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MOLLIERS CHART & REFRIGERANTS

MOLLIERS CHART & REFRIGERANTS. Temperature-Enthalpy Chart. Temperature deg C. Compressor outlet/ Condensor inlet. Condensor outlet/ Expansion valve inlet. Expansion valve outlet/Evaporator inlet. Evaporator outlet/Compressor inlet. (Heat)Specific Enthalpy KJ/Kg. Critical Pressure.

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MOLLIERS CHART & REFRIGERANTS

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  1. MOLLIERS CHART & REFRIGERANTS

  2. Temperature-Enthalpy Chart Temperature deg C Compressor outlet/Condensor inlet Condensor outlet/ Expansion valve inlet Expansion valve outlet/Evaporator inlet Evaporator outlet/Compressor inlet (Heat)Specific Enthalpy KJ/Kg

  3. Critical Pressure Critical Point Pressure (bar) Critical Temperature Line Constant Temperature Lines Physical State Line Pressure – Enthalpy Chart (Phase Diagram) Vapour Phase Liquid Phase HP Compressor outlet/Condensor inlet Condensor outlet/ Expansion valve inlet Saturated Mixture Phase Critical Point: Conditions at which substances stop obeying Gas laws. Distinction between liquids and gases does not exist. Only homogeneous supercritical fluid exists. Gases cannot be condensed by increasing pressure. Dryness Faction lines Constant Temperature Lines LP Expansion valve outlet/Evaporator inlet Evaporator outlet/Compressor inlet (Heat)Specific Enthalpy KJ/Kg

  4. Molliers Chart • Diagrammatic Representation of properties of Refrigerant. Useful for designing the Refrigeration system • Gives enthalpy of refrigerant at various pressures and physical states (liquid, vapour, mixture) • Also called Pressure-Enthalpy Chart. • Enthalpy is the total heat content in a substance BTUs or KJs • Specific Enthalpy is Enthalpy per unit mass BTU/lb or KJ/kg

  5. Constant Entropy LInes Constant Temperature Lines

  6. Pressure-enthalpy (P-H) or Mollier diagram • Line A to B represents the change from high to low pressure, or expansion process • Line B to B’ represents the amount of liquid ‘flashed-off’ in the expansion valve cooling the remaining liquid. • Line B to C represents the evaporation process at constant saturation temperature and pressure in the evaporator. At point C the refrigerant is a dry saturated vapour. • Line C to C’ represents the superheat absorbed by the dry saturated vapour • Line C’ to D represents the compression process. • Line D to E represents the superheat given up by the vapour in the condenser. At point E the refrigerant is a dry saturated vapour. • Line E to F represents the condensation process at constant saturation temperature and pressure. At point F the refrigerant is a saturated liquid. • Line F to A represents the sub cooling of the condensed liquid

  7. Refrigerating effect • The amount of heat absorbed by each unit mass of refrigerant as it flows through an evaporator is known as the refrigerating effect, and is equal to the difference between the enthalpy of the vapour leaving the evaporator and the enthalpy of the liquid at the flow control. • Thus, for the system shown in Fig 3, refrigerating effect,

  8. Refrigerating capacity • The rate at which a system will absorb heat from the refrigerated space or substance is known as the refrigerating capacity, and is expressed as, refrigerating capacity, • where m = mass flow of refrigerant through the evaporator (kg/s). • To achieve a specified refrigerating capacity of 15OkW, say, the required mass flow rate is

  9. Compressor capacity • The capacity of a compressor must be such that it removes the vapour from the evaporator at the same rate as that at which it is formed. To maintain a specified operating condition, a compressor must have a swept volume equal to the volume of vapour formed in the evaporator per unit time (m3/h). • To maintain constant operating conditions and produce the required refrigeration duty would require a compressor with a swept volume: • V = m x v m3 • where v = specific volume of the vapour at the compressor suction inlet, m3/kg, and v at -25°C and 1.32 bar • = 0.18m3/kg. • i.e. V = 0.94 x 0.18 x 3600 = 609 m3/h.

