Bài giảng Kỹ thuật lạnh - Chapter 5: Multi-Stage System - Nguyễn Duy Tuệ

CHAPTER 5: MULTI-STAGE SYSTEM 12/2015 Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ 1 Lecturer : ThS. Nguyễn Duy Tuệ OBJECTIVES In this chapter st dent can, u : - Understand and analyse the basic of multi-stage refrigeration system - Calculate multi-stage refrigeration system 12/2015 2Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ REFERENCES [1] Kỹ th ật lạnh cơ sở Ng ễn Đức Lợi. u - uy [2]. Industrial Refrigeration Handbook – Mc Graw Hill 12/2015 3Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ CONTENT TWO STAGE CYCLE; 1 EXPANSION VALVE, PARTIAL INTERSTAGE DE-SUPERHEAT TWO STAGE CYCLE 2 EXPANSION VALVE ; , PARTIAL INTERSTAGE DE-SUPERHEAT TWO STAGE CYCLE; INTERSTAGE DE-SUPERHEAT WITHOUTH LIQUID SUBCOOL TWO STAGE CYCLE; INTERCOOLER WITH 12/2015 4Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ LIQUID SUBCOOL INTRODUCTION +Investigate this one stage cycle for NH3 12/2015 5Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ INTRODUCTION +Remark: - Temperature after compression is 140oC - That high temperature can damage oil, or refrigerant - Compressor work increase 12/2015 6Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ INTRODUCTION + The reason why to use two stage system: A significant fraction of industrial refrigeration- plants operate with a large difference between evaporating and condensing temperatures— perhaps 50° to 80° C (100° to 150°F). This large temperature lift imposes both problems and opportunities for the system. - An opportunity is to use multistage i hi h lth h i i th fi tcompress on, w c a oug ncreas ng e rs cost over single-stage compression, also alleviates (solve) some problems and can save on total 12/2015 7 compressor power. Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 1 expansion valve, partial interstage de-superheat + This is the simplest among 2 stage cycles. Compressed vapour from low-stage was cooled by water to condensing temperature 12/2015 8Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 1 expansion valve, partial interstage de-superheat + Advantage compared to 1 stage cycle: - Low discharge temperature-> more safety, reliable, high efficiency - Low compression work + Disadvantage: - High cost and complex system for performance B i f F f i-> est us ng or reon re r gerant 12/2015 9Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Principle : G2 (G2-G1) G1 12/2015 10Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Advantage and disadvantage compared to 1 stage cycle: - a specific refrigeration effect increase - a specific work of compression decrease - Low discharge temperature-> more efficiency over one stage cycle A li i l f A i d F+ pp cat on: app y or mon ac an reon 12/2015 11Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Refrigeration cycle calculation: Firstly, give Qo, t tk ; then we must find out mass flowrate of lowo, stage, high stage, condenser heat ejectionQk, COP Mass and heat balance for intercooler: G2.i7=G1.i10 + (G2-G1).i8 G G (i i )/(i i )-> 2 = 1. 8- 10 8- 7 In order to find out 4 state, we have to use mixing equation: (G2-G1).i8 + G1.i3 = G2.i4 12/2015 12Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Diagram often applied in industrial system: a Diagram 1 :. G2 4 5 G2 –G1 G1 Firstly, giving suction temperature of high stage compressor =>G1, G2 by using mixing equation 12/2015 13Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat b. Diagram 2 : - Give t7, Qo, t8=ttg+3~5K - Find out G1 12/2015 14Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat Xem clip 12/2015 15Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat 12/2015 16Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat Using mass and heat balance for subcool tank: (G2-G1) (i -i6) = G1 (i -i8). 7 . 5 -> G2 ? Mixing equation at 3 point: (G2-G1).i7 + G1.i2 = G2.i3 -> Find out i3 12/2015 17Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat c. Diagram 3 : 12/2015 18Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat d. Diagram 4: - Give t10=ttg+5K ; liquid-vapour superheat -> t9 (t8 is saturated vapour or initially given t8) 12/2015 19Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat Heat balance equation: + For liquid-vapour heat exchanger: (G2-G1).(i9-i8) = G2.(i5-i6) -> G2/G1 = (i9-i8)/(i9-i8-i5+i6) (1) + For subcooler: G1.(i6-i10) = (G2-G1).(i8-i7) G /G (i i )/(i i ) (2)-> 2 1 = 8- 10 8- 7 (1)&(2) -> i6 =? Note : i =i 6 7 12/2015 20Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat e. Diagram 5: G2-G1 G1 G2 12/2015 21Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Theory 1: Given Q t8=t +3~5K; t1 state 10 (saturateo, tg , vapour) -> find out state 7? Mass and heat balance for liquid-vapour heat exchanger: G1.(i1-i1’)=G2.(i6-i7) G /G (i i )/(i i ) (1)-> 2 1 = 1- 1’ 6- 7 ; Mass and heat balance for subcooler : G i +(G G ) i = G i +(G G ) i2. 7 2- 1 . 9 2. 8 2- 1 . 10 -> G2/G1 = (i10-i9)/(i10-i7) ; (2) (1)&(2) -> i7 = ? -> G1, G2 ? 12/2015 22 To find out state 4 we have mixing equation: Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle, 2 expansion valve, partial interstage de-superheat + Theory 2: Given Q t8=t +3~5K; t1 point 10 (saturateo, tg , vapour), point 4. Find out G1,G2, point 7? Mixing equation at point 4 : (G2-G1).i10+G1.i2 = G2.i4 -> G2 ? 