Lecture # 2
Dimensions and Units
Dimensions and Units
Dimensions and Units
Short Quiz
Consider a 1.0 lbm watermelon, what is its
weight 𝑊, in the units of:
1. lbf
2. N
gravity: 𝑔 = 9.81 m/s2 (SI) , 𝑔 = 32.17 ft/s2 (USC)
𝑊
If it moves with an acceleration of 1.0 ft/s2,
what is the force 𝐹 on it, in the units of:
1. lbf
2. N
𝐹
?
Force and Weight
ENGR 3327
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Conversion factor
N.B.: British Thermal Units (BTU)  1 BTU = 778 lbf ft
CHAPTER 3
Processes and Process Variables
Processes and Process
Variables
Transfer of Energy
Single or Multiple
Species
Feed
Input
Specifications
Single or Multiple
Species
UNIT
OPERATIONS
Process
Specifications
Product
Output
Specifications
Continuous and Batch Processes
Input
Batch Process
Mass: Kg, lbm
Volume: m3, ft3
Output
Continuous Process
Mass flow rate: Kg/hr, lbm/sec
Volumetric flow rate: m3/min, ft3/min
Density, 𝜌 =
mass
volume
mass/time
mass flow rate
𝑚
Or, 𝜌 =
=
=
volume/time volumetric flow rate 𝑉
Specific Volume =
Specific Gravity:
1
𝜌
𝜌
SG =
𝜌ref
For Liquid, water at 4° C is used as a reference:
of solute
of solution
*
Molality =
gm-mole of solute
Kg of solvent
Atomic and Molecular Weight
• Atomic mass (m): mass of an atom.
• Atomic/Molecular weight (M): is the ratio of the weighted
average mass (naturally-occurring isotopes) of
atoms/molecules. Weight = “Weighted”
Units
SI: kg
non-SI (frequently used): atomic mass unit (amu), unified mass (u),
Dalton (Da), which is defined as 1/12 of the mass of a single carbon12 isotope atom.
Concept of gram-mole (or mol)
gram-mole (mol) = Mass in grams/ Molecular weight
Kg-mole = Mass in Kg/ Molecular Weight
lb-mole = Mass in lbm/Molecular Weight
A gram-mole ( g-mole or mol in SI): Amount of material whose mass in grams is
numerically equal to its molecular weight.
1 g-mole of O2 = 32g of O2
10 lb-mole of H2O = 10x18 = 180 lbm of H2O
Conversion between mass and moles
• Molecular Flow Rate: moles/time
• Mass Fraction (𝑥)
• Mole Fraction (𝑦)
• Average Molecular Weight(M)
Mass
fraction
Mole
fraction
Given
Given
Part per millions (ppm)
ppmi = 𝑦𝑖 × 106
Part per billions (ppb)
ppbi = 𝑦𝑖 × 109
•
•
•
•
Pressure
Manometer Equation
Temperature
Example Problems
Units of Pressure
• Pressure = force per unit area; fluid at rest exerts same
pressure in all directions.
• Units:
–
–
–
–
1 Pascal = 1 Pa = 1 N/m2
1 psi = 1 lbf/in2 (pound-force per square inch )
1 bar = 105 Pa = 0.1 MPa or 100 kPa
mmHg (millimeters of mercury)
Standard Atmosphere:
1 atm = 101,325 Pa ≈ 1.0 bar = 14.696 psi
The pressure excreted by a column of mercury (density equal to
13.591 g/cm3) 760 mm high at a point where the acceleration due to
gravity is equal to 9.80665 m/sec2)
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Pressure—Absolute, Gauge, Vacuum

Absolute: measured from
vacuum or zero, Pabs

Gauge: positive value
above atmospheric, Pgauge

Vacuum: negative value
below atmospheric, Pvacuum
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Example 1:
Example 2
Express 20 psi in terms of cm Hg.
Solution
Pressure Measured by Manometer

Differential Pressure:
Common measurement technique
is a manometer
– Column of liquid of known density
PA – Po = ρg/gc H
–
Unit: mm Hg, cm Hg, in H2o etc.
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Solution
Solution
*
*
*
*
*
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Temperature
t
Pressure Types
Hydrostatic pressure
(stationary)
Fluid pressure
(moving)