CHAPTER 5: COMPOUND REFRIGERATION SYSTEM
VAPOUR
COMPRESSION
5.2 Advan Ad vantag tages es ! C" C"# #$nd $nd %& %& M$ M$'t( 't()st )stage age** Va Va# #$& $& C" C"#&e #&ess( ss(n n +(t, +(t, Inte&-'e&
Following are the main advantages of compound or multistage compression over single stage compression: 1. The work done per kg of refri refrigerant gerant is reduced in compound compou nd compres compression sion with intercooler as compared to single stage compression for the same delivery pressure . 2. It impro improves ves the volu volumet metric ric efficie efficiency ncy for the the given given pressu pressure re ratio. ratio. 3. The sizes of the two cylinders (i.e., high pressure and low pressure may !e ad"usted to suit the volume and pressure of the refrigerant. #. It reduces the leakage loss considera!ly. $. It gives more uniform tor%ue& and hence a smaller size flywheel is needed. '. It provides effective lu!rication !ecause of lower tempe rature range. range. compre ssor. (. It reduces the cost of compressor. 5.
T/# /#es es ! C" C"#$ #$nd nd Va#$ #$& & C"# C"#&e &ess( ss(n n +(t +(t, , Inte Inte&&-' 'e& e&
In compound compression vapour refrigeration systems& the superheated vapour refrigerant leaving the first stage of compression is cooled !y suita!le method !efore !eing fed to the the)) second stage of compression and so on. *uch type of cooling the ref ri ref rigerant gerant i s called intercooling . Though there are many types of compound compression with intercoolers& intercoolers & yet the following are important from the su!"ect point of view : Two o stage compression with li%uid intercooler. i ntercooler. 1. Tw 2.
Two Tw o stage compression with water intercooler.
3. Two Two sta stage ge comp compres ressio sion n wit with h wat water er int interc ercool ooler er&& li% li%uid uid su!cooler and li%u li%uid id flash cham!er. # . Two stage compression with water intercooler& li%uid su!cooler and flash intercooler. $.
Three stage compression with flash cham!ers.
'.
Three stage compression with water intercoolers.
( . Three stage co compression with flash intercoolers. The a!ove mentioned types are now discussed& in detail& one !y one !y one one inthe in the following pages. 5.0 5. 0
T+ Sta Stage ge C" C"#& #&es ess( s(n n +( +(t, t, 1( 1($ $(d (d Int Inte& e&- -' 'e& e&
The arrangement of a two stage compression with li%ui li%u id inter intercooler cooler is shown in Fig. $. $ .1 +a. The corresponding p corresponding p-- h diagram is shown in Fig . $.1 +b +b. The various points on the p-h the p-h diagram are plotted as discussed !elow : 1. First of all& draw a horizontal pressure line representing the p E evaporator pressure +or suction pressure of low pressure
compressor which whic h intersects the ) saturated vapour line at point 1. ,t this point& point & the saturated vapour is supplied to the low pressure compressor. -et&& at point 1& the enthalpy of the saturated vapour is -et h1 s and entropy v 1 . 2. The saturated saturated vapour refrigerant refrigerant admitted at point point 1 is compressed compressed isen is entr trop opic ical ally ly in th the e low pressure compres compressor and deliv del ivers ers the p E r efrigerant efrigerant .in a superheate superheat ed sta tate te.. The pressure ris ris..es from to
p2
. The curve 1) 1 )2 represen represents ts the i sentropic compression compression in the 1ow
pressure compressor. In order to o!tain point 2& draw a line from point 1& sv 1 with with entrop entropy y e%ual e%ual to & along the constant entropy line intersecting the intermediate pressure line
h2
p2
at point 2. -et enthalpy at this point is
.
