Moran M.J., Shapiro H.N. Fundamentals of Engineering Thermodynamics

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Makeup water is supplied at 238C. Atmospheric air enters

the tower at 308C, 1 bar, 35% relative humidity. A saturated

moist air stream exits at 348C, 1 bar. Determine

(a) the mass flow rates of the dry air and makeup water,

each in kg/h.

(b) the rate of exergy destruction within the cooling tower,

in kW, for T

5 238C.

Ignore kinetic and potential energy effects.

12.108 Liquid water at 1108F and a volumetric flow rate of

250 ft

/min enters a cooling tower operating at steady state.

Cooled water exits the cooling tower at 888F. Atmospheric

air enters the tower at 808F, 1 atm, 40% relative humidity,

and saturated moist air at 1058F, 1 atm exits the cooling

tower. Determine

(a) the mass flow rates of the dry air and the cooled water,

each in lb/min.

(b) the rate of exergy destruction within the cooling tower,

in Btu/s, for T

5 778F.

Ignore kinetic and potential energy effects.

c DESIGN & OPEN-ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE

12.1D About half the air we breathe on some airplanes is fresh

air, and the rest is recirculated. Investigate typical equipment

schematics for providing a blend of fresh and recirculated

filtered air to the passenger cabins of commercial airplanes.

What types of filters are used and how do they work? Write

a report including at least three references.

12.2D Identify a campus, commercial, or other building in

your locale with an air-conditioning system installed 20 or

more years ago. Critically evaluate the efficacy of the system

in terms of comfort level provided, operating costs, maintenance

costs, global warming potential of the refrigerant used, and

other pertinent issues. On this basis, recommend specific system

upgrades or a full system replacement, as warranted. Present

your findings in a PowerPoint presentation.

12.3D Study the air-conditioning system for one of the

classrooms you frequent where occupant comfort is

unsatisfactory and describe the system in detail, including

the control strategy used. Propose modifications aimed at

improving occupant satisfaction, including a new heating,

ventilation, and air conditioning (HVAC) system for the room,

if warranted. Compare the proposed and existing systems in

terms of occupant comfort, potential impact on productivity,

and energy requirements. Detail your findings in an executive

summary and PowerPoint presentation.

12.4D Critically evaluate a cooling tower on your campus, or

nearby, in terms of effectiveness in providing the required

range of cooled water, operating costs, maintenance costs,

and other relevant issues. If warranted, recommend cost-

effective upgrades of the existing cooling tower or alternative

cooling technologies to achieve the desired level of performance,

including options for minimizing water loss. Present your

findings in a report including at least three references.

12.5D Using methods in the most current ASHRAE Handbook

of HVAC Applications, estimate the rate of evaporation from

the surface of an Olympic-size swimming pool. The swimming

hall is maintained at 308C with a dew point temperature of

218C. Based on your estimate, determine the load in kW, that

the added moisture places on the air-conditioning system.

Write a memorandum that summarizes your analysis and

include at least three references.

12.6D Write a report explaining the way the human body

regulates its temperature, both under cold and hot climate

conditions. Based on these thermoregulation mechanisms,

discuss designs of personal protective clothing for fire fighters

and other first responders. How are such clothing systems

designed to provide protection against chemical or biological

agents while maintaining sufficient thermal comfort to allow

for strenuous physical activity? Include in your report at

least three references.

12.7D Tunnel-type spray coolers are used to cool vegetables.

In one application, tomatoes on a 4-ft-wide conveyor belt

pass under a 348F water spray and cool from 70 to 408F. The

water is collected in a reservoir below and recirculated

through the evaporator of an ammonia refrigeration unit.

Recommend the length of the tunnel and the conveyor

speed. Estimate the number of tomatoes that can be cooled

in an hour with your design. Write a report including sample

calculations and at least three references.

12.8D Nearly 20 years ago, eight scientists entered Biosphere

2, located in Oracle, Arizona, for a planned two-year period

of isolation. The three-acre biosphere had several ecosystems,

including a desert, tropical rain forest, grassland, and

saltwater wetlands. It also included species of plants and

microorganisms intended to sustain the ecosystems. According

to plan, the scientists would produce their own food using

intensive organic farming, fish farmed in ponds, and a few

barnyard animals. Occupants also would breathe oxygen

produced by the plants and drink water cleansed by natural

processes. Sunlight and a natural gas–fueled generator were to

meet all energy needs. Numerous difficulties were encountered

with the ecosystems and by the scientists, including insufficient

oxygen, hunger and loss of body weight, and animosities

among individuals. Study the record of Biosphere 2 for

lessons that would substantially assist in designing a self-

sustaining enclosed biosphere for human habitation on Mars.

