Refrigeration and air conditioning

Mechanical Engineering (B.E./B.Tech) · Refrigeration and air conditioning · free study notes for exams, placements and interviews

In short

Refrigeration is the process of removing heat from a space or substance and rejecting it to the surroundings, keeping the cooled region below ambient temperature; air conditioning applies the same machinery to control the temperature, humidity, cleanliness and motion of air for comfort or process needs. Both run on the second law: heat will not flow from cold to hot unaided, so work (vapour compression) or heat (vapour absorption) must be supplied. The topic carries heavy weight in GATE ME (cycles, COP, p-h calculations, refrigerant properties) and in university Thermal Engineering / RAC papers. In Indian industry it is the basis of room ACs and chillers (Voltas, Blue Star, Daikin, LG, Carrier, Hitachi), cold chains and ice plants, dairy and pharma plants, Indian Railways coach AC, and HVAC design and service roles governed by BEE star labelling, ECBC and the Ozone Depleting Substances Rules.

Explained

Fundamentals and performance measures

Refrigerator vs heat pump: the same machine; a refrigerator's useful output is the heat *absorbed* at the evaporator, a heat pump's is the heat *rejected* at the condenser.
COP (coefficient of performance) = useful effect / work input. It is a ratio, not a percentage, and is normally greater than 1, so the word "efficiency" is avoided.
Unit of capacity: the tonne of refrigeration (TR) - the rate of heat removal to freeze 1 short ton (2,000 lb) of water at 0 °C into ice at 0 °C in 24 hours = 3.5167 kW (commonly rounded to 3.5 kW).
Relative COP = actual COP / theoretical (Carnot) COP; good vapour-compression plant reaches 40-60% of Carnot.
Reversed Carnot cycle is the ideal benchmark: isentropic compression, isothermal rejection, isentropic expansion, isothermal absorption. COP depends only on the two absolute temperatures, so lifting from a lower evaporator temperature or rejecting to a hotter condenser always costs more work. It is not built in practice (isothermal heat exchange with a gas and two-phase expansion machines are impractical).
Key design rule: raise evaporator temperature and lower condenser temperature wherever the application allows - roughly 2-4% change in COP per °C.

Air refrigeration - the Bell-Coleman (reversed Brayton) cycle

Air is the refrigerant, stays gaseous throughout: compression → cooling in a heat exchanger at constant pressure → expansion in an expander (work recovered) → heat absorption from the cold space.
Processes: 1-2 isentropic compression, 2-3 constant-pressure cooling, 3-4 isentropic expansion, 4-1 constant-pressure refrigeration.
Open system: cold air delivered directly to the space; closed (dense air) system: air circulates sealed at above-atmospheric pressure, allowing smaller components.
Very low COP (typically 1-2) because air has low specific heat and no latent heat, so huge mass flows are needed.
Still used in aircraft air-conditioning (bootstrap, reduced-ambient and regenerative systems) where air is free, weight matters, and bleed air from the engine compressor is already available. Formerly used on ships and in early railway coaches.

Vapour compression refrigeration (VCR) - the working cycle

Compressor: draws dry saturated (or slightly superheated) vapour at evaporator pressure and compresses it to condenser pressure, raising its saturation temperature above ambient.
Condenser: rejects superheat + latent heat to air or water; refrigerant leaves as saturated (or sub-cooled) liquid.
Expansion device: throttles the liquid irreversibly (constant enthalpy, h₃ = h₄) to evaporator pressure; part of it flashes to vapour.
Evaporator: the low-temperature mixture boils at constant pressure absorbing latent heat - this is the refrigerating effect.
Why a throttle and not an expander: expanding a liquid yields very little work, and a two-phase expander is costly and erosion-prone; the simple capillary or valve is accepted even though throttling is the cycle's main irreversibility.
On a p-h (Mollier) chart the cycle is read directly: refrigerating effect and compressor work are horizontal distances, so COP = (h₁−h₄)/(h₂−h₁). T-s and p-V plots are also examined.
Wet vs dry compression: wet compression (compressing a liquid-vapour mixture) risks liquid slugging, washes away cylinder oil and gives poor volumetric efficiency - hence dry compression with a few degrees of suction superheat is standard.
Effect of sub-cooling the liquid: increases refrigerating effect with no extra compressor work → COP rises. Done in the condenser, a liquid sub-cooler, or a liquid-suction heat exchanger.
Effect of superheating the suction vapour: increases refrigerating effect *and* work; net COP change is small and refrigerant-dependent, but it protects the compressor and is always provided in practice.
Actual cycle deviations: pressure drops in suction/discharge lines, valves and heat exchangers; heat gain in the suction line; non-isentropic compression; subcooling; superheating.