2. The superheated vapour refrigerant.ieavinglhe low lo w pressure tompressor at point 2 is cooled +or desuperheated at constant pressure Pi= Pi= p p3 in a li%uid intercooler !y the li%uid refrigerant fr om the condenser :The refrigerantt leaving the li%ui refrigeran li%u id intercooler is in saturated vapour state. state . The line 2)3 represents the cooling or desuperheating process. -et the h3 sv 3 enthalpy and entropy. entropy . at po point 3 is and respectively.
# The dry saturated vapour refriger ant is now supplied to high pressure compressor where it is compressed isentropically from intermediate or p2 pc intercooler pressure to condensor pressure . The cur ve 3)# represents the isentropic compression in the high pressure compressor. The point # on the p-h diagram is o!tained !y dr awing line of entropy e%ual to sv 3 along the constant entropy line as shown in Fig . $.1 (b). -et the enthalpy of superheated vapour refrigerant at point # is
h4
.
$ The superheated vapour refrigerant leaving the high pressure compressor at pc point # is now passed through the condenser at constant pressu r e as shown !y a horizontal line # $.The condensing process #)$ changes the state of refrigerant from superheated vapour to saturated li%uid. ' The high pressure saturated li%uid refrigerant from the condensor is passed to the intercooler where some of li%uid refrigerant evaporates in desuperheating the superheated vapour refrigerant from the low pressure compressor. In order to make up for the li%uid evaporated& i.e. to maintain a E1 constant li%uid level& an e/pansion valve which acts as a float valve& is provided. ( The li%uid refrigerant from the intercooler is first e/panded in an e/pansion E2 valve and then evaporated in the evaporator to saturated vapour condition& as shown in Fig. $.l +! . m1=¿ -et 0ass of refrigerant passing through the evaporator +or low pressure compressor in kgmin& and m2=¿
0ass of refrigerant passing through the condenser
+or high pressure compressor in kgmin .
The high pressure compressor in a given system will compress the mass of m1 refrigerant from low pressure compressor + and the mass of li%uid evaporated in the li%uid intercooler during cooling or desuperheating of m3 superheated vap our refrigerant from low pressure compressor. If
is the
mass of li%uid evaporated in the intercooler& then
m 3= m 2− m 1 The value of
m2
may !e o!tained !y considering the thermal e%uili!rium
for the li%uid intercooler as shown in Fig. $.2& i.e.,
eat taken !y the li%uid intercooler 3 2eat given !y the li%uid intercooler or
Ntes: 3. In case of ammonia& when li%uid refrigerant is used for intercooling& the total power re%uirement will decrease. It is due to the fact that the mass of li%uid evaporated during intercooling is e/tremely small !ecause of its high latent heat of vaporisation and the constant entropy lines of ammonia !ecome very flat in the superheat region. Thus the intercooling !y li%uid refrigerant is commonly used in multi) stage ammonia plants& !ecause of less power re%uirement. . 2. In case of refrigerant 4)12& when li%uid refrigerant is used for intercooling& the total power re%uirements may actually increase. It is due to the fact that the latent heat of vaporisation is small and the constant entropy line of 4)12 does not change very much with the temperature. Thus in 4)12 systems& the saving in work !y performing the compression close to the saturated vapour line does not compensate for the increased mass flow rate through the high stage compressor. Therefore& intercooling !y li%uid refrigerant in 4)12 systems& is never employed. &
5.5 T+ Stage C"#&ess(n +(t, 4ate& Inte&-'e& and 1($(d S$) -'e&
The arrangement of a two)stage compression with water intercooler and li%uid su!)cooler is shown in Fig.$.$ +a. The corresponding p-h diagram is shown in Fig. $.$ (b). The various processes in this system are as follows: p E 1. The saturated vapour refrigerant at the evaporator pressure is admitted to low pressure compressor at point 1. In this compressor& the refrigerant is compressed isentropically from the evaporator pressure p2 p E to the water intercooler pressure & as shown !y the curve 1)2 Fig. $.$+!.
