UNIT 5: REFRIGERATION & AIR CONDITIONING - SHORT NOTES

Based on analysis of past examination papers (Jun 2025, May 2024, May 2023, May 2022).

1.0 FUNDAMENTAL CONCEPTS & PERFORMANCE METRICS

1.1 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.

• Numerical Value:

  • $$\displaystyle 1 \text{ TR} = 3.517 \text{ kW} $$

  • $$\displaystyle 1 \text{ TR} = 12000 \text{ Btu/h} $$

• Calculation Basis: Latent heat of ice = 335 kJ/kg.

  • Refrigeration required per day = $$\displaystyle 1000 \text{ kg} \times 335 \text{ kJ/kg} = 335,000 \text{ kJ} $$

  • Refrigeration per hour = $$\displaystyle \frac{335,000}{24} \approx 13,958 \text{ kJ/h} \approx 3.877 \text{ kW} $$.

  • Standard value adopted: 3.517 kW (based on older imperial definition).

• Exam Problems: Used to find mass of ice produced, power input, or COP when capacity in TR is given.

1.2 Coefficient of Performance (COP)

• Definition: Ratio of desired effect (refrigeration or heating) to the work input required.

  • Refrigerator/Heat Pump: $$\displaystyle \text{COP} = \frac{\text{Refrigeration Effect (} Q_L \text{)}}{\text{Net Work Input (} W_{net} \text{)}} $$

  • Heat Engine: $$\displaystyle \text{COP (or Efficiency)} = \frac{\text{Net Work Output}}{\text{Heat Input}} = \frac{W_{net}}{Q_H} $$

• Carnot COP (Reversible Cycle):

  • Refrigerator/Heat Pump: $$\displaystyle \text{COP}_{\text{Carnot}} = \frac{T_L}{T_H - T_L} $$

  • Heat Engine: $$\displaystyle \eta_{\text{Carnot}} = 1 - \frac{T_L}{T_H} $$

  • Where $$\displaystyle T_L $$ = Lowest absolute temperature (evaporator), $$\displaystyle T_H $$ = Highest absolute temperature (condenser).

• Relationship: For same $$\displaystyle T_L $$ and $$\displaystyle T_H $$, $$\displaystyle \text{COP}_{\text{HP}} = \text{COP}_{\text{Ref}} + 1 $$.

1.3 Refrigeration Effect & Capacity

• Refrigeration Effect ($$\displaystyle q_L $$): Amount of heat absorbed in the evaporator per kg of refrigerant. (kJ/kg)

• Capacity: Rate of refrigeration.

  • $$\displaystyle \text{Capacity (kW)} = \dot{m}_r \times q_L $$

  • $$\displaystyle \text{Capacity (TR)} = \frac{\dot{m}_r \times q_L}{3.517} $$

  • Where $$\displaystyle \dot{m}_r $$ = mass flow rate of refrigerant (kg/s).

[!TIP]

Common Pitfall: Confusing COP with Efficiency. COP can be >1 for refrigeration/heat pumps; efficiency for heat engines is always <1. Always use absolute temperatures (K) for Carnot COP.

2.0 VAPOUR COMPRESSION REFRIGERATION SYSTEMS (VCRS)

2.1 Theoretical (Ideal) Cycle

• Processes (on P-h, T-S diagrams):

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

  2. 2-3: Constant pressure condensation (heat rejection $$\displaystyle Q_H $$).

  3. 3-4: Throttling (isenthalpic, h3 = h4).

  4. 4-1: Constant pressure evaporation (heat absorption $$\displaystyle Q_L $$).

• COP Expression: $$\displaystyle \text{COP} = \frac{h_1 - h_4}{h_2 - h_1} $$

  • $$\displaystyle h_1 $$ = enthalpy at evaporator exit (compressor inlet).

  • $$\displaystyle h_2 $$ = enthalpy at compressor exit.

  • $$\displaystyle h_3 $$ = enthalpy at condenser exit.

  • $$\displaystyle h_4 = h_3 $$ (throttling).

2.2 Practical (Actual) Cycle

• Superheating (1-1'): Vapour at compressor inlet is superheated.

  • Effect: Increases refrigeration effect ($$\displaystyle h_1' - h_4 $$) and compressor work ($$\displaystyle h_2' - h_1' $$). Net effect on COP depends on degree of superheat.

