UNIT 1: REFRIGERATION & AIR CONDITIONING (Exam-Focused Short Notes)

1.0 FUNDAMENTAL CONCEPTS & DEFINITIONS

1.1 Definition of Refrigeration & Air Conditioning

• Refrigeration: The science of lowering the temperature of a space or substance below the surrounding temperature and maintaining it.

• Air Conditioning (AC): The process of treating air to control its temperature, humidity, cleanliness, and distribution to meet the requirements of a conditioned space.

1.2 Unit of Refrigeration: Tonne of Refrigeration (TR)

• Definition: The amount of refrigeration effect required to produce 1 tonne (1000 kg) of ice at 0°C from liquid water at 0°C in 24 hours.

• Derivation:

  • Latent heat of fusion of ice = 335 kJ/kg.

  • Total heat to be removed in 24 hours = 1000 kg × 335 kJ/kg = 335,000 kJ.

  • Refrigeration capacity per hour = 335,000 kJ / 24 h = 13,958.33 kJ/h.

  • Standardized value: 1 TR = 12,000 Btu/hr = 3.516 kW.

• Numerical Relationship:

$$1\ \text{TR} = 3.516\ \text{kW} = 12,000\ \text{Btu/hr}$$

> [!TIP] **Exam Focus:** Numerical problems often involve converting TR to kW or vice-versa, and calculating mass of ice produced or power required using COP.

1.3 Primary vs. Secondary Refrigerants

1.4 Desirable Properties of an Ideal Refrigerant

1. Thermodynamic: Low boiling point, high latent heat of vaporization, high critical temperature & pressure, moderate operating pressures.

2. Chemical: Chemically stable, non-corrosive to metals.

3. Physical: Low specific volume (high density), low viscosity.

4. Safety: Non-toxic, non-flammable, non-explosive.

5. Environmental: Zero Ozone Depletion Potential (ODP), low Global Warming Potential (GWP).

6. Economic & Practical: Low cost, easily available, compatible with lubricants, good leak detection capability.

1.5 Classification of Refrigerants (ASHRAE)

• CFCs (Chlorofluorocarbons): e.g., R-11, R-12. High ODP & GWP. Phased out.

• HCFCs (Hydrochlorofluorocarbons): e.g., R-22. Low ODP, moderate GWP. Transitional substitutes.

• HFCs (Hydrofluorocarbons): e.g., R-134a, R-410A. Zero ODP, high GWP.

• Natural Refrigerants: e.g., Ammonia (R-717), CO₂ (R-744), Hydrocarbons (R-290, R-600a). Zero ODP, low GWP.

• Blends: Zeotropic (temperature glide during phase change, e.g., R-404A) & Azeotropic (constant boiling, e.g., R-500).

1.6 Refrigerant Numbering System (ASHRAE Standard 34)

• Format: R- followed by a number.

• Rule for Simple Compounds:

  • First digit: Number of carbon atoms minus 1.

  • Second digit: Number of hydrogen atoms plus 1.

  • Third digit: Number of fluorine atoms.

  • Remaining atoms are chlorine (if any).

  • Example: R-134a → C₂H₂F₄ (2 C, 4 H, 4 F). The 'a' denotes it's an isomer (different arrangement) of R-134.

• Special Cases:

  • R-717: Ammonia (NH₃). 7 H, 1 N.

  • R-744: Carbon Dioxide (CO₂). 4 O, 1 C.

  • R-290: Propane (C₃H₈).

1.7 Specific Refrigerants: Properties & Applications

2.0 REFRIGERATION CYCLES & PERFORMANCE

2.1 Reversed Carnot Cycle

• Ideal Cycle consisting of two reversible isothermal and two reversible adiabatic processes.

• T-s & P-V Diagrams: Two vertical (adiabatic) and two horizontal (isothermal) lines.

• COP for Refrigeration:

$$\text{COP}_{\text{rev}} = \frac{T_L}{T_H - T_L}$$

where $$\displaystyle T_L $$ = Evaporator (low) temperature (K), $$\displaystyle T_H $$ = Condenser (high) temperature (K).

• COP for Heat Pump:

$$\text{COP}_{\text{HP}} = \frac{T_H}{T_H - T_L} = \text{COP}_{\text{rev}} + 1$$

• COP for Heat Engine:

$$\eta_{\text{rev}} = 1 - \frac{T_L}{T_H}$$

> [!TIP] **Exam Tip:** Carnot COP is the **maximum theoretical COP** for given temperature limits. All practical cycles have lower COP.

