English for thermal engineering

where, Cp is specific heat of the object at constant pressure; and m is mass of the object.. If the heating process occurs while the substance is maintained at a constant volume or is su

Contents

Chapter 1: Introduction to Thermodynamics 4

1 Closed and open system: 4

1.1 Closed system 4

1.2 Open system: 4

2 Pressure 5

2.1 Absolute pressure: 5

2.2 Atmospheric pressure: 5

2.3 Gage pressure (Pg) 6

3 Temperature and the Zeroth Law 7

4 Heat transfer, Q [J] 8

4.1 Latent heat 8

4.2 Sensible heat 8

4.3 Modes of heat transfers: 9

5 Work, W [J] 10

6 Energy E [J] 12

Chapter 2 Energy and Environment 14

2.1 Energy and environment 14

2.2 Acid Rain Formation 15

2.3 The Greenhouse Effect 15

2.4 Global Warming and Climate Change 16

2.5 Negative impacts of global warming 17

2.5.1 Increase in average temperatures and temperature extremes 17

2.5.2 Extreme weather events 17

2.5.3 Ice melt 17

2.5.4 Social effects 17

Chapter 3: The laws of thermodynamics 19

3.1 The first law of thermodynamics 19

3.2 The second law of Thermodynamics 21

3.2.1 Introduction to the second law of thermodynamics 21

3.2.2 The second law of thermodynamics states: 22

3.3 Thermodynamic cycles 22

Chapter 4: Reversed cycles and refrigeration/air conditioning system 24

4.1 The fundamentals of refrigeration 24

4.1.1 THE REVERSED CARNOT CYCLE 25

4.1.2 THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE 26

4.1.3 ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE 29

4.2 Air conditioning and ventilating systems 32

4.2.1 How does an air conditioner work 32

4.2.2 Moisture Removal - Dehumidification 33

4.2.3 Refrigerants 33

4.2.4 Types of commercial air conditioners 35

1) Window or Wall Units 35

2) Split System Air Conditioners 36

3) Ductless Split Systems 37

4) Evaporative Coolers 37

5) Package Air Conditioning Units 39

4.3 Ventilating system 40

CHAPTER 5: BOILER AND THERMAL POWER PLANT 41

5.1 Rankine Cycles 41

5.1.1 The simplest Rankine cycle 41

5.1.2 Rankine cycle with vapor superheating 41

5.1.3 Rankine cycle with vapor reheating 42

5.1.4 Regenerative Rankine Cycle with Feed Water Heating 43

5.1.4 Losses in Rankine Cycles 43

5.1.5 Combined Brayton-Rankine Cycle 43

5.2 Boiler 45

5.2.1 Introduction to the steam generator 45

5.2.2 Reasons for superheating steam 46

5.2.3 Main parts of boiler 46

1) Furnace 46

2) Economizer 47

3) Superheater 47

4) Reheater 48

5) Drum or Separator 48

6) Air heaters 48

7) Types of Pressures 49

a Superheater (SH) Outlet Pressure 49

b Drum Pressure 49

c Design Pressure 50

d Reheater (RH) Inlet and Outlet Pressures 50

e Calculation Pressure 50

8) Outlet Temperatures 50

a SH Outlet Temperature 50

b RH Outlet Temperature 50

c Feed Water Temperature 50

Chapter 1: Introduction to Thermodynamics

The aim of this chapter is to understand the definition of basic thermodynamic parameters

1 Closed and open system:

A system is defined as a quantity of matter or a region in space chosen for study The mass or region outside the system is called the surroundings.The real or imaginary surface that separates the system from its surroundings is called the boundary

Figure 1: System, boundary and surroundings

1.2 Open system:

An open system,or a control volume,as it is often called, is a properly selected region

in space It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle Flow through these devices is best studied by selecting the region within the device as the control volume Both mass and energy can cross the boundary

of a control volume This is illustrated in Fig 3

Figure 2: a closed system

Figure 3: An open system

2 Pressure

2.1 Absolute pressure: The absolute pressure (P) is the force of a fluid acting on unit area In the SI system, the unit for pressure is Pascal (1Pa = 1 N/m2), In the English system, it is psi

