Compressed Air System / Control Systems

Control the blow

A practical look at inlet butterfly valves and inlet guide vanes for compressor capacity control.

By Hank Van Ormer and Scott Van Ormer

The concept of inlet throttling is subject to differing opinions and there is limited public test data available to choose between them. A practical look at inlet butterfly valves and inlet guide vanes for capacity control on constant-speed centrifugal compressors starts with the terminology.

Compressor load

The loads on any air compressor are system frictional resistance, piping backpressure and the head the load imposes. Centrifugal compressors operate at a discharge pressure the system imposes on them. System pressure varies with the air used and the system's pressure drop. Resistance loads in most compressed air systems consist of multiple demands with some additional frictional pressure loss. Basic regulation normally uses constant speed and regulates to discharge pressure. A flow reduction produces a corresponding power consumption reduction. A variable-speed control on a centrifugal compressor generally isn't effective with this type load because pressure varies as the square of the speed.

Power

An electric bill is based on kilowatts and time. At full load, the capacity or unloading control has no effect on that bill. At part load, the capacity control translates reduced air demand into reduced energy input. Proper selection, application and maintenance of these controls help minimize energy costs.

Power is proportional to the head times the mass flow divided by the stage efficiency, plus mechanical losses realized in the compression cycle. Most centrifugal compressor manufacturers use the following definitions.

Brake horsepower is the power input at the compressor shaft needed to compress the air. At least one manufacturer excludes mechanical losses.

Shaft horsepower refers to the power input at the compressor shaft to compress the air and includes mechanical losses, which are machine-specific.

Input power is the shaft horsepower plus mechanical and electrical losses in the drive system. This is the power that determines the electric bill.

The power required for a centrifugal compressor is a function of the mass flow rate of air and the discharge pressure. This implies that:

  • Higher inlet pressure means more mass flow and more power required.
  • Colder inlet air means more mass flow and more power required.
  • Lower temperature cooling water means more mass flow and more power required.

Surge

Each centrifugal compressor has a specific discharge pressure at which the air becomes turbulent at the impeller tips, causing the phenomenon of surge. This increased turndown is referred to as rise-to-surge. Other conditions affect the action of a constant-speed centrifugal capacity control. For example:

  • As the discharge pressure rises, the rated inlet cfm falls.
  • As the pressure rises, the surge point shifts upward to a higher percentage of flow.

Figure 1 illustrates that for a particular compressor operating at its design pressure, the surge point is about 65 percent of flow. When the unit is pushed to 118 percent of design pressure, the surge point rises to 75 percent of rated inlet cfm, which makes control less stable.

Figure 1. At about 118% of design pressure the surge point rises to 75% of flow, making control less stable. (Joy Mfg. Co.)

Inlet butterfly valve

From the late 1970s to the early 1980s, the inlet butterfly valve was the control of choice for industrial multi-stage centrifugal compressors. Mounted on or near the first stage inlet, the valve closes in reaction to a rise in system pressure. The falling pressure on its downstream side is the inlet pressure at the impeller and diffuser. As the pressure drop across the valve increases, the density of the entering air decreases. This results in a lower mass flow in relation to inlet ambient cfm. Power draw falls, but not proportionally to the decrease in mass flow. This implies the specific power (scfm per input kW) falls. Additionally, as the butterfly valve reaches the end of its closure, it produces turbulence, which further reduces the effective flow into the impeller.

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:

  • Constant pressure or baseload: This design matches compressor output to the demand. As the pressure rises, the bypass valve opens to vent excess air to atmosphere. There is no reduction in power draw. This form of base load control is considered obsolete for variable loads.
  • Inlet throttle or modulation control: The inlet butterfly forces the compressor to operate on its characteristic curve when the demand is less than rated capacity. As the inlet valve closes and the rising pressure approaches surge point, the bypass valve matches demand by opening slowly to bleed off excess air. Once the bypass valve is fully open and/or the inlet valve is at full turndown adjustment, there's no further reduction in power.
  • Full unload (auto dual) control: At a predetermined discharge pressure short of the surge point, the compressor unloads as the inlet valve closes and the bypass or unloading valve opens.

Figure 2. Inlet guide vanes are more efficient at partial loads.(Cooper Turbo, Inc.)

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.

Inlet butterfly valve recap

The inlet butterfly valve reduces the centrifugal compressor's power requirement in relation to reduced airflow during part-load conditions while maintaining the design discharge pressure. Because the pressure always decreases across the butterfly valve, it can do nothing to improve compression efficiency or specific power.

Many variables, including the machine and its location, determine idle power. However, good judgment indicates that if you are to run at full idle for long stretches, check your kW. If this unit has an inlet butterfly valve and is drawing more than 20 percent of full power, find out why. Review the factors that affect power draw at idle. Consider an electronic control system to monitor and control the compressor.

Inlet guide vanes

Inlet guide vanes are usually mounted on the compressor's first stage inlet, but are often installed on each stage in larger process units. Like butterfly valves, inlet guide vanes vary the volumetric flow at constant discharge.

