A pplied Thermodynamics

1. Applied Thermodynamics

2. 6. REFRIGERATION Definition Refrigeration is the process of providing and maintaining temperature of the system below that of the surrounding atmosphere. The refrigeration effect can be accomplished by non – cyclic processes, making use of substances at temperature well below the temperature of the surroundings – e.g., ice, snow, dry ice (solid CO2) etc. However, of greater importance are cyclic refrigerationsystems, wherein the cooling substance (called refrigerant) is not consumed and discarded, but used again and again in a thermodynamic cycle.

3. REFRIGERATION

4. A ton of refrigeration is defined as the quantity of heat required to be removed to produce one ton (1000kg) of ice within 24 hours when the initial condition of water is 00C. Consider a refrigerator of T tons capacity, Refrigeration capacity = 3.5 kJ/s Heat removed from refrigerator = Refrigeration effect =R.E. kJ/s Power of the compressor =work/kg of refrigerant x mass flow rate

5. Reversed Heat Engine Cycle A reversed heat engine is a potential refrigerating machine. It receives heat from a low temperature region at T2, discharge heat to a high temperature region at T1, and requires a net inflow of work. Removal of heat from a low temperature region reduces the temperature of that region below the temperature of the surroundings, thus producing refrigeration.

6. According to First Law Q2 – Q1 = -W i.e., Q1 = Q2 + W Such a device is called a Refrigerator or Heat Pump, depending on whether the focus is on heat received from the low temperature region Q2 or the heat discharged to the high temperature region Q1. Q2 is known as the refrigeration effect. The performance of a refrigerator/heat pump is measured by means of its coefficient of performance (COP). COP of a refrigeration/heat pump is defined as The working fluid in a refrigeration cycle is called a Refrigerant.

7. Important application of Refrigeration • Ice plants • Food processing units and transportation, including dairies • Industrial air – conditioning • Comfort air – conditioning • Chemical and related industries. • Hospitals. • Laboratories. • Domestic applications

8. Basic processes (operations) in a Refrigeration Cycle Since a refrigeration cycle is essentially a reversed heat engine cycle, the working substance (refrigerant) will undergo the following basic operations. • Compression - resulting in increase in pressure and temperature. • Heat rejection at high temperature. • Expansion – resulting in reduction in pressure and temperature and • Heat addition at low temperature – during which heat is transferred from the body to be cooled to the refrigerant.

9. Vapour Compression Refrigeration Cycle In this, the refrigerant used is a vapour (e.g., ammonia, Freon-22, Freon-11, Freon -12 etc). The refrigerant undergoes the following operations in a cyclic manner. • Compression in a compressor (Usually reciprocating), with work input. • Condensation of the vapour into liquid in a condenser, wherein heat is rejected to a cooling medium (air, water) at high pressure and temperature. • Expansion of the liquid refrigerant in a suitable device (engine, expansion valve, capillary etc). There may or may not be work output. The liquid may evaporate partially. • Evaporation of the mixture of liquid and vapour in an evaporator where heat is added to the refrigerant from the substance to be cooled, producing the necessary refrigeration effect.

10. Reversed Carnot Cycle as a Refrigeration Cycle 4-1: Reversible adiabatic (isentropic) compression, with work input WC. 1-2: Condensation at constant pressure and temperature with heat Q1 rejected to some cooling medium. 2-3: Reversible adiabatic expansion, with work output WE. 3-4: Evaporation at constant pressure and temperature wherein heat Q2 is absorbed from the substance to be cooled.

12. Q1 = area under 2 – 3 = Tmax (s2 – s3) Q2 = area under 4 – 1 = Tmin (s1 – s4) = Tmin (s2 – s3), s1 = s2 & s3 = s4 Wnet = WC – WE = Q1 – Q2 = (Tmax-Tmin) (s2-s3)

13. These are the maximum values for any refrigerator or heat pump operating between two fixed temperatures Tmax and Tmin. In other words, no refrigerator/ heat pump has a COP greater than that of a Carnot refrigerator/heat pump, operating between the same maximum and minimum temperatures. When the refrigerator/heat pump operates on a cycle other than a Carnot cycle, the heat rejection (condensation) and heat addition (evaporation) process may not be isothermal. Then the COPs are given by Where Tcond = average temperature during condensation. Tevap = average temperature during evaporation. It can be seen that the closer the temperatures Tcond and Tevap, the higher the COP.