2.2 Vapour Compression Refrigeration System (VCRS)

2.2.1 Components & Functions

1. Compressor: Sucks low-pressure vapour, compresses it to high pressure & temperature.

2. Condenser: Rejects heat to surroundings; condenses high-pressure vapour to liquid.

3. Expansion Device (Throttle Valve): Causes pressure drop, produces cold low-pressure liquid-vapour mixture.

4. Evaporator: Absorbs heat from the refrigerated space; evaporates liquid to vapour.

2.2.2 Theoretical Cycle on P-h & T-s Diagrams

• Process 1-2: Isentropic compression (s₁ = s₂).

• Process 2-3: Isobaric heat rejection (condensation).

• Process 3-4: Isenthalpic throttling (h₃ = h₄).

• Process 4-1: Isobaric heat absorption (evaporation).

2.2.3 COP Calculation

$$\text{COP} = \frac{\text{Refrigeration Effect}}{\text{Work Input}} = \frac{h_1 - h_4}{h_2 - h_1}$$

[!TIP] Key: h₁ = enthalpy at compressor inlet (evaporator exit), h₂ = enthalpy at compressor exit, h₃ = enthalpy at condenser exit, h₄ = enthalpy at evaporator inlet (after throttle, h₄ = h₃).

2.2.4 Effect of Suction & Discharge Pressure on COP & Capacity

• Lower Suction Pressure (Lower Evap. Temp):

  • COP ↓ (Refrigeration effect decreases slightly, work input increases significantly).

  • Capacity ↓ (Refrigeration effect per kg decreases, volumetric capacity decreases).

• Higher Discharge Pressure (Higher Cond. Temp):

  • COP ↓ (Work input increases significantly).

  • Capacity ↓ (Refrigeration effect per kg decreases).

2.3 Practical Vapour Compression Cycle

2.3.1 Superheating at Compressor Inlet

• Vapour at evaporator exit is superheated (T₁ > T_evap) to ensure 100% dry vapour enters compressor, preventing liquid slugging.

• Effect:

  • Refrigeration Effect ↑ (h₁ increases).

  • Work Input ↑ (h₂ increases due to larger volume).

  • COP may increase or decrease depending on refrigerant and degree of superheat. Usually slight increase for low superheat.

  • Capacity ↑ (more mass flow for same compressor displacement).

2.3.3 Sub-cooling (Undercooling) of Liquid Refrigerant

• Liquid at condenser exit is cooled below its saturation temperature (T₃ < T_cond).

• Effect:

  • Refrigeration Effect ↑ (h₄ = h₃ decreases, so h₁ - h₄ increases).

  • Work Input unchanged (h₂ - h₁ same).

  • COP ↑.

  • Capacity ↑ (for same mass flow).

2.4 Multistage Compression & Intercooling

2.4.1 Need for Multistage Compression

• Required when pressure ratio is high (> 8-10). Prevents excessive discharge temperature, improves efficiency, reduces work input.

2.4.2 Function & Advantages of Liquid Intercooler

• Function: Cool the refrigerant vapour between compression stages using a separate liquid refrigerant stream from the condenser.

• Advantages:

  • Reduces work of compression (approaches isothermal compression).

  • Reduces discharge temperature.

  • Improves volumetric efficiency & lubrication.

  • Increases COP.

2.4.3 Boot-strap Cycle (Flash Intercooler)

• Description: A special multistage cycle where part of the liquid from the condenser is throttled into the intercooler to provide intercooling by flash evaporation.

• COP Expression (for 2-stage with perfect intercooling & no superheat/sub-cooling):

$$\text{COP} = \frac{h_1 - h_9}{2(h_2 - h_1)}$$

(Where state points follow standard boot-strap cycle notation).

2.5 Cascade Refrigeration System

2.5.1 Necessity

• Required for very low temperatures (below -80°C). A single-stage system would have an impractically high pressure ratio.

2.5.2 Comparison: Multistage vs. Cascade

2.5.3 Cascade System Components

• Low-Temperature (LT) Cycle: Evaporator → LT Compressor → Cascade Condenser (HT Evaporator) → LT Expansion Valve.

