VELTECH Dr.R.R&Dr.S.R TECHNICAL UNIVERSITY AERO ENGINEERING THERMODYNAMICS SEM:IIIYEAR:II PREPARED...

VELTECH Dr.R.R&Dr.S.R TECHNICAL UNIVERSITY

AERO ENGINEERING THERMODYNAMICSSEM:III YEAR:II

PREPARED BYMr.B.Narendhiran

DEPARTMENT OF AERONAUTICALASSISTANT 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

“Classical” means Equipartition Principle applies: each molecule has average energy ½ kT per in thermal equilibrium.

Internal Energy of a Classical ideal gasInternal Energy of a Classical ideal gas

At room temperature, for most gases:

monatomic gas (He, Ne, Ar, …) 3 translational modes (x, y, z)

kTEK2

3

diatomic molecules (N2, O2, CO, …) 3 translational modes (x, y, z) + 2 rotational modes (wx, wy)

kTEK2

5

pVkTNU2

3

2

3

Internal Energy of a Gas

pVkTNU2

3

2

3

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?

J105.705.0105.1 537 mPa

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 = )

WORK done by the system on the environment

Thermal reservoir

HEAT is the transfer of thermal energy into the system from the surroundings

There are two ways to change the internal energy of a system:

Work and Heat are process energies, not state functions.

Wby = -Won

Q

Work Done by An Expanding Gas

The expands slowly enough tomaintain thermodynamic equilibrium.

PAdyFdydW

Increase in volume, dV

PdVdW +dV Positive Work (Work isdone by the gas)

-dV Negative Work (Work isdone on the gas)

A Historical Convention

Energy leaves the systemand goes to the environment.

Energy enters the systemfrom the environment.

+dV Positive Work (Work isdone by the gas)

-dV Negative Work (Work isdone on the gas)

Total Work Done

PdVdW

f

i

V

V

PdVW

To evaluate the integral, we must knowhow the pressure depends (functionally)on the volume.

Pressure as a Function of Volume

f

i

V

V

PdVW

Work is the area underthe curve of a PV-diagram.

Work depends on the pathtaken 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 finalstates and the path taken between these states.

Work done by a Gas

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.

We generally assume quasi-static processes (slow enough that p and T are well defined at all times):

This is just the area under the p-V curve

f

i

V

Vby dVpW

V

p p

V

p

V

dWby = F dx = pA dx = p (A dx)= p dV

Consider a piston with cross-sectional area A filled with gas. For a small displacement dx, the work done by the gas is:

dx

When a gas expands, it does work on its environment

What is Heat?

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 energyQ < 0 system loses thermal energy

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 …

(like cash transfers into and out of your bank account)

Q U

An Extraordinary Fact

The work done depends on the initial and finalstates and the path taken between these states.

BUT, the quantity Q - W does not dependon 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.

-- Heat and work are forms of energy transfer and energy is conserved.

The First Law of Thermodynamics

(FLT)

U = Q + Won

work doneon the system

change intotal internal energy

heat added

to system

or

U = Q - Wby

State Function Process Functions

1st Law of Thermodynamics

• statement of energy conservation for a thermodynamic system• internal energy U is a state variable• W, Q process dependent

system done work : positive

system addedheat : positive

by

to

W

Q

WQU

The First Law of Thermodynamics

bydWdQdE int

What this means: The internal energy of a systemtends to increase if energy is added via heat (Q)and decrease via work (W) done by the system.

ondWdQdE int

. . . and increase via work (W) done on the system.

onby dWdW

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

Isochoric (constant volume)

Thermodynamic processes of an ideal gas( FLT: U = Q - Wby )

V

p

1

2

pVTNkU

0pdVWby

FLT: UQ Q

Temperature changes

Isobaric (constant pressure)

V

p

1 2

VpTNkU FLT: VpWUQ by 1

VppdVWby

Q

p

Temperature and volume change

Isothermal (constant temperature)

2

1

V

V 1

2by V

VnNkTdVpW

0U

FLT: byWQ

p

V

1

2

( FLT: U = Q - Wby )

V

1p

Q

Thermal Reservoir

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


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.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=mcf

2/2 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 system1. The equation

Ein-Eout=ΔEsystem

WQ

§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 system2. Flow work

Wflow=pfΔs=pV

wflow=pvp

V


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.


