Thermodynamics:

Definition

The science that deals with heat and work and those properties of matter that relate to heat and work.

Thermodynamics is study of 3 ES....Energy , Equilibrium, and Entropy.

Historical milestones

Thermodynamics has a long history; unfortunately, it was not blessed with the crispness of development that mechanics realized with Newton. In fact, its growth is filled with false steps, errors, and debate that continues to this day.

Some of the milestones of its development are given here: •

First century AD: Hero of Alexandria documents many early thermal engines.

• 1593: Galileo develops a water thermometer.

• 1650: Otto von Guericke designs and builds the first vacuum pump.

• 1662: Robert Boyle develops his law for isothermal ideal gases

. • 1760s: Joseph Black develops calorimetry.

• 1780s: James Watt improves the steam engine.

• 1848: William Thomson (Lord Kelvin) postulates an absolute zero of temperature.

• 1850: Rudolf Julius Emanuel Clausius formalizes the second law of thermodynamics.

• 1865: Clausius introduces the concept of entropy.

Laws of Thermodynamics

Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics states that if two systems are in thermodynamic equilibrium with a third system, the two original systems are in thermal equilibrium with each other. Basically, if system A is in thermal equilibrium with system C and system B is also in thermal equilibrium with system C, system A and system B are in thermal equilibrium with each other.

First Law of Thermodynamics

The First Law of Thermodynamics states that energy can be converted from one form to another with the interaction of heat, work and internal energy, but it cannot be created nor destroyed, under any circumstances.

Second law of Thermodynamics

There are two statements on the second law of thermodynamics which are;

1. Kelvin- Plank Statement

2. Clausius Statement

Kelvin-Planck Statement

It is impossible for a heat engine to produce a network in a complete cycle if it exchanges heat only with bodies at a single fixed temperature.

Clausius’s Statement

It is impossible to construct a device operating in a cycle that can transfer heat from a colder body to warmer without consuming any work.

Heat pump and Refrigerator works on Clausius’s statement.

Refrigerator

Refrigerator is a device in which heat is absorbed (Q2) from sink to produce cooling effect . For that refrigerator needs to consume power (W) and heat is rejected (Q1) . So refrigerator is power absorbing device.

Solved Examples

Ex. 1 A heat pump uses 300 J of work to remove 400 J of heat from the low-temperature reservoir. How much heat is delivered to a higher temperature reservoir?

Solution:

W = 300 J

Q_C = 400 J

Q_{H =} W + Q_C

Q_{H =} 300 J + 400 J

Q_{H =} 700 J

Heat delivered to the higher temperature reservoir is 700 J.

2. A reversible heat engine receives 4000 KJ of heat from a constant temperature source at 600 K . If the surroundings is at 300K. determine the (a) the availability of heat energy, (b) Unavailable heat.

Solution:

Q₁ = 4000 KJ

T₁ = 600K

T₀ = 300 K

Change in entropy = 4.44 KJ/K

The availability of heat energy,

A = Q₁ – T₀(ΔS)

A = 4000 – 300(4.44)

A = 2,668 KJ.

Unavailable heat (U.A) = T₀ (ΔS)

(U.A) = 300 (4.44)

(U.A) = 1332 KJ.

(a) the availability of heat energy (A) = 2668 KJ

(b)Unavailable heat (U.A) = 1332 KJ

Ex 3 - Heat is transferred to a heat engine from a heat source at a rate of 80 MW. If the rate of waste heat rejection to sink is 50 MW, determine the net power output and the thermal efficiency for this heat engine.

Solution

Given QH = 80 MW

QL = 50 MW.

We know that the net power output is the difference between the heat input and the heat rejected

(cyclic device) Wnet,out =QH + QL = 80 – 50 MW= 30 MW

The net work output is 30 mW.

The thermal efficiency is the ratio of the net work output and the heat input.

ηth = Wnet,out/QH =30/80 = 0.375 •

The thermal efficiency is 0.375 or 37.5 %

Ex 4 .The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2 kW, determine (a) the coefficient of performance of the refrigerator and (b) the rate of heat rejection to the room that houses the refrigerator.

