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Chemcal Engineering Thermodynamics – Ii Unit V – Refriegeration

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Chemcal Engineering Thermodynamics – Ii Unit V – Refriegeration Chemical Engineering thermodynamics – II Unit V - Refrigeration CHEMCAL ENGINEERING THERMODYNAMICS – II UNIT V – REFRIEGERATION Principles of refrigeration Methods of producing Refrigeration Liquefaction Process Coefficient of performance Evaluation of the performance of vapor compression and gas refrigeration cycles REFRIGERATION Refrigeration is process of producing and maintaining a temperature below that of the surrounding atmosphere. This requires continuous absorption of heat at a low temperature level, usually accomplished by evaporation of a liquid in a steady state flow process. The vapor formed may be returned to its original liquid state for re-evaporation, either by compressing and condensing or by absorbing it with a liquid of low volatility from which it is subsequently separated at high pressure. Thus refrigeration is essentially an operation involving the pumping of heat from one temperature to a higher temperature. The complete series of processes that the working fluid - the refrigerant – undergoes constitute a refrigeration cycle. A typical refrigeration cycle includes, evaporation of the liquid refrigerant, compression of the refrigerant vapor, condensation of the vapor into liquid and finally expansion of the liquid. CARNOT REFRIGERATOR A refrigeration cycle is a reversed heat engine cycle. Heat is transferred from a low temperature level to a higher temperature level. According second law of thermodynamics this requires an external source of energy. The ideal refrigerator like the ideal heat engine operates on a Carnot cycle, consisting of two isothermal steps in which heat lQcl is is absorbed at a lower temperature level Tc and heat lQHl is rejected at higher temperature TH and two adiabatic cycles steps. The cycle requires the addition of net work w to the system. Since ∆U of the working fluid is zero from the first law of thermodynamics., W = lQHl –lQcl (1) COEFFICIENT OF PERFORMANCE It is defined as the measure of the performance of a refrigerator and is the ratio of heat absorbed at the lower temperature to the net work. Heat abosorbed at the lower temperature ω = --------------------------------------------------- Net work lQcl = ---------- (2) W Dividing equation (1) by (Qc)., w QH ---- = ----- - 1 Qc Qc St. Joseph’s College of Engineering Department of Chemical Engineering Page 1 Chemical Engineering thermodynamics – II Unit V - Refrigeration lQHl TH But for Carnot refrigerator ------- = ------ lQcl Tc w TH TH - Tc 1 Therefore ----- = ----- - 1 = ------------ = ---- lQcl Tc Tc ω Tc ω = ------------ TH – Tc Applicable to a refrigerator, operating on a Carnot cycle. According to this refrigeration effect per unit of work decreases as the temperature of the refrigerator Tc decreases, and as the temperature of heat rejection TH increases. For refrigeration at a temperature level of 5⁰C and a surroundings of 30⁰C the value of ω for a Carnot refrigerator is 5 + 273.15 ω = ----------------------------------- = 11.13 (30 +273.15) – (5 + 273.15) THE VAPOR COMPRESSION CYCLE St. Joseph’s College of Engineering Department of Chemical Engineering Page 2 Chemical Engineering thermodynamics – II Unit V - Refrigeration A liquid evaporating at constant pressure provides a means of heat absorption at constant temperature. Similarly condensation of the vapor after compression to a higher pressure provides for the rejection of heat at constant temperature. The liquid from the condenser is returned to its original state by an expansion process using a turbine or an expander. When the compression and expansion are isentropic, the sequence of processes constitutes the cycle as shown above. This is just similar to Carnot cycle except that the superheated vapor from the compressor must be cooled to its saturation temperature before condensation begins. On the basis of unit mass of fluid, the heat absorbed in the evaporator is lQcl = ∆H = H2 – H1 Heat rejected lQHl = H3 – H4 St. Joseph’s College of Engineering Department of Chemical Engineering Page 3 Chemical Engineering thermodynamics – II Unit V - Refrigeration But W = lQHl – lQcl = (H3 – H4) – (H2 – H1) Qc H2 – H1 COP = ω = ------- = ---------------------- W (H3 – H4) – (H2 – H1) In small units expansion is accomplished by throttling the liquid from the condenser though a partly opened valve. The pressure drop in this irreversible process results from fluid friction in the valve. But still because of its simplicity and lower cost, it outweighs the energy savings possible with a turbine. The throttling process occurs at constant enthalpy. The vapor compression cycle incorporating an expansion valve is shown above. Line 4- 1 represents the constant enthalpy throttling