Present your results in a report including at least three

references.

12.9D In a 2007 study using influenza-infected guinea pigs in

climate-controlled habitats, researchers investigated the

aerosol transmission of influenza virus while varying the

temperature and humidity within the habitats. The research

showed there were more infections when it was colder and drier,

and based on this work a significant correlation was found between

humidity ratio and influenza. (See BIOCONNECTIONS on

p. 730) For the aerosol transmission experiments, guinea pigs

were housed within cabinets like the one shown in Figure

P12.9D. Each cabinet is fitted with a dedicated compressed-air

Design & Open-Ended Problems: Exploring Engineering Practice 773

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774 Chapter 12

Ideal Gas Mixture and Psychrometric Applications

line and an associated compressed-air dryer that provides

precise and rapid control for the cabinet’s humidity injection

and dehumidification systems. A condensate recircu lator

collects and recycles the condensate that forms in the base of

the chamber and provides continuous, clean, filtered water to

the cabinet’s humidity injection system. The cabinets were

placed within an isolated room with an ambient temperature

of approximately 208C. Experimenters say ambient temperatures

in excess of 258C can result in chamber failure. The objective

of this project is to specify the heating, ventilation, and air

conditioning (HVAC) system for the room housing the

cabinets, assuming the room encompasses 500 ft

and houses

five environmental cabinets with up to eight guinea pigs per

cabinet. Each cabinet delivers a maximum heat transfer rate

of 4000 Btu/h to the room. Document your design in a report

including a minimum of three references that substantiate

assumptions made during the design process.

12.10D Building energy use is significant in the United States,

consuming about 70% of all electricity generated. An increase

in electricity use of about 50% is expected by the end of the

current decade. In response to the adverse impact that buildings

have on energy use and the environment, in 1998 the U.S.

Green Building Council developed LEED

(Leadership in

Energy and Environmental Design), a certification system

aimed at improving the performance of buildings across several

measures, including energy and water use, greenhouse gas

emission, and indoor environmental quality. Thousands of

buildings throughout the world have now gained LEED

certification. Identify a newly constructed LEED

-certified

building on your campus or in a nearby locale. Determine the

level of LEED

certification the building achieves: certified,

silver, gold, or platinum. Prepare a summary of the building’s

design, focusing on elements incorporated to improve building

energy and environmental performance and associated costs.

Present your findings in a written report including at least

three references.

12.11D In 2010, the U.S. Department of Energy focused its

research agenda on innovative technologies to provide energy-

efficient cooling for buildings and reduce greenhouse gas

emissions. Key agenda items include developing the following:

1. Cooling systems using refrigerants with global warming

potential less than or equal to 1

2. Air conditioning systems for warm and humid climates

that increase the coefficient of performance of ventilation

air cooling by 50% or more, based on current technology

3. Hot-climate vapor-compression air-conditioning systems

that condition recirculated air while increasing the

coefficient of performance by 50% or more, based on current

technology

For one such project supported by the Department of

Energy, prepare a report that summarizes the project goals

and objectives, research plan, and expected outcomes. Also

critically evaluate the feasibility of incorporating the resulting

technology into existing cooling systems.

Display panel consists of a

power switch, refrigeration

switch (on/off), temperature

controller, and humidity

controller

Air vent

Guinea pigs housed here

Five adjustable shelves

Port for condensate drain and capture

(unit located on back of cabinet)

Access port

Fig. P12.9D

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12.12D An air-handling system is being designed for a 40 ft 3

40 ft 3 8 ft biological research facility that houses 3000

laboratory mice. The indoor conditions must be maintained at

758F, 60% relative humidity when the outdoor air conditions

are 908F, 70% relative humidity. Develop a preliminary design

of an air conditioning and distribution system that satisfies

National Institute of Health (NIH) standards for animal

facilities. Assume a biological safety level of one (BSL-1), and

that two thirds of the floor space is devoted to animal care.

Since an interruption in ventilation or air conditioning could

place the laboratory animals under stress and compromise the

research under way in the facility, account for redundancy in

your design.

12.13D Adequate levels of ventilation reduce the likelihood of

sick building syndrome. (See BIOCONNECTIONS on p. 727.)

