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.
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.
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.
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: