Điện, điện tử - Chapter 5: Multi - Stage system
+ 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
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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
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