Vapour compression refrigeration and refrigerants

The ideal and actual vapour compression cycle, refrigerating effect, COP, tonnage, subcooling and superheating, automotive AC layout and refrigerant choice (R-134a, R-1234yf, CO₂), with table-based numericals.

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Why it matters

Every car air-conditioner, bus AC, refrigerated truck and EV battery chiller uses the vapour compression cycle. Its COP decides how much engine power or battery energy the AC consumes, and the choice of refrigerant decides the system's pressures, size and environmental impact. Being able to read a refrigerant table and compute capacity, mass flow, power and COP is a core skill for both GATE and automotive HVAC work.

Key ideas

Why a vapour cycle. A refrigerant absorbs heat by evaporating at a low pressure (low saturation temperature) and gives it up by condensing at a high pressure (high saturation temperature). Phase change lets large amounts of heat move at nearly constant temperature, close to the reversed Carnot ideal.

Ideal vapour compression cycle (four steady-flow processes). 1→2 isentropic compression of saturated (or slightly superheated) vapour in the compressor; 2→3 constant-pressure heat rejection in the condenser: desuperheating, condensing, sometimes subcooling; 3→4 throttling in the expansion valve or orifice tube: h₃ = h₄, pressure and temperature drop, and some liquid flashes to vapour; 4→1 constant-pressure heat absorption in the evaporator: the wet mixture evaporates while cooling the cabin air.

Why throttling instead of an expansion engine. Expanding a mostly-liquid stream would give very little work; a valve is cheap, simple and controls flow. The cost is a loss of refrigerating effect, since h₄ = h₃ rather than the lower isentropic value.

Refrigerating effect, capacity and COP. The refrigerating effect is the heat absorbed per kg in the evaporator, h₁ − h₄. Capacity is often quoted in tonnes of refrigeration (TR): 1 TR = 3.517 kW (211 kJ/min). COP = refrigerating effect / compressor work.

Effect of operating conditions.

  • Lower evaporator temperature or higher condenser temperature: higher pressure ratio, more work, lower COP, lower capacity (the suction vapour is less dense).
  • Subcooling liquid below the condensing temperature lowers h₃ = h₄ and increases the refrigerating effect without extra work. It also prevents flash gas before the valve.
  • Superheating in the evaporator protects the compressor from liquid; if the superheat happens inside the cooled space it adds useful refrigeration, but it also raises compressor work and discharge temperature. Thermostatic expansion valves control the superheat.

Actual cycle. Pressure drops in lines and heat exchangers, non-isentropic compression (η_C = (h₂s − h₁)/(h₂ − h₁)), and heat gain in suction lines all lower COP.

Automotive layout. Belt-driven or electric compressor, front-mounted condenser in front of the radiator, receiver-drier with a thermostatic expansion valve, or an accumulator with a fixed orifice tube. EVs often run the same loop as a heat pump and to chill the battery.

Refrigerants. Desirable properties: high latent heat (small mass flow), moderate pressures, high critical temperature, low freezing point, chemical stability, non-toxic, non-flammable, zero ozone depletion potential (ODP) and low global warming potential (GWP).

  • R-12 (CFC): used in older car ACs, high ODP; phased out under the Montreal Protocol.
  • R-134a (HFC): ODP zero, GWP about 1430; the standard car refrigerant for decades, now being replaced.
  • R-1234yf (HFO): GWP below 1, mildly flammable (A2L); now used in many new cars.
  • R-744 (CO₂): GWP 1, very high operating pressures, transcritical cycle; used in some heat-pump systems.
  • R-22 and R-410A are stationary-AC refrigerants. Exact ODP and GWP values come from the relevant standard; quote them from your data book.

Formulas

RE = h₁ − h₄ — refrigerating effect, kJ/kg (h₁: compressor inlet; h₄ = h₃: evaporator inlet).

w_C = h₂ − h₁ — compressor work, kJ/kg (h₂: compressor outlet).

q_cond = h₂ − h₃ — heat rejected in the condenser, kJ/kg; q_cond = RE + w_C.

COP_R = (h₁ − h₄)/(h₂ − h₁)

COP_Carnot = T_L / (T_H − T_L) — reversed Carnot limit; T in K.

ṁ = Q̇_L / (h₁ − h₄) — refrigerant mass flow, kg/s; Q̇_L: capacity, kW.

Ẇ = ṁ (h₂ − h₁) — compressor power, kW.

η_C = (h₂s − h₁)/(h₂ − h₁) — compressor isentropic efficiency.

