Transcript Slide 1

VELTECH Dr.R.R&Dr.S.R
TECHNICAL UNIVERSITY
AERO ENGINEERING THERMODYNAMICS
SEM:III
YEAR:II
PREPARED BY
Mr.B.Narendhiran
DEPARTMENT OF AERONAUTICAL
ASSISTANT PROFESSOR
UNIT-I
Thermodynamic Systems, States and
Processes
Objectives are to:
• define thermodynamics systems and states of systems
• explain how processes affect such systems
• apply the above thermodynamic terms and ideas to the laws of
thermodynamics
Internal Energy of a Classical ideal gas
“Classical” means Equipartition Principle applies: each
molecule has average energy ½ kT per in thermal equilibrium.

At room temperature,
for most gases:
3
KE  kT
2
monatomic gas (He, Ne, Ar, …)
3 translational modes (x, y, z)
diatomic molecules (N2, O2, CO, …)
3 translational modes (x, y, z)
+ 2 rotational modes (wx, wy)
UN
3
3
kT  pV
2
2
5
KE  kT
2
Internal Energy of a Gas
A pressurized gas bottle (V = 0.05 m3), contains
helium gas (an ideal monatomic gas) at a pressure p =
1×107 Pa and temperature T = 300 K. What is the
internal thermal energy of this gas?
3
3
U  N kT  pV
2
2



 1.5  107 Pa  0.05m3  7.5 105 J
Changing the Internal Energy

U is a “state” function --- depends uniquely on the state of the
system in terms of p, V, T etc.
(e.g. For a classical ideal gas, U = NkT )

There are two ways to change the internal energy of a system:
WORK done by the system
on the environment
Wby = -Won
HEAT is the transfer of thermal energy
into the system from the surroundings
Thermal reservoir
Q
Work and Heat are process energies, not state functions.
Work Done by An Expanding Gas
The expands slowly enough to
maintain thermodynamic equilibrium.
dW  Fdy  PAdy
Increase in volume, dV
dW  PdV
+dV Positive Work (Work is
done by the gas)
-dV Negative Work (Work is
done on the gas)
A Historical Convention
+dV Positive Work (Work is
done by the gas)
Energy leaves the system
and goes to the environment.
-dV Negative Work (Work is
done on the gas)
Energy enters the system
from the environment.
Total Work Done
dW  PdV
Vf
W   PdV
Vi
To evaluate the integral, we must know
how the pressure depends (functionally)
on the volume.
Pressure as a Function of Volume
Vf
W   PdV
Vi
Work is the area under
the curve of a PV-diagram.
Work depends on the path
taken in “PV space.”
The precise path serves to
describe the kind of
process that took place.
Different Thermodynamic Paths
The work done depends on the initial and final
states and the path taken between these states.
Work done by a Gas


When a gas expands, it does work on its environment
Consider a piston with cross-sectional area A
filled with gas. For a small displacement dx,
the work done by the gas is:
dWby = F dx = pA dx = p (A dx)= p dV
 We generally assume quasi-static processes (slow
enough that p and T are well defined at all times):
dx
Wby 
This is just the area under the p-V curve
p
 p dV
Vi
p
p
V
Vf
V
V
Note that the amount of work needed to take the system from one
state to another is not unique! It depends on the path taken.
What is Heat?

Up to mid-1800’s heat was considered a substance -- a
“caloric fluid” that could be stored in an object and
transferred between objects. After 1850, kinetic
theory.

A more recent and still common misconception is that
heat is the quantity of thermal energy in an object.

The term Heat (Q) is properly used to describe energy
in transit, thermal energy transferred into or out of a
system from a thermal reservoir …
Q

U
(like cash transfers into and out of your bank account)
Q is not a “state” function --- the heat depends on the
process, not just on the initial and final states of the system
Sign of Q :
Q > 0 system gains thermal energy
Q < 0 system loses thermal energy
An Extraordinary Fact
The work done depends on the initial and final
states and the path taken between these states.
BUT, the quantity Q - W does not depend
on the path taken; it depends only on the initial
and final states.
Only Q - W has this property. Q, W, Q + W,
Q - 2W, etc. do not.
So we give Q - W a name: the internal energy.
The First Law of Thermodynamics
(FLT)
-- Heat and work are forms of energy transfer
and energy is conserved.
U = Q + Won
change in
total internal energy
heat added
to system
State Function
work done
on the system
Process Functions
or
U = Q - Wby
1st Law of Thermodynamics
U  Q  W
positiveQ : heat added to system
positiveW : work done by system
• statement of energy conservation for a thermodynamic
system
• internal energy U is a state variable
• W, Q process dependent
The First Law of Thermodynamics
dEint  dQ  dWby
What this means: The internal energy of a system
tends to increase if energy is added via heat (Q)
and decrease via work (W) done by the system.
dEint  dQ  dWon
. . . and increase via work (W) done on the system.
dWby  dWon
Isoprocesses
• apply 1st law of thermodynamics to closed
system of an ideal gas
• isoprocess is one in which one of the
thermodynamic (state) variables are kept
constant
• use pV diagram to visualise process
Isobaric Process
• process in which pressure is kept
constant
Isochoric Process
• process in which volume is kept
constant
Isothermal Process
• process in which temperature is held
constant
Thermodynamic processes of an ideal gas
( FLT: U = Q - Wby )

