Process Gas Compressor Types Compared: Liquid Ring, Screw, Reciprocating and Centrifugal

Picking a process gas compressor starts with the gas, not the catalogue. Pressure ratio and flow narrow the field, but composition, moisture, liquid carryover, contaminants, and flammability usually decide it. A machine that runs for years on clean nitrogen can fail in months on wet, dirty off-gas.

Two Families: Dynamic and Positive Displacement

All process gas compressors fall into two families, defined by how they add energy to the gas.

Dynamic compressors first accelerate the gas, then convert that velocity into pressure in a diffuser. Centrifugal and axial machines belong here. They deliver roughly constant head across a range of flows.

Positive displacement compressors trap a fixed volume of gas and reduce it mechanically. Reciprocating, screw and liquid ring machines belong here. They deliver roughly constant flow across a range of pressures.

Within the broad middle ground of pressure ratio and flow, every type can compete. That is why the gas itself, rather than pressure and flow alone, so often settles the choice.

Four Process Gas Compressor Types and How They Differ

Reciprocating compressors

A piston moving in a cylinder compresses the gas, with valves controlling inlet and discharge. Reciprocating machines suit high pressure ratios at low to moderate flow. A typical stage ratio is about 3, and multi-stage designs reach overall ratios above 300. They handle gases from hydrogen to chlorine, and the reference standard is API 618. Because liquids are incompressible, suction scrubbers are normally fitted to keep liquid out of the cylinders, and pulsation dampeners smooth the pulsating flow.

Screw compressors

Two meshing helical rotors trap and compress the gas. Screw machines come in two forms: oil-flooded, where oil seals and cools the rotors, and oil-free, where the rotors run without contact and the gas never meets lubricating oil. Oil-free process gas screws often use liquid injection to control discharge temperature, and they tolerate dirty and polymer-forming gases well. A slide valve controls capacity, providing wide turndown, and the reference standard is API 619.

Centrifugal compressors

An impeller spinning at high speed accelerates the gas, and a diffuser then slows it to raise pressure. Centrifugal machines are the usual choice for large, continuous flows of clean, dry gas, with few wearing parts and low maintenance. They must be kept away from surge at low flow, and the reference standard is API 617. Below an inlet flow of roughly 170 m³/h, check vendor support for a centrifugal design on a case-by-case basis.

Liquid ring compressors

An impeller mounted eccentrically in the casing throws a service liquid into a rotating ring, and the shrinking cells between the vanes and the ring compress the gas. Because the gas is in direct contact with the liquid, compression is close to isothermal. A liquid ring compressor handles wet, dirty, corrosive, and flammable gas, and the reference standard is API 681.

Side-by-side comparison

Where Kenflo Liquid Ring Compressors Have the Edge

Each compressor type has its home ground. Very large flows of clean, dry gas suit centrifugal machines, and very high pressure ratios at small flow suit reciprocating ones. In much of the process industry, though, the gas is rarely clean or dry. Off-gas, vent gas and recovered vapour are often wet, saturated, corrosive, flammable or toxic. This is where a Kenflo liquid ring compressor holds clear advantages over the other three types.

Compared with reciprocating compressors

• Liquid is tolerated, not feared. The ring cushions liquids and soft solids that arrive with the gas and washes them through to the separator. No cylinder can be damaged by a liquid slug.
• Fewer wear parts. No valves, piston rings, or rod packing to replace. The impeller assembly is the only rotating part, and there is no metal-to-metal contact in the compression chamber.
• Smooth flow. Gas leaves without the pulsation of a piston machine, so no pulsation dampeners are needed.

Compared with screw compressors

• No lubricating oil in the gas path. Unlike an oil-flooded screw, the gas touches only the service liquid, so the product is never contaminated with oil.
• Cooling built into the principle. Compression is close to isothermal, and the service liquid warms by only about 6–8 °C as it passes through. Low discharge temperature does not depend on a separate injection system.
• A simpler rotor. A single impeller does the work, without the timing gears and oil-free screw needed to keep its rotors apart.
• A service liquid matched to the process. Concentrated sulfuric acid can seal a dry chlorine duty, and a process solvent can seal a recovery duty. Condensable vapours in the gas even add capacity as they condense in the ring.

Compared with centrifugal compressors

• No surge. As a positive displacement machine, a liquid ring compressor delivers stable flow as discharge pressure changes, with no surge-control system to run.
• Wet and saturated gas. Centrifugal machines prefer clean, dry gas. A liquid ring compressor works with gas saturated with water or vapour.
• Small and changing flows. Kenflo liquid ring compressors start from 0.5 m³/min, well below the flows at which centrifugal designs become practical.

Built-in safety for hazardous gas

Low discharge temperature keeps flammable gas far from its ignition point. When a non-flammable liquid such as water forms the ring, tests show the compressor can act as a flame arrester. These features make the liquid ring a trusted choice for flammable or toxic gases, and Kenflo offers CE- and ATEX-certified options for explosive atmospheres.

Kenflo’s liquid ring compressor range

• CBK for high discharge pressure: 0 to 0.7 MPa(G) at 1 to 150 m³/min. A symmetrical double-suction impeller brings the theoretical axial force to zero, SKF bearings support the shaft at both ends, and the shaft can carry a single or double mechanical seal or a dry gas seal.
• CBA2 for high capacity: 0.5 to 1,400 m³/min at up to 0.25 MPa(G), with axial suction and discharge, one rotating part, and wetted-part materials from cast iron to Hastelloy.
• 2BWX tandem package for higher pressure: an ultimate discharge pressure of 1.3 MPa(G).

Kenflo also supplies complete compression packages, including auxiliary equipment, instrumentation and controls, matched to the process.

Efficiency is not its strongest point, so energy efficiency is worth reviewing on large continuous duties, but on wet, dirty, or hazardous gas, reliability is what keeps a plant running.

Conclusion

Centrifugal, reciprocating and screw compressors each have their place, mostly with clean, dry gas. When process gas is wet, corrosive, flammable, toxic, or laden with vapour, a liquid ring compressor offers what the other types cannot: liquid tolerance, low discharge temperature, no oil in the gas path, and no surge.

Guangdong Kenflo Pump Co., Ltd. has built pumps since 1954 and supplies liquid ring compressors and complete compression packages for chemical, petrochemical and oil and gas plants. To discuss whether a liquid ring compressor suits your process gas, contact the Kenflo engineering team.