AtlasCalcsUtility Calculation Suite
2026-07-30 12 min

Compressor Selection Calculator

Select appropriate compressor type and size based on process requirements, gas properties, and operating conditions. Compare compressor technologies.

Calculator

Unit System
Required Flow (Q) [m³/min]
Suction Pressure (p₁) [bar]
Discharge Pressure (p₂) [bar]
Inlet Temperature (T₁) [°C]
Gas Type
Gas Molecular Weight (M) [g/mol]
Specific Heat Ratio (k)
Required Material
Lubrication
Cooling Type
Duty Cycle
Location
Noise Limit [dBA]

Results

Normal

Recommended Compressor Type

Screw-

Optimal compressor technology for given conditions

Estimated Number of Stages

1.0-

Recommended number of compression stages

Estimated Power

63.1kW

Estimated power requirement

Discharge Temperature

257.9°C

Estimated gas temperature at discharge

Interstage Pressure

0.0bar

Optimal interstage pressure for multi-stage design

Estimated Efficiency

75.0%

Expected compressor adiabatic efficiency

Relative Cost

$40-120/CFM

Estimated cost range per unit flow

Package Dimensions Estimate

2×1.5×1.5 m

Approximate package footprint

Recommended Driver

Electric Motor-

Recommended prime mover

Visualizations

Results Overview

1-Estimated Number of Stages
63.1-Estimated Power
257.9-Discharge Temperature
75-Estimated Efficiency
40-Relative Cost
21.5-Package Dimensions Estimate

Formula

Compressor selection is guided by pressure ratio, flow rate, gas properties (molecular weight, specific heat ratio), and application requirements. Each compressor type excels in specific operating ranges.

Selection based on pressure ratio pr = p₂/p₁, flow rate Q, and gas properties

Alternative Form

n_s = n × √Q / H^0.75 for dynamic compressor specific speed

Variables

SymbolNameUnitDescription
prPressure Ratio-Absolute discharge pressure divided by suction pressure
QFlow Ratem³/min / CFMVolumetric flow at suction conditions
p₁Suction Pressurebar / psiAbsolute pressure at compressor inlet
p₂Discharge Pressurebar / psiAbsolute pressure at compressor outlet
T₁Inlet Temperature°CGas temperature at suction flange
kSpecific Heat Ratio-Ratio of specific heats (c_p/c_v)
MMolecular Weightg/molMolecular weight of the gas

Explanation

Compressor selection follows established guidelines from API 617 (centrifugal), API 618 (reciprocating), and API 619 (screw). The pressure ratio determines whether single or multi-stage compression is needed. Flow rate determines the compressor type: reciprocating for low flow (<10 m³/min), screw for medium flow (1-100 m³/min), centrifugal for high flow (>10 m³/min), and axial for very high flow (>200 m³/min). Gas properties including molecular weight and specific heat ratio affect power requirements and discharge temperature.

Derivation

For dynamic compressors: specific speed n_s = n × √Q / H^0.75 guides impeller selection. For positive displacement: displacement volume V_d = Q / (n × η_v). Power for all types: P = (m × R × T₁ × k/(k-1)) × (pr^((k-1)/k) - 1) / η. Discharge temperature: T₂ = T₁ × pr^((k-1)/k).

Step-by-Step Calculation

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

Step 1

Define operating conditions: required flow, suction/discharge pressures, inlet temperature, and gas properties.

Flow:10 m³/min
Pressure Ratio:8.0:1
Gas:Air
Result: Flow 10 m³/min at PR 8.0
Step 2

Evaluate compressor types based on flow and pressure ratio ranges.

Flow Range:10
Pressure Ratio:8.0
Result: Recommended: Screw
Step 3

Determine number of stages, power requirement, and driver selection.

P = (m × R × T₁ × k/(k-1)) × (pr^((k-1)/k) - 1) / η
Stages:1.0
Power:63.1 kW
Efficiency:75.0%
Result: Discharge T: 257.9°C, Driver: Electric Motor
Final Answer
Screw

Worked Examples

See how the calculator works with different scenarios.