• High-Temperature (HT) Cycle: Cascade Condenser (LT Condenser) → HT Compressor → Ambient Condenser → HT Expansion Valve.

• Cascade Heat Exchanger: The interface where LT cycle rejects heat and HT cycle absorbs heat.

2.6 Vapour Absorption Refrigeration System (VARS)

2.6.1 Basic Principle & Comparison with VCRS

• Principle: Uses a thermal compressor (absorber + pump + generator) instead of a mechanical compressor.

• Key Difference:

  | VCRS | VARS | | :--- | :--- | | Compressor (mechanical work) raises pressure of refrigerant vapour. | Absorber absorbs refrigerant vapour into absorbent (creates low pressure). Pump raises pressure of strong solution. Generator separates refrigerant vapour using heat. | | High-grade energy (work). | Low-grade energy (heat, e.g., waste heat, solar). |

2.6.2 Practical Aqua-Ammonia (NH₃-H₂O) System

• Components: Generator, Condenser, Evaporator, Absorber, Solution Pump, Heat Exchangers (Rectifier, Analyser), Expansion Valve.

• Working:

  1. Strong NH₃-H₂O solution from absorber is pumped to generator.

  2. Heat in generator boils off NH₃ vapour (weak solution returns to absorber).

  3. NH₃ vapour condenses in condenser to liquid.

  4. Liquid NH₃ throttles to evaporator, absorbs heat, evaporates.

  5. NH₃ vapour is absorbed by water in absorber (exothermic), creating strong solution, completing cycle.

2.6.3 Lithium Bromide (LiBr-H₂O) System

• Refrigerant: Water (H₂O). Absorbent: LiBr.

• Key Feature: Operates under high vacuum because water's saturation pressure at typical cooling temperatures (5-10°C) is very low.

• Components: Similar to NH₃-H₂O but no rectifier/analyser needed (water is pure refrigerant). Solution pump handles concentrated LiBr solution (corrosive).

2.6.4 Properties of Ideal Refrigerant-Absorbent Combination

1. High Solubility of refrigerant in absorbent at low temperature (absorber).

2. Low Solubility at high temperature (generator) for easy separation.

3. High Latent Heat of Vaporization of refrigerant.

4. Low Viscosity for easy pumping.

5. Chemical Stability & non-corrosive.

6. Large Affinity (strong absorption).

7. Low Cost & Non-toxic.

2.6.5 Advantages of VARS over VCRS

• Utilizes low-grade thermal energy (waste heat, solar, gas flame).

• Quiet operation (no moving parts in the refrigerant circuit, except pump).

• Can be used where electricity is scarce/expensive.

• Disadvantages: Very low COP (0.4-0.7), large size, requires cooling water for absorber & condenser, risk of crystallization (LiBr).

3.0 AIR REFRIGERATION SYSTEMS

3.1 Dense Air Refrigeration System (Reverse Brayton/Joule Cycle)

• Cycle: Compression → Cooling at constant pressure → Expansion (turbine or throttling) → Heating at constant pressure.

• Components: Compressor, Cooler (after compression), Refrigerated Load (expansion turbine or valve), Evaporator/Cooler (before compressor).

• T-s & P-V Diagrams: Two isobaric and two adiabatic (or isentropic) processes.

• Advantages over Open (Bell-Coleman) Cycle:

  1. No moisture freezing in heat exchanger (air is dense, above dew point).

  2. Higher COP (due to cooler after compression, reducing turbine work).

  3. Continuous cooling (no intermittent process).

• Applications: Aircraft refrigeration (primary system), some industrial processes.

3.2 Aircraft Refrigeration

3.2.1 Challenges

• Ram Effect: High-speed flight compresses incoming air, increasing its temperature and pressure.

• Low Ambient Pressure & Temperature at high altitude.

• High Heat Loads from occupants, electronics, solar radiation, and skin friction.

• Weight & Volume Constraints.

3.2.2 Simple Dense Air System for Aircraft

1. Ram air enters compressor (ram pressure rise).

2. Compressed air cooled by ram air in ram air heat exchanger.

3. Cooled dense air expanded in turbine to produce cold air.

4. Cold air supplied to cabin. Turbine exhaust used for cabin pressurization or dumped overboard.

3.2.3 Numerical Problems

• Key Parameters: Ram Pressure Ratio (P₂/P₁), Compression Ratio (P₃/P₂), Mass Flow Rate (ṁ), Cooling Load (Q_L).

