Thermodynamics
Thermodynamics
University of Diyala
College of Engineering
Mechanical Engineering Department
Class: Second Year
Second Semester
ME210 : Thermodynamics
M
E
2
1
0
-
C
H
A
P
T
E
R
8
R
E
F
R
I
G
E
R
A
T
I
O
N
C
Y
C
L
E
S
B
y
L
e
c
t
u
r
e
r
S
a
m
e
e
r
D
.
A
l
i
L
a
y
t
h
A
b
e
d
H
a
s
s
n
a
w
e
2
R
E
F
R
I
G
E
R
A
T
O
R
S
A
N
D
H
E
A
T
P
U
M
P
S
for
f
i
x
ed
va
l
u
e
s
of
Q
an
d
Q
L
H
T
he
o
b
j
e
ctive
of a refri
g
er
a
tor
is to rem
o
ve
h
e
a
t
(
Q
L
)
from
the
cold
me
d
i
u
m;
the
o
b
j
e
ctive
of a h
e
at
p
u
mp
is to
su
p
p
l
y
h
e
at
(
Q
H
)
to a
w
arm
me
d
i
u
m.
T
her
m
od
y
na
m
ic
s
-
CH
11
T
h
e
t
r
a
n
s
f
e
r
o
f
h
e
a
t
f
r
o
m
a
l
o
w
-
t
e
m
p
e
r
a
t
u
r
e
r
e
g
i
o
n
t
o
a
h
i
g
h
-
t
e
m
p
e
r
a
t
u
r
e
o
n
e
r
e
q
u
i
r
e
s
s
p
e
c
i
a
l
d
e
v
i
c
e
s
c
a
l
l
e
d
r
e
f
r
i
g
e
r
a
t
o
r
s
.
A
n
o
t
h
e
r
d
e
v
i
c
e
t
h
a
t
t
r
a
n
s
f
e
r
s
h
e
a
t
f
r
o
m
a
l
o
w
-
t
e
m
p
e
r
a
t
u
r
e
m
e
d
i
u
m
t
o
a
h
i
g
h
-
t
e
m
p
e
r
a
t
u
r
e
o
n
e
i
s
t
h
e
h
e
a
t
p
u
m
p
.
R
e
frig
e
rat
o
rs
a
n
d
h
e
at p
u
mps
are
ess
e
nti
a
l
l
y
the
same
d
e
vic
e
s;
th
e
y d
i
f
fer in
th
e
ir
ob
j
e
ctiv
e
s
on
l
y
.
3
T
H
E
R
E
V
E
R
S
E
D
C
A
R
N
O
T
C
Y
C
L
E
Sc
he
m
a
tic
o
f
a
C
a
rn
o
t
refri
g
er
a
tor
an
d
T
-
s
d
i
a
g
ram
of the
rev
e
rsed
C
a
rn
o
t
c
y
cl
e
.
The
reversed
Carnot
c
y
cle
is the
m
ost
efficient
refrig.
c
y
cle
operating
bet
w
een
T
L
and
T
H
.
It
is not
a
suitab
l
e
model
for
ref
r
igeration
c
y
cles since
proce
s
ses
2
-
3
and 4
-
1
are
not
pract
i
cal
be
c
au
s
e
Process
2
-
3
involves
the
compression
of
a
liqui
d
–vapor
m
i
x
ture,
w
hich
requires
a
compressor
that
w
ill
hand
l
e
t
w
o
pha
s
es,
and
process
4
-
1
involves
the
expans
i
on
of
hig
h
-
moistu
r
e
-
content
r
e
frige
r
ant
in
a
tu
r
bine.
Both C
O
Ps increase as
t
he
d
i
f
f
erence
bet
w
een
t
he
t
w
o
t
empe
r
a
t
ures
dec
r
eases,
t
hat
is,
as
T
L
rises or
T
H
f
a
l
ls.
T
H
E
I
D
E
A
L
V
A
P
O
R
-
C
O
M
P
R
E
S
S
I
O
N
R
E
F
R
I
G
E
R
A
T
I
O
N
C
Y
C
L
E
T
h
e
v
a
p
o
r
-
c
o
m
p
r
e
s
s
i
o
n
r
e
f
r
i
g
e
r
a
t
i
o
n
c
y
c
l
e
i
s
t
h
e
i
d
e
a
l
m
o
d
e
l
f
o
r
r
e
f
r
i
g
e
r
a
t
i
o
n
s
y
s
t
e
m
s
.
U
n
l
i
k
e
t
h
e
r
e
v
e
r
s
e
d
C
a
r
n
o
t
c
y
c
l
e
,
t
h
e
r
e
f
r
i
g
e
r
a
n
t
i
s
v
a
p
o
r
i
z
e
d
c
o
m
p
l
e
t
e
l
y
b
e
f
o
r
e
i
t
i
s
c
o
m
p
r
e
s
s
e
d
a
n
d
t
h
e
t
u
r
b
i
n
e
i
s
r
e
p
l
a
c
e
d
w
i
t
h
a
t
h
r
o
t
t
l
i
n
g
d
e
v
i
c
e
.