Scenario

Select a compressor for 10 m³/min at 8 bar from atmospheric, air at 20°C, continuous duty, oil-free.

Inputs

Flow:10 m³/min
Suction:1 bar
Discharge:8 bar
Gas:Air
Duty:Continuous

Calculation

PR = 8/1 = 8:1 Flow 10 m³/min at PR=8 → Screw compressor P = m × R × T × (k/(k-1)) × (pr^((k-1)/k)-1) / η T₂ = 20 × 8^((1.4-1)/1.4) = 160°C

Result

Screw Compressor, Single Stage, 55 kW, T₂=160°C

Interpretation

A rotary screw compressor with oil-free rotors is ideal for plant air. Two-stage with intercooling would reduce discharge temperature and improve efficiency.

Understanding Your Result

Your Result: Recommended Compressor Type: Screw -

Typical range: Reciprocating: 0.1-100 m³/min, up to 1000+ bar. Screw: 0.5-500 m³/min, up to 15 bar (single) or 45 bar (multi). Centrifugal: 10-5000+ m³/min, up to 100 bar. Axial: 500-50000+ m³/min, up to 10 bar. Scroll: 0.01-1 m³/min, up to 10 bar.

What This Means

Compressor selection identifies the optimal compressor technology, number of stages, power requirement, and driver for the specified gas compression duty.

Why It Matters

Incorrect compressor selection leads to high energy costs, poor reliability, and inability to meet process requirements. The right selection optimizes capital cost, operating cost, and reliability.

Professional Interpretation

Selection must consider more than just flow and pressure. Key factors include: (1) gas composition (corrosive, toxic, flammable), (2) required gas quality (oil-free vs oil-lubricated), (3) capacity control requirements (unloader, VFD, suction throttle), (4) ambient conditions (altitude, temperature range), (5) API standards for critical services. Always verify preliminary selection with compressor manufacturer using detailed gas analysis.

Recommendations

  • •Use reciprocating compressors for high pressure ratio and low flow applications
  • •Use centrifugal compressors for high flow, moderate pressure continuous service
  • •Use screw compressors for medium flow with moderate oil carryover tolerance
  • •Specify intercooling when discharge temperature exceeds 150°C or when efficiency is critical

Educational Content

Introduction

Compressor selection is a systematic process of matching compressor technology to process requirements. The correct choice balances capital cost, operating efficiency, reliability, and maintenance requirements.

Concept Explanation

Compressors are classified as dynamic (centrifugal, axial) or positive displacement (reciprocating, screw, scroll). Dynamic compressors impart velocity energy converted to pressure; PD compressors trap and reduce volume. Selection is primarily based on flow rate and pressure ratio, with gas properties and application requirements further narrowing the choice.

Applications

▸Plant and instrument air systems
▸Natural gas transmission and distribution
▸Refrigeration and air conditioning
▸Chemical and petrochemical processing
▸Oil and gas production (gas lift, injection)
▸Industrial gas supply (N₂, O₂, CO₂)
▸Air separation and gas processing

Benefits

  • •Optimal compressor technology for specific duty reduces energy costs
  • •Proper sizing prevents excessive turndown and associated inefficiency
  • •Correct material selection prevents corrosion and extends life
  • •Appropriate lubrication method ensures required gas quality
  • •Right driver selection optimizes fuel/power consumption

Limitations

  • •Preliminary selection requires manufacturer verification for final design
  • •Does not account for all site-specific conditions (ambient extremes, utility limitations)
  • •Simplified power calculation assumes constant gas properties
  • •Does not model transient or part-load performance in detail

Best Practices

  • •Request API 617/618/619 certified performance data from manufacturers
  • •Include 5-10% margin on flow and pressure for uncertainty
  • •Consider future capacity requirements in initial selection
  • •Evaluate life cycle cost including energy, maintenance, and capital

Common Misconceptions

Myth: Centrifugal compressors are always more efficient than reciprocating.
Truth: Reciprocating compressors often achieve higher efficiency (85-90%) than centrifugal (75-85%) for low-flow, high-pressure applications. Each type excels in its design range.
Myth: Oil-free compressors produce completely oil-free air.
Truth: Oil-free compressors use no oil in the compression chamber, but the term is relative. Most oil-free compressors still have oil-lubricated bearings. True oil-free requires magnetic bearings or labyrinth seals.
Myth: Higher discharge temperature means better compression.
Truth: High discharge temperature reduces efficiency, increases maintenance (valve deposits, oil degradation), and raises safety concerns. Lower temperature through intercooling is generally better.