• Sub-cooling (Undercooling) (3-3'): Liquid at condenser exit is cooled below saturation temperature.

  • Effect: Increases refrigeration effect ($$\displaystyle h_1 - h_4' $$) without increasing compressor work. Improves COP.

• Effect of Pressures:

  • Lower Suction Pressure (Lower $$\displaystyle T_{evap} $$): Decreases refrigeration effect per kg, increases compressor work ratio → COP decreases, capacity decreases.

  • Higher Discharge Pressure (Higher $$\displaystyle T_{cond} $$): Increases compressor work, decreases refrigeration effect → COP decreases.

• Volumetric Efficiency ($$\displaystyle \eta_v $$): $$\displaystyle \eta_v = \frac{\text{Actual volume sucked}}{\text{Piston displacement volume}} $$. Affects effective capacity.

2.3 P-H (Pressure-Enthalpy) Chart

• Construction: Pressure (log scale) vs. Enthalpy. Shows saturation dome (liquid-vapour region).

• Interpretation:

  • Left of dome: compressed liquid.

  • Right of dome: superheated vapour.

  • Inside dome: wet vapour (quality $x$).

  • Constant temperature lines (isotherms) are horizontal in wet region.

  • Constant entropy lines (isentropes) are vertical in wet region.

• Plotting Cycle: Locate points 1 (evap exit), 2 (comp exit, isentropic from 1), 3 (cond exit), 4 (throttle from 3, h4=h3).

• Determining Properties: Read $h$, $P$, $T$, quality from chart.

2.4 Multistage Compression

• Need: High pressure ratios cause high discharge temperature, low volumetric efficiency, high work input.

• Liquid Intercooler:

  • Function: Cool the refrigerant liquid between stages (after condenser of high stage, before entering low-stage evaporator or as separate exchanger).

  • Advantages:

    1. Reduces compressor work (lower $T$ at inlet to second stage).

    2. Improves lubrication (lower discharge temp).

    3. Reduces discharge temperature.

    4. Increases refrigeration effect (sub-cooling effect).

• Boot-strap Cycle (Flash Intercooling):

  • Principle: Use a flash chamber after high-stage compression. Part of high-pressure liquid flashes to vapour, cooling the remaining liquid. The vapour is routed to the low-stage compressor inlet.

  • Working: High-stage discharge → Flash chamber → Liquid (to expansion valve) + Vapour (to low-stage suction).

  • COP Expression: $$\displaystyle \text{COP} = \frac{h_1 - h_4}{(h_2 - h_1) + (h_5 - h_4)} $$ (for two-stage with flash intercooling, where 5 is low-stage inlet from flash chamber).

2.5 Cascade Refrigeration Systems

• Definition: Two or more independent vapour compression cycles operating at different pressure levels, coupled via a cascade heat exchanger.

• Necessity: To achieve very low temperatures (below -80°C) where single-stage compression is inefficient or impractical.

• Comparison with Multistage:

  | Feature | Multistage Compression | Cascade System | | :--- | :--- | :--- | | Compressors | Multiple stages in series, often same refrigerant | Separate compressors, different refrigerants common | | Circuit | Single continuous refrigerant circuit | Multiple independent circuits | | Inter-stage Cooling | Intercooler (liquid or flash) | Cascade heat exchanger (condenser of low cycle = evaporator of high cycle) | | Flexibility | Less flexible | More flexible (can optimize each stage) | | Complexity | Lower | Higher (more components, controls) |

[!TIP]

Key Difference: In multistage, refrigerant is same throughout; in cascade, different refrigerants can be used for low/high stages (e.g., R-23 for low, R-404A for high).

3.0 VAPOUR ABSORPTION REFRIGERATION SYSTEMS (VARS)

3.1 Basic Principle & Comparison with VCRS

• VCRS: Uses a compressor to increase pressure and temperature of refrigerant vapour. Requires significant mechanical work.

• VARS: Replaces compressor with absorber and generator.

  • Absorber: Low-pressure refrigerant vapour is absorbed by a liquid absorbent (e.g., NH₃ into H₂O). This creates a low-pressure solution.

