Post on 02-Jun-2018
transcript
8/11/2019 3-1-1 Heat Transfer Theory
1/13
8/11/2019 3-1-1 Heat Transfer Theory
2/13
Heat Transfer TheoryIntroduction
8/11/2019 3-1-1 Heat Transfer Theory
3/13
The heat lost by the hot mediumis equal to the amountof heat gained by the cold medium, except for losses tothe surroundings.
There must be a temperature differencebetween the twomedia for heat transfer to take place.
Fundamentals of Heat Transfer Theory
Three basic natural laws of physics:
Heat will always be transferred from a hot mediumto acold medium, until equilibrium is reached.
8/11/2019 3-1-1 Heat Transfer Theory
4/13
Modes of Heat Transfer
Three modes:
Radiation = Electromagnetic waves
Conduction = Molecular or atomic vibrations
Convection = Transport of small mass elements
8/11/2019 3-1-1 Heat Transfer Theory
5/13
Convection
Is the most efficient way of transporting heat
This is what we want in the heat exchanger
8/11/2019 3-1-1 Heat Transfer Theory
6/13
Flow Principles: Laminar
Parabolic velocity profile:
friction close to wall -> lower velocity
centre of tube -> higher velocity
Low velocity and low Reynolds number -> low pressure drop
Distinct parallel fluid layers -> no mixing between layers
Only conduction -> poor heat transfer efficiency
Flow profile Velocity profile
8/11/2019 3-1-1 Heat Transfer Theory
7/13
Nearly constant velocity profile
High velocity and high Reynolds number -> high pressure drop
Turbulent flow -> cleaning effect -> less fouling
No orderly flow -> mixing of fluid mass elements
Mainly convection -> high heat transfer efficiency
Flow Principles: Turbulent
Flow profile Velocity profile
8/11/2019 3-1-1 Heat Transfer Theory
8/13
Even in turbulent flow, there is always a layer closest to the wall with
laminar film where heat is transferred only by conduction
Flow Principles: Turbulent
8/11/2019 3-1-1 Heat Transfer Theory
9/13
Plate pattern
Turbulent flow
Herring bone pattern -> many
contact points where the fluid
is forced to turn direction ->
induced turbulence ->convection -> high heat
transfer efficiency
Cleaning effect
High pressure drop
8/11/2019 3-1-1 Heat Transfer Theory
10/13
Laminar Film
Thickness of the laminar film depends on
Fluid velocity
Fluid viscosity
Plate pattern
The thinner the laminar film, the less resistance to heat transfer,
the more efficient heat exchanger
8/11/2019 3-1-1 Heat Transfer Theory
11/13
Two heat exchanger types
Direct
Principle: Product and service medium are in direct contact
Example: Water and air in a cooling tower
Indirect
Principle: Product and service medium are separated by a wall
Example: Hot water and product in a plate heat exchanger
8/11/2019 3-1-1 Heat Transfer Theory
12/13
Counter-current v.s Co-current
Counter current flow:
medium driving force all over plate
possible with crossing temperatures
larger LMTD -> smaller area
T2outT1out
Co current flow:
high driving force in beginning
very low in the endbetter control of wall temperature
lower pressure drop in
condensing duties
T2outT1out
T2in
T1in
T2in
T1in
8/11/2019 3-1-1 Heat Transfer Theory
13/13