What are centrifugal air compressors?

Centrifugal machines, everyone in the compressor business knows about them, but many people have never touched one. Screw machines are everywhere. Centrifugal machines only show up on big projects.

Compression principle is different. Screw machines are positive displacement, two rotors mesh together squeezing air out.

Centrifugal machines are dynamic type, impeller spins at high speed flinging air outward, flung into a diffuser where velocity turns into pressure.

Surge

Working with centrifugal machines, can't avoid talking about surge.

Flow drops below the critical point, gas backflows in the impeller, whole machine pulsates violently, sound comes in waves, like someone gasping for breath. Run in surge too long, impeller blades will crack, thrust bearing takes the hit first, axial displacement exceeding limits means major overhaul. Seen an old Shengu unit, anti-surge protection failed, ran in surge less than a minute, bearings burned out. Afterward replacing bearings, doing dynamic balancing, realignment, down for nearly two months.

So centrifugal machines must have anti-surge systems. Same approach everywhere, detect flow approaching surge line, open blowoff valve or recirculation valve, pull flow back up. Question is how to define the surge line, how fast the response is.

Old days units used fixed surge line, manufacturer gives a curve, PLC follows it. Very conservative, starts blowing off when still far from actual surge point, high energy consumption. Some projects try to save electricity, have instrument techs adjust surge line lower. Then it actually surges. Not worth it.

Now mainstream is dynamic anti-surge. Inlet temperature changes, speed changes, surge line adjusts accordingly. Atlas Copco's units are aggressive on this, small surge margin, good efficiency numbers, but heard occasional false triggers in the field. Ingersoll Rand's Centac series relatively conservative. Domestic units, Shengu and Shaangu big machines have mature anti-surge logic, small and medium centrifugal machines not many choices, control precision and response speed still gap with imports.

Surge problem determines centrifugal machine applications. Can't use centrifugal machines where flow fluctuates a lot. Anti-surge system constantly activating, blowoff losses too big. Those shops with air demand jumping up and down, stick with screw machines. Centrifugal machines need stable load situations. Steel mill power stations, chemical plant air separation units, running 24 hours continuously, load curves flat as can be. That's centrifugal territory.

Industrial compressor — Centrifugal compressor technology

Flow Range Selection

Below 1,200-1,400 CFM, screw machines dominate completely. Centrifugal machines have no efficiency advantage at this flow range, prices much higher, nobody picks them.

Above around 1,800 CFM, options start appearing. Single screw machine can't do it, need to parallel. Two or three screws in parallel, big footprint, complex piping, control is also troublesome. One centrifugal machine solves the problem, later maintenance also simpler. This flow range, both centrifugal and screw have their markets, depends on specific project requirements.

Past about 3,500 CFM, basically no choice, only centrifugal. Screw machines can't reach that capacity as single units. Paralleling too many not realistic.

One easy pitfall is altitude correction. Centrifugal machines sensitive to inlet conditions. Higher altitude, higher temperature, flow drops. Rated 2,100 CFM at standard conditions, gets to Denver might only deliver around 1,800 CFM. Manufacturer catalog parameters all standard conditions. Selection must factor in correction coefficients. Seen a project, design institute selected straight from catalog, unit arrived on site, found flow wasn't enough, had to add a screw machine to fill the gap. Project review didn't catch this beforehand.

Startup

Centrifugal startup is troublesome. Cold machine needs barring, check if rotor is seized, if there's accumulated liquid. Warmup takes time, speed ramp has requirements. Imported units have comprehensive protection logic, startup process highly automated, press the button and wait. Some older units or small manufacturer products, startup steps have to be done manually one by one, miss one step and it alarms and trips. From pressing start button to full load air delivery, short ones fifteen minutes, long ones half an hour.

Screw machines are simple. Press start button, few dozen seconds to load.

This difference is critical for backup machine selection. Main machine trips, backup needs to take over. Screw machines respond fast, centrifugal startup too slow. Big air supply stations typically configure centrifugal machines as main units, screw machines as backup and peak-shaving.

Compressor station — Air supply station configuration

Maintenance

Centrifugal machine maintenance intervals are long, no question. No oil circuit system, no oil changes, no oil filter changes, daily work is just watching vibration, checking shaft displacement, testing seal gas. Major overhaul interval starts at 15,000-20,000 hours, well-maintained ones running 25,000 hours before opening up also happens.

Screw machines have more to do. Oil change every 3,000-4,000 hours, regular oil filter, air filter, oil separator changes, airend itself has wear cycle.

But one thing, centrifugal machines have problems, repair threshold is high. Screw machine repairs ordinary mechanics can do, change an oil separator element, change a bearing, parts available everywhere. Centrifugal machines different. Involves impeller dynamic balancing, rotor alignment, seal clearance adjustment, can't do it without specialized teams. Parts supply lead times also long. Some imported unit impellers, ordering and waiting three to four months is normal. Major overhaul either calls the manufacturer or specialized overhaul companies. Low-volume models, might not even find parts after ten years.

Maintenance work — Specialized maintenance requirements

Investment costs and such I won't elaborate on. Centrifugal is expensive, installation requirements high, payback period long. Owners can do that math themselves.

Surge

Flow Range Selection

Startup

Maintenance