Understanding Rotary Screw Compressor KPIs in Enalysis

Enalysis calculates several key performance indicators for rotary screw compressors that help users understand capacity, power efficiency, operating conditions, and potential optimization opportunities.

Important screw compressor KPIs include compression ratio, optimum Vi setting, slide valve adjusted capacity, volumetric efficiency, over or under compression, compressor flow, and oil flow. These values should be reviewed together with the compressor operating conditions, equipment setup, and report history.

Which rotary screw compressor KPIs does Enalysis calculate?

For rotary screw compressors, Enalysis can calculate information including:

• Compression ratio
• Optimum Vi setting
• Slide valve adjusted capacity
• Volumetric efficiency
• Over or under compression
• Compressor flow
• Oil flow

Enalysis also uses operating information such as suction pressure, discharge pressure, speed, slide valve position, bypass position, temperatures, and available flow measurements to provide context for compressor performance.

What is compression ratio and why does it matter?

Compression ratio is an important operating condition for a rotary screw compressor because it influences both capacity and the power required for compression.

The Enalysis training material notes that volumetric efficiency decreases as compression ratio increases. Higher compression ratios increase gas slippage backward across the rotor tips, which reduces the amount of gas the compressor can effectively move.

Compression ratio is also one of the factors that increases the theoretical power required for compression. Review changes in compression ratio together with suction pressure, discharge pressure, flow, and power utilization.

 What is volumetric efficiency?

For a rotary screw compressor, volumetric efficiency is always less than 100 percent. It represents the effect of operating and mechanical conditions on how much gas the compressor can effectively move compared with its physical displacement.

The source training identifies the following factors that affect rotary screw volumetric efficiency:

• Gas characteristics
• Physical gap between the rotors and resulting gas slippage
• Compression ratio
• Rotor tip speed
• Slide valve position
• Over compression occurs when the Vi setting is too high for the current field conditions.
• Under compression occurs when the Vi setting is too low for the current field conditions.

Because volumetric efficiency affects compressor capacity, changes in this value can help explain changes in calculated compressor flow or available throughput.

What is Vi?

Vi, or volumetric ratio, describes the internal volumetric reduction of a rotary screw compressor. It is the ratio of the gas volume when compression begins to the volume of that same quantity of gas at the discharge port.

Vi is not the same as compression ratio. The compressor may have a fixed Vi, a selectable variable Vi, or an infinitely variable Vi depending on the compressor design and controls.

Enalysis can calculate an optimum Vi setting based on the operating conditions. The training material notes that matching the Vi setting as closely as possible to the field conditions can improve power efficiency.

What do over compression and under compression mean?

Over compression and under compression indicate that the compressor internal Vi does not closely match the current field conditions.

• Over compression occurs when the Vi setting is too high for the current field conditions.
• Under compression occurs when the Vi setting is too low for the current field conditions.

Both conditions can reduce compression efficiency and increase unnecessary power consumption. Review the Enalysis report, the current Vi setting, and the operating pressures before considering a configuration change.

 What does slide valve adjusted capacity tell me?

The slide valve is one of the primary capacity controls on a rotary screw compressor. The source training describes slide valve control from approximately 100 percent capacity down to about 10 percent, depending on the compressor.

Slide valve position and actual capacity are not necessarily linear. Opening the slide valve reduces the effective inlet volume, and on many compressors it also reduces Vi.

A slide valve that is open more than required can reduce throughput. Enalysis uses slide valve information together with other operating data to help identify potential capacity gains.

What can indicate a potential capacity gain?

The Enalysis rotary screw training identifies several conditions that can point to unused capacity or production potential:

• A large pressure drop between station inlet pressure and compressor suction pressure
• Reported compressor flow below calculated compressor flow
• Compressor speed below the maximum available speed
• The slide valve reported open
• The bypass or recycle valve reported open

The training also notes that inlet restrictions, speed below rated, an unnecessarily open slide valve, unnecessarily high compression ratios, and an open bypass or recycle valve can reduce rotary screw compressor throughput.

 How are power and efficiency related to these KPIs?

The power required by a rotary screw compressor is influenced by inlet suction pressure, gas flow rate, compression ratio, Vi, and gas specific heat ratio.

The relationship between Vi and the required field compression ratio has a strong influence on screw compressor efficiency. The source training recommends setting Vi correctly for field conditions, identifying unnecessary pressure drops, minimizing backpressure when appropriate, and loading the slide valve as much as possible within the available driver load and operating limits.

For maximum throughput, the training describes an optimized condition as one where available capacity and or horsepower are fully utilized, the suction control valve is fully open, speed is at the appropriate maximum, the slide valve is fully loaded, the bypass valve is closed, and Vi is as close as possible to the optimum setting.

How should I review a flagged rotary screw compressor KPI?

1. Confirm that the operating data in the report is accurate, especially pressures, temperatures, speed, slide valve position, Vi setting, and bypass position.
2. Identify the KPI or operating condition that is flagged and review the associated measured and calculated values.
3. Compare the current report with previous reports and trends to determine whether the condition is new, persistent, or changing.
4. Review related capacity and power information, including compressor flow, slide valve position, compression ratio, Vi, and power utilization.
5. Check for operating restrictions such as inlet pressure drop, backpressure, an open bypass valve, or a partially loaded slide valve.
6. Contact your Detechtion Engineering Account Manager when a condition is severe, persistent, approaching an equipment limit, or difficult to interpret.

 What about screw compressor warning and severe limits?

Enalysis can flag rotary screw compressor conditions such as over compression, under compression, high discharge temperature, oil injection temperature, filter differential pressure, internal discharge pressure, thrust load, and other operating limits. The exact criteria can depend on the compressor setup and configured limits.

Use the warning or severe flag as a prompt for additional review rather than making a compressor configuration change based on a single value.