2 The refrigerant leaving the low pressure compressor at point 2 is in superheated state. This superheated vapour refrigerant is now passed through the water intercooler at constant pressure& in order to reduce the degree of superheat. The line 2)3 r epresents the water intercooling or desuperheating process. 3 The refrigerant leaving the water intercooler at point 3 +which is still in the superheated state is compressed isentr opically in the high pressure pc com pr essor to the condenser pressure . The curve 3)# shows the isentro pic compression in high pressure compressor. # The discharge from the high pressure compressor is now passed through the condenser which changes the state of refrigerant from superheated
vapour .to saturated li%uid as shown !y process #)$. $ The temperature of the saturated li%uid refriger a nt f urther reduced !y passing it through a li%uid su!)cooler as shown !y process $)'. ' The li%uid refrigerant from the su!)cooler is now e/pa nde d in an e/pansion valve +process ')( !efore !eing sent to the ev aporator for evaporation +process ()1. It may !e noted that water intercooling reduces the work to !e done in high pressure compressor. It also reduces the specific volume of the refrigerant which re%uires a compressor of less capacity +or stroke volume. The complete desuperheating of the vapour refrigerant is not possi!le in case of water intercooling. It is due to the fact that temperature of the cooling water used in the water intercooler is not availa!le sufficiently low so as to desuperheat the vapour completely. -et
5 -oad on the evaporator in tonnes of refrigeration.
0ass of refrigerant passing through the evaporator + or passing through the
∴
-.6. compressor&
m=
210 Q
h1−h 7
=
210 Q
h 1−hf 6
kg / min
*ince the mass of refrigerant passing through the compressors is same& therefore& total work done in !oth the compression& 7
=
7ork done in -.6. compressor 8 7or k done in .6. compressor
¿ m ( h −h ) + m ( h − h ) = m [ ( h − h ) + ( h − h ) ] 2
1
4
3
2
1
4
3
6ower re%uired to drive the system&
∴
P=
m [ ( h2 − h 1 ) + ( h4 − h3 ) ] 60
kW
7e know that refrigerating effect&
R E=m ( h1− hf 6 ) =210 Q kJ / min 9..6. of the system
∴
¿
m ( h1− hf 6 )
R E
=
W
=
210 Q
[ ( h −h ) + ( h − h ) ] P x 60 2
1
4
3
5.6 T+ Stage C"#&ess(n +(t, 4ate& 'nte&-'e&7 1($(d S$)-'e& and 1($(d F'as, C,a"e& The arrangement of a two stage compression with water intercooler& li%uid su!)cooler and li%uid flash cham!er is shown in Fig. $.( (a). The corresponding p-h diagram is shown in Fig. $.( +!. The various processes& in this system& are as follows: p E 1. The saturated vapour refrigerant at the evaporator pressure is
admitted to low pressure; compressor at point 1. In this compressor& the refrigerant is compressed isentropically from evaporator pressure 6< to water intercooler +or flash cham!er pressure the curve 1)2 in Fig. $.( (b).
p F
as shown !y
2. The superheated vapour refrigerant leaving the low pressure compressor at point 2 is now passed through the water intercooler at p F constant pressure in order to reduce the degree of superheat (i.e., from temperature
t 2
to
t 3
. The line 2)3 represents the water
intercooling or de)superheating process. 3. The superheated vapour refrigerant leaving the water intercooler at point 3 is mi/ed with the vapour refrigerant supplied !y the flash cham!er at point =. The condition of refrigerant after mi/ing is shown !y point # which is in superheated state. -et the temperature at this t 4 point is . #. The superheated vapour refrigerant admitted at point # to the high pressure compressor is compressed isentropically from the intercooler p F or flash cham!er pressure to condenser pressure pc as shown !y the curve #)$. The temperature rises from
t 4
to
t 5
.
$. The superheated vapour leaving the high pressure compressor at pressure pC is passed through a condenser at constant pressure as shown !y a horizontal line $)'. The condensing process $)' changes the state of refrigerant from superheated vapour to saturated li%uid.