  • Generator: The strong solution is pumped to a high-pressure generator and heated (by waste heat, steam, etc.). Refrigerant vapour is driven off at high pressure.

  • Pump: Requires little work to circulate the liquid solution (vs. compressing vapour).

• Advantages of VARS:

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

  • Quiet operation (no moving parts in vapour circuit).

  • Can be used where electricity is scarce/expensive.

3.2 Practical Vapour Absorption Cycle (Aqua-Ammonia)

• Components:

  1. Generator: Heat input → strong NH₃-H₂O solution → high-pressure NH₃ vapour + weak solution.

  2. Condenser: NH₃ vapour condenses to liquid.

  3. Evaporator: Liquid NH₃ evaporates at low pressure → refrigeration effect.

  4. Absorber: Evaporated NH₃ vapour absorbed by cool water → strong solution.

  5. Solution Pump: Pressurizes strong solution to generator pressure.

  6. Throttle Valve: Weak solution from generator to absorber.

  7. Heat Exchanger (optional): Economizer to pre-heat strong solution / cool weak solution.

• Working: Follows the path on T-s or P-h diagram: 1 (weak sol) → 2 (strong sol, pump) → 3 (vapour + weak sol, generator) → 4 (liquid, condenser) → 5 (vapour, evaporator) → 6 (strong sol, absorber).

3.3 Refrigerant-Absorbent Combination

• Desirable Properties:

  1. High affinity (refrigerant readily absorbed).

  2. Large difference in boiling points (refrigerant boils off easily in generator).

  3. Low viscosity, high thermal conductivity.

  4. Chemical stability (no decomposition).

  5. Non-corrosive.

  6. Low freezing point (for absorbent).

  7. Low vapour pressure at operating temperatures (for absorbent).

• Common Pairs:

  • Ammonia (Refrigerant) - Water (Absorbent): Most common for industrial/commercial. High COP.

  • Lithium Bromide (Absorbent) - Water (Refrigerant): Used for large building AC. Requires vacuum operation, risk of crystallization.

4.0 AIR REFRIGERATION SYSTEMS (Dense Air Cycle)

4.1 Dense Air vs. Open Air System

• Dense Air (Closed) Cycle: Air is compressed, cooled, expanded in a turbine (or throttled), then cooled in the evaporator. Air is re-circulated in a closed loop. Used in aircraft.

• Open Air Cycle: Ambient air is compressed, cooled, expanded, and ejected overboard. Simple but inefficient.

• Advantages of Dense Air System:

  • No loss of refrigerant (air).

  • Can maintain cabin pressure independently.

  • More efficient than open cycle (uses expansion work).

4.2 Aircraft Refrigeration Systems

• Need: At high altitude, ambient air is very cold but at low pressure. Cabin must be pressurized and cooled.

• Process: Ambient air → Ram effect (compression due to aircraft speed) → Main compressor → Heat exchanger (cooling with ambient air) → Turbine (expansion to cabin pressure, produces cooling) → Cabin.

• Calculations (per kg/s of air flow):

  1. Power for Pressurization ($$\displaystyle W_p $$): Work to compress air from ambient pressure $$\displaystyle P_a $$ to cabin pressure $$\displaystyle P_c $$.

$$W_p = C_p (T_2 - T_1)$$

    (Isentropic compression from state 1 to $$\displaystyle P_c $$, then cooling to $$\displaystyle T_2 $$).

2.  **Additional Power for Refrigeration ($$\displaystyle W_t $$):** Work extracted in turbine to expand from $$\displaystyle P_c $$ to $$\displaystyle P_a $$.

$$W_t = C_p (T_3 - T_4)$$

    (Isentropic expansion from state 3 to $$\displaystyle P_a $$).

3.  **Net Power Input:** $$\displaystyle W_{net} = W_p - W_t $$ (if turbine drives compressor) or total if separate.

4.  **Refrigerating Effect ($$\displaystyle q_L $$):** $$\displaystyle q_L = C_p (T_4 - T_{ambient}) $$ or $$\displaystyle C_p (T_4 - T_5) $$ where $$\displaystyle T_5 $$ is supply air temp.

• Key Point: Air acts as the refrigerant in a reversed Brayton (Bell Coleman) cycle.

