2 History Of Refrigeration Refrigeration relates to the cooling of air or liquids, thus providing lower temperature to preserve food, cool beverages, make ice and for many other .Most evidence indicate that the Chinese were the first to store natural ice and snow to cool wine and other delicacies.Ancient people of India and Egypt cooled liquids in porous earthen jars.In 1834, Jacob Perkins, an American, developed a closed refrigeration system using liquid expansion and then compression to produce cooling. He used Ether as refrigerant, in a hand- operated compressor, a water-cooled condenser and an evaporator in liquid cooler.
3 Refrigerantion Principle Modern refrigeration and air-conditioning equipment is dominated by vapour compression refrigeration technology built upon the thermodynamic principles of the reverse Carnot cycle.Refrigerant Changes phases during cooling and used again and again.
4 What is a RefrigerantRefrigerants are used as working substances in a Refrigeration systems.Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants.Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems.These fluids provide refrigeration by undergoing a phase change process in the evaporator.Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes
5 What is ChloroFloroCarcons Today’s refrigerants are predominantly from a group of compounds called halocarbons (halogenated hydrocarbons) or specifically fluorocarbons.Chlorofluorocarbons were first developed by General Motor’s researchers in the 1920’s and commercialized by Dupont as “Freons”.
6 Halocarbon Refrigerants Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants.Examples :CFC’s : R11, R12, R113, R114, R115
7 Freon Group Refrigerants Application and ODP Values Areas of ApplicationODPCFC 11(R11)CFC 12 ( R 12 )CFC 13 (R 13)CFC113 ( R113 )CFC114 ( R114 )Blend of R22 and R115 (R502)Air-conditioning Systems ranging from 200 to 2000 tons in capacity. It is used where low freezing point and non-corrosive properties are important.It is used for most of the applications. Air-conditioning plants, refrigerators, freezers, ice-cream cabinets, water coolers, window air-conditioners, automobile air conditioners.For low temp refrigeration up to – 90 C in cascade systemSmall to medium air-conditioning system and industrial coolingIn household refrigerators and in large industrial coolingFrozen food ice-cream display cases and warehouses and food freezing plants. An excellent general low temp refrigerant1.01.070.80.34
8 What is Ozone LayerOzone is an isotope of oxygen with three atoms instead of normal two. It is naturally occurring gas which is created by high energy radiation from the Sun.The greatest concentration of ozone are found from 12 km to 50 km above the earth forming a layer in the stratosphere which is called the ozone layer.This layer, which forms a semi-permeable blanket, protects the earth by reducing the intensity of harmful ultra-violet (UV) radiation from the sun.
9 Ozone Layer DepletionIn the early70’s,scientists Sherwood Roland and Mario Molina at the University of California at Irvine were the first to discover the loss of ozone in stratosphere while investigating the ozone layer from highflying aircraft and spacecraft.They postulated the theory that exceptionally stable chlorine containing fluorocarbons could, overtime, migrate to the upper reaches of the atmosphere and be broken by the intense radiation and release chlorine atoms responsible for catalytic ozone depletion.