Multi-stage, multi-evaporator and absorption systems

Why stage: a single stage over a large temperature lift gives a high pressure ratio → high discharge temperature (oil breakdown), poor volumetric efficiency, large throttling loss.
Two-stage compound compression with a flash chamber (removes flash vapour so only liquid is throttled onward) and/or a water intercooler or flash intercooler between stages. Optimum intermediate pressure p_i = √(p₁p₂) equalises stage pressure ratios and minimises work.
Cascade system: two or more separate circuits with *different* refrigerants linked by a cascade condenser (the HT evaporator is the LT condenser). Needed below about −40 °C and down to −100 °C, e.g. R-404A/R-23 cascades for pharma and vaccine storage.
Multi-evaporator systems: several evaporators at different temperatures served by a single compressor (with back-pressure/pressure-reducing valves) or by individual compressors - as in a refrigerator with freezer and fresh-food compartments.
Vapour absorption refrigeration (VAR): the compressor is replaced by absorber-pump-generator. Heat (steam, hot water, flue gas, solar) drives the refrigerant out of solution in the generator; the pump handles liquid, so electrical input is tiny.
- Aqua-ammonia (NH₃ refrigerant, water absorbent): needs an analyser and rectifier to strip water vapour from the ammonia; works below 0 °C.
- Lithium bromide-water (water is the refrigerant, LiBr the absorbent): cannot go below ~4 °C (water freezes) and risks crystallisation; used for large chillers. Single-effect COP ≈ 0.6-0.8, double-effect ≈ 1.1-1.3.
- Electrolux (three-fluid) system: NH₃-water-hydrogen, no pump at all, heated by LPG or a heater - used in hotel minibars and kerosene/gas refrigerators.
Other methods: steam-jet (ejector) refrigeration for water chilling; thermoelectric (Peltier) cooling for small, silent, vibration-free duty; vortex tube (Ranque-Hilsch) for spot cooling with compressed air; adiabatic demagnetisation for cryogenic research; evaporative cooling (desert coolers) which adds moisture and works only in dry climates.

Refrigerants, environment and Indian regulation

Numbering: R-11, R-12 (CFCs); R-22, R-123 (HCFCs); R-134a, R-32, R-125 (HFCs); R-1234yf, R-1234ze (HFOs); R-400 series = zeotropic blends (temperature glide, must be charged as liquid, e.g. R-404A, R-407C, R-410A); R-500 series = azeotropic; R-600 series = hydrocarbons (R-600a isobutane, R-290 propane); R-700 series = inorganic with the number following the molecular mass (R-717 ammonia, R-718 water, R-744 CO₂).
Selection criteria: suitable saturation pressures at the working temperatures (evaporator above atmospheric to avoid air leaking in), high latent heat, low specific volume at suction, low discharge temperature, non-toxic, non-flammable, chemically stable, miscible/compatible with oil, easy leak detection, low ODP and GWP, cheap and available.
Safety classification (ASHRAE 34 / ISO 817): A1 non-toxic non-flammable (R-134a, R-410A), A2L mildly flammable (R-32, R-1234yf), A3 highly flammable (R-290, R-600a), B2L toxic and mildly flammable (R-717 ammonia).
Montreal Protocol (1987): phased out ozone-depleting CFCs; India completed its CFC phase-out on 1 August 2008, ahead of the 1 January 2010 Article-5 deadline. HCFC-22 is under phase-out (its use in new room-AC manufacture was stopped around 2016 in India; complete phase-out follows the Article-5 schedule towards 2030 with a small servicing tail - check the current HPMP stage, as dates have been revised).
Kigali Amendment (adopted 2016, ratified by India in September 2021): HFCs are phased *down* for their high GWP. India's indicative schedule is a freeze around 2028 followed by stepped cuts to about 85% reduction by 2047 - treat the exact percentages as subject to revision.
Indian instruments: Ozone Depleting Substances (Regulation and Control) Rules, 2000 under the Environment (Protection) Act, 1986; the India Cooling Action Plan (ICAP, 2019); BEE star labelling (mandatory for fixed- and variable-speed room ACs, rated by ISEER), the BEE default thermostat setting of 24 °C for room ACs; ECSBC 2024 (successor to ECBC 2017; States adopt it in their own versions) and Eco-Niwas Samhita for building envelopes and HVAC; IS 1391 (room air conditioners), IS 8148 (packaged/ducted units), IS 660 (safety code for mechanical refrigeration) and ISHRAE/ASHRAE handbooks for design practice.

