Refrigeration & Air Conditioning (ME-801) - Unit 4 Short Notes

1.0 FUNDAMENTALS OF REFRIGERATION & CYCLE ANALYSIS

1.1 Tonne of Refrigeration (TR)

• Definition: Cooling effect equivalent to melting 1 tonne (1000 kg) of ice at 0°C in 24 hours.

• Units:

  • 1 TR = 3.516 kW (SI)

  • 1 TR = 12,000 Btu/h

• Calculation:

$$ Q = m \times L_f = 1000 \, \text{kg} \times 335 \, \text{kJ/kg} = 335,000 \, \text{kJ/day} $$

$$ \text{TR} = \frac{335,000}{24 \times 3600} = 3.88 \, \text{kW} \approx 3.516 \, \text{kW (standard)} $$

• [!TIP] Always use 3.516 kW/TR for conversions. Ice latent heat often taken as 335 kJ/kg or 144 Btu/lb.

1.2 Carnot Refrigeration Cycle & Heat Pump

• COP Formulas (Temperatures in Kelvin):

  • Refrigeration: $$\displaystyle \boxed{\text{COP}_R = \frac{T_L}{T_H - T_L}} $$

  • Heat Pump: $$\displaystyle \boxed{\text{COP}_{HP} = \frac{T_H}{T_H - T_L}} $$

  • Heat Engine: $$\displaystyle \boxed{\eta = 1 - \frac{T_L}{T_H}} $$ (not COP)

• Problem Solving:

  • Given $$\displaystyle T_L $$, $$\displaystyle T_H $$, and $$\displaystyle Q_{in} $$ (refrigeration) or $$\displaystyle Q_{out} $$ (heat pump):

    • Work input: $$\displaystyle W = \frac{Q_{in}}{\text{COP}_R} $$

    • For heat pump: $$\displaystyle Q_{out} = Q_{in} + W = Q_{in} \cdot \text{COP}_{HP} $$

• [!TIP] COP_HP = COP_R + 1. Ensure temperatures are absolute (K).

1.3 Reversed Carnot Cycle - Ice Plant Problems

• Process: Water cooled from initial temp → 0°C → frozen → subcooled ice.

• Refrigeration Effect per kg:

$$ q_{evap} = C_{p,w}(T_{initial} - 0) + L_f + C_{p,ice}(0 - T_{storage}) $$

• Mass of Ice Produced:

$$ m_{ice} = \frac{Q_{total}}{q_{evap}} \quad \text{where } Q_{total} = \text{COP}_R \times W_{input} $$

• Power Requirement:

$$ W = \frac{m_{ice} \times q_{evap}}{\text{COP}_R} $$

• [!TIP] If power given, find COP first from Carnot formula, then compute ice production.

1.4 Natural & Elementary Methods of Refrigeration

• Ice Harvesting: Collecting natural ice from frozen lakes/rivers in winter, storing in insulated icehouses.

• Evaporative Cooling: Water evaporation absorbs latent heat, cooling air. Effective in dry climates.

• Nocturnal Radiation: Radiative cooling at night by exposing surfaces to sky (used in desert coolers).

• Throttling (Joule-Thomson) Effect: Adiabatic expansion through a valve. Cooling occurs if gas is below inversion temperature (e.g., CO₂, NH₃ at room temp). Used in Linde process for liquefaction.

2.0 VAPOUR COMPRESSION REFRIGERATION SYSTEM (VCRS)

2.1 Theoretical Cycle Analysis on P-h, T-s, and P-V Diagrams

• Processes:

  1. 1-2: Isentropic compression ($$\displaystyle s_1 = s_2 $$)

  2. 2-3: Constant pressure condensation (heat rejection)

  3. 3-4: Throttling ($$\displaystyle h_3 = h_4 $$, constant enthalpy)

  4. 4-1: Constant pressure evaporation (heat absorption)

• Key Diagrams:

  • P-h: Throttling is vertical line; evaporation/condensation horizontal.

  • T-s: Isentropic compression vertical; throttling constant enthalpy diagonal.

  • P-V: Compression increases pressure, decreases volume.

• [!TIP] On P-h chart, read enthalpies directly: $$\displaystyle h_1 $$ (evaporator exit), $$\displaystyle h_2 $$ (compressor exit), $$\displaystyle h_3 $$ (condenser exit), $$\displaystyle h_4 = h_3 $$ (after throttle).