Isochoric (constant volume)
Wby   pdV 0
U   Nk T   V p
FLT:
2
p
Q  U
Q
1
Temperature
changes
V

Isobaric (constant pressure)
Wby   pdV  pV
U   Nk T   p V
FLT: Q  U  Wby    1 p V
p
1
2
p
Q
V
Temperature and
volume change
( FLT: U = Q - Wby )

Isothermal (constant temperature)
U  0
p
1
V2
V2
Wby   p dV  NkT n
V1
V
2
p
Q
1
FLT:
1
V
Thermal Reservoir
Q Wby
V
T
Volume and
pressure change
The First Law Of Thermodynamics
§2-1.The central point of first law
§2-2. Internal energy and total energy
§2-3.The equation of the first law
§2-4.The first law for closed system
§2-5.The first law for open system
§2-6.Application of the energy equation
§2-1.The
central point of first law
1.Expression
In a cyclic process, the algebraic sum of the
work transfers is proportional to the algebraic
sum of the heat transfers.
Energy can be neither created nor destroyed;
it can only change forms.
The first law of thermodynamics is simply a
statement of energy principle.
§2-1.The
central point of first
law
2.Central point
The energy conservation law is used to
conservation between work and heat.
Perpetual motion machines of the first
kind.(PMM1)
Heat: see chapter 1;
Work: see chapter 1;
§2-2.Internal
Energy
1.Definition:
Internal energy is all kinds of micro-energy in system.
2. Internal energy is property
It include:
a) Kinetic energy of molecule (translational kinetic,
vibration, rotational energy)
b) Potential energy
c) Chemical energy
d) Nuclear energy
§2-2.Internal
Energy
3.The symbol
u: specific internal energy, unit –J/kg, kJ/kg ;
U: total internal energy,
unit – J, kJ;
4.Total energy of system
E=Ek+Ep+U
Ek=mcf2/2
Ep=mgz
ΔE=ΔEk+ΔEp+ΔU
per unit mass:
e=ek+ep+u
Δe=Δek+Δep+Δu
§2-3.
The equation of the first law
1. The equation
( inlet energy of system) – (outlet energy of
system) = (the change of the total energy of the
system)
Ein-Eout=ΔEsystem
§2-4.The first law in closed system
1. The equation
Ein-Eout=ΔEsystem
Q
W
§2-4.The first law in closed system
Q-W=ΔEsystem=ΔU
Q=ΔU+W
Per unit mass:
q= Δu+w
dq=du+dw
If the process is reversible, then:
dq=du+pdv
This is the first equation of the first law.
Here q, w, Δu is algebraic.
§2-4.The first law in closed system
The only way of the heat change to mechanical
energy is expansion of working fluid.
§2-5.
The first law in open system
1. Stead flow
For stead flow, the following conditions are
fulfilled:
① The matter of system is flowing steadily, so that
the flow rate across any section of the flow has
the same value;
② The state of the matter at any point remains
constant;
③ Q, W flow remains constant;
§2-5.
The first law in open system
2. Flow work
Wflow=pfΔs=pV
wflow=pv
p
V
§2-5.
The first law in open system
3. 技术功
“ Wt” are expansion work and the
change of flow work for open system.
4. 轴功
“ Ws” is “ Wt” and the change of kinetic
and potential energy of fluid for open
system.
§2-5.
The first law in open system
5. Enthalpy
for flow fluid energy:
U+pV +mcf2/2+mgz
H =U+pV
unit: J, kJ
For Per unit mass:
h=u+pv
unit: J/kg, kJ/kg
§2-5.
The first law in open system
6. Energy equation for steady flow open system
, 1mc
U1+p
H
V1 f12/2, mgz1
W
Q
U2+p
H
V2 f22/2, mgz2
, 2mc
§2-5.
The first law in open system
1
2
Ein  Q  H1  m1c f 1  mgz 1
2
1
2
Eout  Ws  H 2  m2c f 2  mgz 2
2
Esystem  0
1
1
2
2
(Q  H1  m1c f 1  mgz 1 )  (Ws  H 2  mc f 2  mgz 2 )  0
2
2
§2-5.
The first law in open system
1
2
Q  H  mc f  mg z  Ws
2
Per unit mass:
1 2
1 2
(q  h1  c f 1  gz1 )  ( ws  h2  c f 2  gz 2 )  0
2
2
1
2
q  h  c f  gz  ws
2
§2-5.
The first law in open system
If neglect kinetic energy and potential energy , then:
q  h  wt
If the process is reversible, then:
q  h  vdp
This is the second equation of the first law.
§2-5.
The first law in open system
7. Energy equation for the open system
Q
Inlet flows
Out flows
1
1
2
Open system
2
……
……
i
j
W
§2-5.
The first law in open system
Energy equation for the open system
n
.
.
.
.
.
.
1 2
1 2
Q Ws   (hi  c fi  g zi ) mi   ( h j  c fj  g z j ) m j   Esystem
2
2
i
i
.
.
n
.
§2-6. Application of The Energy Equation
1. Engine
a). Turbines energy equation:
Ein-Eout=Esystem=0
Wi=H2-H1
wi=h2-h1
Q
Q≈0
, 1mc
U1+p
H
V1 f12/2, mgz1 =0
Wi
U2+p
H22V2
mcf22/2, mgz2=0
§2-6. Application of The Energy Equation
1. Engine
b). Cylinder engine energy equation:
Wt=H2-H1+Q=(U+pV) 2-(U+pV) 1 +Q
Ek1, Ep1≈0
H2
Q
H1
Ek1, Ep1≈0
Wt
§2-6. Application of The Energy Equation
2. Compressors
Energy equation:
Wc=- Wt =H2-H1
Ek1, Ep1≈0
H2
Wc
H1
Ek1, Ep1≈0
Q≈0
§2-6. Application of The Energy Equation
3. Mixing chambers
Energy equation:
m1h1 + m2h2 -m3h3=0
Mixing water:
m3h3
hot water: m2h2
Cold water: m1h1
§2-6. Application of The Energy Equation
4. Heat exchangers
Energy equation:
m3h3
m２h２
m5h5
m1h1
m4h4
m6h6
(m1h1 + m2h2 + m3h3)-(m4h4 + m5h5 + m6h6)= 0
§2-6. Application of The Energy Equation
5. Throttling valves
Energy equation:
h1 -h2 =0
h2
h1
Unit - II
Air Cycles
OTTO CYCLE
OTTO CYCLE
Efficiency is given by
 1
1
r
 1
Efficiency increases with increase in
compression ratio and specific heat
ratio (γ) and is independent of load,
amount of heat added and initial
conditions.