' The saturated li%uid refrigerant from the condenser is now cooled in t 7 li%uid su!)cooler to a temperature& say .The line ')( represent a su!)cooling process. ( The li%uid refrigerant leaving the su!)cooler at pressure e/panded in an e/pansion valve cham!er pressure
p F
E1
pc
is
to a pressure e%ual .to the flash
, as shown !y vertical line ()>. The e/panded
refrigerant which is a mi/ture of vapour and li%uid refrigerants is admitted to a flash cham!er at point >. The flash cham!er separates the vapour and li%uid refrigerants at pressure. The vapour refrigerant from the flash cham!er at point = is mi/ed with the refrigerant from the water intercooler. The li%uid refrigerant from the flash cham!er at point E2 1? is further e/panded in an e/pansion valve as shown !y the vertical line 1?)11. > The li%uid refrigerant leaving the e/pansion valve
E2
is evaporated in
the evaporator at the evaporator pressure 6< +usually 2 ! ar as shown !y the horizontal line 11)1 in Fig. $.( +!. m2=¿ -et 0ass of refrigerant passin g through the condenser +or high pressure compressor& and
m3=¿
0ass of vapour refrigerant formed in the flash
cham!er. 0ass of refrigerant passing through the evaporator +or low pressure compressor&
∴
m 1= m 2 − m 3
If 5 tonne of refrigeration is the load on the evaporator& then the mass of refrigerant passing through evaporator&
@ow let us consider the thermal e%uili!rium of the flash cham!er . *ince the ash cham!er is an insulated vessel& therefore there is no heat e/change !etween the flash cham!er and atmosphere. In other words& the heat taken and given !y the flash cham!er are same. 0athematically&
The vapour refrigerant from the water intercooler +represented !y point 3 is mi/ed with vapour refrigerant m3 from the flash cham!er +represented !y point = at the same pressure !efore entering the high pressure compressor. The enthalpy of the mi/ed refrigerant +represented !y point # may !e calculated !y using the e%uation&
6
T+ Stage C"#&ess(n +(t, 4ate& Inte&-'e&7)1($(d S$)-'e& and F'as, Inte&-'e&
, two stage compression with water intercooler& li%uid su!)cooler and flash intercooler is shown in Fig. $.= +a. The corresponding . p)h diagram is shown in Fig. $.= +!.
+a Two stage compression with water intercooler& li%uid su!)cooler and flash intercooler .
+ !
p)h diagram Fig. $.=
7e have seen in the previous article that when the vapour refrigerant from the low pressure compressor is passed through the water intercooler& its temperature does not reduce the saturated vapour line or even very near to it& !efore admitting it to the high pressure compressor A4efer point# of Fig. $.( +!B. In fact& with water cooling there may !e no saving of work in compression. Cut the improvement in performance and the reduction in compression work may !e achieved !y using a flash cham!er as an intercooler as well as flash separator& as shown in Fig. $.= +a.The
corresponding p-h diagram is shown in F g. $.= +!. The various processes& in this system& are as follows: p E 1. The saturated vapour refrigerant at the evaporator pressure is admitted to the low pressure compressor at point 1. In this compressor & the refrigerant is compressed isentropically from evaporator pressure p E p E to )the flash intercooler pressure as shown !y the curve 1) 2 in Fig. $.= +!. 2. The superheated vapour refrigerant leaving the low pressure compressor at point 2 is now passed through the water intercooler at p F constant pressure in order to reduce the degree of superheat (i.e. from temperature t2 to t3. The line 2)3 represents the water intercooling or desuperheating process. 3. The superheated vapour refrigerant leaving the water intercooler at point 3 is passed through a flash intercooler which cools the superheated vapour refrigerant to saturated vapour refrigerant as shown !y the line 3)#. The cooling of superheated vapour refrigerant is done !y the evaporation of a part of the li%uid refrigerant from the flash intercooler placed at point >. #. The saturated vapour refrigerant leaving the flash intercooler enters the high pressure compressor at point # where it is compressed p F isentropically from flash intercooler pressure to condenser pressure $. The
pc
& as shown !y the curve #)$.