The outdoor air used for ventilation must be conditioned, and

this requires energy. Consider the air-handling system for the

commercial building illustrated in Figure P12.13D, consisting

of ducting, two dampers labeled A and B, a vapor-compression

dehumidifier, and a heater. The system supplies 25 m

/s of

conditioned air at 208C and a relative humidity of 55% to

maintain the interior space at 258C and a relative humidity of

50%. The recirculated air has the same conditions as the air

in the interior space. A minimum of 5 m

/s of outdoor air is

required to provide adequate ventilation. Dampers A and B

can be set to provide alternative operating modes to maintain

required ventilation rates. On a given summer day when the

outside air dry-bulb temperature and relative humidity are 258C

and 60%, respectively, which of the following three operating

modes is best from the standpoint of minimizing the total heat

transfer of energy from the conditioned air to the cooling coil

and to the conditioned air from the heating coil?

1. Dampers A and B closed.

2. Damper A open and damper B closed with outside air

contributing one-quarter of the total supply air.

3. Dampers A and B open. One-quarter of the conditioned

air comes from outside air and one-third of the recirculated

air bypasses the dehumidifier via open damper B; the rest

flows through damper A.

Present your recommendation together with your reasoning in

a PowerPoint presentation suitable for your class. Additionally,

in an accompanying memorandum, provide well-documented

sample calculations in support of your recommendation.

Supply air,

25 m

/s at 20°C,

f = 55%

Interior

air-conditioned

space at

25°C, f = 50%

Outside air

at 25°C,

f = 60%

Recirculated air

Damper B

Heating coil

Cooling coil

Condensate

Damper A

Fig. P12.13D

Design & Open-Ended Problems: Exploring Engineering Practice 775

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776

Combustion fundamentals are introduced in Sec. 13.1. © Digital Vision/Age Fotostock America, Inc.

ENGINEERING CONTEXT The objective of this chapter is to study systems involving chemical

reactions. Since the combustion of hydrocarbon fuels occurs in most power-producing devices (Chaps. 8 and 9),

combustion is emphasized.

The thermodynamic analysis of reacting systems is primarily an extension of principles introduced thus far.

The concepts applied in the first part of the chapter dealing with combustion fundamentals remain the same:

conservation of mass, conservation of energy, and the second law. It is necessary, though, to modify the

methods used to evaluate specific enthalpy, internal energy, and entropy, by accounting for changing chem-

ical composition. Only the manner in which these properties are evaluated represents a departure from

previous practice, for once appropriate values are determined they are used as in earlier chapters in the

energy and entropy balances for the system under consideration. In the second part of the chapter, the

exergy concept of Chap. 7 is extended by introducing chemical exergy.

The principles developed in this chapter allow the equilibrium composition of a mixture of chemical sub-

stances to be determined. This topic is studied in Chap. 14. The subject of dissociation is also deferred until

then. Prediction of reaction rates is not within the scope of classical thermodynamics, so the topic of chem-

ical kinetics, which deals with reaction rates, is not discussed in this text.

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Reacting Mixtures

and Combustion

When you complete your study of this chapter, you will be able to…

demonstrate understanding of key concepts, including complete combustion, theoretical

air, enthalpy of formation, and adiabatic flame temperature.

determine balanced reaction equations for combustion of hydrocarbon fuels.

apply mass, energy, and entropy balances to closed systems and control volumes involving

chemical reactions.

perform exergy analyses, including chemical exergy and the evaluation of exergetic

efficiencies.

LEARNING OUTCOMES

777

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778 Chapter 13

Reacting Mixtures and Combustion

Combustion Fundamentals

13.1 Introducing Combustion

When a chemical reaction occurs, the bonds within molecules of the reactants are

broken, and atoms and electrons rearrange to form products. In combustion reac-

tions, rapid oxidation of combustible elements of the fuel results in energy release

as combustion products are formed. The three major combustible chemical ele-

ments in most common fuels are carbon, hydrogen, and sulfur. Sulfur is usually a

relatively unimportant contributor to the energy released, but it can be a significant

cause of pollution and corrosion problems. Combustion is complete when all the

carbon present in the fuel is burned to carbon dioxide, all the hydrogen is burned

to water, all the sulfur is burned to sulfur dioxide, and all other combustible ele-

ments are fully oxidized. When these conditions are not fulfilled, combustion is

incomplete.

