Compressors: Types, Design & Selection for Process Industries

Compressors are mechanical devices used to increase the pressure of a gas by reducing its volume. They are widely used in industrial settings, particularly in oil & gas, petrochemical, power generation, and refrigeration systems. Their primary purpose is to deliver pressurized gases to process units, pipelines, or storage vessels, playing a crucial role in energy conversion and utility supply.

In thermodynamic terms, compressors increase the enthalpy of the gas. This results in a rise in pressure and temperature, accompanied by a decrease in volume. The energy required to perform this task is usually provided by an electric motor, gas turbine, or diesel engine.

Fundamental Concepts

Understanding a compressor’s behavior requires knowledge of fluid dynamics and thermodynamics. The following parameters are essential in compressor design and operation:

Inlet Pressure and Temperature

These are the pressure and temperature of the gas before entering the compressor. They significantly affect gas density, power consumption, and compressor efficiency.

Discharge Pressure

This is the desired outlet pressure after compression. It determines the compression ratio and the size or staging of the compressor.

Compression Ratio (CR)

The compression ratio is a critical parameter in compressor performance and energy efficiency. It is defined as the ratio of the absolute discharge pressure to the absolute inlet pressure.

Formula:
CR = P₍discharge₎ / P₍inlet₎

Where:

P₍discharge₎ = Discharge pressure (absolute)
P₍inlet₎ = Inlet pressure (absolute)

A higher compression ratio indicates that the gas is being compressed more significantly, which generally requires more power and may result in higher outlet temperatures. In such cases, multi-stage compression with intercooling is often implemented to reduce energy consumption and protect compressor components from thermal stress.

Flow Rate

Flow rate is the volume of gas delivered over time, typically expressed in Nm³/h, ACFM, or kg/h. It determines the size and type of compressor needed.

Specific Heat Ratio (k = Cp/Cv)

This ratio influences the amount of work required for compression. For air and most hydrocarbons, $k≈1.4k \approx 1.4$ .

Isentropic and Polytropic Compression

Compression can be modeled as isentropic (ideal) or polytropic (realistic). Isentropic models assume no heat exchange, while polytropic includes some heat losses and is closer to actual performance.

Classification of Compressors

Compressors are generally classified based on the mechanism used to compress the gas:

Positive Displacement Compressors – Trap a fixed amount of gas and compress it by reducing its volume.
Dynamic (or Turbo) Compressors – Use high-speed impellers to impart kinetic energy to gas, which is then converted into pressure.

Positive Displacement Compressors

1. Reciprocating Compressors

Reciprocating compressors use pistons moving inside cylinders to compress gas. As the piston moves downward, it creates a vacuum drawing gas in. As it moves upward, the gas is compressed and discharged at high pressure.

Key Components and Operation:

Cylinders and Pistons
Gas enters and is compressed in cylinders through the movement of pistons. The number of cylinders determines whether it is single-acting or double-acting.
Crankshaft and Connecting Rod
These convert rotary motion into reciprocating motion. Power is transmitted through these components from the driver (motor or engine).
Valves
Automatic suction and discharge valves control gas flow. These are spring-loaded and open based on pressure differential.
Cooling Systems
Due to heat generated during compression, cylinders often have water or air cooling systems. Intercoolers may be added between stages to reduce work in multistage designs.

Common Features:

Multistage Options
Allows handling of very high pressures in a series of smaller compressions, increasing overall efficiency.
High Pressure Capability
Can reach discharge pressures exceeding 30,000 psi, making them suitable for challenging applications like gas injection and CNG compression.
Pulsation Dampening Needed
As reciprocating compressors create pressure pulsations, dampeners or pulsation bottles are often used.

Applications:

CNG Filling Stations
Compress natural gas for vehicle fueling.
Gas Lift Systems in Oil Fields
Deliver high-pressure gas to lift hydrocarbons from the wellbore.
Air and Nitrogen Compression
For instrumentation, control systems, and purging.

2. Screw Compressors

Screw compressors utilize two intermeshing helical rotors to compress gas as it travels along the screw length. The gas is trapped between the rotors and the casing, compressed as the space between the rotors reduces.