Components and air-conditioning equipment

Compressors: reciprocating (hermetic in fridges, semi-hermetic/open in plant), rotary rolling-piston and vane (room ACs and small splits), scroll (quiet, 3-50 TR, dominant in ductable splits, cassettes, VRF and small chillers), screw (50-500 TR), centrifugal (above ~200 TR, large chillers). Inverter (variable-speed) drives modulate capacity instead of cycling on-off, cutting energy use sharply.
Condensers: air-cooled (fin-and-tube, most room ACs), water-cooled shell-and-tube with a cooling tower, evaporative. Approach and fouling control condenser pressure and hence power.
Expansion devices: capillary tube - fixed bore, no moving parts, fixed-charge sealed systems; thermostatic expansion valve (TEV) - maintains constant evaporator superheat via a bulb-charged diaphragm, with external equaliser for large coils; electronic expansion valve (EEV) - stepper-driven, essential in inverter and VRF systems; float valves for flooded evaporators.
Evaporators: direct-expansion (DX) fin-and-tube coils, flooded shell-and-tube, plate and bare-tube for liquid chilling.
Accessories: filter-drier, liquid receiver, suction accumulator, oil separator, sight glass with moisture indicator, solenoid valve, HP/LP cut-outs, thermostat.
Air-conditioning system types: window (unitary); split - hi-wall, cassette, ducted; multi-split and VRF/VRV with one outdoor unit serving many indoor units and heat-recovery variants; packaged units; central chilled-water plant (chiller + pumps + AHUs/FCUs + cooling tower + ducting), used in malls, hospitals and airports; district cooling (e.g. GIFT City, Gandhinagar); thermal-energy-storage (ice-bank) plants that shift load off peak.
Air distribution: AHU with filters, cooling coil and blower; supply and return ducts; all-air, all-water and air-water classifications; variable air volume (VAV) boxes for part-load control.

Key points

Refrigeration - continuous removal of heat from a body below ambient, maintained by work or heat input.
COP_R = Q_e/W - refrigerating effect per unit work; COP_HP = COP_R + 1.
1 TR - 3.5167 kW ≈ 211 kJ/min ≈ 12,000 Btu/h ≈ 3,024 kcal/h.
Reversed Carnot COP = T_e/(T_c − T_e) - the maximum possible, with temperatures in kelvin.
Bell-Coleman cycle - gas cycle using air; COP = 1/(r_p^(γ−1)/γ − 1); low COP, used in aircraft.
VCR cycle - compressor, condenser, throttle, evaporator; the workhorse of all commercial refrigeration.
Throttling - isenthalpic (h₃ = h₄), irreversible; the single largest loss in the VCR cycle.
Dry compression - slight suction superheat avoids liquid slugging and oil washing in the compressor.
Sub-cooling - raises refrigerating effect at no extra work, so COP improves.
Flash chamber - removes the vapour formed on throttling so the second-stage throttle handles only liquid.
Optimum intermediate pressure - p_i = √(p₁p₂) for two-stage compression.
Cascade system - different refrigerants in coupled circuits for temperatures below about −40 °C.
VAR - heat-operated; LiBr-water for chillers above 4 °C, aqua-ammonia for sub-zero duty; COP ≈ 0.6-1.3.
ODP and GWP - ozone depletion potential (R-11 = 1) and global warming potential (CO₂ = 1) decide a refrigerant's legality.
R-32 and R-290 - the low-GWP replacements in Indian room ACs and refrigerators; both flammable, needing charge limits and trained service.
ISEER - BEE's seasonal performance index for room ACs = annual cooling output / annual electricity input, over 1,600 operating hours.
Capillary vs TEV - fixed restriction for sealed fixed-load units; superheat-controlled valve for variable load.
Inverter compressor - variable speed matches capacity to load, avoiding cycling losses.

