AtlasCalcsUtility Calculation Suite
2026-07-30 10 min

Compressor Capacity Calculator

Calculate compressor displacement capacity for reciprocating, screw, and centrifugal compressors based on geometry and operating parameters.

Calculator

Unit System
Compressor Type
Cylinder Bore (D) [mm]
Stroke Length (L) [mm]
Number of Cylinders
Speed (n) [RPM]
Clearance Volume Ratio [%]
Number of Stages

Results

Normal

Displacement Volume

47.124m³/min

Theoretical piston displacement per minute

Theoretical Flow

47.124m³/min

Ideal flow without losses

Volumetric Efficiency (η_v)

84.927%

Ratio of actual to theoretical flow

Actual Flow

40.021m³/min

Flow after accounting for volumetric efficiency

FAD

38.020m³/min

Free Air Delivery

Swept Volume per Revolution

0.785L

Displacement per revolution

Visualizations

Results Overview

47.1m³/minDisplacement Volume
47.1m³/minTheoretical Flow
84.9m³/minVolumetric Efficiency
40m³/minActual Flow
38m³/minFAD
0.8m³/minSwept Volume per Revolution

Formula

Compressor capacity depends on geometry, speed, and volumetric efficiency factors.

V = π/4 × D² × L × n × N (reciprocating displacement)

Alternative Form

Q = C_d × A × U (centrifugal capacity)

Variables

SymbolNameUnitDescription
DCylinder Boremm / inCylinder inner diameter
LStroke Lengthmm / inPiston travel distance
nSpeedRPMRotational speed
NCylinder Count-Number of cylinders
η_vVolumetric Efficiency%Ratio of actual to theoretical flow

Explanation

The displacement volume of a reciprocating compressor is determined by the swept volume of all cylinders multiplied by speed. Volumetric efficiency accounts for losses from clearance gas expansion, gas heating, leakage, and valve pressure drops.

Derivation

For a single cylinder: swept volume = π/4 × D² × L. Total displacement = swept volume × N_cylinders × N_stages × speed. Volumetric efficiency: η_v = 1 - c[(p₂/p₁)^(1/k) - 1], where c is the clearance ratio. This is known as the clearance factor equation.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Calculate swept volume of one cylinder using bore and stroke dimensions.

V_cylinder = π/4 × D² × L
D:100 mm
L:100 mm
Intermediate: V_cyl = 0.000785 m³
Result: Swept volume per cylinder
Step 2

Multiply by number of cylinders, speed, and stages for total displacement.

Total Displacement = V_cylinder × N_cyl × speed × stages
Cylinders:1
Speed:1000 RPM
Stages:1
Result: Displacement = 47.124 m³/min
Step 3

Apply volumetric efficiency to get actual flow.

Q_actual = Displacement × η_v
Clearance Ratio:5%
η_v:84.927%
Result: Actual Flow = 40.021 m³/min
Final Answer
47.124

Worked Examples

See how the calculator works with different scenarios.

Scenario

A single-cylinder reciprocating compressor has bore 100 mm, stroke 120 mm, speed 1000 RPM, 5% clearance ratio.

Inputs

Bore:100 mm
Stroke:120 mm
Cylinders:1
Speed:1000 RPM
Clearance:5%

Calculation

Swept vol = π/4 × 0.1² × 0.12 = 0.0009425 m³ Displacement = 0.0009425 × 1 × 1000 = 0.9425 m³/min η_v = 1 - 0.05 × [(7/1)^(1/1.4) - 1] = 0.05 × (4.73 - 1) = 0.813 Actual = 0.9425 × 0.813 = 0.766 m³/min

Result

0.766 m³/min

Interpretation

The compressor delivers 0.766 m³/min actual flow. The volumetric efficiency of 81.3% is typical for a reciprocating compressor with moderate pressure ratio.

Understanding Your Result

Your Result: Displacement Volume: 47.124 m³/min

Typical range: Small reciprocating: 0.1-2 m³/min. Industrial screw: 2-50 m³/min. Large centrifugal: 50-5000 m³/min. Pipeline: up to 50000 m³/min.

What This Means

Compressor capacity determines how much gas the compressor can move. It is directly related to geometric dimensions and operating speed, reduced by volumetric efficiency losses.