T
h
i
s
is the
most
w
i
d
ely
us
e
d
c
y
cle
for refri
g
er
a
tors,
A
-C s
y
stems,
a
n
d
hea
t
pu
m
p
s.
Schematic
and
T
-
s
d
i
agr
a
m
for
t
he i
d
eal
vap
o
r
-compress
i
on
refri
g
er
a
tion
c
y
cl
e
.
4
An
or
d
i
n
ary
hou
s
eho
ld
refri
g
er
a
to
r
.
T
he
P
-
h
d
i
a
g
ram
of an
id
e
al
va
p
or-
com
p
ress
i
on
refri
g
er
a
tion
c
y
cl
e
.
T
he
ideal vapo
r
-
comp
r
ess
i
on
r
e
f
rigera
t
ion
c
y
cle
invo
l
ves
an
ir
r
eversib
l
e
(
t
hro
tt
ling)
process
t
o
make
it a
mo
r
e
real
i
s
t
ic
model
f
or the ac
t
ual
s
y
s
t
ems.
Rep
l
ac
i
ng
t
he
e
x
pans
i
on
va
l
ve
by
a
t
urbine
is
not prac
t
ical since
t
he
added benefits cannot
jus
t
i
f
y
t
he
added
cost and
comple
x
i
t
y
.
Stead
y
-
flow
energy
balan
c
e
5
6
A
C
T
U
A
L
V
A
P
O
R
-
C
O
M
P
R
E
S
S
I
O
N
R
E
F
R
I
G
E
R
A
T
I
O
N
C
Y
C
L
E
An
ac
t
ual vapor
-
comp
r
ess
i
on
r
e
f
rigera
t
ion
c
y
cle
d
i
f
f
ers f
r
om the ideal one
o
w
ing
mos
t
ly
t
o
t
he
ir
r
eversib
i
l
it
ies
t
hat
occur
i
n
various componen
t
s,
main
l
y
due
t
o
f
lu
i
d
f
riction
(causes pressu
r
e
drops) and
heat
t
rans
f
er
t
o
or f
r
om the sur
r
ound
i
ng
s
.
Sc
he
m
a
tic
an
d
T
-
s
d
i
a
g
ram
for
the
actu
a
l
va
p
o
r
-
com
p
ress
i
on
r
e
fri
ge
r
a
ti
o
n
c
y
cl
e
.
D
I
F
F
E
R
E
N
C
E
S
N
o
n
-is
e
ntr
o
p
i
c
com
p
ress
i
on
S
u
p
e
rh
e
at
e
d
va
p
or
at
ev
a
p
o
rat
o
r
e
x
it
S
u
bc
o
o
l
ed
l
i
q
u
id
at
co
n
d
e
ns
e
r
e
x
it
Pr
e
ss
u
re
d
r
op
s in c
onden
s
e
r
an
d
e
v
apo
r
a
tor
T
he
COP
dec
r
eases as
a result
of ir
r
eversib
i
l
it
ies.
Refrigeration cycles involving refrigerators and heat pumps are explored, including the ideal vapor-compression cycle and the reversed Carnot cycle. Learn about the efficiency and practicality of different cycles in transferring heat between temperature regions.
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Presentation Transcript
University of Diyala College of Engineering Mechanical Engineering Department Class: Second Year Second Semester ME210 : Thermodynamics ME210- CHAPTER 8 REFRIGERATION CYCLES By Lecturer Sameer D. Ali Layth Abed Hassnawe Thermodynamics
REFRIGERATORSAND HEAT PUMPS The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators. Another device that transfers heat from a low-temperature medium to a high- temperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium. for fixed values of Q and Q L H 2 Thermodynamics-CH11
THE REVERSED CARNOT CYCLE The reversed Carnot cycle is the most efficient refrig. cycle operating between TLand TH. It is not a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical because Process 2-3 involves the compression of a liquid vapor mixture, which requires a compressor that will handle two phases, and process 4-1 involves the expansion of high- moisture-content refrigerant in a turbine. Both COPs increase as the difference between the two temperatures decreases, that is, as TL rises or TH falls. Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle. 3
THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE The vapor-compression refrigeration cycle is the ideal model for refrigeration systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device. This is the most widely used cycle for refrigerators, A-C systems, and heat pumps. Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle. 4
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems. Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added cost and complexity. Steady-flow energy balance An ordinary household refrigerator. The P-h diagram of an ideal vapor- compression refrigeration cycle. 5
ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE An actual vapor-compression refrigeration cycle differs from the ideal one owing mostly to the irreversibilities that occur in various components, mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. The COP decreases as a result of irreversibilities. DIFFERENCES Non-isentropic compression Superheated vapor at evaporator exit Subcooled liquid at condenser exit Pressure drops in condenser and evaporator Schematic and T-s diagram for the actual vapor- compression refrigeration cycle. 6