2.2 Performance Parameters

• COP (ideal cycle):

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

• Effect of Suction Pressure (Evaporator temperature):

  • Lower suction pressure → lower $$\displaystyle h_1 $$ (less refrigeration effect) and higher pressure ratio → higher $$\displaystyle h_2 $$ (more work) → COP decreases.

• Effect of Discharge Pressure (Condenser temperature):

  • Higher discharge pressure → higher $$\displaystyle h_2 $$ (more work) and possibly lower $$\displaystyle h_1 $$ if superheat changes → COP decreases.

• [!TIP] For fixed condenser temp, lower evaporator temp always reduces COP. Optimize evaporator temp based on load.

2.3 Multistage Compression Systems

• Purpose: Reduce discharge temperature, improve efficiency for high pressure ratios.

• Liquid Intercooler:

  • Liquid from condenser is subcooled before entering second stage.

  • Reduces work input because vapor entering second stage has lower enthalpy.

• Boot-strap Cycle:

  • Uses flash intercooling: liquid from condenser flashes in intercooler to cool vapor from first stage.

  • COP (approx):

$$ \text{COP} = \frac{h_1 - h_8}{(h_2 - h_1) + (h_4 - h_3)} $$

(States: 1=evap exit, 2=1st stage comp exit, 3=after intercooler liquid, 4=flash vapor, 8=throttle exit)

• DiagramCANVAS: Boot-strap cycle with two compressors, intercooler with flash chamber, condenser, evaporator, throttle valves. Show pressure levels: P1 (evap), P2 (intermediate), P3 (condenser).

• [!TIP] Boot-strap avoids external intercooling; uses internal flash cooling.

2.4 Comparison: Multi-stage Compression vs. Cascade System

2.5 Practical Vapour Compression Cycle

• Superheating:

  • Vapor at evaporator exit > saturation temperature.

  • Increases refrigeration effect ($$\displaystyle h_1 $$ increases) but also work ($$\displaystyle h_2 $$ increases).

  • Net COP may increase or decrease; generally beneficial if superheat is moderate.

• Subcooling (Undercooling):

  • Liquid at condenser exit < saturation temperature.

  • Increases refrigeration effect ($$\displaystyle h_4 $$ decreases) without extra work → COP increases.

• Combined Effect: Superheat + subcooling usually improves COP significantly.

3.0 VAPOUR ABSORPTION REFRIGERATION SYSTEM (VARS)

3.1 Basic Principle & Comparison with VCRS

• VCRS: Mechanical compressor raises pressure of refrigerant vapor.

• VARS:

  • Absorber dissolves refrigerant vapor in absorbent (low pressure).

  • Pump increases pressure of solution (liquid, so little work).

  • Generator heats solution to separate refrigerant vapor.

• Function Achieved: Compressor replaced by absorber (low side) and pump + generator (high side).

3.2 Practical Vapour Absorption Cycle

• Components:

  • Generator: Heat input → strong solution → weak solution + vapor.

  • Condenser: Vapor condenses to liquid.

  • Evaporator: Liquid evaporates, providing refrigeration.

  • Absorber: Vapor absorbed by weak solution → strong solution.

  • Pump: Pressurizes strong solution to generator pressure.

  • Throttle valve: Drops pressure from generator to absorber.

  • Solution heat exchanger: Recovers heat from hot strong solution to preheat weak solution → improves COP.

• [!TIP] COP of VARS is lower than VCRS (0.5-0.7 vs 3-6) but economical with waste heat/solar.

3.3 Aqua-Ammonia (NH₃-H₂O) System

• Refrigerant: Ammonia (NH₃)

• Absorbent: Water (H₂O)

• Working:

  1. Strong NH₃-H₂O solution pumped to generator.

  2. Heated (by steam/solar) → NH₃ vapor separates, weak solution returns to absorber.

  3. NH₃ vapor condenses, throttles, evaporates in evaporator.

  4. NH₃ vapor absorbed in absorber by weak solution, releasing heat.

• DiagramCANVAS: Aqua-ammonia absorption system: generator (top), condenser (right), evaporator (bottom), absorber (left). Solution pump between absorber and generator, throttle between generator and absorber, solution heat exchanger in solution line. Arrows show flow: strong solution up to generator, weak solution down to absorber, refrigerant vapor from generator to condenser to evaporator to absorber.

• Properties of Ideal Pair: High solubility, low heat of mixing, chemical stability, no crystallization.

3.4 Lithium Bromide (LiBr-H₂O) System

• Refrigerant: Water (H₂O)

• Absorbent: Lithium Bromide (LiBr)

• Working: Similar to aqua-ammonia but water evaporates at low pressure. LiBr solution absorbs water vapor.

