Refrigeration & Air Conditioning (ME-801) - Unit 2 Short Notes

UNIT 2: REFRIGERATION & AIR CONDITIONING

I. Fundamentals of Refrigeration

A. Basic Concepts and Terminology

• Tonne of Refrigeration (TR): The amount of refrigeration effect required to freeze 1 tonne (1000 kg) of water at 0°C into ice at 0°C in 24 hours.

  • Definition: 1 TR = 3.516 kW ≈ 12,000 Btu/h.

  • Formula: $$\displaystyle \text{TR} = \frac{Q}{3.516} $$ where $Q$ is cooling capacity in kW.

• Coefficient of Performance (COP): Measure of cycle efficiency.

  • Refrigeration Cycle: $$\displaystyle \text{COP}_{\text{ref}} = \frac{\text{Refrigerating Effect (RE)}}{\text{Work Input (W)}} = \frac{Q_L}{W_{\text{net}}} $$

  • Heat Pump Cycle: $$\displaystyle \text{COP}_{\text{HP}} = \frac{\text{Heat Rejected (Q_H)}}{\text{Work Input (W)}} = \frac{Q_H}{W_{\text{net}}} = 1 + \text{COP}_{\text{ref}} $$

  • Heat Engine: $$\displaystyle \text{COP}_{\text{HE}} = \frac{W_{\text{net}}}{Q_H} $$ (Note: COP for heat engine is actually thermal efficiency, $\eta$).

• Refrigerating Effect (RE): The amount of heat absorbed in the evaporator per kg of refrigerant (kJ/kg). It is the useful cooling output.

• Refrigeration Capacity: Total cooling provided by the system, $$\displaystyle Q_L = \dot{m} \times \text{RE} $$ (kW or TR).

[!TIP] Exam Focus: COP is always > 1 for refrigeration/heat pump cycles. For a Carnot cycle, $$\displaystyle \text{COP}_{\text{Carnot, ref}} = \frac{T_L}{T_H - T_L} $$.

B. Reversible Cycles and Carnot Refrigeration Cycle

• Carnot Refrigeration Cycle: Consists of two reversible isothermal processes (heat addition at $$\displaystyle T_L $$, heat rejection at $$\displaystyle T_H $$) and two reversible adiabatic processes.

• COP Calculation: For a Carnot cycle operating between two temperature reservoirs:

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

where $$\displaystyle T_L $$ and $$\displaystyle T_H $$ are in **Kelvin**.

• Application: Sets the maximum theoretical COP for any refrigeration cycle operating between the same temperature limits. Real cycles have lower COP due to irreversibilities.

C. Natural and Simple Refrigeration Methods

1. Evaporative Cooling: Cooling by evaporation of water. Air is cooled at constant wet-bulb temperature. Limited to humid climates.

2. Ice Refrigeration: Using melting of ice (latent heat absorption). Simple but limited to 0°C.

3. Air Refrigeration:

   • Open (Bell-Coleman) Cycle: Air is compressed, cooled at constant pressure, expanded in an expansion engine, and absorbs heat. Air is discharged after one cycle.

   • Dense Air System: Air is compressed, cooled, expanded in a turbine to produce low temperature, and the cold dense air is circulated in a closed loop. The working fluid (air) is reused.

   • Advantage of Dense Air over Open Air:

     • No continuous loss of working fluid (air).

     • No need for continuous supply of fresh air from atmosphere.

     • More compact and efficient for aircraft applications.

D. Throttling Process and Joule-Thomson Effect

• Throttling (Isenthalpic Process): A steady-flow adiabatic process through a restriction (valve, porous plug). $$\displaystyle h_1 = h_2 $$, $W \approx 0$.

• Joule-Thomson Effect: The temperature change of a gas during throttling at constant enthalpy.

  • Joule-Thomson Coefficient: $$\displaystyle \mu = \left( \frac{\partial T}{\partial P} \right)_H $$.

  • If $$\displaystyle \mu > 0 $$, gas cools upon expansion (most gases at room temperature).

  • If $$\displaystyle \mu < 0 $$, gas heats upon expansion (H₂, He at room temp).

  • Inversion Curve: Separates cooling and heating regions on P-T diagram.

