AIR REFRIGERATION SYSTEMS

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AIR REFRIGERATION SYSTEMS:

AIR REFRIGERATION SYSTEMS MECHANICAL ENGG. DEPT . MITS,RAYAGADA Nityanand Singh 2/23/2011 1 LECTUR:7/ Mr.S.K HOTTA

SELECTION OF REFRIGERATIONSYSTEM FOR AIR CRAFT:

SELECTION OF REFRIGERATIONSYSTEM FOR AIR CRAFT The advent of high-speed passenger aircraft, jet aircraft and missiles has introduced the need for compact, lightweight and simple refrigeration system. Air cycle refrigeration systems are employed for air conditioning the cockpit and cabin space of an airplane . 2/23/2011 2 LECTUR:7/Mr.S.K HOTTA

Methods of Air Refrigeration System:

Methods of Air Refrigeration System 1. Simple air-cooling system. 2. Simple air evaporative cooling system. 3. Boot-strap air cooling system. 4. Boot-Strap air evaporative cooling system. 5. Reduced ambient air-cooling system. 6. Regenerative air-cooling system. 2/23/2011 3 LECTUR:7/Mr.S.K HOTTA

SIMPLE AIR-COOLING SYSTEM:

SIMPLE AIR-COOLING SYSTEM This system is good for ground surface cooling & for low flight speed . 2/23/2011 4 LECTUR:7/Mr.S.K HOTTA

T-S DIAGRAM FOR SIMPLE AIR COOLING SYSTEM:

T-S DIAGRAM FOR SIMPLE AIR COOLING SYSTEM 2/23/2011 5 LECTUR:7/Mr.S.K HOTTA

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2/23/2011 6 LECTUR:7/Mr.S.K HOTTA Ramming Process -The ambient air is rammed isentropically from p1&T1 to p2 &T2.This ideal ramming is shown by process 1-2.Actual ramming process is shown by curve 1-2’ which is adiabatic not isentropic.(because of friction due to irreversibility).The pressure & temp. of the rammed air is now P2’ & T2’ res. During the actual or ideal ramming process, the total energy remains constant : h 2 = h 2 ’ & T 2 = T 2 ’ 2. Compression Process -The isentropic compression of air in the main compressor is represented by line 2’-3.In actual pactice , due to irreversibilities (because of friction etc.) is represented by curve 2’-3’ 3 Cooling Process- The compressed air is cooled by the ram air in the heat exchanger, shown by curve 3’-4. 4. Expansion Process -The cooled air now expanded isentropically in the cooling turbine as shown by curve 4-5.In actual practice, because of irreversibility due to friction etc, the actual expansion is shown by 4-5’. 5. Refrigeration Process- (5’-6) -The air from cooling turbine (i.e. after expansion) is sent to the cabin & cockpit where this cold air absorbs heat thereby producing cooling effect in the cabin.

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2/23/2011 LECTUR:7/ Mr.S.K HOTTA 7 MACH NUMBER Mach number (Ma or M ) (generally pronounced /ˈmɑːk/ , sometimes /ˈmɑːx/ or /ˈmæk/) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance. It is commonly used to represent an object's (such as an aircraft or missile) speed, when it is travelling at (or at multiples of) the speed of sound . where M is the Mach number v S is the speed of the source (the object relative to the medium) and U is the speed of sound in the medium Subsoni c : M < 1, Sonic : M=1, Transonic : 0.8 < M < 1.2, Supersonic : 1.2 < M < 5, Hypersonic : M > 5

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STGNATION POINT: I n fluid dynamics , a stagnation point is a point in a flow field where the local velocity of the fluid is zero. [ Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object. 2/23/2011 LECTUR:7/Mr.S.K HOTTA 8 The Bernoulli equation shows that the static pressure is highest when the velocity is zero and hence static pressure is at its maximum value at stagnation points. This static pressure is called the stagnation pressure

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MECHANICAL ENGG. DEPT . MITS,RAYAGADA LECTURE:8 BY Mr. S.K.HOTTA 2/23/2011 10 LECTUR:7/ Mr.S.K HOTTA SIMPLE AIR EVAPORATIVE COOLING SYSTEM

SIMPLE AIR EVAPORATIVE COOLING SYSTEM:

SIMPLE AIR EVAPORATIVE COOLING SYSTEM It is similar to the simple cooling system except that a evaporator is added between the heat exchanger & cooling turbine. The evaporator provides an additional cooling effect through evaporation of a refrigerant such as water. The various processes are same as discussed in previous article, except that cooling process in the evaporator. (shown by 4-4’). 2/23/2011 LECTUR:7/Mr.S.K HOTTA 11

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MECHANICAL ENGG. DEPT . MITS,RAYAGADA LECTURE:9 BY Mr. S.K.HOTTA 2/23/2011 14 LECTUR:9/ Mr.S.K HOTTA BOOT-STRAP AIR COOLING SYSTEM

BOOT STRAP AIR COOLING SYSTEM:

