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7/28/2019 Chap. 1 Introduction to Engineering Calculations
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Introduction to EngineeringCalculations
Chapter1
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After completing this chapter, you should
be able to:
Identify units commonly used to express both
mass and weight in SI and Americanengineering system
Perform unit conversions
Apply the concept of dimensional homogeneityof equation
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Units and Dimension
Conversions of Unit
Systems of Units
Force, Weight and Mass
Whats in this chapter
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Introduction
Describe the basic techniques for the handling of
units and dimensions in calculations.
Describe the basic techniques for expressing the
values of process variables and for setting up andsolving equations that relate these variables.
Develop an ability to analyze and work engineering
problems by practice.
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CHEMISTVS
CHEMICAL ENGINEER
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SEPARATION PROCESS
REACTION PROCESS
SEPARATION PROCESS
RAW MATERIALS
INTERMEDIATE PRODUCT
INTERMEDIATE PRODUCT
FINAL PRODUCT
Chemical process
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You need to:
Minimize production of unwanted
byproducts
Separate the good (product) from the
bad (byproducts)
Recover the unused reactants
Maximize profit, minimize energy
consumption, minimize impact on the
environment
Role of chemical engineer
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petroleum and petrochemical
pharmaceuticals
polymers
energy
food
consumer products
biotechnology
electronic and optical materials.
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Dimensions are: properties that can be measured such as length,
time, mass, temperature,
properties that can calculated by multiplying ordividing other dimensions, such as velocity(length/time), volume, density
Units are used forexpressing the dimensions such asfeet or meter for length, hours/seconds for time.
Every valid equation must be dimensionallyhomogeneous: that is, all additive terms on both sidesof the equation must have the same unit
1.1 Units and dimension
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1.2 Conversion of units
A measured quantity can be expressed in terms ofany units having the appropriate dimension
To convert a quantity expressed in terms of one unitto equivalent in terms of another unit, multiply thegiven quantity by the conversion factor
Conversion factor a ratio of equivalent values of aquantity expressed in different units
Let say to convert 36 mg to gram
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1.2.1 Dimensional equation
1. Write the given quantity and units on left
2. Write the units of conversion factors that cancel
the old unit and replace them with the desired
unit3. Fill the value of the conversion factors
4. Carry out the arithmetic value
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Convert 1 cm/s2 to km/yr2
1 cm s2 h2 day2 m km
s2 h2 day2 yr2 cm m
1 cm 36002 s2 242 h2 3652
day21 m 1 km
s2 12 h2 12 day2 12 yr2 100 cm 1000 m
(3600 x 24 x 365) 2 km=
9.95 x 109 km/ yr2
100 x 1000 yr2
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1.3 Systems of units
Components of a system of units: Base units - units for the dimensions ofmass, length, time,
temperature, electrical current, and light intensity.
Multiple units- multiple or fractions of base unit
E.g.: for time can be hours, millisecond, year, etc.
Derived units - units that are obtained in one or two ways;
a) By multiplying and dividing base units; also referred to as
compound units
Example: ft/min (velocity), cm2(area), kg.m/s2 (force)
b) As defined equivalent of compound unit (Newton = 1 kg.m/s2)
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3 systems of unit:
a) SI system
b) American engineering system
c) CGS system
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Base Units
Quantity SI Symbol American Symbol CGS Symbol
Length meter m foot ft centimeter cm
Mass kilogram kgpoundmass lbm gram g
Molesgram-mole mole pound mole lbmole gram-mole mole
Time second s second s second s
Temperature Kelvin K Rankine R Kelvin K
Table 1: SI, American and CGS Units
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Multiple Unit Preferences
tera (T) = 10 12 centi (c) = 10 -2
giga (G) = 10 9 milli (m) = 10 -3
mega (M) = 10 6 micro () = 10 -6
kilo (k) = 10 12 nano (n) = 10 -9
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Derived SI Units
Quantity Unit Symbol Equivalent to the Base Unit
Volume Liter L 0.001m3 = 1000 cm3
Force Newton
(SI)Dyne(CGS)
N 1 kg.m/s2
1 g.cm/s2
Pressure Pascal Pa 1 N/m2
Energy/WorkJouleCalorie
Jcal
1 N.m = 1 kg.m2/s2
4.184 J =4.184 kg.m2/s2
Power Watt W 1 J/s = 1 kg.m2/s3
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1.4 Force and weight
Force is proportional to product of mass and acceleration
Usually defined using derived units ;
1 Newton (N) = 1 kg.m/s2
1 dyne = 1 g.cm/s2
1 Ibf = 32.174 Ibm.ft/s2
Weight of an object is force exerted on the object by
gravitational attraction of the earth i.e. force of gravity, g.
Value of gravitational acceleration:
g = 9.8066 m/s2
= 980.66 cm/s2
= 32.174 ft/s2
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gc is used to denote the conversion factor
from a natural force unit to a derived
force unit.
gc = 1kg.m/s2= 32.174
lbm.ft/s2
1N 1 lbf
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1.5 Dimensional homogeneity and
dimensionless quantities
Every valid equation must be dimensionally
homogeneous: that is, all additive terms on both
sides of the equation must have the samedimensions
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Exercises
Problems 2.1, 2.2, 2.8, 2.9, 2.26