Reciprocating Compressor Theory

1. Introduction

Reciprocating compressors are positive displacement machines that use a piston-cylinder arrangement to compress gas. They are among the oldest and most widely used compressor types in the oil and gas industry, valued for their high efficiency, flexibility, and ability to achieve high compression ratios.

Key Principle: A reciprocating compressor traps a fixed volume of gas in a cylinder and reduces that volume by piston motion, thereby increasing the gas pressure. The compression ratio is determined by the cylinder geometry and operating pressures.

The name "reciprocating" comes from the back-and-forth motion of the piston within the cylinder. This motion is driven by a crankshaft connected to the piston via a connecting rod, converting rotary motion from a driver (engine or motor) into linear piston motion.

2. Positive Displacement Principle

Reciprocating compressors belong to the positive displacement family of compressors. This classification describes the fundamental operating principle:

Fixed volume

Trapped gas

A specific volume of gas is trapped in the cylinder each cycle.

Volume reduction

Mechanical compression

Piston movement physically reduces the gas volume.

Pressure rise

From confinement

Pressure increases as volume decreases per gas laws.

Comparison with Dynamic Compressors

Unlike centrifugal (dynamic) compressors that use velocity to create pressure, reciprocating compressors directly reduce volume:

Characteristic	Reciprocating	Centrifugal
Compression method	Volume reduction	Velocity conversion
Flow type	Pulsating	Continuous
Pressure ratio/stage	Up to 10:1	Typically 1.5-3:1
Flow capacity	Low to moderate	High
Efficiency	80-95%	70-85%

3. The Compression Cycle

Each revolution of the crankshaft produces one complete compression cycle consisting of four distinct phases. Understanding these phases is essential for analyzing compressor performance and diagnosing problems.

Phase 1: Suction (Intake)

As the piston moves away from the head end (toward BDC), the cylinder volume increases. When cylinder pressure drops below suction line pressure, the suction valve opens and gas flows into the cylinder.

Piston moves from TDC toward BDC
Suction valve opens (automatic, pressure-actuated)
Gas fills the expanding cylinder
Continues until piston reaches BDC

Phase 2: Compression

At BDC, piston direction reverses and begins moving toward the head end. The suction valve closes and trapped gas is compressed.

Piston moves from BDC toward TDC
Both valves closed - gas trapped
Volume decreases, pressure increases
Temperature rises (heat of compression)

Phase 3: Discharge

When cylinder pressure exceeds discharge line pressure (plus valve spring force), the discharge valve opens and compressed gas is expelled.

Piston continues toward TDC
Discharge valve opens
Compressed gas flows to discharge header
Continues until piston reaches TDC

Phase 4: Expansion (Re-expansion)

At TDC, a small amount of gas remains trapped in the clearance volume. As the piston reverses, this trapped gas expands before suction can begin.

Piston begins moving toward BDC
Clearance gas expands
Cylinder pressure decreases
Suction valve opens when P < P_suction

Clearance Volume Impact: The expansion phase reduces effective cylinder capacity. Higher clearance means more gas to re-expand, reducing volumetric efficiency. This is why clearance volume is a critical design parameter.

4. Piston Motion

The piston follows a sinusoidal-like motion pattern driven by the crankshaft rotation. Two key positions define the limits of travel:

Position

TDC (Top Dead Center)

Piston at closest approach to cylinder head. Minimum volume, maximum pressure.

Position

BDC (Bottom Dead Center)

Piston at maximum distance from head. Maximum volume, minimum pressure.

Stroke and Displacement

The distance the piston travels between TDC and BDC is called the stroke. Combined with bore diameter, this determines cylinder displacement:

Cylinder Displacement: V_displacement = (π/4) × D² × L Where: D = Bore diameter (inches) L = Stroke length (inches) V = Swept volume (in³)

Speed and Frequency

Compressor speed directly affects capacity and mechanical stress:

Speed Class	RPM Range	Application
Slow speed	200-450	Integral engine units, long life
Medium speed	450-900	Separable units, balanced design
High speed	900-1800	Compact packages, motor-driven

5. Valve Operation

Compressor valves are critical components that control gas flow into and out of the cylinder. Unlike engine valves, compressor valves are automatic - they open and close based on pressure differential, not mechanical actuation.

Valve Types

Plate valves: Flat circular plates, common in low-speed applications
Ring valves: Concentric rings, good for medium speeds
Poppet valves: Individual discs, excellent for high-speed and high-pressure
Channel valves: Fingers in a plate, good all-around performance

Valve Operation Sequence

Automatic Operation: Valves open when the pressure differential across them exceeds the spring preload force. They close when flow reverses or pressure differential decreases.

Phase	Suction Valve	Discharge Valve
Suction	Open	Closed
Compression	Closed	Closed
Discharge	Closed	Open
Expansion	Closed	Closed

Valve Losses: Valves introduce pressure drops that reduce compressor efficiency. Typical valve losses are 2-5% of pressure. Damaged or dirty valves significantly increase losses and reduce capacity.

6. Thermodynamics of Compression

Gas compression follows thermodynamic principles that govern the relationship between pressure, volume, and temperature. Understanding these relationships is essential for predicting compressor performance.

Ideal Gas Law

Ideal Gas Relationship: PV = nRT or PV/T = constant (for fixed mass) Where: P = Absolute pressure V = Volume n = Number of moles R = Universal gas constant T = Absolute temperature

Compression Processes

Real compression falls between two theoretical limits:

Process

Isothermal (PV = const)

Constant temperature, minimum work. Not achievable in practice.

Process

Adiabatic (PV^k = const)

No heat transfer, maximum temperature rise. Theoretical limit.

Actual

Polytropic (PV^n = const)

Real compression with some heat loss. n typically between 1 and k.

Temperature Rise

Compression increases gas temperature according to the compression ratio and process:

Adiabatic Temperature Rise: T2/T1 = (P2/P1)^((k-1)/k) Where: T1, T2 = Absolute temperatures (suction, discharge) P1, P2 = Absolute pressures (suction, discharge) k = Specific heat ratio (Cp/Cv)

7. Pressure-Volume Diagrams

The P-V diagram (indicator card) is the most important tool for analyzing reciprocating compressor performance. It plots cylinder pressure against piston position (volume) through a complete cycle.

Theoretical P-V Diagram

An ideal cycle shows four distinct phases:

1→2: Compression - pressure rises as volume decreases
2→3: Discharge - gas expelled at constant high pressure
3→4: Expansion - clearance gas re-expands
4→1: Suction - cylinder fills at constant low pressure

Diagram Area = Work: The area enclosed by the P-V curve represents the work done per cycle. Deviations from the theoretical shape indicate losses or problems.

Common Abnormalities

Observation	Indicates
Rounded corners	Valve losses, late opening/closing
Suction line above/below horizontal	Pulsation, restriction
Discharge line not flat	Discharge valve problems
Compression curve shifted	Ring leakage, valve leakage