superheated
vapour refrigerant leaving the high pressure pc compressor at pressure is passed through a condenser at constant
pressure. The condensing process as shown !y line $)' changes the state of refrigerant from superheated vapour to saturated li%uid . '. The saturated li%uid refrigerant leaving the condenser a.t point ' is pc now cooled at constant pressure in the li%uid s.u!)cooler to
a temperature
t 7
as shown in Fig. $.= +!. The line ')( shows
the su!)cooling process. (. The li%uid refrigerant leaving the su!)cooler at point ( is e/panded in E1 an e/pansion valve to a pressure e%ual to the flash intercooler pressure PF &as shown !y the vertical line ()>. The e/panded refrigerant +which is a mi/ture of vapour and li%uid refrigerant is admitted to flash intercooler at point > which also acts as a flash separator. >. The li%uid refrigerant leaving the flash intercooler at point = is passed E2 through the second e/pansion valve +process =)1? and then evaporated in the evaporator as shown !y the horizontal line 1?)1. m1 -et 0ass of the refrigerant passing through the evaporator or low pressure compressor& and
m2
= 0ass of the refrigerant passing through the
condenser +or high pressure compressor. If Q tonne of refrigeration is the load on the evaporator& then the mass of refrigerant passing through the evaporator is given !y&
5.8
T,&ee Stage C"#&ess(n +(t, 4ate& 'nte&-'e&s
%a* T,&ee stage -"#&ess(n +(t, +ate& (nte&-'e&s.
%* p-h d(ag&a" F(g. 5.33
The arrangement of a three stage compression with water intercoolers is shown in Fig. $.11+a. The corresponding p-h diagram is shown in Fig .$.ll ( b).The work done in the high pressure compressor may !e greatly reduced with such an arrangement. The water intercooling !etween the stages reduces the degree of superheat of the refrigerant. It also reduces the specific volume of the refrigerant which re%uires a compressor of less capacity +or stroke volume. 7e see from p-h diagram that the water intercoolers reduce the temperature of superheated vapour to its saturation value after each stage& as shown !y points 3 and 5 in Fig. $.l 1 +!. It may !e noted that the complete desuperheating of the vap?ur is not possi!le !ecause the temperature of cooling water used in water intercoolers is not availa!le sufficiently low so as to desuperheat the vapour completely. 7e know that for a load of Q tonnes of refrigeration on the evaporator& the mass of refrigerant circulating through the evaporator is given !y
5.9
T,&ee Stage C"#&ess(n +(t, F'as, C,a"e&s The arrangement of three compressors with multiple e/pansion valves E1 , E 2 , E3 and two flash cham!ers F 1 and F 2 is shown in Fig. 5.13 (a).