• Power Calculation:

$$W_{\text{comp}} = \dot{m} C_p (T_3 - T_2)$$

$$W_{\text{turb}} = \dot{m} C_p (T_4 - T_5)$$

$$W_{\text{net}} = W_{\text{comp}} - W_{\text{turb}}$$

• Refrigeration Effect:

$$Q_L = \dot{m} C_p (T_{\text{cabin}} - T_4)$$

> [!TIP] **Assumptions:** Often assume isentropic compressor/turbine, ideal gas for air (Cp = 1.005 kJ/kgK, γ = 1.4). Use ram equations to find T₂, P₂ from flight speed & altitude.

3.3 Steam Jet Refrigeration System

3.3.1 Principle

• Uses momentum transfer from a high-velocity steam jet (primary fluid) to entrain and compress low-pressure vapour (secondary fluid - usually the same steam after condensation) in an ejector.

• Throttling & Evaporative Cooling: The compressed vapour is condensed; the remaining low-pressure vapour provides refrigeration by evaporating at low temperature in the evaporator.

3.3.2 Components & Working

1. Steam Boiler: Produces high-pressure motive steam.

2. Ejector: Steam jet creates suction, compresses vapour from evaporator.

3. Condenser: Condenses compressed vapour (steam + refrigerant vapour).

4. Separator: Separates condensate (water) from steam.

5. Evaporator: Low-pressure refrigerant evaporates, absorbing heat.

6. Pump/Throttle: Returns condensate to boiler or throttles to evaporator.

3.3.3 T-s & H-s Diagrams

• T-s: Process 1-2 (throttling of motive steam), 2-3 (mixing in ejector), 3-4 (condensation), 4-5 (throttling to evaporator), 5-1 (evaporation).

• H-s (Mollier): Steep drop in enthalpy across ejector due to inefficiency.

3.3.4 Applications & Limitations

• Applications: Where cheap steam is available (cogeneration plants, sugar mills), large cold storage, ice plants.

• Limitations: Very low COP (0.1-0.3), requires large steam flow, noisy, high maintenance of ejector nozzles.

4.0 PSYCHROMETRY & PSYCHROMETRIC PROCESSES

4.1 Psychrometric Terms & Definitions

4.2 Psychrometric Processes on Chart

• Sensible Heating/Cooling: Constant ω (horizontal line). Heat added/removed without moisture change.

$$Q_{\text{sensible}} = \dot{m}_{da} C_{p,da} (t_2 - t_1)$$

• Humidification:

  • Constant DBT: Steam injection (vertical line up).

  • Constant WBT: Adiabatic humidification (e.g., spray tower) - follows adiabatic saturation line.

• Dehumidification/Cooling & Dehumidification: Cooling below dew point. Constant WBT (approx.) for simple cooling coil. Process follows constant wet-bulb line until air is saturated.

• Mixing of Two Air Streams:

  • Mass Balance: $$\displaystyle \dot{m}_{da1} + \dot{m}_{da2} = \dot{m}_{da3} $$

  • Moisture Balance: $$\displaystyle \dot{m}_{da1}\omega_1 + \dot{m}_{da2}\omega_2 = \dot{m}_{da3}\omega_3 $$

  • Graphical: Mixing point lies on straight line joining two inlet states. Distance from point inversely proportional to mass flow.

• Fog Formation: Occurs when RH > 100% (supersaturation). On chart, state lies to the right of 100% RH line. Caused by rapid cooling or excessive humidification.

4.3 Cooling Coil Performance

4.3.1 Apparatus Dew Point (ADP)

• The temperature at which air would be saturated if cooled adiabatically to saturation. It is the effective surface temperature of the cooling coil.

• On chart, it's the intersection of the constant wet-bulb line (process line) with the saturation curve.

4.3.2 By-pass Factor (BPF)

• Definition: Fraction of air that by-passes the coil without being affected (i.e., leaves at entering condition).

• Formula:

$$\text{BPF} = \frac{t_1 - t_2}{t_1 - t_{ADP}} = \frac{\omega_1 - \omega_2}{\omega_1 - \omega_{ADP}}$$

where $$\displaystyle t_1, \omega_1 $$ = inlet air state; $$\displaystyle t_2, \omega_2 $$ = outlet air state; $$\displaystyle t_{ADP}, \omega_{ADP} $$ = ADP state.