5.0 REFRIGERANTS

5.1 Classification

5.2 Desirable Properties of an Ideal Refrigerant

1. Low boiling point (at atmospheric pressure).

2. High latent heat of vaporization (large refrigeration effect per kg).

3. Low specific volume (reduces compressor size).

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

5. Chemically stable (no decomposition under operating conditions).

6. Good thermal conductivity (improves heat exchanger performance).

7. Miscible with lubricating oil (ensures proper lubrication).

8. Low cost and easily available.

9. Zero ODP and low GWP (modern requirement).

5.3 Selection Factors

• Safety: Toxicity, flammability, explosiveness.

• Environmental Impact: ODP (Ozone Depletion Potential), GWP (Global Warming Potential).

• Thermodynamic Properties: Critical temperature, pressure, latent heat, specific heat.

• Compatibility: With materials (metals, elastomers), lubricating oil.

• Cost & Availability.

5.4 Specific Refrigerants

• ASHRAE Numbering: e.g., CHClF₂ → R-22 (add 90 to number of H atoms: 1+90=91, subtract Cl atoms: 91-2=89? Actually standard: CHClF₂ has 1 C, 1 H, 1 Cl, 2 F → R-22. Rule: For methane-based, R(Number of H atoms + 90) - (Number of Cl atoms) - (Number of Br atoms) etc. Simplified: CHClF₂ → 1 H → 91, minus 1 Cl → 90? Wait, standard is R-22. Better to memorize common ones: R-12=CCl2F2, R-22=CHClF2, R-134a=CH2FCF3.*

5.5 Environment-Friendly Refrigerants

• Alternatives to CFCs/HCFCs:

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

  • HFOs: Zero ODP, very low GWP (e.g., R-1234yf for car AC).

  • Natural Refrigerants: NH₃ (R-717), CO₂ (R-744), Hydrocarbons (R-290, R-600a). Very low GWP, but may have toxicity/flammability.

• Concepts:

  • ODP (Ozone Depletion Potential): Relative to R-11 (ODP=1). Measures potential to destroy stratospheric ozone.

  • GWP (Global Warming Potential): Relative to CO₂ (GWP=1) over 100 years. Measures contribution to climate change.

6.0 PSYCHROMETRY & AIR CONDITIONING PROCESSES

6.1 Psychrometric Terms & Chart

• Psychrometric Chart: Plot of $\omega$ vs. $t$. Lines: constant RH (curved), constant $h$ (straight, sloped), constant WBT (approx. straight), constant volume (approx. straight).

6.2 Psychrometric Processes on Chart

• Sensible Heating/Cooling: Constant $\omega$ (horizontal line). Heat added/removed: $$\displaystyle Q_s = m_a C_{pa} \Delta t $$.

• Humidification (Adding Moisture): $\omega$ increases. Usually constant $t$ (vertical up) or constant WBT.

• Dehumidification (Removing Moisture): $\omega$ decreases. Usually constant $t$ (vertical down) or constant WBT.

• Heating & Humidification: Combination (right/up).

• Cooling & Dehumidification: Most common in AC. Air cooled below its dew point. Condensation occurs. Process follows constant WBT line approximately until ADP, then sensible cooling.

6.3 Cooling & Dehumidification Process

• Process: Air passes over cooling coil with surface temperature below air's dew point.

  1. Initial cooling (sensible) until dew point.

  2. Condensation starts → both sensible and latent heat removal → follows approx. constant WBT line.

  3. Final state depends on coil surface temperature.

• Apparatus Dew Point (ADP): Temperature of air leaving the coil if it were brought to saturation by contact with coil (theoretical exit condition).

• Bypass Factor (BF):

  • Definition: Fraction of air that bypasses the coil without being affected.

  • Formula:

$$ BF = \frac{h_1 - h_2}{h_1 - h_{ADP}} = \frac{t_1 - t_2}{t_1 - t_{ADP}} \text{ (if constant } \omega \text{ process)} $$

    Where:

    *   $$\displaystyle h_1, t_1 $$ = Inlet air enthalpy/DBT.

    *   $$\displaystyle h_2, t_2 $$ = **Actual** outlet air enthalpy/DBT.

    *   $$\displaystyle h_{ADP}, t_{ADP} $$ = Enthalpy/DBT of air at ADP (saturated at coil surface temp).