process. Line 2 - 3 represents an sloping in the direction of increasing entropy. The dashed line 2 – 3’ is the path of isentropic compression. For this cycle H2 – H1 COP = ω = ---------------------- (H3 – H4) – (H2 – H1) But H4 = H1 throttling being a isentropic process H2 – H1 COP = ω = ------------------ (H3 – H2) St. Joseph’s College of Engineering Department of Chemical Engineering Page 4 Chemical Engineering thermodynamics – II Unit V - Refrigeration Design of the evaporator, compressor condenser requires the knowledge of the rate of circulation of refrigerant m lQcl Mass flow rate of refrigerant = ---------- H2 – H1 The vapor compression cycle shown above can also be represented on P H diagram as shown above. P H diagrams will give enthalpies directly. Although the evaporation and condensation processes are represented by constant pressure paths, small pressure drops do occur because of fluid friction. Usually refrigeration equipments are commonly rated in tons of refrigeration. A ton of refrigeration is defined as heat absorption at the rate of 12, 000 BTU per hour. This rate corresponds to the rate of heat removal that is required to freeze 1 ton of water at 273 K in a day. One ton of refrigeration is equivalent to a refrigeration rate of 12, 600 KJ/h in SI units. A Carnot refrigerator will have highest value of COP for a given set of values Tc and TH. A vapor compression cycle with expansion in a throttle valve has somewhat lower value and this is further reduced when compression in not isentropic. THE CHOICE OF THE REFRIGERANT Efficiency of Carnot engine and COP of Carnot refrigerator is independent of the refrigerant, however irreversibilities inherent in the vapor compression cycle cause the COP of the practical refrigerators depend on refrigerant. Toxicity, Flammability, Cost, corrosion properties and vapor pressure in relation to temperature are of greater importance in the choice of refrigerant. The most widely used refrigerants are a group of halogenated hydrocarbons marketed under the various proprietary names Freon – 12, genetron, arctron, isotron, frigen, mafron etc. These are either methane based or ethane based, where the hydrogen atoms are replaced by chlorine or fluorine atoms. For methane based refrigerants, the name is represented by two digits. First digit minus 1 represents the number of hydrogen atoms, and 2 digit represents the number of fluorine atoms. Eg: R – 12 CCl2F2 Dichlro difluoro methane. R – 10 CCl4 Carbon tetrachloride. For ethane based refrigerants, a three digit number is assigned where the first digit is always 1, the second minus 1 represents hydrogen atoms and the third digit represents number of fluorine atoms. Eg: R-113 C2F3Cl3 Trifluoro trichloro ethane. R – 142 C2H3F2Cl Difluoro chloro ethane. But these halogenated hydrocarbons being largely insoluble in water will move up and react with ozone layer and deplete it. Ammonia ia also a popular refrigerant but it is toxic and inflammable. St. Joseph’s College of Engineering Department of Chemical Engineering Page 5 Chemical Engineering thermodynamics – II Unit V - Refrigeration MULTI-STAGE VAPOR COMPRESSION SYSTEMS Limit placed on the operating pressures of the evaporator and condenser of a refrigeration system also limit the temperature difference TH – Tc over which a vapor compression cycle can operate. TH is fixed by the temperature of the surroundings ie., the temperature of the cooling water available. This limit in TH – Tc can be handled by the operation of two or more refrigeration cycles employing different refrigerators, in a cascade. A two stage cascade is shown in figure. Here the two cycles operate so that heat absorbed by the refrigerant of the higher temperature cycle (cycle 2) in the interchanger serves to condense the refrigerant of the lower temperature cycle (cycle 1). The two refrigerators are so chosen, so that at the required temperature levels each cycle operates at reasonable pressures. St. Joseph’s College of Engineering Department of Chemical Engineering Page 6 Chemical Engineering thermodynamics – II Unit V - Refrigeration ABSORPTION REFRIGERATION The work that is needed to operate a refrigerator cycle can also be obtained by operating a heat engine at a higher temperature level. The work required by a Carnot refrigerator absorbing heat at temperature Tc and rejection heat at the temperature of the surroundings TS lQcl Tc ω = ------- = ----------------------- W Ts – Tc Ts –Tc W = -----------lQcl (1) Tc When a source heat is available at a temperature above that of surroundings TH, then work can be obtained from a Carnot engine operating between this temperature and the surroundings temperature Ts. The heat required lQHl for the production of work lWl is found as follows St. Joseph’s College of Engineering Department of Chemical Engineering Page 7 Chemical Engineering thermodynamics
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