termo-Chapter 2

THERMODYNAMICS

CHAPTER 1 BASIC CONCEPTS

1.0 INTRODUCTION

• THERMODYNAMICS IS THE SCIENCE THAT

DEALS WITH HEAT AND WORK AND

PROPERTIES OF SUBSTANCES RELATED TO

HEAT AND WORK

• THERMODYNAMICS DEALS WITH THE

NECESSARY METHODS FOR CONVERTING

HEAT ENERGY FROM AVAILABLE

RESOURCES (SUCH AS CHEMICAL FUEL) TO

MECHANICAL WORK

• RELATIONSHIPS BETWEEN HEAT, WORK AND

THE PROPERTIES OF SUBSTANCES ARE

EXPRESSED IN THE FORM OF BASIC LAWS

KNOWN AS THE LAWS OF THERMODYNAMICS

• THE BASIS OF THERMODYNAMICS IS

EXPERIMENTAL OBSERVATION

• IT CAN BE OBSERVED THAT HEAT AND WORK

ARE TWO FORMS OF ENERGY THAT ARE

CLOSELY RELATED TO EACH OTHER AND

THIS OBSERVATION PROVIDES THE BASIS

FOR THE FIRST LAW OF THERMODYNAMICS

• IT CAN ALSO BE OBSERVED THAT HEAT

FLOWS FROM A HIGH TEMPERATURE OBJECT

TO A LOW TEMPERATURE OBJECT AND THE

REVERSE PROCESS IS ONLY POSSIBLE WITH

SOME HELP (IN THE FORM OF WORK). THIS

CONCEPT IS IMPORTANT IN THE

DEVELOPMENT OF THE SECOND LAW OF

THERMODYNAMICS

• EXAMPLES OF ENGINEERING PLANT WHICH

INVOLVES THERMODYNAMIC ANALYSIS ARE

THE STEAM POWER PLANT, THE INTERNAL

COMBUSTION ENGINE, REFRIGERATION

SYSTEMS, ETC.

1.1 BASIC CONCEPTS AND DEFINITIONS

• SYSTEM:

o A thermodynamic system is defined as a

quantity of matter or a region in space chosen

for study

• SURROUNDINGS:

o The mass or region outside the system

• BOUNDARY:

o The surface that separates the system from its

surroundings

o Boundaries can be fixed or movable

o Boundaries can be real (cylinder walls and

piston surfaces in an internal combustion

engine)

o Boundaries can also be imaginary (cross

sections of pipes at the entrance and exit of

turbines)

• CLOSED SYSTEM

o Consists of a fixed amount of mass

o No mass can cross its boundary

o No mass can enter or leave a closed system

o Volume does not have to be fixed

o But energy, in the form of heat or work can

cross the boundary

o Example as shown in Figure 1.1 where the gas

in the cylinder is considered the system

o If a Bunsen burner is placed under the

cylinder, the temperature of the gas will

increase and the piston will rise

o As the piston rises, the boundary of the system

moves

o Heat and work crosses the boundary of the

system during this process but not mass

Weights/load

Piston

Gas System’s boundary

(system)

Figure 1.1 Example of a closed system

o In the special case where even energy is not

allowed to cross the boundary, that system is

called an ISOLATED SYSTEM

• OPEN SYSTEM OR CONTROL VOLUME

o A properly selected region in space

o Used when the analysis involves devices with

mass flow into and/or out of the device such as

a compressor, turbine, nozzle, heat exchanger,

etc.

o The procedure in such an analysis is to specify

a control volume that surrounds the device

under consideration

o The boundaries of a control volume are called

a control surface

o Mass as well as heat and work can flow across

the control surface

o Figure 1.2 shows an example of an open

system.

control surface AIR COMPRESSOR MOTOR

Figure 1.2 Example of a Control Volume

• MICROSCOPIC POINT OF VIEW

o Considers behaviours of individual molecules

o Involves a large number of equations to

explain the behaviour of a system

• MACROSCOPIC POINT OF VIEW

o Concerned with the gross or average effects of

many molecules

o These effects can be perceived by our senses

and measured by instruments such as a

pressure gauge

Heat

Air out

(high pressure) Air in

(low pressure)