Why It Matters

Capacity determines whether the compressor can meet system demand. Undersized compressors cause pressure drops and inadequate supply. Oversizing wastes capital and energy.

Professional Interpretation

Volumetric efficiency is the key performance parameter. It is affected by clearance volume, pressure ratio, specific heat ratio, valve losses, and gas temperature. For reciprocating compressors, η_v typically ranges from 70-90%. For screw compressors, it can reach 95% due to the absence of clearance volume.

Recommendations

  • •Minimize clearance volume through proper valve and piston ring maintenance
  • •For high pressure ratios, use multistage compression to maintain acceptable volumetric efficiency
  • •Consider screw compressors for applications where high volumetric efficiency is critical
  • •Monitor volumetric efficiency over time as an indicator of wear
  • •Select compressor speed based on valve dynamics and gas properties

Educational Content

Introduction

Compressor capacity is a fundamental specification that determines how much gas a compressor can move. It depends on the physical dimensions of the compressor and its operating speed.

Concept Explanation

Positive displacement compressors (reciprocating, screw, scroll) trap gas in chambers and reduce volume to increase pressure. Their capacity is directly proportional to displacement volume times speed. Dynamic compressors (centrifugal, axial) impart velocity to the gas and convert kinetic energy to pressure, with capacity depending on impeller geometry and flow coefficients.

Applications

▸Compressor selection and specification
▸Performance evaluation and troubleshooting
▸Maintenance planning (capacity loss indicates wear)
▸System design and pipe sizing
▸Energy efficiency optimization

Benefits

  • •Enables accurate compressor sizing for specific applications
  • •Facilitates performance comparison between compressor types
  • •Helps diagnose capacity loss from wear or damage
  • •Supports multistage compressor design optimization

Limitations

  • •Volumetric efficiency estimation is approximate without specific valve data
  • •Does not account for pulsation effects in reciprocating compressors
  • •Screw compressor capacity depends on rotor profile details beyond simple geometry
  • •Centrifugal compressor modeling requires detailed impeller analysis

Best Practices

  • •Use actual measured bore and stroke, not nominal values
  • •Include valve losses and gas heating effects in volumetric efficiency estimate
  • •Consider altitude and suction conditions for actual delivered capacity
  • •Verify capacity with flow measurement after installation
  • •Account for clearance changes due to wear over compressor life

Common Misconceptions

Myth: Theoretical displacement equals actual compressor capacity.
Truth: Actual capacity is always less than theoretical displacement due to clearance volume, gas heating, leakage, and valve losses. Volumetric efficiency typically ranges from 70-95%.
Myth: Doubling speed doubles compressor capacity.
Truth: While displacement is proportional to speed, volumetric efficiency decreases at higher speeds due to increased valve losses and gas heating. The actual capacity gain is less than proportional.
Myth: All cylinders in a multistage compressor contribute equally to capacity.
Truth: In multistage compressors, only the first stage cylinders determine total capacity. Later stages compress the gas but do not increase volumetric throughput.

Practical Tips

  • •A 1% increase in clearance ratio typically reduces volumetric efficiency by 2-3%
  • •Valve plate lift and spring tension significantly affect volumetric efficiency at high speeds
  • •Suction valve pressure drop of 0.1 bar can reduce capacity by 2-3%
  • •Gas heating during suction stroke can reduce capacity by 2-5%
  • •Screw compressors have no clearance volume - their volumetric efficiency can exceed 95%
  • •Centrifugal compressors follow affinity laws: capacity ∝ speed, head ∝ speed², power ∝ speed³
  • •For reciprocating compressors, double-acting cylinders nearly double capacity compared to single-acting

Common Mistakes

Using nominal bore instead of actual measured bore
Always use actual cylinder bore dimensions, which differ from nominal due to wear and manufacturing tolerances. Measure bore at multiple points.
Neglecting gas heating in volumetric efficiency
Gas absorbs heat from cylinder walls during the suction stroke, reducing density and effective capacity. Include a heating factor of 0.95-0.98 in volumetric efficiency.
Assuming constant volumetric efficiency across all conditions
Volumetric efficiency varies significantly with pressure ratio, speed, and gas properties. Recalculate for each operating condition rather than using a single value.
Confusing displacement with actual flow for screw compressors
Screw compressor displacement is determined by rotor geometry, but actual flow is reduced by internal leakage (slip) which increases with pressure differential. Slip can be 5-15% of displacement.

