Refrigeration and Air Conditioning Lab Experiments — Complete Procedure and COP Calculations

The Refrigeration and Air Conditioning laboratory is a core practical component of B.Tech Mechanical Engineering, Chemical Engineering, and HVAC programmes. Students study the vapour compression cycle, measure COP, analyse the p-h diagram, and study components including compressors, condensers, evaporators, and expansion valves. This guide provides complete procedures for the three most important refrigeration lab experiments.

Experiment 1: Vapour Compression Refrigeration — COP Measurement

Aim

To determine the Coefficient of Performance (COP) of a vapour compression refrigeration system and compare it with the ideal Carnot COP.

Apparatus

• Refrigeration training unit (R-134a or R-404a refrigerant, hermetic compressor, finned condenser, thermostatic expansion valve, evaporator cabinet)
• Digital temperature indicators — at compressor suction, compressor discharge, condenser outlet, evaporator outlet
• Pressure gauges — low side (suction) and high side (discharge)
• Wattmeter (for compressor power input measurement)
• Refrigerant property tables or p-h chart for the refrigerant used

Procedure

1. Check the refrigerant charge level and system integrity before starting. Ensure all valves are in the correct position.
2. Switch on the refrigeration unit. Allow the system to reach steady state — wait until temperatures and pressures stabilise (approximately 20–30 minutes).
3. Record the following at steady state:
   — Compressor suction pressure P₁ (kPa) and temperature T₁ (°C)
   — Compressor discharge pressure P₂ (kPa) and temperature T₂ (°C)
   — Condenser outlet temperature T₃ (°C) — should be subcooled liquid
   — Evaporator inlet temperature T₄ (°C) — after expansion valve
   — Compressor power input W (Watts) from wattmeter
4. Using the refrigerant property tables (p-h chart), read off the enthalpies at each state point: h₁ (compressor inlet), h₂ (compressor outlet), h₃ (condenser outlet), h₄ = h₃ (throttling — constant enthalpy).
5. Calculate COP and compare with Carnot COP.
6. Vary the evaporator load (by adjusting the heater inside the evaporator cabinet) and repeat for 3 different load conditions.

Observation Table

Calculations

From p-h chart or tables: read h₁, h₂, h₃, h₄ = h₃

Refrigerating effect: q_E = h₁ − h₄ (kJ/kg)

Work of compression: w_c = h₂ − h₁ (kJ/kg)

Heat rejected at condenser: q_C = h₂ − h₃ (kJ/kg)

Theoretical COP: COP_th = q_E / w_c = (h₁ − h₄)/(h₂ − h₁)

Actual COP: COP_actual = Q_E / W_compressor (from heater watts / wattmeter watts)

Carnot COP: COP_Carnot = T_L / (T_H − T_L) where T in Kelvin

Refrigerating efficiency: η_R = COP_actual / COP_Carnot × 100%

Sample Calculation (R-134a)

At steady state (from p-h chart for R-134a):
P₁ = 3.0 bar (suction) → T_sat = 0°C → T₁ = 5°C (5°C superheat)
P₂ = 12.0 bar (discharge) → T₂ = 65°C (superheated)
T₃ = 38°C (subcooled by 2°C at condenser exit)
h₁ = 398 kJ/kg, h₂ = 435 kJ/kg, h₃ = h₄ = 260 kJ/kg

q_E = 398 − 260 = 138 kJ/kg
w_c = 435 − 398 = 37 kJ/kg
COP_th = 138/37 = 3.73

T_L = 0 + 273 = 273 K; T_H = 40 + 273 = 313 K
COP_Carnot = 273/(313−273) = 6.83

Refrigerating efficiency = 3.73/6.83 × 100% = 54.6%

Experiment 2: Determination of Dryness Fraction — Separating and Throttling Calorimeter

Aim

To determine the dryness fraction (quality) of wet steam using a separating and throttling calorimeter.

Theory

Wet steam is a mixture of saturated vapour and saturated liquid. The dryness fraction x = m_vapour / (m_vapour + m_liquid). A value of x = 1 means dry saturated steam; x = 0 means saturated liquid. The separating calorimeter mechanically separates the water droplets; the throttling calorimeter superheats the remaining steam so its state can be determined.

Combined dryness fraction: x = x_s × x_t + (1 − x_s)

where x_s = dryness after separator, x_t = dryness through throttling valve

Apparatus

• Steam separating calorimeter
• Throttling calorimeter (with pressure gauge and thermometer)
• Steam supply line with isolating valves
• Condensate collecting vessel (graduated) and stopwatch
• Barometer (for atmospheric pressure)

Procedure

1. Allow steam to flow through the separating calorimeter at the main line pressure P₁. Note P₁ and the temperature.
2. After separator, steam passes through the throttling valve to a lower pressure P₂ (close to atmospheric). Note P₂ and the throttled steam temperature T₂.
3. From the steam tables, read h₂ (enthalpy of superheated steam at P₂, T₂).
4. Collect condensate from the separator (m_s) over a timed period using the graduated vessel.
5. Collect condensate from the calorimeter outlet (m_c) over the same period.
6. Calculate dryness fraction x_t = (h₂ − h_f1) / h_fg1 at pressure P₁, where h_f1 and h_fg1 are from steam tables at P₁.
7. Calculate combined dryness: x = (m_c × x_t) / (m_s + m_c)

Experiment 3: Air Conditioning Unit — Psychrometric Analysis

Aim

To study the working of a window/split air conditioning unit and plot the air conditioning process on a psychrometric chart.

Measurements Required

• Return air (room) DBT (dry bulb temperature) and WBT (wet bulb temperature)
• Supply air DBT and WBT at the unit outlet
• Air flow rate through the unit (using an anemometer at the supply grille)
• Refrigerant suction and discharge pressures
• Compressor power input (wattmeter)

Psychrometric Properties

From the psychrometric chart or ASHRAE tables, read at each measurement point:
DBT, WBT → Specific humidity W (kg/kg dry air) → Enthalpy h (kJ/kg dry air) → Relative humidity RH%

Cooling capacity: Q = ṁ_air × (h_return − h_supply) kW
where ṁ_air = air mass flow rate (kg/s) = ρ_air × V̇ (volume flow from anemometer)

Sensible Heat Ratio: SHR = Q_sensible / Q_total = (1.0216 × ṁ_air × ΔDBT) / Q_total

Energy Efficiency Ratio (EER): EER = Q_cooling (BTU/hr) / P_compressor (Watts) — converted using 1 kW = 3412 BTU/hr

Viva Questions

1. What is the function of the expansion valve in a vapour compression system?
2. Why is the COP of an actual refrigeration system always less than the Carnot COP?
3. What effect does condensing temperature have on COP?
4. Define dryness fraction. What is its significance for steam turbines?
5. What is the difference between DBT and WBT? Why does WBT indicate humidity?
6. What is EER? How does it differ from COP?
7. Why is R-134a used instead of R-12 in modern refrigeration systems?

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