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The Application of Heat Exchangers
Transcript of The Application of Heat Exchangers
What is a
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?
What factors affect the performance of a heat exchanger?
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:
Q = -k.A.dT/dx
Q = h.A.(T - T)
There are a variety of types of cross flow in heat exchange:
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)
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.
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?
Common types of heat exchanger
Oil and gas industry
Cooling tower isolation
Thermal storage systems
Shell and Tube
AERO GROUP 6
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