Formulas and examples

COP_R = Q_e / W_net ; COP_HP = Q_c / W_net = COP_R + 1 (Q in kW, W in kW)
1 TR = 3.5167 kW = 211 kJ/min = 12,000 Btu/h ≈ 3,024 kcal/h
Reversed Carnot: COP = T_e / (T_c − T_e), T in kelvin
VCR: refrigerating effect RE = h₁ − h₄ (kJ/kg); work w = h₂ − h₁ (kJ/kg); COP = (h₁ − h₄)/(h₂ − h₁)
Mass flow ṁ = Q_e / (h₁ − h₄) kg/s ; Compressor power P = ṁ (h₂ − h₁) kW ; Heat rejected Q_c = ṁ (h₂ − h₃)
Suction volume flow V = ṁ v₁ (m³/s) ; Piston displacement = V / η_vol
Bell-Coleman: COP = 1 / [r_p^(γ−1)/γ − 1] ; RE = c_p (T₁ − T₄) ; w_net = c_p[(T₂ − T₁) − (T₃ − T₄)]
Two-stage: p_i = √(p₁ p₂) ; dryness after throttling x = (h_before − h_f)/h_fg
VAR: COP = Q_e / Q_generator ; COP_max = (1 − T₀/T_g) × T_e/(T₀ − T_e)
EER = cooling capacity (W) / power input (W) ; ISEER = CSTL (Wh) / CSEC (Wh)

Example 1 - Carnot limit and minimum power. A cold store is held at −10 °C and heat is rejected at 40 °C; the load is 10 TR.

T_e = 263 K, T_c = 313 K → COP_Carnot = 263/(313 − 263) = 263/50 = 5.26
Q_e = 10 × 3.5167 = 35.17 kW → W_min = 35.17/5.26 = 6.69 kW
A real plant at 45% of Carnot (COP ≈ 2.37) would need about 14.8 kW - the realistic design figure.

Example 2 - VCR cycle on R-134a (simple saturation cycle). Evaporator −10 °C, condenser 40 °C, dry saturated suction, saturated liquid leaving condenser, load 5 TR. Using approximate table values h₁ = 392.4, h₂ = 424.5, h₃ = h₄ = 256.4 kJ/kg and v₁ = 0.0994 m³/kg (use your own tables in the exam):

RE = 392.4 − 256.4 = 136.0 kJ/kg; w = 424.5 − 392.4 = 32.1 kJ/kg
COP = 136.0/32.1 = 4.24 (Carnot for the same temperatures = 263/50 = 5.26, so relative COP ≈ 81% for this idealised cycle)
Q_e = 5 × 3.5167 = 17.58 kW → ṁ = 17.58/136.0 = 0.1293 kg/s (465 kg/h)
Power = 0.1293 × 32.1 = 4.15 kW; Q_c = 0.1293 × 168.1 = 21.7 kW (= 17.58 + 4.15, as it must)
Suction volume = 0.1293 × 0.0994 = 0.01285 m³/s = 0.771 m³/min; with η_vol = 0.80, piston displacement = 0.964 m³/min
Pressure ratio ≈ 10.17/2.00 ≈ 5.1 - acceptable for a single stage.

