The workhorse of plant air
If you walk through a typical manufacturing plant and find a compressor room, there is a good chance the main machines are rotary screw compressors. They are popular because they are built to run for long hours, deliver a smooth stream of air and are relatively compact for the amount of air they produce.
How the machine works
A screw compressor has a pair of rotors with helical lobes, one male and one female, that turn in a close-fitting housing. Air enters at the intake end and is trapped between the lobes and the housing. As the rotors turn, the trapped pocket becomes smaller and the air is squeezed, then pushed out at the discharge end at higher pressure.
Most industrial units are oil-injected. Oil is sprayed into the compression chamber to seal the small gaps, cool the air and lubricate the rotors. The air leaving the compressor head is therefore a mix of air and oil. It passes through an air/oil separator that removes the oil, which is cooled, filtered and recirculated. The air continues to an aftercooler and onward to the dryer and the system.
Oil-free (or dry) screw machines use close tolerances and special coatings and do not inject oil into the compression chamber. They are used where oil contamination of the product is a concern, although they typically cost more and require specialist service. Whether air is oil-free at the point of use also depends on the rest of the system.
When a screw compressor suits
Screw compressors tend to make sense when:
- Demand is steady or runs for many hours a day.
- A smooth, pulsation-free flow is preferred.
- The plant wants a reliable machine that can run through several shifts.
- Noise should be lower than that of a comparable piston machine, especially with an enclosed design.
- Space is limited, since they are compact for their output.
They may be less ideal when:
- Use is very light and occasional, since a small piston or scroll unit can be simpler and cheaper.
- The pressure needed is very high; other designs reach higher pressures more readily.
- Frequent start-stop operation is expected, because screw machines are normally designed to run for extended periods rather than cycle often.
Capacity control
How a screw compressor responds to changing demand has a big effect on energy use. Common methods include:
- Load/unload. The machine runs loaded to build pressure, then unloads and idles when the upper pressure is reached. An unloaded machine still draws some power, so it is only partly efficient. A receiver gives more time between cycles.
- Modulation. An inlet valve throttles the intake to match demand. It is smooth but not always efficient at low loads.
- Variable speed. A drive changes motor speed to follow demand, which is useful where demand varies a lot. It makes sense to read about variable speed drives before choosing it. Where demand is steady, it may bring little benefit.
- Start/stop on small units, with timers to prevent too many starts.
Multiple machines are often combined, with a controller that ensures only the needed number run, and often one trim machine that follows the changing load.
Sizing and installation basics
Machines are chosen by delivered air flow at the working pressure, and by the electrical or engine power supply. Installation needs good ventilation to carry away heat, clean intake air, a level base, correct electrical protection, drainage for condensate, and room for service. Fitting a wet receiver, dryer and filters is normal practice. Room ventilation needs special attention because a large part of the input energy appears as heat.
Maintenance of screw compressors
Everything below is a general outline. The manual governs.
Oil
The oil lubricates, seals and cools. It ages with heat and contamination, so it needs periodic replacement at intervals set by the maker and the oil type. Use the specified oil. Mixing types or using unsuitable oil can cause foaming, deposits and shortened life. Check the oil level regularly.
Oil filter
Replaced along with the oil or at its own interval. A blocked filter can starve the machine of oil, which is why a bypass design is often used.
Air/oil separator
The separator element removes oil from the air. As it loads up, pressure drop rises, which increases energy use. If it fails, oil gets into the air system. It is replaced per the manual or when the pressure drop indicates.
Intake filter
Dust reaching the rotors causes wear. Keep the intake filter clean or replace it on schedule, and more frequently in dusty places.
Cooling
Air-cooled units rely on clean cooler surfaces and fans. Dirt on coolers is a leading cause of high temperature shutdowns. Water-cooled units need good water quality and flow.
Drive
Belt-driven units need belt checks. Direct or gear-driven designs need coupling and bearing inspection. Motors need their bearings lubricated as the maker specifies, and electrical connections should be checked for signs of heat. Bearing care in general terms follows the same logic described in guidance on bearing wear patterns.
Condensate and drains
Check drains daily or confirm that automatic ones work.
Controls and safety devices
Check that temperature and pressure sensors, relief valves and alarms function. Regular calibration of gauges and sensors helps keep the controller honest.
Signs that service is due
- Rising discharge temperature.
- Increasing pressure drop across filters or separator.
- Oil in the air lines or rising oil consumption.
- New vibration or noise.
- The machine runs loaded longer for the same demand.
- Alarms on the controller.
Safety
Screw compressors store pressure in the separator vessel and contain hot oil. Always shut down, isolate electrically, lock out and fully depressurize before opening any part. Allow time to cool. Use qualified technicians for internal work, and dispose of used oil responsibly. This guide is general and not engineering advice.