r
1
0
2
0.242
3
0.356
4
0.426
5
0.475
6
0.512
7
0.541
8
0.565
9
0.585
10
0.602
16
0.67
20
0.698
50
0.791
CR ↑from 2 to 4, efficiency ↑ is 76%
CR from 4 to 8 efficiency is 32.6
CR
from 8 to 16 efficiency
18.6
OTTO CYCLE
Mean Effective Pressure
It is that constant pressure which, if exerted
on the piston for the whole outward stroke,
would yield work equal to the work of the
cycle. It is given by
W
m ep 
V1  V2

 Q23
V1  V2
OTTO CYCLE
Mean Effective Pressure
We have:
 V2 
V1  V2  V1 1  
 V1 
 1
 V1 1  
r

Eq. of state:
To give:
R0 T1
V1  M
m p1
m ep  
p1m
MR0T1
1
1
r
Q23
OTTO CYCLE
Mean Effective Pressure
The quantity Q2-3/M is heat added/unit
mass equal to Q’, so
p1m
Q
R0T1
m ep  
1
1
r
OTTO CYCLE
Mean Effective Pressure
Non-dimensionalizing mep with p1 we get


 1   Q m 
m ep




1
p1
1    R0 T1 
 r
Since:
R0
 cv   1
m
OTTO CYCLE
Mean Effective Pressure
We get
m ep
Q
1

p1
cvT1  1 
1  r   1
Mep/p1 is a function of heat added, initial
temperature, compression ratio and
properties of air, namely, cv and γ
Choice of Q’
We have
Q23
Q 
M
For an actual engine: Q23  M f Qc
 FM a Qc in kJ / cycle
F=fuel-air ratio, Mf/Ma
Ma=Mass of air,
Qc=fuel calorific value
Choice of Q’
FM
Q
a
c
We now get: Q 
M
M a V1  V2
Now