10 OZONE LAYER DEPLETION N0RMAL REACTION O2 = O + O O2 + O = O3 But CFC refrigerants leaked during the manufacturing and normal operation or at the time of servicing or repair, mix with surrounding air and rise to troposphere and then into stratosphere due to normal wind or storm. The Ultraviolet rays act on CFC releasing Cl atom, which retards the normal reaction:RETARDED REACTIONO = O2 + OCCL2F = CCLF2 + CLO3 + CL = CLO + O2O + CLO = CL + O2
11 Harmful consequences of ozone depletion For Humans Increase inskin cancersnow blindnesscataractsLess immunity toinfectious diseasesmalariaherpesFor plantssmaller sizelower yieldincreased toxicityaltered formFor marine lifeReducedplanktonjuvenile fishlarval crabs and shrimps
12 MONTREAL PROTOCOLSIGNED IN 1987 UNDER THE ‘UNEP’, AFTER MUCH DISCUSSIONSMORE THAN 170 COUNTRIES HAVE RATIFIEDINDIA RATIFIED ON SEPT 17,1992ONE OF MOST SUCCESSFUL EXAMPLE OF INTERNATIONAL COOPERATION IN UN HISTORY
13 Montreal protocol- Control Schedule ozone depleting substancedeveloped countriesdeveloping countriesCFCsphased out end of 1995total phase out by 2010halonsphased out end of 1993HCFCstotal phase out by 2020total phase out by 2040
14 CFC Phase-out in India What is to be phased out? CFC-11, CFC-12 & CFC-113a. How much and when? Year 1999 22,588 MT,294 MTo MTHow to achieve the target? Production is controlled through a production quota allocated to each producer every year. The Ozone Cell conducts audits twice a year to monitor the production. How much has been Phaseout? CFC has been completely phased out as on 1st August, 2008
15 Vapor compression refrigeration System In 1834 an American inventor named Jacob Perkins obtained the first patent for a vapor-compression refrigeration system, it used ether in a vapor compression cycle.Joule-Thomson (Kelvin) expansionLow pressure (1.5 atm) low temperature (-10 to +15 ℃) insideHigh pressure (7.5 atm) high temperature (+15 to +40 ℃) outside
17 Circulation of Refrigerant Compressor cold vapor from the evaporator is compressed, raising it temperature and boiling point adiabatic compression T, b.p. ~ P work done on the gasCondenser hot vapor from the compressor condenses outside the cold box, releasing latent heat isothermal, isobaric condensation (horizontal line on PV diagram) high temperature T (hot) latent heat of vaporization Q (hot)Expansion valve (throttling valve) hot liquid from the condenser is depressurized, lowering its temperature and boiling point adiabatic, isochoric expansion (vertical line on PV diagram) T, b.p. ~ P no work done W = 0Evaporator cold liquid from the expansion valve boils inside the cold box, absorbing latent heat isothermal, isobaric boiling (horizontal line on PV diagram) low temperature T (cold) latent heat of vaporization Q (cold)
18 Importance of Refrigerant The thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures.However, important practical issues such as the system design, size, initial and operating costs, safety, reliability, and serviceability etc. depend very much on the type of refrigerant selected for a given application.Due to several environmental issues such as ozone layer depletion and global warming and their relation to the various refrigerants used, the selection of suitable refrigerant has become one of the most important issues in recent times.
19 Refrigerant selection criteria Selection of refrigerant for a particular application is based on the following requirements:i. Thermodynamic and thermo-physical propertiesii. Environmental and safety propertiesIii. Economics
20 Thermodynamic and thermo-physical properties The requirements are:a) 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 displacementb) 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.c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumptiond) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small
21 Thermodynamic and thermo-physical properties In addition to the above properties; the following properties are also important:e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be smallf) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inletg) Vapour specific heat: Should be large so that the degree of superheating will be smallh) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficientsi) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure dropsThe thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature, molecular weight and structure.
22 Environmental and safety properties At present the environment friendliness of the refrigerant is a major factor in deciding the usefulness of a particular refrigerant. The important environmental and safety properties are:a) 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
23 Environmental Effects of Refrigerants Global warming :Refrigerants directly contributing to global warming when released to the atmosphereIndirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations )
24 Environmental and safety properties b) 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.c) 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.
25 Environmental and safety properties d) Toxicity: Ideally, refrigerants used in a refrigeration system should be non-toxic. 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. 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. 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
26 Environmental and safety properties e) Flammability: The refrigerants should preferably be non-flammable and non-explosive. For flammable refrigerants special precautions should be taken to avoid accidents.f) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system.g) Compatibility with common materials of construction (both metals and non-metals)h) 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(R12).
27 Environmental and safety properties Ease of leak detection: In the event of leakage of refrigerant from the system, it should be easy to detect the leaks.Economic properties:The refrigerant used should preferably be inexpensive and easily available.