Solution:

Given

QL = 360 kJ/min = 6 kJ/Sec (kW)

W net = 2 kW

COP of the refrigerator,

COPR=Desired effect/work input=QL/Wnet,in

=(360/60 kJ/s)/2 = 3

The COP of the refrigerator is 3(3 kJ of heat is removed per kJ of work supplied).

The rate of heat rejection can be obtained by applying the first law of thermodynamics

QH=QL+Wnet,in=6 kW + 2 KW = 8 kW.

Modes of Heat Transfer

Introduction:

The science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and makes no reference to how long the process will take. But in engineering, we are often interested in the rate of heat transfer, which is the topic of the science of heat transfer. Heat energy can be transferred from one body to the other or from one location in a body to the other. Study of the techniques and methods adopted to transfer heat energy is known as ‘Heat Transfer’. To facilitate heat transfer between 2 bodies there needs to be a temperature difference between them.This means that these bodies must be a 2 different temperatures one higher than the other to allow heat to flow from one body to the other.

This means that no heat transfer occurs between 2 bodies which are at the same temperature. At the same time, it is very important to note that heat only flows from a body at higher temperature to a body at a lower temperature. Although this may look obvious, this law is very important from the point of view of thermodynamics.

There are three modes of heat transfer namely Conduction, Convection and Radiation.

Conduction

Conduction refers to the heat transfer that occurs across the medium. Medium can be solid Thermal conduction is the transfer of internal energy by microscopic collisions of particles and movement of electrons within a body. Conduction takes place in all phases: solid, liquid, and gas. The rate at which energy is conducted as the heat between two bodies depends on the temperature difference (and hence temperature gradient) between the two bodies and the properties of the conductive interface through which the heat is transferred.

Heat transfer by Conduction

Convection

Convection refers to the heat transfer that will occur between a surface and moving fluid when they are at different temperatures.Convection is the transfer of heat due to the bulk movement of molecules within fluids (gases and liquids), including molten rock . Convection includes sub-mechanisms of advection (directional bulk-flow transfer of heat), and diffusion (non-directional transfer of energy or mass particles along a concentration gradient).

Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but that is called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other.

Radiation

In Radiation in the absence of intervening medium, there is net heat transfer between two surfaces at different temperatures in the form of electromagnetic waves.

Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. Light, radio, and microwaves are types of radiation. Atoms with unstable nuclei are said to be radioactive. In order to reach stability, these atoms give off, or emit, the excess energy or mass. These emissions are called radiation.

Fourier’s Law of Heat Conduction

According to Fourier’s law, the rate of heat flow, q , through a homogeneous solid is directly proportional to the area A, of the section at the right angles to the direction of the heat flow, and to the temperature difference ∇T along the path of heat flow.

Mathematically, it can be written as q = − k ∇T

Thermal conductivity

Thermal conductivity (k) is the intrinsic property of a material which relates its ability to conduct heat. Thermal conductivity is defined as the quantity of heat (Q) transmitted through a unit thickness (x) in a direction normal to a surface of unit area (A) due to a unit temperature gradient (∆T) under steady state conditions and when the heat transfer is dependant only on temperature gradient.

So mathematically it can be written as dT • dx = q k (W/m K)

where q = Rate of heat flow .

q = −k • ∇T , where k is the thermal conductivity .

Newtons law of Cooling

Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings. The law is frequently qualified to include the condition that the temperature difference is small and the nature of heat transfer mechanism remains the same. As such, it is equivalent to a statement that the heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. In convective heat transfer, Newton's Law is followed for forced air or pumped fluid cooling, where the properties of the fluid do not vary strongly with temperature.

Internal Combustion Engine

Heat engine:

A heat engine is a device which transforms the chemical energy of a fuel into thermal energy

and uses this energy to produce mechanical work. It is classified into two types-

(a) External combustion engine

(b) Internal combustion engine

External combustion engine:

In this engine, the products of combustion of air and fuel transfer heat to a second fluid which

is the working fluid of the cycle.

