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Refrigeration Outline • Purpose of refrigeration • Examples and applications • Choice of coolant and refrigerants • Phase diagram of water and CO2 • Vapor compression refrigeration system • Pressure-enthalpy diagram for refrigerants • Refrigerator, air conditioner, thermoelectric cooler, heat pump • Designation, choice, criteria for selection, and characteristics of refrigerants • Alternatives to vapor compression refrigeration system • Heat transfer in refrigeration applications 2 Purpose of Refrigeration • To slow down rates of detrimental reactions – Microbial spoilage – Enzyme activity – Nutrient loss – Sensorial changes Guideline: Generally, rates of reactions double for every 10 °C rise in temperature 3 Examples/Applications of Cooling • Cooling engine of a car – Coolant/water • Cooling food/beverage during prolonged period of transportation in a car (vacation trip) – Ice, dry ice in an insulated container • Cooling interior of car – Car AC unit • Cooling interior of room/house – Window AC unit – Whole house unit (can it be used for heating also???) • Cooling food in a refrigerator/freezer 4 Cooling of Engine of Car HOT Engine Head Finned Radiator Coolant High cp Low freezing pt. Coolant Reservoir Air flow from outside Coolant/water is pumped through pipes to hot engine; coolant absorbs heat; fins on radiator results in high surface area (A); as car moves, air flow and hence ‘h’ increases due to forced convection Q = h A (∆T); high ‘A’ and high ‘h’ results in high Q or heat loss from engine to outside air Note: During prolonged idling of car, engine can overheat due to low ‘h’ by free convection 5 Room (or Car) Air Conditioner 6 Household Refrigerator HEAT Are there parts Extracted from in a refrigerator food inside where you can refrigerator get burnt? Can you cool the kitchen by keeping Extracted HEAT the refrigerator door open? Moved to the outside 7 Evaporative (Swamp) Cooler Water 8 Refrigerants/Coolants • Cold water (at say, 0 °C) – Heat extracted from product is used as sensible heat and increases water temperature • Ice (at 0 °C) – Heat extracted from product is used as latent heat and melts ice (λfusion = 334.94 kJ/kg at 1 atm, 0 °C); it can then additionally extract heat from product and use it as sensible heat to increase the temperature of water • Dry ice (Solid CO2) – Heat extracted from product is used as latent heat and sublimates dry ice (λsublimation = 571 kJ/kg at 1 atm, -78.5 °C) • Liquid nitrogen – Heat extracted from product is used as latent heat and evaporates liquid N2 (λvaporization = 199 kJ/kg at 1 atm, -195.8 °C) Why does dry ice sublimate while “regular” ice melt under ambient conditions? 9 Phase Diagram Water CO2 Solid Liquid Gas Solid Liquid Gas ) ) atm atm Melting point 1.0 Pressure ( Pressure Triple point ( Pressure Triple point Boiling point 0.006 5.1 1.0 0.01 100 -78.5 -56.6 Temperature (°C) Temperature (°C) 10 Drawback of Ice/Dry-Ice as Refrigerant • Neither can be re-used – Ice melts – Dry-ice sublimates • Expensive and cumbersome technique 11 Alternatives to Ice/Dry-Ice • Blue ice or gel packs (cellulose, silica gel etc) – Low freezing point – Though it isn’t “lost”, it has to be re-frozen • Endothermic reaction (Ammonium nitrate/chloride and water) • Evaporation of “refrigerant” Cooled Air (After λvap of refrigerant is absorbed by the refrigerant from air) Liquid Gaseous Refrigerant Refrigerant Warm Ambient Air Fan Can boiling/evaporation of water serve as a refrigeration method? 12 Re-Utilization of Refrigerant High Pr. Liq. High Pr. Gas Condense the Gas High Pr. Liq. High Pr. Gas Cooled Air (after λvap of refrigerant is Expand the Liquid absorbed by refrigerant from air) Compress the Gas Low Pr. Liq. Low Pr. Gas Liquid Refrigerant Gaseous Refrigerant Warm Ambient Air Fan 13 Vapor Compression Refrigeration System Energy Output d Liquid c Vapor Condenser IDEAL CONDITIONS b High Pressure Side Expansion Valve Compressor Low Pressure Side Energy Input Evaporator Condensing: Constant Pr. (P2) Liquid + Vapor e Vapor a Expansion: Constant Enthalpy (H1) Energy Input Evaporation: Constant Pr. (P1) Compression: Constant Entropy (S) Critical Point Saturated Liquid Line . Saturated Vapor Line Constant Temperature Line Left of dome: Vertical d Condenser c b P 2 ~ 30 °C Within dome: Horizontal Expansion Valve Compressor Right of dome: Curved down ~ -15 °C P1 e Evaporator a IDEAL CONDITIONS SUB-COOLED LIQUID + VAPOR SUPERHEATED LIQUID VAPOR Refrigerant is 100% vapor at end of evap. AND H1 H2 H3 Refrigerant is 100% liquid at end of condenser Enthalpy (kJ/kg) 14 Vapor Compression Refrigeration System Energy Output d Liquid c Vapor Condenser b High