29 Halocarbon Refrigerants Halocarbon Refrigerant are all synthetically produced and were developed as the Freon family of refrigerants.Examples :CFC’s : R11, R12, R113, R114, R115HCFC’s : R22, R123HFC’s : R134a, R404a, R407C, R410a
30 HFCs Remain a popular choice especially for R22 phase outGood efforts at improving leakage performancee.g. Real Zero projectInterest in R407A to replace R404A50% reduction in GWPF Gas Stakeholder Group, 14th October 2009Slide 30
33 HCFC Transitional compounds with low ODP Partially halogenated compounds of hydrocarbonRemaining hydrogen atom allows Hydrolysis and can be absorbed.R22, R123
34 HCFC Production frozen at 1996 level 35% cut by 2005,65% by 2010 10 year grace period for developing countries.
35 R22 ODP-0.05, GWP-1700 R22 has 40% more refrigerating capacity Higher pressure and discharge temp and not suitable for low temp applicationExtensively used in commercial air-conditioning and frozen food storage and display cases
36 R123ODP-0.02,GWP-90As a replacement for R11 as similar thermodynamic properties.Very short atmospheric life but classified as carcinogenRetrofit alternative to R11
37 HFC Zero ODP as no chlorine atom contains only Hydrogen and Flurodine Very small GWP valuesNo phase out date in Montreal ProtocolR134a and R152 a – Very popular refrigerantsHFC refrigerants are costly refrigerants
38 R134aODP-0, GWP-1300Used as a substitute for R12 and to a limited range for R22Good performance in medium and high temp applicationToxicity is very lowNot miscible with mineral oil
39 R152aODP-0,GWP-140R152a is another attractive HFC with similar properties to R12.GWP is one order less than HFC134a but it is slightly flammable.Also it has lower energy consumption. Hence the Environmental Protection Agency of Europe prefers HFC152a to HFC134a
40 Hydrocarbon Very promising non-halogenated organic compounds With no ODP and very small GWP valuesTheir efficiency is slightly better than other leading alternative refrigerantsThey are fully compatible with lubricating oils conventionally used with CFC12.
41 Hydrocarbon Refrigerants Extraordinary reliability- The most convincing argument is the reliability of the hydrocarbon system because of fewer compressor failures.But most of the hydrocarbons are highly flammable and require additional safety precaution during its use as refrigerants.Virtually no refrigerant lossesHydrocarbons have been used since the beginning of the century and now being considered as long term solutions to environmental problems,
42 HydrocarbonsDominant in domestic market like household refrigerators and freezersGrowing use in very small commercial systems like car air-conditioning systemExamples: R170, Ethane, C2H6R290 , Propane C3H3R600, Butane, C4H10R600a, Isobutane, C4H10Blends of the above GasesF Gas Stakeholder Group, 14th October 2009Slide 42
43 R290 ODP-0,GWP-3 Compatible with copper.Miscible with mineral oil Highest latent heat and largest vapour densityA third of original charge only is required when replacing halocarbons refrigerant in existing equipmentEnergy saving : up to 20% due to lower molecular mass and vapour pressure
44 R 600aODP-0,GWP-3Higher boiling point hence lower evaporator pressureDischarge temp is lowestVery good compatibility with mineral oil
46 Modifications of Electrical Equipment Replaced with solid state equivalentsSealed to ensure that any sparks do not come into contact with leaking gasRelocated to a position where the component would not come into contact with leaking gas
47 Modifications of Electrical Equipment Faulty components.Poor, corroded, loose, or dirty electrical connections.Missing or broken insulation which could cause arcing/sparks.Friction sparks, like a metal fan blade hitting a metal enclosure.