Work

• CONTINUUM

o Concerned with systems that contain many

molecules

o Since we are not concerned with the behaviour

of individual molecules, we can treat the

substance as being continuous and this is

called a continuum

o This is valid as long as the size of the system

we deal with is large relative to the space

between the molecules

o Not valid in high-vacuum technology

1.2 PROPERTIES AND STATE OF A

SUBSTANCE

• If we consider a given mass of water, we recognize

that this water can exist in various forms

• If it is a liquid initially, it may become a vapour

when it is heated, or a solid when it is cooled

• Thus we speak of the different phases of a

substance

• A phase is defined as a quantity of matter that is

homogeneous throughout

• When more than one phase is present, the phases

are separated from each other by the phase

boundaries

• In each phase the substance may exist at various

pressures and temperatures or to use the

thermodynamic term, in various states

• The state may be identified or described by certain

observable, macroscopic properties

• Examples of these properties are temperature,

pressure and density

• Each of the properties of a substance in a given

state has only one definite value, and these

properties always have the same value for a given

state

• A property is defined as any quantity that depends

on the state of the system and is independent of the

path by which the system arrived at the given state

• The state is specified or described by the properties

• The number of properties required to fix the state of

a system is given by the STATE POSTULATE:

The state of a simple compressible system is

completely specified by two independent, intensive

properties

• A system is called a simple compressible system

when surface effects, magnetic effects, and

electrical effects are not significant

• Thermodynamic properties can be divided into 2

general classes, intensive and extensive properties

• Intensive properties are those that are independent

of the size of the system, such as temperature,

pressure and density

• Extensive properties are those whose values depend

on the size of the system. Mass(m), Volume(V) and

total Energy(E) are extensive properties

• If a quantity of matter in a given state is divided

into 2 equal parts, each part will have the same

value of intensive properties as the original, and

half the value of the extensive properties

• Extensive properties per unit mass are called

specific properties

• Examples are specific volume ( v = V/m) and

specific total energy (e = E/m)

• Thermodynamics deals with equilibrium states

• The word equilibrium implies a state of balance

• In an equilibrium state there are no unbalanced

potentials (or driving forces) within the system

• There are many types of equilibrium

• A system is in thermal equilibrium if the

temperature is the same throughout the entire

system

• A system is in mechanical equilibrium if there is no

change in pressure at any point of the system with

time

• A system is in chemical equilibrium if its chemical

composition does not change with time

• A phase equilibrium is when the mass of each phase

of the system (if the system involves two phases)

