Thermodynamic Cycle

      According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area ; work is required to achieve this. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. The operating principles of the refrigeration cycle was described mathematically by sadi carnot  in 1824 as a heat engine. A heat pump can be thought of as a heat engine which is operating in reverse.
      Heat pump and refrigeration cycles can be classified as vapor compression , vapor absorption , gas cycle or stirling cycle types.

Heat pump and refrigeration cycle

      Thermodynamic heat pump cycles or refrigeration cycles are the models for heat pumps and refrigerators. A heat pump is a machine or device that moves heat from one location (the 'source') at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. Thus a heat pump may be thought of a "heater" if the objectives is to warm an area , or a "refrigerator" if the objective is to cool an area ; regardless the operating principles are identical.

Refrigerants

  

Refrigerants
   A modern R-134a refrigeration compressor

     "Freon" is a trade name for a family of haloalkane refrigerants manufactured by Dupont and other companies. These refrigerants were commonly used due to their superior stability and safety properties. However , these  chlorine-bearing refrigerants reach the upper atmosphere when they escape. Once the refrigerant reaches the stratosphere , UV radiation from the sun cleaves the chlorine-Carbon bond , Yielding a chlorine radical. These chlorine atoms catalyze the breakdown of ozone into diatomic oxygen , depleting the ozone layer that shields the Earth's surface from strong UV radiation. Each chlorine radical remains active as a catalyst unless it binds with another chlorine radical , forming a stable molecule and breaking the chain reaction. The use of CFC as a refrigerant was once common , being used in the refrigerants R-11 and R-12. In most countries the manufacture and use of CFCs has been banned or severely restricted due to concerns about ozone depletion. In light of these environmental concerns , beginning on November 14 , 1994 , the Environmental Protection Agency has restricted the sale , possession and use of refrigerant to only licensed technicians , per rules 608 and 609 of the EPA rules and regulations; failure to comply may result in criminal and civil sanctions. Newer and more environmentally-safe refrigerants such as HCFCs (R-22,  used in most homes today) and HFCs (R-134a , used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal protocol and replaced by  hydrofluorocarbons (HFCs) such as R-410A , which lack chlorine. Carbon dioxide (R-744) is being rapidly adopted as a refrigerant in Europe and Japan. R-744 is an effective refrigerant with a global warming potential of 1. It must use higher compression to produce an equivalent cooling effect.    

Air conditioning system basic and theories - Humidity

Humidity

     Air conditioning equipment usually reduces the humidity of  the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air , much as cold drink will condense water on the outside of a glass. The water is drained , removing water vapor from the cooled space and thereby lowering its relative humidity. Since humans perspire to provide natural cooling by the evaporation of  perspiration from the skin , drier air (up to a point) improves the comfort provided. The comfort air conditioner  is designed to create a 40% to 60% relative humidity in the occupied space. In food retail establishments , large , open chiller cabinets act as highly effective dehumidifiers.
     Some air conditioning units dry the air without cooling it. These work like a normal air conditioner , except that a heat exchanger is placed between the intake and exhaust. In combination with convection fans , they achieve a similar level of comfort as an air cooler in humid tropical climates , but only consume about one-third the energy. They are also preferred by those who find the draft created by air coolers uncomfortable.

Air conditioning system basics and theories

Refrigeration cycle

    A simple stylized diagram of the refrigeration cycle :

        01). Condensing coil
        02). Expansion valve
        03). Evaporator coil
        04). Compressor

A simple stylized diagram of the refrigeration cycle: 1) condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor.

    Capillary expansion valve connection to evaporator inlet. Notice frost formation.

Capillary expansion valve connection to evaporator inlet. Notice frost formation.

    In the refrigeration cycle a heat pump transfers heat from a lower-temperature heat source onto a higher-temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way , as it pumps the heat out of the interior and into the room in which it stands.
    This cycle takes advantage of the way phase changes work , where latent heat is released at a constant temperature during a liquid/gas phase change and where varying the pressure of a pure substance also varies its condensation/boiling point.
    The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a belt over a pulley , the belt being driven by the engine's crankshaft (similar to the driving of the pulleys for the alternator , power steering , etc.. ). Whether in a car or building , both use electric fan motors for air circulation. Since evaporation occurs when heat is absorbed , and condensation occurs when heat is released , air conditioners use a compressor to cause pressure changes between two compartments and actively condense and pump a refrigerant around. A refrigerant is pumped into the evaporator coil , located in the compartment to be cooled , where the low pressure causes the refrigerant to evaporate into a vapor , taking heat with it. At the opposite side of the cycle is the condenser , which is located out side of the cooled compartment , where the refrigerant vapor is compressed and forced through another heat exchange coil , condensing the refrigerant into a liquid , thus rejecting the heat previously absorbed from the cooled space.