• Applications: Large capacity air conditioning (100+ TR) in hotels, hospitals, where waste heat or solar available.

• Challenges: LiBr crystallizes at high concentration, requires vacuum operation, corrosive → need inhibitors.

• DiagramCANVAS: LiBr-H₂O absorption system: similar layout but refrigerant is water. Evaporator provides chilled water for AC. Generator heated by steam/ hot water. Absorber cooled by cooling water.

3.5 Advantages of VARS over VCRS

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

• Quiet operation (no compressor noise).

• Fewer moving parts (only pump and valves) → less maintenance.

• Suitable for remote areas with heat source but no reliable electricity.

• [!TIP] VARS ideal where electricity is expensive/unavailable but heat is free (e.g., industrial waste heat, solar thermal).

4.0 REFRIGERANTS

4.1 Classification of Refrigerants

• Primary vs Secondary:

  • Primary: Directly circulate in refrigeration system (e.g., R-12, NH₃).

  • Secondary: Carry heat to primary (e.g., brine, water, glycol solutions).

• Chemical Classification:

  • Halocarbons:

    • CFCs (R-11, R-12) – ODP high, banned.

    • HCFCs (R-22) – ODP low, transitional.

    • HFCs (R-134a, R-404A) – ODP zero, GWP high.

  • Azeotropes: Mixtures behaving as single substance (e.g., R-500 = R-12/R-152a).

  • Inorganic: NH₃ (R-717), CO₂ (R-744), H₂O (R-718).

  • Hydrocarbons: Propane (R-290), Isobutane (R-600a) – flammable, low GWP.

4.2 Desirable Properties of an Ideal Refrigerant

• Thermodynamic:

  • Low boiling point (for low evaporator temp).

  • High latent heat (large refrigeration effect per kg).

  • High critical temperature/pressure (allows higher condensing temp, better COP).

  • Low specific volume (reduces compressor size).

• Chemical:

  • Non-toxic, non-flammable, non-corrosive.

  • Chemically stable (no decomposition at high temps).

  • Miscible with lubricating oil.

• Physical:

  • Low viscosity, high thermal conductivity.

  • Low specific heat (for better throttling cooling effect).

4.3 Factors for Refrigerant Selection

• Application:

  • Cold storage: NH₃, R-404A.

  • Air conditioning: R-22, R-134a, R-410A.

  • Cascade systems: Low GWP refrigerants (CO₂, NH₃).

  • Domestic refrigerators: HCs (R-600a), HFCs (R-134a).

• Safety: Toxicity, flammability, ASHRAE safety classifications (A1, A2L, B2, etc.).

• Environmental Impact: ODP, GWP.

• Compatibility: With materials (elastomers, metals) and lubricants.

• Cost and Availability.

4.4 Specific Refrigerants

• Ammonia (NH₃, R-717):

  • Properties: High latent heat (1370 kJ/kg at -10°C), good thermophysical, toxic (TLV 25 ppm), flammable (flammable at 15-28% in air), strong odor (detectable).

  • Applications: Industrial refrigeration, cold storage, ice plants.

  • Advantages: High COP, leaks easily detectable, inexpensive.

  • Disadvantages: Toxic, corrosive to copper/brass, requires steel components, not for domestic AC.

• R-12 (CCl₂F₂):

  • ODP = 1.0 (reference), GWP = 10,900. Banned by Montreal Protocol.

  • Used in old domestic refrigerators, car AC.

• R-22 (CHClF₂):

  • ODP = 0.05, GWP ≈ 1810. Being phased out (HCFC).

  • Used in AC, chillers, heat pumps.

• Refrigerant Numbering:

  • R-22: CHClF₂ → 1 carbon, 1 H, 1 Cl, 1 F → 22.

  • R-134a: CH₂FCF₃ → C₂H₂F₄ → 134a (1=C-1, 3=H+1, 4=F, 'a' isomer).

  • R-717: 7 = inorganic, 17 = molecular weight of NH₃ (17).

4.5 Environment-Friendly Refrigerants (Eco-friendly)

• HFCs (e.g., R-134a, R-404A, R-410A):

  • ODP = 0, but high GWP (R-134a GWP=1430).

  • Used as CFC/HCFC replacements but being phased down under Kigali Amendment.

• Hydrocarbons (HCs) (e.g., R-290 propane, R-600a isobutane):

  • ODP = 0, GWP very low (R-290 GWP=3), flammable.