• Significance in Refrigeration: The expansion valve in vapour compression cycles is a throttling process. It causes a drop in pressure and temperature, producing a mixture of liquid and vapour at the evaporator inlet.

E. Cooling Load Calculations for Food Preservation

• Key Principle: Total heat to be removed = Sensible Heat + Latent Heat.

• Ice Production from Water:

$$Q_{\text{total}} = m_w \left[ C_{p,w} (T_{\text{initial}} - 0) + \lambda_{\text{ice}} \right]$$

where $$\displaystyle m_w $$ = mass of water, $$\displaystyle C_{p,w} $$ = specific heat of water, $$\displaystyle \lambda_{\text{ice}} $$ = latent heat of fusion.

• Cooling of Perishables (e.g., Fish) with Phase Change:

  1. Cooling from initial temp ($$\displaystyle T_i $$) to freezing point ($$\displaystyle T_f $$): Sensible heat, $$\displaystyle Q_1 = m C_{p,\text{solid}} (T_i - T_f) $$.

  2. Freezing at $$\displaystyle T_f $$: Latent heat of fusion, $$\displaystyle Q_2 = m \lambda_f $$.

  3. Cooling from $$\displaystyle T_f $$ to final storage temp ($$\displaystyle T_s $$): Sensible heat, $$\displaystyle Q_3 = m C_{p,\text{frozen}} (T_f - T_s) $$.

$$\boxed{Q_{\text{total}} = m \left[ C_{p,\text{solid}} (T_i - T_f) + \lambda_f + C_{p,\text{frozen}} (T_f - T_s) \right]}$$

*   **Power Requirement:** $$\displaystyle P = \frac{Q_{\text{total}} / \text{time}}{\text{COP}} $$.

II. Vapour Compression Refrigeration Systems

A. Simple Vapour Compression Cycle

• Components: Compressor → Condenser → Expansion Valve → Evaporator → Compressor.

• Processes:

  1. 1-2: Isentropic compression (s = constant).

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

  3. 3-4: Isenthalpic throttling ($$\displaystyle h_3 = h_4 $$).

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

• P-h Diagram: Shows cycle as a closed loop. Key points: 1 (evaporator exit/compressor inlet), 2 (compressor exit/condenser inlet), 3 (condenser exit/expansion valve inlet), 4 (expansion valve exit/evaporator inlet).

• Performance:

  • $$\displaystyle \text{RE} = h_1 - h_4 $$

  • $$\displaystyle W_{\text{comp}} = h_2 - h_1 $$

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

  • Discharge temperature $$\displaystyle T_2 $$ found from $$\displaystyle h_2 $$ and pressure $$\displaystyle P_2 $$.

B. Practical Considerations

• Effect of Suction Pressure (Superheat):

  • Superheat: Temperature of vapour above saturation at a given pressure ($$\displaystyle T_1 > T_{\text{sat at } P_1} $$).

  • Advantages: Prevents liquid slugging in compressor, increases RE slightly (as $$\displaystyle h_1 $$ increases), but also increases $$\displaystyle W_{\text{comp}} $$ slightly. Net effect on COP is usually small.

• Effect of Discharge Pressure (Subcooling):

  • Subcooling: Temperature of liquid below saturation at condenser pressure ($$\displaystyle T_3 < T_{\text{sat at } P_2} $$).

  • Advantages: Increases RE (as $$\displaystyle h_3 $$ decreases, so $$\displaystyle h_4 = h_3 $$ decreases), no extra work input. Increases COP.

  • Provides liquid reserve for expansion valve.

• Superheating & Subcooling in Real Cycles: Standard practice to ensure safe compressor operation and improve performance.

C. Multistage Vapour Compression

• Need: High pressure ratios ($$\displaystyle P_{\text{cond}}/P_{\text{evap}} $$) cause high discharge temperatures, low volumetric efficiency, and high work input.

• Liquid Intercooler:

  • Function: Cool the refrigerant liquid between stages (after condenser of first stage, before entering second stage compressor).

  • Advantages: Reduces work input (intermediate pressure optimization), reduces discharge temperature of second stage, allows use of different refrigerants per stage.

