ITI/NCVT RAC Mechanic (AITT Theory) cheat sheet

Personal protective equipment for RAC work: safety goggles when brazing or using nitrogen, heat-resistant gloves for hot lines, insulated gloves (rated 1000 V) for electrical work, ear protection in compressor rooms, safety shoes with steel toe.

Refrigerant cylinder safety: never fill a cylinder above 80% of its water capacity; cylinders are hydro-tested (typically every 5 years), stored upright, below 52 °C, away from heat and oxidizers, and secured against falling.

Oxygen and acetylene: store separately with at least 3 m between them and 3 m from open flame; use flashback arrestors on both regulators; never use oil or grease on oxygen fittings - spontaneous ignition risk.

Nitrogen pressure testing: use dry nitrogen with a pressure regulator, never connect a cylinder directly to the system; test pressure per manufacturer (commonly 1.5× working pressure); oxygen-free nitrogen (OFN) only, never oxygen.

Asphyxiation hazard: refrigerants are heavier than air and displace oxygen in pits and basements; R-12/R-22/R-134a/R-410A all require ventilation and O2 monitoring before entry into confined spaces.

Decomposition products: brazing or a flame on refrigerant creates phosgene (from CFC/HCFC) and hydrogen fluoride; ventilate, never inhale smoke from a burnt refrigerant line, and evacuate on a strong pungent odour.

Electrical lockout/tagout: isolate, lock, tag, verify dead with a tester on the actual conductors, and discharge capacitors in inverter and PSC circuits before touching terminals.

First aid for refrigerant contact: frostbite - flush with lukewarm (not hot) water, do not rub; eye splash - irrigate 15 minutes; liquid refrigerant on skin causes instant freeze burns.

Fire classes for the trade: Class A (paper/wood), B (flammable liquids and gases including LPG and A2L refrigerants), C (electrical), D (metals); CO2 extinguishers suit electrical, dry powder suits gas.

Micrometer reading: main scale (sleeve) in 0.5 mm increments + thimble graduation × 0.01 mm; total = sleeve + thimble reading. Example: 5.5 mm sleeve + 24 thimble = 5.74 mm.

Vernier calliper: least count = 1 main scale division (1 mm) ÷ number of vernier divisions (commonly 20) = 0.05 mm, or 50 divisions = 0.02 mm. Reading = main scale before zero + vernier division coinciding × LC.

Dial gauge/vernier height gauge used to check shaft runout, coupling alignment and compressor shaft end play; magnetic base holds the dial indicator steady.

Tube cutters: score-and-break is not acceptable for refrigeration copper; use a wheel cutter and ream the burr, then blow with dry nitrogen to remove chips before assembly.

Flaring: flare height must fill the cone; over-flaring cracks the flare, under-flaring leaks; use a proper flaring block, lubricate the cone, and torque per manufacturer (e.g., 1/4 in ≈ 15–20 N·m).

Torque wrench: for flare nuts and valve bodies; over-torque deforms copper and causes leaks, under-torque leaks; always back off to zero after use.

Brazing filler selection: copper-to-copper uses BCuP (phosphorus) with no flux; copper-to-brass/steel uses BAg (silver) with flux; never use soft solder (Sn-Pb) on high-side discharge lines.

Leak detection tools: electronic halogen leak detector (sensitivity in g/yr), soap solution on joints, UV dye with black light, and pressure hold test; never use a flame (halide torch) on A2L refrigerants.

Double-stage vacuum: pull below 500 microns (0.5 mm Hg) and hold with the pump isolated; a rising micron gauge indicates moisture or a leak. Micron = 1/1000 mm Hg.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In a series circuit current is common, voltage divides; in parallel voltage is common, current divides.

Single-phase power: P = V × I × cos φ; three-phase: P = √3 × V × I × cos φ. cos φ is power factor; a fully resistive heater has cos φ = 1.

Motor current rules of thumb: single-phase ≈ 5–6 A per kW at 230 V; three-phase ≈ 1.5–2 A per kW at 415 V; locked-rotor current is typically 5–7× running current.

Resistance temperature dependence: for copper and most metals R rises with temperature; a hot compressor winding reads higher resistance than a cold one - compare with manufacturer values at 25 °C.

Insulation resistance: use a 500 V megohmmeter; a healthy motor reads ≥ 1 MΩ to earth (often ≥ 50–100 MΩ when dry); a low value indicates moisture or winding breakdown.