Practical Tips

  • •Each 1 bar of unnecessary discharge pressure increases power consumption by approximately 5-7%
  • •For variable flow, consider multiple compressors in parallel or VFD control for centrifugal compressors
  • •Reciprocating compressors require significant foundation mass (3-5× compressor weight) for vibration control
  • •Screw compressors are the most tolerant of liquid carryover among the common types
  • •Centrifugal compressor surge occurs below minimum flow; anti-surge control systems are essential
  • •Hydrogen compression requires special materials due to hydrogen embrittlement risk (NACE MR0175)
  • •For air compressors, intake air at 10°C cooler reduces power consumption by approximately 3%

Common Mistakes

Oversizing compressor for future expansion
Select compressor for current duty; add parallel unit for future expansion. Running oversized compressors at part load is very inefficient.
Ignoring altitude effects on compressor performance
At higher altitude, lower suction density reduces mass flow for the same volumetric flow. Specify suction conditions (pressure, temperature) accurately.
Selecting compressor without considering capacity control method
Specify how flow will be regulated: VFD (centrifugal), step unloaders (reciprocating), slide valve (screw), or suction throttle (general).
Not specifying gas composition accurately for chemical service
Provide complete gas analysis including trace components, moisture content, and potential for condensation. Small impurities can cause major material or performance issues.

Assumptions

This calculator makes the following assumptions to provide estimates:

Adiabatic Compression: Assumes adiabatic (isentropic) compression without heat transfer. Actual polytropic efficiency is accounted for in power calculation.
Ideal Gas Behavior: Assumes ideal gas behavior. Real gas effects become significant at high pressure (>50 bar) and require compressibility factor correction.
Constant Gas Properties: Assumes constant specific heat ratio and molecular weight. These may vary with temperature and pressure in real gases.
Single Operating Point: Selection is based on a single design operating point. Actual systems may require off-design performance evaluation.

Limitations

Be aware of these limitations when using this calculator:

Preliminary Sizing Only: This provides preliminary selection guidance. Detailed design requires compressor manufacturer engineering.
Simplified Thermodynamics: Power calculation uses simplified isentropic model. Polytropic analysis with real gas properties is needed for final design.
No Pulsation Analysis: Does not account for pulsation effects in reciprocating compressors, which require detailed acoustic analysis.
Limited API Scope: Does not fully address all API 617/618/619 requirements for materials, auxiliaries, and testing.

Frequently Asked Questions

Answers to common questions about this calculator.