BOOT STRAP AIR COOLING SYSTEM 2/23/2011 LECTUR:7/Mr.S.K HOTTA 15

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1-2 represents isentropic ramming of ambient air. 1-2’ Actual ramming process (due to internal friction etc.) 2’-3 represents isentropic compression of the air in main compressor. 2’-3’ represents actual compression. 3’-4 represents the cooling by ram air in the first heat exchanger. The pressure drop in the heat exchanger is neglected. 4-5 represents compression of cooled air from first heat exchanger, in the secondary compressor. 4-5’ is actual compression process. 5’-6 represents the cooling by ram air in the secondary heat exchanger. The pressure in heat exchanger is neglected 6-7 represents isentropic expansion of cooled air in the cooling turbine up to cabin pressure. 6-7’ is the actual expansion of cooled air in the cooling turbine. 7’-8 represents cooling effect produced by cooled air taking heat from the cabin, thereby producing refrigerating effect in cabin. If Q tonnes of refrigeration is the cooling load in the cabin , then the quantity of air required for the refrigeration purpose will be ma = 210 Q/Cp(T8-T7’) Kg/min Power required for the refrigeration system, P = ma Cp (T3’-T2’)/60 KW And C.O.P of refrigerating system = 210 Q/ [ma Cp (T3’-T2’)] = 210/(P x 60) 2/23/2011 LECTUR:7/ Mr.S.K HOTTA 17

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LECTURE:9 BY Mr. S.K.HOTTA 2/23/2011 18 LECTUR:7/ Mr.S.K HOTTA BOOT-STRAP AIR EVAPORATIVE COOLING SYSTEM

BOOT-STRAP AIR EVAPORATIVE COOLING SYSTEM:

BOOT-STRAP AIR EVAPORATIVE COOLING SYSTEM 2/23/2011 LECTUR:7/Mr.S.K HOTTA 19

The addition of an evaporator between the heat exchanger cooling turbine provides an additional cooling effect. This evaporant absorbs the latent heat of air in evaporator chamber and cools it before letting it to enter into cooling turbine. This after passing from cooling chamber produces additional cooling effect in the cockpit:

2/23/2011 LECTUR:7/Mr.S.K HOTTA 20 The addition of an evaporator between the heat exchanger cooling turbine provides an additional cooling effect. This evaporant absorbs the latent heat of air in evaporator chamber and cools it before letting it to enter into cooling turbine. This after passing from cooling chamber produces additional cooling effect in the cockpit

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If Q tonnes of refrigeration is the cooling load in the cabin , then the quantity of air required for the refrigeration purpose will be m = 210 Q/ [Cp (T8-T7’)] kg/min Power required for the refrigeration system is given by P = ma Cp (T3’-T2’)/60 Kw And C.O.P of refrigeration system = 210 Q/ [ma Cp (T3’-T2’)] kW = 210 Q/(P x 60) 2/23/2011 LECTUR:7/Mr.S.K HOTTA 21

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MECHANICAL ENGG. DEPT . MITS,RAYAGADA LECTURE:10 BY Mr. S.K.HOTTA 2/23/2011 22 LECTUR:10/ Mr.S.K HOTTA NUMERICALS OF BOOT-STRAP AIR COOLING SYSTEM

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MECHANICAL ENGG. DEPT . MITS,RAYAGADA LECTURE:11 BY Mr. S.K.HOTTA 2/23/2011 31 LECTUR:7/ Mr.S.K HOTTA SIMPLE VAPPOUR COMPRESSION REFRIGERATION SYSTEM

SIMPLE VAPOUR COMPRESSION CYCLE:

SIMPLE VAPOUR COMPRESSION CYCLE A simple vapour compression cycle is a combination of system by the use of which cooling effect is produced. The system consists of a compressor, a condenser, an expansion device and an evaporator . Introduction A vapor compression refrigeration system is a improved type of air refrigeration system in which a suitable working substance ,termed as refrigerant is used. It condenses and evaporates at a temp. close to the atmospheric pressure and temp.The refrigerant used here dose not leave the system but it circulated through the system alternately condensing and evaporating. In evaporating ,it absorbs its latent heat from the brine, which is used for circulating it around the cold chamber While condensing it gives out the latent heat to the circulating water or to the cooler. therefore the vapour comp. ref. system is a latent heat pump. 2/23/2011 LECTUR:7/Mr.S.K HOTTA 32

Advantages of vcrs over air-refrigeration system:

Advantages of vcrs over air-refrigeration system 1. For a given capacity of system it is of smaller size. 2. It has less running cost. 3. Larger range of temperature can be obtained. 4. We can have high coefficient of performance. Disadvantages of VCRS 1. Initial cost is high. 2. Refrigerant leakage is the main problem faced in this system. The system consists of a compressor, a condenser, an expansion device and an evaporator . 2/23/2011 LECTUR:7/ Mr.S.K HOTTA 33

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2/23/2011 LECTUR:7/Mr.S.K HOTTA 36 Compressor As we all know, the work of compressor is to compress the gas upto high pressure and temperature. Condenser The condenser (which is placed after the compressor) is a place in which high pressure and temperature refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding medium, which is generally air. In condenser the vapour refrigerant changes into liquid refrigerant i.e. condensation takes place. Expansion Device Generally a throttle valve or expansion valve is used in simple vapor compression system. It may be also called as refrigerant control valve. The function of expansion valve is to reduce the pressure and Temp.of the liquid refrigerant and allow it to pass at controlled rate. Some of the liquid refrigerant evaporates as it passes over expansion valve, but majority of the portion is still in liquid state while passing through expansion valve.