Example 3 - Bell-Coleman cycle. Air enters the compressor at 1 bar, 268 K and is compressed to 4 bar, cooled to 300 K, then expanded back to 1 bar. γ = 1.4, c_p = 1.005 kJ/kg·K, load 10 TR.

r_p^(γ−1)/γ = 4^0.2857 = 1.486
T₂ = 268 × 1.486 = 398.2 K; T₄ = 300/1.486 = 201.9 K
RE = 1.005 × (268 − 201.9) = 66.4 kJ/kg
w_net = 1.005 × [(398.2 − 268) − (300 − 201.9)] = 1.005 × 32.1 = 32.3 kJ/kg
COP = 66.4/32.3 = 2.06 (check: 1/(1.486 − 1) = 2.06)
ṁ = 35.17/66.4 = 0.53 kg/s of air; power = 35.17/2.06 = 17.1 kW - note the mass flow is four times that of the R-134a case in Example 2 for twice the load, which is why air systems are bulky.

Example 4 - staging and an absorption limit.

(a) An ammonia plant works between an evaporator at 1.2 bar and a condenser at 12 bar. p_i = √(1.2 × 12) = √14.4 = 3.80 bar; each stage then has a pressure ratio of 3.16 instead of a single-stage 10.
(b) A LiBr chiller is driven by steam at 100 °C, rejects heat at 30 °C and chills water to 5 °C. COP_max = (1 − 303/373) × 278/(303 − 278) = 0.1877 × 11.12 = 2.09. A real single-effect machine gives about 0.7 - roughly 33% of the thermodynamic limit, which is still attractive when the heat is waste heat.

Example 5 - star rating and running cost. A split AC of rated cooling capacity 5,000 W is tested over BEE's 1,600 annual hours.

CSTL = 5,000 × 1,600 = 8,000,000 Wh = 8,000 kWh of cooling
At ISEER 5.0 → CSEC = 8,000/5.0 = 1,600 kWh/yr; at ISEER 3.5 → 8,000/3.5 = 2,286 kWh/yr
Saving = 686 kWh ≈ ₹5,490 per year at ₹8/kWh - a payback of about two to three years on the price difference. (Star thresholds are revised periodically by BEE, so confirm the current table.)

	Vapour compression	Vapour absorption
Energy input	Shaft work (electric motor)	Heat (steam, hot water, flue gas, solar) + small pump work
COP	2.5-6	0.6-0.8 single effect, 1.1-1.3 double effect
Moving parts	Compressor - noise, vibration, wear	Pump only - quiet, long life
Part load	Poor without inverter/VFD	Good
Capital cost / space	Lower, compact	Higher, bulky
Best suited to	Almost all duties	Sites with cheap waste heat or costly power

Refrigerant	Type	ODP	GWP (approx.)	Safety	Typical Indian use
R-12	CFC	1.0	10,900	A1	Banned; old fridges
R-22	HCFC	0.055	1,810	A1	Legacy ACs, servicing only
R-134a	HFC	0	1,430	A1	Car AC, water coolers, chillers
R-410A	HFC blend	0	2,088	A1	Splits and VRF (being replaced)
R-32	HFC	0	675	A2L	Most new Indian split ACs
R-290	Hydrocarbon	0	3	A3	Low-charge room ACs, chillers
R-600a	Hydrocarbon	0	3	A3	Domestic refrigerators
R-717 (NH₃)	Inorganic	0	0	B2L	Cold stores, ice plants, dairies
R-744 (CO₂)	Inorganic	0	1	A1	Transcritical supermarket racks

Where it is used

Room air conditioning and refrigerators: manufacturing, testing and service at Voltas, Blue Star, Daikin, LG, Hitachi-Johnson Controls, Godrej and Whirlpool plants; BEE star-rating compliance and ISEER testing in NABL labs.
Commercial and institutional HVAC: chillers, VRF and AHU design for malls, metro stations, airports, IT parks and hospitals (Blue Star, Voltas projects, L&T, ISHRAE member consultancies); ECBC-compliant design and commissioning.
Cold chain and food processing: cold storages for potato and apple (Punjab, UP, Himachal), ammonia ice plants, milk chilling and dairy plants (Amul, Mother Dairy), reefer containers and insulated trucks, NHB/MoFPI-supported cold-chain projects.
Pharmaceutical, vaccine and blood cold chain: 2-8 °C cold rooms, −20 °C and −80 °C cascade freezers, validated temperature mapping; Serum Institute and hospital blood banks.
Transport: Indian Railways AC coach and LHB roof-mounted packaged units, automotive AC (R-134a and R-1234yf), aircraft air-cycle machines.
Job roles: HVAC design engineer, service and commissioning engineer, chiller plant operator, energy auditor (BEE-certified), product development and reliability engineer, cold-chain project engineer.