14. In practice, an expansion engine is not used in a vapour compression refrigeration unit. This is because; the power output of such an engine is too small to justify its cost. Instead, some kind of expansion device – like a throttling valve or a capillary tube – is used to reduce the pressure and temperature of the refrigerant.

15. The most convenient property diagram.

17. Process 1-2 or 1’–2’: Reversible adiabatic compression. Process 1–2, starting with saturated vapour (state 1) and ending in the superheated region (state 2) is called Dry compression. Process 1’-2’, starting with wet vapour (state 1’) and ending as saturated vapour (state 2’) is called wet compression. Dry compression is always preferred to wet compression. .

18. Process 2-3 (or 2’–3): Reversible constant pressure heat rejection, at the end of which the refrigerant is in saturated liquid state. 2–2’ is desuperheating, and 2’-3 is condensation.

19. Process 3-4: Adiabatic throttling process, for which enthalpy before is equal to enthalpy after throttling. This process is adiabatic but not isentropic. Since it is irreversible, it cannot be shown on a property diagram. States 3 and 4 are equilibrium points and are simply joined by a dotted line following a constant enthalpy line.

20. Process 3-4: Adiabatic throttling process, for which enthalpy before is equal to enthalpy after throttling. This process is adiabatic but not isentropic. Since it is irreversible, it cannot be shown on a property diagram. States 3 and 4 are equilibrium points and are simply joined by a dotted line following a constant enthalpy line.

21. Analysis: The compressor, the condenser and the evaporator can be treated as steady–flow devices, governed by the Steady Flow Energy Equation. Application of S.F.E.E. to these devices results in: Compressor: Process 1–2 isentropic Q1-2 = 0 W1-2 = - ∆h W1-2 = - (h2 - h1) Compressor work WC = (h2-h1) kJ/kg, on a unit mass basis. If mr is the mass flow rate of the refrigerant in kg/sec, then the power input in the compressor is given by Power input = mr (h2 – h1) kW

22. Condenser: Process 2–3: reversible constant pressure process W2-3 = 0 Q2-3 = ∆h = (h3 – h2) kJ/kg This is negative i.e., heat rejected. Heat rejected per unit mass of the refrigerant is Q1 = (h2 - h3) kJ/kg. Rate of heat rejection Q1 = mr(h2 - h3) kJ/sec

23. Evaporator: Process 4 – 1: reversible constant pressure process , W4-1 = 0 Q4-1 = ∆h = (h1 - h4) kJ/kg Heat received by unit mass of the refrigerant = heat received from the substance being cooled = Q2 = (h1 – h4) kJ/kg of refrigerant. Rate of heat removed = Refrigerating effect = Q2 = m r(h1 - h4) kJ/sec Refrigerating Effect in terms of refrigeration

24. Expansion: for process 3 – 4, h3 = h4 but, it is not a constant enthalpy process. Note: Values of enthalpy h1, h2, h3 & h4 can be obtained from property Tables or Property Charts (Diagrams).

25. Actual Vapour Compression Refrigeration Cycle: A constant amount of superheating of the vapour before it enters the compressor is recommended. This is to ensure that no liquid refrigerant droplets enter the compressor. Further, a small degree of sub cooling (under cooling) of the liquid refrigerant at the condenser exit is desirable, in order to reduce the mass of vapour formed during expansion. Excessive formation of vapour bubbles may obstruct the flow of liquid refrigerant through the expansion valve.