Examples:

Steam engine or a steam turbine plant,

Internal combustion engine:

In this engine, the combustion of air and fuels take place inside the cylinder and are used as

the direct motive force. It can be classified into the following types:

1. According to the basic engine design- (a) Reciprocating engine (Use of cylinder piston

arrangement), (b) Rotary engine (Use of turbine)

2. According to the type of fuel used- (a) Petrol engine, (b) diesel engine, (c) gas engine

(CNG, LPG), (d) Alcohol engine (ethanol, methanol etc)

3. According to the number of strokes per cycle- (a) Four stroke and (b) Two stroke engine

4. According to the method of igniting the fuel- (a) Spark ignition engine, (b) compression

ignition engine and (c) hot spot ignition engine

5. According to the working cycle- (a) Otto cycle (constant volume cycle) engine, (b) diesel

cycle (constant pressure cycle) engine, (c) dual combustion cycle (semi diesel cycle) engine.

6. According to the fuel supply and mixture preparation- (a) Carburetted type (fuel supplied

through the carburettor), (b) Injection type (fuel injected into inlet ports or inlet manifold,

fuel injected into the cylinder just before ignition).

7. According to the number of cylinder- (a) Single cylinder and (b) multi-cylinder engine

8. Method of cooling- water cooled or air cooled

9. Speed of the engine- Slow speed, medium speed and high speed engine

10. Cylinder arrangement-Vertical, horizontal, inline, V-type, radial, opposed cylinder or

piston engines.

11. Valve or port design and location- Overhead (I head), side valve (L head); in two stroke

engines: cross scavenging, loop scavenging, uniflow scavenging.

12. Method governing- Hit and miss governed engines, quantitatively governed engines and

qualitatively governed engine

14. Application- Automotive engines for land transport, marine engines for propulsion of

ships, aircraft engines for aircraft propulsion, industrial engines, prime movers for electrical

generators.

Main components IC engines:

Cylinder: It is the main part of the engine inside which piston reciprocates to and fro.

The ordinary engine is made of cast iron and heavy duty engines are made of steel

alloys or aluminum alloys. In the multi-cylinder engine, the cylinders are cast in one block

known as cylinder block.

Cylinder head: The top end of the cylinder is covered by cylinder head over which inlet and

exhaust valve, spark plug or injectors are mounted. A copper or asbestos gasket is provided

between the engine cylinder and cylinder head to make an air tight joint.

Piston: Transmit the force exerted by the burning of charge to the connecting rod. Usually

made of aluminium alloy which has good heat conducting property and greater strength at

higher temperature.

Figure 1 shows the different components of IC engine.

Fig. 1. Different parts of IC engine

Piston rings: These are housed in the circumferential grooves provided on the outer surface

of the piston and made of steel alloys which retain elastic properties even at high temperature.

Connecting rod: It converts reciprocating motion of the piston into circular motion of the

crank shaft, in the working stroke. The special steel alloys or aluminium alloys are used for the manufacture of connecting rod.

Crankshaft: It converts the reciprocating motion of the piston into the rotary motion with the

help of connecting rod. The special steel alloys are used for the manufacturing of the

crankshaft. It consists of eccentric portion called crank.

Crank case: It houses cylinder and crankshaft of the IC engine and also serves as sump for

the lubricating oil.

Flywheel: It is big wheel mounted on the crankshaft, whose function is to maintain its speed

constant. It is done by storing excess energy during the power stroke, which is returned

during other stroke.

Terminology used in IC engine:

1. Cylinder bore (D): The nominal inner diameter of the working cylinder.

2. Piston area (A): The area of circle of diameter equal to the cylinder bore.

3. Stroke (L): The nominal distance through which a working piston moves between two

successive reversals of its direction of motion.

4. Dead centre: The position of the working piston and the moving parts which are

mechanically connected to it at the moment when the direction of the piston motion is

reversed (at either end point of the stroke).

(a) Bottom dead centre (BDC): Dead centre when the piston is nearest to the crankshaft.

(b) Top dead centre (TDC): Dead centre when the position is farthest from the crankshaft.

5. Displacement volume or swept volume (Vs): The nominal volume generated by the

working piston when travelling from the one dead centre to next one and given as,

Vs=A × L

6. Clearance volume (Vc): the nominal volume of the space on the combustion side of the

piston at the top dead centre.