  • Used in small appliances (refrigerators, ACs < 5 TR).

• Natural Refrigerants:

  • CO₂ (R-744): ODP=0, GWP=1, high pressure (up to 100 bar), used in cascade and transcritical systems.

  • NH₃ (R-717): ODP=0, GWP=0, toxic but excellent thermophysical.

  • H₂O (R-718): ODP=0, GWP=0, used in steam jet and absorption.

• Regulations:

  • Montreal Protocol (1987): Phase-out of CFCs, HCFCs.

  • Kyoto Protocol (1997): Targets for GHG reduction, affecting high-GWP HFCs.

  • Kigali Amendment (2016): Phase-down of HFCs globally.

• [!TIP] For new systems, prefer low-GWP options: HCs for small capacity, CO₂ for commercial, NH₃ for industrial. Check safety class (A2L, A3).

5.0 PSYCHROMETRICS & AIR CONDITIONING PROCESSES

5.1 Psychrometric Terms & Definitions

• Dry Bulb Temperature (DBT): Measured by ordinary thermometer.

• Wet Bulb Temperature (WBT): Temperature read by thermometer with wet wick, indicates moisture content.

• Dew Point Temperature (DPT): Temperature at which air becomes saturated (RH=100%) when cooled at constant pressure.

• Relative Humidity (RH): $$\displaystyle \phi = \frac{p_v}{p_{sat}(T_{DB})} \times 100\% $$, where $$\displaystyle p_v $$ = partial pressure of vapor.

• Specific Humidity (Humidity Ratio, ω): Mass of water vapor per kg dry air:

$$ \omega = 0.622 \frac{p_v}{p - p_v} \quad \text{(kg water/kg dry air)} $$

• Saturated Air: RH = 100%, DBT = DPT = WBT.

• Enthalpy of Moist Air (approx):

$$ h = 1.006 T_{DB} + \omega (2501 + 1.86 T_{DB}) \quad \text{(kJ/kg dry air)} $$

5.2 Psychrometric Processes on Chart

• Sensible Heating/Cooling: Constant ω (vertical line on chart with DBT vertical, ω horizontal). Heat added/removed without moisture change.

• Humidification: Adding moisture. At constant DBT → horizontal line to right; at constant WBT → along constant wet-bulb line.

• Dehumidification: Removing moisture. Cooling below dew point → condensation.

• Cooling and Dehumidification Process:

  • Air cooled below dew point, moisture condenses.

  • Process line is straight with negative slope, parallel to constant wet-bulb lines.

  • Ends at saturation if coil temperature below dew point.

  • DiagramCANVAS: Psychrometric chart showing cooling and dehumidification from state A to state B. Line slopes downward left, crossing saturation curve at Apparatus Dew Point (ADP).

• Heating and Dehumidification: Heating while removing moisture (e.g., heating coil with dehumidification) – process moves right and up.

5.3 Air Mixing Problems

• Two-stream mixing (recirculated + fresh air):

  • Mass balance dry air: $$\displaystyle m_{da,1} + m_{da,2} = m_{da,m} $$

  • Humidity ratio: $$\displaystyle \omega_m = \frac{m_1 \omega_1 + m_2 \omega_2}{m_1 + m_2} $$

  • Enthalpy: $$\displaystyle h_m = \frac{m_1 h_1 + m_2 h_2}{m_1 + m_2} $$

  • Find $$\displaystyle T_m $$, $$\displaystyle \phi_m $$ from chart using $$\displaystyle \omega_m $$ and $$\displaystyle h_m $$.

• [!TIP] For volume flow rates (m³/min), convert to mass flow using density at given conditions or use volume-based mixing if pressures equal.

5.4 Cooling Coil Performance

• Apparatus Dew Point (ADP):

  • Temperature at which air would be saturated if cooled along constant wet-bulb line to saturation.

  • Represents effective coil surface temperature if coil is 100% effective.

• Bypass Factor (BPF):

  • Fraction of air that bypasses coil without contacting it.

$$ \text{BPF} = \frac{h_1 - h_3}{h_1 - h_2} = \frac{\omega_1 - \omega_3}{\omega_1 - \omega_2} = \frac{T_1 - T_3}{T_1 - T_2} \text{ (approx)} $$

• Where: 1 = inlet, 2 = ADP state (saturated at ADP), 3 = outlet.

• Outlet State given BPF and ADP:

  • $$\displaystyle h_3 = h_1 - \text{BPF} (h_1 - h_2) $$

  • $$\displaystyle \omega_3 = \omega_1 - \text{BPF} (\omega_1 - \omega_2) $$

  • Find $$\displaystyle T_3 $$, $$\displaystyle \phi_3 $$ from chart.

