Air Compressor (motor) — Most Asked Questions
6 questions asked about air compressor (motor) across the General and Fn 4b papers, heaviest first.
Learn the system first
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1 How does unloader work and when does drain unloader open and close?
Answer
The unloader holds the compressor at part load so it does not start and stop: it opens when the receiver is full and closes as it discharges.
- The purpose is to avoid frequent starting and stopping, so the machine holds a steady load instead of cycling.
- The unload valve opens when the discharge reaches the setting — commonly around 24 bar — relieving the pump to the atmosphere.
- It closes again as the receiver falls to the cut-in pressure, so the compressor picks up its full delivery.
- A drain unloader works differently: it opens to drain condensate from the receiver and closes on the next load.
Surveyor follow-up: compressor starting and stopping every few minutes with the unloader proved free — is it the unloader or the cut-in?
Full theory: Air Compressor Working — Cycle, Volumetric Efficiency, Coolers & Capacity -
2 Include this also Unloader activates how
Answer
The unloader holds the compressor off load until delivery pressure is built, then lets it load — by pressure, not by a signal.
- With the compressor stationary the machine is unloaded: with no air pressure, springs keep the unloaders depressed.
- The compressor is started and runs light, and air bleeds through the orifice to the diaphragm.
- The diaphragm then pushes the unloader valves off their seats, the unloaders cease to function and the machine runs on load.
- So the unloader activates on pressure, and the loss of that pressure — or a stuck unloader — is why a compressor will not come on load.
Surveyor follow-up: compressor started but never loading with full delivery pressure — is the orifice, the diaphragm or the unloader?
Full theory: Air Compressor Operations — Lubrication, Drains & Running -
3 Reefer unloader valve qstn (confuses a lot
Answer
On a reefer the unloader is a capacity control: it lets the compressor idle at low load so the box does not get too cold.
- Capacity control is the automation provided to match compressor output to the varying refrigerating load.
- The methods are on/off motor cut-out, variable motor speed, cylinder unloading by cutting in or out cylinders, hot gas bypass to the evaporator, and hot gas injection.
- An unloader valve is the hot gas bypass arrangement — a portion of the discharge gas passes straight to the evaporator, bypassing the condenser.
- That reduces effective capacity without stopping the compressor, which is why it confuses people who expect the compressor to stop.
Surveyor follow-up: reefer box at −22 °C with the compressor running continuously — which capacity control has failed open?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
4 Why heaters in AC compressor Crankcase
Answer
Crankcase heaters keep the oil warm and the machine ready: a compressor started cold is a compressor started with no oil film.
- The heater keeps the sump above the point where the oil is too thick to flow, so the compressor can be started on demand.
- A cold start means the oil is dragged rather than circulated, so the bearings run without their film.
- It also keeps condensate and water from the air outgassing collecting, and stops the crankcase from being cold-started daily.
- The heater runs on a thermostat or is left on continuously when the compressor is laid up, not run only before starting.
Surveyor follow-up: compressor restarted cold with the heater found dead and the oil clean — is the machine at risk, and how?
Full theory: Air Compressor Working — Cycle, Volumetric Efficiency, Coolers & Capacity -
5 Why is ac compressor belt driven?
Also asked: “Why refer compressor are belt driven”
Answer
Belt drive is for isolated mounts and shock: a hermetic unit is direct-coupled, and a belt isolates the motor from the crankcase.
- Air-cooled compressors have fins cast as part of the cylinder casting, with cooling air drawn across them.
- Water-cooled versions have fins forming the cylinder casting with cooling water circulating in the heads.
- Belt drive lets the motor be mounted off the frame, absorbing shock and letting the compressor be removed as a unit.
- Hermetic compressors are direct-coupled with no belt at all — which is why the belt tells you the machine is not hermetic.
Surveyor follow-up: hermetic compressor with a belt drive — is the machine hermetic, or is the drive wrong?
Full theory: Air Compressor Construction — Pistons, Rings, Valves & Running Gear -
6 Leakage in discharge and suction valves Fault or leakage in the unloader Leakage from the relief valve Increase in bumping clearance Wrong setting of compressor auto cut-in and cut-out (too close
Answer
These are the five reasons a compressor loses capacity or never reaches pressure — read the symptom and each one names its own cause.
- First stage suction valve leakage sends air back to the cylinder, so the first stage shows increased delivery pressure from the extra back pressure.
- Second stage delivery valve leakage sends air back during suction, so the second stage shows increased suction pressure and raised temperatures.
- A leaking unloader means the machine never comes properly on load, and a leaking relief valve means the discharge pressure is never held.
- Increased bumping clearance reduces the volume actually compressed, and cut-in and cut-out set too close together means the machine hunts without building pressure.
Surveyor follow-up: first stage delivery pressure high with the second stage suction pressure high — which valve is leaking?
Full theory: Air Compressor Troubleshooting — Interstage Pressure & Fault Finding
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.