• Boot-strap Cycle (Open Intercooling):

  • Configuration: Two compressors (low-stage, high-stage). Flash chamber or intermediate cooler between them. Vapour from flash chamber (at intermediate pressure) is mixed with suction vapour from evaporator and fed to low-stage compressor.

  • Operation: Utilizes flash gas from intercooler to boost refrigeration effect and improve efficiency.

D. Cascade Refrigeration Systems

• Configuration: Two or more independent vapour compression cycles using different refrigerants. The condenser of the low-temperature cycle (LT) is thermally coupled to the evaporator of the high-temperature cycle (HT) via a cascade condenser.

• Comparison with Multistage Compression:

  | Feature | Multistage Compression | Cascade System | | :--- | :--- | :--- | | Refrigerant | Same throughout | Different per stage (e.g., R-23 for LT, R-404A for HT) | | Compressors | Multiple in series | Separate, independent circuits | | Pressure | Continuous drop | Abrupt drop at cascade interface | | Complexity | Simpler piping | More complex, needs two systems | | Application | Moderate low temps (e.g., -40°C) | Very low temps (e.g., -80°C, -100°C) | | Flexibility | Less flexible | Highly flexible refrigerant choice |

E. Cycle Analysis with Refrigerant Property Tables

• Steps:

  1. Identify states (1,2,3,4) on P-h or T-s diagram.

  2. Use given pressures/temperatures to find properties ($h$, $s$, $T$) from tables for the refrigerant (e.g., R-12, NH₃).

  3. For isentropic compression: $$\displaystyle s_1 = s_2 $$. Find $$\displaystyle h_2 $$ at $$\displaystyle P_2 $$ with $$\displaystyle s=s_1 $$.

  4. For throttling: $$\displaystyle h_3 = h_4 $$.

  5. Calculate: RE = $$\displaystyle h_1 - h_4 $$, $$\displaystyle W = h_2 - h_1 $$, COP = RE/W.

  6. Discharge temp $$\displaystyle T_2 $$ from $$\displaystyle h_2 $$ and $$\displaystyle P_2 $$.

[!TIP] Common Pitfall: For throttling, always use $$\displaystyle h_3 = h_4 $$. Do not assume $$\displaystyle T_3 = T_4 $$.

III. Vapour Absorption Refrigeration Systems

A. Principle and Basic Cycle

• Core Difference: Replaces the mechanical compressor of vapour compression cycle with a thermal compressor (absorber + generator + pump).

• Components: Generator, Condenser, Expansion Valve, Evaporator, Absorber, Pump.

• Working Fluid Pair: Refrigerant (e.g., NH₃) + Absorbent (e.g., H₂O for NH₃ system).

• Process: Weak solution from absorber is pumped to generator. Heat input boils refrigerant vapour from solution. Strong solution returns to absorber. Refrigerant vapour condenses, throttles, evaporates to provide cooling. Vapour from evaporator is absorbed by strong solution in absorber, releasing heat.

B. Practical Vapour Absorption Systems

1. Aqua-Ammonia (NH₃-H₂O) System:

   • Refrigerant: Ammonia (NH₃)

   • Absorbent: Water (H₂O)

   • Working: Standard single-effect cycle. Water absorbs NH₃ vapour readily. Requires rectification column to ensure pure NH₃ enters condenser.

2. Lithium Bromide (LiBr-H₂O) System:

   • Refrigerant: Water (H₂O)

   • Absorbent: Lithium Bromide (LiBr)

   • Working: Used for large capacity air conditioning ( > 100 TR). LiBr is solid, so system operates under high vacuum. Water evaporates at low temperatures. No rectification needed as LiBr is non-volatile.

3. Electrolux Absorption System:

   • Uses Hydrogen (H₂) as inert carrier gas. No pump needed. Uses NH₃-H₂O pair. Hydrogen circulates with refrigerant vapour, reducing partial pressure of NH₃ in evaporator, allowing evaporation at low temp. Used in domestic refrigerators.

C. Properties of Ideal Refrigerant-Absorbent Combination

• High solubility of refrigerant in absorbent at low temperature (absorber).

• Low solubility at high temperature (generator).

• Large boiling point difference between refrigerant and absorbent.

• Chemical stability, non-corrosiveness, non-toxicity, low cost.

• High latent heat of vaporization of refrigerant.