5. Enthalpy

for flow fluid energy:

+mcf2/2+mgzU+pV

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 system6. Energy equation for steady flow open system

U1+p1V1H1, mcf1

2/2, mgz1

U2+p2V2H2 , mcf22/2, mgz2

QW


12

111 2

1mgzcmHQE fin

22

222 2

1mgzcmHWE fsout

0 systemE

0)2

1()

2

1( 2

2221

2111 mgzmcHWmgzcmHQ fsf


sf WzmgcmHQ 2

2

1

0)2

1()

2

1( 2

2221

211 gzchwgzchq fsf

Per unit mass:

sf wzgchq 2

2

1


vdphq

If neglect kinetic energy and potential energy , then:

twhq

If the process is reversible, then:

This is the second equation of the first law.

W

§2-5. The first law in open system7. Energy equation for the open system

Inlet flows Out flows

Q

1

2

… …

i

Open system

1

2

……

j

§2-5. The first law in open systemEnergy equation for the open system

...2

...2

...

)2

1()

2

1( systemjjfjj

n

i

iifi

n

iis EmzgchmzgchWQ

§2-6. Application of The Energy Equation1. Enginea). Turbines energy equation:

Ein-Eout=Esystem=0

Wi=H2-H1

wi=h2-h1

U1+p1V1H1, mcf1

2/2, mgz1

U2+p2V2H2

mcf22/2, mgz2

Q WiQ≈0

=0

=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

Q Wt

H1

H2

Ek1, Ep1≈0

Ek1, Ep1≈0


2. Compressors

Energy equation:

Wc=- Wt =H2-H1

Wc

H1

H2

Ek1, Ep1≈0

Ek1, Ep1≈0

Q≈0


3. Mixing chambers

Energy equation:

m1h1 + m2h2 -m3h3=0

Cold water: m1h1

hot water: m2h2

Mixing water: m3h3


4. Heat exchangers

Energy equation:

m1h1

m ２ h ２

m3h3

m4h4

m5h5

m6h6

(m1h1 + m2h2 + m3h3)-(m4h4 + m5h5 + m6h6)= 0


5. Throttling valves

Energy equation:

h1 -h2 =0

h1

h2

Unit - II

Air Cycles

OTTO CYCLE

OTTO CYCLE

Efficiency is given by

Efficiency increases with increase in compression ratio and specific heat ratio (γ) and is independent of load, amount of heat added and initial conditions.

1

11

r

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 CYCLEMean 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

21

32

21

VV

Q

VV

Wmep


We have:

Eq. of state:

To give:

rV

V

VVVV

11

1

1

1

2121

1

101 p

T

m

RMV

r

TMRmp

Q

mep1

1

10

132


The quantity Q2-3/M is heat added/unit mass equal to Q’, so

r

TRmp

Q

mep1

1

10

1


Non-dimensionalizing mep with p1 we get

Since:

1011

1

1

TR

mQ

rp

mep

10 vcm

R


We get

Mep/p1 is a function of heat added, initial temperature, compression ratio and properties of air, namely, cv and γ

11

1

1

11

rTc

Q

p

mep

v

Choice of Q’

We have

For an actual engine:

F=fuel-air ratio, Mf/Ma

Ma=Mass of air,

Qc=fuel calorific value

M

QQ 32

cyclekJinQFM

QMQ

ca

cf

/

32

Choice of Q’

We now get:

Thus:

M

QFMQ ca

rV

VVAnd

V

VV

M

MNow a

11

1

21

1

21

rFQQ c

11

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


Another parameter, which is of importance, is the quantity mep/p3. This can be obtained from the following expression:

1

11

11

13

rTcQrp

mep

p

mep

v

Diesel CycleThermal Efficiency of cycle is given by

rc is the cut-ff ratio, V3/V2

We can write rc in terms of Q’:

1

111

1c

c

r

r

r

11

1

rTc

Qr

pc

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:

11

1

1

11

rTc

Q

p

mep

v

Diesel CycleWe can write the mep in terms of γ, r and

rc:

The expression for mep/p3 is:

11

11

1

r

rrrr

p

mep cc

rp

mep

p

mep 1

13

DUAL CYCLE

Dual Cycle

The Efficiency is given by

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

11

111

1cpp

cp

rrr

rr

r

Dual Cycle

For the dual cycle, the expression for mep/p3

is as follows:

11

111

1

r

rrrrrrrr

p

mep cppcp

Dual Cycle

For the dual cycle, the expression for mep/p3

is as follows:

11

111

1

r

rrrrrrrr

p

mep cppcp

3

1

13 p

p

p

mep

p

mep

Dual Cycle

We can write an expression for rp the pressure ratio in terms of the peak pressure which is a known quantity:

We can obtain an expression for rc in terms of Q’ and rp and other known quantities as follows:

rp

prp

1

1

3

Dual Cycle

We can also obtain an expression for rp in terms of Q’ and rc and other known quantities as follows:

