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Heat Transfer

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Reading Materials: Chapter 9 Heat transfer results from a temperature difference. LAST LECTURE * * CONVECTION HEAT TRANSFER Modes forced flow induced by external ... – PowerPoint PPT presentation

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Title: Heat Transfer

1
Heat Transfer CONVECTION
Heat transfer results from a temperature
difference.
LAST LECTURE
2
CONVECTION HEAT TRANSFER
• Modes
• forced
• flow induced by external agency e.g. pump
• eg. forced-draught air cooler, evaporators
• natural
• fluid motion caused by temperature-induced
• Examples
• air flow over hot steam pipe, fireplace
circulation, cooling electronic devices

3
CONVECTION HEAT TRANSFER
Figure Natural convection flow over a heated
steam pipe
4
Modelling Convection
Forced convection generally most-effective
transport of energy from solid to fluid.
• Engineer's prime concern
• ?
• rate of convection
• ?
• enables sizing of equipment

5
Modelling Convection
Experimentally found that
h - convective heat transfer coefficient.
Main problem predict h value for variety
fluids flow rates range of shapes
6
Resistance Concept
Rate equation Written in same form as Ohms Law
(Ts-Tb) driving force (1/hA) thermal
resistance (R) for convection heat transfer.
7
TYPICAL UNITS FOR h
S.I. W m-2K-1 or J s-1m-2K-1
British Btu hr-1ft-2(F deg)-1
Conversion 1 W m-2K-1 0.176 Btu hr-1ft-2(F deg)-1
Typical Values
free convection (air) 5 - 60
forced convection (air) 25 - 300
forced convection (water) 200 - 10,000
boiling water 2,000 - 25,000
condensing steam 4,000 - 110,000
8
Illustration 27.1
Air at 20C is blown over an electrical resistor
to keep it cool. The resistor is rated at
40,000 ohm and has a potential difference of 200
volts applied across it. The expected mean heat
transfer coefficient between the resistor surface
and the air is 50 W m-2K-1. What will be the
surface temperature of the resistor, which has a
surface area of 2 cm2?
9
SOLUTION
Energy Balance Generation heat loss by
convection Rate of heat generation Convective
loss
10
Determining the size (H/T area) of the exchanger
(10.25)
(10.26)
Figure 1. Heat transfer between two flowing
fluids separated by a rectangular
11
Determining the size (H/T area) of the exchanger
Figure 2 Heat transfer between two flowing
fluids separated by a cylindrical wall
(10.27)
12
Overall heat-transfer coefficient
As a short-hand method of describing
heat-exchanger performance, we use the overall
heat-transfer coefficient,
(10.28)
13
Determining the size (H/T area) of the exchanger
14
Illustration 27.2
Consider the kettle below. For the conditions
given, find the flame temperature for the
following values of the heat transfer
coefficients hi (boiling) 4000 W m-2K-1
ho (gas flame) 40 W m-2K-1
15
Solution
Plane slab- area constant, eliminate A
16
Solution
17
Determining the size (H/T area) of the exchanger
(10.28)
• The term ?Tavg in equation 10.28 represents the
temperature difference between the hot and cold
streams averaged.
• For single-pass exchangers, the appropriate form
of ?Tavg is the log-mean temperature difference,
?Tlog mean (often abbreviated LMTD), defined as

(10.29)
18
Determining the size (H/T area) of the exchanger
(10. 30)
For shell-and-tube exchangers, the inside area
(Ai) of the tubes is smaller than the outside
area (Ao). However, the differences between Ai
and Ao will be neglected.
19
Example
Balance on cold stream
20
Example
How much area is required for the counter-current
heat exchanger in Example 10.5?
1
2
21
Example
From table 10.5
22
Example
How much area is required for the co-current heat
exchanger in Example 10.5?
1
Saturated water, 280oF, mstream
Hot
Saturated Steam, 280oF, mstream
2
Cold
Oil, 110oF, 960 lbm/min
Oil, 35oF, 960 lbm/min
23
Example
From table 10.5
24
Summary