7/28/2019 Heat Exchanger2.Ppt

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Heat Transfer with Phase Change

So far we have discussed heat transfer due to a

temperature gradient/difference

( )wbx

x

TThA

qdx

dTk

A

q

=

= conduction

convection

When a phase change takes place, the temperature on

one side is CONSTANT, but the presence of

boiling/condensing fluids affects heat transfer. Important in evaporation, distillation

LARGE h

Faith A. Morrison, Michigan Tech U.

vapor phase

liquid phase

satT

satTT>

Boilingresistance to heat transfer

At low T, few

bubbles form, and

heat transfer is by

natural convection.

As Tincreases,

more bubbles form,

increasing convection

(flow) in the liquid

phase, increasing h.

heat

Faith A. Morrison, Michigan Tech U.

7/28/2019 Heat Exchanger2.Ppt

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vapor phase

liquid phase

satT

satTT>

Film Boiling

resistance to heat transfer

As Tincreases

further, a film appears

and grows, increasing

resistance to heat

transfer

heat

Faith A. Morrison, Michigan Tech U.

Geankoplis, p260

natural

convection

nucleateboiling

transitionboiling

film

boilingRegimes of HeatTransfer during

Boiling

25.0T

radiation

becomes

important

There are

correlationsfor h for

each

regime.

Faith A. Morrison, Michigan Tech U.

7/28/2019 Heat Exchanger2.Ppt

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vapor phase

condensed phase

satTsatTT