26. Both the superheating at the evaporator outlet and the subcooling at the condenser outlet contribute to an increase in the refrigerating effect. However, the load on the condenser also increases. There will be an increase in the compressor discharge temperature. Since the compressor input more or less remains unchanged, the COP of the cycle appears to increase due to this superheating/subcooling. However, for a fixed temperature of the refrigerated space, the evaporation temperature must be lowered (i.e., Tevap is reduced). Further, for a fixed temperature of the cooling medium, the condensation temperature must be raised (i.e., Tcond will be higher). Hence COP will reduce.

27. Refrigerants and desirable properties: The most commonly used refrigerants are a group of halogenated hydrocarbons, marketed under various proprietary names of freon, genetron, arcton etc. Among them Freon–22 (Mono-chloroDifluoro Methane), Freon–11 (Tri-chloro – mono-fluoro methane) & Freon–12 (DichloroDifluoro methane) are extensively used. Ammonia is another commonly used refrigerant. Other refrigerants include CO2, SO2, Methyl chloride, Methylene chloride, Ethyl chloride etc.

28. Desirable properties of a good refrigerant: Thermodynamic properties Low boiling point Low freezing point Positive gauge pressure in condenser and evaporator, but not very high High latent heat of vaporization  Chemical properties Non–toxic Non–inflammable & non–explosive Non–corrosive Chemically stable No effect on quality of stored products

29. Desirable properties of a good refrigerant: Physical properties. Low specific volume of vapour Low specific heat High thermal conductivity Low viscosity Other properties Ease of leakage detection Cost Ease of handling

30. Ammonia is a good refrigerant with the highest refrigerating effect per unit mass. It is relatively cheap. But it is toxic and corrosive. Leakage can be easily detected because if its pungent odour. Freons are Non–toxic & non–inflammable. Leakage cannot be detected easily as they are odour less and colour less. Some coloured additives are sometimes mixed with Freons to facilitate detection of leakage.

31. Gas (Air) Cycle Refrigeration: Refrigeration can also be accomplished by means of a gas cycle, the most common being the one using air as a refrigerant. In such a cycle, a throttle valve cannot be used for expansion of the working fluid. During the throttling process, enthalpy at the beginning is equal to enthalpy at the end. For an ideal gas, (all gases including air are assumed to be ideal), enthalpy is a function of temperature only. Hence, during throttling temperature at the beginning will be equal to temperature at the end.

32. Gas (Air) Cycle Refrigeration contin…..: Since there is no cooling of air during expansion, refrigeration is not possible. In place of a throttle valve, an expander is used. Work output obtained from the expander can be utilized for compression, thus decreasing the net work input. In a gas refrigeration cycle, the refrigerant (gas/air) remains in a gaseous state throughout the cycle. Since there is no phase change, the terms ‘condenser’ and ‘evaporator’ are not appropriate. The device in which heat is rejected at a higher temperature can be called a cooler, while the device in which heat is absorbed at a lower temperature is called the ‘refrigerator’.

33. Reversed Carnot Cycle A reversed Carnot cycle using air as the working substance can be a Refrigeration cycle, through it is not practicable. 1 – 2: isentropic compression. 2 – 3: heat rejection at constant temperature. 3 – 4: Expansion 4 – 1: heat addition at constant temperature (refrigeration)

36. Heat rejected during process 2 – 3 = Q1 = Tmax (s2-s3) = Tmax (s1-s4) Heat received during process 4 – 1 = Q2 = Tmin (s1-s4) Wnet = WC - WE = Q1 - Q2 (First Law) = (Tmax - Tmin) (s1-s4) These COPs are the maximum possible COPs for given maximum and minimum temperatures.

37. Reversed Brayton Cycle. A reversed Brayton cycle with air as the working substance is a more practical refrigeration cycle. 1 – 2: isentropic compression 2 – 3: constant pressure heat rejection 3 – 4: isentropic expansion 4 - 1: constant pressure heat addition.

