Bài giảng Chapter 5 multi stage system [chuẩn SEO]

CHAPTER 5: MULTI-STAGE SYSTEM

Lecturer : ThS. Nguyễn Duy Tuệ

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Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ

OBJECTIVES

In this chapter, student can : In this chapter st dent can - Understand and analyse the basic of multi-stage refrigeration system refrigeration system - Calculate multi-stage refrigeration system

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REFERENCES

[1]. Kỹ thuật lạnh cơ sở - Nguyễn Đức Lợi [1] Kỹ th ật lạnh cơ sở Ng ễn Đức Lợi [2]. Industrial Refrigeration Handbook – Mc Graw HillHill

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CONTENT

TWO STAGE CYCLE; 1 EXPANSION VALVE,

PARTIAL INTERSTAGE DE-SUPERHEAT

TWO STAGE CYCLE; 2 EXPANSION VALVE, TWO STAGE CYCLE 2 EXPANSION VALVE

PARTIAL INTERSTAGE DE-SUPERHEAT

TWO STAGE CYCLE; INTERSTAGE

DE-SUPERHEAT WITHOUTH LIQUID SUBCOOL

TWO STAGE CYCLE; INTERCOOLER WITH

LIQUID SUBCOOL

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INTRODUCTION

+Investigate this one stage cycle for NH3

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INTRODUCTION

- Temperature after compression is 140oC - That high temperature can damage oil, or

+Remark:

- Compressor work increase

refrigerant…

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INTRODUCTION

+ The reason why to use two stage system:

and and condensing condensing

- A significant fraction of industrial refrigeration A significant fraction of industrial refrigeration plants operate with a large difference between evaporating temperatures temperatures— evaporating perhaps 50° to 80° C (100° to 150°F). This large imposes both problems and temperature lift opportunities for the system. is

opportunity hi h - An i to h i lth fi i

use multistage compression, which although increasing the first t th cost over single-stage compression, also alleviates (solve) some problems and can save on total (solve) some problems and can save on total compressor power.

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Two stage cycle, 1 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

+ This is the simplest among 2 stage cycles. Compressed vapour from low-stage was cooled by Compressed vapour from low-stage was cooled by water to condensing temperature

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Two stage cycle, 1 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

+ Advantage compared to 1 stage cycle: - Low discharge temperature-> more safety, - Low discharge temperature-> more safety

reliable, high efficiency

- Low compression work + Disadvantage: - High cost and complex system for performance -> Best using for Freon refrigerant f F f i B i

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

G2 G2

(G2-G1)

GG1

+ Principle :

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

+ Advantage and disadvantage compared to 1

stage cycle: stage cycle:

p p

- a specific refrigeration effect increase - a specific work of compression decrease - Low discharge temperature-> more efficiency

over one stage cycle

+ Application: apply for Amoniac and Freon A li d F A f i l i

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

+ Refrigeration cycle calculation: Firstly, give Qo, to, tk ; then we must find out mass flowrate of low tk ; then we must find out mass flowrate of low t stage, high stage, condenser heat ejectionQk, COP…

)/(i -> G2 = G1.(i8-i10)/(i8-i7) i )

Mass and heat balance for intercooler: G2.i7=G1.i10 + (G2-G1).i8 G G (i i In order to find out 4 state, we have to use

mixing equation: mixing equation:

(G2-G1).i8 + G1.i3 = G2.i4

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

+ Diagram often applied in industrial system:

G2 –G1

Firstly, giving suction temperature of high stage

compressor =>G1, G2 by using mixing equation

a. Diagram 1 : a Diagram 1 :

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

- Give t7, Qo, t8=ttg+3~5K - Find out G1

b. Diagram 2 :

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

Xem clip

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

Using mass and heat balance for subcool tank:

(G2-G1).(i7-i6) = G1.(i5-i8) (G2-G1) (i -i6) = G1 (i -i8)

-> G2 ? Mixing equation at 3 point: g q p

(G2-G1).i7 + G1.i2 = G2.i3

-> Find out i3

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

c. Diagram 3 :

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

d. Diagram 4:

y g

- Give t10=ttg+5K ; liquid-vapour superheat -> t9 (t8 is saturated vapour or initially given t8) 8) p 9 ( 8

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

Heat balance equation: + For liquid-vapour heat exchanger: + For liquid-vapour heat exchanger:

(G2-G1).(i9-i8) = G2.(i5-i6) -> G2/G1 = (i9-i8)/(i9-i8-i5+i6) (1) ( ) 6) ( 9 8) ( 9 8 5

+ For subcooler:

i )

G1.(i6-i10) = (G2-G1).(i8-i7) G /G (i -> G2/G1 = (i8-i10)/(i8-i7) (2) (2) )/(i i (1)&(2) -> i6 =?