2.2 Atmospheric pressure: The air surrounding the earth can be treated as

a homogeneous gas, called atmosphere Atmospheric pressure (Pa)

is the pressure due to the force by the atmosphere’s mass Standard atmospheric pressure is 101.325 kPa (at sea level and the temperature is 150C)

Figure 4: Atmosphric pressure

2.3 Gage pressure (Pg) is the difference between the absolute pressure and the atmospheric pressure if the difference is positive If the difference is negative, it is called vacuum pressure (Pv)

Barometer is a device used to measure the atmospheric pressure

Pa = ρ g h (1) where:

ρ = The density of the working liquid, kg/m3

g = The acceleration of gravity, 9.8 m/s2

h = The height of the working liquid in the tube, m

1 bar = 105 Pa; 1 at = 1 kG/cm2 = 10 mH2O = 9.807×104 Pa; 1 atm = 101.325 kPa

Figure 6: Absolute, gage and vacuum pressures

Figure 5: a basic Barometer

3 Temperature and the Zeroth Law

a Definition: The measurement of the degree of hotness or coolness is temperature

If two bodies at different temperatures are brought together, the hot body will warm up the cold one At the same time, the cold body will cool down the hot one This process will end when the two bodies have the same temperatures At that point, the two bodies are said to have reached thermal equilibrium

b Temperature scales: Celsius (0C), Fahrenheit (0F), Kelvin (K)…

Conversion of temperature scales: 0C = 5/9(0F-32); K = 0C + 273.15; 0F = 9/50C + 32 The Zeroth Law of thermodynamics states: Two bodies each in thermal equilibrium with a third body will be in thermal equilibrium with each other

Notice: The Zeroth Law of thermodynamics is a basis for the validity of temperature measurement

c Thermometers measure temperature, by using materials that change in some way when they are heated or cooled In a mercury or alcohol thermometer the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column becomes longer or shorter depending on the temperature Modern thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius Three practical points for using thermometer are:

 The thermometer should be isolated to everything except the body which temperature is measured The general method is to immerse the thermometer in

a hole in a solid body, or directly in a fluid body

 When thermal equilibrium is reached, the thermometer can indicate its own temperature as well as the body measured The thermometer should be small relative to the body so that it only has a small effect upon the body

 The thermometer must not be subject to effects such as pressure changes, which might change the volume independently of temperature

Digital thermometers almost replace the mercury ones in nowadays because they are more accurate and easier to use

4 Heat transfer, Q [J]

a Definition: Heat is energy

transferred from one system to another solely by reason of a temperature difference between the systems Heat exists only as it crosses the boundary of a system and the direction of heat transfer is from higher temperature to lower temperature

b Heat Unit: Heat is measured in terms of calorie, defined as the amount of heat necessary to raise the temperature of 1 g of water at a pressure of 1 atm by 1 degree Celsius (ºC) In mechanical engineering practice in the United State and the United Kingdom, heat is measured in British thermal unit or Btu One Btu is the quantity of heat required to raise the temperature of 1 lb of water to 1 degree Fahrenheit (ºF) and is equal to 252 cal

Unit conversion: 1 Btu = 1055.056 J; 1 cal = 4.184 J; 1N.m = 1 J

For thermodynamics sign convention, heat transferred to a system is positive (Q

> 0); Heat transferred from a system is negative (Q < 0) The heat needed to raise an object's temperature from T1 to T2 is:

Figure 7: Thermometers (a) Liquid-in-glass (b) Infrared - sensing ear thermometer

where, Cp is specific heat of the object at constant pressure; and m is mass of the

object

+ Specific heat C: the heat capacity, or the measure of the amount of heat required to raise the temperature of a unit mass of a substance one degree is known as specific heat If the heating process occurs while the substance is maintained at a constant volume or is subjected to a constant pressure the measure is referred to as a specific heat at constant volume or at constant pressure The latter is always larger than, or at least equal to, the former for each substance In case of water and other approximately incompressible substances,

it is not necessary to distinguish between the constant-volume and constant pressure specific heat, as they are approximately equal Generally, the two specific heats of a substance depend on the temperature

