A machine that draws in atmospheric air and compresses it into a receiver for use as plant utility air.
01 / Definition
What a Air Compressor is
An air compressor turns electrical power into stored pressure. In a reciprocating machine a piston draws air in through an inlet valve on its down stroke and pushes it past a discharge valve on the up stroke, into a receiver tank. The receiver holds that air so the compressor can cycle on and off rather than run constantly, and a pressure switch starts it at the cut-in setting and stops it at cut-out.
Two consequences follow. Compressing air heats it, and that heat has to go somewhere — into the cooling fins, the oil, and eventually the room. And the water that was in the air condenses as the compressed air cools, which is why every receiver needs draining and why a dryer exists downstream. Most compressor problems trace back to one of those two: heat, or water.
Also called
02 / Anatomy
The parts that matter
The components a technician reaches for when this machine stops working.
Pump block and pistons
Does the compressing. Ring and bore wear shows up as falling delivery and rising oil consumption.
Inlet and discharge valves
Thin reed or plate valves that open and close on every stroke. Among the first components to fail and the easiest to overlook.
Air receiver
Stores compressed air so the motor cycles instead of running continuously. A pressure vessel with statutory inspection requirements.
Pressure switch
Sets cut-in and cut-out. Worn contacts here cause short cycling, which is hard on motor and pump alike.
Unloader valve
Vents head pressure so the motor can restart against no load. A failed unloader is the classic cause of start-up tripping.
Drive and cooling
Belt or direct drive, plus the fan and fins that reject the heat of compression.
03 / Applications
Where it is used
Pneumatic cylinders and tooling, blowing and cleaning, instrument air, and spray or conveying systems.
04 / Failure modes
How it fails
What actually goes wrong on Indian shop floors, and what each fault looks like to the operator.
FM_01
Trips on start
Usually a failed unloader leaving head pressure on the piston, so the motor cannot accelerate. Also check belt tension and supply voltage.
FM_02
Falling delivery
Worn valves, worn rings and bore, a choked inlet filter, or leaks in the plant's own pipework rather than in the compressor.
FM_03
Short cycling
Pressure switch differential too narrow, receiver waterlogged and so effectively smaller, or a system leak the compressor keeps chasing.
FM_04
Oil carryover into the air
Worn rings or an overfilled sump. Downstream it contaminates tools, valves and anything the air touches.
FM_05
Running hot
Blocked fins, failed fan, high ambient temperature, or a machine undersized for a duty that has grown over the years.
05 / Onsite
What our technician checks
A methodical path to root cause, not a scattergun of part swaps.
- 01Record actual cut-in and cut-out pressures and compare them against the plate — not against what the gauge is assumed to read
- 02Listen for air escaping at the unloader after shutdown; it should vent briefly and then stop
- 03Check inlet filter condition and the state of the cooling fins and fan
- 04Drain the receiver and note how much water comes out — a waterlogged receiver shortens cycle time
- 05Measure motor current against nameplate on load and check belt tension and alignment
- 06Walk the distribution pipework with the compressor off and listen for leaks before blaming the machine for low delivery
06 / Get it fixed
Related services
The MachineryFix services that cover this equipment.
07 / Questions
Frequently asked questions
Why does my compressor trip the breaker when it starts?+
Most often a failed unloader valve. If head pressure is not vented after shutdown, the motor has to start against full load and draws heavy current. Check that the unloader vents audibly at stop, then check belt tension and supply voltage.
Why is the compressor running constantly but pressure stays low?+
Either it is no longer delivering — worn valves or rings, a blocked inlet filter — or the plant is leaking more than it makes. Shut the compressor down with the system pressurised and listen: if pressure falls quickly, the problem is in your pipework, not the machine.
How much water should come out of the receiver drain?+
More than most people expect, because compression condenses the moisture already in the air, and humid weather makes it worse. If a receiver has not been drained for weeks it may be part-full, which reduces storage and makes the compressor cycle more often.
Reciprocating or screw — which should I have?+
A reciprocating machine suits intermittent duty and lower volumes and costs less to buy. A screw compressor suits continuous demand, where a piston machine would overheat. If yours runs more than it rests, you have outgrown the type.
Written & reviewed by MachineryFix — last reviewed — questions? Call +91 70133 33007