Pressure Side Expansion Valve Compressor Low Pressure Side Energy Input Evaporator Condensing: Constant Pr. (P2) Liquid + Vapor e Vapor a Expansion: Constant Enthalpy (H1) Energy Input Evaporation: Constant Pr. (P1) Compression: Constant Entropy (S) Critical Point Saturated Liquid Line . Saturated Vapor Line Constant Temperature Line Left of dome: Vertical d’ d Condenser c b b’ P 2 ~ 30 °C Within dome: Horizontal Expansion Valve Compressor Right of dome: Curved down ~ -15 °C P1 e’ e Evaporator a a’ Ideal: Solid line SUB-COOLED Real/Non-ideal: Dotted line LIQUID + VAPOR SUPERHEATED LIQUID VAPOR (Super-heating in evaporator, H1 H2 H3 sub-cooling in condenser) Enthalpy (kJ/kg) 15 Functions of Components of a Vapor Compression Refrigeration System • Evaporator – Extract heat from the product/air and use it as the latent heat of vaporization of the refrigerant • Compressor – Raise temperature of refrigerant to well above that of surroundings to facilitate transfer of energy to surroundings in condenser • Condenser – Transfer energy from the refrigerant to the surroundings (air/water) – Slightly sub-cool the refrigerant to minimize amount of vapor generated as it passes through the expansion valve • Expansion valve – Serve as metering device for flow of refrigerant – Expand the liquid refrigerant from the compressor pressure to the evaporator pressure (with minimal conversion to vapor) 16 Evaporator Types: Plate (coil brazed onto plate) Flooded (coil) 17 Compressor Types: Positive disp. (piston, screw, scroll/spiral) Centrifugal 18 Condenser Types: Air-cooled, water-cooled, evaporative 19 Expansion Valve Types: Manual, automatic const. pr. (AXV), thermostatic (TXV) For nearly constant load, AXV is used; else, TXV is used 20 Vapor Compression Refrigeration System Condenser Evaporator (5 °F) Expansion valve Compressor 21 Industrial Refrigeration System 22 Pressure-Enthalpy Diagram for R-12 Constant Pressure Line Horizontal Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Specific Enthalpy (kJ/kg) 23 Pressure-Enthalpy Diagram for R-12 Constant Enthalpy Line Vertical Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Specific Enthalpy (kJ/kg) 24 Pressure-Enthalpy Diagram for R-12 Constant Temperature Line Left of dome: Vertical Within dome: Horizontal Right of dome: Curved down Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Specific Enthalpy (kJ/kg) 25 Pressure-Enthalpy Diagram for R-12 Constant Entropy Line ~60 °angled line: North-Northeast (superheated region) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Specific Enthalpy (kJ/kg) 26 Pressure-Enthalpy Diagram for R-12 Constant Dryness Fraction Curved (within dome) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Dryness fraction (similar concept as steam quality) ranges from 0 on Saturated Liquid Line to 1 on Saturated Vapor Line Specific Enthalpy (kJ/kg) 27 Pressure-Enthalpy Diagram for R-12 Lines of Constant Values for Various Parameters Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute Const. Pressure Const. Enthalpy Const. Temp. Const. Entropy Const. Dryness Fraction Specific Enthalpy (kJ/kg) 28 Pressure-Enthalpy Table for R-12 P-H Diagram for Ideal Conditions e H1 = hf based on temperature at ‘d’ (exit of condenser) H2 = hg based on temperature at ‘a’ (exit of evaporator) Note 1: If there is super-heating in the evaporator, H2 can not be obtained from P-H table Note 2: If there is sub-cooling in the condenser, H1 can not be obtained from P-H table Note 3: For ideal or non-ideal conditions, H3 can not be obtained from P-H table (For the above 3 conditions, use the P-H Diagram to determine the enthalpy value) 29 P-H Diagram for Superheated R-12 Saturated Vapor Line Liquid + Vapor Mixture Superheated Vapor Constant Entropy Line 30 Pressure-Enthalpy Diagram for R-12 Ideal Conditions Condenser Pressure Condensation Expansion Compression Evaporation Evaporator Pressure Absolute Pressure (bar) Pressure Absolute Specific Enthalpy (kJ/kg) 31 Pressure-Enthalpy Diagram for R-12 Real/NonIdeal- IdealConditions Conditions (Determination of Enthalpies) Degree of sub-cooling Condenser Pressure AnimatedCondensation Slide Expansion Compression (See next slideEvaporation for static version of slide) Evaporator Pressure . Qe = m. (H2 – H1) Qw = m. (H3 – H2) Absolute Pressure (bar) Pressure Absolute Degree of super-heating Qc = m (H3 – H1) Note: Q = Q + Q c e w C.O.P. = Qe/Qw H1 H2 H3 = (H2 – H1)/(H3 – H2) Specific Enthalpy (kJ/kg) 32 Pressure-Enthalpy Diagram for R-12 Real/NonIdeal- IdealConditions Conditions Degree of sub-cooling Condenser Pressure Condensation Expansion Compression Evaporation . Evaporator Pressure Q = m
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