4.3 Modes of heat transfers:

Conduction: Heat transferred between two bodies in direct contact

Convection: Heat transfer between a solid surface and an adjacent gas or liquid

It is the combination of conduction and flow motion Heat transferred from a solid surface to a liquid adjacent is conduction And then heat is brought away

by the flow motion

Radiation: The energy emitted by matter in the form of electromagnetic waves

as a result of the changes in the electronic configurations of the atoms or molecules

Figure 8: Three types of heat transfer

The work expression can be written as: δW = Vdp (3)

For a change in volume from V1 to V2, the work is obtained by integrating (3):

Schematic and Given Data: The given p–Vrelationship and the given data for pressure and volume can be used to construct the accompanying pressure– volume diagram of the process:

Assumptions:

1 The gas is a closed system

2 The moving boundary is the only work mode

3 The expansion is a polytropic process

Analysis: The required values for the work are obtained by integration of Eq (4) using the given pressure–volume relation

(a) Introducing the relationship p = into Eq (4) and performing the integration:

The constant in this expression can be evaluated at either end state p =

p = p : The work expression then becomes

To evaluate W, the pressure at state 2 is required This can be found by using which on rearrangement yields

(b) For n = 1.0, the pressure–volume relationship is p = or p =

/ The work is Substituting values

(c) For n = 0, the pressure–volume relation reduces to p = constant, and the integral becomes W = p(V2 - V1), which is a special case of the expression found in part (a) Substituting values and converting units as above,W =

30 kJ

6 Energy E [J]

a Definition: Energy is the capacity of doing work It may exist in a variety of forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear All forms of energy are interconvertible by appropriate processes In the process of transformation either kinetic or potential energy may

be lost or gained, but the sum total of the two remains always the same This is known as the principle of conservation of energy: energy cannot be created or destroyed; it is transferred from one type of energy to another For example, Heating water by gas: Chemical energy -> thermal energy

Heating water by electricity: electric energy -> thermal energy

Running nuclear power plant: Nuclear energy -> electric energy

Flying rocket: Chemical energy -> thermal Energy -> Kinetic Energy

Total energy of a system consists of the kinetic, potential, and internal energies and is expressed as:

E = U + KE + PE (6)

or, on a unit mass basis,

e = u + ke + pe (7)

Closed systems whose velocity and elevation of the center of gravity remain constant during a process are frequently referred to asstationary systems.The change in the total energy E of a stationary system is identical to the change in its internal energy U

b Classification:

- Kinetic Energy (KE): The energy that a system possesses as a result of its motion

KE = mv2/2 (8) where, m = mass of the system; v = velocity of the system

- Potential Energy (PE): The energy that a system possesses as a result of its elevation in a gravitational field or change of configurations

- Internal energy (U): The energy associated with the random, disordered motion

of molecules It is the sum of the kinetic and potential energies of all molecules Note: In the thermodynamics, total energy of a stationary system is its internal energy

- Enthalpy, H:

H = U + pV

In the thermodynaics, we usually find this group For simplicity, they were gathered into a new property called Enthalpy It has the same properties of internal energy

Homeworks:

1 The temperature of 150 kg of water is raised from 150C to 850C by addition of heat How much heat energy in kilojoules is supplied?

2 20 kg of water at 650C are supplied with 5000 kJ of heat energy What the mass

of water will be vaporized? Note: r = 2256.54 kJ/kg; Cp = 4.184 kJ/kgK

3 Determine the quantity of heat in kilojoule required to vaporize 50 kg of water

at a saturation temperature of 1000C

Chapter 2 Energy and Environment The aim of this chapter is to give a background about energy, environment, the greenhouse effect and climate change

(Source: Fundamentals of thermal-fluid science, Y A Çengel)

2.1 Energy and environment

The conversion of energy from one form to another often affects the environment and the air

we breathe in many ways, and thus the study of energy is not complete without considering its impact on the environment Fossil fuels such as coal, oil, and natural gas have been powering the industrial development and the amenities of modern life that we enjoy since the 1700s, but this has not been without any undesirable side effects From the soil we farm and the water we drink to the air we breathe, the environment has been paying a heavy toll for it Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming and climate change The environmental pollution has reached such high levels that it became a serious threat to vegetation, wild life, and human health Air pollution has been the cause of numerous health problems including asthma and cancer It is estimated that over 60,000 people in the United States alone die each year due to heart and lung diseases related to air pollution

