RRB ALP Mechanic RAC Trade — Complete CBT-2 Study Guide

• The basic vapor-compression refrigeration cycle has four main components in sequence: Compressor (compresses low-pressure refrigerant vapor into high-pressure, high-temperature vapor), Condenser (rejects heat to surroundings, refrigerant condenses to high-pressure liquid), Expansion device (throttles high-pressure liquid to low pressure, causing a temperature drop), Evaporator (absorbs heat from the space being cooled, refrigerant vaporizes back to low-pressure vapor, and the cycle repeats). • Heat always flows from a warmer body to a cooler one naturally — refrigeration forces heat to flow the OPPOSITE direction (from the cooled space to the warmer surroundings) by doing mechanical work on the refrigerant via the compressor. • Superheat (temperature of vapor above its saturation/boiling point at that pressure) at the evaporator outlet ensures only vapor (no liquid droplets) enters the compressor — liquid refrigerant entering a compressor can cause serious mechanical damage ("liquid slugging"). • Subcooling (temperature of liquid below its saturation point at that pressure) at the condenser outlet ensures only liquid (no vapor bubbles) reaches the expansion device, for efficient, predictable throttling.

• Compressor types: Reciprocating (piston-based, common in window/split ACs and smaller systems), Rotary (compact, common in modern split ACs, generally quieter and more efficient than reciprocating at similar capacity), Scroll (two interleaving spiral elements, used in many modern residential/commercial systems, smooth operation with fewer moving parts than reciprocating). • Split AC system separates the noisy/hot components (compressor, condenser) into an outdoor unit and the cooling components (evaporator, blower) into an indoor unit, connected by refrigerant piping — reduces indoor noise and heat compared to a window AC unit. • Capacity of an AC unit is commonly rated in "Tons of Refrigeration" (1 Ton = 3.5 kW cooling capacity, historically defined as the cooling effect of melting one ton of ice in 24 hours) — correct capacity sizing for room size is essential for both comfort and energy efficiency. • Energy Efficiency Ratio (EER) or the modern ISEER (Indian Seasonal Energy Efficiency Ratio) rates how much cooling is delivered per unit of electrical energy consumed — higher EER/ISEER means better energy efficiency for the same cooling output.

• Refrigerants have evolved due to environmental concerns: CFCs (Chlorofluorocarbons, e.g. R-12) — ozone-depleting, now banned; HCFCs (e.g. R-22) — less ozone-depleting but still being phased out; HFCs (e.g. R-410A, R-134a) — do not deplete ozone but have high Global Warming Potential (GWP); newer refrigerants (R-32, R-290/propane) offer lower GWP alternatives. • A good refrigerant should have: appropriate boiling point/pressure characteristics for the application, chemical stability, non-toxicity, non-flammability (though some newer low-GWP refrigerants like R-290 ARE mildly flammable, requiring specific handling precautions), and good compatibility with system materials/lubricants. • Refrigerant charge (correct quantity in the system) is critical — undercharging reduces cooling capacity and can cause the evaporator to ice up; overcharging can cause liquid to enter the compressor and elevated discharge pressure/temperature. • Refrigerant recovery (capturing refrigerant from a system before servicing, rather than releasing it to atmosphere) is both an environmental requirement and standard professional practice in modern RAC servicing.

• Insufficient cooling — common causes: low refrigerant charge, dirty/blocked condenser or evaporator coils (restricting heat exchange), faulty/undersized compressor, or a malfunctioning expansion device (not metering refrigerant correctly). • Evaporator coil icing up — usually indicates low refrigerant charge, restricted airflow (dirty air filter, blocked evaporator fins), or a faulty expansion device allowing too little refrigerant flow — all reduce evaporator temperature below the normal operating range. • High discharge pressure — commonly caused by a dirty/blocked condenser (poor heat rejection), refrigerant overcharge, or non-condensable gases (air) trapped in the system. • Compressor not starting — check for power supply issues first, then overload protector trip (may indicate an underlying electrical or mechanical fault), then capacitor failure (common in single-phase compressor motors that rely on a start/run capacitor).