Types:

Oil-Injected
Oil is introduced to seal gaps, cool the rotors, and lubricate bearings. The oil is later removed using separators and filters.
Oil-Free
Designed for applications requiring pure, uncontaminated air. These use specialized coatings and tight clearances instead of oil.

Design Features:

Continuous, Non-Pulsating Flow
Screw compressors provide a smooth and constant flow, making them suitable for process systems sensitive to pressure fluctuations.
Low Vibration and Noise Levels
Thanks to balanced rotor movement, these compressors are quieter and require less foundation support.
Compact and Modular
Screw compressors have fewer moving parts and can be easily packaged with integrated dryers, separators, and control panels.

Applications:

Plant Air Supply
Provides compressed air for tools, actuators, and controls in process facilities.
Refineries and Chemical Plants
Used in utility systems, flare gas recovery, and gas boosting applications.
Pharmaceutical and Food Industries
Especially oil-free variants, used where air purity is critical.

Centrifugal Compressors

Centrifugal compressors use high-speed rotating impellers to add kinetic energy to the gas, which is then converted into pressure energy in a diffuser. Multiple stages can be used to achieve high overall pressure ratios.

Key Design Aspects:

Impellers and Diffusers
Gas is accelerated by impellers and slowed in diffusers, converting velocity into pressure.
Multistage Configuration
When high pressures are required, multiple impellers and diffusers are arranged in series with intercooling stages.
Inlet Guide Vanes
Control flow rate and improve part-load efficiency.
Surge Control System
Protects the compressor from unstable operation by controlling flow during low demand.

Performance Characteristics:

High Flow, Moderate Pressure
Ideal for applications requiring large volumes of gas at pressures up to ~50 bar.
Best Efficiency at Design Point
Efficiency drops off sharply at off-design conditions, making flow control important.
Low Maintenance
With fewer wear parts, centrifugal compressors offer high availability and low life cycle cost.

Applications:

LNG Production
Used to compress natural gas prior to liquefaction.
Pipeline Gas Compression
For transporting gas over long distances.
Chemical Reactors
Recycle compressors for circulating gases.

Compressor Type Comparison: Detailed Technical Matrix

Compressors must be selected based on gas type, pressure, flow rate, and process requirements. The table below highlights the technical characteristics of the three main compressor types.

Parameter	Reciprocating	Screw	Centrifugal
Compression Mechanism	Piston motion compresses trapped volume	Helical screws trap and compress gas continuously	Impellers accelerate gas; diffusers convert velocity to pressure
Typical Gases	Natural gas, hydrogen, CO₂, flare gas	Fuel gas, nitrogen, utility air, process gas blends	Wet gas, dry gas, hydrogen, cracked gas, acid gas
Pressure Range	Very High (up to 30,000 psi)	Moderate (up to 3,000 psi)	Medium (up to 1,500 psi per stage)
Flow Rate	Low to medium	Medium to high	Very high
Efficiency (at design)	High	Moderate to high	High
Oil-Free Options	Yes (with special rings and coatings)	Yes (expensive and limited duty)	Yes (oil-free by design, with dry gas seals)
Maintenance Frequency	Frequent (wear-prone parts)	Moderate (less wear, requires oil management)	Low (minimal moving parts)
Installation Footprint	Large (especially for multistage)	Compact	Compact for high throughput
Noise and Vibration	High (pulsating discharge)	Low (smooth flow)	Very low
Cost	Moderate to high (scales with stage and size)	High (especially oil-free variants)	Very high (but lower per unit throughput)

Applications of Compressors in Oil & Gas and Chemical Industries

Compressors in process industries handle a wide range of gases—natural gas, hydrogen, nitrogen, CO₂, flare gas, ammonia, chlorine, cracked gas, etc. The gas properties (molecular weight, reactivity, corrosivity) directly affect compressor design, material selection, and sealing systems.

Natural Gas Compression

Used extensively for boosting pressure in gas pipelines, gas lift operations, and wellhead treatment. Natural gas composition varies and may contain heavier hydrocarbons, water vapor, H₂S, and CO₂, requiring materials that resist sour service corrosion.

Hydrogen Compression

Hydrogen’s low molecular weight and high diffusivity present challenges in sealing and compression. Reciprocating and centrifugal compressors are preferred, using specialized dry gas seals and leak-tight housings.

