Screw Compressor Sizing Calculator: Power, Flow, Pressure Ratio & Performance

Sizing a rotary screw compressor involves more than matching airflow to a motor rating.

Pressure ratio, inlet conditions, compression efficiency, discharge temperature, rotor speed, specific energy, driver sizing and cooling load all affect the final selection.

The Ruhify Screw Compressor Calculator brings these variables together in one preliminary sizing workflow.

Here is what a typical result looks like for an oil-flooded, single-stage rotary screw compressor handling 2,500 m³/h of inlet air.

These numbers should not be evaluated independently. They describe one compressor operating point and the relationships between flow, pressure, temperature and power.

The compressor increases pressure from:

1.01 bar(a) → 8.01 bar(a)

This gives a pressure ratio of:

8.01 / 1.01 = 7.91

Pressure ratio has a major influence on compressor power and discharge temperature.

Higher discharge pressure generally means more compression work. That can increase:

shaft power;

discharge temperature;

specific energy;

cooling requirement;

and sensitivity to the compressor's built-in volume ratio.

This is why compressor sizing should start with the actual suction and discharge conditions rather than only a nominal "8 bar compressor" requirement.

For this operating point, calculated compressor shaft power is:

251 kW

Gas power is approximately:

244 kW

The difference accounts for mechanical effects between the compression process and the shaft.

The sizing result then selects a:

315 kW electric motor

rather than matching the motor directly to the calculated 251 kW shaft requirement.

The additional capacity provides operating margin for real package conditions and variations around the design point.

Estimated total electrical input is:

275 kW

including the variable-speed drive and cooling fan.

This distinction is important:

Compressor shaft power ≠ motor nameplate rating ≠ total package electrical input.

Calculated discharge temperature is:

93.8°C

The displayed operating limit is approximately:

100–110°C

The same compression process without liquid injection would produce a calculated adiabatic temperature of around:

309°C

That difference shows why oil injection is fundamental to an oil-flooded screw compressor.

The injected oil helps:

remove compression heat;

lubricate the rotors and bearings;

improve sealing;

and control discharge temperature.

For this operating point, estimated injected liquid flow is:

163 L/min

Temperature therefore needs to be evaluated together with pressure ratio and oil circulation, not as a standalone output.

The compressor handles:

2,500 m³/h inlet volume

At the specified suction conditions, calculated suction density is:

1.198 kg/m³

That produces a mass flow of approximately:

2,995 kg/h

or:

0.832 kg/s

This matters because volumetric flow changes meaning depending on the inlet pressure and temperature.

Two operating conditions with the same m³/h value can produce different mass flows.

For preliminary compressor sizing, inlet conditions therefore matter just as much as the airflow number itself.

The calculated specific energy is:

0.1006 kWh/m³

or approximately:

6.59 kW/(m³/min)

Specific energy connects compressor power consumption to delivered air.

It is useful when comparing different compressor selections or operating points.

A larger motor does not automatically mean a less efficient compressor. What matters is how much power is required to produce the required flow at the required pressure.

Specific energy provides a useful metric for making that comparison.

The calculated stage performance includes:

Isentropic head: 238 kJ/kg

Polytropic head: 251 kJ/kg

with:

Isentropic efficiency: 81.2%

Polytropic efficiency: 85.7%

An ideal compressor would require less energy than a real machine.

Leakage, mechanical losses, aerodynamic effects and other inefficiencies increase the actual power required.

Efficiency assumptions allow the sizing model to move from an ideal thermodynamic calculation toward a more realistic compressor operating point.

The preliminary rotor calculation gives:

Male rotor diameter: 321 mm
Rotor length: 433 mm
L/D ratio: 1.35
Rotor speed: 2,116 rpm
Tip speed: 35.6 m/s
Estimated optimum tip speed: 38.4 m/s
Rotor tip Mach number: 0.103

Calculated volumetric efficiency is:

90.6%

These outputs connect the required capacity to the physical compressor geometry.

A sizing calculation should therefore answer more than:

How much airflow is required?

It should also test whether a practical rotor size and operating speed can deliver that airflow efficiently.

For this case:

Selected Vi: 4.2

Required Vi: 4.39

The built-in volume ratio, or Vi, describes how much the trapped gas volume is reduced inside the screw compressor before the discharge port opens.

Matching Vi reasonably closely to the operating pressure ratio helps reduce under-compression or over-compression.

A poor Vi match can increase energy consumption even when the compressor still achieves the required discharge pressure.

This makes Vi an important parameter when evaluating screw compressor efficiency.

Compressors generate substantial heat.

For this package, calculated heat rejection is:

225 kW

including approximately:

184 kW through the oil system;

41.3 kW through aftercooling.

The selected package is air cooled, so no cooling-water requirement is shown.

Heat rejection matters during compressor-room and package design because it affects:

ventilation;

ambient temperature;

cooler sizing;

equipment layout;

and potentially HVAC requirements.

Selecting the compressor without considering its heat load can create installation problems later.

The Ruhify Screw Compressor Calculator also generates performance curves to show how the compressor behaves away from the primary design point.

Shows the relationship between:

inlet flow, rotor speed and discharge pressure.

This helps evaluate whether the selected compressor can cover different operating requirements.

Shows how compressor power changes as capacity changes.

This is particularly relevant for variable-speed-drive (VSD) compressors, where the machine may operate for long periods below full load.

Shows calculated compressor temperatures against the configured temperature limit.

This makes thermal margin easier to evaluate than a single discharge-temperature value.

Shows the relationship between pressure ratio, efficiency and built-in volume ratio.

This helps identify whether the selected Vi is reasonably matched to the required operating condition.

The Ruhify Screw Compressor Calculator provides a fast way to explore preliminary compressor sizing and see the engineering assumptions behind the result.

Open the Screw Compressor Calculator →

You can review pressure ratio, flow, shaft power, driver size, temperature, efficiency, rotor conditions, Vi matching, performance curves and cooling requirements in one workflow.

The calculator is intended for preliminary engineering and equipment evaluation. Final compressor selection should use validated process data, manufacturer performance information and appropriate engineering review.

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