8. process variables-temperature

8/14/2019 8. Process Variables-Temperature

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PROCESS VARIABLES: TEMPERATURE

Temperature is defined as the degree of hotness or

coldness of a substance measured on some definite

scale. Temperature is a measure of the average kineticenergy of the molecules of a substance.

To compare the hotness (temperature) of two

substances need to define a scale of relative

temperatures - assign values to two points anddividing up the interval between into smaller intervals

called "degrees".

Temperature Scales

Fahrenheit scale and the Celsius scale - examples ofrelative Temperature scales.

Gabriel Daniel Fahrenheit(1686-1736), a German

physicist, fixed one point using a mixture of salt,

water, and ice (0

o

F) and the other using bodytemperature (96oF) -- chosen because it is divisible by

2, 3, 4, 6, 8). On this scale, water freezes at 32oF and

boils at 212oF.


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Anders Celsius (1701-1744), a Swedish astronomer

fixed the freezing point of water (0oC) and the boiling

point of water (100oC). Because it has 100 degrees,

this scale has also been called the centigrade scale.

To convert between the scales, first look at the size of

the degrees:

and then the start point

32oF = 0

oC

Thus to convert from degC to degF

EXAMPLE: What is the temperature in Fahrenheit

when it is 70

o

C?

212 - 32 1801.8

100 - 0 100

o o oo

oo o o

F F F F

CC C C! !

1.8 32

- 32

1.8

o o

o

o

T F T C

T F

T C

!

!


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Absolute Temperature Scales

Temperature is based on molecular motion -

Theoretically can exist a condition of no molecular

motion (so cold that the molecules stop moving, orzero kinetic energy in the molecules). This point is

called absolute zero, and is the lowest conceivable

temperature.

Useful to define temperature scales which began at

absolute zero - called absolute Temperature scales.

Two scales are commonly used - set up so that the

degree intervals are the same size as the relative

scales.

The Kelvin scale has the same size degree as the

Celsius scale. Thus,

0 K = - 273.15oC or 0

oC = 273.15 K

The Rankine scale has the same size degree as theFahrenheit scale, so:

0oR = - 459.67

oF or 0

oF = 459.67

oR

Celsius and Kelvin (or Fahrenheit and Rankine)

degrees have same "thickness" - make an "additive"

conversion to adjust between the two.


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EXAMPLE: What is the boiling point of water on the

absolute temperature scales?

To convert from Kelvin to Rankine - both start atabsolute zero - only use the "multiplicative"

conversion to switch (1.8 R/K).

Temperature Intervals

Degree is both a temperature and a temperature

interval

When converting an interval don't need to compensate

for the zero shifts.

EXAMPLE: You have a mixture at 50oF and increaseits temperature by 30

oC. What is the final

temperature?


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Converting Temperature

Interrelation Formula

Celsius to Fahrenheit T(F) = 1.8T(C) + 32

Fahrenheit to Celsius T(C) = [T(F) - 32]/1.8Celsius to Kelvin T(K) = T(C) + 273.15

Fahrenheit to Rankine T(R) = T(F) + 459.67

Kelvin to Rankine T(R) = 1.8T(K)

Rankine to Kelvin T(K) = T(R)/1.8

Converting Temperature Changes

Interrelation Formula

Fahrenheit to Celsius T(F) = 1.8T(C)

Celsius to Kelvin T(K) = T(C)Rankine to Kelvin T(K) = 1.8T(R)

8. process variables-temperature

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