Lecture 2

Processes and Process variables

1 Dr. A. Alim

Processes and Process variables:

Density and Specific gravity

Mass and Volumetric Flow Rate

Moles and Molecular Weight

Concentration

Pressure

Temperature

2

Process Variables – Density and Specific Gravity

Summary:

Density (ρ) = Mass / Volume

Specific gravity (solids and liquids) = Density / Density of water

Specific gravity (gases) = Density / Density of air

Question: what are examples of units for density and for

specific gravity?

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Process Variables – Mass and Volumetric

Flow Rate Measurements

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Process Variables – Moles and Molecular

Weight

A more accurate definition of atomic weight :

A dimensionless ratio of the average atomic mass (average of all present isotopes) to

1/12 the atomic mass of carbon -12

Note that “weight” here does not mean the usual definition of “weight” as the force exerted

by gravity we discussed earlier.

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Process Variables – Moles and Molecular

Weight

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• What is the average molecular weight :

From equation 3.3-7:

Av mol weight = (0.15)(32) + (0.044)(28) + (0.12)(44) + (0.69)(28) = 30.5

From equation 3.3-8:

Av mol weight = 1/[0.16/32 + 0.04/28 + 0.17/44 + 0.63/28] = 30.5

Or simply:

Av mol weight = total mass / total moles = 100 / 3.279 = 30.5

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Mole fraction-Mass fraction- Volume fraction:

The process for converting mass fraction to mole fraction and vice versa can

Also be extended to include conversions to and from volume fractions.

Key equation for each material is : volume = mass/density

Example:

A mixture of A and B has 30% by mass of A (sp.gravity 0.6) and the rest is B

(sp. Gravity 0.7). What are the volume fractions of A and B in the mixture?

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Mole fraction-Mass fraction- Volume fraction:

The process for converting mass fraction to mole fraction and vice versa can

Also be extended to include conversions to and from volume fractions.

Key equation for each material is : volume = mass/density

Example:

A mixture of A and B has 30% by mass of A (sp.gravity 0.6) and the rest is B

(sp. Gravity 0.7). What are the volume fractions of A and B in the mixture?

Basis 100 grams of mixture:

Material Mass (g) density(g/cm3) volume(cm3) Volume fraction

A 30 0.6 50 50/100 = 0.33

B 70 0.7 100 100/150 = 0.67

Total 100 150

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Process Variables – Concentration

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Process Variables – Pressure

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Pressure as Fluid head

• In addition to being expressed as force per unit area, a

pressure may also be expressed as head of a particular fluid.

• This is the equivalent height of a hypothetical column of this

fluid that would exert the given pressure at its base if the

pressure at the top were zero.

• With P0 = 0 , we can write P = ρ. h . (g/gc)

• Therefore Ph = h = (P. gc) / (ρ . g)

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The Barometer An instrument to measure atmospheric (barometric) pressure

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Pressure - is the ratio of force to the area on which the force acts; has

dimensions of (Force/length2)

Units in SI system = N/m2 or Pascal (Pa)

Units in CGS system = dynes/cm2

Units in British system = Ibf/ft2 (psf), or more commonly Ibf/in

2 (psi)

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The Bar:

The Bar is a widely used metric unit of measurement for pressure and 1

bar equals precisely 100,000 Pascals. Even though Bar is not an SI unit it

has been adopted as one of the most popular pressure units particularly

in European countries where most pressure measurement instruments

are specified with pressure ranges in bar.

The value of 1 bar is a close approximation to atmospheric pressure and

is often used to represent atmospheric pressure rather than standard

atmosphere (101325 Pascals) which is the correct value used by the

scientific and engineering community.

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IMPORTANT : Only absolute pressure is used in thermodynamic calculations !

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IMPORTANT : Only absolute pressure is used in thermodynamic calculations !

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Gage +ve

Gage –ve

Or Vac.

Abs.

Fluid Pressure Measurements

• Elastic elements methods - Bourdon

gauge

• Liquid column methods – Manometers

• Electrical methods – strain gauges,

transducers

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29 2. Resistance Transducer

3. 1.

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Resistance Transducer

31

The Manometer

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The Manometer

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DO NOT FORGET

gC !!!

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Manometers: Special Cases

Note: For gases, you may assume the gas density is zero

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Manometers: Special Cases

IMPORTANT : Pressure difference does not depend on manometer shape

diameter, inclination,…etc.

The ONLY important factor is the vertical distance between fluid surfaces.

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Process Variables – Temperature

• Is a measure of the average molecular

kinetic energy.

• Measurements:

Thermometers (Liquid expansion)

Thermocouples (Resistance)

Pyrometers (Radiation)

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Industrial thermometer

Pyrometer

Thermocouples

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Temperature conversion Temperature Interval conversion

Example: Water freezes at 0 oC which is the same as 1.8X0 + 32 = 32 oF

Example: The gas constant R is 1.987 cal/g mole K. Convert into Btu/lb mole oR. R = (1.987 cal/g mole K).(1Btu/252 cal) .(454 g mole/1 lb mole).(1 K/1.8 oR) = 1.987 Btu/lb mole oR.

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Home Work # 1

45

Home Work # 1

Thursday, June 7, 2012

Problems:

2.1 page 31

2.2 page 31

2.3 page 31

2.6 page 31

2.7 page 32

2.8 page 32

2.9 page 32

2.10 page 32

2.25 page 35

Example 2.5-2 page 19

2.18 page 33

2.31 page 36

2.45 page 41 (parts a, b, and c only)

3.6 page 65

3.16 page 68

3.25 page 69

3.36 page 73

3.41 page 74

3.42 page 74

3.48 page 77

3.46 page 76

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Solutions

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3rd ed. , Wiley 2005

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