V̇ = ṁ v₁ — volume flow at compressor suction, m³/s; v₁: specific volume at suction.

1 TR = 3.517 kW

Worked examples

Example 1 (standard, ideal cycle). An R-134a car AC works on the ideal cycle with evaporator at −10 °C and condenser at 40 °C, and must remove 5 kW from the cabin. From R-134a tables: h₁ = h_g(−10 °C) = 244.5 kJ/kg, s₁ = 0.9377 kJ/kg·K; at the condenser pressure (1.017 MPa) and s = 0.9377, h₂ = 278.3 kJ/kg; h₃ = h_f(40 °C) = 108.3 kJ/kg. Find COP, mass flow and power.

  1. h₄ = h₃ = 108.3 kJ/kg (throttling).
  2. RE = h₁ − h₄ = 244.5 − 108.3 = 136.2 kJ/kg; w_C = h₂ − h₁ = 278.3 − 244.5 = 33.8 kJ/kg.
  3. COP = RE/w_C = 136.2/33.8 = 4.03.
  4. ṁ = Q̇_L/RE = 5/136.2 = 0.0367 kg/s.
  5. Ẇ = ṁ w_C = 0.0367 × 33.8 = 1.24 kW.
  6. Compare: Carnot COP = 263.15/50 = 5.26. The ideal vapour cycle reaches about 77% of it, the shortfall coming from throttling and the superheated discharge.

Example 2 (GATE level, actual cycle). The same system now has 5 K of superheat at the evaporator exit (useful), 5 K of subcooling at the condenser exit, and a compressor isentropic efficiency of 0.80. Capacity is 3 TR. Data: h₁ = 248.8 kJ/kg (−5 °C at 200.6 kPa), h₂s = 283.5 kJ/kg, h₃ = 100.9 kJ/kg (liquid at 35 °C). Find mass flow, power and COP.

  1. Capacity: Q̇_L = 3 × 3.517 = 10.55 kW.
  2. Actual discharge: h₂ = h₁ + (h₂s − h₁)/η_C = 248.8 + (283.5 − 248.8)/0.80 = 248.8 + 43.4 = 292.2 kJ/kg.
  3. RE = h₁ − h₄ = 248.8 − 100.9 = 147.9 kJ/kg (larger than ideal because of subcooling and useful superheat).
  4. ṁ = 10.55/147.9 = 0.0713 kg/s.
  5. Ẇ = ṁ (h₂ − h₁) = 0.0713 × 43.4 = 3.10 kW.
  6. COP = 10.55/3.10 = 3.41.

Common mistakes

  • Computing compressor work from the evaporator inlet enthalpy instead of the compressor inlet (h₂ − h₁, not h₂ − h₄).
  • Taking the refrigerating effect as h₁ alone, or as h₁ − h_f at the evaporator temperature, instead of h₁ − h₄ with h₄ = h₃.
  • Assuming the throttling valve is isothermal or isentropic. It is isenthalpic.
  • Using °C in the Carnot COP.
  • Mixing refrigerant tables with different reference states (ASHRAE vs IIR); enthalpy differences agree, absolute values do not.
  • Treating 1 TR as 3.5 kW in one step and 211 kJ/min in another; be consistent.

For GATE ME

Questions give a p–h or T–s diagram or table values and ask for refrigerating effect, COP, mass flow per TR, compressor power, condenser heat rejection or quality after throttling. Conceptual MCQs cover the effect of subcooling, superheating, evaporator and condenser temperatures on COP, why a throttle valve is used, and refrigerant properties (ODP, GWP). Practise sketching the cycle on the p–h chart; every quantity is a horizontal distance on it.

Quick check

  1. Evaporator 10 kW, compressor 2.5 kW. What is the COP?
  2. What property stays constant through the expansion valve?
  3. Does subcooling increase or decrease the refrigerating effect?
  4. What is the Carnot COP between −10 °C and 40 °C?

Answers: 1. 4.0. 2. Enthalpy. 3. Increases it (h₄ falls). 4. 263.15/50 = 5.26.

Interview questions

All Thermodynamics and Heat Transfer interview questions

Try answering each one aloud before you open it.

  1. 1.What is vapour compression refrigeration?Concept

    Vapour compression refrigeration is a process that uses a refrigerant to absorb heat from a low-temperature environment and release it at a higher temperature. It involves four main components: a compressor, a condenser, an expansion valve, and an evaporator. The refrigerant circulates through these components, changing phases from liquid to vapour and back, to transfer heat effectively.