39. On a unit mass basis, Compressor work input = WC = h2 - h1 = Cp (T2 - T1) Expansion work output = WE = h3 - h4 = Cp (T3 - T4) Heat rejected at constant pressure = Q1 = h2 - h3 = Cp (T2 - T3) Heat received at constant pressure = Q2 = h1 - h4 = Cp (T1 - T4)

40. On a unit mass basis, Compressor work input = WC = h2 - h1 = Cp (T2 - T1) Expansion work output = WE = h3 - h4 = Cp (T3 - T4) Heat rejected at constant pressure = Q1 = h2 - h3 = Cp (T2 - T3) Heat received at constant pressure = Q2 = h1 - h4 = Cp (T1 - T4)

41. For the isentropic process 1 – 2, For the isentropic process 3-4,

42. The COP of a gas cycle refrigeration system is low. The power required per unit capacity is high. Its main application is in aircrafts and missiles, where a vapour compression refrigeration system becomes heavy and bulky. Another application of gas cycle refrigeration is in the liquefaction of gases. Shown below is a schematic flow diagram of an open cycle air refrigeration system.

43. A small amount of compressed air is blend from the main compressor of a turbojet or a supercharged aircraft engine, and is cooled by rejecting heat to large amounts of cooler ambient air. The cooled compressed air expands in an expander, and as a result cools further. The cool air enters the cabin. The output of the expander is used to run a blower which sucks the ambient air in. In addition to cooling, replacement of stale air in the cabin is possible. At high altitudes the pressurization of the cabin is also possible. Because of this consideration air cycle refrigeration is extensively used in aircrafts.

44. Vapour Absorption Refrigeration A vapour absorption refrigeration system uses a refrigerant as well as an absorbent which can be a liquid or solid. Possibly the best known combination is ammonia as the refrigerant and water as the absorbent. A vapour absorption refrigeration system does not have a compressor. The compressor is replaced by a combination of generator, an absorber and a pump.

46. Working: The generator acts as a reservoir for the solution of ammonia in water. Heat from an external source QG is supplied to the solution, leading to evaporation of ammonia and water. The mixture of ammonia vapour and water vapour rises through the analyzer, where most of the water vapour condenses, gets separated from NH3 and drops back into the generator. The analyzer is a direct–contact heat exchanger consisting of a series of trays mounted above the generator. The strong solution of NH3 from the absorber flows down over the trays, comes into contact with and cools the rising vapours. Since the saturation temperature of water is higher than that of NH3 at a given pressure, water vapour will condense first.

47. As the vapour passes upward through the analyzer, it is cooled and enriched by ammonia. The ammonia vapour leaving the analyzer may still contain traces of water vapour. If allowed to flow through the condenser and expansion valve, the water vapour will freeze and block the expansion valve. Traces of water vapour are separated from ammonia vapour in the rectifier. The rectifier is a water cooled heat exchanger, wherein all of the remaining water vapour and some ammonia vapour condense and return to the generator through the drip line.

48. The net result is that pure ammonia vapour flows into the condenser and condenses to form saturated or slightly under cooled liquid. The refrigerant then expands through the valve, resulting in a drop in its pressure and temperature. The cold refrigerant then flows through the evaporator, extracting heat from the substance to be cooled. Saturated or slightly saturated ammonia vapour from evaporator flows into the absorber. The weak solution of ammonia (with low concentration of ammonia in water) coming from the generator is sprayed into the absorber. The ammonia vapour comes into contact with the weak solution, and gets readily absorbed, releasing the latent heat of condensation.

49. This heat QA taken away by cooling water, thereby maintaining the temperature in the absorber constant. The resulting strong NH3 solution is pumped to the generator, where heat Qais supplied to it from an external source. The weak solution leaving the generator and the pressurized strong solution going to the generator flow through a heat exchanger. In this heat exchanger, the strong solution is preheated while the weak solution is pre-cooled, reducing both Qa, the heat to be supplied in the generator and QA the heat to be removed in the absorber.

50. The combination of the generator and absorber is equivalent to a heat engine, which does the job of the compressor, namely, receiving from the evaporator at low pressure, comparatively low temperature ammonia vapour and delivering high pressure, higher temperature ammonia vapour to the condenser. This is shown in the diagram.