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The Application of Heat Exchangers

A presentation covering heat exchangers, factors affecting their efficiency and a focus on one particular industrial application.
by

Luke McCarthy

on 16 December 2013

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Transcript of The Application of Heat Exchangers

Application of
Heat Exchangers

What is a
heat exchanger?
A heat exchanger is a device used to efficiently transfer heat from one medium to another. The media may be either:

- Separated by a solid wall

- In direct contact
What are the common types of flow associated with heat exchangers?
Parallel Flow
Counter Flow
Cross Flow
What factors affect the performance of a heat exchanger?
x
HOT IN
COLD OUT
HOT OUT
COLD IN
T
x
Hot fluid
Cold Fluid
x
HOT IN
COLD IN
HOT OUT
COLD OUT
T
x
Hot fluid
Cold Fluid
As the name suggests, the two fluid media flow parallel to each other. They are divided by a solid wall. Heat transfer occurs from the hot fluid to the wall by convection, through the wall by conduction and from the wall to the cold fluid by convection.
In counter flow, the two fluids flow in opposite directions, and are again separated by a solid wall. This helps to maintain a greater temperature gradient across the wall, allowing for faster heat transfer. Heat is transferred in the same way as in a parallel flow system.
Let's Quantify "Performance"
A heat exchanger must be able to transfer heat quickly and efficiently. We must therefore maximise both:
.
Conduction:
Q = -k.A.dT/dx
Convection:
Q = h.A.(T - T)
1
.
.
2
There are a variety of types of cross flow in heat exchange:
mixed
unmixed
We try to minimise the thickness of the boundary wall to maximise the temperature gradient.
We try to maximise the surface area available for heat transfer.
Q = U.A.(T - T)
1
.
2
AND
X
Y
THE DYSON HOT
Air from the surroundings is drawn into the Dyson Hot by an impeller.
Instead of a large open vent, many small holes are used to prevent large impurities from entering.
Fouling
Over time, impurities may be deposited on the heat transfer surface.

These deposits create additional resistance to heat transfer due to:

reduced fluid velocity.
poor thermal contact between media.

If fouling is likely, it must be accounted for in engineering design.
The air is forced into a looping chamber at high pressure and at a high Reynold's Number.
Why do we want a high Re?
Element
Fins
Ceramic
Air
Common types of heat exchanger
Double Pipe
Oil and gas industry
Food processing
Chemical processing
Plate
Water heaters
Cooling tower isolation
Thermal storage systems
Shell and Tube
Refrigeration
Dairy industry
Compressor cooling
AERO GROUP 6
solid
hollow
References
To keep the casing cool to the touch, the material used needs to have a low heat conduction coefficient, giving it a low rate of heat exchange.
Low Reynolds number as the air leaves the Dyson Hot
http://www.nytimes.com/interactive/2011/12/29/technology/personaltech/a-new-way-to-get-more-comfort-out-of-hot-air.html?nl=technology&emc=ctb1

http://excelcalculations.blogspot.co.uk/2011/11/double-pipe-heat-exchanger.html

http://www.graham-mfg.com/index.asp?pageId=76

http://www.shell-tube.com/

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-engineering-principles-and-heat-transfer/steam-consumption-of-heat-exchangers.asp

http://www.dyson.co.uk/fans/fansandheaters/air-multiplier-technology.aspx

http://www.appliancesonlineblog.com.au/appliance-news/the-new-dyson-hotcool-fan/

http://www.likecool.com/Dyson_Hot--Appliance--Home.html
Full transcript