The corresponding p-h diagram is shown in Fig. $.13 (b) -et m kgmin !e the mass of refrigerant leaving the condenser at point (. E3 This refrigerant while passing through the e/pansion valve reduces its pressure from
pc
to
p F 2
. The refrigerant leaving the e/pansion valve
at point > is separated !y the flash cham!er
F 2
. If
x 8
E3
is the dryness fraction
of the refrigerant at point >& then mass of saturated vapour refrigerant separated at point > and delivered to high pressure compressor at point $ will !e m5 m / x 8 kgmin
0ass of saturated li%uid refrigerant leaving the flash cham!er
∴
point =& m 9=m− m5 =m −m x x 8=m ( 1 − x 8 ) kg / min
F 2
at
D+∴ This saturated li%uid refrigerant thr ough the second e/pansion valve
p F 2
to
p F 1
(i.e. E2
m9
kgmin
F 1
. If
x 10
is now
passed
where its pressure reduces from
. The refrigerant leaving the e/pansion valve
!y the flash cham!er
m 5= m x x 8
E2
is separated
is the dryness fraction of the refrigerant at
point 1?& then mass of saturated vapour separated at point 1? and delivered to intermediate pressure compressor at point 3 will !e m3= m9− x 10=m ( 1− x 8 ) x 10 kg / min m9=m ( 1− x 8 ) D+∴
0ass of saturated li%uid refrigerant leaving the flash cham!er
∴
11&
m 11= ( m9−m 3 )=m ( 1− x 8 )− m ( 1− x 8 ) x 10 kg / min
F 1
at point
This refrigerant mi i kgmin is passing through the e/pansion valve where its pressure reduces from e/pansion valve
E1
p F 1
to
p E
E1
. The refrigerant leaving the
at point 12 is passed through the evaporator and then to low
pressure compressor at point 1. *ince same mass of refrigerant is supplied to evaporator or low pressure compressor at point 1 as that of saturated li%uid leaving the F 1 flash cham!er at point 11& therefore 0ass of refrigerant passing through the evaporator or low pressure compressor& m1=m 11= m ( 1− x 8 ) ( 1− x 10 ) kg / min D+i If 5 tonnes of refrigeration is the load on the evaporator& then mass of refrigerant passing through the evaporator or low pressure compressor& 210 Q 210 Q = m1= kg / min h =h f 11 D+∴ 12 h1−h12 h1−h f 11 +ii
D
Ntes: 3. Cy using multiple e/pansion valves in the a!ove arrangement& the refrigerant can !e e/panded close to the li%uid line and !y using the flash cham!er& the total work done per kg of refrigerant is reduced. 2. Thermodynamically& this arrangement leads to more 9 ..6. as compared to simple saturation cycle for the operating pressure range& !ecause of decrease in total compression work without affecting the refrigerating effect produced at the evaporator.
5.3:
T,&ee Stage C"# n +(t, F'as, Inte&-'e&s
7e have already discussed that due to non)availa!ility of c ling water at tow temperature for intercooling& the superheated vapour at the end of each stage cannot !e completely desuperheated. This difficulty can !e overcome F 1 F 2 !y adopting the flash intercoolers and !etween the stages as shown in Fig. 5.1$ +a. The p-h diagram for the arrangement is shown in Fig. 5.1$ +!. The superheated vapour from low pressure compressor +as represented !y point 2 is cooled to saturated vapour in the flash intercooler F 1 !y the li%uid refrigerant from e/pansion valve
E2
+as represented !y point
=. The flash intercooling process for the first stage as represented !y 2)3 is p F 1 carried out at pressure . *imilarly& the superheated vapour from intermediate pressure compressor +as represented !y point # is cooled to
saturated vapour in the flash intercooler e/pansion valve
E3
F 2
!y the li%uid refrigerant from
+as represented !y point >. The flash intercooling
process for the second stage as represented !y #)$ is carried out at pressure p F 2 . If Q tonnes of refrigeration is the load on the evaporator& then the mass of refrigerant passing through the evaporator or low pressure compressor at point l&
The intermediate pressure compressor will compress the mass of refrigerant m1 from the low pressure compressor (i.e., kgmin and the mass of li%uid refrigerant evaporated in
the
flash intercooler
F 1
during
cooling
or
desuperheating of superheated vapour refrigerant from the low pressure m2 compressor. If is the mass of li%uid refrigerant evaporated in the flash intercooler
F 2
& then
m 3 = m 1+ m 2
m2
The value of
may !e o!tained !y considering the thermal
F 1
e%uili!rium of the flash intercooler
eat taken !y the flash intercooler
& i.e.
