Bài giảng chapter 4 single stage cycle

CHAPTER 4: SINGLE STAGE CYCLE 12/2015 Chapter : Single stage cylce OBJECTIVES After this Aft thi charpter, h t student t d t can : - Analyze and perform cyclic calculations for C Carnot t refrigeration fi ti cycle l and d others th - Analyze and perform cyclic calculations for standard t d d vapour compression i refrigeration fi ti systems - Analyze A l advantage d t and d disadvantage di d t off some refrigeration cycles 12/2015 Chapter : Single stage cylce CONTENTS CARNOT REFRIGERATION CYCLE STANDARD VAPOUR COMPRESSION REFRIGERATION SYSTEM (VCRS) SUBCOOLING AND SUPERHEATING CYCLE LIQUID – SUCTION HEAT EXCHANGER 12/2015 Chapter : Single stage cylce CONTENTS ACTUAL STANDARD VAPOUR COMPRESSION REFRIGERATION SYSTEM 12/2015 Chapter : Single stage cylce REFERENCES [1] 4O LESSONS ON REFRIGERATION AND AIR CONDITIONING FROM IIT KHARAGPUR ( Useful t i i training material t i l for f mechanical h i l engineering i i students/college, or reference for engineer ) - Indian I tit t off Technology Institute T h l (IIT) [2] Kỹ thuật lạnh sở - Nguyễn Đức Lợi 12/2015 Chapter : Single stage cylce CARNOT REFRIGERATION CYCLE Definition: Carnot refrigeration cycle is a completely y , hence is used as a model of reversible cycle, perfection for a refrigeration cycle operating p heat source and between a constant temperature sink It is used as reference against which the real y are compared p cycles 12/2015 Chapter : Single stage cylce CARNOT REFRIGERATION CYCLE Description p : Refer to (page 155, [1]) : -Process P 4-1 : vaporization i i in evaporator -Process P 1-2 : compression i in compressor -Process P 2-3 : condensing d i in condenser -Process P 3-4 : expansion i iin turbin 12/2015 Chapter : Single stage cylce CARNOT REFRIGERATION CYCLE qc − qe = wnet 12/2015 Chapter : Single stage cylce CARNOT REFRIGERATION CYCLE Refer (p (page g 155,, [[1]) ]) : 12/2015 Chapter : Single stage cylce CARNOT REFRIGERATION CYCLE The Coefficient of Performance ((COP)) is g given by: y - The COP of Carnot refrigeration g cycle y is a function of evaporator and condenser temperatures y and is independent p of the nature of the working g only substance - From Carnot’s theorems,, for the same heat source and sink temperatures, no irreversible cycle g than that of Carnot COP can have COP higher 12/2015 Chapter : Single stage cylce 10 LIQUID-SUCTION HEAT-EXCHANGER CYCLE Effect of superheat p on system y COP: - When the refrigerant is superheated usefully ((either in the LSHX or the evaporator p itself), ), the refrigeration effect increases p also increases,, - The work of compression primarily due to increase in specific volume of the g due to superheat p refrigerant 12/2015 Chapter : Single stage cylce 44 Actual Standard Vapour Compression Refrigeration System The cycles y considered so far are internallyy reversible and no change of refrigerant state takes place in the connecting p gp pipelines p However,, in actual VCRS several irreversibilities exist These are due to: - Pressure drops in evaporator, condenser and LSHX - Pressure drop across suction and discharge p valves of the compressor - Heat transfer in compressor p and heat transfer in connecting g - Pressure drop pipe lines 12/2015 Chapter : Single stage cylce 45 Actual Standard Vapour Compression Refrigeration System The cycles y considered so far are internallyy reversible and no change of refrigerant state takes place in the connecting p gp pipelines p However,, in actual VCRS several irreversibilities exist These are due to: - Pressure drops in evaporator, condenser and LSHX - Pressure drop across suction and discharge p valves of the compressor - Heat transfer in compressor p and heat transfer in connecting g - Pressure drop pipe lines 12/2015 Chapter : Single stage cylce 46 Actual Standard Vapour Compression Refrigeration System - The p pressure drop p in the evaporator, p , in the suction line and across the suction valve has a g effect on system y performance because p significant suction side pressure drop increases the specific p ratio and discharge g volume at suction,, compression temperature increase -> reduction in system p y, increase in p power input p and also affect the capacity, life of the compressor due to higher discharge p -> this p pressure drop p should be as temperature small as possible for good performance - 12/2015 Chapter : Single stage cylce 47 Actual Standard Vapour Compression Refrigeration System - The p pressure drop p depends p on the refrigerant g velocity, length of refrigerant tubing and layout ((bends,, jjoints etc.) ) Pressure drop p can be reduced by reducing refrigerant velocity (e.g by increasing g tubes) ) the inner diameter of the refrigerant However, this affects the heat transfer coefficient in p and the carring g of the lubricating g oil evaporator back to the compressor p across the valves of the - Pressure drops compressor increase the work of compression and p reduce the volumetric efficiencyy of the compressor Hence they should be as small as possible 12/2015 Chapter : Single stage cylce 48 Actual Standard Vapour Compression Refrigeration System - Heat transfer in the suction line is detrimental as it reduces the density of refrigerant vapour and g temperature p of the increases the discharge compressor Hence, the suction lines are normally insulated to minimize heat transfer - Actual systems there are the presence of g matter : lubricating g oil,, water,, air,, p particulate foreign matter inside the system presence of oil in system y - We can’t avoid the p but we must return oil