*   **Significance:** Measures coil effectiveness. Lower BF = better coil performance. Affected by coil design, air velocity, fin spacing.

*   **Outlet Conditions:** $$\displaystyle h_2 = h_1 - BF (h_1 - h_{ADP}) $$.

6.4 Air Mixing Problems

• Mass Balance (Dry Air): $$\displaystyle m_{a1} + m_{a2} = m_a $$

• Moisture Balance: $$\displaystyle m_{a1} \omega_1 + m_{a2} \omega_2 = m_a \omega $$

• Enthalpy Balance: $$\displaystyle m_{a1} h_1 + m_{a2} h_2 = m_a h $$

• Graphical Method: On psychrometric chart, join states 1 and 2. Mixed state lies on line connecting them. Distance ratio inversely proportional to mass flow rates.

• Fog Condition: If mixing line enters the two-phase region (inside dome), fog (visible moisture droplets) forms. Mixed state is on 100% RH line at same enthalpy as theoretical mix.

6.5 Summer Air Conditioning Load Calculations

• Sensible Heat Load ($$\displaystyle Q_s $$): Heat to be removed to lower DBT.

$$ Q_s = m_a C_{pa} (t_r - t_s) \quad \text{or} \quad Q_s = m_a (h_r - h_s)_{sensible} $$

• Latent Heat Load ($$\displaystyle Q_L $$): Heat to be removed to lower humidity.

$$ Q_L = m_a (\omega_r - \omega_s) h_{fg} \quad \text{or} \quad Q_L = m_a (h_r - h_s)_{latent} $$

• Total Heat Load ($$\displaystyle Q_T $$): $$\displaystyle Q_T = Q_s + Q_L = m_a (h_r - h_s) $$

• Sensible Heat Factor (SHF):

$$ \boxed{SHF = \frac{Q_s}{Q_T}} $$

*   Ratio of sensible to total load. Determines slope of process line on psychrometric chart.

• Grand Sensible Heat Factor (GSHF): For multiple rooms/zoned systems. Overall SHF for entire plant.

$$ GSHF = \frac{\sum Q_s}{\sum Q_T} $$

• Factors for Summer Load: Solar radiation through walls/windows, occupants, lighting, equipment, infiltration, ventilation air.

[!TIP]

Critical Skill: Always use enthalpy difference ($$\displaystyle h_r - h_s $$) for total load and mass flow rate of dry air ($$\displaystyle m_a $$). SHF determines supply air state: from room condition, follow line with slope = SHF to saturation line (or to supply condition if over-cooling).

7.0 COMFORT & INDUSTRIAL AIR CONDITIONING

7.1 Comfort Air Conditioning

• Objective: Maintain indoor environment for human comfort and health.

• ASHRAE Comfort Zone: Defined by ranges of:

  • Dry Bulb Temperature (typically 20-24°C summer, 20-24°C winter).

  • Relative Humidity (typically 30-60%).

  • Air Velocity (low, < 0.2 m/s).

  • Mean Radiant Temperature (close to DBT).

  • Clothing insulation and metabolic rate (activity level).

• Focus: Overall thermal comfort (PMV/PPD indices).

7.2 Industrial (Process) Air Conditioning

• Objective: Maintain environment for product/process requirements, not human comfort.

• Differences from Comfort AC:

  | Comfort AC | Industrial/Process AC | | :--- | :--- | | Conditions for human comfort | Conditions for product quality, process yield, equipment operation. | | Moderate latent loads (from occupants) | Often high latent loads (from process moisture). | | Wider allowable ranges | Very strict temperature/humidity control (±1°C, ±5% RH). | | Focus on air quality (ventilation) | May require high air cleanliness (e.g., semiconductor, pharma). | | 24/7 operation typical | May have specific operating schedules. |

7.3 Design Conditions & Supply Air State

• Design Conditions: Specified indoor (room) DBT, RH, and outdoor design conditions.

• Supply Air State Determination:

  1. Plot room condition ($$\displaystyle t_r, \omega_r $$) on psychrometric chart.

  2. Calculate room sensible and latent loads → find SHF.

  3. From room condition, draw line with slope = SHF towards saturation line (or below if over-cooling).

  4. Intersection with saturation line gives Apparatus Dew Point (ADP).

  5. Supply air state ($$\displaystyle t_s, \omega_s $$) is located on this line such that $$\displaystyle m_a (h_r - h_s) = Q_T $$.

