Odisha CHSE +2 VOC Field Technician Air Conditioner cheat sheet

Tonn of refrigeration (TR) = 3.517 kW = 210 kJ/min = 12,000 BTU/h; 1 TR is the heat absorbed to melt 1 short ton (907 kg) of ice in 24 h.

Carnot COP (refrigerator) = T_L / (T_H − T_L); Carnot COP (heat pump) = T_H / (T_H − T_L); temperatures must be in kelvin.

Second law of thermodynamics: heat flows spontaneously from hot to cold; a compressor is required to move heat from low-temperature to high-temperature space.

Pressure–enthalpy (p–h) diagram: isenthalpic expansion is a vertical line; compressor work = h2 − h1; refrigerating effect = h1 − h4.

Sensible heat Q = m·c·ΔT; latent heat Q = m·h_fg; total heat = sensible + latent.

Dry-bulb temperature (DBT) is measured by an ordinary thermometer; wet-bulb temperature (WBT) is the lowest temperature attainable by evaporative cooling; DBT ≥ WBT always.

Relative humidity RH = (partial pressure of water vapour / saturation pressure at same DBT) × 100%; dew point is the temperature at which air becomes saturated (RH = 100%).

Psychrometric chart: horizontal axis = DBT, curved saturation line = 100% RH; comfort zone in India is typically 24–27 °C DBT and 40–60% RH.

Air-conditioning processes: cooling and dehumidification (cooling coil below dew point), heating and humidification, evaporative cooling (increases humidity, lowers DBT).

ASHRAE numbering: R-12 CCl2F2, R-22 CHClF2, R-134a CH2FCF3, R-32 CH2F2, R-410A is a 50/50 mass blend of R-32 and R-125; R-407C is a zeotropic blend of R-32/R-125/R-134a.

CFC refrigerants (R-11, R-12, R-502) have high ODP; HCFC (R-22) has ODP ≈ 0.055; HFC refrigerants (R-134a, R-410A, R-32) have zero ODP.

ODP is defined relative to CFC-11 = 1.0; GWP is defined relative to CO2 = 1.0. R-134a GWP ≈ 1430, R-410A GWP ≈ 2088, R-32 GWP ≈ 675.

R-22 is being phased out under the Montreal Protocol (HCFC phase-out); HFCs are controlled under the Kigali Amendment.

R-410A operates at roughly 1.6 times the pressure of R-22 and requires POE (polyolester) oil; mineral oil is not suitable.

R-134a is a low-pressure refrigerant used in automotive and centrifugal chillers; it requires synthetic POE oil and larger compressor displacement than R-22.

Refrigerant cylinder colour code (ISO/ASHRAE): R-22 green, R-134a light blue, R-410A rose/pink, R-32 light green.

Never charge a blended refrigerant (R-410A, R-407C) as a liquid slug into a running compressor; always charge liquid from the cylinder to avoid fractionation.

Refrigerant must never be released to atmosphere; recovery, recycling, and reclaim are mandatory under the Montreal Protocol.

Ozone layer is in the stratosphere (15–35 km); it absorbs UV-B and UV-C radiation.

Ozone depletion is caused by chlorine and bromine radicals from CFCs and halons; one chlorine atom can destroy thousands of ozone molecules.

Montreal Protocol (1987) phases out ODS; Kigali Amendment (2016) targets HFC phase-down for GWP control.

Asphyxiation risk: refrigerant vapour is heavier than air and displaces oxygen in low-lying areas; never enter a pit or basement with suspected refrigerant leak without SCBA/ventilation.

Refrigerant contact with skin causes frostbite (boiling point of R-134a ≈ −26 °C); flush with lukewarm water, not hot water, and seek medical help.

Phosgene gas forms when refrigerant contacts an open flame; never use a flame leak detector on CFC/HCFC systems.

Personal protective equipment: safety goggles, insulated gloves, safety shoes, and a respirator when handling refrigerant or brazing.

Brazing safety: purge nitrogen through the pipe, use a fire extinguisher nearby, keep a flashback arrestor on the acetylene line, and do not braze near refrigerant vapour.

Hermetic compressor: motor and compressor sealed in one shell; semi-hermetic allows access via bolted heads; open type uses external shaft seal.

Capillary tube is a fixed restriction used in small window/split ACs; TXV (thermostatic expansion valve) modulates flow based on evaporator superheat.

TXV controls superheat (typically 5–8 K) using bulb pressure vs. evaporator pressure and spring force.

Condenser types: air-cooled (fin-and-tube), water-cooled (shell-and-tube), evaporative; condenser removes latent heat of condensation.

Evaporator absorbs heat from room air; it must be kept above 0 °C to avoid frost formation (unless it is a defrost-cycle freezer).

Compressor types: reciprocating, rotary, scroll, screw, centrifugal; rotary and scroll dominate domestic split ACs.

Controls: thermostat (room temperature), LP/HP pressure switch (safety cut-out), overload protector (Klixon), thermal overload in compressor, and capacitor for motor start/run.

Four-way reversing valve changes refrigerant direction to switch between cooling and heating in a heat pump.

Filter-drier removes moisture and dirt from the liquid line; moisture indicator (sight glass) shows green (dry) or yellow (wet).

Practical skill sequence: pipe cutting → deburring → flaring/swaging → brazing → pressure test → evacuation → charging → commissioning.

Brazing copper-to-copper: use 5% silver-phos filler with flux not required for copper-to-copper; nitrogen purge prevents internal oxidation (black scale).

Flare nut torque and concentricity determine leak-free joints; a 45° flare angle matches the SAE standard.

Leak test procedure: nitrogen at 350–550 psi, hold 24 h, then vacuum to 500 microns and hold 10 min.

Charging by weight is the most accurate method; charging by superheat/subcooling is used when weight is unknown.

Project report structure: title, objective, tools used, procedure, observations, result, conclusion, and safety precautions followed.

Typical project: install and commission a 1.5 TR split AC - record pipe length, additional charge, superheat, subcooling, and running current.

Viva questions: define TR, COP, superheat, subcooling, ODP, GWP, and explain the function of the TXV and capillary tube.

Assessment weighting: practical skills and project work carry significant marks; keep a signed work diary and observation book.