Capacitors: run capacitor stays in circuit; start capacitor is switched out by a potential relay or centrifugal switch. A bulged or leaky capacitor must be replaced with the same µF and equal/higher voltage rating.

AC vs DC: alternating current reverses at supply frequency (50 Hz in India = 50 cycles/s, 20 ms period); RMS × √2 = peak value; 230 V RMS has ≈ 325 V peak.

Star-delta: in star each winding sees V/√3 and current is 1/3 of delta line current; delta gives full voltage per winding. Star-delta starting reduces starting current to about one-third.

Contactor and relay: contactor switches the load, relay switches the coil circuit; overload relay (thermal or electronic) protects the motor, not the wiring.

Multimeter usage: select function and range above the expected value; voltage measured in parallel (high impedance), current in series (low impedance) - never measure current across a supply.

Clamp meter: measures current by magnetic field around a single conductor; clamp only one conductor at a time - clamping live and neutral together reads near zero.

Megohmmeter (insulation tester): test between winding and earth and between windings with the motor disconnected; never test electronic boards or inverter modules with high voltage.

Thermistor (NTC) sensors: resistance falls as temperature rises; typical refrigerator sensor ≈ 5–10 kΩ at 25 °C; a drift causes wrong defrost or compressor cycling.

Thermostat: opens and closes on temperature with a differential; a cold control thermostat with a differential of 2–4 K starts and stops the compressor within that band.

Pressure switch (LP/HP): low-pressure switch protects against low charge and low suction; high-pressure switch trips on condenser blockage or fan failure; both have fixed cut-out/cut-in settings.

Defrost timer: motorised or electronic, cycles compressor off and defrost heater on for a set period (e.g., 8 h cumulative run, 20–30 min defrost) with a termination thermostat.

PTC and NTC starters: PTC start device increases resistance when hot, removing the start winding; used on small hermetic compressors; check with an ohmmeter for open or short.

Overload protector: bimetal disc or electronic; opens on current or temperature; a tripped protector must be allowed to cool before testing; repeated trips indicate a failing compressor or wrong supply voltage.

Sensible heat changes temperature: Q = m × c × ΔT. Latent heat changes state at constant temperature: Q = m × h_fg. Total = sensible + latent.

1 tonne of refrigeration (TR) = 3.517 kW = 3024 kcal/h = 12,000 BTU/h = 12000/200 = 60 kcal/min. This is heat absorbed to melt 1 tonne of ice at 0 °C in 24 h.

Pressure units: 1 bar = 100 kPa = 14.5 psi = 750 mm Hg; 1 atm = 1.013 bar = 760 mm Hg; gauge pressure + atmospheric = absolute pressure.

Temperature conversions: °F = (°C × 9/5) + 32; K = °C + 273.15; a temperature difference of 1 K equals 1 °C.

Psychrometry: dry-bulb (DB) is ordinary temperature; wet-bulb (WB) is the lowest temperature achievable by evaporation; dew point is the temperature at which air saturates (RH = 100%).

Relative humidity = (actual vapour pressure ÷ saturation vapour pressure at same DB) × 100; RH falls when air is heated without adding moisture.

Sling psychrometer: wet-bulb depression (DB − WB) plus DB gives RH from a psychrometric chart; a large depression means dry air.

Dew point and coil temperature: if the evaporator surface is above the dew point, no condensate forms (sensible cooling only); below dew point gives dehumidification.

Superheat = suction temperature − evaporating (saturation) temperature; subcooling = condensing (saturation) temperature − liquid line temperature. These are the key charging indicators.

Vapour compression cycle stages: (1) compression, (2) condensation (reject heat, high pressure), (3) expansion (throttle to low pressure), (4) evaporation (absorb heat).

Four key pressures/temperatures: evaporating temperature, condensing temperature, suction pressure, discharge pressure; superheat at evaporator outlet, subcooling at condenser outlet.

COP = refrigerating effect ÷ work of compression = (h1 − h4) ÷ (h2 − h1). Higher evaporating temperature and lower condensing temperature improve COP.

Compression ratio = absolute discharge pressure ÷ absolute suction pressure. Above about 10:1 single-stage becomes inefficient - use two-stage or compound systems.

Refrigerating effect = h1 − h4 (enthalpy of vapour leaving evaporator minus enthalpy of liquid entering evaporator).

Liquid line: filter-drier removes moisture and dirt; sight glass shows moisture indicator and bubbles (bubbles = undercharge or flash gas).

Suction line: accumulator prevents liquid floodback; suction filter protects the compressor; crankcase heater keeps oil warm and drives off refrigerant during off cycle.