M
V1
V1  V2
1
And
1
V1
r
Thus:
 1
Q  FQc 1  
r

Choice of Q’
For isooctane, FQc at stoichiometric
conditions is equal to 2975 kJ/kg, thus
Q’ = 2975(r – 1)/r
At an ambient temperature, T1 of 300K and
cv for air is assumed to be 0.718 kJ/kgK,
we get a value of Q’/cvT1 = 13.8(r – 1)/r.
Under fuel rich conditions, φ = 1.2, Q’/ cvT1 =
16.6(r – 1)/r.
Under fuel lean conditions, φ = 0.8, Q’/ cvT1
= 11.1(r – 1)/r
OTTO CYCLE
Mean Effective Pressure
Another parameter, which is of importance,
is the quantity mep/p3. This can be
obtained from the following expression:
m ep m ep 1


p3
p1 r
1
Q
1
 1
cvT1r
Diesel Cycle
Thermal Efficiency of cycle is given by
1  rc  1 
  1   1 

r   rc  1

rc is the cut-ff ratio, V3/V2
We can write rc in terms of Q’:
Q
rc 
1
 1
c pT1r
We can write the mep formula for the
diesel cycle like that for the Otto cycle in
terms of the η, Q’, γ, cv and T1:
m ep
Q
1

p1
cvT1  1 


1



1
 r 
Diesel Cycle
We can write the mep in terms of γ, r and
rc:




mep  r rc  1  r rc  1

r  1  1
p1
The expression for mep/p3 is:
m ep m ep  1 

 
p3
p1  r 
DUAL CYCLE
Dual Cycle
The Efficiency is given by

r r 1
1 
  1   1 

r  rp  1   rp rc  1

p c
We can use the same expression as
before to obtain the mep.
To obtain the mep in terms of the cut-off
and pressure ratios we have the
following expression
Dual Cycle

mep  rp r rc  1  r rp  1  r rp rc  1

r  1  1
p1




For the dual cycle, the expression for mep/p3
is as follows:
Dual Cycle

mep  rp r rc  1  r rp  1  r rp rc  1

r  1  1
p1



For the dual cycle, the expression for mep/p3
is as follows:
m ep m ep  p1 
 

p3
p1  p3 

Dual Cycle
We can write an expression for rp the
pressure ratio in terms of the peak
pressure which is a known quantity:
p3  1 
rp    
p1  r 
We can obtain an expression for rc in terms
of Q’ and rp and other known quantities as
follows:
Dual Cycle

1   Q  1 



rc 



1




 1


   cvT1r  rp 

We can also obtain an expression for rp in
terms of Q’ and rc and other known
quantities as follows:
 Q

 c T r   1  1
v 1


rp 
1   rc  
First Law of Thermodynamics Review
Steady- State First Law :
2
2




V
V
e
i


Q  W   m  he 
 gze    m  hi 
 gzi 




2
2




if only1 fluid streamexists &
kineticand potentialenergychangesare negligible:
Q  W  he  hi
rate of heat transfer Q  Qm where Q has unitsof kJ/kg in SI
power W  Wm where W (work)has unitsof kJ/kg in SI
Vapor Power Cycles
• In these types of
cycles, a fluid
evaporates and
condenses.
• Ideal cycle is the
Carnot
• Which processes here
would cause
problems?
Ideal Rankine Cycle
• This cycle follows the idea of the Carnot
cycle but can be practically implemented.
1-2 isentropic pump
3-4 isentropic turbine
2-3 constant pressure heat addition
4-1 constant pressure heat rejection
Ideal Cycle Analysis
• h1=hf@ low pressure (saturated liquid)
• Wpump (ideal)=h2-h1=vf(Phigh-Plow)
– vf=specific volume of saturated liquid at low
pressure
• Qin=h3-h2
value)
heat added in boiler (positive
– Rate of heat transfer = Q*mass flow rate
– Usually either Qin will be specified or else the
high temperature and pressure (so you can
find h3)
Ideal Cycle Analysis, cont.
• Qout=h4-h1 heat removed from condenser
(here h4 and h1 signs have been switched
to keep this a positive value)
• Wturbine=h3-h4 turbine work
– Power = work * mass flow rate
• h4@ low pressure and s4=s3
Deviations from Ideal in Real Cycles
• Pump is not ideal
 pump 
Wideal
Wactual
Wactual 
v f P2  P1 
 pump
• Turbine is not ideal
turbine  Wactual W
ideal
notethat thisis an inverseof the pump equation
• There will be a pressure drop across the boiler and
condenser
• Subcool the liquid in the condenser to prevent cavitation
in the pump. For example, if you subcool it 5°C, that
means that the temperauture entering the pump is 5°C
below the saturation temperature.
Reheat Cycle
• Allows us to increase boiler pressure
without problems of low quality at turbine
exit
Regeneration
• Preheats steam entering boiler using a
feedwater heater, improving efficiency
– Also deaerates the fluid and reduces large
volume flow rates at turbine exit.
Unit – III
AIR-COMPRESSORS