• Significance: Measures coil effectiveness. Lower BPF = better coil performance.

• Factors Affecting BPF: Coil surface area, fin density, air velocity, number of rows, ADP.

4.3.3 Cooling & Dehumidification Coil Calculation

1. Find outlet state ($$\displaystyle t_2, \omega_2 $$) using BPF formula.

2. Sensible Cooling Load:

$$Q_S = \dot{m}_{da} C_{p,da} (t_1 - t_2)$$

3. Latent Cooling Load (Condensation rate):

$$\dot{m}_{\text{cond}} = \dot{m}_{da} (\omega_1 - \omega_2)$$

$$Q_L = \dot{m}_{\text{cond}} \cdot h_{fg} \approx \dot{m}_{\text{cond}} \times 2501\ \text{kJ/kg}$$

4. Total Cooling Load:

$$Q_T = Q_S + Q_L = \dot{m}_{da} (h_1 - h_2)$$

5. Cooling Capacity in TR:

$$\text{Capacity (TR)} = \frac{Q_T\ (\text{kW})}{3.516}$$

5.0 SUMMER AIR CONDITIONING & LOAD CALCULATION

5.1 Basic Requirements of AC

• Temperature Control

• Humidity Control

• Air Motion & Distribution

• Air Purity (filtration, ventilation)

5.2 Comfort vs. Industrial AC

5.3 Summer Air Conditioning Load Calculation

5.3.1 Sensible Heat Load Sources

• Transmission through walls, roof, windows: $$\displaystyle Q = U A \Delta t $$.

• Solar Radiation through windows: $$\displaystyle Q = A \cdot SHGF \cdot SC $$.

• Occupants: Sensible part ≈ 60 W/person.

• Lighting & Equipment: All sensible.

5.3.2 Latent Heat Load Sources

• Infiltration (outside air entry): $$\displaystyle \dot{m}_v \cdot \omega_{\text{outside}} $$.

• Occupants: Latent part ≈ 45 W/person (≈ 0.05 kg/h water vapour).

• Processes: Cooking, drying, etc.

5.3.3 Grand Sensible Heat Factor (GSHF) & Room Sensible Heat Factor (RSHF)

• RSHF: Ratio of room sensible heat to total room heat (sensible + latent).

$$\text{RSHF} = \frac{Q_{S,\text{room}}}{Q_{S,\text{room}} + Q_{L,\text{room}}}$$

• GSHF: Ratio of total heat to be removed by coil (room heat + outdoor air heat) to total sensible heat to be removed.

$$\text{GSHF} = \frac{Q_{T,\text{total}}}{Q_{S,\text{total}}}$$

where $$\displaystyle Q_{T,\text{total}} = (Q_S + Q_L)_{\text{room}} + \dot{m}_v (h_{\text{mix}} - h_{\text{return}}) $$.

• Use on Psychrometric Chart: Plot RSHF line from Room Condition point. Plot GSHF line from Mixed Air Condition point (after mixing return & outside air). Intersection with saturation curve gives Supply Air Condition.

5.3.4 Outside Air Factor (OAF) for Ventilation

• Fraction of total supply air that is fresh outside air.

$$\text{OAF} = \frac{\dot{m}_v}{\dot{m}_{\text{total}}}$$

where $$\displaystyle \dot{m}_v $$ = mass flow rate of outside air for ventilation.

5.3.5 Total Cooling Load & Supply Air Mass Flow Rate

1. Find Mixed Air Condition (from return air, outside air, OAF).

2. Plot RSHF line from Room Condition.

3. Plot GSHF line from Mixed Air Condition.

4. Intersection point = Supply Air Condition ($$\displaystyle t_s, \omega_s $$).

5. Total Cooling Load:

$$Q_T = \dot{m}_{da,\text{total}} (h_{\text{mixed}} - h_s)$$

6. Supply Air Mass Flow Rate (dry air basis):

$$\dot{m}_{da,\text{total}} = \frac{Q_S}{C_{p,da} (t_{\text{room}} - t_s)} = \frac{Q_T}{(h_{\text{mixed}} - h_s)}$$

6.0 SPECIAL TOPICS & APPLICATIONS

6.1 Refrigeration for Food Preservation

• Spoilage Mechanisms:

  1. Microbial Growth: Bacteria, yeast, mold multiply rapidly above 10°C.

  2. Enzymatic Action: Natural enzymes cause ripening, discoloration, off-flavors.

  3. Chemical Reactions: Oxidation, non-enzymatic browning.

• How Refrigeration Controls Spoilage: Reduces temperature, which exponentially decreases reaction rates (Q₁₀ rule: rate halves for every 10°C drop). Slows microbial growth, enzyme activity, and chemical reactions.

