Bài giảng Chapter 4: Single stage cycle

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Sau khi học xong "Bài giảng Chapter 4: Single stage cycle" sinh viên có thể Analyze and perform cyclic calculations for refrigerati on cycle and others; Analyze and perform cyclic calculations for standard vapou r compressi on refrigeration systems;... Cùng tìm hiểu để nắm bắt nội dung thông tin tài liệu.

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Nội dung Text: Bài giảng Chapter 4: Single stage cycle

CHAPTER 4: SINGLE STAGE CYCLE 12/2015 Chapter 4 : Single stage cylce 1
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 andd 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 4 : Single stage cylce 2
CONTENTS CARNOT REFRIGERATION CYCLE STANDARD VAPOUR COMPRESSION REFRIGERATION SYSTEM (VCRS) SUBCOOLING AND SUPERHEATING CYCLE LIQUID – SUCTION HEAT EXCHANGER 12/2015 Chapter 4 : Single stage cylce 3
CONTENTS ACTUAL STANDARD VAPOUR COMPRESSION REFRIGERATION SYSTEM 12/2015 Chapter 4 : Single stage cylce 4
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 cơ sở - Nguyễn Đức Lợi 12/2015 Chapter 4 : Single stage cylce 5
CARNOT REFRIGERATION CYCLE 1. Definition: Carnot refrigeration cycle is a completely y , hence is used as a model of reversible cycle, perfection for a refrigeration cycle operating p between a constant temperature heat source and sink. It is used as reference against which the real y cycles are compared. p 12/2015 Chapter 4 : Single stage cylce 6
CARNOT REFRIGERATION CYCLE 2. Description p : Refer to (page 155, [1]) : -Process P 4-1 4 1 : vaporization i i in evaporator -Process P 1-2 1 2 : compression i in compressor -Process P 2-3 2 3 : condensing d i in condenser -Process P 3-4 3 4 : expansion i iin turbin 12/2015 Chapter 4 : Single stage cylce 7
CARNOT REFRIGERATION CYCLE qc − qe = wnet 12/2015 Chapter 4 : Single stage cylce 8
CARNOT REFRIGERATION CYCLE Refer (p (page g 155,, [[1]) ]) : 12/2015 Chapter 4 : Single stage cylce 9
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 only p of the nature of the working g substance. - From Carnot’s theorems,, for the same heat source and sink temperatures, no irreversible cycle g can have COP higher than that of Carnot COP. 12/2015 Chapter 4 : Single stage cylce 10
CARNOT REFRIGERATION CYCLE 3. Practical difficulties with Carnot refrigeration g system: gp - During process 1-2,, a mixture consistingg of liquid q and vapour have to be compressed isentropically in p the compressor -> compressor p will be damaged g - Using a turbine and extracting work from the y system during g the isentropic p expansion p of liquid q refrigerant is not economically feasible, particularly in case of small capacity p y systems. y 12/2015 Chapter 4 : Single stage cylce 11
CARNOT REFRIGERATION CYCLE - This is due to the fact that the specific p work output (per kilogram of refrigerant) from the turbine is g given by: y - The specific p volume of liquid q is much smaller compared to the specific volume of a vapour/gas, p from the turbine in case of the liquid the work output q will be small. In addition, the inefficiencies of the turbine -> then the net output p will be further reduced. 12/2015 Chapter 4 : Single stage cylce 12
Standard Vapour Compression Refrigeration System (VCRS) In practical p considerations,, the Carnot refrigeration system need to be modified: - Dryy compression p with a single g compressor p is possible if the isothermal heat rejection process is p replaced byy isobaric heat rejection j process p -The isentropic expansion process can be p replaced byy an isenthalpic p throttlinggpprocess. 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,, the coolingg effect reduces and work input increases, thus reducing the y system COP. This can be explained p easily y with the 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 replaced byy isenthalpic p throttling g p process of VCRS cycle, this reduction is equal to the area d-4-4’-c-d ((area A2)) and is known as throttling g loss. - It is easy to show that the loss in refrigeration p effect increases as the evaporator temperature p decreases and/or condenser temperature increases. Ap practical consequence q of this is a requirement q of higher 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 cycley also increases when compared to Carnot cycle. j - The heat rejection in case of Carnot cycle y ((1-2’’-3- 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 Compression Refrigeration System (VCRS) Hence the increase in heat rejection j rate of VCRS compared to Carnot cycle is equal to the area 2’’-2-2’ ((area A1). ) This region g is known as superheat p horn, and is due to the replacement of isothermal j heat rejection process of Carnot cycle p y byy isobaric 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 Compression Refrigeration System (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