What type of compressor should I use for plant air?
For general plant air at 7-10 bar: screw compressors are most common for 1-100 m³/min; reciprocating for smaller flows; centrifugal for very large plants (>100 m³/min). Oil-free screw is preferred for critical applications.
What is the difference between single-stage and multi-stage compression?
Single-stage: one compression step, limited pressure ratio (typically <4:1 centrifugal, <10:1 reciprocating). Multi-stage: compression in series with intercooling, higher pressure ratio, better efficiency, lower discharge temperature.
How do I calculate compressor power?
P = (m × R × T₁ × k/(k-1)) × (pr^((k-1)/k) - 1) / η where m is mass flow, R is gas constant, T₁ is inlet temperature, k is specific heat ratio, pr is pressure ratio, and η is efficiency.
What is a good compressor discharge temperature?
For reciprocating: 135-165°C typical for air; max 200°C for safety. For centrifugal: typically 80-150°C. For screw: 60-100°C. High temperature degrades lubricants and increases maintenance.
When should I use an oil-free compressor?
Oil-free is required when: (1) air/gas contacts food or pharmaceuticals, (2) instrument air for critical controls, (3) process requires pure gas, (4) catalytic processes poisoned by oil, (5) breathing air applications.
What is compressor surging?
Surge is a dangerous condition in dynamic compressors occurring when flow drops below the surge limit. Flow reverses rapidly, causing violent vibration, temperature spikes, and potential mechanical damage. Anti-surge control systems prevent this.
How do I select between air-cooled and water-cooled compressors?
Air-cooled: lower installation cost, no cooling water required, higher ambient noise. Water-cooled: better efficiency, consistent cooling, lower discharge temperature, suitable for high ambient temperatures and indoor installations.
What is the typical efficiency of different compressor types?
Reciprocating: 80-90%, Centrifugal: 75-85%, Screw: 70-80%, Axial: 82-88%, Scroll: 60-75%. Efficiency depends on size, operating point, and design quality.
How does molecular weight affect compressor selection?
Lower molecular weight (e.g., hydrogen, M=2) requires more stages for the same pressure ratio, develops less pressure per stage in dynamic compressors, and has higher discharge temperature. High MW gases develop more pressure per stage.
What is the maximum pressure ratio per stage?
Centrifugal: 1.5-3:1 per stage (up to 8:1 with special designs). Axial: 1.1-1.5:1 per stage. Reciprocating: 3-10:1 per stage. Screw: 5-15:1. Multi-stage designs achieve much higher overall ratios.
How do I select compressors for hydrogen service?
Hydrogen requires: (1) materials resistant to hydrogen embrittlement (NACE MR0175), (2) reciprocating for high pressure or centrifugal for high flow, (3) special seal systems to prevent leakage, (4) consideration of high diffusivity and low molecular weight.
What is the difference between API 617 and API 618?
API 617 covers centrifugal and axial compressors. API 618 covers reciprocating compressors. API 619 covers rotary positive displacement (screw) compressors. Each specifies design, materials, testing, and auxiliary requirements.
How does intercooling improve compressor efficiency?
Intercooling reduces gas temperature between stages, bringing it closer to isothermal compression (the most efficient process). Each intercooler can improve overall efficiency by 5-10% by reducing the work of subsequent stages.
What capacity control methods are available?
Reciprocating: step unloaders, VFD, clearance pockets. Screw: slide valve, VFD. Centrifugal: VFD, inlet guide vanes, suction throttle. Each method has different turndown capability and efficiency characteristics.
How do I size a compressor for intermittent duty?
For intermittent duty, size based on required flow during on-cycle plus storage receiver capacity. The compressor runs to fill the receiver, then shuts off. Duty cycle = (on-time / total cycle time). Select for run time ≥ 10 minutes minimum.
What is the effect of altitude on compressor performance?
Altitude reduces atmospheric pressure and air density. A compressor delivers the same volumetric flow but less mass flow at altitude. For a given mass flow requirement, increase the size by approximately 3-4% per 300m elevation.
How do I select materials for corrosive gas service?
Consider: (1) ASTM material selection guidelines for the specific gas, (2) NACE MR0175 for H₂S service, (3) temperature limits for materials, (4) coatings and cladding for corrosion resistance, (5) galvanic corrosion in wet environments.
What is the foundation requirement for compressors?
Reciprocating: massive concrete block (3-5× compressor weight) for vibration isolation. Centrifugal: lighter foundation (1-2× weight) with proper grouting. Screw: relatively light foundation. Skid-mounted packages are common for smaller units.
How do I select between lubricated and non-lubricated compressors?
Non-lubricated: 100% oil-free gas, higher maintenance (piston rings, rider bands), higher cost. Lubricated: lower maintenance, oil in gas stream requires separation, suitable when oil contamination is acceptable. Oil-free is preferred for instrument air and process purity.
What is the recommended margin on compressor flow?
Include 5-10% margin on flow for uncertainties in demand, instrument error, and compressor degradation over time. For critical processes, consider 15% margin or N+1 configuration with standby compressor.
How do I evaluate compressor life cycle cost?
LCC = Initial cost + Energy cost (PV of lifetime) + Maintenance cost (PV) + Installation cost. Energy cost typically dominates (50-70% of LCC). A more efficient compressor with higher first cost often has lower LCC.
What is the typical noise level of compressors?
Reciprocating: 85-110 dBA, Screw: 75-95 dBA, Centrifugal: 80-95 dBA, Scroll: 65-75 dBA. Noise enclosures can reduce levels by 15-25 dBA. Always specify noise requirements clearly in the purchase specification.
How do I select the right driver for a compressor?
Consider: (1) power requirement, (2) speed requirements, (3) available utilities, (4) hazardous area classification, (5) speed control needs. Electric motors are most common; gas turbines for large pipeline; steam turbines for cogeneration.
What is the maximum allowable discharge temperature?
API 618 limits: max 150°C for lubricated, 200°C for non-lubricated reciprocating. API 617: depends on material, typically 150-200°C. Higher temperatures accelerate lubricant degradation, valve wear, and pose safety risks.
How does specific heat ratio (k) affect compression?
Higher k (e.g., H₂ k=1.41, He k=1.66) results in higher discharge temperature for the same pressure ratio. Lower k (e.g., natural gas k=1.3) gives lower discharge temperature. Power requirement also varies with k.
What is a dry gas seal?
A dry gas seal uses a thin gas film between rotating and stationary faces to seal the compressor shaft. They eliminate seal oil systems, reduce power loss, and are standard for modern centrifugal compressors, especially in gas service.
How do I select compressor auxiliaries?
Key auxiliaries: (1) lube oil system (API 614), (2) seal gas system, (3) cooling system, (4) control system (PLC/DCS), (5) surge control (centrifugal), (6) pulsation dampeners (reciprocating), (7) moisture separators.
What is the turndown capability of different compressor types?
Reciprocating: 10-100% (step unloaders), Screw (slide valve): 25-100%, Centrifugal (VFD): 30-100%, Centrifugal (IGV): 50-100%. Below minimum turndown, efficiency drops significantly or the compressor may not operate stably.
How do I select compressors for flammable gas service?
Follow: (1) API 617/618/619, (2) ATEX/IECEx for hazardous areas, (3) Gas detection and ventilation, (4) Fire-safe valves and piping, (5) Emergency shutdown system, (6) Proper material selection for the specific gas.