Hot gas line and discharge line: oil separator returns oil to the crankcase; discharge muffler reduces pulsation; fusible plug or relief valve protects against overpressure.

Heat pumps and defrost: reversing valve swaps evaporator and condenser for heating; defrost is by hot gas, electric heater or off-cycle - terminated by a thermostat or pressure switch.

Refrigerant families: CFC (R-12), HCFC (R-22), HFC (R-134a, R-32, R-410A blend of R-32/R-125 50/50), HFO (R-1234yf, R-1234ze, low GWP).

Ozone depletion potential (ODP): CFC ≈ 1, HCFC ≈ 0.05, HFC and HFO = 0. Global warming potential (GWP, CO2 = 1): R-12 ≈ 10,900, R-22 ≈ 1,810, R-134a ≈ 1,430, R-410A ≈ 2,088, R-32 ≈ 675, R-1234yf ≈ 4.

Montreal Protocol phases out ozone-depleting substances; the Kigali Amendment targets HFC phase-down. India's HCFC phase-out schedule follows the accelerated phase-out.

R-22 operates with mineral oil; R-134a, R-410A and R-32 require synthetic POE/polyolester oil; POE is hygroscopic - keep containers sealed and change the filter-drier after a burn-out.

Never mix refrigerants or top up a system with a different refrigerant; blends such as R-410A must be charged as liquid (not vapour) to avoid fractionation.

R-410A operates at about 1.6× the pressure of R-22 and requires POE oil and dedicated gauges/manifold; never use R-22 gauge sets or hoses on R-410A.

A2L refrigerants (R-32, R-1234yf) are mildly flammable: no open flame near the system, no halide torch leak testing, ventilate, and use spark-free tools.

Recovery and recycling: recover refrigerant into a recovery cylinder, never vent to atmosphere; label the recovered refrigerant; reclaim to AHRI 700 purity before reuse.

Oil properties: viscosity grade (e.g., 32, 68), miscibility with refrigerant, dielectric strength, floc point and wax content; wrong oil causes carbon deposits, bearing wear and acid formation.

Reciprocating compressor: piston and valve plates; valves leak causes low capacity and high discharge temperature; check valve plate, reed valves and gaskets.

Rotary (rolling piston) compressor: used in small room ACs; fewer parts, higher volumetric efficiency; check for worn roller or vane and for correct oil charge.

Scroll compressor: two interleaved spirals, no suction or discharge valves; quieter and higher efficiency; reverse rotation gives no pressure build-up - check phase sequence.

Hermetic compressor motor cooling: suction gas cools the motor; low charge or high superheat causes motor overheating; winding resistance balanced across terminals in a healthy three-phase motor.

Condenser types: air-cooled (natural/forced), water-cooled (shell and tube, tube-in-tube), evaporative; approach = condensing temperature − ambient (air) or − leaving water (water).

Evaporator types: flooded, dry expansion (DX), fin-and-tube, plate, shell-and-tube; starved evaporator from a clogged expansion valve gives high superheat and low capacity.

TXV (thermostatic expansion valve): controls superheat using bulb pressure vs evaporator pressure and spring; bulb must be clamped on the suction line at the evaporator outlet, insulated, and not near the liquid line.

Capillary tube: fixed restriction; length and bore matched to the system; a kinked capillary gives high head pressure and low suction; never cut or braze a capillary without restoring its length.

Accessories: filter-drier, sight glass, solenoid valve, check valve, receiver, accumulator, oil separator, pressure switches, and vibration eliminator; each has a defined location in the circuit.

Domestic refrigerator: single-door (direct cool, defrost manually) and frost-free (no-frost, automatic defrost with timer, heater and evaporator fan).

Direct cool: evaporator is the freezer plate; frost builds on it; defrost by switching off or a push-button; capillary tube and small hermetic compressor with PTC start.

Frost-free: evaporator hidden behind a panel; a defrost timer/heater/termination thermostat cycle removes frost; a failed defrost heater causes ice build-up and no cooling.

Deep freezer: chest (top-opening, holds cold air, longer holdover) and upright (front-opening, shelves, more cold-air loss on opening); operating temperature typically −18 °C to −24 °C.

Water cooler: refrigeration circuit plus a water tank and bubbler; a thermostat controls water temperature; scale in the tank reduces capacity; check the tank for leaks and the bubbler for hygiene.

Bottle cooler and visi cooler: glass-door display units with forced-air evaporator; condensation on glass is normal; a failed heater or door gasket causes fogging and higher load.