D. Advantages and Disadvantages of Absorption Systems

IV. Refrigerants

A. Classification

• Primary Refrigerants: Used in vapour compression/absorption cycles. Undergo phase change (e.g., R-12, NH₃, R-134a, CO₂).

• Secondary Refrigerants (Coolants): Do not undergo phase change. Transfer heat from source to refrigerant (e.g., water, brine, glycol solutions, liquid CO₂).

B. Desirable Properties of an Ideal Refrigerant

• Thermodynamic: Low boiling point, high latent heat of vaporization, moderate critical temperature & pressure, low specific volume (high density).

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

• Safety: Low toxicity, non-flammable, non-explosive.

• Compatibility: Miscible with lubricating oil, chemically stable, non-corrosive to metals.

C. Specific Refrigerants and Characteristics

D. Selection Criteria

• Application: Domestic (R-134a, R-600a), Commercial (R-404A, R-507), Industrial (NH₃), Automotive (R-134a, R-1234yf).

• Environmental Regulations: Montreal Protocol (ODP), Kigali Amendment (HFC phase-down).

• Safety: Toxicity & flammability classification (ASHRAE 34: A1, A2L, B1, etc.).

• System Design: Operating pressure, temperature glide (for blends), material compatibility.

E. Environment-Friendly Refrigerants

• Zero ODP, Low GWP: HFOs (e.g., R-1234yf, R-1234ze), Natural refrigerants (NH₃, CO₂, HCs).

• Challenges:

  • NH₃: Toxicity limits use in occupied spaces.

  • HCs: Flammability limits charge size (safety standards).

  • CO₂: Very high operating pressures (up to 100 bar), requires robust components.

• Future Trends: Use of natural refrigerants with safety systems, low-GWP HFO/HFC blends, and CO₂ transcritical systems for commercial/industrial.

V. Psychrometry and Air Conditioning

A. Psychrometric Terms and Definitions

• Dry Bulb Temperature (DBT): Measured by ordinary thermometer. True temperature of air.

• Wet Bulb Temperature (WBT): Temperature indicated by a thermometer with a wet wick, when air is passed over it at a velocity > 3.5 m/s. Represents adiabatic saturation temperature.

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

• Relative Humidity (RH): $$\displaystyle \phi = \frac{P_v}{P_{\text{sat}}}} \times 100\% $$, where $$\displaystyle P_v $$ = partial pressure of vapour, $$\displaystyle P_{\text{sat}} $$ = saturation pressure at DBT.

• Specific Humidity (ω): Mass of water vapour per kg of dry air. $$\displaystyle \omega = 0.622 \frac{P_v}{P - P_v} $$ (kg water/kg dry air).

• Enthalpy of Moist Air: $$\displaystyle h = 1.005 T_{\text{DBT}} + \omega (2501 + 1.88 T_{\text{DBT}}) $$ (kJ/kg dry air).

• Degree of Saturation (μ): Ratio of actual humidity to humidity at saturation at same DBT. $$\displaystyle \mu = \frac{\omega}{\omega_s} = \frac{\phi P_{\text{sat}}(T_{\text{WB}})}{P_{\text{sat}}(T_{\text{DBT}})} $$ (approximate).

B. Psychrometric Chart

• Construction: DBT on horizontal axis, specific humidity on vertical axis. Lines of constant RH (curved), constant WBT (diagonal ~ -1 slope), constant enthalpy (straight, ~ -0.24 slope), constant volume (steep curves).

• Use: Graphical representation of state and processes of moist air.

C. Psychrometric Processes

[!TIP] Fog Formation: When two air streams mix to a state where ω > ω_s at the mixture's DBT. State lies above saturation curve. Represented by a line from mixing point that goes upward to meet saturation curve.

D. Air Conditioning Processes and Coils

• Cooling Coil:

  • Apparatus Dew Point (ADP): The temperature at which air would be saturated if cooled along a constant wet-bulb line to saturation. It is the effective surface temperature of the coil.

  • Bypass Factor (BPF): Fraction of air that passes through the coil without contacting the coil surface.

$$\text{BPF} = \frac{T_{\text{mix}} - T_{\text{ADP}}}{T_{\text{in}} - T_{\text{ADP}}} = \frac{\omega_{\text{mix}} - \omega_{\text{ADP}}}{\omega_{\text{in}} - \omega_{\text{ADP}}}$$

    (for cooling & dehumidifying coil, using enthalpy or humidity ratio).