  10. Heat of compression • The energy input from the compressor motor to raise the pressure of the vapour to the required condensing temperature is known as the heat of compression, and is equal to the difference between the enthalpy of the vapour at the compressor outlet and inlet. • Heat of compression,

  11. Condenser duty • The rate of heat transfer from the refrigerant in the condenser to the cooling medium is known as the condenser duty, and is expressed as, • = 0.94 (470 – 230.3) = 225.3 kW

  12. Coefficient of performance • The ratio of refrigerating effect to the heat of compression is known as the coefficient of performance (COP). • Thus for the system shown in Fig 3,

  13. Generation of Refrigerants • First Generation 1830-1930s ex: CO2; NH3; HCs;SO2 etc. usefulness of volatile compounds • Second Generation: 1931-1990s ex: CFCs; HCFCs; safety & durability • Third Generation; 1990-2010s ex: HCFCs and HFCs; Ozone Layer protection • Fourth Generation; 2010 onwards ex: Pure & Blended HCs; Global warming; high efficiency

  14. Refrigerant selection criteria • Selection of refrigerant for a particular application is based on the following requirements: • Thermodynamic and thermo-physical properties • Environmental and safety properties, and • Economics

  15. Thermodynamic and thermo-physical properties • The requirements are: • Suction pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection. Higher suction pressure is better as it leads to smaller compressor displacement • Discharge pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow light-weight construction of compressor, condenser etc.

  16. 3. Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumption 4. Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small 5. Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small

  17. Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet • Vapour specific heat: Should be large so that the degree of superheating will be small • Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients

  18. Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops • The freezing point of the refrigerant should be lower than the lowest operating temperature of the cycle to prevent blockage of refrigerant pipelines. • High critical temperature

  19. Environmental properties • The important environmental properties are: • Ozone Depletion Potential (ODP) • Global Warming Potential (GWP) • Total Equivalent Warming Index (TEWI) • Atmospheric Lifetime • Chlorine Leading Potential

  20. Safety and Other Properties • TLV (Threshold Limit Value) • Toxicity • Flammability • Chemical stability • Compatibility with common materials • Miscibility with lubricating oils • Dielectric strength • Ease of leak detection

  21. Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having non-zero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in near-future (e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations

  22. Global Warming Potential (GWP) Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.

  23. Total Equivalent Warming Index (TEWI) • The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view.

  24. Atmospheric Lifetime • HFC125, the major component of HFC blend refrigerants, has an atmospheric life of 29 years, while the atmospheric life of HFC32 is only five years.

  25. Toxicity Ideally, refrigerants used in a refrigeration system should be non-toxic. However, all fluids other than air can be called as toxic as they will cause suffocation when their concentration is large enough. Thus toxicity is a relative term, which becomes meaningful only when the degree of concentration and time of exposure required to produce harmful effects are specified.

  26. Toxicity • Some fluids are toxic even in small concentrations. Some fluids are mildly toxic, i.e., they are dangerous only when the concentration is large and duration of exposure is long. Some refrigerants such as CFCs and HCFCs are non-toxic when mixed with air in normal condition. However, when they come in contact with an open flame or an electrical heating element, they decompose forming highly toxic elements (e.g. phosgene-COCl2).

  27. Toxicity • In general the degree of hazard depends on: • Amount of refrigerant used vs total space • Type of occupancy • Presence of open flames • Odor of refrigerant, and • Maintenance condition • Thus from toxicity point-of-view, the usefulness of a particular refrigerant depends on the specific application.

  28. Flammability • The refrigerants should preferably be non-flammable and non-explosive. For flammable refrigerants special precautions should be taken to avoid accidents.

  29. Chemical stability & Compatibility • The refrigerants should be chemically stable as long as they are inside the refrigeration system. • Compatibility with common materials of construction (both metals and non-metals)

  30. Miscibility with lubricating oils • Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils are easier to handle (e.g. R12). However, for refrigerants with limited solubility (e.g. R 22) special precautions should be taken while designing the system to ensure oil return to the compressor

  31. Dielectric strength • This is an important property for systems using hermetic compressors. For these systems the refrigerants should have as high a dielectric strength as possible

  32. Ease of leak detection • In the event of leakage of refrigerant from the system, it should be easy to detect the leaks.