48 Blends & MixturesLimited no of pure refrigerants with low ODP & GWP valuesTo try a mixture of pure refrigerants to meet specific requirement
49 Azeotropic Refrigerants A stable mixture of two or several refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures.Examples : R-500 : 73.8% R12 and 26.2% R152R-502 : 8.8% R22 and 51.2% R115R-503 : 40.1% R23 and 59.9% R13
50 Zeotropic Refrigerants A zeotropic mixture is one whose composition in liquid phase differs to that in vapour phase. Zeotropic refrigerants therefore do not boil at constant temperatures unlike azeotropic refrigerants.Examples :R404a : R125/143a/134a (44%,52%,4%)R407c : R32/125/134a (23%, 25%, 52%)R410a : R32/125 (50%, 50%)R413a : R600a/218/134a (3%, 9%, 88%)The fact that zeotropic refrigerants do not boil and condense and condense at constant temperature, can present an advantage. This phenomenon known as temperature glide is used to match the pressure drop in heat in heat exchangers thereby increasing their efficiency. This in turn results in an improved COP of the refrigeration cycle.
52 Carbon Dioxide Zero ODP & GWP Non Flammable, Non toxic Inexpensive and widely availableIts high operating pressure provides potential for system size and weight reducing potential.Drawbacks:Operating pressure (high side) : 80 barsLow efficiency
53 Ammonia –A Natural Refrigerant Ammonia is produced in a natural way by human beings and animals; 17 grams/day for humans.Natural production3000 million tons/yearProduction in factories120 million tons/yearUsed in refrigeration6 million tons/year
54 Ammonia as Refrigerant ODP = 0GWP = 0Excellent thermodynamic characteristics: small molecular mass, large latent heat, large vapour density and excellent heat transfer characteristicsHigh critical temperature (132C) : highly efficient cycles at high condensing temperaturesIts smell causes leaks to be detected and fixed before reaching dangerous concentrationRelatively Low price
55 Some Drawbacks of Ammonia as Refrigerant ToxicFlammable ( 16 – 28% concentration )Not compatible with copperTemperature on discharge side of compressor is higher compared to other refrigerants
56 WaterZero ODP & GWPWater as refrigerant is used in absorption system .New developing technology has created space for it for use in compression cycles also.But higher than normal working pressure in the system can be a factor in restricted use of water as refrigerant
57 Application of New Eco-friendly Refrigerants Application HFCs used Possible Eco-friendly refrigerantDomestic refrigeration R134a,R152a HC600a and blendsCommercial refrigeration R134a,R404A,R407C HC blends,NH3 ,CO2 **Cold storage ,food processingAnd industrial refrigeration R134a,R404A,R507A NH3 ,HCs,CO2 **Unitary air conditioners R410A,R407C CO2 , HC sCentralized AC (chillers) R134a,R410A,R407C NH3 ,HCs,CO2, water **Transport refrigeration R134a,R404A CO 2,Mobile air conditioner R134a CO2 ,HCsHeat pumps R134a,R152a,R404A NH3 ,HCs,CO2, water **R407C,R410A
58 General Safety measures for refrigerating plants Reduction of refrigerant contents:Components with reduced contentsIndirect systems with secondary refrigerant: distinction between generation and transport of coldScheduled maintenance and leak testingGovernmental surveillance – Refrigerant Audits forsystems operating with HFC’s. Recovery, Stock of used refrigerants, Recycling of refrigerants.For the Netherlands, the combined measures resulted in a leak rate reduction of 35% (1995) to 8% (2001) for R22-systems
59 Survey Of Refrigerants GroupAtmospheric lifeODPGWPR11CFC13014000R128500R22HCFC15.051500R134aHFC161300R404a3260R410a1720R5073300R717NH3-R744CO2R290HC< 18R600a
61 To ConcludeIn the aftermath of the Montreal protocole HFC’s have predominantly replaced CFC’s and HCFC’s in RAC equipment.Due to their high GWP, HFC’s are not a good replacement solution.The solution are the natural refrigerants :Ammonia, Hydrocarbons and Carbon dioxideSystem need to have low TEWI factorHigh efficiency with ammonia and lower power consumption with hydrocarbons
62 Environmental Effects of Refrigerants Global warming :Refrigerants directly contributing to global warming when released to the atmosphereIndirect contribution based on the energy consumption of among others the compressors (CO2 produced by power stations )
63 THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE The vapor-compression refrigeration cycle is the ideal model for refrigeration systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device.This is the most widely used cycle for refrigerators, A-C systems, and heat pumps.Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.
