Heat Exchangers Flashcards
Conduction
- heat transfer through a solid
- like the handle of a kettle becoming hot while the kettle is heating
Convection
- heat transfer through a liquid
- can not occur in a solid
Radiation ( radiant heat)
-can transfer across a vacuum or a gas
Gas(air) to gas(air)
-commonly used in compressors with intercoolers and after coolers
Liquid to gas
Liquid to liquid
-both sides of the tubes have liquid flowing through
Three types of heaters
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Type of flow paths
-parallel, counter, cross flow, mixed flow
Type of exchangers
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-parallel flow
- both fluids flowing in the same direction
- 2 fluids reach equilibrium at the halfway point between the two temperature extremes
Counter flow
- most efficient
- can reach the two temperature extremes if given enough time
Cross flow
-can produce lots of turbulence which makes for greater efficiency for a given size exchanger
Mixed flow
-common in heat exchangers with baffles that cause the flows to switch between parallel and counterflow
Finned tube exchanger (radiator)
- most common exchanger as it is a car radiator
- fins are added to increase surface that in turns transfers more heat
Plate heat exchanger
- high turbidity
- a number of pressed metal plates clamped together on a frame with gaskets. The fluid flows between the plates
- great for heat transfer because of the amount of surface area
- can’t Handle high pressure because of the gaskets between the plates
Spiral heat exchanger
- uses counterflow which helps make it the most efficient and has many spirals inside it which creates turbulence.
- also has lots of surface area because of the spirals.
Natural or mechanical draft Cooling towers
- natural relies on wind conditions
- mechanical use induced or forced draft just like fans. They have mist eliminators that help collect the water in the airstream
Double pipe heat exchanger
- uses counter current flow and because of the piping it can be used for high pressure
- these can be hooked together in series on a wall and take up very little space
- most common is the hairpin because it can handle differential thermal expansion
Shell & Tube baffles
- segmental - drilled plates equally spaced causing the fluid to flow back and forth across the tubes
- disc & doughnut - has a relatively smooth flow
- impingement - reduce erosion by entering fluid
- longitudinal - used to split the shell side flow into two or more passes
- 3 longitudinal baffles make a 4 pass shell
Baffle Main functions
-support the tubes and create turbulence to increase heat transfer