7. Cylinder volume (V): Total volume of the cylinder. V= Vs + Vc

1. Compression ratio (r): It is ratio of total volume to the clearance volume of cylinder.

Four stroke engine:- Working

Cycle of operation completed in four strokes of the piston or two revolution of the piston.

1. Suction stroke (suction valve open, exhaust valve closed)-charge consisting of

fresh air mixed with the fuel is drawn into the cylinder due to the vacuum pressure

created by the movement of the piston from TDC to BDC.

2. Compression stroke (both valves closed)-fresh charge is compressed into clearance volume by the return stroke of the piston and ignited by the spark for combustion. Hence pressure and temperature is increased due to the combustion of fuel.

3. Expansion stroke (both valves closed)-high pressure of the burnt gases force the

piston towards BDC and hence power is obtained at the crankshaft.

4. Exhaust stroke (exhaust valve open, suction valve closed)- burned gases expel out

due to the movement of piston from BDC to TDC.

Two stroke Engine - Working

Cycle of operation completed in two strokes of the piston and one revolution of crankshaft.

No piston stroke for suction and exhaust operations

-Suction is accomplished by air compressed in crankcase or by a blower

-Induction of compressed air removes the products of combustion through exhaust ports

-Transfer port is there to supply the fresh charge into combustion chamber

Figure represents operation of two stroke engine.

Steam Generator (Boiler)

Definition

A steam boiler or steam generator is a closed vessel in which water is heated, vaporized and converted into steam at a pressure higher than atmospheric pressure.

A Boiler is the biggest and most critical part of a thermal power plant.

Definition of Boiler according to IBR Act 1923 (Indian Boiler Regulation), A steam boiler means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure.

Applications of Steam Generators

· Operating steam engines.

· Operating steam turbines.

· Operating reciprocating pumps.

· Industrial process work in chemical engineering.

· For producing hot water required to be supplied to room in very cold areas.

· In thermal power stations.

· The heat content of the steam is large and thus it is suitable for process heating in many industries like sugar mills, textile mills, dairy industry and also in chemical industries.

Impotant Terms in Steam Generator

1. Boiler shell: The boiler shell consists of a hollow cylindrical body made up of steel plates riveted or welded together.

2. Furnace: Furnace is that part of the boiler in which the fuel is conveniently burned to produce heat. This heat is utilized in generating steam in the boiler.

3. Grate: The grate is a space on which the fuel is burnt. It consist of a combination of several cast-iron bars so arranged that the fuel may be placed on it. Some space is always provided in between two consecutive bars so that may flow to the fuel from below the great and ashes may drop into the ash pit provided beneath the Grate. Grate may be circular or rectangular in shape.

4. Grate area: The area of the great upon which the fuel burns is called great area. Grate area is always measured in square meters.

5. Heating surface: The heating surface is the surface of a boiler which is exposed to hot gases on one side and water of the other.

6. Water space and steam space: Water space is the volume of the boiler which is occupied by water. The remaining space is called steam space because it is needed for storage of steam in the boiler until it id s drawn off through the steam pipe.

7. Flue gases: Flue gases are hot gases produced due to the combination of fuel in the boiler furnace. Flue gas usually contains water vapor (H2O), Carbon dioxide (CO2), Carbon monoxide (CO), Nitrogen (N2). Flue gas includes complete and incomplete products of combustion of fuels.

Classification of Steam Genearators

· According to the circulation of gases:

1. Fire-tube boiler

2. Water-tube boiler

Fire-tube boiler:

Fire tube boilers are those boilers in which hot gases produced by the combination of fuel in the boiler furnace while on their way to chimney pass through a number of tubes (called fuel tubes or smoke tubes) which are immersed in water.

Heat is transferred from the hot gasses to water through the walls of tubes.

Example of fire tube boilers are Cochran boiler, locomotive boiler etc.

Fire tubes boilers are also known as a smoke tube boiler.

Water-tube boiler:

Water-tube boilers are those boilers in which water flows through a number of tubes (called water tubes) and the hot gases produced by the combustion of fuel in the boiler furnace while on their way to chimney pass surrounding the tubes.

The heat from the hot gases is transferred to the water through the walls of the water tubes.

