Reciprocating Compressors: The Power of Pistons

Positive Displacement vs. Centrifugal

While centrifugal compressors dominate massive-flow applications (like LNG pipelines), they struggle to generate extreme pressures efficiently.

When you need to push a small amount of gas to an astronomically high pressure (e.g., compressing hydrogen to 200 bar for a hydrocracker, or $CO_2$ to 150 bar for supercritical injection), you must use a Positive Displacement machine.

The king of positive displacement is the Reciprocating Compressor. It operates using pistons driven by a crankshaft, exactly like a car engine in reverse. It traps a specific volume of gas in a cylinder, physically crushes it into a smaller volume, and forces it out through a discharge valve.

Unlike a centrifugal compressor, a reciprocating compressor has absolutely no Surge line and no Stonewall limit. It will relentlessly force the gas forward against any downstream pressure. If a discharge block valve is accidentally closed, the compressor will not surge—it will simply continue crushing the gas until the discharge pipe violently explodes or the connecting rod snaps. (This is why every single cylinder MUST have a dedicated mechanical Safety Relief Valve).

The P-V Diagram (The Indicator Card)

The thermodynamic cycle of a reciprocating cylinder is mapped on a Pressure-Volume (P-V) diagram. One complete revolution of the crankshaft consists of four distinct phases:

1. Suction (Intake): The piston moves backward. The cylinder pressure drops below the suction header pressure. The spring-loaded suction valve pops open, drawing fresh gas into the cylinder.
2. Compression: The piston reaches bottom-dead-center (BDC) and reverses direction. The suction valve slams shut. The trapped gas is physically crushed, causing pressure and temperature to skyrocket along an adiabatic curve.
3. Discharge: The pressure inside the cylinder finally exceeds the pressure in the discharge header. The discharge valve pops open, and the piston forces the high-pressure gas out into the pipe.
4. Expansion (Clearance Volume): The piston reaches top-dead-center (TDC). However, the piston cannot perfectly touch the cylinder head (otherwise it would smash it). A small "clearance volume" of highly compressed gas remains trapped. As the piston pulls back, this trapped gas expands until its pressure drops low enough for the suction valve to open again.

Volumetric Efficiency ($\eta_v$)

Because of that trapped "clearance volume" expanding during the intake stroke, the piston must travel a significant distance backward before fresh gas can enter.

This means the cylinder never draws in its full physical volume of gas. The ratio of the actual volume of gas drawn in to the physical swept volume of the piston is the Volumetric Efficiency ($\eta_v$).

Volumetric efficiency is highly sensitive to the Compression Ratio ($R_c$). If the compression ratio is extremely high, the trapped clearance gas is at such an immense pressure that it expands and fills almost the entire cylinder during the pullback stroke! The compressor draws in almost zero fresh gas.

This is why reciprocating compressors are limited to a compression ratio of about 3.0 to 5.0 per stage. Above this, the volumetric efficiency plummets toward zero, and the discharge temperature exceeds safe metallurgical limits.