Hundreds of elements and compounds such as benzene and formaldehyde are known to be emitted during the combustion of coal, oil, natural gas, and wood in electric power plants, engines of vehicles, furnaces, and even fireplaces Some compounds are added to liquid fuels for various reasons (such as MTBE to raise the octane number of the fuel and also to oxygenate the fuel in winter months to reduce urban smog) The largest source of air pollution is the motor vehicles, and the pollutants released by the vehicles are usually grouped as hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) The HC emissions are a large component

of volatile organic compounds (VOC) emissions, and the two terms are generally used interchangeably for motor vehicle emissions A significant portion of the VOC or HC emissions are caused by the evaporation of fuels during refueling or spillage during spit back

or by evaporation from gas tanks with faulty caps that do not close tightly The solvents, propellants, and household cleaning products that contain benzene, butane, or other HC products are also significant sources of HC emissions

2.2 Acid Rain Formation

Fossil fuels are mixtures of various chemicals, including

small amounts of sulfur The sulfur in the fuel reacts

with oxygen to form sulfur dioxide (SO2), which is an

air pollutant The main source of SO2 is the electric

power plants that burn high-sulfur coal The Clean Air

Act of 1970 has limited the SO2 emissions severely,

which forced the plants to install SO2 scrubbers, to

switch to low-sulfur coal, or to gasify the coal and

recover the sulfur Motor vehicles also contribute to SO2

emissions since gasoline and diesel fuel also contain

small amounts of sulfur Volcanic eruptions and hot

springs also release sulfur oxides (the cause of the rotten

egg smell)

The sulfur oxides and nitric oxides react with water

vapor and other chemicals high in the atmosphere in the

presence of sunlight to form sulfuric and nitric acids

(Fig 2.1) The acids formed usually dissolve in the suspended water droplets in clouds or fog These acid-laden droplets, which can be as acidic as lemon juice, are washed from the air on

to the soil by rain or snow This is known as acid rain The soil is capable of neutralizing a certain amount of acid, but the amounts produced by the power plants using inexpensive high-sulfur coal has exceeded this capability, and as a result many lakes and rivers in industrial areas such as New York, Pennsylvania, and Michigan have become too acidic for fish to grow Forests in those areas also experience a slow death due to absorbing the acids through their leaves, needles, and roots Even marble structures deteriorate due to acid rain

2.3 The Greenhouse Effect

You have probably noticed that when you leave your car under direct sunlight on a sunny day, the interior of the car gets much warmer than the air outside, and you may have wondered why the car acts like a heat trap This is because glass at thicknesses encountered in practice transmits over 90 percent of radiation in the visible range and is practically opaque (nontransparent) to radiation in the longer wavelength infrared regions Therefore, glass allows the solar radiation to enter freely but blocks the infrared radiation emitted by the interior surfaces This causes a rise in the interior temperature as a result of the energy buildup in the car This heating effect is known as the greenhouse effect, since it is utilized primarily in greenhouses

Figure 2.1: Formation of acid rain

The greenhouse effect is also experienced on a larger

scale on earth The surface of the earth, which warms

up during the day as a result of the absorption of solar

energy, cools down at night by radiating part of its

energy into deep space as infrared radiation Carbon

dioxide (CO2), water vapor, and trace amounts of some

other gases such as methane and nitrogen oxides act

like a blanket and keep the earth warm at night by

blocking the heat radiated from the earth Therefore,

they are called “greenhouse gases,” with CO2 being the

primary component Water vapor is usually taken out

of this list since it comes down as rain or snow as part

of the water cycle and human activities in producing

water (such as the burning of fossil fuels) do not make

much difference on its concentration in the atmosphere

(which is mostly due to evaporation from rivers, lakes,

oceans, etc.) CO2 is different, however, in that

people’s activities do make a difference in CO2

concentration in the atmosphere

2.4 Global Warming and Climate Change

The greenhouse effect makes life on earth possible by keeping the earth warm (about 30 0C warmer) However, excessive amounts of these gases disturb the delicate balance by trapping too much energy, which causes the average temperature of the earth to rise and the climate at some localities to change These undesirable consequences of the greenhouse effect are referred