• [!TIP] BPF = 0 → all air contacts coil, outlet = ADP state. BPF = 1 → no contact, outlet = inlet.

5.5 Summer Air Conditioning Load Calculation

• Sensible Heat Factor (SHF):

$$ \text{SHF} = \frac{Q_{sens}}{Q_{total}} = \frac{C_{p,da} (T_{mixed} - T_{supply})}{h_{mixed} - h_{supply}} $$

• Grand Sensible Heat Factor (GSHF): SHF of mixed air before coil (considering outdoor + recirculated).

• Total Cooling Load:

$$ Q_{total} = \dot{m}_{da} (h_{mixed} - h_{supply}) \quad \text{(kW)} $$

• Sensible Load:

$$ Q_{sens} = \dot{m}_{da} C_{p,da} (T_{mixed} - T_{supply}) \quad \text{(kW)} $$

• Latent Load:

$$ Q_{lat} = Q_{total} - Q_{sens} $$

• Mass of Water Condensed:

$$ \dot{m}_{cond} = \dot{m}_{da} (\omega_{mixed} - \omega_{supply}) \quad \text{(kg/h or kg/s)} $$

• Cooling Coil Capacity in TR:

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

• [!TIP] Always compute $$\displaystyle \omega_{mixed} $$ and $$\displaystyle h_{mixed} $$ from mixing calculation, then apply SHF/GSHF to find supply conditions or loads.

5.6 Comfort vs. Industrial Air Conditioning

6.0 SPECIAL REFRIGERATION SYSTEMS & APPLICATIONS

6.1 Steam Jet Refrigeration System

• Working Principle:

  • High-pressure steam expands through nozzle, creates vacuum in mixing chamber.

  • Low-pressure vapor from evaporator is entrained and compressed by steam jet.

  • Mixed vapor condenses in condenser; condensate returned to boiler.

  • DiagramCANVAS: Steam jet refrigeration: steam nozzle, mixing chamber, evaporator, condenser. Arrows: steam → nozzle → mixing → condenser; refrigerant vapor from evaporator → mixing → condenser.

• T-s and H-s Diagrams:

  • T-s: Steam expansion isenthalpic? Actually steam expands isentropically in nozzle, then mixes. Refrigerant vapor compressed.

  • H-s: Shows steam expansion and refrigerant compression.

• Advantages: No moving parts, simple, uses waste steam.

• Limitations: Very low COP (0.1–0.3), high steam consumption, requires high-pressure steam source.

• [!TIP] Used where waste steam is available, e.g., sugar mills, chemical plants.

6.2 Cascade Refrigeration System

• Working:

  • Two independent cycles with different refrigerants.

  • Low-Temperature (LT) Cycle: Evaporator cools space, condenser rejects heat to cascade heat exchanger.

  • High-Temperature (HT) Cycle: Evaporator absorbs heat from cascade heat exchanger, condenser rejects to ambient.

  • Cascade Heat Exchanger: Allows heat transfer between LT condenser and HT evaporator.

• DiagramCANVAS: Cascade system: LT cycle (evaporator → LT comp → LT condenser → cascade HX → throttle → LT evap). HT cycle (cascade HX → HT comp → HT condenser → throttle → HT evap). Cascade HX between LT condenser and HT evaporator.

• Comparison with Multistage:

  • Cascade: Different refrigerants, separate compressors, intermediate heat exchanger. For very low temps (< -40°C).

  • Multistage: Same refrigerant, intercooling between stages. For moderate pressure ratios.

• [!TIP] Cascade avoids high compression ratios in one stage; allows optimal refrigerants for each temperature level.

6.3 Aircraft Refrigeration (Air Cycle System)

• Problem: High altitude air is cold (e.g., 263 K at 8000 m) but at low pressure; cabin needs pressurization and cooling.

• Bootstrap Cycle:

  • Bleed air from engine compressor → aftercooler → compressor (increases pressure) → aftercooler → expansion turbine (cools) → air to cabin.

  • Some air bled off for cooling before turbine (reduced ambient).

  • DiagramCANVAS: Bootstrap cycle: ambient air → compressor → aftercooler → turbine → cabin. Cooling turbine provides refrigeration. Bleed air from compressor may be used for cabin pressurization.

• Reduced Ambient Cooling: Use ram air (high-altitude cold air) directly for cooling without compression.

