ITI/NCVT RAC Mechanic (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Tonne of refrigeration

A cooling rate of 3.517 kW, equal to 3024 kcal/h or 12,000 BTU/h, defined as the heat needed to melt one tonne of ice at 0 °C in 24 hours.

Superheat

The difference between the actual suction line temperature and the saturation temperature at suction pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding correctly.

Subcooling

The difference between the condensing saturation temperature and the liquid line temperature. It indicates how much liquid refrigerant has been cooled below its condensing point and helps verify charge.

Compression ratio

Absolute discharge pressure divided by absolute suction pressure. Ratios above about 10:1 reduce efficiency and call for two-stage compression; always use absolute, not gauge, pressures.

COP of a refrigerator

Coefficient of performance equals refrigerating effect divided by compressor work, (h1 − h4) ÷ (h2 − h1). Higher evaporating temperature and lower condensing temperature raise the COP.

Relative humidity

The ratio of actual water vapour pressure in air to the saturation vapour pressure at the same dry-bulb temperature, expressed as a percentage. Heating air without adding moisture lowers it.

Dew point temperature

The temperature to which air must be cooled at constant pressure for it to become saturated, giving 100% relative humidity. An evaporator below dew point produces condensate and dehumidifies.

Wet-bulb depression

Dry-bulb temperature minus wet-bulb temperature. A large depression means dry air and high evaporative potential; used with a psychrometric chart to find relative humidity.

Gauge versus absolute pressure

Gauge pressure is measured above atmospheric; absolute pressure equals gauge plus atmospheric (about 1.013 bar). Refrigerant tables and compression ratios require absolute pressure.

Micron and evacuation

One micron equals 1/1000 mm Hg. Evacuation below 500 microns with a holding test confirms removal of moisture and non-condensables before charging a system.

Vernier calliper least count

One main scale division divided by the number of vernier divisions. With 1 mm main divisions and 20 vernier divisions the least count is 0.05 mm; with 50 divisions it is 0.02 mm.

Micrometer reading

Add the sleeve reading in 0.5 mm steps to the thimble graduation multiplied by 0.01 mm. A sleeve at 5.5 mm and thimble at 24 gives 5.74 mm.

Nitrogen pressure testing

Leak testing with dry, oil-free nitrogen through a regulator at about 1.5 times working pressure. Never use oxygen or compressed air, which can ignite oil and cause an explosion.

Flashback arrestor

A device fitted on oxygen and acetylene regulator outlets and torch inlets that stops a flame from travelling back into the cylinder, preventing regulator and cylinder explosion.

Asphyxiation risk

Refrigerant vapour is heavier than air and displaces oxygen in pits, basements and plant rooms. Test the atmosphere and ventilate before entry; never rely on smell alone.

Phosgene formation

Burning CFC or HCFC refrigerant in the presence of a flame produces phosgene and hydrogen fluoride, which are highly toxic. Ventilate and evacuate if refrigerant burns.

Ohm's law and power

V equals I times R; power equals V times I, also I²R or V²/R. In series circuits current is common; in parallel circuits voltage is common across branches.

Insulation resistance test

A 500 V megohmmeter test between winding and earth with the motor disconnected. A healthy motor reads at least 1 MΩ; low readings indicate moisture or winding failure.

Run versus start capacitor

The run capacitor stays in circuit with the auxiliary winding; the start capacitor is switched out by a centrifugal switch or potential relay once the motor accelerates.

Star-delta starting

The motor starts in star, where each winding sees voltage divided by √3 and line current is one-third of delta, then changes to delta for full-voltage running.

NTC thermistor sensor

A temperature sensor whose resistance falls as temperature rises, typically 5–10 kΩ at 25 °C. Drift or open circuit causes wrong defrost, fan or compressor control.

Low-pressure switch

A control that cuts out the compressor when suction pressure falls below a set value, protecting against low charge, a restriction or low load; it resets automatically when pressure returns.

High-pressure switch

A safety control that trips the compressor when discharge pressure exceeds its setting, as with a dirty condenser, fan failure or overcharge. Some types require manual reset.

Defrost timer

A motorised or electronic control that periodically stops the compressor and energises the defrost heater in frost-free refrigerators, ending defrost when the termination thermostat reaches its set temperature.

Frost-free refrigerator

A refrigerator with a hidden evaporator, evaporator fan and automatic defrost cycle using a timer, heater and termination thermostat. Ice build-up usually means a failed heater or timer.

Chest versus upright freezer

A chest freezer opens at the top, retains cold air and has longer holdover after power failure. An upright has shelves and loses cold air whenever the door opens.

TXV bulb location

The thermostatic expansion valve bulb is clamped on the suction line at the evaporator outlet, insulated, and kept away from the liquid line so it senses true superheat.

Filter-drier function

A device in the liquid line that removes moisture, acid and dirt from the refrigerant. It must be replaced after a burn-out or whenever the system has been opened.

Refrigerant GWP and ODP

Ozone depletion potential ranks damage to the ozone layer (CFC about 1, HFC zero); global warming potential ranks climate impact with CO2 as 1, for example R-410A about 2088.

POE lubricant handling

Polyolester oil is used with R-134a, R-410A and R-32. It is hygroscopic, so containers stay sealed, the system is evacuated well, and a new filter-drier is fitted after opening.

A2L refrigerant safety

A mildly flammable classification including R-32 and R-1234yf. No open flame or halide torch near the system, ventilate the area, and use spark-free tools and leak detectors.

Refrigerant recovery

Removing refrigerant into a labelled recovery cylinder with a recovery machine instead of venting it to atmosphere. Reclaim to AHRI 700 purity before reuse in another system.