CHAPTER 1 INTRODUCTION TO ENGINEERING CALCULATIONS

Transcription

1 CHAPTER 1 INTRODUCTION TO ENGINEERING CALCULATIONS Sem 1, 2016/2017 ERT 214 Material and Energy Balance / Imbangan Bahan dan Tenaga

2 After completing this chapter, you should be able to do the following: Convert a quantity expressed in one set of units into its equivalent in any other dimensionally consistent units using conversion factor tables. Identify the units commonly used to express both mass and weight in SI, CGS, and American Engineering units. Identify the number of significant figures in a given value expressed in either decimal or scientific notation and state the precision with which the value is known based on its significant figures. Explain the concept of dimensional homogeneity of equations. Given tabulated data for two variables (x and y), use linear interpolation between two data points to estimate the value of one variable for a given value of the other. Given a two-parameter expression relating two variables [such as y = a sin(2x) + b or P =1/(aQ3 + b) and two adjustable parameters (a and b), state what you would plot versus what to generate a straight line. Given data for x and y, generate the plot and estimate the parameters a and b.

3 Units and Dimensions Objectives: Convert one set of units in a function or equation into another equivalent set for mass, length, area, volume, time, energy and force Specify the basic and derived units in the SI and American engineering system for mass, length, volume, density, time, and their equivalence. Explain the difference between weight and mass Apply the concepts of dimensional consistency to determine the units of any term in a function

4 Dimensions are: properties that can be measured such as length, time, mass, temperature, properties that can be calculated by multiplying or dividing other dimensions, such as velocity (length/time), volume, density Units are used for expressing the dimensions such as feet or meter for length, hours/seconds for time. Every valid equation must be dimensionally homogeneous: that is, all additive terms on both sides of the equation must have the same unit

5 The numerical values of two quantities may be added or subtracted only if the units are the same. On the other hand, numerical values and their corresponding units may always be combined by multiplication or division.

6 Conversion of Units A measured quantity can be expressed in terms of any units having the appropriate dimension To convert a quantity expressed in terms of one unit to equivalent in terms of another unit, multiply the given quantity by the conversion factor Conversion factor a ratio of equivalent values of a quantity expressed in different units (new unit/old unit) Let s say to convert 36 mg to gram 36 mg 1 g = g 1000 mg Conversion factor

7 Example Convert an acceleration of 1 cm/s 2 to its equivalent in km/yr 2 1 d=24hr 1hr=60min 1min=60s So, 1hr=3600s A principle illustrated in this example is that raising a quantity (in particular, a conversion factor) to a power raises its units to the same power. The conversion factor for h 2 /day 2 is therefore the square of the factor for h/day:

8 Dimensional Equation Eg: Convert 1 cm/s 2 to km/yr 2 1. Write the given quantity and units on the left 2. Write the units of conversion factors that cancel the old unit and replace them with the desired unit 1 cm s 2 h 2 day 2 m km s 2 h 2 day 2 yr 2 cm m 3. Fill the value of the conversion factors 1 cm s h day 2 1 m 1 km s h day yr cm 1000 m 4. Carry out the arithmetic value (3600 x 24 x 365) 2 km = 9.95 x 109 km/ yr x 1000 yr 2

9 TEST YOURSELF 1) What is a conversion factor? 2) What is the conversion factor for s/min (s=second)? 3) What is the conversion factor for min 2 /s 2? 4) What is the conversion factor for m 3 /cm 3?

10 TEST YOURSELF (Ans) What is a conversion factor? a ratio of equivalent values of a quantity expressed in different units (new unit/old unit) What is the conversion factor for s/min (s=second)? 60s/1 min What is the conversion factor for min 2 /s 2? (1min) 2 /(60s) 2 What is the conversion factor for m 3 /cm 3? 1m 3 /10 6 cm 3

11 Systems of Units Components of a system of units: Base units - units for the dimensions of mass, 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), cm 2 (area), kg.m/s 2 (force) b) As defined equivalent of compound unit (Newton = 1 kg.m/s 2 )

12 3 Systems of Unit: a) SI system System International eg: meter (m) for length, kilogram (kg) for mass and second (s) for time. b) American engineering system Are the foot (ft) for length, the pound mass (lbm) for mass and the seconds (s) for time. c) CGS system Almost identical to SI. Principles different being that grams (g) and centimeters (cm) are used instead of kg and m as the base units of mass and length.

13 Base Units Base Units Quantity SI Symbol American Symbol CGS Symbol Length meter m foot ft centimeter cm Mass kilogram kg pound mass lbm gram g Moles grammole mole pound mole lbmole gram-mole mole Time second s second s second s Temperature Kelvin K Rankine R Kelvin K Base units - units for the dimensions of mass, length, time, temperature, electrical current, and light intensity.

