ITI/NCVT RAC Mechanic (AITT Theory) key terms

The quantity of heat absorbed by one kilogram of a liquid at its boiling point when it changes completely into saturated vapour without any rise in temperature. In a refrigeration system this heat is absorbed in the evaporator from the space being cooled.

Heat added to or removed from a substance that changes its temperature but not its state. In air conditioning, cooling air from 30 °C to 20 °C without condensation is sensible cooling, measured by the dry-bulb thermometer.

The ordinary air temperature indicated by a thermometer whose bulb is dry and shielded from radiation. It is used together with wet-bulb temperature to fix the state of moist air on a psychrometric chart.

The lowest temperature reached by a wetted thermometer bulb exposed to moving air, caused by evaporative cooling. It indicates the cooling limit achievable by evaporation and, with dry-bulb temperature, allows humidity and enthalpy to be read from a psychrometric chart.

The ratio, expressed as a percentage, of the actual water vapour present in air to the maximum vapour the air could hold at the same dry-bulb temperature. It describes how close the air is to saturation and affects comfort and evaporator condensation.

The temperature to which moist air must be cooled, at constant pressure and moisture content, for water vapour to begin condensing. A surface colder than the dew point collects condensate, which is why evaporator coils drip water during cooling.

The temperature rise of refrigerant vapour above its saturation temperature at the same pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding correctly and protects the compressor from liquid slugging; typical values are 5 to 8 K.

The temperature drop of liquid refrigerant below its saturation temperature at the same pressure. Measured at the condenser outlet, it shows how much liquid is fully condensed and how much charge is present; typical values are 5 to 10 K.

The four-stage process in which a compressor raises refrigerant vapour pressure, the condenser rejects heat and liquefies it, an expansion device drops its pressure, and the evaporator boils it while absorbing heat from the load. It is the basis of most refrigeration and air-conditioning plants.

The machine that draws low-pressure refrigerant vapour from the evaporator, raises its pressure and temperature, and delivers it to the condenser. Common trade types are reciprocating, rotary, scroll and screw, each with different starting torque and capacity-control characteristics.

The heat exchanger in which high-pressure refrigerant vapour gives up heat to ambient air or water and condenses into liquid. Types include air-cooled wire-and-tube, forced-draught fin-and-tube, and water-cooled shell-and-tube; dirty surfaces raise head pressure and power consumption.

The heat exchanger in which low-pressure liquid refrigerant boils and absorbs heat from the refrigerated space or chilled water. Its surface temperature must stay below the dew point for dehumidification, and below 0 °C it needs periodic defrosting.

A modulating metering device that controls refrigerant flow into the evaporator to maintain a nearly constant superheat at the outlet, using a sensing bulb clamped to the suction line. It also closes to stop flow when the compressor stops.

A long, small-bore tube used as a fixed restriction to drop refrigerant pressure from condenser to evaporator level. It is cheap and reliable but cannot adjust to load changes, so charge quantity must be exact and the system must be fully off before charging.

The volatile fluid that circulates in a refrigeration system, absorbing heat by evaporating at low pressure and releasing it by condensing at high pressure. Selection considers boiling point, latent heat, oil miscibility, toxicity, flammability and ozone and global-warming impact.

A relative number expressing how strongly a substance destroys stratospheric ozone compared with CFC-11, which is taken as 1.0. HCFCs such as R-22 are about 0.055; HFCs such as R-134a are zero, which is why CFCs and HCFCs are phased out.

A relative measure of the heat trapped in the atmosphere over a set period by a gas compared with carbon dioxide, which is 1. Refrigerant HFCs such as R-404A and R-410A have values in the thousands, driving the shift to lower-GWP alternatives.

The international treaty, agreed in 1987 and since amended, that schedules the phase-out of ozone-depleting substances including CFCs and HCFCs. Its Kigali Amendment later added controls on high-GWP hydrofluorocarbons, shaping refrigerant choice in trade practice.

The 2016 amendment to the Montreal Protocol that sets a phased global reduction in the production and use of hydrofluorocarbons, which are potent greenhouse gases. It obliges technicians to handle HFCs carefully and to adopt low-GWP refrigerants.

The exact mass of refrigerant a system must contain for correct operation, stated on the nameplate. Undercharge causes low suction pressure and bubbling in the sight glass; overcharge raises head pressure and can flood the compressor with liquid.