Reciprocating Compressors
Scroll Compressors
Screw Compressors
Turbo Compressors
Roller Type Compressors
Vane Type Compressors
Refrigeration Technology
Chapter10. Compressors
Main Types of Compressors



The compressor is the heart of a mechanical refrigeration system.
There is the need for many types of compressors because of the
variety of refrigerants and the capacity, location and application of
the systems.
Generally, the compressor can be classified into two basic types:
positive displacement and roto-dynamic.
Refrigeration Technology
Chapter10. Compressors



As shown in Fig.10-1, the positive
displacement family includes
reciprocating compressors and
rotary compressors.
According to the movement of
compression components, the
rotary compressors can be further
classified as scroll, screw, rollertype and vane type.
The roto-dynamic compressor
which is also called centrifugal or
turbo compressor, is classified as
radial flow and axial flow types
according to the flow
arrangement.
compressor
s
Positive
Displacement
Reciprocati
ng
Scroll
Roto-dynamic
Turbo/Centri
fugal
Rotary
Screw
rollertype
vane-type
rotary
Fig.10-1 .The classification of compressors
92
Refrigeration Technology
Chapter10. Compressors
10-1.Reciprocating Compressors
93
Refrigeration Technology
Chapter10. Compressors
1. The Construction of Reciprocating
Compressors


Reciprocating compressor
compresses the vapor by
moving piston in cylinder to
change the volume of the
compression chamber, as shown
in Fig.10-2.
The main elements of a
reciprocating compressor
include piston, cylinder, valves,
connecting rod, crankshaft and
casing.
Fig.10-2 Cutaway view of small two-cylinder reciprocating compressor
[12]
Refrigeration Technology
Chapter10. Compressors


A wide variety of compressor designs can be used on the
separable unit including horizontal, vertical, semi-radial and Vtype.
However, the most common design is the horizontal, balancedopposed compressor because of its stability and reduced
vibration.
Refrigeration Technology
Chapter10. Compressors
2. Principle of Operation




Fig. 10-3 shows single-acting piston actions in the cylinder of a
reciprocating compressor.
The piston is driven by a crank shaft via a connecting rod.
At the top of the cylinder are a suction valve and a discharge valve.
A reciprocating compressor usually has two, three, four, or six
cylinders in it.
Fig.10-3 The compression cycle
[13]
Refrigeration Technology
Chapter10. Compressors

The states of the refrigerant in a reciprocating compressor can be expressed
by four lines on a PV diagram as shown in Fig.10-4.
Discharge
volume
3
2
pressure
Clearan
ce
Suction intake volume
4
1
Piston displacement
Total cylinder volume
volume
Fig.10-4 Principle of operation of a reciprocating compressor
97
Refrigeration Technology
Chapter10. Compressors
3. Clearance Space and Clearance
Fraction



In order to prevent the piston from striking the valve plate, a
clearance volume must be allowed at the end of the piston
compression stroke.
Manufacturing design tolerances require this to allow for reasonable
bearing wear, which would effectively lengthen the stroke.
The space between the bottom and top of the valve assembly adds
extra to the clearance volume.
Refrigeration Technology
Chapter10. Compressors






The clearance volume will cause the vapor not being completely
discharged after compression.
The remaining vapor trapped in the clearance volume will re-expend
in the next suction stroke.
As a result, the volume of the vapor sucked in by the compressor in
each stroke is less than the volume the piston swept through.
So the compressor volumetric displacement must be greater than
the volume of vapor to be drawn in.
Other factors that cause reduction to the compressor capacity are:
pressure drop through valves which reduces the amount of vapor
sucked or discharged; vapor leaks around closed valves or between
the piston and cylinder; refrigerant evaporating out of oil in the
cylinder space; the vapor heated by the cylinder walls, thus,
increasing its specific volume.
99
Refrigeration Technology
Chapter10. Compressors


The performance of reciprocating compressors can be described by
volumetric efficiency.
Here we only consider the actual and the clearance volumetric
efficiencies. The actual volumetric efficiency is defined as
volume  flow  rate  entering  compressor , m3 s
va 
displacement  rate  of  compressor , m3 s
Refrigeration Technology
Chapter10. Compressors
2. Advantages and limitation






