ITI AC Technician Interview Questions and Answers (Refrigeration and AC)

Explain the vapour compression refrigeration cycle.

The compressor compresses low-pressure refrigerant vapour into high-pressure, high-temperature vapour. In the condenser it gives up heat to the outside air and condenses into high-pressure liquid. The expansion device (a capillary tube or expansion valve) drops its pressure. In the evaporator the low-pressure liquid absorbs heat from the room and evaporates, and the vapour returns to the compressor.

How to say it: Go round the cycle in order, saying what happens to pressure and heat at each part.

What is the difference between a capillary tube and an expansion valve?

A capillary tube is a fixed restriction, used in small units such as window ACs and refrigerators. A thermostatic expansion valve (TXV) adjusts the flow by sensing superheat at the evaporator outlet; inverter units commonly use an electronic expansion valve (EEV) controlled by the circuit board.

What are superheat and subcooling?

Superheat is how much the suction vapour temperature is above the refrigerant's boiling (saturation) temperature at evaporator pressure. Subcooling is how much the liquid temperature is below the condensing (saturation) temperature. Both are used to judge whether the charge is correct.

What is one ton of refrigeration?

A cooling capacity of 12,000 BTU per hour, about 3.5 kW.

What is the difference between an inverter and a non-inverter AC?

A non-inverter AC runs its compressor at one speed and switches it on and off with the thermostat. An inverter AC varies the compressor speed to match the load, so it runs more steadily and uses less power at part load.

Which refrigerants have you worked with?

Older ACs use R-22; newer split ACs use R-32 or R-410A, and some use R-290. Domestic refrigerators use R-134a or R-600a. R-32 is mildly flammable, and R-290 and R-600a are flammable, so no open flame near them and good ventilation. Never mix refrigerants: charge only the one on the nameplate.

How to say it: Name only the ones you have actually handled, and mention the nameplate.

Why do you vacuum a system before charging?

To remove air and moisture. Air raises the pressure and reduces cooling; moisture can freeze at the capillary or expansion valve and choke it, and it reacts with oil and refrigerant to form acid that damages the compressor. Use a vacuum pump with the gauge manifold and hold the vacuum to check it does not rise.

How do you test for leaks?

Pressurise the system with dry nitrogen (never oxygen or compressed air, because oxygen with oil can explode), and check every joint with soap solution or an electronic leak detector. Watch the gauge for a pressure drop over time.

How to say it: "Never oxygen" is the safety point they are listening for.

How do you charge refrigerant?

After vacuum, charge the quantity given on the nameplate, by weight on a charging scale, then check pressures, running current and the temperature difference across the indoor coil. R-410A is a blend, so it is charged as liquid.

Why should refrigerant be recovered and not vented?

Many refrigerants damage the ozone layer or add to global warming, and venting wastes refrigerant that can be recovered with a recovery machine into a recovery cylinder.

How do you braze copper pipes?

Clean and fit the joint, then braze copper to copper with a phosphorus-copper rod, which needs no flux. Copper to brass or steel needs a silver brazing rod with flux. Flow a little nitrogen through the pipe while brazing so no oxide scale forms inside.

What is flaring and swaging?

Flaring spreads the pipe end into a 45° cone for a flare-nut joint, as at split AC service valves. Swaging enlarges one pipe end so another pipe of the same size slides into it for brazing. Tighten flare nuts with the right spanners or a torque wrench: over-tightening cracks the flare.

How do you install a split AC?

Mount the indoor unit level on its plate, drill the wall hole with a slight slope outward for the drain, run the pipes, drain and cable, place the outdoor unit on a firm stand with space for airflow, make the flare joints, pressure-test with nitrogen, vacuum, open the service valves, then check current, pressures, drain flow and the air temperature difference.

How to say it: Saying "nitrogen test and vacuum before opening the valves" separates you from installers who skip it.

An AC is running but not cooling. What will you check?

Dirty filter or coils first, then the fans, then the compressor: is it running? Then the capacitor, the gas pressure (a low pressure points to a leak), a choked capillary or filter drier, and whether the AC is the right size for the room.

What does the capacitor do in an AC?

It helps the compressor and fan motors start and run. A weak or bulged capacitor can make the compressor hum and trip on overload, or the fan run slowly. Discharge a capacitor before handling it.

Which tools do you use?

Gauge manifold, vacuum pump, charging scale, nitrogen cylinder with regulator, flaring and swaging tools, tube cutter, pinch-off tool, brazing torch, clamp meter, multimeter, leak detector and a thermometer.

What safety do you follow?

Isolate the power before opening a unit and discharge capacitors. Wear gloves and goggles: liquid refrigerant causes frostbite. No open flame near flammable refrigerants, and ventilate the room. Use a safety harness when installing an outdoor unit at height, and never pressurise with oxygen.