intro thermodynamics

8/4/2019 Intro Thermodynamics

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Prepared by: Engr.Alexander Hamilton S. Atienza

Introduction to

Thermodynamics



Introduction

Thermodynamics

-It is a branch of physical science that treats various

phenomena of energy and other related properties

of matter



Def inition of terms

System

-It is the term given to the collection of matter

under consideration enclosed within the boundarySurrounding-It is the region outside the boundary of the space

and matter external to a system



Def inition of Terms

Surrounding

-sunlight,air

System

Boundary

System



Application of Thermodynamics to

building structures

-Heat Transfer

-Air Conditioning

-Refrigeration

-Application on Machines(Pump, Compressor,Turbine,Condenser,Evaporator,Turbine

etc)




Types of Systems

Open System

-It is a system in whichthere is flow of matter

through the boundary




Types of Systems

Closed system

-It is a system in whichthere is no transfer of matter

across the boundary




Types of Systems

Isolated system

-It is a system in which neither mass nor energycross the boundaries and it is not influenced by

the surroundings




State-It is a set of values of properties of a system thatmust be specified to reproduce the system

-The individual parameters are known as statevariables, state parameters orthermodynamic variables-O

nce a sufficient set of thermodynamic variableshave been specified, values of all other properties of the system are uniquely determined.




State functions-State functions, also called thermodynamic

variables, state quantities, or a functions of state describe the momentary condition of a

thermodynamic system.



Definition of Terms

State Functions- Regardless of the path by

which a system goes from

one state to another ³ i.e.,

the sequence of intermediate

states ³ the total change in

any state variable will be the same

2

Initial State

Final State



Application-Building Structure

y Refrigeration Systems

y Airconditioning Systems

y Pump and Compressor systems

y Heat Engines



Ref rig eration System



Airconditioning System



Parts Identif ication

Evaporator-It is a device used to turn

(or allow to turn) the liquid form of

some chemical into its gaseous form.

For example, an evaporator is used inan air conditioning system to allow the

compressed cooling chemical (for

example, Freon) to evaporate from

liquid to gas, absorbing heat in the

process.




Condenser-It is a device or unit used to condense a substance

from its gaseous to its liquid state, typically by cooling it




Compressor- is a mechanical device that increases the

pressure of a gas by reducing its volume




Expansion Valve-It is a

component in refrigeration and

air conditioning systems that

controls the amount of

refrigerant flow into the

evaporator thereby controlling

the superheating at the outlet

of the evaporator. Thermal

expansion valves are oftenreferred to generically as

"metering devices"



Simple Steam Power Plant



Steam Power Plant Cycle

Power plants generate electrical power by using fuels like coal,

oil or natural gas. A simple power plant consists of a boiler,

turbine, condenser and a pump. Fuel, burned in the boiler

and superheater, heats the water to generate steam.

The steam is then heated to a superheated state in the

superheater. This steam is used to rotate the turbine which

powers the generator.

Electrical energy is generated when the generator windings

rotate in a strong magnetic field. After the steam leaves the

turbine it is cooled to its liquid state in the condenser. The

liquid is pressurized by the pump prior to going back to the

boiler.



Main Parts of a Steam Power Plant

A simple power plant is described by a Rankine

Cycle




Steam turbine-It is is a

mechanical device that

extracts thermal energy from

pressurized steam, and

converts it into rotary

motion. Its modern

manifestation was invented

by Sir Charles Parsons in

1884.




Condenser-It is the commonly

used term for a water-cooled

shell and tube heat exchanger

installed on the exhaust steam

from a steam turbine in

thermal power stations




Boiler ² It is a closed

vessel in which

water or other

fluid is heated. The

heated or

vaporized fluid

exits the boilerfor use in various

processes orheating

applications.




Pump- It is a device

used to move fluids,

such as liquids, gases

or slurries.A pumpdisplaces a volume by

physical or

mechanical action.



Heat Engine

heat engine-It is a system that

performs the conversion of

heator thermal energy to

mechanical work. It does this

by bringing a working

substance from a high

temperature state to a lower

temperature state



Heat Engines

Carnot heatengine is a

hypothetical enginethat operates on the

reversible Carnotcycle. The basic

model for this

engine was

developed byNicolas Léonard Sadi

Carnot in 1824




State functionsPressure

TemperatureVolume

Internal Energy

E

nthalpyEntropy




State functionsPressure-I

t is is an effect whichoccurs when a force is

applied on a surface.

pressure is the amount

of force acting on a unit

area.

Area

Force




Temperature-It indicates the hotness or coldness of a body

Absolute zero-It is the temperature at which the molecules stop

moving

Absolute temperature-It is the temperature measured from absolute zero




Pressure(P)Atmospheric Pressure

-It is the pressure obtained from barometric reading

Gage Pressure-It is the pressure measured from the level of atmospheric

pressure by most pressure recording instrument like

pressure gage




Pressure(P)Absolute Pressure

-It is the true pressure measured above a perfect vacuum.