Refrigerator capacity rule: compressor running continuously but not cooling - check for leak/low charge, clogged capillary, faulty compressor valve, or defrost heater stuck on.

Temperature settings: fresh food compartment 3–5 °C, freezer −18 °C or lower; a refrigerator that is overfilled or has a bad door gasket loses capacity.

Window AC: single package, evaporator indoors and condenser outdoors in one casing; check for correct tilt for condensate drain, clean filter, and condenser coil.

Split AC: indoor evaporator unit and outdoor condensing unit connected by flare or brazed lines; typical line lengths and elevation limits per manufacturer; nitrogen purge while brazing.

Cassette and ductable units: ceiling-mounted cassette distributes air four ways; ductable has a static pressure rating and needs correctly sized ducts, dampers and return air.

Package unit: all components in one cabinet, air-cooled condenser outside; used for large halls; check belt, blower and economiser.

Central plant: chilled water system with chiller, pumps, AHUs, cooling tower and piping; chilled water supply/return typically 7/12 °C; cooling tower approach and range are the key performance figures.

Cooling load factors: orientation, glass area, occupancy, lighting, equipment, infiltration; 1 TR per 10–15 m² is a rough rule for Indian conditions (varies with load).

Air distribution: supply, return and fresh air; check static pressure, air velocity and balancing dampers; a dirty filter reduces airflow and can freeze the evaporator coil.

Charging a split system: weigh-in for critical charge, superheat method for TXV, subcooling method for fixed orifice; never charge by guesswork on a running system.

Installation sequence: site survey, bracket/stand fixing, indoor unit mounting with drain slope, line routing, brazing with nitrogen purge, pressure test, evacuation, charging, then electrical connection and commissioning.

Evacuation: use a two-stage vacuum pump and a micron gauge; pull to below 500 microns and hold for 10–15 min; a rise indicates a leak or moisture.

Pressure test: dry nitrogen at 1.5× working pressure (commonly 350–550 psi for R-410A), hold and check for leaks; never use oxygen or compressed air.

Commissioning checks: supply voltage, running current vs nameplate, superheat/subcooling, air flow, temperature difference across evaporator (typically 8–12 °C), and drain test.

Troubleshooting rules: high head pressure - dirty condenser, fan failure, overcharge, non-condensable gas; low suction - low charge, restriction, faulty TXV, weak compressor.

Compressor trips on overload: check supply voltage, capacitor, winding resistance, insulation resistance, and mechanical condition (locked rotor); a hot compressor must cool before resistance reading.

Ice on suction line: low charge, restricted capillary, or blower/fan failure; ice on the whole evaporator often means low airflow, not low refrigerant.

Preventive maintenance: clean condenser and evaporator coils, clean or replace filters, check refrigerant charge and superheat, tighten electrical terminals, check drain pan and gaskets, and record readings.

Refrigerant recovery before service: recover, do not vent; use recovery machine and cylinder; check for leaks with electronic detector or soap solution after reassembly.

Single-phase PSC motor: run capacitor in series with the auxiliary winding; a weak capacitor causes low torque and overheating; test with a capacitance meter (µF) and check for bulging.

CSIR and CSCR motors: start capacitor switched by a centrifugal switch or potential relay; a failed start capacitor gives no starting torque and a humming motor.

Three-phase motor: check phase sequence for scroll and screw compressors; reverse rotation gives no pressure build-up and can damage the compressor; use a phase sequence tester.

Starters: DOL for small motors (up to about 5 HP), star-delta for larger, soft starter and VFD for controlled acceleration; overload relay setting = motor full-load current.

Wiring diagram reading: L and N supply, contactor coil circuit, thermostat in series with the compressor contactor coil, overload contact in series, fan and valve outputs; trace from supply to load.

PCB in split ACs: indoor PCB handles display, fan, louver and communication; outdoor PCB handles compressor, fan and sensors; check for burnt tracks, swollen capacitors, and communication error codes.

Inverter AC: converts AC to DC, then variable frequency and voltage to the compressor; a three-phase inverter compressor cannot be checked with a simple ohmmeter alone - check winding balance and insulation, and read the error code.

Sensors and error codes: room and coil thermistors, high-pressure switch, and communication faults; always check sensor resistance against the temperature-resistance table before replacing the PCB.

Earthing and supply: verify earth continuity, correct voltage (230 V ±10% single phase, 415 V three phase), and use an MCB/RCCB of correct rating; loose neutral causes erratic control operation.