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Introduction to Heat Transfer

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Introduction to Heat Transfer Powered By Docstoc
					Heat Transfer Lecture
Tutor: Paul Jensen email: Paulj@cheque.uq.edu.au Phone Ex. 58398

Objectives
• Perform an energy balance over the heat exchanger • Identify key variables affecting heat transfer and quantify the nature of any significant effects • Develop appropriate models to describe heat transfer characteristics • Recommend future experiments to investigate heat exchanger performance.

Introduction to Heat Transfer
• The driving force for heat transfer is temperature gradient • Three Mechanisms of Heat transfer – Conduction, Convection, Radiation

Conduction
Refers to transfer of energy through a medium via direct contact, without intermixing or flow of any material and is expressed by Fourier’s law

Fourier’s Law
dT  qx  kA dx
Where:
 q x rate of heat transfer in x direction k Thermal conductivity A Cross sectional area dT Temperature gradient in x direction
dx

Convection
Heat is transferred by the motion of a fluid. If the fluid is forced (i.e. Pump) then we talk of forced convection . If the motion of the fluid is induced by temperature we talk of natural convection. (i.e. Hot air rises).

Convection
In order to account for convective heat transfer we use the concept of a boundary layer which contains all the resistance to heat transfer.

The heat transfer coefficient h, is a function of the fluid properties cp,, , k, the flow conditions and the system geometry.

Radiation
Radiation refers to light, infrared, ultraviolet and radio waves which emanate from a hot body and are absorbed by a cooler body.

Introduction to Heat Exchangers
What Are Heat Exchangers?
Heat exchangers are units designed to transfer heat from a hot flowing stream to a cold flowing stream.

Why Use Heat Exchangers?
Heat exchangers and heat recovery is often used to improve process efficiency.

Types of Heat Exchangers
There are three broad categories: • The recuperator, or through-the-wall non storing exchanger • The direct contact non storing exchanger • The regenerator, accumulator, or heat storage exchanger

Recuperators

Direct Contact

Regenerators

Heat Transfer Within a Heat Exchanger
Heat transfer within a heat exchanger typically involves a combination of

conduction and convection
The overall heat transfer coefficient U accounts for the overall resistance to

heat transfer from convection and conduction

1 1 1 x    UA h1 A1 h2 A2 kAmean

Heat Exchanger Driving Force
The temperature difference and thus the driving force for heat transfer varies throughout the heat exchanger.

Log mean Temperature Difference
The temperature difference at each end of the exchanger is calculated and combined using the following equation to give the log mean temperature difference:

T2  T1 Tlm  T2 ln T1

Heat Exchanger Energy Balance
Rate of heat transfer by exchanger Rate of heat loss by hot fluid Rate of heat gain by cold fluid

=

=

   q  UA Tlm   mhot Chot Thot ,in  Thot ,out   mcold Ccold Tcold ,in  Tcold ,out 

Heat Exchanger Diagram
Steam In P1 Steam Valve T1

Water Out T4
Water In

Rotameter

Double Pipe Heat Exchanger

T3

T2

Steam Out