Scroll compressors can deliver high compression pressure ratio.
The pressure ratio is increased by adding spiral wraps to the scroll.
Scroll compressors are true rotary motion and can be dynamically
balanced for smooth, vibration-free, quiet operation.
They have no inlet or discharge valves to break or make noise and
no associated valve losses.
Although scroll compressors continue to expand into larger and
smaller size compressor market, some weak points of scroll
compressors could limit this trend.
One of them is that the effect of leakage at the apex of the crescent
shaped pokets could become so significant in small size
compressors that scoll compressors can not be constructed much
smaller.
Refrigeration Technology
Chapter10. Compressors
10-3. Screw Compressors
102
Refrigeration Technology
Chapter10. Compressors
2. Advantages of the screw compressor



Screw compressors are reliable and compact.
Compressor rotors can be manufactured with very small
clearances at an economic cost.
In many applications, the screw compressor offers significant
advantages over reciprocating compressors.
1.
Its fewer moving parts mean less maintenance. There is no need to
service the items such as compressor valves, packing and piston
rings, and the associated downtime for replacement.
2.
The absence of reciprocating inertial forces allows the screw
compressor to run at high speeds. So, it could be constructed more
compact.
Refrigeration Technology
Chapter10. Compressors

3.
The continuous flow of cooling lubricant allows much higher singlestage compression ratios.
4.
The compactness tends to reduce package costs.
5.
Low vibration due to reducing or eliminating pulsations by screw
technology
6.
Higher speeds and compression ratios help to maximize available
production horsepower.
A major problem with screw compressors is that the pressure
difference between entry and exit creates very large radial and
axial forces on the rotors whose magnitude and direction is
independent of the direction of rotation.
104
Refrigeration Technology
Chapter10. Compressors
10-4. Turbo Compressors
105
Refrigeration Technology
Chapter10. Compressors
1. The construction and operation of
turbo Compressors






“Turbo compressor” as understood in refrigeration industry usually refers to
a centrifugal compressor.
A schematic diagram of the centrifugal compressor is shown in Fig.10-14.
Vapor enters axially at the centre wheel 1
and flows through the passage 3 in the
impeller 2.
The pressure and absolute velocity of the
vapor rises when it passes the impeller
because of the centrifugal force.
In the stationary diffuser 4 the flow of vapor
is decelerated to further raise the vapor
pressure.
The compressed vapor is collected in the
scroll or volute 5 and discharged to the
Fig.10-14 Schematic diagram of the centrifugal compressor
1-eye, inlet cavity. 2–impeller (wheel). 3-blades (or vanes).
4-diffuser. 5-volute (scroll). 6- outlet cavity.
Refrigeration Technology
Chapter10. Compressors


Fig. 10-13 Two-stage centrifugal compressor
1-Second-stage variable inlet guide vane. 2-First-stage impeller.
3-Second-stage impeller. 4-Water-cooled motor.
5-Base, oil tank, and lubricating oil pump assembly.
6-First-stage guide vanes and capacity control.
7-Labyrinth seal. 8-Cross-over connection. 9-Guide vane actuator.
10-Volute casing. 11-Pressure-lubricated sleeve bearing. The discharge
opening is not shown.
The major elements of a
centrifugal compressor are
shown in Fig.10-13.
A turbo compressor
consists of a housing and
at least one rotor of which
the shaft is pivotally
supported by the housing,
with a free shaft end and
with a rotor connected with
the other end of the rotor
shaft.
Refrigeration Technology
Chapter10. Compressors


Fig. 10-13 Two-stage centrifugal compressor
1-Second-stage variable inlet guide vane. 2-First-stage impeller.
3-Second-stage impeller. 4-Water-cooled motor.
5-Base, oil tank, and lubricating oil pump assembly.
6-First-stage guide vanes and capacity control.
7-Labyrinth seal. 8-Cross-over connection. 9-Guide vane actuator.
10-Volute casing. 11-Pressure-lubricated sleeve bearing. The discharge
opening is not shown.
The free end of the rotor shaft
facing away from the rotor
projects into a pressure chamber
connected with the housing, and
is acted upon by a pressurized
fluid whose force of pressure
compensates for the force of the
axial thrust acting on the rotor.
Thus, the starting friction of the
compressor is lower and drive
motors of lower output target
can be utilized.
108
Refrigeration Technology
Chapter10. Compressors
10-5. Roller Type Compressors
Refrigeration Technology
Chapter10. Compressors

The roller type compressor，which is also called as “blade-type rotary compressor”
by some companies, compresses gases by revolving a steel cylindrical roller on an
eccentric shaft which is mounted concentrically in a cylinder (Fig.10-15).
Fig.10-15 Roller-type compressor [18]
110
Refrigeration Technology
Chapter10. Compressors



