chap 1 basic concepts in thermodynamics

8/4/2019 Chap 1 Basic Concepts in Thermodynamics

1/10

CLB 20703Chemical EngineeringThermodynamics

Chapter 1:

Basic Concepts in Thermodynamics

Objective of Chapter 1

To introduce students to some of the

fundamental concepts and definitions thatare used in the study of engineering

thermodynamics

Outline

Introduction

Dimensions and units Measure of amount

Force

Temperature

Pressure

Work Energy

Heat


2/10

What is Thermodynamics?

Stems from the Greek words therme (heat)

and dynamics (force or power)

In early 19th century: consideration of themotive power of heat and the capacity ofhot bodies to produce work

Today: dealing generally with energy andwith relationships among the properties ofmatter

Scopes of Thermodynamics

First and second laws of thermodynamics

To cope with variety of problems especiallyin the calculation of energy changes, heatand work requirements for processes

Property values are essential to applicationof thermodynamics

Development of generalized correlations

to provide property estimates in theabsence of data

System in Thermodynamics

Always starts with the identification of aparticular body of matter

This body of matter is called the system


3/10

Property in Thermodynamics

Thermodynamic state

is defined by a fewmeasurablemacroscopic properties

The fundamentaldimensions, e.g.

length, time, mass,temperature etc.

Dimensions and Units

The dimensions require the definition ofscales of measure specific units of size

Primary units are codified as theInternational System of Units (SI)

Multiples and decimal fractions of SI unitsare designated by prefixes

Other systems of units (e.g. English

engineering system) are related to SI unitsby fixed conversion factors

SI Units and Prefixes

Unit Symbol

second s

meter m

kilogram kg

kelvin K

mole mol

Mu lt ip le Pr efix Sy mb ol

10-15 femto f

10-12 pico p

10-9 nano n

10-6 micro

10-3 milli m

10-2 centi c

102 hecto h

103 kilo k

106 mega M

109 giga G

1012 tera T

1015 peta P


4/10

Measures of Amount/Size

Three common measures of amount/size:

Mass, mNumber of moles, n = m/M

Total volume, Vt

Intensive thermodynamic variables: Independent of size of system

Specific volume, V = Vt/m

Molar volume, V = Vt/n

Specific density, = V-1 = m/Vt

Force

From Newtons second law:

Force = mass x acceleration (F = ma)

where gc = 32.174 (Ibm)(ft)(Ibf)-1(s)-2

Force (F) SI EES

Equation F = ma F = ma/gcUnit N / kg m s-2 (Ibf)

Example 1:

A box weighs 730 N in Melaka, where the

local acceleration of gravity is g = 9.792m/s2.

What are the boxs mass and weight on

the moon, where g = 1.67 m/s2?


5/10

Temperature

Four temperature scales:

(i) Kelvin scale (ii) Celsius scale

(iii) Rankine scale (iv) Fahrenheit scale

Relations among temperature scales:

tC = TK 273.15

T (R) = 1.8 TK

t (F) = T (R) 459.67

t (F) = 1.8 tC + 32

Temperature (contd)

Pressure

P is defined as the normal force exerted by afluid per unit area of the surface

P = F/A = mg/A

Pressure (P) SI EES

Unit N m-2/Pa (Pascal) (psi)


6/10

Pressure (contd)

When using manometers for pressure

measurement, P is also expressed as theequivalent height of a fluid column

P = F/A

= mg/A

= (AL)g/A

= Lg

Pressure (contd)

Most pressure gauges only give readings of

gauge pressures

Absolute P = Gauge P + Atmosphere P

Absolute P must be used in thermodynamicscalculations

Example 2:

A dead-weight gauge with 1-cm-diameterpiston is used to measure pressures veryaccurately. In a particular instance a mass

of 6.14 kg (including piston and pan)brings it into balance.

(i) If the local acceleration of gravity is 9.82m/s2, what is the gauge pressure beingmeasured?

(ii) If the barometric pressure is 748 (torr),what is the absolute pressure?


7/10

Example 3:

At 27 C the reading on a manometer filled

with mercury is 60.5 cm. The localacceleration of gravity is 9.784 m/s2 and the

density of mercury is 13.53 g/cm3 at 27 C.

To what pressure does this height of mercury

correspond?

Work

W is performed whenever a force actsthrough a distance

W is positive when the displacement is inthe same direction as the applied force orvice versa

dlFdW =

Work (W) SI EES

Unit N m / J (Joule) (ft Ibf)

Work (contd)

W is also performed when there is achange in volume of fluid (compression or

expansion)

The minus sign is required because thevolume change is negative

A

VdPAdW

t

=

tdVPdW = =

t

t

V

V

tdVPW

2

1


8/10

Work (contd)

Energy

Kinetic energy

Work is done when a body acceleratesfrom velocity of u

1to u

2, which is equal to

the change in kinetic energy of the body

2

2

1muEK =

==

222

22

1

2

2mumumu

W

Kinetic Energy (EK) SI EES

Equation EK = mu2/2 EK = mu

2/2gc


Energy (contd)

Potential energy

Work required to raise a body is the

product of force exerted and the change in

elevation from z1

to z2

Potential Energy (EP) SI EES

Equation EP = mzg EP = mzg/gc


mzgEP=

( )mzggmzgmzW ==12


9/10

Energy Conservation

Work done on a body is equal to the

change in a quantity of energy

Work performed can be recovered bycarrying out reverse process and returning

the body to its initial condition

( )mzgEW P ==

==

2

2muEW K

0=+PK

EE

Energy Conservation (contd)

Work is energy in transit, not residing in abody, and can be converted into anotherform of energy

Work exists only during energy transferfrom the surroundings to the system, or thereverse

In contrast, kinetic and potential energy

reside with the system

Example 4:

An elevator with a mass of 2,500 kg rests at alevel 10 m above the base of an elevator shaft. Itis raised to 100 m above the base of the shaft,where the cable holding it breaks.

The elevator falls freely to the base of the shaftand strikes a strong spring. The spring isdesigned to bring the elevator to rest and holdthe elevator at the position of maximum springcompression.

Assuming the entire process to be frictionless,and taking g = 9.8 m/s2, calculate:


10/10

Example 4:

a. The potential energy of the elevator in its initial

position relative to the base of the shaft

b. The work done in raising the elevator

c. The potential energy of the elevator in itshighest position relative to the base of the shaft

d. The velocity and kinetic energy of the elevatorjust before it strikes the spring

e. The potential energy of the compressed spring

Heat

Heat always flows from higher temperatureto a lower one

Rate of heat transfer is proportional to thetemperature difference between two bodies

Heat is never stored within a body

It exists only as energy in transit between a

system and its surroundings

chap 1 basic concepts in thermodynamics

Documents