12/2015 23Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler + Disadvantage of 1 stage cycle: vapour drawn to high stage is superheat -> compression work and discharge temperature are high -> use this cycle with completely interstage de-superheat 12/2015 24Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler 12/2015 25Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler The two principal advantages of interstage desuperheating or intercooling are the saving in, , compressor power and the reduction in discharge temperature from the low-stage compressor. 12/2015 26Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler Two of the methods of achieving intercooling are shown : The traditional method this figure is to, , immerse the outlet of the discharge line of the low- stage compressor below the liquid level in the intercooler providing bubbling of vapor through the liquid 12/2015 27Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler Some of the liquid in the vessel evaporates to provide desuperheating of the vapor The method. will usually achieve a close approach of the temperature of the discharge vapor to the liquid temperature. But it have some disadvantages: - The outlet of the low-stage discharge line should b b 0 6 1 2 (2 4 f ) b l h fe etween . to . m to t e ow t e sur ace of the liquid. Due to the static head of the liquid, the point of discharge will be at a slightly higher pressure than at the surface of the liquid. The compressor must, therefore, expend more energy to 12/2015 28 overcome this additional pressure. Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler 12/2015 29Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler From mass and energy balance of the flash tank: m7 + m3 = m8 + m4 m7.h7 + m3.h3 = m8.h8 + m4.h4 From mass and energy balance across evaporator: Qe= m1.(h1- h9 ) F d b l lrom mass an energy a ance across ow-stage compressor, Compressor-I: W = m (h h )I I 2- 1 Calculation for compressor 2 is same as comp 1 12/2015 30Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler From the above set of equations, it can be easily shown that for the flash tank: It can be seen from the above expression that h f i fl h h h hi ht e re r gerant ow t roug t e g stage compression can be reduced by reducing the enthalpy of refrigerant vapour entering into the flash tank, h3 from the water-cooled intercooler. 12/2015 31Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler The amount of additional vapour generated due to de-superheating of the refrigerant vapour from the water-cooled intercooler is given by: Th h d ill b ifus t e vapour mgen generate w e zero, the refrigerant vapour is completely desuperheated in the water cooled intercooler itself However this- . , may not be possible in practice. 12/2015 32Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler For the above system, the COP is given by: 12/2015 33Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler The above system offers several advantages, a) Quality of refrigerant entering the evaporator reduces thus giving rise to higher refrigerating effect, lower pressure drop and better heat transfer in the evaporator b) Throttling losses are reduced as vapour d d i h li f P P i dgenerate ur ng t rott ng rom c to i s separate in the flash tank and recompressed by CompressorII. c) Volumetric efficiency of compressors will be high due to reduced pressure ratios 12/2015 34 d) Compressor discharge temperature is reduced considerably. Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid subcooler However, one disadvantage of the above system is that since refrigerant liquid in the flash tank is saturated, there is a possibility of liquid flashing ahead of the expansion valve due to pressure drop or heat transfer in the pipelines connecting the flash tank to the expansion device. Sometimes this bl i kl d b i i h li idpro em s tac e y us ng a system w t a qu subcooler. 12/2015 35Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler While usually not able to achieve the degree of desuperheating possible in the bubbler the spray, method of this figure causes less disturbance in the vessel. 12/2015 36Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler In some cases the supply of liquid comes directly from the condenser through an expansion valve. The superheat-control (thermostatic) expansion valve mounts a sensor on the vapor line to the high-stage compressor, and if the vapor temperature is too high, the valve opens to admit f imore re r gerant. 12/2015 37Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler In some cases the supply of liquid comes directly from the condenser through an expansion valve. The superheat-control (thermostatic) expansion valve mounts a sensor on the vapor line to the high-stage compressor, and if the vapor temperature is too high, the valve opens to admit f imore re r gerant. 12/2015 38Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler Example : An R-507 two-stage system with flash-gas removal and intercooling provides 200 kW of refrigeration at an evaporating temperature of - 40°C when operating with a condensing temperature of 35°C. The intermediate temperature is -5°C ( ) Wh h h l i f h f i lla at are t e ent a p es o t e re r gerant at a points in the system? (b) Compute the flow rates through each compressor. (c) What are the power requirements of the 12/2015 39 compressors? Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler (d) What would be the power required in a single-stage R-507 system with these evaporating and condensing temperatures, and the percentage saving in power through the use of a two-stage system? 12/2015 40Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler + Solution : a The enthalpies of R-507 at the positions. designated in above figures are : 12/2015 41Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler 12/2015 42Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler 12/2015 43Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler We can put evaporator with interstage pressure 12/2015 44Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler 12/2015 45Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler The COP of this system is given by: where mI and meII are the refrigerant mass flow rates through evaporator I and II respectively. They i bare g ven y: 12/2015 46Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat without liquid-subcooler mII is the mass flow rate of refrigerant through the high-stage compressor which can be obtained by taking a control volume which includes the flash tank and high temperature evaporator (as shown by dashed line in the schematic) and applying mass and energy balance: b l+ mass a ance: + Energy balance: 12/2015 47Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with liquid-subcooler + Method : - A popular method of subcooling shown in this, figure, immerses a pipe coil in the liquid of the intermediate-pressure vessel. Warm liquid from the condenser enters the heatexchanger coil and transfers heat to the lower-temperature liquid. Li id b li i f f fl h- qu su coo ng s a orm o as -gas removal, because some liquid in the vessel vaporizes and is drawn off at the intermediate pressure. 12/2015 48Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with liquid-subcooler Compared to direct flashing of this figure, the liquid subcooler has the advantage of maintaining the liquid at a high pressure. Therefore, the subcooled liquid can travel long distances and endure some drops in pressure without flashing into vapor. The liquid leaving a direct flash tank is d d fl h isaturate an as es nto pressure. 12/2015 49Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with liquid-subcooler The disadvantage of the subcooler in comparison to the direct flash tank is that liquid cannot be cooled all the way down to the saturation temperature of the liquid because the heat exchanger must operate with a temperature difference between the leaving subcooled liquid and h i di li idt e nterme ate-temperature qu . 12/2015 50Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with liquid-subcooler The selection of the length of tube of the immersed subcooler is not usually the result of a detailed heat-transfer calculation. Instead, the fabricator often inserts as much tube as convenient, and the system lives with the result. In certain cases it may be profitable to design the coil more carefully b l h i ll d i h i Thto a ance t e nsta e cost aga nst t e sav ng. e overall-heat-transfer coefficient, which is the U-value in the equation: 12/2015 51Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler The U-value is a function of the boiling heat- transfer coefficient at the outside of the tube and the convection coefficient of the flowing liquid refrigerant inside the tube. Using some typical values of boiling heat-transfer coefficients suggested by Ayub for R- 22 and ammonia, approximate U-values can be l l d h i T blca cu ate , as s own n a e 12/2015 52Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler A guideline sometimes used by one designer is to install a heat-transfer area of immersed coil of 2 5. m2 for every 100 kW (1 ft2 per ton) of refrigeration capacity at the evaporator. A heat exchanger of this size conforms with the data in the table to provide a reasonable temperature drop of liquid. 12/2015 53Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler + Diagram : 12/2015 54Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler + Thermodynamic calculation: - Given Qo state point Find out G1 G2, . , , compressor work, Qk. - With G1, write heat balance equation at interstage subcool -> G2=G1.(i2 – i10)/(i3-i7) - Calculate the remain parameters 12/2015 55Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler We can put evaporator with interstage pressure 12/2015 56Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler Liquid subcooling with an external shell-and- tube heat exchanger with boiling refrigerant controlled by an expansion valve. 12/2015 57Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler 12/2015 58Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ Two stage cycle,interstage de- superheat with subcooler The benefit is compact, but discharge temp is high We have energy balance equation:. G1.i5+(G2-G1).i6+G1.i2 = G2.i3+G1.i7 G G (i i +i i )/(i i ) (k / )2 = 1. 5- 6 2- 7 3- 6 , g s Due to i5=i6 : (i i )/(i i ) (k / ) 12/2015 59Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ G2=G1. 2- 7 3- 6 , g s