Refrigeration & Air Conditioning Notes

Refrigeration & Air Conditioning

Refrigeration fundamentals — refrigerating effect, tonne of refrigeration, COP; reversed Carnot cycle; vapour compression refrigeration system — components, working, T–s and p–h diagrams, COP from enthalpies, effect of subcooling and superheating; vapour absorption refrigeration (ammonia–water, lithium bromide–water); refrigerants — properties, ODP and GWP, CFC/HCFC/HFC phase-out and natural refrigerants; psychrometry — dry bulb, wet bulb and dew point temperatures, specific and relative humidity, psychrometric chart and processes; human comfort; air conditioning systems — window, split, packaged, central (chilled water), VRF; cooling load estimation, ventilation and building design considerations — with fully worked numericals.

Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 9 min read

Refrigeration fundamentals

Refrigeration — removing heat from a space or substance to maintain its temperature below that of the surroundings, which requires work input (second law).

FormulaBasic definitions

Refrigerating effect (RE) — heat removed from the space per unit mass of refrigerant (kJ/kg)

Tonne of refrigeration (TR) — heat extraction rate needed to freeze 1 short ton (907 kg) of water at 0 °C into ice in 24 hours:

1 TR \approx 3.517 kW \approx 211 kJ/min

(often rounded to 3.5 kW or 210 kJ/min)

Coefficient of performance: $C O P_{R} = \frac{Refrigerating effect}{Work input}$ Heat pump: $C O P_{H P} = C O P_{R} + 1$ Energy efficiency ratio (EER) = cooling output (W) ÷ electrical input (W); ISEER is a seasonal rating used for Indian star labels

Reversed Carnot cycle

The ideal refrigeration cycle between $T_{L}$ and $T_{H}$ :

C O P_{C a r n o t} = \frac{T _{L}}{T _{H} - T _{L}}

COP increases when the evaporator temperature is higher and the condenser temperature is lower (smaller temperature lift).
Not practical (wet compression, isothermal heat transfer difficulties), but sets the upper limit.

Vapour compression refrigeration system (VCRS)

The most widely used system — domestic refrigerators, air conditioners, cold storages, water coolers.

Components and processes

Component	Process	State change
Compressor	Isentropic compression (1–2)	Low-pressure vapour → high-pressure, high-temperature superheated vapour
Condenser	Heat rejection at constant pressure (2–3)	Vapour → high-pressure liquid (heat released to air/water)
Expansion device (capillary tube, thermostatic expansion valve)	Throttling (3–4), isenthalpic $h_{3} = h_{4}$	High-pressure liquid → low-pressure, low-temperature liquid–vapour mixture
Evaporator	Heat absorption at constant pressure (4–1)	Mixture evaporates, absorbing heat from the space (refrigerating effect)

FormulaVCRS performance (p–h diagram)

R E = h_{1} - h_{4}, W = h_{2} - h_{1}, Q_{co n d} = h_{2} - h_{3}

C O P = \frac{h _{1} - h _{4}}{h _{2} - h _{1}}

Refrigerant mass flow rate $\overset{m}{˙} = \frac{Capacity (kW)}{R E}$ ; compressor power $= \overset{m}{˙} (h_{2} - h_{1})$

Subcooling the liquid after the condenser increases RE (and COP).
Superheating the vapour in the evaporator increases RE and protects the compressor from liquid; effect on COP depends on refrigerant.
Pressure–enthalpy (p–h) chart is the standard tool for analysis.

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Refrigeration fundamentals

Reversed Carnot cycle

Vapour compression refrigeration system (VCRS)

Components and processes

This chapter is in the syllabus of