Inlet guide vanes produce a swirl in the airflow, usually in the direction of impeller rotation. When throttling flow, the vanes shift from being parallel to the air stream to fully perpendicular. The effect is to reduce the work required to produce the same air discharge condition. The net result is reduced input power and improved specific power at low flow. Inlet guide vanes also can increase the flow when oriented in the over-throttle position (flow against rotation). There appears to be no fixed amount of flow gain, but it is estimated to be as high as 20 percent. This increased flow requires commensurate additional horsepower.

Inlet guide vane performance

Inlet guide vanes reduce the power required to produce a lower-than-design flow at the same pressure more than an inlet butterfly valve can reduce it. Simply put, inlet guide vanes are more efficient at turndown control than butterfly valves. Basic guide vane facts include:

  • The efficiency improvement from having inlet guide vanes only on the first stage decreases as the number of compression stages increase.
  • The better the inlet guide vanes are adjusted, the greater their impact on performance.
  • Other than over-throttle flow increase, inlet guide vanes offer no benefit at full load. 
  • Inlet guide vanes should be mounted no more than one pipe diameter away from the inlet, but not so close that the vane swirl sets up harmonics with the impeller's vane pass frequency.
  • Although inlet guide vanes don't increase turndown, they do enhance efficiency during turndown.

Inlet guide vane benefits

Figure 2 shows typical performance curves for inlet guide vanes and an inlet butterfly valve. Note the 4.2 percent to 8 percent difference in specific power. The greatest improvement is at the fully closed position. Many factors affect this fully unloaded valuenot the least being the minimum mass flow. The actual power at idle with inlet guide vanes will probably be no lower than 20 percent at full load and can rise as high as 30 percent.

Figure 3 reflects performance at various ambient conditions. The design flow and discharge is always stated at the worst case conditionshigh-temperature air and warm cooling water. For example, extreme summer conditions (air at 95 F and 60 percent RH, water at 85 F) maximizes the work required to compress air. The point is that the compressor runs at these conditions only for a limited time. An accurate benefit evaluation should include performance in colder weather and on average days.

These curves show inlet guide vanes perform from 3 percent to 6 percent better in hot design conditions. At lower temperatures, better performance also can be expected. The basic performance at colder conditions (air at 30 F, water at 70 F) now goes from 5.2 percent to 8 percent. On an average day with air at 80 F and water at 80 F, the gain is from 4.2 percent to 8 percent. Most engineers cite an average overall enhancement under real world operating conditions of 2 percent to 3 percent with a maximum of 4 percent.

To determine the actual advantage of inlet guide vanes over inlet butterfly valves in recovered electrical (or other) energy cost, determine the duration of turndown. Measure the actual kW across the turndown range. Deduct the appropriate percent from this total kW. Then multiply by the number of operating hours and the power rate to determine the projected energy cost. When evaluating a new machine, go through the same exercise using the machine-specific performance curve.

Inlet guide vanes and idle power draw

The idle power draw is a function of the mass flow of the air and the discharge pressure at each stage. This appears to be controlled by the manufacturer's desire to maintain a minimum pressure.

There is no question that some inlet guide vanes don't seal as completely as a high-quality, full-seat butterfly valve. But the technology exists for guide vanes to seal more than enough for any required minimum flow. The inlet guide vane's turndown operational flow is more stable than the inlet butterfly valve's.

Valve vs. vane recap

Other than not being able to reach total closure, there's no reason that inlet guide vanes can't be manufactured to hold the same idle power draw as the inlet butterfly valve on most machines. Designers seem to agree on an average idle draw of 20 percent with inlet butterfly valves and perhaps as high as 30 percent with inlet guide vanes.

Determining the initial cost and energy savings from inlet guide vanes is relatively easy. But if you use them, there are some important points to remember:

  • Selecting an inlet guide vane control to take advantage of its over-throttle or counter-rotation operating characteristics can provide as much as 20 percent more flow if the motor horsepower is available. You'll give up a little efficiency at full load, but can throttle back into a high-efficiency range.
  • Inlet guide vanes are excellent when system resistance is primarily frictional. Vanes tend to follow the system resistance load better than inlet butterfly valves and are better at avoiding blow off.
  • Inlet guide vanes offer a stable turndown, which often allows better use of full turndown when compared to the higher turbulent flow from an inlet butterfly valve.
  • Building in a high rise-to-surge will force the unit to operate down the curve in a less efficient region. Good throttle range and high efficiency at design are difficult to achieve simultaneously. Inlet guide vanes allow wider throttle range at higher efficiencies than inlet butterfly valves.
  • With the increased turndown efficiency, there is more air flow at the same brake horsepower, and during colder weather significantly more air is available (as much as 20 percent) if the motor horsepower is available.
  • Under certain conditions this can be important, particularly if the extra air flow allows shutting down a unit that would otherwise be operating, particularly at part load.
  • Inlet guide vanes vary in shape, material and drive mechanism. One type of control may be more precise than another or operate better than others under certain conditions.

Hank van Ormer and Scott van Ormer own and operate AirPower USA, Inc. They can be reached at hank@airpowerusainc.com or 740-862-4112.