Assumptions

This calculator makes the following assumptions to provide estimates:

Geometric Idealization: Cylinder geometry is assumed perfectly cylindrical with uniform bore. Actual ovality and taper affect swept volume slightly.
Constant Clearance Volume: Clearance volume ratio is constant. In practice, valve recesses and piston top geometry contribute to clearance.
Uniform Temperature: Cylinder wall temperature is assumed uniform. Actual temperature gradients affect volumetric efficiency.
Ideal Gas Behavior: Gas follows ideal gas law. Real gas effects at high pressure affect volumetric efficiency calculation.

Limitations

Be aware of these limitations when using this calculator:

Simple Volumetric Efficiency Model: The clearance factor formula provides an estimate only. Actual volumetric efficiency depends on valve dynamics, gas heating, and leakage that require detailed modeling.
No Valve Dynamics: Valve behavior (opening delay, flutter, and sealing) significantly affects capacity but is not modeled. Dynamic simulation is needed for accurate valve analysis.
Type-Specific Modeling: Each compressor type requires different capacity modeling approach. Screw compressors need rotor profile analysis. Centrifugal compressors need impeller and diffuser modeling.
No Pulsation Effects: Reciprocating compressor flow is pulsating, affecting instantaneous capacity. Pulsation dampeners smooth the flow but don't change average capacity.

Frequently Asked Questions

Answers to common questions about this calculator.