A modern R-134a refrigeration compressor

    By placing the condenser (where the heat is rejected) inside a compartment and the evaporator (which absorbs heat) in the ambient environment (such as outside) or merely running a normal air conditioner's refrigerant in the opposite direction , the overall effect is the opposite , and the compartment is heated. This is usually called a heat pump and is capable of heating a home to comfortable temperatures (25°C ; 70°F) , even when the outside air is below the freezing point of water (0°C ; 32°F).
    Cylinder unloaders are a method of load control used mainly in commercial air conditioning systems. On a semi-hermetic (or open) compressor , the heads can be fitted with unloaders which remove a portion of the load from the compressor so that it can run better when full cooling is not needed. Unloaders can be electrical or mechanical.      
                     
  

Indvirkning på livsstil

Indvirkning på livsstil

Køleskabet giver den moderne familie til at holde maden frisk i meget længere tid end før. Dette, sammen med den moderne supermarked, giver de fleste familier, uden en stor have til at dyrke grøntsager og opdrætte dyr, en langt mere varieret kost og forbedret sundhed som følge af bedre ernæring. [Redigér] Mejeriprodukter, kød, fisk, fjerkræ og grøntsager kan opbevares nedkølet i det samme rum i køkkenet (selv om råt kød skal holdes adskilt fra andre fødevarer af hygiejniske grunde).Køleskabet lader folk spiser mere salat, frisk frugt og grøntsager, uden at eje en have eller en frugtplantage. Eksotiske fødevarer fra fjerne lande, der er importeret ved hjælp af køle kan nydes i hjemmet på grund af interne køling.Frysere give husholdningerne til at købe fødevarer i bulk: Det kan spises i ro og mag, og samlet indkøb sparer penge (se stordriftsfordele). Is, en populær vare i det 20. århundrede, kunne tidligere kun fås ved at rejse lange afstande, hvor produktet blev lavet frisk, og måtte blive spist på stedet. Nu er det en fælles mad element. Ice-on-demand ikke kun bidrager til at nyde kolde drikke, men er nyttig for førstehjælp, og for koldt pakninger, der kan opbevares nedfrosset til camping eller i tilfælde af en nødsituation.

Energieffektivitet

Energieffektivitet

En auto-afrimning enhed bruger en blæser fan til at holde fugten ude af enheden. Det har også en varmespiral under fordamperen at jævne varmer fryseren og smelter eventuelle is oprustning. Nogle enheder har også varmeapparater i siden af døren for at holde enheden fra "græde." Manual afrimning apparater til rådighed i brugte-apparat butikker eller ved særlige orden. Køleskabe, der anvendes til at forbruge mere energi end nogen anden hjem apparat, men i de sidste tyve år store fremskridt der er gjort for at gøre køleskabe mere energieffektive. I begyndelsen af 1990'erne en konkurrence blev afholdt mellem de store producenter til at fremme energieffektivitet. Nuværende modeller, der er Energy Star kvalificeret bruge 50 procent mindre energi end modeller lavet før 1993. [11] Det mest energieffektive enhed i USA er designet til at køre på 120 eller 110 volt, og bruger omkring en halv kilowatt-timer per dag. [12] Men selv almindelige enheder er ganske effektiv, og nogle mindre enheder bruger mindre end 0,5 kilowatt-timer per dag. Større enheder, især dem med store frysere og icemakers, kan bruge så meget som 4 kWh per dag. Blandt de forskellige stilarter af køleskabe, er top-fryseskab modeller mere effektiv end bottom-fryseskab modeller med samme kapacitet, som igen er mere effektiv end side-fryseskab modeller. Modeller med through-the-dørs isen enheder er mindre effektive end dem uden. [13] Dr. Tom Chalko i Australien har udviklet en ekstern termostat til at konvertere en kummefryser i en kiste køleskab med kun ca 0.1kWh per dag mængden af energi, der anvendes af en 100 watt pære i en time. [14] En lignende anordning, fremstilles af Johnson Controls [15]. Forskere på Oxford University har rekonstrueret et køleskab opfundet i 1930 af Albert Einstein i deres bestræbelser på at erstatte de nuværende teknologier med energi effektiv grøn teknologi. Den Einstein køleskab fungerer uden elektricitet og bruger ingen bevægelige dele eller drivhusgasser [16].