1304 332 Unit Operation in Heat Transfer

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1304 332 Unit Operation in Heat Transfer

• Instructor Dr.Chakkrit Umpuch
• Department of Chemical Engineering
• Faculty of Engineering
• Ubon Rachathani University
• Presented to
• Chemical and Biological Engineering Program

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1304 332 Unit Operation in Heat Transfer

• Class EN6304 (4.00-7.00 pm)
• Assessment
• Homework 10 (At least 80 submitted)
• Attendance 10 (Attend class and class
  intention)
• Midterm exam 30 (Closed book exam)
• Final exam 40 (Closed book exam)
• Warning
• Compulsory, students must have attended 80 of
  lectures in a course in order to sit for the
  final exam.
• Text
• - Holman, Heat Transfer, 8th edition,
  McGraw-Hill, 1997.
• - Gebhart, Heat conduction mass diffusion,
  McGraw-Hill, 1993.
• - Incropera Dewitt, Fundamental of heat and
  mass transfer, John-
• Wiley, 1996.

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Course Outline

• Introduction to Heat Transfer
• Chapter 1 Heat Conduction
• Chapter 2 Heat Convection
• Chapter 3 Heat Exchangers
• Chapter 4 Binary Distillation
• Chapter 5 Liquid-liquid Extraction
• Chapter 6 Humidification and Cooling Tower

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What is heat transfer?

• Heat transfer is a science that studies the
  energy transfer between two bodies due to
  temperature difference.
• Thermodynamics is used to predict the amount of
  energy required to change a system from one
  equilibrium state to another.
• Heat Transfer is used to predict how fast a
  change will take place since system is not
  equilibrium during the process.

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Three modes of heat transfer

• 1.Heat conduction Heat goes through a static
  material (medium).
• 2.Heat convection Heat goes through a moving
  medium or is carried away
• by a moving medium (fluid).
• 3.Heat radiation Heat travels through a
  space with or without a medium.

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Heat conduction
Rate of heat transfer
where T temperature at any point in the wall
x thickness of the wall Aarea of the wall
k thermal conductivity (Btu/hr.ft.ºF) -dT/dx
temperature gradient
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Thermal Conductivity
Metals
Silver 410 W/m. C 237 Btu/h.ft.F
Copper 385 W/m. C 223 Btu/h.ft.F
Nonmetallic solids
Diamond 2300 W/m. C 1329 Btu/h.ft.F
Quartz 41.6 W/m. C 24 Btu/h.ft.F
Sandstone 0.78 W/m. C 0.45 Btu/h.ft.F
Liquids
Ammonia 0.147 W/m. C 0.085 Btu/h.ft.F
Water 0.556 W/m. C 0.327 Btu/h.ft.F
Gases
Hydrogen 0.175 W/m. C 0.101 Btu/h.ft.F
Air 0.0206 W/m. C 0.0119 Btu/h.ft.F
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Thermal conductivity is strongly
temperature-dependent.
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Example 01 One face of a copper 3 cm thick is
maintained at 400 C, and the other face is
maintained at 100 C. How much heat is
transferred through the plate?
solution
Copper
Rate of heat transfer
400 ºC
100 ºC
kcopper 370 w/m.ºC
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Heat convection
Rate of heat transfer
where h heat transfer coefficient
Heated air rises, cools, then falls. Air near
heater is replaced by cooler air, and the cycle
repeats.
Hot water rises, cools, and falls
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Heat convection
Force Heat Convection
Natural or Free Heat Convection
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Convection heat-transfer coefficients
Free Convection
Vertical plate 0.3m high in air 4.5 W/m2. C 0.79 Btu/h.ft2.F
Horizontal cylinder, 2-cm diameter, in air 6.5 W/m. C 1.14Btu/h.ft.F
Forced Convection
Airflow at 2m/s over 0.2-m square plate 12 W/m2. C 2.1 Btu/h.ft2.F
Airflow at 2m/s over 0.2-m square plate 75 W/m. C 13.2 Btu/h.ft.F
Boiling water
In a pool or container 2,500-35,000 W/m2. C 400-600 Btu/h.ft2.F
Condensation of water vapor
Vertical surfaces 4,000-11,300W/m2. C 700-2,000 Btu/h.ft2.F
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Convection heat transfer from a plate
Flow
Since the velocity of the fluid layer at the wall
will be zero, the heat must be transferred only
by conduction at that point.
Newtons law of cooling
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Example 02 Air at 20 ºC blows over a hot plate 50
by 75 cm maintained at 250 ºC. The convection
heat-transfer coefficient is 25 W/m2.ºC.
Calculate the heat transfer.
solution
Rate of heat transfer
Air
Too 20 ºC
h 25 w/m2.ºC
Tw 250 ºC
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Heat radiation
Radiation is heat from the sun that travels in
the form of waves.
Rate of heat transfer
where
s Stefan-Boltzmann constant (5.669 x 10-8
W/m2.K4) e emissivity T absolute temperature
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Example 03 A heated plate of D 0.2 m diameter
has one of its surface insulated, and the other
is maintained at Tw550 K. If the hot surface has
an emissivity e 0.9 and is exposed to a
surrounding area at Ts300 K with atmospheric air
being the intervening medium, calculate the heat
loss by radiation from the hot plate to the
surroundings.
solution
Rate of heat transfer
Ts300 K
r 0.1m
Tw550 K
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Three modes of heat transfer
Radiant energy
Surrounding at Ts
Flow, T8
Tw
Heat conducted through wall
The heat conducted through the plate is removed
from the plate surface by a combination of
convection and radiation.
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Example 04 Assuming that the plate in Example 02
is made of carbon steel (1) 2 cm thick and that
300 W is lost from the plate surface by
radiation, calculate the inside plate temperature.
Air
Too 20 ºC
Tw 250 ºC
h 25 w/m2.ºC
Kcarbon steel 43 W/mC
Ti ? ºC
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Dimensions and Units
Unit symbol English SI Heat
flow Q BTU/h W Heat flux per unit
area q BTU/h.ft2 W/m2 Thermal conductivity k BTU/h
.ft.ºF W/m. ºC Temperature T ºF
ºC Length L ft m
Do you know how to convert unit from one to the
other systems?
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Practice in class

• 1nm ? mm
• 1ft ? mm (1ft 0.3048m)
• 900 ºR ? ºK
• ºF 9/5ºC32
• ºR ºF459.69
• ºK ºC273.16
• ºR 9/5ºK
• 4) 1 BTU/hr.ft2 ? W/m2 (1BTU 1.05504 kJ
  WJ/s)

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Homework1

• 1. A temperature difference of 85 ºC is
  impressed across a fiberglass layer of 13 cm
  thickness. The thermal conductivity of the
  fiberglass is 0.035 W/m. ºC. Compute the heat
  transferred through the material per hour per
  unit area.
• 2. One side of a plane wall is maintained at
  100C, while the other side is exposed to a
  convection environment having T10C and
  h10W/m2. C. The wall has k1.6W/m. C and is 40
  cm thick. Calculate the heat-transfer rate
  through the wall.