A small transparent window fitted in the liquid line, often with a moisture indicator, used to observe refrigerant flow. Bubbles suggest a shortage of charge or a restriction, while a colour change in the indicator warns of moisture in the circuit.

A replaceable device in the liquid line containing a desiccant and strainer that traps moisture, acid and dirt circulating with the refrigerant. It protects the expansion device and compressor, and must be replaced whenever the circuit has been opened.

The process of removing air and moisture from a refrigeration circuit with a vacuum pump before charging, usually down to 500 microns or less. A deep, held vacuum confirms no leaks and prevents acid formation and ice blocking at the metering device.

Checking a circuit for refrigerant loss using soap solution, electronic leak detectors, fluorescent dye or nitrogen pressure tests. It is done before evacuation and charging, and its results decide whether joints must be remade by brazing.

The discharge pressure of the compressor, read on the high-pressure gauge, which corresponds to the condensing temperature. It rises with dirty condensers, high ambient temperature, air in the system or overcharge, increasing current draw and shortening compressor life.

The low-side pressure at the compressor inlet, corresponding to evaporating temperature. It falls with a restricted expansion device, frosted coil, low load or undercharge, and rises with overcharge or a faulty compressor valve.

An electrical resistance heater fitted around the compressor sump to keep oil warm while the compressor is stopped, preventing refrigerant from dissolving in the oil. It reduces oil dilution and liquid slugging on start-up and should be energised before starting.

Oil charged into the compressor to lubricate bearings, seal clearances and carry away heat. Mineral oil suits CFC and HCFC systems, while synthetic polyolester oil is required with HFC refrigerants because it is hygroscopic and must not be left exposed to air.

The removal of frost from an evaporator operating below 0 °C, done by stopping the compressor, by electric heaters, or by hot gas. A timer or demand control initiates it and a termination thermostat ends it, preventing loss of airflow and capacity.

A temperature-actuated switch that opens or closes an electrical circuit to hold a space or component within a set range. In domestic refrigerators it cycles the compressor; in air conditioners it may be part of a room control or PCB input.

An electrically operated switch with a coil and heavy-current main contacts, used to start and stop compressors, fans and heaters from a control signal. The coil circuit is protected by a fuse or MCB, and pitted contacts cause voltage drop and overheating.

A protective device that senses excess current or temperature in a motor circuit and opens the control circuit to stop the motor. Thermal types use a bimetal strip; they must be reset only after the cause of overload is found.

A capacitor connected in series with the start winding of a single-phase motor to increase starting torque, then disconnected by a centrifugal switch or relay. A failed capacitor causes the motor to hum and trip on overload without rotating.

A continuously energised capacitor that keeps a permanent-split capacitor motor in phase, improving torque, power factor and quiet running. It stays in circuit during normal operation, unlike the start capacitor, and a weak one reduces speed and raises current.

A portable instrument combining voltmeter, ammeter and ohmmeter functions, used to check supply voltage, motor current draw and winding continuity. Correct range and function selection before probing a live circuit is an essential safety habit.

An instrument with split jaws that measure alternating current by surrounding a single conductor, without breaking the circuit. It is used to check compressor and fan running current against the nameplate value when diagnosing overload trips.

An insulation resistance tester that applies a high DC voltage, commonly 500 V, between a winding and earth to measure leakage in megohms. Low readings indicate moisture, damaged insulation or a grounded compressor, and the circuit must be isolated and discharged before testing.

A two-gauge, two-valve tool with colour-coded hoses connected to low and high side service ports. It reads suction and discharge pressures and allows evacuation, charging and pressure testing while the hoses and valves control the direction of flow.

A safety procedure in which the electrical supply is switched off, the isolator is locked and a warning tag is attached before work begins. It prevents accidental energising of equipment during servicing and is mandatory before opening any control panel.

Wearable safety gear such as goggles, gloves, safety shoes and ear protection that shields the technician from specific hazards. For refrigeration work it includes face and hand protection against liquid refrigerant burns and eye protection during brazing and grinding.

Joining copper pipe by heating it and melting a filler alloy, such as silver-phosphorus rod, which flows into the joint by capillary action at a temperature above 450 °C. Nitrogen purging during brazing prevents internal oxide scale that would clog the circuit.