In this chapter, we deal with combustion reactions expressed by chemical equa-

tions of the form

reactants S products

fuel 1 oxidizer S products

When dealing with chemical reactions, it is necessary to remember that mass is con-

served, so the mass of the products equals the mass of the reactants. The total mass

of each chemical element must be the same on both sides of the equation, even though

the elements exist in different chemical compounds in the reactants and products.

However, the number of moles of products may differ from the number of moles of

reactants.

consider the complete combustion of hydrogen with oxygen

O (13.1)

In this case, the reactants are hydrogen and oxygen. Hydrogen is the fuel and oxygen

is the oxidizer. Water is the only product of the reaction. The numerical coefficients

in the equation, which precede the chemical symbols to give equal amounts of each

chemical element on both sides of the equation, are called stoichiometric coefficients.

In words, Eq. 13.1 states

1 kmol H

kmol O

1 kmol H

or in English units

1 lbmol H

lbmol O

1 lbmol H

Note that the total numbers of moles on the left and right sides of Eq. 13.1 are not

equal. However, because mass is conserved, the total mass of reactants must equal

the total mass of products. Since 1 kmol of H

equals 2 kg,

kmol of O

equals 16 kg,

and 1 kmol of H

O equals 18 kg, Eq. 13.1 can be interpreted as stating

2 kg H

1 16 kg O

18 kg H

or in English units

2 lb H

1 16 lb O

18 lb H

O b b b b b

In the remainder of this section, consideration is given to the makeup of the fuel,

oxidizer, and combustion products typically involved in engineering combustion

applications.

reactants

products

complete combustion

stoichiometric coefficients

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13.1.1

Fuels

A fuel is simply a combustible substance. In this chapter emphasis is on hydrocarbon

fuels, which contain hydrogen and carbon. Sulfur and other chemical substances also

may be present. Hydrocarbon fuels can exist as liquids, gases, and solids.

Liquid hydrocarbon fuels are commonly derived from crude oil through distillation

and cracking processes. Examples are gasoline, diesel fuel, kerosene, and other types

of fuel oils. Most liquid fuels are mixtures of hydrocarbons for which compositions

are usually given in terms of mass fractions. For simplicity in combustion calculations,

gasoline is often modeled as octane, C

, and diesel fuel as dodecane, C

Gaseous hydrocarbon fuels are obtained from natural gas wells or are produced

in certain chemical processes. Natural gas normally consists of several different hydro-

carbons, with the major constituent being methane, CH

. The compositions of gaseous

fuels are usually given in terms of mole fractions. Both gaseous and liquid hydrocar-

bon fuels can be synthesized from coal, oil shale, and tar sands.

Coal is a familiar solid fuel. Its composition varies considerably with the location

from which it is mined. For combustion calculations, the composition of coal is usually

expressed as an ultimate analysis. The ultimate analysis gives the composition on a

mass basis in terms of the relative amounts of chemical elements (carbon, sulfur,

hydrogen, nitrogen, oxygen) and ash.

13.1.2

Modeling Combustion Air

Oxygen is required in every combustion reaction. Pure oxygen is used only in special

applications such as cutting and welding. In most combustion applications, air pro-

vides the needed oxygen. The composition of a typical sample of dry air is given in

Table 12.1. For the combustion calculations of this book, however, the following

model is used for simplicity:

c All components of dry air other than oxygen are lumped together with nitrogen.

Accordingly, air is considered to be 21% oxygen and 79% nitrogen on a molar basis.

With this idealization the molar ratio of the nitrogen to the oxygen is 0.79/0.21 5 3.76.

When air supplies the oxygen in a combustion reaction, therefore, every mole of

oxygen is accompanied by 3.76 moles of nitrogen.

c We also assume that the nitrogen present in the combustion air does not undergo

chemical reaction. That is, nitrogen is regarded as inert. The nitrogen in the prod-

ucts is at the same temperature as the other products, however. Accordingly, nitrogen

undergoes a change of state if the products are at a temperature other than the

reactant air temperature. If a high enough product temperature is attained, nitrogen

can form compounds such as nitric oxide and nitrogen dioxide. Even trace amounts

of oxides of nitrogen appearing in the exhaust of internal combustion engines can

be a source of air pollution.

Air–Fuel Ratio

Two parameters that are frequently used to quantify the amounts of fuel and air in a

particular combustion process are the air–fuel ratio and its reciprocal, the fuel–air ratio.