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PRESSURE ENTHALPY CHART (ALSO CALLED AS P-H CHART)- 2/23/2011 LECTUR:7/Mr.S.K HOTTA 37

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The vapour compression refrigeration cycle is classified as follows : 1. Cycle with dry saturated vapour after compression, 2. Cycle with wet vapor after compression, 3. Cycle with superheated vapor after compression, 4. Cycle with superheated vapor before compression, 5. Cycle with under-cooling or sub-cooling of refrigerant. Theoretical vapor compression cycle with dry saturated vapour after compression TYPES OF VAPOUR COMPRESSION CYCLE

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1-2 Compression Process During compression process, pressure and temperature increases from p1 T1 to p2 T2 (as shown in the curve). 2-3 Condensation Process The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure and temperature shown by the horizontal line 2-3 on T-s & p-h diagram. The refrigerant, while passing through the condenser, gives its latent heat to the surrounding condensing medium. The vapour refrigerant is changed into liquid refrigerant. 3-4 Expansion Process The liquid refrigerant from the condenser is expanded in the expansion valve through throttling process. The pressure decreases in this process. Keep in mind that no heat is absorbed or rejected by the liquid refrigerant.

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4-1 Vaporising Process During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporization from the medium to be cooled. This heat, which is absorbed by the refrigerant is the refrigerating effect which we wanted to achieve. Refrigerating effect or the heat extracted by the liquid-vapour refrigerant during evaporation per kg of refrigerant is given by : R E = h 1 - h 4 = h 1 - h f3 h f3 is nothing but enthalpy of liquid refrigerant leaving the condenser. It is also called as sensible heat at temperature T3 . We know that-- C.O.P = Refrigerating effect/ workdone = ( h 1 - h 4 )/(h 2 -h 1 ) = ( h 1 - h f3 )/ (h 2 -h 1 )

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Ex. The temperature limits of an ammonia refrigerating system are 25 o C and -10 0 C. If the gas is dry and saturated at the end of the compression, calculate the coefficient of performances of the cycle assuming no under-cooling of the liquid ammonia. Use the following table for properties of Ammonia.

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CYCLE WITH WET VAPOUR AFTER COMPRESSION The only change in this system is the condition of refrigerant after compression. In previous article we were having dry saturated vapour after compression. As the name suggest, the outlet condition of refrigerant in this type of cycle ,after compression is wet vapour. C.O.P = (h 1 – hf 3 )/ (h 2 -h 1 ) 2/23/2011 LECTUR:7/Mr.S.K HOTTA 44

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CYCLE WITH SUPERHEATED VAPOR AFTER COMPRESSION In this type of cycle we get superheated vapour after compression.. The enthalpy at point 2 is found out with the help of degree of superheat. But the question is how would you find the degree of superheat? We can find it by equating entropies at points 1 & 2. C.O.P = Refrigerating effect / work done= (h 1 – hf 3 )/ (h 2 -h 1 ) 2/23/2011 LECTUR:7/Mr.S.K HOTTA 45

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Ex - A vapour compression refrigerator uses R-40 and operates between limits of -10 0 C and 45 o C. At entry to the compressor, the refrigerant is dry saturated and after compression it acquires a temperature of 60 0 C. Find the C.O.P of the refrigerator. The properties related to R-40 are : 2/23/2011 LECTUR:7/Mr.S.K HOTTA 46

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THEORITICAL VAPOUR COMPRESSION CYCLE WITH SUPERHEATED VAPOUR BEFORE COMPRESSION In this cycle we have superheated vapour before compression. The evaporation starts at point 4 and continues upvto point 1’, when it is dry saturated. The vapour is now superheated before entering the compressor up to point 1. C.O.P = Refrigerating effect /work done = (h1-hf3)/(h2-h1) 2/23/2011 LECTUR:7/Mr.S.K HOTTA 48

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THEORITICAL VAPOUR COMPRESSION CYCLE WITH SUB-COOLING OR UNDER COOLING OF VAPOUR REFRIGERANT Under cooling or sub cooling is the process when refrigerant is cooled below the saturation temperature T3 ’ before throttling. Due to under cooling, coefficient of performance increases under same set of conditions. Now, the question is how we achieve under cooling? We achieve under cooling by circulating more quantity of cooling water through condenser. We can sometimes also use heat exchanger to serve the purpose . 2/23/2011 LECTUR:7/Mr.S.K HOTTA 49

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