reaches an equilibrium level and stays there

• When a system is in equilibrium as regards all

possible changes of state, we say that the system is

in thermodynamic equilibrium

1.3 PROCESSES AND CYCLES

• Any change that a system undergoes from one

equilibrium state to another is called a process

• The series of states through which a system passes

during a process is called the path of the process

• To describe a process completely, one should

specify the initial and final states of the process, the

path it follows and the interactions with the

surroundings

Property A

State 2

Process path

State 1

Property B

Figure 1.3 A Process

• When a process proceeds in such a manner that the

system remains infinitesimally close to an

equilibrium state at all times, it is called a quasi-

static or quasi-equilibrium process

• a quasi-equilibrium process can be viewed as a

sufficiently slow process that allows the system to

adjust itself internally so that properties in one part

of the system do not change any faster than those at

other parts

• All the states the system passes through during a

quasi-equilibrium process may be considered as

equilibrium states

• Referring back to Figure 1, if the weights on the

piston are small and are taken off one by one, the

process could be considered as quasi-equilibrium

• If all the weights were removed at once, the piston

would rise rapidly until it hits the stops. This would

be a non-quasi-equilibrium process

• For a non-quasi-equilibrium process, the system

would not be in equilibrium at any time

• For a non-quasi-equilibrium process, we are limited

to a description of the system before the process

occurs and after the process is completed and

equilibrium is restored

• For a non-quasi-equilibrium process, we are not

able to specify each state through which the system

passes and this process is denoted by a dashed line

between the initial and final states instead of a solid

line

• The prefix iso- is used to designate a process for

which a particular property remains constant

• An isothermal process is a process during which the

temperature T remains constant

• An isobaric process is a process during which the

pressure P remains constant

• An isochoric/isometric process is a process during

which the specific volume v remains constant

• When a system in a given initial state goes through

a number of different changes of state or processes

and finally returns to its initial state, the system has

undergone a THERMODYNAMIC CYCLE

• At the end of a cycle, all the properties have the

same value they had at the beginning

• Steam(water) that circulates through a steam power

plant undergoes a cycle

Property A

2

1

3

Property B

Figure 1.4 A Thermodynamic cycle

1.4 UNITS

• In SI, the units for mass, length and time are as

follows:

• mass : kilogram (kg)

• length : meter (m)

• time : seconds (s)

• In SI, the force unit is the newton (N):

o It is derived from Newton’s second law

o F = m x a

o Force = mass x acceleration

o It is defined as the force required to accelerate

a mass of 1 kg at a rate of 1m/s2

o 1 N = 1 kg.m/s2

o Weight is the gravitational force applied to a

body and is given by, W = mg (N)

o The weight of a unit volume of a substance is

called the specific weight w and is determined

from w = ρg where ρ is the density

o Prefixes

Prefix Symbol Multiple

Factor

Example

Kilo K 103 1KN=103N

Mega M 106 1MW=106W

Giga G 109 1 Gpa=109Pa

Tera T 1012 1 TW=1012W

• In engineering, all equations must be dimensionally

homogeneous. That is, every term in an equation

must have the same unit

• Checking units can serve as a valuable tool to spot

errors/mistakes

1.5 DENSITY AND VOLUME

• Density is defined as mass per unit volume

ρ = m/v (kg/m3)

• Density is the reciprocal of specific volume v

v = V/m = 1/ρ (m3/kg)

• ρ is an intensive property

• Unit for v is m3/kg

• Unit for ρ is kg/m3

• Unit for Volume (V) is m3 or another frequently

used unit for volume is litre (L)

• 1 Litre = 10-3 m3 or 0.001 m3

1.6 TEMPERATURE AND THE ZEROTH

LAW OF THERMODYNAMICS

• It is not easy to give an exact definition for

temperature

• Fortunately, several properties of materials change

with temperature in a repeatable and predictable way,

and this forms the basis for accurate temperature

measurement

• The commonly used mercury-in-glass thermometer,

for example, is based on the expansion of mercury

with temperature

• The equality of temperature is the only requirement

for thermal equilibrium

• The zeroth law of thermodynamics states that if two

bodies are in thermal equilibrium with a third body,

they are also in thermal equilibrium with each other

• By replacing the third body with a thermometer, the

zeroth law can be restated as two bodies are in thermal

equilibrium if both have the same temperature reading

even if they are not in contact

Temperature Scales

• Temperature scales enable us to use a common basis

for temperature measurement

• Temperature scales are based on some easily

reproducible states such as the freezing and boiling

points of water, which are also called the ice point and

the steam point, respectively.

• The temperature scales used in the SI and in the

English system today are the Celcius scale and the

Fahrenheit scale

• On the Celcius scale, the ice and steam point are

assigned the values of 0 and 100°C, respectively

• In thermodynamics, it is very desirable to have a

temperature scale that is independent of the properties

of any substance or substances

• Such a temperature scale is called a thermodynamic

temperature scale, which is developed later in

conjunction with the second law of thermodynamics

• The thermodynamic temperature scale in the SI is the

Kelvin scale

• The temperature unit on this scale is the Kelvin

• The lowest temperature on the Kelvin scale is 0 K

• The thermodynamic temperature scale in the English

system is the Rankine scale

• A temperature scale that turns out to be identical to the

Kelvin scale is the ideal gas temperature scale

• The Kelvin scale is related to the Celcius scale by

T(K) = T(°C) + 273.15

• The Rankine scale is related to the Fahrenheit scale by

T(R) = T(°F) + 459.67

• The temperature scales in the two unit systems are

related by

T(R) = 1.8 T(K)