to as global warming or global climate change

The global climate change is due to the excessive use of fossil fuels such as coal, petroleum products, and natural gas in electric power generation, transportation, buildings, and manufacturing, and it has been a concern in recent decades In 1995, a total of 6.5 billion tons

of carbon was released to the atmosphere as CO2 The current concentration of CO2 in the atmosphere is about 360 ppm (or 0.36 percent) This is 20 percent higher than the level a century ago, and it is projected to increase to over 700 ppm by the year 2100 Under normal conditions, vegetation consumes CO2 and releases O2 during the photosynthesis process, and thus keeps the CO2 concentration in the atmosphere in check A mature, growing tree consumes about 12 kg of CO2 a year and exhales enough oxygen to support a family of four However, deforestation and the huge increase in the CO2 production in recent decades disturbed this balance

In a 1995 report, the world’s leading climate scientists concluded that the earth has already warmed about 0.5 0C during the last century, and they estimate that the earth’s temperature will rise another 2 0C by the year 2100 A rise of this magnitude is feared to cause severe changes

in weather patterns with storms and heavy rains and flooding at some parts and drought in others, major floods due to the melting of ice at the poles, loss of wetlands and coastal areas due to rising sea levels, variations in water supply, changes in the ecosystem due to the inability

of some animal and plant species to adjust to the changes, increases in epidemic diseases due

to the warmer temperatures, and adverse side effects on human health and socioeconomic conditions in some areas

Figure 2.2: Greenhouse effect

In the 1997 meeting in Kyoto (Japan), the world’s industrialized countries adopted the Kyoto protocol and committed to reduce their CO2 and other greenhouse gas emissions by 5 percent below the 1990 levels by 2008 to 2012 This can be done by increasing conservation efforts and improving conversion efficiencies, while meeting new energy demands by the use of renewable energy (such as hydroelectric, solar, wind, and geothermal energy) rather than by fossil fuels

2.5 Negative impacts of global warming

2.5.1 Increase in average temperatures and temperature extremes

One of the most immediate and obvious effects of global warming is the increase in temperatures around the world The average global temperature has increased by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past 100 years, according to the National Oceanic and Atmospheric Administration (NOAA) In 2014, some cities in the United States had the warmest summers on record, according to Scientific American A report by the World Meteorological Organization released July 3, 2014, said that deaths from heat increased by more than 2,000 percent over the previous decade

2.5.2 Extreme weather events

Extreme weather is an effect of global warming While experiencing some of the hottest summers on record, much of the United States also has been experiencing colder than normal winters Global warming may also lead to extreme weather other than cold or heat extremes For example, hurricane formations will change Though this is still a subject of active scientific research, current computer models of the atmosphere indicate that hurricanes are more likely

to become less frequent on a global basis, though the hurricanes that do form may be more intense This is because hurricanes get their energy from the temperature difference between the warm tropical ocean and the cold upper atmosphere Global warming increases that temperature difference

Scientists project that extreme weather events, such as heat waves, droughts and rainstorms will continue to occur more often and with greater intensity due to global warming, according

to Climate Central Climate models forecast that global warming will cause climate patterns worldwide to experience significant changes These changes will likely include major shifts in wind patterns, annual precipitation and seasonal temperatures variations

2.5.3 Ice melt

One of the most dramatic effects of global warming is the reduction in Arctic sea ice: In 2012, scientists saw the smallest amount of Arctic ice cover ever recorded Most analyses project that, within a matter of years, the Arctic Sea will be completely ice-free during the summer months

In addition to less nutritious food, the effect of global warming on human health is also expected to be serious The American Medical Association has reported an increase in mosquito-borne diseases like malaria and dengue fever, as well as a rise in cases of chronic conditions like asthma, are already occurring, most likely as a direct result of global warming

Chapter 3: The laws of thermodynamics

(Source: Fundamentals of thermal-fluid science, Y A Çengel)