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Glossary

Technical terms used in mortgage calculations explained in plain language.

Adiabatic Efficiency
Ratio of isentropic (ideal) work to actual work input. The most common efficiency metric for compressors.
API 617
American Petroleum Institute standard for centrifugal and axial compressors for refinery and petrochemical service.
API 618
API standard for reciprocating compressors for petroleum, chemical, and gas industry services.
Intercooling
Cooling of gas between compression stages to reduce temperature and improve overall efficiency.
NACE MR0175
Standard for materials resistant to sulfide stress cracking in H₂S-containing oil and gas environments.
Polytropic Efficiency
Efficiency accounting for actual compression path with heat transfer, used for centrifugal compressor analysis.
Pressure Ratio
Ratio of absolute discharge pressure to absolute suction pressure; determines compression work and stage requirements.
Specific Heat Ratio (k)
Ratio of specific heat at constant pressure to constant volume (c_p/c_v); affects discharge temperature and power.
Surge
Unstable flow condition in dynamic compressors at low flow, causing flow reversal and potential mechanical damage.
Turndown
The minimum flow at which a compressor can operate stably as a percentage of design flow.
Unloader
A device that reduces or eliminates compressor capacity by holding valves open or bypassing gas.
VFD (Variable Frequency Drive)
Electronic speed controller for compressor motors, providing efficient capacity control for dynamic compressors.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

API 617API 618API 619ISO 5389NACE MR0175/ISO 15156ASME PTC 10

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Page Information

Last Updated:

2026-07-30

Version:

2.0.0

License:

Free to use for educational and professional purposes.