T(°F) = 1.8 T(°C) + 32

• Therefore, when we are dealing with temperature

differences ∆Τ, the temperature interval on both scales

is the same

• Raising the temperature of a substance by 10°C is the

same as raising it by 10 K

∆T(K) = ∆T(°C)

∆T(R) = ∆T(°F)

• Some thermodynamic relations involve the

temperature T and often the question arises of whether

it is in K or °C

• If the relation involves temperature differences (such

as a = b∆Τ ) , it makes no difference and either can be

used

• However, if the relation involves temperatures only

instead of temperature differences (such as a = bΤ ) then K must be used

1.7 PRESSURE • Pressure is defined as the force exerted by a fluid per

unit area

• We speak of pressure only when we deal with a gas or

a liquid

• The counterpart of pressure in solids is stress

• Since pressure is defined as force per unit area, it has

the unit of newtons per square meter (N/m2), which is

called a pascal (Pa), that is

1 Pa = 1 N/m2

• Three other pressure units commonly used in practice

are bar, standard atmosphere, kilogram-force per

square centimeter

1 bar = 105 Pa = 0.1 Mpa = 100kPa

1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars

1 kgf/cm2 = 9.807 N/cm2 = 9.807 × 104 N/m2 = 9.807×104 Pa

= 0.9807 bar

= 0.96788 atm

• The actual pressure at a given position is called the

absolute pressure

• It is measured relative to absolute vacuum (i.e.,

absolute zero pressure)

• Most pressure-measuring devices, indicate the

difference between the absolute pressure and the local

atmospheric pressure. This difference is called the

gage pressure

• Pressures below atmospheric pressure are called

vacuum pressure and are measured by vacuum gages

that indicate the difference between the atmospheric

pressure and the absolute pressure

• Absolute, gage, and vacuum pressures are all positive

quantities and are related to each other by

Pgage = Pabs – Patm (for pressures above Patm)

Pvac = Patm – Pabs (for pressures below Patm)

• In thermodynamic relations and tables, absolute

pressure is almost always used

• Pressure is the compressive force per unit area, and it

gives the impression of being a vector

• However, pressure at any point in a fluid is the same

in all directions. That is, it has magnitude but not a

specific direction, and thus a scalar quantity

1.8 Variation of Pressure with Depth

• To obtain a relation for the variation of pressure with

depth, consider a rectangular fluid element of height

∆z, length ∆x, and unit depth (into the paper) in

equilibrium, as shown in the Figure 1.5 below:

z

x

Figure 1.5 Free Body Diagram of a rectangular fluid

element in equilibrium

W

P1

P2

∆z

∆x

• Assuming the density of the fluid ρ to be constant, a

force balance in the vertical z-direction gives

∑ FZ = maz = 0 : P2 ∆x – P1 ∆x – ρg ∆x ∆z = 0

where W = mg = ρg ∆x ∆z is the weight of the fluid

element. Dividing by ∆x and rearranging gives

∆P = P2 – P1 = ρg ∆z = γ ∆z

where γ = ρg is the specific weight of the fluid

• Pressure in a fluid increases linearly with depth

• For a given fluid, the vertical distance ∆z is sometimes

used as a measure of pressure, and it is called the

pressure head

• For small to moderate distances, the variation of

pressure with height is negligible for gases because of

their low density. The pressure in a tank containing a

gas, for example, may be considered to be uniform

since the weight of the gas is too small to make a

significant difference. Also, the pressure in a room

filled with air may be assumed to be constant (Figure

1.6 below)

Figure 1.6 Pressure in tank containing a gas

AIR

(a 5 metre high room)

Ptop = 1 atm

Pbottom = 1.006 atm

• Pressure in a fluid is independent of the shape or cross

section of the container. It changes with the vertical

distances, but remains constant in other directions

• Therefore, the pressure is the same at all points on a

horizontal plane in a given fluid

• The pressures are not the same if two points cannot be

interconnected by the same fluid, although they are at

the same depth

1.9 THE MANOMETER

• We notice that an elevation change of ∆z of a fluid

corresponds to ∆P/ρg, which suggests that a fluid

column can be used to measure pressure differences

• A device based on this principle is called a

manometer

• It is commonly used to measure small and moderate

pressure differences

• A manometer mainly consists of a glass or plastic U-

tube containing one or more fluids such as mercury,

water, alcohol or oil.