The air–fuel ratio is simply the ratio of the amount of air in a reaction to the amount of

fuel. The ratio can be written on a molar basis (moles of air divided by moles of fuel)

or on a mass basis (mass of air divided by mass of fuel). Conversion between these

values is accomplished using the molecular weights of the air, M

air

, and fuel, M

fuel

mass of air

mass of fuel

moles of air 3 M

air

moles of fuel 3 M

fuel

moles of air

moles of fuel

air

fuel

ultimate analysis

air–fuel ratio

TAKE NOTE...

In this model, air is

assumed to contain no

water vapor. When moist air

is involved in combustion,

the water vapor present

must be considered in

writing the combustion

equation.

13.1 Introducing Combustion 779

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780 Chapter 13 Reacting Mixtures and Combustion

AF 5 AF a

air

fuel

(13.2)

where AF is the air–fuel ratio on a molar basis and AF is the ratio on a mass basis.

For the combustion calculations of this book the molecular weight of air is taken as

28.97. Tables A-1 provide the molecular weights of several important hydrocarbons.

Since AF is a ratio, it has the same value whether the quantities of air and fuel are

expressed in SI units or English units.

Theoretical Air

The minimum amount of air that supplies sufficient oxygen for the complete combus-

tion of all the carbon, hydrogen, and sulfur present in the fuel is called the theoretical

amount of air

. For complete combustion with the theoretical amount of air, the products

consist of carbon dioxide, water, sulfur dioxide, the nitrogen accompanying the oxy-

gen in the air, and any nitrogen contained in the fuel. No free oxygen appears in the

products.

let us determine the theoretical amount of air for the complete

combustion of methane. For this reaction, the products contain only carbon dioxide,

water, and nitrogen. The reaction is

1 a1O

1 3.76N

2S bCO

1 cH

O 1 dN

(13.3)

where a, b, c, and d represent the numbers of moles of oxygen, carbon dioxide, water,

and nitrogen. In writing the left side of Eq. 13.3, 3.76 moles of nitrogen are considered

to accompany each mole of oxygen. Applying the conservation of mass principle to

the carbon, hydrogen, oxygen, and nitrogen, respectively, results in four equations

among the four unknowns

C: b 5 1

H: 2 c 5 4

O: 2 b 1 c 5 2a

N: d 5 3.76a

Solving these equations, the balanced chemical equation is

1 21O

1 3.76N

1 2H

O 1 7.52N

(13.4)

The coefficient 2 before the term (O

1 3.76N

) in Eq. 13.4 is the number of moles

of oxygen in the combustion air, per mole of fuel, and not the amount of air. The

amount of combustion air is 2 moles of oxygen plus 2 3 3.76 moles of nitrogen, giv-

ing a total of 9.52 moles of air per mole of fuel. Thus, for the reaction given by Eq.

13.4 the air–fuel ratio on a molar basis is 9.52. To calculate the air–fuel ratio on a

mass basis, use Eq. 13.2 to write

AF 5 AF a

air

fuel

b5 9.52 a

28.9

16.04

b5 17.19

b b b b b

Normally the amount of air supplied is either greater or less than the theoretical

amount. The amount of air actually supplied is commonly expressed in terms of the

percent of theoretical air. For example, 150% of theoretical air means that the air actu-

ally supplied is 1.5 times the theoretical amount of air. The amount of air supplied

can be expressed alternatively as a percent excess air or a percent deficiency of air.

Thus, 150% of theoretical air is equivalent to 50% excess air, and 80% of theoretical

air is the same as a 20% deficiency of air.

theoretical air

percent of theoretical air

percent excess air

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consider the complete combustion of methane with 150% theo-

retical air (50% excess air). The balanced chemical reaction equation is

1.5

1 3.76N

1 2H

O 1 O

1 11.28N

(13.5)

In this equation, the amount of air per mole of fuel is 1.5 times the theoretical amount

determined by Eq. 13.4. Accordingly, the air–fuel ratio is 1.5 times the air–fuel ratio

determined for Eq. 13.4. Since complete combustion is assumed, the products contain

only carbon dioxide, water, nitrogen, and oxygen. The excess air supplied appears in

the products as uncombined oxygen and a greater amount of nitrogen than in Eq.

13.4, based on the theoretical amount of air.

b b b b b

The equivalence ratio is the ratio of the actual fuel–air ratio to the fuel–air ratio

for complete combustion with the theoretical amount of air. The reactants are said

to form a lean mixture when the equivalence ratio is less than unity. When the ratio

is greater than unity, the reactants are said to form a rich mixture.