Nitrogen Compression

Used for blanketing, purging, and pressure testing. As an inert gas, nitrogen is non-reactive but requires careful control due to its expansion potential.

Flare Gas Recovery

Screw and reciprocating compressors are used to capture low-pressure waste gas and return it to the fuel gas or processing system, improving plant efficiency and environmental compliance.

Ammonia, Chlorine, and Other Toxic Gases

Special handling and materials (e.g., PTFE seals, Hastelloy internals) are used. Leakage prevention is critical due to toxicity and reactivity.

LNG and Cryogenic Service

Compressors operate under very low temperatures, requiring materials with high fracture toughness and special lubrication systems.

Compressor Selection Criteria

Selecting a compressor involves a deep understanding of process requirements, gas properties, and site conditions.

Type of Gas

Gases like H₂S, CO₂, or Cl₂ may require corrosion-resistant materials (e.g., stainless steel, Monel). Hydrogen requires special sealing due to its small molecular size. Refrigerants and cryogenic gases require compressors designed for low temperatures and low specific volumes.

Flow Rate & Pressure Requirements

Higher pressures may need multistage reciprocating or centrifugal compressors. Very high flow, low-pressure services often favor centrifugal types.

Gas Molecular Weight

Affects compression work and stage design. Light gases (hydrogen, helium) behave differently than heavy gases (propane, butane).

Process Duty

Whether continuous or intermittent, with or without fluctuations, will impact selection. Screw compressors handle variable flow better than centrifugal types.

Contaminants in Gas

Liquids, particulates, or corrosive elements require pre-filtration, gas conditioning, or special internals like wear-resistant coatings.

Oil-Free vs Oil-Injected

Oil-free designs are critical for gases that must remain pure or cannot tolerate contamination (e.g., oxygen, pharmaceutical-grade gases).

Design Considerations

Designing a compressor for industrial use is a multidisciplinary task involving thermodynamics, mechanical integrity, safety, and reliability.

Number of Stages

High compression ratios are achieved in stages to reduce power requirements and limit discharge temperatures. Intercoolers reduce the gas temperature between stages, increasing efficiency.

Driver Selection

Compressors are typically driven by electric motors, steam turbines, or gas turbines. The selection depends on available utilities, load variation, and plant layout.

Cooling System

Compression generates heat. Intercoolers (between stages) and aftercoolers (after discharge) are used to control gas temperature and improve efficiency.

Lubrication and Sealing

Proper lubrication of moving parts and effective sealing (e.g., mechanical seals, dry gas seals, packing) are essential, especially in toxic or flammable gas applications.

Key Compressor Formulas

Isentropic Compression Work:

W = (k / (k - 1)) * (P₁ * V₁) * [ (P₂ / P₁)^((k - 1)/k) - 1 ] / η

Where:

$W$ = Work (kW)
$k$ = Cp/Cv (specific heat ratio)
$P₁$ = Inlet pressure (Pa)
$V₁$ = Inlet volumetric flow rate (m³/s)
$P₂$ = Discharge pressure (Pa)
$η$ = Isentropic efficiency (decimal)

Brake Horsepower (BHP) for Reciprocating Compressors:

BHP = [ 144 × P × V × k × ( (P₂ / P₁)^((k - 1)/k) - 1 ) ] / [ 33000 × η × (k - 1) ]

Where:

$P$ = Inlet pressure (psia)
$V$ = Flow rate (cfm)
$k$ = Cp/Cv
$η$ = Efficiency

Polytropic Head (for Centrifugal Compressors):

H = (n / (n - 1)) * R * T₁ * [ (P₂ / P₁)^((n - 1)/n) - 1 ]

Where:

$H$ = Polytropic head (J/kg)
$n$ = Polytropic exponent
$R$ = Gas constant
$T₁$ = Inlet temperature (K)
$P₁, P₂$ = Pressures (Pa)

Control Systems and Surge Management

Compressors, especially dynamic types like centrifugal compressors, require advanced control systems to ensure safe, stable, and efficient operation across varying process conditions.