Note : i6=i7 Note : i =i

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

G2-G1 1 2

e. Diagram 5:

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

t1, state 10 (saturate t1 state 10 (saturate

+ Theory 1: t8=ttg+3 5K; Given Qo, Given Q t8=t +3~5K; vapour) -> find out state 7?

Mass and heat balance for liquid-vapour heat q p

exchanger:

i )/(i (1) (1) G1.(i1-i1’)=G2.(i6-i7) -> G2/G1 = (i1- i1’)/(i6-i7); i ) G /G (i

Mass and heat balance for subcooler : G2.i7+(G2-G1).i9 = G2.i8+(G2-G1).i10 G i +(G G ) i = G i +(G G ) i -> G2/G1 = (i10-i9)/(i10-i7) ; (2) ? G1, G2 ? (1)&(2) (1)&(2) -> i7 = ? -> G1, G2 ? To find out state 4 we have mixing equation:

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Two stage cycle, 2 expansion valve, te stage de supe eat partial interstage de-superheat pa t a

t1, point 10 (saturate t1 point 10 (saturate

+ Theory 2: t8=ttg+3 5K; Given Q t8=t +3~5K; Given Qo, vapour), point 4. Find out G1,G2, point 7?

Mixing equation at point 4 : g q (G2-G1).i10+G1.i2 = G2.i4

-> G2 ?

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

+ Disadvantage of 1 stage cycle: vapour drawn to high stage is superheat -> compression work and to high stage is superheat -> compression work and discharge temperature are high -> use this cycle with completely interstage de-superheat p g p y

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

The two principal advantages of

interstage desuperheating, or intercooling, are the saving in desuperheating or intercooling are the saving in compressor power and the reduction in discharge temperature from the low-stage compressor. p g p

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

q g p

Two of the methods of achieving intercooling are shown : The traditional method, this figure, is to 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

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

p g

Some of the liquid in the vessel evaporates to provide desuperheating of the vapor. The method provide desuperheating of the vapor The method the will usually achieve a close approach of temperature of q p the discharge vapor to the liquid temperature. But it have some disadvantages:

1 2 0 6 4 f ) b l h f

- The outlet of the low-stage discharge line should b b be between 0.6 to 1.2 m (2 to 4 ft) below the surface (2 of the liquid. Due to the static head of the liquid, the point of discharge will be at a slightly higher point of discharge will be at a slightly higher pressure than at the liquid. The the surface of compressor must, therefore, expend more energy to compressor must, therefore, expend more energy to overcome this additional pressure.

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

From mass and energy balance of the flash

tank: tank:

8 From mass and energy balance across evaporator:

m7 + m3 = m8 + m4 m7.h7 + m3.h3 = m8.h8 + m4.h4 4 8 3

l Qe= m1.(h1- h9 ) b l d

F From mass and energy balance across low-stage compressor, Compressor-I:

WI= mI (h2- h1 ) W = m (h h )

Calculation for compressor 2 is same as comp 1

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

From the above set of equations, it can be

easily shown that for the flash tank: easily shown that for the flash tank:

f i the h

It can be seen from the above expression that h highstage hi h refrigerant flow through fl h h the compression can be reduced by reducing the enthalpy of refrigerant vapour entering into the flash enthalpy of refrigerant vapour entering into the flash tank, h3 from the water-cooled intercooler.

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

The amount of additional vapour generated due to de-superheating of the refrigerant vapour from to de-superheating of the refrigerant vapour from the water-cooled intercooler is given by:

d ill b h Thus the vapour mgen generated will be zero, if Th if

the refrigerant vapour is completely desuperheated in the water-cooled intercooler itself. However, this in the water cooled intercooler itself However this may not be possible in practice.

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

For the above system, the COP is given by:

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

The above system offers several advantages,

, p

P P i recompressed d d i flash h li tank

Chapter 5 : Multi-Stage System - ThS.Nguyễn Duy Tuệ

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refrigerant entering the evaporator refrigerant entering the evaporator a) Quality of a) Quality of refrigerating reduces thus giving rise to higher effect, lower pressure drop and better heat transfer p in the evaporator vapour b) Throttling losses are reduced as d f generated during throttling from Pc to Pi is separated by and the in CompressorII. CompressorII c) Volumetric efficiency of compressors will be high due to reduced pressure ratios due to reduced pressure ratios d) Compressor discharge temperature is reduced considerably.

Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid subcooler supe eat

there is a possibility of

kl d b i h li

However, one disadvantage of the above system is that since refrigerant liquid in the flash tank is is that since refrigerant liquid in the flash tank is liquid flashing saturated, ahead of the expansion valve due to pressure drop p p or heat transfer in the pipelines connecting the flash tank to the expansion device. Sometimes this problem is tackled by using a system with a liquid id i i bl subcooler.

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

While usually not able to achieve the degree of desuperheating possible in the bubbler, the spray the spray desuperheating possible in the bubbler method of this figure causes less disturbance in the vessel.

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

In some cases the supply of

The

liquid comes directly from the condenser through an expansion directly from the condenser through an expansion (thermostatic) superheat-control valve. expansion valve mounts a sensor on the vapor line p p the vapor to the high-stage compressor, and if temperature is too high, the valve opens to admit more refrigerant. f i

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

In some cases the supply of

The

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

p a

f h h l f i h

flow rates flow rates through through each each

requirements of requirements of the the

Example : An R-507 two-stage system with flash-gas removal and intercooling provides 200 kW flash-gas removal and intercooling provides 200 kW of refrigeration at an evaporating temperature of - 40°C when operating with g condensing g temperature of 35°C. The intermediate temperature is -5°C ( ) Wh (a) What are the enthalpies of the refrigerant at all ll i points in the system? the the (b) Compute (b) Compute compressor. (c) What are the power (c) What are the power compressors?

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

g p g g

(d) What would be the power required in a single-stage R-507 system with these evaporating 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?

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

+ Solution :

a. The enthalpies of R-507 at a The enthalpies of R-507 at the positions the positions

designated in above figures are :

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

We can put evaporator with interstage pressure

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

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 are given by: i b

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Two stage cycle,interstage de- t out qu d subcoo e superheat without liquid-subcooler supe eat

g p p (

mII is the mass flow rate of refrigerant through the high-stage compressor which can be obtained 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 y dashed line in the schematic) and applying mass and energy balance: + mass balance: b l

+ Energy balance: + Energy balance:

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Two stage cycle,interstage de- qu d subcoo e superheat with liquid-subcooler supe eat

t

immerses a pipe coil

p in the liquid of q

+ Method : - A popular method of subcooling, shown in this - A popular method of subcooling shown in this the figure, intermediate-pressure vessel. Warm liquid from the condenser enters the heatexchanger coil and transfers heat to the lower-temperature liquid. - Liquid subcooling is a form of f li Li id b f i

flash-gas fl h removal, because some liquid in the vessel the intermediate the intermediate vaporizes and is drawn off at vaporizes and is drawn off at pressure.

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Two stage cycle,interstage de- qu d subcoo e superheat with liquid-subcooler supe eat

t

q g

Compared to direct flashing of this figure, the liquid subcooler has the advantage of maintaining liquid subcooler has the advantage of maintaining the the liquid at a high pressure. Therefore, subcooled liquid can travel long distances and endure some drops in pressure without flashing into flash tank is vapor. The liquid leaving a direct saturated and flashes into pressure. d fl h d i

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Two stage cycle,interstage de- qu d subcoo e superheat with liquid-subcooler supe eat

t

the The subcooler disadvantage

of flash tank is that flash tank is that

in liquid comparison to the direct liquid comparison to the direct cannot be cooled all the way down to the saturation temperature of q the liquid because the heat exchanger must operate with a temperature difference between the leaving subcooled liquid and the intermediate-temperature liquid. h i id di li

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Two stage cycle,interstage de- qu d subcoo e superheat with liquid-subcooler supe eat

t

tube of the length of The selection of

calculation. Instead,

h i ll d b l Th h i i

the immersed subcooler is not usually the result of a immersed subcooler is not usually the result of a the detailed heat-transfer 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 to balance the installed cost against the saving. The overall-heat-transfer coefficient, which is the U-value in the equation: in the equation:

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

yp g

The U-value is a function of the boiling heat- transfer coefficient at the outside of the tube and the 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 g heat-transfer coefficients suggested by Ayub for R- 22 and ammonia, approximate U-values can be calculated, as shown in Table i T bl d h l l

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

p p g y

A guideline sometimes used by one designer is to install a heat-transfer area of immersed coil of 2.5 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.

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

+ Diagram :

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

+ Thermodynamic calculation: - Given Qo, state point. Find out G1,G2, - Given Qo state point Find out G1 G2

compressor work, Qk….

- With G1, write heat balance equation at

q interstage subcool -> G2=G1.(i2 – i10)/(i3-i7) - Calculate the remain parameters

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

We can put evaporator with interstage pressure

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Two stage cycle,interstage de- t subcoo e superheat with subcooler supe eat

Liquid subcooling with an external shell-and- tube heat exchanger with boiling refrigerant tube heat exchanger with boiling refrigerant controlled by an expansion valve.