In Example 13.1, we use conservation of mass to obtain balanced chemical reac-

tions. The air–fuel ratio for each of the reactions is also calculated.

equivalence ratio

Determining the Air–Fuel Ratio for Complete Combustion of Octane

c c c c EXAMPLE 13.1 c

Determine the air–fuel ratio on both a molar and mass basis for the complete combustion of octane, C

, with

(a) the theoretical amount of air, (b) 150% theoretical air (50% excess air).

SOLUTION

Known:

Octane, C

, is burned completely with (a) the theoretical amount of air, (b) 150% theoretical air.

Find: Determine the air–fuel ratio on a molar and a mass basis.

Engineering Model:

Each mole of oxygen in the combustion air is accompanied by 3.76 moles of nitrogen.

2. The nitrogen is inert.

3. Combustion is complete.

Analysis:

(a)

For complete combustion of C

with the theoretical amount of air, the products contain carbon dioxide,

water, and nitrogen only. That is

1 a1O

1 3.76N

bCO

1 cH

O 1 dN

Applying the conservation of mass principle to the carbon, hydrogen, oxygen, and nitrogen, respectively, gives

C: b 5 8

H: 2 c 5 18

O: 2 b 1 c 5 2a

N: d 5 3.76a

Solving these equations, a 5 12.5, b 5 8, c 5 9, d 5 47. The balanced chemical equation is

1 12.51O

1 3.76N

2S 8CO

1 9H

O 1 47N

The air–fuel ratio on a molar basis is

AF 5

12.5 1 12.513.7

12.514.762

5 59.5

kmol 1air2

kmol 1fuel2

13.1 Introducing Combustion 781

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782 Chapter 13 Reacting Mixtures and Combustion

13.1.3

Determining Products of Combustion

In each of the illustrations given above, complete combustion is assumed. For a hydro-

carbon fuel, this means that the only allowed products are CO

, H

O, and N

, with

also present when excess air is supplied. If the fuel is specified and combustion is

complete, the respective amounts of the products can be determined by applying the

conservation of mass principle to the chemical equation. The procedure for obtaining

the balanced reaction equation of an actual reaction where combustion is incomplete

is not always so straightforward.

Combustion is the result of a series of very complicated and rapid chemical reactions,

and the products formed depend on many factors. When fuel is burned in the cylinder

of an internal combustion engine, the products of the reaction vary with the tempera-

ture and pressure in the cylinder. In combustion equipment of all kinds, the degree of

mixing of the fuel and air is a controlling factor in the reactions that occur once the

fuel and air mixture is ignited. Although the amount of air supplied in an actual

combustion process may exceed the theoretical amount, it is not uncommon for some

carbon monoxide and unburned oxygen to appear in the products. This can be due

to incomplete mixing, insufficient time for complete combustion, and other factors.

When the amount of air supplied is less than the theoretical amount of air, the products

The air–fuel ratio expressed on a mass basis is

AF 5 c59.5

kmol 1air

kmol 1fuel2

d≥

28.97

kg 1air

kmol 1air

114.22

kg 1fuel

kmol 1fuel2

¥5 15.1

kg 1air

kg 1fuel2

(b) For 150% theoretical air, the chemical equation for complete combustion takes the form

➊ C

1 1.5112.521O

1 3.76N

bCO

1 cH

O 1 dN

1 eO

Applying conservation of mass

C: b 5 8

H: 2 c 5 18

O: 2 b 1 c 1 2e 5 11.52112.52122

N: d 5 11.52112.5213.762

Solving this set of equations, b 5 8, c 5 9, d 5 70.5, e 5 6.25, giving a balanced chemical equation

1 18.751O

1 3.76N

2S 8CO

1 9H

O 1 70.5N

1 6.25O

The air–fuel ratio on a molar basis is

AF 5

18.7514.76

5 89.25

kmol 1air2

kmol 1fuel2

On a mass basis, the air–fuel ratio is 22.6 kg (air)/kg (fuel), as can be verified.

➊ When complete combustion occurs with excess air, oxygen appears in the

products, in addition to carbon dioxide, water, and nitrogen.

Ability to…

❑

balance a chemical reaction

equation for complete com-

bustion with theoretical air

and with excess air.

❑

apply definitions of air–fuel

ratio on mass and molar

bases.

✓

Skills Developed

For the condition in part (b), determine the equivalence ratio.

Ans. 0.67.

TAKE NOTE...

In actual combustion pro-

cesses, the products of

combustion and their relative

amounts can be determined

only through measurement.

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