to compressor properly 12/2015 Chapter : Single stage cylce 49 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 50 Actual Standard Vapour Compression Refrigeration System Exercise : In a R22 based refrigeration system, a liquid-tog ((LSHX)) with an suction heat exchanger effectiveness of 0.65 is used The evaporating and g temperatures p are 7.2oC and 54.4oC condensing respectively Assuming the compression process to p , find: be isentropic, a) Specific refrigeration effect, g effect,, b)) Volumic refrigeration c) Specific work of compression y , d)) COP of the system, 12/2015 Chapter : Single stage cylce 51 Actual Standard Vapour Compression Refrigeration System e)) Temperature p of vapour p at the exit of the compressor p g the Comment on the use of LSHX byy comparing performance of the system with a SSS cycle p g between the same evaporator p and operating condensing temperatures 12/2015 Chapter : Single stage cylce 52 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 53 Actual Standard Vapour Compression Refrigeration System e)) Temperature p of vapour p at the exit of the compressor p g the Comment on the use of LSHX byy comparing performance of the system with a SSS cycle p g between the same evaporator p and operating condensing temperatures 12/2015 Chapter : Single stage cylce 54 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 55 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 56 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 57 Actual Standard Vapour Compression Refrigeration System 12/2015 Chapter : Single stage cylce 58 [...]... refrigerant mass flow rate 12/2015 Chapter 4 : Single stage cylce 16 Standard Vapour Compression Refrigeration System (VCRS) The heat rejection j in case of VCRS cycle y also increases when compared to Carnot cycle j in case of Carnot cycle y ((1-2’’-3- The heat rejection 4 ) is given by: - In case of VCRS cycle, y , the heat rejection j rate is given by: 12/2015 Chapter 4 : Single stage cylce 17 Standard Vapour... SUPERHEATING CYCLE + Advantage g of subcooling g cycle y : 12/2015 Chapter 4 : Single stage cylce 33 SUBCOOLING AND SUPERHEATING CYCLE - Increases the refrigeration g effect by y reducing g the throttling loss at no additional specific work p input - Without subcooling the throttling loss is equal to the hatched area b -4 -4- c - With subcooling the throttling loss is given by the area a -4 -4 -b -The refrigeration... of VCRS compared to Carnot cycle is equal to the area ) This region g is known as superheat p 2’’-2-2’ ((area A1) horn, and is due to the replacement of isothermal j process of Carnot cycle p y byy isobaric heat rejection heat rejection in case of VCRS 12/2015 Chapter 4 : Single stage cylce 18 Standard Vapour Compression Refrigeration System (VCRS) 12/2015 Chapter 4 : Single stage cylce 19 Standard Vapour... (VCRS) The net work input p in case of Carnot and VCRS cycles are given by: wnet,Carnot = (qc − qe )Carnot = area 1− 2' '−3 − 4' −1 wnet,VCRS = (qc − qe )VCRS = area 1− 2 − 3 − 4' −c − d − 4 −1 12/2015 Chapter 4 : Single stage cylce 20 Standard Vapour Compression Refrigeration System (VCRS) The COP of VCRS cycle y is g given by: y Unlike Carnot COP, the cycle efficiency depends y much on the shape p of T-s... expansion process can be p byy an isenthalpic p throttling gp process replaced 12/2015 Chapter 4 : Single stage cylce 13 Standard Vapour Compression Refrigeration System (VCRS) This is the theoretical cycle y on which the actual vapour compression refrigeration systems are based 12/2015 Chapter 4 : Single stage cylce 14 Standard Vapour Compression Refrigeration System (VCRS) Due to these irreversibilities,,... subcooling the throttling loss is given by the area a -4 -4 -b -The refrigeration effect increases by an amount q to ((h4-h 44 ) = ((h3-h33’)) equal - Less vapour at the inlet to the evaporator 12/2015 Chapter 4 : Single stage cylce 34 SUBCOOLING AND SUPERHEATING CYCLE + Advantage g of superheating p g cycle y : It prevents the entry of liquid droplets into the p compressor + Disadvantage of superheating : In... y with the system help of the cycle diagrams on T-s charts 12/2015 Chapter 4 : Single stage cylce 15 Standard Vapour Compression Refrigeration System (VCRS) - There is a reduction in refrigeration g effect when the isentropic expansion process of Carnot cycle is p byy isenthalpic p throttling g p process of VCRS replaced cycle, this reduction is equal to the area d -4- 4’-c-d ((area A2)) and is known... refrigeration effect 12/2015 Chapter 4 : Single stage cylce 31 SUBCOOLING AND SUPERHEATING CYCLE - It is p possible for the refrigerant g vapour p to become superheated by exchanging heat with the g as it flows through g the connecting g surroundings pipelines Such a superheating is called as useless p g as it does not increase refrigeration g superheating effect 12/2015 Chapter 4 : Single stage cylce 32 SUBCOOLING... ((ratio of refrigeration work of compression) may or may not increase with p , depending p g mainlyy upon p the nature of the superheat, working fluid 12/2015 Chapter 4 : Single stage cylce 35 SUBCOOLING AND SUPERHEATING CYCLE 12/2015 Chapter 4 : Single stage cylce 36 ... 12/2015 Chapter 4 : Single stage cylce 21 Standard Vapour Compression Refrigeration System (VCRS) As mentioned before,, the losses due to superheat (area A1) and throttling (area A2 ≈ A3) p veryy much on the shape p of the vapor p dome depend (saturation liquid and vapour curves) on T s g The shape p of the saturation curves diagram depends on the nature of refrigerant 12/2015 Chapter 4 : Single stage