It is the sum of the atmospheric and gage pressure

Pabs=Patm+Pgage




Temperature(T)-It indicates the hotness or coldness of a body

Absolute zero-It is the temperature at which the molecules stop

moving

Absolute temperature-It is the temperature measured from absolute zero




Volume(V)-it refers to the volume of the working fluid in the

system

Internal Energy(U)It is the energy stored within the body. It is the sum of

all the kinetic energies of all of its constituents

particles plus the sum of all the potential energies of interaction among these particles




Enthalpy(H)-It is the heat energy transferred to a substance at aconstant pressure process

H=U + PV

where H = Enthalpy P=Absolute PressureU= Internal Energy V= Volume




Entropy(S)-It is the measure of randomness of the molecules of

the substance




Process-It describes that a system undergoes any changeA thermodynamic process can be visualized by graphicallyplotting the changes to the system's state variables. Theseprocesses can be represented by the PV (Pressure ² Volume) Diagram and T-S( Temperature- Entropy)Diagram




Isobaric Process-It is a thermodynamic process wherein pressure stays

constant

P

V

1 2

T

S

1

2




Isochoric Process-It is a thermodynamic process wherein volume stays

constant

P

V

1

2

T

S

1

2




Isothermal Process-It is a thermodynamic process wherein temperature

stays constant

P

V

1 2

T

S

1

2




Isentropic Process-It is a thermodynamic process wherein entropy stays

constant

P

V

1

2

T

S

1

2



Law of Thermodynamics

Zeroth Law of Thermodynamics- If two systems are each in thermal equilibrium with

a third, they are also in thermal equilibrium with each

other



Law of Thermodynamics

First Law of Thermodynamics� Energy Entering=

� Energy Leaving

System

KE1PE1H1

KE2PE2H2

Q WQ+KE1+PE1+H1=W+KE2+PE2+H2

Where Q = Heat

KE= Kinetic Energy

PE=Potential Energy

H =U +PV= Enthalpy

W=Work



Law of ThermodynamicsFirst Law of Thermodynamics

Heat(Q)= it is a form of energy associated

kinetic random motion of large number of

molecules

Q is positive if heat is added on the system

otherwise, it the heat is released from the system,

Q is negative

Work(W)= is the energy transferred by the

system to another that is accounted for by

changes in the external generalized mechanical

constraints on the system. As such,

thermodynamic work is a generalization of the

concept of mechanical work in mechanics.

W is positive if work is done by the system and

negative if work is done on the system

System

+Q +W

-W-Q



Laws of Thermodynamics

Second Law of ThermodynamicsThe statement of the second law is facilitated by usingthe concept of heat engines. Heat engines work in a

cycle and convert heat into work. A thermal reservoir isdefined as a system which is in equilibrium and largeenough so that heat transferred to and from it doesnot change its temperature appreciably.



Laws of ThermodynamicsSecond Law of Thermodynamicsy Heat Enginey Heat engines usually work between two thermal reservoirs, the

low temperature reservoir and the high temperature reservoir. Theperformance of a heat engine is measured by its thermal efficiency ,which is defined as the ratio of work output to heat input, i.e., =

W/Q 1, where W is the net work done, and Q 1 is heat transferredfrom the high temperature reservoir.

y Heat Pumps

y H eat pumps transfer heat from a low temperature reservoir to ahigh temperature reservoir using external work, and can beconsidered as reversed heat engines.



Laws of ThermodynamicsKevin Planck Statement- It is impossible to construct a

heat engine which will operate

continuously and convert all the

heat it draws from a reservoir into

work.

Clausius Statement-It is impossible to construct a heat

pump which will transfer heat from

a low temperature reservoir to a high

temperature reservoir without using

external work.

TH

TLSystem

QH W

TH TL

SystemQH Q

L




Second Law of ThermodynamicsA change in the entropy (S) of a system is the

infinitesimal transfer of heat (Q ) to a closed system

driving a reversible process, divided by theequilibrium temperature (T ) of the system

S=Q/T




Third Law of Thermodynamics-The total entropy of pure substance approaches zero

as the absolute thermodynamic temperature

approaches zeroAs Tabs zero

Stotal zero



Formula

Carnot Efficiency:

e=(TH-TL)/TH=(QH-QL)/QH

First Law of Thermodynamics:

Q+KE1+PE1+H1=W+KE2+PE2+H2

Second Law of Thermodynamics:

S= Q/T

Work:

W=PV



Problems

1. A closed vessel contains air at a temperature of 30ÜC. It

was heated at 60ÜC with pressure of 759 mm Hg, what is the

initial pressure of the vessel

2. A heat engine is operated between limits of 1370ÜC and

260ÜC. Engine is supplied with 14,142 J. Find the Carnot

efficiency.

3. A volume of 450 cm3 of air is measured at a pressure of 740

mm Hg and a temperature of 20 ÜC.



Problems

1. A closed vessel contains air at a temperature of 30ÜC. It

was heated at 60ÜC with pressure of 759 mm Hg, what is the

initial pressure of the vessel

2. A heat engine is operated between limits of 1370ÜC and

260ÜC. Engine is supplied with 14,142 J. Find the Carnot

efficiency.

3. A volume of 450 cm3 of air is measured at a pressure of 740

mm Hg and a temperature of 20 ÜC.Find the moles of air



Problems

4. The thermal efficiency of a particular engine operating on an

ideal cycle is 35%. Calculate the heat supplied per 4320 kJ of

work developed.

5. Determine the average Cp value in kJ/kg-K of a 1 kg gas if

522 kJ of heat is necessary to raise the temperature from 300

K to 800 K making the pressure constant.

6. The enthalpy of air is increased by 140 J in a compressor. The

mass of air is 1kg. The power input is 48 J. What is the heat

loss from the compressor in J.



Problems

7. What is the maximum thermal efficiency possible for a

power cycle operating between 1200ÜF and 225ÜF.

8. A pressure gage registers 50 psig in a region where the

barometer is 14.25 psia. Find the absolute pressure in psia.

9. An air compressor delivers 0.20 m3 of air at a pressure of

850 kPa and 31ÜC into an air reservoir. Taking the gas

constant R of air as .287 kJ/kg-K, calculate the mole of air

delivered.

10. Gas is enclosed in a cylinder with a weighted piston as the

top boundary. The gas is heated and expands from a volume

of 0.04m 3to 0.10m 3at a constant pressure of 200 kPa.

Calculate the work done by the system

intro thermodynamics

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