Surge and Surge Protection

Surge is a condition where the compressor cannot overcome downstream pressure, causing flow reversal and violent oscillations. It can damage bearings, seals, and impellers in seconds.

To prevent this:

Anti-surge valves are installed to bypass flow during low demand.
Flow and pressure transmitters monitor real-time data for automatic control.
Recycle loops maintain minimum flow requirements during turndown.

Surge control is critical for compressors handling light gases (e.g., hydrogen, natural gas), where small density changes significantly impact flow behavior.

Load Control Strategies

Compressor performance must be matched to fluctuating process demand. Common methods include:

Inlet Guide Vanes (IGVs):
Mechanically adjust the angle of gas entering centrifugal compressors to modulate flow without changing speed. Useful for part-load efficiency.
Variable Speed Drives (VSDs):
Adjust compressor speed based on flow requirements. These are energy efficient, especially for screw compressors in fluctuating-duty applications.
Step Load/Unload Control:
Used in reciprocating compressors by enabling/disabling cylinders or adjusting clearance pockets.
Bypass Control (Recycle):
Excess flow is recycled back to suction to maintain stability during low demand periods.

Efficiency and Performance Curves

Understanding compressor performance across a range of pressures and flow rates is essential for proper selection, control, and optimization.

Compressor Map (Performance Curve)

A typical compressor map plots flow rate vs discharge pressure, overlaid with:

Constant speed lines
Efficiency contours
Surge line (left boundary)
Stonewall/choke line (right boundary)

These maps help visualize:

Best efficiency point (BEP)
Operating range
Load control strategies

Types of Efficiency

Isentropic Efficiency (ηₛ):
Ratio of ideal work (isentropic compression) to actual work done.

ηₛ = W_ideal / W_actual

Polytropic Efficiency (ηₚ):
Used for multistage dynamic compressors where temperature and pressure change gradually. Provides more accurate performance representation.
Mechanical Efficiency:
Accounts for power losses due to friction and mechanical components.

Best Practices in Compressor Operation and Maintenance

To ensure long-term reliability and efficiency, compressors in oil & gas industries must follow rigorous maintenance and operational guidelines.

1. Clean and Dry Suction Gas

Dirty or wet gas leads to fouling, corrosion, and seal damage. Proper filtration, knock-out drums, or dryers should be used to condition gas before compression.

2. Monitor Vibration and Temperature

Early detection of misalignment, imbalance, or bearing wear can prevent catastrophic failures. Use predictive maintenance tools like vibration analysis and infrared thermography.

3. Lubrication Management

Oil-injected compressors need regular oil analysis and scheduled changes. Contaminated oil can degrade seals and rotors, especially under high pressure.

4. Avoid Frequent Start-Stop Cycles

Compressors experience the most wear during start-up. Use proper control systems to reduce unnecessary cycling.

5. Perform Regular Leak Tests

Especially for hydrogen, CO₂, and toxic gases, undetected leaks can pose serious safety and environmental risks. Use ultrasonic or pressure decay tests.

6. Log Operational Parameters

Regularly track suction/discharge pressure, temperature, flow, and power consumption. Deviation from baseline trends can signal inefficiencies or early-stage faults.

Additional Considerations for Special Gas Handling

Compressors in process industries must often handle gases that differ from air in significant ways:

Gas Type	Impact on Design
Hydrogen	Low molecular weight → high leakage risk → dry gas seals and tight clearances
CO₂	High compressibility → careful thermodynamic modeling → affects stage design
H₂S	Corrosive → materials like stainless steel, Monel, Inconel are required
Wet Gas	Risk of liquid carryover → separators and knock-out drums must precede compressor
Oxygen	Highly reactive → oil-free designs mandatory to avoid fire risk
Refrigerants (R22, R134a)	Low boiling point → compression at cryogenic temperatures → specialized lubrication and seals

Summary and Conclusion

Compressors are essential components of oil, gas, and chemical processing facilities. Unlike general-purpose air compressors, industrial compressors must handle a wide variety of gases—each with unique chemical, thermal, and physical properties.

From reciprocating compressors offering high pressure for critical services, to screw compressors with stable flow for plant utilities, and centrifugal compressors moving enormous volumes of gas across plants and pipelines—each type has its domain of application.