12.1 Air Compressor Types, Operation, and Maintenance
Key Takeaways
- Compressors divide into positive displacement types, such as reciprocating and rotary screw, and dynamic types, such as centrifugal.
- Multi-staging with intercooling reduces the work of compression and keeps discharge temperature within safe limits.
- A reciprocating compressor is intermittent-duty by nature, while an oil-flooded rotary screw is designed for continuous full-load running.
- Surge is the characteristic instability of a centrifugal compressor operating at too low a flow, and it is prevented with an antisurge or blowoff control.
- Valve failure is the dominant maintenance item on reciprocating compressors and shows up as high discharge temperature and lost capacity.
Compressors sit inside the Steam Systems domain
Module 32403, Compressors and Pneumatic Systems, is grouped by the specification into the 16-item Steam Systems domain along with the boiler and tower modules. A candidate who studies "steam" and skips compressors has left part of the joint-largest domain uncovered.
The compressor families
| Family | Type | Principle | Character |
|---|---|---|---|
| Positive displacement | Reciprocating (piston) | A piston reduces a trapped volume | High pressure capability; pulsating flow; intermittent duty |
| Rotary screw | Meshing helical rotors carry and compress air | Continuous duty, smooth flow, compact | |
| Rotary vane | Sliding vanes in an eccentric rotor | Simple, moderate capacity | |
| Scroll | Two interleaved spirals | Oil-free, quiet, small capacity | |
| Dynamic | Centrifugal | An impeller adds velocity, a diffuser converts it to pressure | Very high volume; oil-free air; subject to surge |
| Axial | Rows of rotating and stationary blades | Very high volume at moderate ratio |
The organizing idea matches the pump chapter: a positive displacement machine delivers a nearly fixed volume per cycle regardless of discharge pressure, while a dynamic machine's flow varies with the pressure it is working against.
Staging and intercooling
Compressing air heats it, and the heat of compression both wastes energy and limits how far a single stage can go. Multi-staging splits the compression ratio between two or more stages with an intercooler between them.
Cooling the air between stages does two things:
- Reduces the volume entering the next stage, so less work is needed for the same final pressure.
- Keeps the discharge temperature within safe limits for the lubricant and the valves.
An aftercooler then cools the final discharge, which is the first and most important moisture removal step in the whole system — cooling the air condenses out a large fraction of the water it is carrying.
Reciprocating compressors
| Component | Function and maintenance note |
|---|---|
| Suction and discharge valves | Automatic, pressure-actuated plate or channel valves; the dominant wear item |
| Piston and rings | Compression rings plus rider rings in non-lubricated designs |
| Crosshead | Separates the rod's side load from the piston in a heavy-duty design |
| Packing (rod packing) | Seals the piston rod where it leaves the cylinder |
| Unloaders | Hold suction valves open so the compressor turns without compressing, for unloaded starting and capacity control |
| Intercooler and aftercooler | Remove heat between and after stages |
Valve failure is the diagnosis to reach for first on a reciprocating machine. A leaking discharge valve recirculates hot air back into the cylinder, so the symptoms are high discharge temperature, reduced capacity, and higher power for the same output. A leaking suction valve gives similar symptoms with heat concentrated on the suction side. Comparing cylinder discharge temperatures between identical cylinders is a fast field diagnostic.
Other reciprocating specifics:
- Machines are intermittent duty. A typical air-cooled reciprocating unit is designed for a duty cycle well under 100 percent, and running it continuously overheats it.
- Crankcase oil level and type matter; compressor oil is not motor oil.
- Excess condensate in the crankcase points to short cycling that never lets the unit reach operating temperature.
- Knocking may be a loose rod bearing, excessive clearance, a broken valve, or liquid carryover.
Rotary screw compressors
Two meshing helical rotors carry air along and reduce the volume between the lobes.
- Oil-flooded (lubricated) designs inject oil for sealing, cooling, and lubrication, then recover it in an oil separator. They are the industrial standard for continuous plant air.
- Oil-free designs use timing gears so the rotors do not touch, and produce air with no lubricant carryover for instrument and process service.
Key components: the airend, the inlet (modulating) valve, the minimum pressure / check valve that maintains enough internal pressure for oil circulation, the oil separator element, the oil cooler and aftercooler, and the thermal valve that bypasses the oil cooler until the unit warms up.
Maintenance revolves around the separator element differential pressure, oil condition and change intervals, coolers kept clean, and air filter condition. High oil carryover points to a failing separator element, an overfilled sump, or a scavenge line blockage.
Centrifugal compressors and surge
A centrifugal compressor's flow falls as the discharge pressure it must overcome rises. Below a certain flow, the machine can no longer sustain the pressure it has built, and the flow momentarily reverses — then re-establishes, then reverses again. This oscillation is surge.
Surge is violent. It produces loud pulsation, rapid axial thrust reversals, and can destroy the thrust bearing and the impeller in a short time. It is prevented with an antisurge control that maintains minimum flow by recycling or blowing off air whenever operating flow approaches the surge line.
A related but distinct condition is choke (stonewall), at the opposite extreme, where flow reaches sonic velocity in the passages and cannot increase further.
Capacity control
| Method | How it works | Efficiency at part load |
|---|---|---|
| Start / stop | Motor cycles on demand | Good if cycling is infrequent; hard on the motor |
| Load / unload | Machine runs continuously, unloading at setpoint | Moderate; depends heavily on receiver volume |
| Inlet modulation | Throttles the inlet valve | Poor; the machine still absorbs substantial power unloaded |
| Variable speed drive | Motor speed tracks demand | Best across a wide range of part loads |
| Multi-step (unloaders) | Steps a reciprocating unit between capacities | Good on reciprocating machines |
Adequate receiver volume is what makes load/unload and start/stop control work. Too little storage and the machine cycles rapidly, which wastes energy and shortens component life.
A two-stage reciprocating compressor shows rising discharge temperature, reduced delivered capacity, and higher power draw for the same output. What should be investigated first?
Why is intercooling used between the stages of a multi-stage compressor?
A centrifugal air compressor begins to pulsate loudly with rapid pressure oscillation as plant demand falls. What is occurring, and what prevents it?