Examples of water tube boilers are Bab-cock and Wilcox boiler, Benson boiler, etc.

· According to Circulation of water:

1. Free circulation

2. Forced circulation

Free circulation:

In any water heating vessel heat is transmitted from one place to another not by condition but by convection because water is a bad conductor of heat.

Let vessel containing water be heated at its bottom, as the water in the bottom portion is heated therefore its density becomes reduced in comparison to the density of water in the upper portion of the vessel, as a result, the less dense water at the bottom portion of the vessel rise up and comparatively more dense and cold water at the upper portion of the vessel comes down to take its place and thus a convection current is set up in the water until temperature off all water becomes the same.

The method of circulation of water described above is known as free circulation.

In boilers like Lancashire, Babcock, and Wilcox, etc. free circulation of water takes place.

he advantages of free circulation are:

1. Free circulation of water helps to maintain a uniform temperature true everywhere within the boiler so that unequal expansion of various parts of the boiler is prevented.

2. Free circulation of water facilities the escape of steam from the heating surface as soon as it formed. If steam does not escape quickly after its formation the boilerplates do not remain constantly in touch with water and as a result, these plates may be overheated.

Forced Circulation:

In Forced circulation, pumps are used to maintains the continuous flow of water in the boiler. In such a case, the circulation of water takes place due to pressure created by the pump.

The forced circulation system is adopted in more high pressure, high capacity boilers of all of which are water tube type boiler.

· Advantages of forced circulation:

The advantages of forced circulation are:

1. The rate of heat transfer from the flue gases to the water higher.

2. Tubes having comparatively smaller diameters can be used. This reduces the overall weight of the boiler.

3. The number of boiler drums required may be reduced.

4. less scale formation in the boilers is required.

5. Steam can be quickly generated.

6. The fluctuation of load can be easily met without taking the help of any complicated controlled device.

7. Chance of overheating of the boilerplates in minimum.

8. Weight per unit mass of steam generated is less.

· According to the number of tubes used:

According to the number of tubes, Boilers may be classified as:

1. Single tube boiler

2. Multi-tube boiler

Single tube boiler:

Cornish boiler may be termed as a single tumbler boiler because it has only one flue tube.

Multi-tube boiler:

Cochran boiler may be termed as multi-tube boiler because it has a number of flue tubes.

According to nature use, boilers are classified as

1. Stationary boilers

2. locomotive boilers

3. Marine boilers.

Stationary boilers:

For the generation of thermal power and for process work (in chemical, sager and textile industries) boilers used are called stationary boiler.

Locomotive boilers:

Boilers used in locomotive steam engines are called locomotive boilers.

Marine boilers:

Boilers used in steamships are called marine boilers.

· According to the nature of the fuels used:

According to the nature of the fuel used boiler may be:

1. Fuel-fired

2. Gas fired

3. Liquid fuel fired

4. Electrically fired

5. Nuclear fired

Volex boilers use oil fuel.

· According to the pressure of the boiler:

1. High-pressure boiler

2. Medium-pressure boiler

3. Low-pressure boiler

High-pressure boiler:

The pressure of the boiler above 80 bar.

Medium-pressure boiler:

It has a working pressure of steam from 20 bar to 80 bar. It is used for power generation or process heating.

Low-pressure boiler:

This type of boiler produces steam at 15-20 bar pressure. This is used for process heating.

· According to the position of the axis of the boiler shell:

According to the position of the axis of the boiler shell, boilers are classified as:

1. Vertical boiler

2. Horizontal boiler

Vertical boiler:

If the boiler axis is vertical, it is called a vertical boiler. For example, Cochran boiler.

Horizontal boiler:

If the boiler axis is horizontal, it is called a horizontal boiler.

For example, Lancashire boiler.

Types of Fuel Used in Boiler:

· Solid Fuels:

Wood, Coal, Briquettes (a block of compressed coal dust ), Pet Coke, Rice Husk.

· Liquid Fuels:

LDO (Light Diesel Oil), Furnace oil.

· Gaseous Fuels:

LPG (Liquified Petroleum Gas), LNG (Liquified Natural Gas), PNG (Piped Natural Gas) can be used to carry out the combustion for a specific purpose.