*   **Outlet Conditions:** $$\displaystyle T_{\text{out}} = T_{\text{ADP}} + \text{BPF} (T_{\text{in}} - T_{\text{ADP}}) $$, similarly for $$\displaystyle \omega_{\text{out}} $$.

• Grand Sensible Heat Factor (GSHF):

  • Ratio of total sensible heat to total heat (sensible + latent) of the air stream entering the coil.

$$\boxed{\text{GSHF} = \frac{\text{Total Sensible Heat Load}}{\text{Total Heat Load}} = \frac{Q_s}{Q_s + Q_l}}$$

*   **Significance:** Used in system design to select cooling coil ADP. GSHF line on psychrometric chart is drawn from room condition parallel to the constant wet-bulb line of the **entering air** to the saturation curve. The intersection gives the required ADP.

E. Air Mixing and Load Calculations

• Mixing of Two Streams (by mass):

$$\omega_{\text{mix}} = \frac{\dot{m}_1 \omega_1 + \dot{m}_2 \omega_2}{\dot{m}_1 + \dot{m}_2}$$

$$h_{\text{mix}} = \frac{\dot{m}_1 h_1 + \dot{m}_2 h_2}{\dot{m}_1 + \dot{m}_2}$$

(Volumetric flow rates can be used if densities are similar).

• Summer Air-Conditioning Load Calculation:

  1. Determine outside and inside design conditions (DBT, RH).

  2. Find mixed air condition (outdoor + recirculated) using mass balance.

  3. Room Heat Gain: $$\displaystyle Q_{\text{room}} = \dot{m}_{\text{supply}} (h_{\text{mix}} - h_{\text{room}}) $$.

  4. Coil Load: $$\displaystyle Q_{\text{coil}} = \dot{m}_{\text{supply}} (h_{\text{mix}} - h_{\text{ADP}}) $$.

  5. Sensible & Latent Loads:

     $$\displaystyle Q_s = \dot{m}_{\text{supply}} \times 1.005 \times (T_{\text{mix}} - T_{\text{ADP}}) $$

     $$\displaystyle Q_l = Q_{\text{coil}} - Q_s $$

  6. Sensible Heat Factor (SHF): For the room, $$\displaystyle \text{SHF} = \frac{Q_s}{Q_s + Q_l} $$. Used to find coil ADP from psychrometric chart.

  7. Cooling Capacity: $$\displaystyle Q_{\text{coil}} $$ in kW or TR ($$\displaystyle Q_{\text{coil}} / 3.516 $$).

F. Comfort vs. Industrial Air Conditioning

VI. Special Refrigeration Systems

A. Steam Jet Refrigeration System

• Working Principle: Uses high-pressure steam (6-10 bar) passing through a nozzle to create a vacuum in the evaporator.

  1. Steam expands in nozzle to supersonic velocity, pressure drops.

  2. Low pressure entrains vapour from evaporator (flash chamber) in mixing throat.

  3. Mixture enters diffuser, velocity decreases, pressure rises to condenser pressure.

  4. Condensate from evaporator (flashed refrigerant, usually water) is pumped back.

• Refrigerant: Usually water (flash evaporation).

• T-s & H-s Diagrams: Show constant enthalpy expansion in nozzle (h₁ = h₂), mixing at constant pressure, compression in diffuser (increase in enthalpy).

• Applications: Large capacity, low-temperature (0-5°C) cooling for breweries, chemical plants, ice plants where waste steam is available.

• Limitations: Very low COP (0.1-0.3), requires high-quality steam, noisy.

B. Cascade Refrigeration Systems (Detailed)

• Configuration: Two or more independent cycles. Cascade condenser acts as evaporator for HT cycle and condenser for LT cycle. Cascade heat exchanger (or condenser/evaporator) is the interface.

• Operation: HT cycle rejects heat to ambient. LT cycle rejects heat to HT cycle's condenser. Refrigerants chosen based on their operating temperature ranges (e.g., R-23 for -80°C, R-404A for -30°C).

• Advantages over Multistage:

  • Can achieve much lower temperatures (< -80°C).