What is compressor capacity?
Compressor capacity is the volume of gas delivered per unit time, typically expressed as FAD (Free Air Delivery) in m³/min or CFM. It depends on compressor geometry, speed, and volumetric efficiency.
How do I calculate compressor displacement?
For reciprocating: V = π/4 × D² × L × n × N. For screw: V = rotor swept area × rotor length × speed. For centrifugal: Q = C_d × A × U.
What is volumetric efficiency?
Volumetric efficiency is the ratio of actual gas intake to piston displacement. It accounts for clearance volume, gas heating, leakage, and valve losses. Typical values: 70-95%.
How does clearance volume affect capacity?
Clearance volume reduces capacity because high-pressure gas trapped in the clearance re-expands during the suction stroke, reducing the volume of fresh gas drawn in. Higher clearance = lower capacity.
What is the difference between capacity and displacement?
Displacement is the geometric volume swept by pistons (theoretical maximum). Capacity (actual flow) is displacement reduced by volumetric efficiency losses.
How does compressor speed affect capacity?
Capacity is approximately proportional to speed for positive displacement compressors. However, volumetric efficiency decreases at higher speeds, so the relationship is not perfectly linear.
What is FAD?
Free Air Delivery is the volume of air delivered, expressed at inlet pressure and temperature conditions. It is the standard measure of compressor capacity.
How do I increase compressor capacity?
Increase speed (within limits), reduce clearance volume, improve valve performance, reduce suction pressure drop, or add cylinders/stages. For centrifugal: increase speed or modify impeller.
What causes capacity loss in compressors?
Common causes: worn piston rings (increased leakage), valve damage or fouling, increased clearance from wear, suction filter blockage, and elevated inlet temperature.
How does gas type affect capacity?
Gas molecular weight and specific heat ratio affect volumetric efficiency. Heavier gases (higher molecular weight) reduce capacity for a given volumetric displacement due to density effects.
What is the role of suction pressure in capacity?
Lower suction pressure reduces gas density at the inlet, reducing mass flow for the same volumetric displacement. For a given capacity rating, higher suction pressure increases mass flow.
How do I size a compressor for my application?
Calculate total tool/equipment air demand, apply load and usage factors (typically 0.5-0.8), add leakage allowance (10-30%), and add future expansion margin (10-20%). Select compressor capacity exceeding peak demand.
What is the effect of altitude on capacity?
At higher altitudes, lower atmospheric pressure reduces inlet density. Compressor capacity de-rating: approximately 3% per 300 m above sea level for mass flow.
How does cylinder cooling affect capacity?
Cylinder cooling reduces gas heating during compression and suction, improving volumetric efficiency. Water-cooled cylinders typically have 2-5% higher capacity than air-cooled.
What is the capacity of a screw compressor?
Screw compressor capacity depends on rotor length, diameter, and speed. Typical range: 1-100 m³/min for industrial screw compressors. Capacity is controlled by variable-speed drive or inlet valve modulation.
How does suction valve design affect capacity?
Suction valves with larger flow area and lower pressure drop improve capacity. Valve lift, spring force, and damping characteristics affect dynamic behavior and effective flow area.
What is the capacity of a scroll compressor?
Scroll compressors have capacity typically 0.1-30 m³/min. Their volumetric efficiency is high (90-95%) due to continuous compression without clearance volume or valves.
How does intercooling affect multistage capacity?
Intercooling increases gas density entering the next stage, slightly increasing mass flow. The primary benefit is reduced power, not increased capacity.
What is the capacity turndown ratio?
Turndown ratio is the range of capacity control. Reciprocating: 0-100% via clearance pockets or valve unloading. Screw: 10-100% with VFD. Centrifugal: 30-100% with inlet guide vanes.
How do I measure actual compressor capacity?
Use flow measurement devices (orifice plate, vortex meter, thermal mass flow) in the discharge line. For reciprocating compressors, use pulsation-dampened measurements or calorimetric methods.
What is the difference between single-acting and double-acting capacity?
Double-acting compressors compress on both sides of the piston, nearly doubling capacity. For a given cylinder, double-acting capacity ≈ 1.8 × single-acting capacity (accounting for piston rod area).
How does valve leakage affect capacity?
Valve leakage causes high-pressure gas to flow back into the cylinder or suction line, reducing capacity. A leaking discharge valve can reduce capacity by 10-30%.
What is the effect of piston ring wear on capacity?
Worn piston rings increase blow-by, reducing compression efficiency and capacity. Ring wear typically reduces capacity by 1-3% per year of operation before replacement is needed.
How do clearance pockets affect capacity?
Clearance pockets are variable volume spaces added to the cylinder to reduce capacity. Opening a clearance pocket increases effective clearance, reducing volumetric efficiency and capacity.
What is the capacity of a centrifugal compressor stage?
A single centrifugal stage typically handles 50-5000 m³/min. Multi-stage centrifugal compressors can reach 50000 m³/min. Capacity depends on impeller diameter, width, and rotational speed.
How does suction drum pulsation affect capacity?
Suction pulsations can reduce capacity by interfering with the natural filling of the cylinder. Proper suction drum and piping design minimize pulsation effects on capacity.
What is the relationship between capacity and pressure ratio?
For positive displacement compressors, capacity is relatively independent of pressure ratio (slight decrease due to increased leakage). For centrifugal compressors, capacity decreases significantly as pressure ratio increases.
How do I compare capacity across different compressor types?
Compare FAD at the same reference conditions (pressure, temperature, humidity). Also compare mass flow for a true capacity comparison independent of ambient conditions.

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Glossary

Technical terms used in mortgage calculations explained in plain language.

Displacement
The volume swept by all pistons per unit time, representing the theoretical maximum gas handling capacity.
Clearance Volume
Volume remaining in the cylinder at top dead center between piston and cylinder head.
Volumetric Efficiency
Ratio of actual volume of gas drawn into the cylinder to the piston displacement volume.
Swept Volume
Volume displaced by the piston in one stroke: π/4 × D² × L.
Blow-by
Gas leakage past piston rings from high-pressure to low-pressure side during compression.
Valve Loss
Pressure drop across suction and discharge valves that reduces effective flow area and capacity.
Clearance Pocket
Variable volume chamber connected to the cylinder for capacity regulation.
FAD (Free Air Delivery)
Volume of air delivered expressed at inlet pressure and temperature conditions.
Pulsation
Cyclic variation in flow and pressure due to intermittent gas intake and discharge in reciprocating compressors.
Affinity Laws
Relationships for centrifugal compressors: Q ∝ N, H ∝ N², P ∝ N³, where N is rotational speed.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

API 618API 619ISO 1217ASME PTC 9CAGI Performance

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Page Information

Last Updated:

2026-07-30

Version:

2.0.0

License:

Free to use for educational and professional purposes.