Fig.10-15 Roller-type compressor [18]
Because of the shaft being
eccentric, the cylinder roller
is eccentric with the cylinder
as well.
The cylinder roller touches
the cylinder wall at the point
of minimum clearance.
As the shaft turns, the roller
rolls around the cylinder wall
in the direction of shaft
rotation, always maintaining
contact with the cylinder
wall.
111
Refrigeration Technology
Chapter10. Compressors



Fig.10-15 Roller-type compressor [18]
With relation to the camshaft,
the inside surface of the
cylinder roller moves counter to
the direction of shaft rotation in
the manner of a crankpin
bearing.
A spring-loaded blade mounted
in a slot in the cylinder wall,
bears firmly against the roller at
all times.
The blade moves in and out of
the cylinder slot to follow the
roller as the latter rolls around
the cylinder wall.
wu wei-dong
112
Refrigeration Technology
Chapter10. Compressors




Cylinder heads or end-plates are used to close the cylinder at each
end and to serve as supports for the camshaft.
Both the roller and blade extend the full length of the cylinder with
only working clearance being allowed between these parts and the
end-plates.
Suction and discharge ports are located in the cylinder wall near the
blade slot, but on opposite sides.
The flow of vapor through both the suction and discharge ports is
continuous, except for the instant that the cylinder at the point of
contact between the blade and roller on one side and between the
roller and cylinder wall on the other side.
Refrigeration Technology
Chapter10. Compressors
10-6. Vane Type Compressors
114
Refrigeration Technology
Chapter10. Compressors

The vane type compressor，which is also called as “sliding vane
compressor”or “multi-vane compressor” by some companies,
employs a series of rotating vanes or blades which are installed
equidistant around the periphery of a slotted rotor (Fig.10-16).
Fig.10-16 vane-type rotary compressor.[19]
Refrigeration Technology
Chapter10. Compressors






The rotor shaft is mounted eccentrically in a steel cylinder so that the rotor
nearly touches the cylinder wall on one side, the two being separated only
by an oil film at this point.
Directly opposite this point the clearance between the rotor and the cylinder
wall is maximum.
Heads or end-plates are installed on the ends of the cylinder and to hold the
rotor shaft.
The vanes move back and forth radially in the
rotor slots as they follow the contour of the
cylinder wall when the rotor is turning.
The vanes are held firmly against the cylinder
wall by action of the centrifugal force developed
by the rotating rotor.
In some instances, the blades are spring-loaded to
obtain a more positive seal against the cylinder
wall.
Fig.10-16 vane-type rotary compressor.[19]
116
Refrigeration Technology
Chapter10. Compressors



The suction vapor drawn into the cylinder through suction ports in
the cylinder wall is entrapped between adjacent rotating vanes.
The vapor is compressed by the reduction in volume that results as
the vanes rotate from the point of maximum rotor clearance to the
point of minimum rotor clearance.
The compressed vapor is discharged from the cylinder through ports
located in the cylinder wall near the point of minimum rotor
clearance.
Refrigeration Technology
Chapter10. Compressors