Thermodynamic, a field of physics that describes and correlates the physical properties of macroscopic systems of matter and energy The principle of thermodynamics are of fundamental importance to all branches of science and engineering

A central concept of thermodynamic is the macroscopic system The state of a macroscopic system in equilibrium can be described in terms of such measurable properties as temperature, pressure and volume, which are known as thermodynamic variables Many other variables (such as density, specific heat, compressibility, and the coefficient of thermal expansion) can

be identified and correlated, to produce a more complete description of an object and its relationship to its environment

When a macroscopic system moves from one state of equilibrium to another, a thermodynamic process is said to take place Some processes are reversible and other are irreversible The laws

of thermodynamics govern the nature of all thermodynamic processes and place limits on them 3.1 The first law of thermodynamics

We have considered various forms of energy such as heat Q, work W, and total energy E individually, and no attempt has been made to relate them to each other during a process The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationships among the various forms of energy and energy interactions Based on experimental observations, the first law of thermodynamics states that energy can be neither created nor destroyed; it can only change forms Therefore, every bit of energy should be accounted for during a process

We all know that a rock at some elevation possesses some potential energy, and part of this potential energy is converted to kinetic energy as the rock falls Experimental data show that the decrease in potential energy exactly equals the increase in kinetic energy when the air resistance is negligible, thus confirming the conservation of energy principle

Consider a system undergoing a series of adiabatic processes from a specified state 1 to another specified state 2 Being adiabatic, these processes obviously cannot involve any heat transfer, but they may involve several kinds of work interactions Careful measurements during these experiments indicate the following: For all adiabatic processes between two specified states of

a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process

Considering that there are an infinite number of ways to perform work interactions under adiabatic conditions, this statement appears to be very powerful, with a potential for far reaching implications This statement, which is largely based

on the experiments of Joule in the first half of the nineteenth century, cannot be drawn from any other known physical principle and is recognized as a fundamental principle This principle is called the first law of thermodynamics or just the first law

A major consequence of the first law is the existence and the definition of the property total energy E Considering that the network is the same for all adiabatic processes of a closed system between two specified states, the value of the network must depend on the end states of the system only, and thus it must correspond to a change in a property of the system This property is the total energy Note that the first law makes no reference to the value of the total energy of a closed system at

a state It simply states that the change in the total energy during an adiabatic process must be equal to the net work done Therefore, any convenient arbitrary value can be assigned to total energy at a specified state to serve as a reference point

The first law of thermodynamics is essentially an expression of the conservation of energy principle, also called the energy balance The general mass and energy balances for any system undergoing any process can be expressed as:

They can also be expressed in the rate form as:

Taking heat transfer tothe system and work done bythe system to be positive quantities, the energy balance for a closed system can also be expressed as:

Figure 9: The energy change of a system during a

process is equal to the network and heat transfer

between the system and its surroundings.

For a constant-pressure process, Wb +U = H.Thus,

3.2 The second law of Thermodynamics

3.2.1 Introduction to the second law of thermodynamics

We applied the first law of thermodynamics, or the conservation of energy principle, to processes involving closed and open systems As pointed out above, energy is a conserved property, and no process is known to have taken place in violation of the first law of thermodynamics

Therefore, it is reasonable to conclude that a process must

satisfy the first law to occur However, as explained here,

satisfying the first law alone does not ensure that the

process will actually take place It is common experience

that a cup of hot coffee left in a cooler room eventually

cools off (Fig 2) This process satisfies the first law of

thermodynamics since the amount of energy lost by the

coffee is equal to the amount gained by the surrounding air

Now let us consider the reverse process—the hot coffee

getting even hotter in a cooler room as a result of heat

transfer from the room air We all know that this process

never takes place Yet, doing so would not violate the first

law as long as the amount of energy lost by the air is equal to the amount gained by the coffee

It is clear from this argument that processes proceed in a certain direction and not in the reverse direction The first law places no restriction on the direction of a process, but satisfying the first law does not ensure that the process will actually occur This inadequacy of the first law

to identify whether a process can take place is remedied by introducing another general

principle, the second law of thermodynamics

The use of the second law of thermodynamics is not limited to identifying the direction of

processes The second law also asserts that energy has quality as well as quantity The first

law is concerned with the quantity of energy and the transformations of energy from one form

to another with no regard to its quality Preserving the quality of energy is a major concern to engineers, and the second law provides the necessary means to determine the quality as well

as the degree of degradation of energy during a process

Figure 10: A cup of hot coffee does not get hotter

3.2.2 The second law of thermodynamics states:

- Kelvin–Planck statement: “It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.”