  6. If bypass factor (BF) is considered, actual supply state is between ADP and inlet state: $$\displaystyle h_s = h_1 - BF (h_1 - h_{ADP}) $$.

8.0 SPECIAL REFRIGERATION SYSTEMS & APPLICATIONS

8.1 Steam Jet Refrigeration System

• Principle: Uses a high-pressure steam jet as the motive fluid. Steam expands through a nozzle, creating a vacuum in the evaporator. Low-pressure refrigerant (usually water) flashes/evaporates at low temperature, producing refrigeration.

• Components: Steam generator, steam nozzle, evaporator (at low pressure), condenser, condensate pump, ejector.

• Working (T-s/H-s Diagram):

  1. High-pressure steam (1) expands isentropically in nozzle to low pressure (2).

  2. Low pressure at nozzle exit induces refrigerant vapour from evaporator (3) and mixes.

  3. Mixture (4) is condensed in condenser.

  4. Condensate (5) pumped to evaporator pressure.

• Applications: Where cheap/waste steam is available (e.g., chemical plants, ice plants, ship refrigeration).

• Limitations: Low COP (typically 0.5-0.7), requires high-quality steam, large water flow rate.

8.2 Production of Dry Ice (Solid CO₂)

• Process:

  1. Liquid CO₂ is stored under pressure (≈ 20-25 bar) at ambient temperature.

  2. When released to atmospheric pressure through a nozzle or plate, it flash evaporates.

  3. Latent heat of vaporization is drawn from the remaining liquid, causing a portion to freeze into solid snow-like CO₂.

  4. This snow is compressed into blocks or pellets (dry ice).

• Key Point: Uses the Joule-Thomson effect (throttling) of CO₂ at its critical temperature region.

8.3 Applications of Refrigeration for Food Preservation

• Control of Microbial Growth: Low temperatures slow down or inhibit growth of bacteria, yeasts, molds.

• Control of Enzymatic Activity: Enzymes causing ripening, spoilage, texture change are slowed.

• Moisture Loss Prevention: Low temperature reduces evaporation from food, maintaining weight and quality.

• Other Applications: Ice making, cold storage (fruits, vegetables, meat, fish), refrigerated transport, display cabinets, food processing (freezing, chilling).

9.0 AUXILIARY & MAINTENANCE TOPICS

9.1 Leak Detection Methods

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

• Electronic Leak Detectors: Sniffing devices sensitive to refrigerant gases. Common for HFCs/HCFCs.

• UV Dye: Add dye to system; leaks show under UV light. Good for hard-to-find leaks.

• Halide Torch: Uses copper flame color change (green for CFCs/HCFCs). Obsolete due to toxicity/CFCs.

• Pressure Decay Test: Pressurize system with inert gas (N₂); monitor pressure drop over time.

• Ultrasonic: Detects sound of gas escaping.

9.2 By-Pass Factor of Cooling Coil

• Definition: Fraction of air that bypasses the coil without any change in state.

$$ BF = \frac{\text{Enthalpy difference (inlet to outlet)}}{\text{Enthalpy difference (inlet to ADP)}} $$

• Factors Affecting BF:

  • Coil surface area (larger area → lower BF).

  • Air velocity over coil (higher velocity → higher BF).

  • Fin spacing and design.

  • Number of rows.

• Significance: Determines actual outlet air condition. A coil with BF=0.2 means 20% of air is unaffected, 80% leaves at ADP.

9.3 Fog in Air

• Definition: Condition where air contains visible water droplets suspended. Occurs when air is supersaturated (RH > 100%) at a DBT above the dew point.

• Cause on Psychrometric Chart: When two air streams are mixed and the mixing line enters the two-phase region (inside the saturation dome). The mixture state is forced onto the 100% RH line (fog line) at the same enthalpy as the theoretical mix.

• Representation: Mixed state point lies on the saturation curve (100% RH line), even though DBT > original dew point of either stream.

[!TIP]

Exam Focus: Be able to identify fog condition on a psychrometric chart: mixing line crossing inside the dome → outlet on 100% RH line. Calculate using enthalpy balance and setting RH=100%.