Control the blow

At full idle, the inlet butterfly valve closes, and the inlet bypass valve or unloader valve opens. Theoretically, the compressor is now moving just enough air for cooling, avoiding vacuum and minimizing the power draw.

Ability to control this flow precisely under varying inlet conditions is a function of the specific equipment or valves used, as well as how they are adjusted and maintained. Some inlet butterfly valves are non-seating with actuator stops at full open and full close. But the term "full close" is misleading. Air must bleed around valve to flow through the unit to eliminate first-stage vacuum while minimizing mass flow and discharge pressure. Other inlet butterfly valves use a full seating design with a machined opening for bypass air.

Regardless of terminology, the power the compressor requires at idle is a function of the mass flow and the discharge pressure at each stage. The mass flow thus is a function of the pressure at the entrance to the first stage.

Most designers believe a partly open butterfly valve that's controlled by stop adjustment is not efficient. They argue the margin for error is too great, and the turbulent flow around the valve may cause unpredictable results. There's no doubt that restricted flow won't fill the impeller as effectively as would be the case in the wide-open position; thus, the specific power decreases.

Regardless of type, the inlet butterfly valve is applied in several ways:

On most equipment, the inlet throttling valve can be controlled to a very narrow band to achieve a constant pressure.

So, what's the idle power input with a properly applied throttle valve? The most common answer is a high of 30 percent with an average of 20 percent, although sometimes it can be as low as 15 percent.

Before you ask if that's as good as it gets, consider the following:

The mechanical fixed power draw is not an industry average. It's specific to a frame size.

The mass flow of air at closureand thus the powervaries as the inlet conditions change. Without adjustment, it'll be higher in colder weather and lower when a storm approaches.

Backpressure on the unloader or blowoff valve is significant.

A maladjusted control linkage that leaves the valve more open than necessary passes more mass flow and consumes additional power.

Most compressors require a minimum flow and pressure at idle for cooling and to avoid pulling a vacuum on the first stage, which could pull oil from the drive train into the compressor. Most designs avoid this by blowing high-pressure air towards the gear case along the pinion shaft and through the seal.

At least one manufacturer manifolds the discharge of all stages at idle. Even if the first stage pulls a vacuum, the positive pressure in the following stages buffer the vacuum to preclude oil migration. This allows the first stage to start at a lower pressure and, therefore, requires a potentially lower base power draw at full idle. However, with a good inlet butterfly drive, it still uses about 20 percent power at idle.

Well applied, high-quality inlet butterfly valves have a much better ability to stop the air flow than most inlet guide vanes. The inlet butterfly valve is predictable, and its performance is predictable and repeatable.

Some compressors may require more mass flow to control or eliminate thrust loads that could damage critical components. These units have a higher power draw at idle.