  33. Economic properties • The refrigerant used should preferably be inexpensive and easily available.

  34. Halocarbon Refrigerants • Methane (CH4)Based will have a two-digit number ex:R22 First figure shows no. of Hydrogen atoms plus 1 Second figure shows no. of fluorine Atoms R22=CHClF2 Chlorofluoromethane. The total number of Hydrogen and replacement atoms should be 4; R11; R12; R13; R23 etc. • Ethane (C2H6) Based will have a three-digit number ex: R113. First figure shows no. of Carbon atoms minus 1; Second figure shows no. of Hydrogen atoms plus 1; Third figure shows no. of fluorine Atoms R113 =CCl2FCClF2 Tricholorofluoroethane; R114; R115 etc.

  35. R134a=C2H2F4 Tetrafluoro ethane The letter a signifies isomer ( having same chemical composition but different atomic arrangement) • Refrigerants starting with 4 and 5 indicate blended refrigerants

  36. The most important members of the group have been – • Chlorofluorocarbons (CFC) • Hydrochlorofluorocarbon (HCFC) • Hydrofluorocarbon(HFC)

  37. Chlorofluorocarbons (CFC) • CCl2F2 - Dichlorodifluoromethane (Freon 12 or R12) • CCl3F -Trichlorofluoromethane (Freon 11 or R11) • C2Cl2F4 - Dichlorotetrafluoroethane (Freon 114 or R114) • C2Cl3F3 - Trichlorotrifluoroethane (Freon 113 or R113)

  38. Hydrochlorofluorocarbon (HCFC) • CHClF2 – monochlorodifluoromethane (Freon 22 or R22) • R123 – C2HCl2F3 • R124 - C2HClF4

  39. Hydrofluorocarbon(HFC) • C2H2F4 -R134a • C2H4F2 - R152a • C2HF5 - R125 • CH2F2 - R32 • C2H3F3 - R143a

  40. Inorganic refrigerants • These are designated by number 7 followed by the molecular weight of the refrigerant (rounded-off).

  41. Mixtures • Azeotropic mixtures (containing two gases with same boiling point ) are designated by 500 series, where as zeotropic (containing two or more gases not having same boiling point ) refrigerants (e.g. non-azeotropic mixtures) are designated by 400 series.

  42. Pure Hydrocarbon Refrigerants

  43. Depletion of stratospheric ozone layer • The depletion of stratospheric ozone layer was attributed to chlorine and bromine containing chemicals such as CFCs, HCFCs. If released to atmosphere, they are broken down by photolysis to release chlorine atoms, which catalytically destroy ozone, the stratospheric gas which acts as a filter to ultra violet (UV) light from the sun.

  44. Effect of UV light • Scientists predict that increased UV light on earth as a result of ozone depletion will, amongst other possible consequences, cause skin cancer, interfere with immune systems and harm aquatic systems and crops. Strong pressure was exerted to phase out CFCs and HCFCs, resulted in the Montreal Protocol being adopted in 1987.

  45. Regulation 12(2) of Annex VI of MARPOL 73/78 • New installations which contain ozone-depleting substances shall be prohibited on all ships, except that new installations containing hydrochloroflourocarbons (HCFCs) are permitted till 1 January 2020.

  46. Alternate refrigerants • They can be classified into two broad groups: • Non-ODS, synthetic refrigerants based on Hydro-Fluoro-Carbons (HFCs) and their blends • Natural refrigerants including ammonia, carbon dioxide, hydrocarbons and their blends

  47. Refrigeration oils Lubricating oils for refrigeration compressors are selected for their suitability with the different refrigerant, compressor type and the plant’s operating temperatures. Refrigeration oils should possess the following properties: • Good chemical stability • Good thermal stability • Low viscosity • Low wax content • Low pour point • Low Floc Point • Moisture free

  48. Good chemical and thermal stability • Good chemical stability: There should be little or no chemical reaction with the refrigerant or materials normally found in the system. • Good thermal stability: They should not form hard carbon deposits at hot spots in the compressor (such as valves or discharge ports).

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