Interview questions

Q: Why is COP used instead of efficiency for a refrigerator? A: Because the useful heat moved is normally larger than the work input, so the ratio exceeds 1 and calling it an efficiency would wrongly suggest a violation of the first law.
Q: What exactly is one tonne of refrigeration? A: The heat removal rate needed to freeze one short ton of water at 0 °C into ice at 0 °C in 24 hours - 3.5167 kW, or about 12,000 Btu/h.
Q: Why is the reversed Carnot cycle never built? A: Isothermal heat exchange is impractically slow, and expanding a two-phase refrigerant in a machine causes erosion and yields little work.
Q: Why is throttling used instead of an expander in a VCR plant? A: The liquid expansion would recover very little work, and the valve or capillary is cheap, reliable and also acts as a flow controller - the irreversibility is accepted.
Q: What is the difference between a capillary tube and a TEV? A: A capillary is a fixed-bore restriction suited to a sealed, fixed-charge, constant-load unit; a TEV senses evaporator outlet superheat and varies the opening, so it suits variable loads and prevents liquid floodback.
Q: Does superheating the suction vapour improve COP? A: It raises both refrigerating effect and compressor work, so the net COP change is small and refrigerant-dependent; it is provided mainly to keep liquid out of the compressor.
Q: Why is sub-cooling always beneficial? A: It increases the refrigerating effect per kilogram with no change in compressor work, so COP rises and mass flow falls.
Q: Why does a cold store need two-stage or cascade refrigeration at −40 °C? A: The single-stage pressure ratio becomes very high, giving excessive discharge temperature, low volumetric efficiency and large throttling loss; staging or cascading splits the lift.
Q: Why can a LiBr-water absorption chiller not produce chilled water below about 4 °C? A: Water is the refrigerant, so evaporating below 0 °C would freeze it; a practical margin keeps the limit around 4-5 °C.
Q: Why is ammonia still used in large plants despite being toxic? A: It has very high latent heat, zero ODP and zero GWP, low cost and excellent COP; leaks are self-announcing by smell, and plants are designed under safety codes with machine rooms and detectors.
Q: Why are R-32 and R-290 replacing R-410A and R-22 in India? A: R-22 depletes ozone and R-410A has a GWP above 2,000; R-32 (675) and R-290 (3) meet Montreal/Kigali obligations, with mild or high flammability managed by charge limits and trained servicing.
Q: How does an inverter AC save energy? A: A variable-frequency drive modulates compressor speed to match the load instead of cycling on and off, so it runs longer at higher part-load COP and avoids start-up and cycling losses.

Common mistakes

Using °C instead of kelvin in the Carnot COP expression, or writing T_c/(T_c − T_e) for a refrigerator instead of T_e/(T_c − T_e).
Taking 1 TR as 3.5 kJ/s in one line and 211 kJ/min in another without checking consistency, or mixing kcal/h with kW.
Forgetting that throttling is constant enthalpy, not constant entropy, and so drawing 3-4 as a vertical line on a T-s diagram.
Reading h₃ as saturated vapour instead of saturated liquid at condenser pressure, which inflates the refrigerating effect.
Omitting volumetric efficiency when converting suction volume flow to compressor piston displacement.
Treating the compressor power as the heat rejected, or forgetting that Q_c = Q_e + W in every energy balance check.
Charging a zeotropic blend such as R-410A or R-407C as vapour, which shifts its composition - these must be charged as liquid; and quoting banned refrigerants (R-12, R-22) as current choices for new Indian equipment.