64 An ordinary household refrigerator. The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems.Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added cost and complexity.Steady-flow energy balanceAn ordinary household refrigerator.The P-h diagram of an ideal vapor-compression refrigeration cycle.
65 ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE An actual vapor-compression refrigeration cycle differs from the ideal one in several ways, owing mostly to the irreversibilities that occur in various components, mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. The COP decreases as a result of irreversibilities.DIFFERENCESNon-isentropic compressionSuperheated vapor at evaporator exitSubcooled liquid at condenser exitPressure drops in condenser and evaporatorSchematic and T-s diagram for the actual vapor-compression refrigeration cycle.
66 SELECTING THE RIGHT REFRIGERANT Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.The industrial and heavy-commercial sectors use ammonia (it is toxic).R-11 is used in large-capacity water chillers serving A-C systems in buildings.R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and freezers, as well as automotive air conditioners.R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to the ozone layer have been developed.Two important parameters that need to be considered in the selection of a refrigerant are the temperatures of the two media (the refrigerated space and the environment) with which the refrigerant exchanges heat.
67 HEAT PUMP SYSTEMSThe most common energy source for heat pumps is atmospheric air (air-to- air systems).Water-source systems usually use well water and ground-source (geothermal) heat pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install.Both the capacity and the efficiency of a heat pump fall significantly at low temperatures. Therefore, most air-source heat pumps require a supplementary heating system such as electric resistance heaters or a gas furnace.Heat pumps are most competitive in areas that have a large cooling load during the cooling season and a relatively small heating load during the heating season. In these areas, the heat pump can meet the entire cooling and heating needs of residential or commercial buildings.A heat pump can be used to heat a house in winter and to cool it in summer.
68 INNOVATIVE VAPOR-COMPRESSION REFRIGERATION SYSTEMS The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and it is adequate for most refrigeration applications.The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free.However, for large industrial applications efficiency, not simplicity, is the major concern.Also, for some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified.For moderately and very low temperature applications some innovative refrigeration systems are used. The following cycles will be discussed:Cascade refrigeration systemsMultistage compression refrigeration systemsMultipurpose refrigeration systems with a single compressorLiquefaction of gases
69 Cascade Refrigeration Systems Some industrial applications require moderately low temperatures, and thetemperature range they involve may be too large for a single vapor-compressionrefrigeration cycle to be practical. The solution is cascading.Cascading improves the COP of a refrigeration system.Some systems use three or four stages of cascading.A two-stage cascade refrigeration system with the same refrigerant in both stages.
70 Multistage Compression Refrigeration Systems When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics.A two-stage compression refrigeration system with a flash chamber.
71 Multipurpose Refrigeration Systems with a Single Compressor Some applications require refrigeration at more than one temperature. A practical and economical approach is to route all the exit streams from the evaporators to a single compressor and let it handle the compression process for the entire system.Schematic and T-s diagram for a refrigerator–freezer unit with one compressor.
72 Liquefaction of Gases Linde-Hampson system for liquefying gases. Many important scientific and engineering processes at cryogenic temperatures (below about 100°C) depend on liquefied gases including the separation of oxygen and nitrogen from air, preparation of liquid propellants for rockets, the study of material properties at low temperatures, and the study of superconductivity.The storage (i.e., hydrogen) and transportation of some gases (i.e., natural gas) are done after they are liquefied at very low temperatures. Several innovative cycles are used for the liquefaction of gases.Linde-Hampson system for liquefying gases.
73 GAS REFRIGERATION CYCLES The reversed Brayton cycle (the gas refrigeration cycle) can be used for refrigeration.Simple gas refrigeration cycle.