111

11

pvc rrTc

Qr

c

vp r

rTcQ

r1

111

First Law of Thermodynamics Review

SIin kJ/kg of units has (work) W wherepower

SIin kJ/kg of units has Q wherefer heat trans of rate

:negligible are changesenergy potential and kinetic

& exists stream fluid 1only if

22

:LawFirst State-Steady22

mWW

mQQ

hhWQ

gzV

hmgzV

hmWQ

ie

ii

iee

e

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 2-3 constant pressure heat

addition

3-4 isentropic turbine 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 heat added in boiler (positive value)– 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

• Turbine is not ideal

• 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.

pump

factual

actual

idealpump

PPvWW

W 12

equation pump theof inversean is that thisnote ideal

actualturbine W

W

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

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.

Chapter10. Compressors

92


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, roller-type 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.


Rotary Turbo/Centrifugal

Roto-dynamicPositive Displacement

Reciprocating

compressors

roller-type

vane-type rotary

Scroll Screw

Fig.10-1 .The classification of compressors

93


10-1.Reciprocating Compressors



1. The Construction of Reciprocating Compressors

Fig.10-2 Cutaway view of small two-cylinder reciprocating compressor[12]


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.


A wide variety of compressor designs can be used on the separable unit including horizontal, vertical, semi-radial and V-type.

However, the most common design is the horizontal, balanced-opposed compressor because of its stability and reduced vibration.



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]


97


Fig.10-4 Principle of operation of a reciprocating compressor


2

4

3

Clearance

Discharge volume

Suction intake volume

Total cylinder volume

Piston displacement

volume

pressure

1

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.


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.


99


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.



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

3

3

,

,va

volume flow rate entering compressor m s

displacement rate of compressor m s



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.


102


10-3. Screw 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.


104


3. The continuous flow of cooling lubricant allows much higher single-stage 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.


105


10-4. Turbo 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 delivery pipe 6.

Fig.10-14 Schematic diagram of the centrifugal compressor1-eye, inlet cavity. 2–impeller (wheel). 3-blades (or vanes).4-diffuser. 5-volute (scroll). 6- outlet cavity.


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.


Fig. 10-13 Two-stage centrifugal compressor1-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.

108

Refrigeration TechnologyChapter10. Compressors

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.

Fig. 10-13 Two-stage centrifugal compressor1-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.


10-5. Roller Type Compressors


110


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]

111



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.

112


wu wei-dong



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.


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.


114


10-6. Vane Type 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]


116


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.

Fig.10-16 vane-type rotary compressor.[19]


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.


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.


118


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.


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

TypicalRefrigeration

Cycle

Components

• Refrigerant • Evaporator/Chiller • 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/non-explosive)– Ease of leak detection– Low cost– Readily available

• Commonly use FREON (R-12, R-114, etc.)

Evaporator/Chiller

• Located in space to be refrigerated

• Cooling coil acts as an indirect heat exchanger

• Absorbs heat from surroundings and vaporizes– Latent Heat of Vaporization– Sensible Heat of surroundings

• Slightly superheated (10°F) - ensures no liquid carryover 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 :

222111 AuAu

s

dt

sdv

The momentum equation :

s s

sdpdfdt

vvsdv

)(

)(

2222221

21111 )( 2

1

AuApApdAuAp x

A

A

Automatically balainced

X-dir

Y-dir

The energy equation

s

dvfsdvpdq )(

s

dsvV

edV

et

)

2()]

2([

22

consthu

hu

h 0

22

2

21

1 22

peh

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 Relation0)( uAd

uA

AudAduudA

0 udu

d

d

dPdP

0A

dA

u

dud

u

duM

ua

duu

a

udud 2

2

2

2

∵ adiabatic & inviscid no dissipation mechanism∴

→ isentropic

u

duM

A

dA)1( 2

Important information1. M→0 incompressible flow

Au=const consistent with the familiar continuity eq for incompressible flow

2. 0 M≦ ＜ 1 subsonic flow

an increase in velocity (du ， +) is associated with a decrease in area (dA，－ ) and vice versa.

3. M>1 supersonic flowan increase in velocity is associated with an increase in area , and vice versa

4. 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

VELTECH Dr.R.R&Dr.S.R TECHNICAL UNIVERSITY AERO ENGINEERING THERMODYNAMICS SEM:IIIYEAR:II PREPARED...

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Transcript of VELTECH Dr.R.R&Dr.S.R TECHNICAL UNIVERSITY AERO ENGINEERING THERMODYNAMICS SEM:IIIYEAR:II PREPARED...