• Heavy fluids such as mercury are used if large

pressure differences are anticipated

• Consider the manometer shown in the figure below

that is used to measure the pressure in the tank. The

pressure anywhere in the tank and at position 1 has the

same value

Figure 1.7 The Basic Manometer

• Since pressure in a fluid does not vary in the

horizontal direction within a fluid, the pressure at

point 2 is the same as the pressure at 1, P2 = P1. The

pressure at point 2 is

P2 = Patm + ρgh

where ρ is the density of the fluid in the tube

• For manometers with multiple fluids of different

densities stacked on top of each other, remember:

• (1) the pressure change across a fluid column of height

h is ∆P = ρgh ,

• (2) pressure increases downward in a given fluid and

decreases upward (i.e., Pbottom > Ptop), and

• (3) two points at the same elevation in a continuous

fluid at rest are at the same pressure

• The pressure at the bottom of the tank in the figure

below can be determined by starting at the free surface

where the pressure is Patm, and moving downwards

until we reach point 1 at the bottom, and setting the

result equal to P1. It gives

Patm + ρ1gh1 + ρ2gh2 + ρ3gh3 = P1

Figure 1.8 Stacked-up fluid layers

Fluid 1

Fluid 2

Fluid 3

1

P atm

h1

h2

h3

• Manometers are particularly well-suited to measure

pressure drops across a horizontal flow section

between two specified points due to the presence of a

device such as a valve or heat exchanger or any

resistance to flow

• The working fluid can be either a gas or a liquid

whose density is ρ1. The density of the manometer

fluid is ρ2, and the differential fluid height is h

• A relation for the pressure difference P1 – P2 can be

obtained by starting at point 1 with P1 and moving

along the tube by adding or subtracting the ρgh terms

until we reach point 2, and setting the result equal to

P2

P1 + ρ1g (a + h) – ρ2gh – ρ1ga = P2

P1 – P2 = ( ρ2 – ρ1 ) gh

• When the fluid flowing in the pipe is a gas, then ρ1<ρ2

and the relation above simplifies to P1 – P2 = ρ2gh

1.10 OTHER PRESSURE MEASUREMENT

DEVICE

• Another type of commonly used mechanical pressure

measurement device is the Bourdon tube, which

consists of a hollow metal tube bent like a hook whose

end is closed and connected to a dial indicator needle

• When the tube is open to the atmosphere, the tube is

undeflected, and the needle on the dial at this state is

calibrated to read zero (gage pressure). When the fluid

inside the tube is pressurized, the tube stretches and

moves the needle in proportion to the pressure applied

• Modern pressure sensors, called pressure

transducers, are made of semiconductor materials

such as silicon and convert the pressure effect to an

electrical effect such as change in voltage, resistance,

or capacitance

• Pressure transducers are smaller and faster, and they

are more sensitive, reliable, and precise than their

mechanical counterparts. They can measure pressure

from less than a millionth of 1 atm to several

thousands of atm

1.11 BAROMETER AND THE

ATMOSPHERIC PRESSURE

• The atmospheric pressure is measured by a device

called a barometer; thus, the atmospheric pressure is

often referred to as the barometric pressure

• A frequently used pressure unit is the standard

atmosphere, which is defined as the pressure produced

by a column of mercury 760 mm in height at 0ºC

(ρHg = 13,595 kg/m3) under standard gravitational

acceleration (g = 9.807 m/s2)

• The standard atmospheric pressure for example, is 760

mmHg (29.92 inHg) at 0ºC. The unit mmHg is also

called the torr

• The standard atmospheric pressure Patm changes from

101.325 kPa at sea level to 89.88, 79.50, 54.05, 26.5,

and 5.53 kPa at altitudes of 1000, 2000, 5000, 10,000,

and 20,000 meters, respectively

• Remember that the atmospheric pressure at a location

is simply the weight of the air above that location per

unit surface area

• For a given temperature, the density of air is lower at

high altitudes, and a given volume contains less air

and less oxygen. So, we tire more easily and

experience breathing problems at high altitudes