14 Multiple Units Preferences Multiple Unit Preferences tera (T) = centi (c) = 10-2 giga (G) = 10 9 milli (m) = 10-3 mega (M) = 10 6 micro (μ) = 10-6 kilo (k) = 10 3 nano (n) = 10-9 Multiple units- multiple or fractions of base unit E.g.: for time can be hours, millisecond, year, etc

15 Derivatives SI Units Derived SI Units Quantity Unit Symbol Equivalent to the Base Unit Volume (L x W x H) Force (F=ma) Pressure (force/area) Energy/ Work (E=1/2*MV 2 ) depends on mass n speed Power (force x distance)/time Liter L 0.001m 3 = 1000 cm 3 Newton (SI) Dyne (CGS) N 1 kg.m/s 2 1 g.cm/s 2 Pascal Pa 1 N/m 2 Joule Calorie J cal 1 N.m = 1 kg.m 2 /s J =1 cal Watt W 1 J/s = 1 kg.m 2 /s 3 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), cm 2 (area), kg.m/s 2 (force) b) As defined equivalent of compound unit (Newton = 1 kg.m/s 2 )

16 Example Conversion Between Systems of Units Convert 23 Ibm. ft/min 2 to its equivalent in kg cm/s 2. Ans: As before, begin by writing the dimensional equation, fill in the units of conversion factors (new/old) and then the numerical values of these factors, and then do the arithmetic.

17 TEST YOURSELF 1. What are the factors (numerical values and units) needed to convert? (a) meters to millimeters? (b) nanoseconds to seconds? (c) square centimeters to square meters? (d) cubic feet to cubic meters (use the conversion factor table on the inside front cover)? (e) horsepower to British thermal units per second? 2. What is the derived SI unit for velocity? The velocity unit in the CGS system? In the American engineering system?

18 TEST YOURSELF (Ans) 1. What are the factors (numerical values and units) needed to convert? (a) meters to millimeters? 1000mm/1m (b) nanoseconds to seconds? 10-9 s/1ns (c) square centimeters to square meters? 1m 2 /10 4 cm 2 (d) cubic feet to cubic meters (use the conversion factor table on the inside front cover)? 1m 3 / ft 3 (e) horsepower to British thermal units per second? (9.486x10-4 Btu/s)/(1.341x10-3 hp) 2. What is the derived SI unit for velocity? The velocity unit in the CGS system? In the American engineering system? m/s, cm/s, ft/s

19 Force and Weight Force is proportional to product of mass and acceleration (according Newton second law of motion) kg.m/s2 (SI unit), g.cm/s 2 (CGS) and lbm.ft/s 2 (American engineering) Usually defined using derived units ; 1 Newton (N) = 1 kg.m/s 2 1 dyne = 1 g.cm/s 2 1 Ib f = Ib m.ft/s 2 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 = m/s 2 = cm/s 2 = ft/s 2

20 Example The force in newtons required to accelerate a mass of 4.00 kg at a rate of 9m/s 2 is 4 kg 9m 1N s 2 1kg.m/s 2 = 36.0 N The force in lb f required to accelerate a mass of 4.00 Ib m at a rate of 9.00 ft/s 2 is 4 lb m 9ft 1lb f s lb m.ft/s 2 = 1.12lb f

21 g c is used to denote the conversion factor from a natural force unit to a derived force unit. g c = 1 kg.m/s 2 = lb m.ft/s 2 1N 1 lb f

22 The weight of an object is the force exerted on the object by gravitational attraction. Suppose that an object of mass m is subjected to a gravitational force W (W is by definition the weight of the object) and that if this object were falling freely its acceleration would be g. The weight, mass, and free-fall acceleration of the object are related by W=mg. The value of g at sea level and 45'" latitude is given below in each system of units:

23 Sample Mean Variations in sampling and chemical analysis procedures invariably introduce scatter in measured values (X). We might ask two questions about the system at this point. 1. What is the true value of X? 2. How can we estimate of the true value of X? So, we collect N (numb of measured values of X (X l, X 2,..., X N ) and then calculate

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25 Two-Point Linear Interpolation Given tabulated data for two variables (x and y), use linear interpolation between two data points to estimate the value of one variable for a given value of the other. The equation of the line through (X l, y 1 ) and (X 2, Y 2 ) on a plot of y versus x is You may use this equation to estimate y for an x between Xl and X2; You may also use it to estimate y for an x outside of this range (i.e., to extrapolate the data), but with a much greater risk of inaccuracy.

26 Generate a Straight Line Given a two-parameter expression relating two variables [such as y = a sin(2x) + b or P =1/(aQ3 + b) and two adjustable parameters (a and b), state what you would plot versus what to generate a straight line. Given data for x and y, generate the plot and estimate the parameters a and b. Example:

27 References: Felder, Rousseau Elementary Principles of Chemical Processes John-Wiley, 3rd Edition Murphy, R.M. Introduction to Chemical Processes: Principles, Analysis, Synthesis McGraw Hill, Himmelblau, Riggs Basic Principles and Calculations in Chemical Engineering, Prentice Hall, 7 th Edition, J.F. Richardson Chemical Engineering, Volume 3 Prentice Hall, Pauline Doran Bioprocess Engineering Principles Elsevier Science, 1995.

28 THANK YOU