74 An open-cycle aircraft cooling system. The gas refrigeration cycles have lower COPs relative to the vapor-compression refrigeration cycles or the reversed Carnot cycle.The reversed Carnot cycle consumes a fraction of the net work (area 1A3B) but produces a greater amount of refrigeration (triangular area under B1).An open-cycle aircraft cooling system.Despite their relatively low COPs, the gas refrigeration cycles involve simple, lighter components, which make them suitable for aircraft cooling, and they can incorporate regeneration
75 Gas refrigeration cycle with regeneration. Without regeneration, the lowest turbine inlet temperature is T0, the temperature of the surroundings or any other cooling medium.With regeneration, the high-pressure gas is further cooled to T4 before expanding in the turbine.Lowering the turbine inlet temperature automatically lowers the turbine exit temperature, which is the minimum temperature in the cycle.Extremely low temperatures can be achieved by repeating regeneration process.Gas refrigeration cycle with regeneration.
76 ABSORPTION REFRIGERATION SYSTEMS When there is a source of inexpensive thermal energy at a temperature of 100 to 200°C is absorption refrigeration.Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price.Ammonia absorption refrigeration cycle.
77 Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport medium.The most widely used system is the ammonia–water system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.Other systems include water–lithium bromide and water–lithium chloride systems, where water serves as the refrigerant. These systems are limited to applications such as A-C where the minimum temperature is above the freezing point of water.Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis.ARS are often classified as heat-driven systems.ARS are much more expensive than the vapor-compression refrigeration systems. They are more complex and occupy more space, they are much less efficient thus requiring much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common.Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.ARS are primarily used in large commercial and industrial installations.
78 The COP of actual absorption refrigeration systems is usually less than 1. Air-conditioning systems based on absorption refrigeration, called absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop.Determining the maximum COP of an absorption refrigeration system.
79 Summary Refrigerators and Heat Pumps The Reversed Carnot Cycle The Ideal Vapor-CompressionRefrigeration CycleActual Vapor-CompressionSelecting the Right RefrigerantHeat Pump SystemsInnovative Vapor-CompressionRefrigeration Systems
80 IntroductionThe mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system.Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.
81 RefrigerationThe term refrigeration may be defined as the process of removing heat from a substance under controlled conditions.It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.
82 Contd….In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.
83 Refrigerator & Refrigerant A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body.The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temp. system & delivers it to a higher temp. system.
84 Refrigeration CycleIn refrigeration system the heat is being generally pumped from low level to higher one & rejected at that temp.This rejection of heat from low level to higher level of temp. can only be performed with the help of external work according to second law of thermodynamics.
85 Contd….The total amount of heat being rejected to the outside body consist of two parts:-- the heat extracted from the body to be cooled .- the heat equivalent to the mechanical work required for extracting it.
87 Contd….A refrigerator is a reverse heat engine run in the reverse direction by means of external aid.Every type of refrigeration system used for producing cold must have the following four basic units:-
88 Contd….Low temp. thermal sink to which the heat is rejected for cooling the space.Means of extracting the heat energy from the sink, raising its level of temp. before delivering it to heat receiver.A receiver is a storage to which the heat is transferred from the high temp., high pressure refrigerant.
89 Contd…..Means of reducing the pressure & temp. of the refrigerant before it return to the sink.The processes of the cycle are evaporation, compression, condensation & expansion.By reversing the heat engine cycle completely & by changing the working agent, a refrigeration cycle is obtained.
90 Refrigeration Systems Vapour compression refrigeration systemVapour absorption refrigeration systemThermo electric refrigeration system
91 Vapour Compression Refrigeration This is the most important system from the point of commercial & domestic utility & most practical form of refrigeration.The working fluid refrigerant used in this refrigeration system readily evaporates & condenses or changes alternatively between the vapour & liquid phases without leaving the refrigerating plant
92 Contd….During evaporation it absorbs heat from the cold body or in condensing or cooling it rejects heat to the external hot body .The heat absorbed from cold body during evaporation is used as its latent heat for converting it from liquid to vapour.Thus a cooling effect is created in working fluid.
94 Contd….This system of refrigeration thus act as latent heat pump since its pump its latent heat from the cold body or brine & rejects it or deliver it to the external hot body or the cooling medium.According to the law of thermodynamics , this can be done only on the expenditure of energy which is supplied to the system in the form of electrical energy driving the compressor.