  2. 2.Explain the role of the compressor in a vapour compression refrigeration system.Concept

    The compressor in a vapour compression refrigeration system is responsible for compressing the refrigerant vapour, increasing its pressure and temperature. This allows the refrigerant to release absorbed heat when it reaches the condenser. The compressor essentially drives the refrigerant through the system, maintaining the cycle of heat absorption and release.

  3. 3.Why was R-134a used in car air-conditioners, and why is it being replaced?Application

    R-134a replaced R-12 because it has zero ozone depletion potential while having similar pressures and a suitable boiling point (about −26 °C at atmospheric pressure), and it is non-flammable and of low toxicity. Its weakness is a high global warming potential, about 1430, which matters because car systems leak and are vented at service and scrapping. Regulations on mobile air-conditioning refrigerants have therefore pushed new cars to R-1234yf (GWP below 1, mildly flammable) and, in some heat-pump systems, CO₂ (R-744).

  4. 4.What happens if the expansion valve in a refrigeration system is not functioning properly?Application

    If the valve sticks open or overfeeds, too much refrigerant enters the evaporator, superheat at the evaporator exit drops to zero and liquid can reach the compressor, risking slugging and valve damage; suction pressure rises and cooling falls. If it sticks closed or underfeeds, the evaporator is starved: suction pressure and superheat become high, capacity drops, discharge temperature rises, and the low evaporator pressure can bring the coil below 0 °C so that it ices up. Both show up as poor cooling, but the gauge readings (suction pressure and superheat) tell them apart.

  5. 5.Explain the term 'subcooling' in the context of refrigeration.Concept

    Subcooling refers to the process of cooling the refrigerant liquid below its saturation temperature after it has condensed in the condenser. This ensures that only liquid refrigerant enters the expansion valve, improving the efficiency of the refrigeration cycle by preventing flash gas formation and enhancing the cooling capacity.

  6. 6.How does the choice of refrigerant affect the efficiency of a vapour compression refrigeration system?Application

    For the same evaporator and condenser temperatures, refrigerants differ in COP mainly through the losses in throttling and in the superheated discharge, which depend on the shape of their saturation dome and their critical temperature. A high latent heat gives a large refrigerating effect and a small mass flow, while the vapour density at suction sets the volumetric capacity and so the compressor size. Saturation pressures should be moderate: above atmospheric in the evaporator so air does not leak in, and not so high in the condenser that components become heavy. A refrigerant close to its critical point, like CO₂ in hot weather, loses COP sharply. Safety, ODP and GWP then narrow the choice.

  7. 7.What is the purpose of the condenser in a vapour compression refrigeration system?Concept

    The condenser in a vapour compression refrigeration system is responsible for releasing the heat absorbed by the refrigerant in the evaporator. It cools and condenses the high-pressure refrigerant vapour into a liquid by transferring heat to the surrounding environment, typically using air or water as a cooling medium.

  8. 8.Calculate the coefficient of performance (COP) of a refrigeration system if the refrigerant absorbs 200 kJ of heat from the evaporator and the work input to the compressor is 50 kJ.Numerical

    The coefficient of performance (COP) of a refrigeration system is calculated using the formula: COP = Qc / W, where Qc is the heat absorbed by the refrigerant and W is the work input. Here, COP = 200 kJ / 50 kJ = 4. This means the system is four times more effective in transferring heat than the energy input required.

  9. 9.What are the environmental concerns associated with the use of certain refrigerants?Application

    CFCs such as R-12 and, to a lesser degree, HCFCs such as R-22 contain chlorine that destroys stratospheric ozone, so the Montreal Protocol phased them out. Their HFC replacements, such as R-134a and R-410A, have zero ozone depletion potential but high global warming potential, and the Kigali Amendment now phases down HFCs. Current choices are low-GWP HFOs (R-1234yf), CO₂ and hydrocarbons, each trading off flammability, pressure or efficiency. Leakage during service and end-of-life recovery are major practical concerns for vehicle systems.

  10. 10.If a refrigeration system operates between a condenser temperature of 40°C and an evaporator temperature of -10°C, what is the ideal Carnot COP?Numerical

    The ideal Carnot COP is calculated using the formula: COP = Tc / (Th - Tc), where Tc is the absolute temperature of the cold reservoir (evaporator) and Th is the absolute temperature of the hot reservoir (condenser). Convert temperatures to Kelvin: Tc = 263 K (-10°C) and Th = 313 K (40°C). COP = 263 / (313 - 263) = 263 / 50 = 5.26.

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