• Calculations (per kg air):

  • Power for Pressurization: Compressor work $$\displaystyle W_c = C_p (T_{2s} - T_1)/\eta_c $$ (isentropic efficiency).

  • Refrigeration Effect: $$\displaystyle q_{evap} = C_p (T_3 - T_4) $$ after expansion (isentropic efficiency considered).

  • Given: altitude pressure $$\displaystyle p_a $$, temperature $$\displaystyle T_a $$, aircraft speed $V$ (ram effect), compression ratio $$\displaystyle r_p $$, cabin pressure $$\displaystyle p_c $$, cabin temperature $$\displaystyle T_c $$.

• [!TIP] In aircraft, air is the refrigerant (dense air system). Use isentropic relations for compression and expansion with efficiencies.

6.4 Applications of Refrigeration for Food Preservation

• Controls:

  • Microbial Growth: Low temperature slows bacterial/mold growth.

  • Enzyme Activity: Reduces enzymatic browning, ripening.

  • Respiration: Slows metabolic rate in fresh produce.

• Methods:

  • Cold Storage: 0–4°C for vegetables, fruits.

  • Chilling: 0–4°C for meat, fish.

  • Freezing: -18°C to -30°C for long-term storage.

  • Ice Production: For direct cooling, preservation during transport.

• [!TIP] Freezing point depression for fish/meat; blast freezing for quality.

7.0 AUXILIARY TOPICS & MISCELLANEOUS

7.1 Fog in Air Conditioning

• Definition: Visible mist formed when air becomes supersaturated (RH > 100%) and water vapor condenses into tiny droplets.

• Cause:

  • Over-cooling air below its dew point in cooling coil.

  • Mixing of cold, dry air with warm, humid air.

• Psychrometric Representation:

  • When two air streams mix, if the mixed state lies on the saturation curve, fog forms.

  • Cooling coil process: If ADP is very low and BPF high, outlet may be on saturation curve → fog at coil exit.

• [!TIP] Fog occurs when air is saturated and contains excess moisture; avoid by ensuring outlet air RH < 100% (control ADP or BPF).

7.2 Leak Detection Methods for Refrigerants

1. Soap Solution: Apply to joints; bubbles indicate leak. Simple, cheap.

2. Electronic Leak Detector: Sniffs refrigerant, gives audible/visual signal. Sensitive to halocarbons.

3. UV Dye: Add fluorescent dye to system; inspect with UV light after circulation.

4. Halide Torch: For halocarbons (CFCs/HCFCs); flame turns green if refrigerant present.

• [!TIP] For HFCs (no chlorine), use electronic detector or UV dye. Soap solution works for any gas.

7.3 Production of Dry Ice (Solid CO₂)

• Process:

  1. Liquefaction: CO₂ gas compressed and cooled to liquid at high pressure (~60 bar).

  2. Flashing: Liquid CO₂ released to atmospheric pressure → flashes to solid (snow) and gas (sublimation).

  3. Pelletizing/Blocking: Solid compressed into pellets or blocks.

• Applications: Food transport, medical cooling, fog effects.

• [!TIP] Dry ice sublimes at -78.5°C; handle with gloves to avoid frostbite.

7.4 Electrolux (Water-Lithium Bromide Absorption) System - Brief Note

• Type: LiBr-H₂O absorption refrigeration.

• Working: Similar to aqua-ammonia but refrigerant is water, absorbent is LiBr.

• Features:

  • Generator heated by steam/solar.

  • Evaporator produces chilled water for AC.

  • Absorber cooled by cooling water.

  • Used in large-capacity air conditioning (e.g., hotels, hospitals).

• Advantages: Quiet, uses waste heat.

• Disadvantages: LiBr crystallizes, requires vacuum, corrosive.

• DiagramCANVAS: Electrolux system: generator (heated), condenser, evaporator (chilled water), absorber (cooling water), solution pump, throttle, solution heat exchanger.

7.5 Dense Air vs. Open Air Refrigeration System (Aircraft)

• Open Air System: Ambient air at altitude (low pressure) used directly for cooling after expansion. Requires large volumes, inefficient.

• Dense Air System: Air compressed first (increasing pressure and density), then cooled and expanded. More compact, efficient.

• Aircraft Application: Bootstrap cycle is a dense air system.

  • Advantages: Higher density → smaller components, better control of cabin pressure and temperature.

  • Disadvantage: Requires compressor work (bleed air from engine).

• [!TIP] All modern aircraft use dense air (boot-strap or reduced ambient) for both pressurization and cooling.