95 Contd….The vapour compression cycle is used in most of the modern refrigeration systems in large industrial plants.The vapour in this cycle is circulated through the various components of the system, where it undergoes a number of changes in its state or condition.
96 Contd….Each cycle of operation consists of the four fundamental changes of state or processes:-ExpansionVaporisationCompressionCondensation
98 CompressorThe low pressure & temp. refrigerant from evaporator is drawn into the compressor through the inlet or suction valve , where it is compressed to a high pressure & temp.The high pressure & temp vapour refrigerant is discharged into the condenser through the delivery or discharge valve.
99 CondenserThe condenser or the cooler consists of coils of pipe in which the high pressure & temp. vapour refrigerant is cooled & condensed.The refrigerant while passing through the condenser, rejects its latent heat to surrounding condensing medium which is normally air or water.Thus hot refrigerant vapour received from compressor is converted into liquid form in condenser.
100 ReceiverThe condensed liquid refrigerant from the condenser is stored in a vessel, known as receiver, from where it is supplied to the expansion valve or refrigerant control valve.
101 Expansion ValveThe function of this valve is to allow the liquid refrigerant under high pressure & temp. to pass at a controlled rate after reducing its pressure & temp.some of liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure & temp.
102 EvaporatorAn evaporator consists of coils of pipes in which the liquid vapour refrigerant at low pressure & temp. is evaporated & changed into vapour refrigerant at low pressure & temp.During evaporation process, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium which is to be cooled.
103 Advantages Smaller size for a given refrigerating capacity Higher coeff. of performanceLower power requirements for a given capacityLess complexity in both design & operationIt can be used over large of temp.
104 Domestic Refrigerator The application of refrigeration for domestic purposes are mainly in the form of domestic refrigerators & home freezers.The main purpose of this type of refrigeration is to provide low temp. for storage & distribution of foods & drinks.
105 Contd….It represents a significant portion of the refrigeration industry due to the use of these units in large number.For domestic preservation, the storage is generally short term. The domestic refrigerators used for the purposes are usually small in sizes with rating in ranges from 1/20 to ½ tonne.
106 Contd…. The unit is usually self contained and hermetically sealed. Due to short term storage the domestic refrigerator load is intermittent.
107 Contd…. The requirement of domestic refrigerator is that:- it should be simple in constructionautomatic in actionnominal in initial cost
108 Contd….dependable and without any necessity of expert inspection & repair.Non irritant & non toxic refrigerant should be used.Generally methylene chloride, freon-12, freon -11 are used as refrigerants.
109 Contd…The common type of domestic refrigerator have a cabinet shaped with compressor motor-fan assembly, the condensed and receiver fitted in their basement.The expansion valve evaporator coils are exposed in the storage cabinet with the piping, carrying liquid refrigerant passing through the body.
110 Contd….The heat of the bodies to be cooled is carried to the evaporator coils by means of air trapped in the cabinet.Refrigeration is not only provided with double walled cabinet packed with materials having high thermal insulation such as fibre glass or expanded rubber but also all around the inside of door flap soft rubber seal is used which makes rubber air tight.
111 Electrical CircuitRefrigerator is provided with a door push switch, which closes on opening of refrigerator and puts the lamp on.Capacitor start single phase induction motor is used in open type refrigerators and split phase induction motor is used in sealed unit refrigerators.Electromagnetic relay is provided to connect auxiliary winding on the start & disconnect it when the motor picks up the speed.
113 Contd…..Thermal overload release is provided to protect the motor from damage against flow of over current.Thermostat switch is provided to control the temp. inside the refrigerator.Temp. inside the refrigerator can be adjusted by means of temp. control screw.
114 Contd…To protect the motor against under voltage use of automatic voltage regulator is essential since in case of fall in applied voltage, motor will draw heavy current to develop the required torque and will become hot, thermal overload relay will therefore repeatedly disconnect and connect the motor to supply, eventually burning it out.