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REFRIGERATION THERMODYNAMICS OF THE REFRIGERATION CYCLE BASIC KNOWLEDGE THERMODYNAMICS OF THE REFRIGERATION CYCLE Set-up and function of a compression The refrigeration cycle refrigeration system For operating media which can have different phases, The refrigerant in a compression refrigeration system Heat dissipation such as water or refrigerant, the T-ss diagram looks Liquid flows through a closed cycle with the following four during condensation different. supercooled stations: Highg pressure It has an area on the left (grey), in which the operating Evaporation A medium is liquid and supercooled. In the centre (blue) Condensation Compres- Compression B there is a mixture of steam and liquid, the wet steam. sion Condensation C s On the right of it (orange) the operating medium is in Expansionnnn Expansion D ou pure steam form and superheated. The cooling takes place in the evaporator (A). The se Compressor The real refrigeration cycle with its typical phase tran- Evaporation Gaseous evaporation takes place at low pressures and tempera- Ga power sitions can also be represented in this T-ss diagram. Wet steam superheated tures. Here the refrigerant absorbs heat from the envi- The cycle has many similarities to the familiar steam boiling temperature ronment and thus cools it. power cycle. The major difference is that the cycle is The still cold refrigerant steam is aspirated by a d anticlockwise. Thus the processes of evaporation and compressor (B) and subjected to higher pressure by condensation and expansion and compression (pump- using mechanical energy. The refrigerant steam heats Liqui ing) swap places. up due to the compression. The enclosed area (green)corresponds to the compres- Refrigeration cycle in the T-s diagram The hot refrigerant steam is cooled down in a condenser sor work added to the cycle. (C) and condenses while discharging heat to the envi- ronment. The liquid pressurised refrigerant is then expanded to LLow pressure the low evaporation pressure in an expansion element The log p-h diagram for refrigerant (D) and returned to the evaporator. Heat absorption In the log p-h diagram the pressure pp is plotted above the The refrigerant evaporates again and thus completes during evaporation Liquid enthalpy h. the circuit. supercooled In the centre (blue) is the wet steam area. Here the Cyclic process of a simple compression boiling temperaturegp temperature corresponds to the boiling temperature for refrigeration system the pressure. The wet steam area is surrounded by limit curves with the steam content x=0.0 and x=1.0. Steam contentttx To the left of it (grey) the refrigerant is liquid. The tempera- ture is below the boiling temperature for the pressure; the A cyclic process can be represented very clearly in the T-ss diagram. Gaseous refrigerant is supercooled. Here the temperature TT of the operating medium is plotted above superheated On the right (orange) the refrigerant is gaseous and the Isothermal compression the entropy s. The area enclosed by the change of state of the log p-h diagram for refrigerant temperature is above the boiling temperature. The refrig- operating medium corresponds to the work realised in the cyclic erant is superheated. process. Every refrigerant has its own log p-h diagram. The cyclic process with the highest possible efficiency is the The log p-h diagram is better suited to represent the refrig- Carnot cycle, here the enclosed area is a rectangle. This cycle is eration cycle than the T-s diagram and is therefore used Isothermal expansionp often used as a comparison cycle to describe the quality of the predominantly. cyclic process. The direction of the cyclic process in the T-ss diagram determines Because energies exchanged with the refrigerant modify whether this is a heat pump cycle (refrigeration cycle) or a work the enthalpy h of the refrigerant, energy flows can be read machine cycle (steam power cycle). Refrigeration cycles are anti- directly from the diagram as horizontal lines. Ideal cyclic process (Carnot cycle) of a clockwise and the work represented by the green area is added gaseous medium in the T-s diagram to the cycle. 27 REFRIGERATION THERMODYNAMICS OF THE REFRIGERATION CYCLE BASIC KNOWLEDGE THERMODYNAMICS OF THE REFRIGERATION CYCLE The refrigeration cycle in the log p-h diagram The refrigerant The real refrigeration cycle consists of the following changes of state: Every cyclic process requires an operating medium which The different refrigerants are marked with an RR followed in the refrigeration cycle is the refrigerant. In the refrigera- by a number. 1 – 2 polytropic compression on the tion cycle the refrigerant has the purpose of transporting The water often used in technical cycles is not suitable for condensation pressure (for comparison heat. Here the high absorption of energy during evapora- the refrigeration cycle. At the low temperatures prevail- 1 – 2’ isentropic compression) tion or discharge of energy during the condensation of ing in a refrigeration system the evaporation pressure is a liquid is utilised. To achieve this at the temperatures extremely low and there is a risk of the water freezing. 2 – 2’’ isobaric cooling, deheating of the prevailing in a refrigeration system at well manageable pressures, liquids with a low boiling point, such as differ- The use of CO is technically demanding. Due to its low superheated steam 2 ent fluorocarbons (FC), ammonia (NH3), carbon dioxide boiling temperature a very high pressure level results. (CO ) or hydrocarbons such as butane or propane, are This means that common components from refrigera- 2’’ – 3’ isobaric condensation 2 used as operating medium. tion technology, such as valves, compressors or heat 3’ – 3 isobaric cooling, supercooling of the exchangers, cannot be used. For NH there are also special components, because liquid Boiling 3 Name temperature materials containing copper are not resistant against 3 – 4 isenthalpic expansion to the ammonia. evaporation pressure FC R134a Pure substance Ts = -26°C FC R404a Mixture Ts = -47℃ 4 – 1’ isobaric evaporation FC R407a Mixture Ts = -39...-45°C NH R717 Pure substance Ts = -33°C 1’ – 1 isobaric heating, superheating of the 3 steam The refrigeration cycle in the log p-h diagram Isobutane R600a Pure substance Ts = -12℃ CO R744 Pure substance Ts = -78°C In addition there are also pressure losses in the real refrigeration cycle, which means that evaporation and condensation 2 are not exactly horizontal (isobaric). Energy considerations in the log p-h diagram Important for a good operation is the steam pressure The horizontal distances of the key cycle points in the curve of the operating medium. It should be gaseous log p-h diagram correspond to the enthalpy differences. In the C at low pressures and at the desired cooling tempera- simple refrigeration cycle without branched off mass flows these tures and liquid at high pressures and temperatures. result in the energy flows or capacities of the ideal system when The pressure levels should also be easy to manage multiplied with the refrigerant mass flow. The distances in the technically. log p-h diagram are therefore a direct measure for the energy The diagram shows the steam pressure curve of the flows exchanged. emperature in ° The distance 4 – 1 corresponds to the cooling capacity and is the T well suited FC R134a. Typical freezing temperatures net capacity of the refrigeration system. The distance 1 – 2 is the of -26°C in the evaporator can be implemented with drive power exerted via the compressor. The distance 2 – 3 corre- pressures around 1bar while for condensing only a sponds to the heat capacity discharged via the condenser. This is pressure of 17bar at 60°C is required. the waste heat of the refrigeration system. Pressure in bar a While in pure substances, such as NH , propane and Steam pressure curve of FC R134a 3 CO , the steam pressure curve is fixed, it can be From the ratio of the net capacity and the drive power the coef- 2, ficient of performance COP can be calculated. adapted in FC within wide boundaries to meet require- ments by mixing different base grades. Energy flows in the refrigeration cycle h - h COP = 1 4 cooling capacity absorbed h2 - h1 compressor drive power The coefficient of performance can be compared to the efficiency heat capacity discharged in a work machine. 29
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