• Applications: Cold storage (chill 0-4°C, freeze -18°C to -30°C), refrigerated transport, display cabinets, food processing.

6.2 Production of Dry Ice (Solid CO₂)

• Process: From liquid CO₂ stored in high-pressure vessels.

  1. Liquid CO₂ is forced through a nozzle or pelletizer into a chamber at atmospheric pressure.

  2. Due to throttling (Joule-Thomson effect), part of liquid flashes to solid (snow) and cold vapour.

  3. The solid is compressed into blocks, pellets, or nuggets.

• Key Point: CO₂ sublimes at -78.5°C at atmospheric pressure (no liquid phase at 1 atm).

6.3 Environment Friendly Refrigerants (Natural Refrigerants)

6.4 Leak Detection Methods

1. Soap Solution: Apply to joints; bubbles indicate leak. Simple, for large leaks.

2. Electronic Leak Detectors: Sniffers sensitive to refrigerant molecules. Most common for HVAC/R.

3. UV Dye: Inject dye into system; leaks show under UV light. Good for hard-to-find leaks.

4. Halide Torch (for CFCs/HCFCs): Flame color changes (green) in presence of halogen. Obsolete, hazardous.

5. Pressure Decay/Vacuum Decay: Monitor pressure change in isolated section.

6.5 Refrigeration System Components – Focus on Compressor

6.5.1 Function in VCRS

• Primary Function: Increase pressure and temperature of refrigerant vapour.

• How: By doing work on the refrigerant (mechanical compression - piston, scroll, screw, centrifugal).

• Result: Creates pressure difference (high side to low side) enabling continuous cycle.

6.5.2 How Function is Achieved in VARS

• No mechanical compressor for refrigerant vapour.

• Pressure increase is achieved in two steps:

  1. Absorber: Refrigerant vapour is absorbed into absorbent (e.g., NH₃ into H₂O). This creates a very low partial pressure of refrigerant in the absorber, equivalent to the evaporator pressure.

  2. Solution Pump: The strong solution (refrigerant + absorbent) is pumped to the high-pressure side (generator pressure).

  3. Generator: Heat is applied to the strong solution, distilling refrigerant vapour at high pressure.

• Analogy: Absorber creates "suction", pump increases solution pressure, generator "compresses" refrigerant by thermal means.

6.6 Refrigeration of Food – Cooling Load Calculation

• Total Cooling Load = Sensible Heat + Latent Heat of Freezing + Sensible Heat below Freezing.

$$Q_{\text{total}} = \dot{m} \left[ C_{\text{above}} (t_{\text{initial}} - t_f) + \lambda_f + C_{\text{below}} (t_f - t_{\text{final}}) \right]$$

where:

*   $\dot{m}$ = mass flow rate of food (kg/s or kg/h).

*   $$\displaystyle C_{\text{above}} $$ = specific heat above freezing point (kJ/kg·K).

*   $$\displaystyle t_f $$ = freezing point temperature (°C).

*   $$\displaystyle \lambda_f $$ = latent heat of freezing (kJ/kg).

*   $$\displaystyle C_{\text{below}} $$ = specific heat below freezing point (kJ/kg·K).

• Example: Cooling fish from 27°C to -23°C, freezing point -3°C.

$$Q = \dot{m} [C_1(27 - (-3)) + \lambda + C_2((-3) - (-23))]$$

• Power Required:

$$P = \frac{Q_{\text{total}}}{\text{COP}}$$

> [!TIP] If actual COP is given (e.g., half of Carnot), use it. Carnot COP uses evaporator & condenser temperatures corresponding to final product temp & ambient.

END OF UNIT 1 NOTES
Aligned with RGPV past paper pattern (Jun 2025, May 2024, May 2023, May 2022). Focus on definitions, derivations (TR, COP), cycle analysis (VCRS, VARS, Cascade), psychrometric calculations (BPF, ADP, mixing, loads), and special applications.