Compressor Inputs
Example Data Table
| Parameter | Example Value | Unit |
|---|---|---|
| Cylinder bore | 120 | mm |
| Stroke length | 100 | mm |
| Cylinders | 4 | - |
| Speed | 900 | RPM |
| Acting type | Single-acting | - |
| Clearance ratio | 8 | % |
| Polytropic exponent | 1.30 | - |
| Capacity factor | 95 | % |
| Suction pressure | 1.013 | bar(a) |
| Discharge pressure | 7.5 | bar(a) |
| Suction temperature | 25 | °C |
| Compressibility factor | 1.00 | - |
| Molecular weight | 28.97 | kg/kmol |
Formula Used
This page uses an engineering approximation for reciprocating compressor capacity. All pressures are absolute. Double-acting service is idealized unless corrected by the capacity factor.
Swept volume per cylinder = (π/4) × Bore² × Stroke
Swept volume per revolution = Swept volume per cylinder × Cylinders × Acting factor
Theoretical displacement = Swept volume per revolution × RPM
Pressure ratio, r = P₂ / P₁
Volumetric efficiency = [1 + C − C × r^(1/n)] × Capacity factor
Specific gas constant, Rₛ = Rᵤ / Molecular weight
Inlet density = P₁ / (Z × Rₛ × T₁)
Mass flow = Inlet density × Actual inlet volumetric flow
Standardized flow = Mass flow / Standard density
Estimated discharge temperature = T₁ × r^((n−1)/n)
Gas power = ṁ × [n/(n−1)] × Z × Rₛ × T₁ × [r^((n−1)/n) − 1]
Shaft power = Gas power / Mechanical efficiency
This model is very useful for screening calculations, preliminary sizing, and trend studies. Final design work should still be validated with vendor curves and detailed thermodynamic analysis.
How to Use This Calculator
- Enter the cylinder bore, stroke, cylinder count, speed, and acting type.
- Add clearance ratio, polytropic exponent, and capacity factor to reflect real machine behavior.
- Use absolute suction and discharge pressures, then provide suction temperature and compressibility factor.
- Set gas molecular weight and your preferred standard reference pressure and temperature.
- Press Calculate Flow Rate to see results directly below the header.
- Review volumetric efficiency, actual inlet flow, standardized flow, mass flow, outlet conditions, and power estimates.
- Use the chart to study sensitivity to discharge pressure.
- Export the results with the CSV or PDF buttons.
Frequently Asked Questions
1) What type of compressor does this calculator model?
It models a reciprocating compressor using swept volume, clearance re-expansion, gas density, and a polytropic temperature relation. It is best for engineering estimates, comparisons, and early-stage sizing.
2) Why must I use absolute pressure?
Gas density and pressure ratio calculations require absolute pressure. Gauge pressure would distort density, volumetric efficiency, discharge temperature, and power values, especially near atmospheric suction conditions.
3) What does the capacity factor represent?
It combines real losses not fully captured by the basic clearance model, such as valve losses, leakage, pulsation effects, and practical delivery reductions. Lower values reduce calculated flow.
4) Why does volumetric efficiency fall at higher discharge pressure?
Higher pressure ratio increases the re-expansion effect of clearance gas. That leaves less fresh gas volume available during suction, so actual intake flow falls as discharge pressure rises.
5) What is the difference between actual flow and standardized flow?
Actual flow is based on inlet operating conditions. Standardized flow expresses the same mass flow at a chosen reference pressure and temperature, which makes comparison easier across different operating cases.
6) Can I use this for gases other than air?
Yes. Enter the correct molecular weight and a suitable compressibility factor. For highly non-ideal gases, you should validate the result with a more detailed property method or vendor software.
7) Is the power output motor power?
The calculator estimates gas power first, then divides by mechanical efficiency to estimate shaft power. Motor input power can be higher after transmission, drive, and motor efficiency losses.
8) Can this replace manufacturer performance curves?
No. It is a practical engineering calculator for fast estimates and trend checks. Final equipment selection should always rely on certified vendor data, test corrections, and application-specific limits.