  • Flexibility in refrigerant selection for each stage.

  • No mixing of refrigerants.

  • Intermediate pressure can be optimized independently.

• Disadvantages: Higher initial cost, more complex, needs two compressors, two sets of controls.

C. Aircraft Refrigeration (Air Cycle System)

• Dense Air Refrigeration: Uses air as refrigerant in a reverse Brayton (Bell-Coleman) cycle.

• Requirements: Cabin pressurization (from ambient ~0.3 bar at 10,000 m to 0.8-1.0 bar) + cooling (to ~25°C).

• Process:

  1. Ram Air: Ambient air compressed by engine-driven compressor or turbine.

  2. Primary Heat Exchanger: Ram air cools compressed air (approx. constant pressure).

  3. Compressor: Further compression increases temperature.

  4. Secondary Heat Exchanger (Cooling Turbine): Compressed air expanded in a turbine to produce cold air. Turbine work drives the compressor or a fan.

  5. Mixing: Cold air from turbine mixed with hot bleed air from engine to achieve desired cabin temperature.

• Calculations (for 1 kg/s air, $$\displaystyle C_p=1.005 $$ kJ/kgK, $$\displaystyle \gamma=1.4 $$):

  • Power for Pressurization: $$\displaystyle W_{\text{comp}} = C_p (T_2 - T_1) $$, where $$\displaystyle T_1 $$ is ambient, $$\displaystyle T_2 $$ after compression to cabin pressure (isentropic).

  • Refrigerating Effect: $$\displaystyle RE = C_p (T_3 - T_4) $$, where $$\displaystyle T_3 $$ is after secondary heat exchanger (before turbine), $$\displaystyle T_4 $$ after turbine expansion to cabin pressure.

  • Total Power: $$\displaystyle W_{\text{total}} = W_{\text{comp}} - W_{\text{turb}} $$ (if turbine drives compressor) or sum if separate.

D. Production of Dry Ice

• Process:

  1. Liquefaction: Liquid CO₂ stored at high pressure (~60 bar) in cylinder.

  2. Flashing: Liquid CO₂ released to atmospheric pressure through a nozzle or hydraulic head. Part of it flashes to solid (snow) and cold vapour (at -78.5°C).

  3. Compaction: The CO₂ snow is compressed in a hydraulic press into solid blocks, pellets, or nuggets.

• Applications: Transportation of frozen goods, stage effects, cleaning (dry ice blasting), medical transport.

VII. Applications and System Considerations

A. Refrigeration for Food Preservation

• Spoilage Mechanisms:

  • Microbial Growth: Bacteria, yeast, mold. Reduced significantly below 4°C (chilling) and stopped below -18°C (freezing).

  • Enzymatic Reactions: Natural enzymes cause ripening, browning, texture loss. Slowed by low temperature.

  • Oxidation: Rancidity of fats. Slowed by low temp.

  • Moisture Loss (Desiccation): Prevented by high RH in storage.

• Temperature Ranges:

  • Chilling: 0°C to 4°C (meat, fish, dairy, fruits, vegetables). Slows microbial growth.

  • Freezing: -18°C to -30°C (frozen foods, ice cream). Stops microbial activity and enzyme action.

  • Deep Freezing: -30°C to -80°C (biological samples, premium ice cream). For long-term storage.

B. Leak Detection Methods in Refrigeration Systems

C. Environmental Impact and Safety

• ODP (Ozone Depletion Potential): Relative rate of ozone depletion compared to R-11 (ODP=1). CFCs (high) > HCFCs (low) > HFCs/Naturals (zero).

• GWP (Global Warming Potential): Relative warming effect compared to CO₂ (GWP=1) over 100 years. HFCs (high) > HFOs/Naturals (low).

• Handling Precautions:

  • NH₃: Toxic, flammable. Use in well-ventilated areas, leak detectors, PPE. Avoid skin contact.

  • CO₂: High pressure. Use pressure-rated components. Asphyxiation risk in confined spaces.

  • Hydrocarbons (HCs): Flammable. Limit charge size, no ignition sources, proper ventilation.

  • HFCs: Generally safe (A1), but high GWP mandates recovery and recycling.

END OF UNIT 2 NOTES