The discharge ports are so located as to allow discharge of the
compressed vapor at the desired point which is the design point of
the compressor during the compressing process.
Operation of the compressor at compression ratios above or below
the design point will result in compression losses and increasing
power consumptions.
Current practice limits compression ratios to a maximum of 7 to 1.
118
Unit – IV
Refrigeration &
Air Conditioning
Objectives
• Basic operation of refrigeration and AC
systems
• Principle components of refrigeration and
AC systems
• Thermodynamic principles of refrigeration
cycle
• Safety considerations
Uses of Systems
• Cooling of food stores and cargo
• Cooling of electronic spaces and
equipment
– CIC (computers and consoles)
– Radio (communications gear)
– Radars
– ESGN/RLGN
– Sonar
• Cooling of magazines
• Air conditioning for crew comfort
Definition Review
• Specific heat (cp): Amount of heat
required to raise the temperature of 1 lb
of substance 1°F (BTU/lb) – how much
for water?
• Sensible heat vs Latent heat
• LHV/LHF
• Second Law of Thermodynamics: must
expend energy to get process to work
Refrigeration Cycle
• Refrigeration - Cooling of an object and
maintenance of its temp below that of
surroundings
• Working substance must alternate b/t
colder and hotter regions
• Most common: vapor compression
– Reverse of power cycle
– Heat absorbed in low temp region and
released in high temp region
Generic Refrigeration Cycle
Thermodynamic Cycle
Typical
Refrigeration
Cycle
Components
• Refrigerant
• Evaporator/Chille
r
• Compressor
• Condenser
• Receiver
• Thermostatic
expansion valve
(TXV)
Refrigerant
• Desirable properties:
– High latent heat of vaporization - max cooling
– Non-toxicity (no health hazard)
– Desirable saturation temp (for operating
pressure)
– Chemical stability (non-flammable/nonexplosive)
– Ease of leak detection
– Low cost
– Readily available
Evaporator/Chiller
• Located in space to be refrigerated
• Cooling coil acts as an indirect heat
exchanger
• Absorbs heat from surroundings and
vaporizes
– Latentsuperheated
Heat of Vaporization
Slightly
(10°F) – Sensible
Heatcarryover
of surroundings
ensures
no liquid
into
compressor
•
Compressor
• Superheated Vapor:
– Enters as low press, low temp vapor
– Exits as high press, high temp vapor
•
Temp: creates differential (T)
promotes heat transfer
• Press: Tsat
allows for
condensation at warmer temps
• Increase in energy provides the driving
force to circulate refrigerant through the
system
Condenser
• Refrigerant rejects latent heat to cooling
medium
• Latent heat of condensation (LHC)
• Indirect heat exchanger: seawater absorbs
the heat and discharges it overboard
Receiver
• Temporary storage space & surge
volume for the sub-cooled refrigerant
• Serves as a vapor seal to prevent vapor
from entering the expansion valve
Expansion Device
• Thermostatic Expansion Valve (TXV)
• Liquid Freon enters the expansion valve
at high pressure and leaves as a low
pressure wet vapor (vapor forms as
refrigerant enters saturation region)
• Controls:
– Pressure reduction
– Amount of refrigerant entering evaporator
controls capacity
Air Conditioning
• Purpose: maintain the atmosphere of an
enclosed space at a required temp,
humidity and purity
• Refrigeration system is at heart of AC
system
• Heaters in ventilation system
• Types Used:
• Self-contained
• Refrigerant circulating
• Chill water circulating
AC System Types
• Self-Contained System
– Add-on to ships that originally did not have
AC plants
– Not located in ventilation system (window
unit)
• Refrigerant circulating system
– Hot air passed over refrigerant cooling coils
directly
• Chilled water circulating system
– Refrigerant cools chill water
– Hot air passes over chill water cooling coils
Basic AC System
Safety Precautions
• Phosgene gas hazard
– Lethal
– Created when refrigerant is exposed to high
temperatures
• Handling procedures
– Wear goggles and gloves to avoid eye irritation and
frostbite
• Asphyxiation hazard in non-ventilated spaces
(bilges since heavier than air)
• Handling of compressed gas bottles
Unit – V
One Dimensional Compressible
flow
5.1 Introduction
Good approximation for practicing gas dynamicists
eq. nozzle flow、flow through wind tunnel & rocket engines
5.2 Governing Equations
• For a steady,quasi-1D flow
The continuity equation :
  
  v  ds 
t
s
  d


1u1 A1  2u2 A2
The momentum equation :


( v )
  

d   f  d   p  ds
s (v  ds )v  
t


s
X-dir
Y-dir
p1 A1   u A  (A
2
1 1 1
A2
1

pdA) x  p2 A2  2u22 A2
Automatically balainced
The energy equation
 
 
 q  d   pv  ds    ( f  v )d


s

V2
V2 
  [  (e  )]d    (e  )v  ds
2
2
 t
s
h  e p

2
1
2
2
u
u
h1 
 h2 
 h0  const
2
2
total enthalpy is
constant along the flow
Actually, the total enthalpy is constant along a streamline in any
adiabatic steady flow
5.3 Area-Velocity Relation
d ( uA)  0
dP d

 udu
 d 
dP
udA Adu  Aud  0
uA
∵ adiabatic & inviscid
∴ no dissipation mechanism
→ isentropic
d
du dA


0

u
A
dA
du
2
 ( M  1)
A
u
d
2
udu
u du
2 du
  2   2  M

a
au
u
Important information
1.
2.
3.
4.
M→0 incompressible flow
Au=const consistent with the familiar continuity eq for
incompressible flow
0≦M＜1 subsonic flow
an increase in velocity (du，+) is associated with a decrease
in area (dA，－) and vice versa.
M>1 supersonic flow
an increase in velocity is associated with an increase in area ,
and vice versa
M=1 sonic flow →dA/A=0
a minimum or maximum in the area
A subsonic flow is to be
accelerated isentropically
from
subsonic
to
supersonic
Supersonic flow is to be
decelercted isentropically
from
supersonic
to
subsonic
Application of area-velocity
relation
1.Rocket engines