F 1
eat given !y the flash intercooler
F 1
m1 h 2+ m2 h9= m3 h3
r
m1 h 2+ m 2 h9= ( m1 + m 2 ) h3
m 2=
∴
m1 ( h2−h3 ) h3 −h9
=
m1 (h 2−h3 ) h 3−h f 7
kg / min
D+
h9 =hf 7
*imilarly& the high pressure compressor will compress the mass of) refrigerant from the intermediate pressure compressor (i.e. m 3 kgmin and the F 2 mass of li%uid evaporated in the flash intercooler during cooling or desuperheating of superheated vapour refrigerant from the intermediate pressure m4 compressor. If kgmin is the mass of li%uid refrigerant evaporated in the flash intercooler
F 2
& then
m5=m3 + m4
The value of
m4
may !e o!tained !y considering the thermal e%uili!rium
of the flash intercooler
F 2
& i.e.
eat taken !y the flash intercooler
F 2
eat given !y the flash intercooler
F 2
m 3 h 4 + m 4 h8 = m 5 h5
r
m (¿ ¿ 3 + m4 ) h5 m3 h 4 + m4 h8 =¿ m 4=
∴
h8= hf 7
m 3 ( h 4− h 5) h5 −h8
=
m3 ( h4 −h5 ) h5− hf 7
kg / min
D+∴
5.33 T,&ee Stage C"#&ess(n +(t, M$'t(#'e E;#ans(n Va'ves and F'as, Inte&-'e&s In the previous article of three stage compression with flash intercoolers& a single e/pansion valve was used along the flow line to evaporator. Cut in this arrangement& multi)e/pansion valves are used to increase the coefficient of performance of the system as shown in Fig . $.1( +a.
If Q tonnes of refrigeration is the load on the evaporator& then the mass of refrigerant passing through the evaporator or low pressure compressor&
m1=
210 Q
h1− h12
=
210 Q
h1−h f 11
kg / min
D+∴
h12=h f 11
From Fig. $.1( +a& we see that the superheated vapour refrigerant discharged F 1 from the low pressure compressor is !rought into the flash intercooler where it is desuperheated !y the li%uid refrigerant received from the e/pansion E1 valve at point 1o. Euring the process of desuperheating& some of the li%uid refrigerant gets evaporated and supplied to the intermediate pressure compressor. Thus the mass of vapour refrigerant passing through the intermediate compressor +or second stage of compression& m 2 = 0ass of vapour refrigerant from -.6. compressor 8 0ass of vapour refrigerant or
flash
resulting
from
E2
e/pansion valve
8 0ass of vapour
refrigerant resulting from evaporation in the flash F 1 intercooler
where
E2
x 10
m 1+ m1
m1 [1 +
( ) x10
1
− x
− x
1
10
x 10 1
+m (
+ 10
h 2− h3 h3−h10
h 2− h3 h3− h10
)
]
is the dryness fraction of refrigerant leaving the e/pansion valve
at point 1?.
*imilarly& the mass of vapour refrigerant passing through the high pressure compression +or third stage of compression& m3=¿ 0ass of vapour refrigerant from I.6. compressor 8 0ass of vapour refrigerant or flash resulting from e/pansion E3 valve 8 0ass of vapour refrigerant resulting from evaporation in the flash intercooler
m 2+
m1 x 8
( 1− x )( 1− x ) 10
m2 [ 1 +
h4 −h5 h5 −h8
+m ( 2
8
]+
7ork done in I.6. compressor& W I =m2 ( h 4−h3 ) 7ork done in .6. compressor&
W H =m3 ( h6 −h5 ) ,nd total work done in three compressors&
W = W L + W I + W H
h5 − h8
m 1 x 8
(1− x )( 1− x ) 10
7e know that work done in -.6. compressor&
W L= m1 ( h 2−h1 )
h4 − h5
8
)
F 2
.
¿ m ( h − h ) + m ( h −h ) + m ( h −h ) 1
2
1
6ower re%uired to drive the compressors&
∴
6 7'? k7 7e know that refrigerating effect&
R E=m1 ( h1−h f 11 ) =210 Q kJ / min 9..6. of the system
∴
¿
R E
=
210 Q
W P x 60
2
4
3
3
6
5