- The Clausius statement is expressed as follows: “It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher- temperature body.” (See Figure 3)

Figure 11: A refrigerator that violates the Clausius statement of the second law

3.3 Thermodynamic cycles

All important thermodynamic relations used in engineering are derived from the first and second laws of thermodynamics One useful way of discussing thermodynamic processes is in terms of cycles – processes that return a system to its original state after a number of stages, thus restoring the original values for all the relevant thermodynamic variables In a complete cycle the internal energy of a system depends solely on these variables and cannot change Therefore, the total net heat transferred to the system must equal the total net work delivered from the system

An ideal cycle would be performed by a perfectly efficient heat engine – that is, all the heat would be converted to mechanical work The 19th century, Nicolas Carnot, who conceived a thermodynamic cycle that is the basic cycle of all heat engines, showed that such an ideal engine cannot exist Any heat engine must expend some fraction of its heat input as exhaust The second law of thermodynamic places an upper limit on the efficiency of engines; that upper limit is less than 100 percent The limiting case is known as a Carnot cycle

Carnot cycle is composed of four reversible processes—two isothermal and two adiabatic—and it can be executed either in a closed or a steady-flow system Carnot cycle principles:

1 The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs

2 The efficiencies of all reversible heat engines operating between the same two reservoirs are the same

Figure 12: Execution of the Carnot cycle in a closed system

Figure 13: P-Vdiagram of the Carnot cycle

Chapter 4: Reversed cycles and refrigeration/air

conditioning system

(Source: Fundamentals of thermal-fluid science, Y A Çengel)

4.1 The fundamentals of refrigeration

We all know from experience that heat flows in the direction of decreasing temperature, that

is, from high-temperature regions to low-temperature ones This heat transfer process occurs

in nature without requiring any devices The reverse process, however, cannot occur by itself The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators

Refrigerators are cyclic devices, and the working fluids used in the refrigeration cycles are called refrigerants A refrigerator is shown schematically in Fig 1a Here, QL is the magnitude

of the heat removed from the refrigerated space at temperature TL; QH is the magnitude of the heat rejected to the warm space at temperature TH, and Wnet,in is the net work input to the refrigerator

Another device that transfers heat from a low-temperature medium to a high-temperature one

is the heat pump (Fig 1b) Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it Meanwhile, the objective of a heat pump, however, is to maintain a heated space at a high temperature

Figure 14 The schematic of Refrigerator and heat pump

The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP), which was defined as:

4.1.1 THE REVERSED CARNOT CYCLE

The Carnot cycle is a totally reversible cycle that consists of two reversible isothermal and two isentropic processes It has the maximum thermal efficiency for given temperature limits, and

it serves as a standard against which actual power cycles can be compared

Since it is a reversible cycle, all four processes that comprise the Carnot cycle can be reversed Reversing the cycle does also reverse the directions of any heat and work interactions The

result is a cycle that operates in the counterclockwise direction on a T-s diagram, which is

called the reversed Carnot cycle A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump

Figure 15 The schematic of Carnot Refrigerator and its T-s diagram

Consider a reversed Carnot cycle executed within the saturation dome of a refrigerant, as shown

in Fig 2 The refrigerant absorbs heat isothermally from a low-temperature source at T L in the

amount of Q (process 1-2), is compressed isentropically to state 3 (temperature rises to T L),

rejects heat isothermally to a high-temperature sink at T H in the amount of Q H (process 3-4),

and expands isentropically to state 1 (temperature drops to T H) The refrigerant changes from a saturated vapor state to a saturated liquid state in the condenser during process 3-4

The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms

of temperatures as: