within 1.5 psi. The truth is that VSD compressors are only capable of maintaining the pressure at their discharge within 1.5 psi over their turndown range. You must properly integrate the VSD compressor into your system if you want to prevent larger pressure swings.

For example, a VSD compressor requires a signal from a pressure transducer located downstream of the cleanup equipment if it is to prevent changes in pressure drop across the cleanup equipment from increasing system pressure swings. If the control signal is on the compressor, the smallest pressure swing possible is 7.8 psi within the VSD compressor’s turndown range. To reduce the pressure swings, we can either oversize the dryer or just move the control signal to the downstream side of the cleanup equipment, which can reduce the maximum pressure swing, within the compressors’ turndown range, to 3.0 psi.

If you’re operating within the turndown range of the VSD compressor, storage isn’t required, except to ride through sizeable system events. As Table 1 shows, you can’t prevent larger pressure swings from occurring when the VSD compressor operates outside its turndown range or when base-load compressors load and unload.

Storage

Adding sufficient storage minimizes the pressure swings that occur when the VSD compressor operates below minimum turndown or when base-load compressors load and unload. Understanding why pressure swings occur requires an explanation of how VSD compressors operate below minimum speed.

Consider a single compressor system that uses a VSD unit that stops when demand is less than its minimum capacity. As demand decreases, the compressor reduces motor speed to its minimum. If demand continues to fall, the pressure rises to the stop point, which is normally set 5 psi above the target pressure and the VSD compressor shuts down.

Some lubricated VSD compressors blow down the sump when they stop. When the pressure drops to the target pressure, the compressor starts immediately, but the VFD drive needs approximately 10 seconds to ramp up to minimum speed.