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Boiling & condensation
1. PRESENTED BY : Ziad Zohdy
SUBMITTED TO : Dr. Mohamed abo el-azm
BOILING & CONDENSATION
2. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
3. What is meant by Boiling
• the transition of a liquid into vapor .
• Boiling begins when the liquid is heated to the point at
which the pressure of the saturated vapor above its
surface becomes equal to the external pressure
4. To keep a liquid boiling , the heat that is expended
in the formation of bubbles must be supplied. Thus,
boiling is inseparably associated with heat transfer,
which results in the transmission of heat from the
heating surface to the liquid. The heat exchange
during boiling is one of the forms of convective heat
transfer.
6. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
8. Pool boiling
The fluid isn’t forced to flow by a mover like a pump
Any motion of the fluid is due to natural convection
The surface Temperature is greater than the
saturation temperature so the boiling will occur
9. Flow boiling
Liquid is forced to move by an external source like a
pump
Liquid is forced through the tube with temperature
Ts
Heat is added through the wall of the tube ( hot
liquid passes outside or electrical heater)
10. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
12. Natural convection
A super heated liquid rising to the surface and
evaporation takes place from the free surface
The fluid motion in this mode of boiling is governed by
natural convection and the heat transfer from the heated
surface to the fluid also by natural convection.
13. Nucleate Boiling
From A-B : isolated bubbles are formed on the
heated surface, but these bubbles are dissipated in
the liquid shortly after they separate from the
surface.
From B-C : the heater temperature is increased, so
we get large number of nucleation sites and large
14. These bubbles move up to the free surface
where they break up and release their vapor
content.
Nucleate boiling is the most desirable boiling
regime in practice because high heat transfer
rates can be achieved in this regime
16. Transition Boiling
The temperature difference is increased at point
C, the heat flux decreases.
This is because a large fraction of the heater
surface is covered by a vapor film, which acts as an
insulation due to the low thermal conductivity of the
vapor
18. Film Boiling
In this region the heater surface is completely
covered by a continuous stable vapor film.
Point D, where the heat flux reaches a minimum, is
called the Leidenfrost point.
20. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
22. Nucleate boiling
The rate of heat transfer depends on :
the number of active nucleation sites on the
surface.
the rate of bubble formation at each site
It is observed that irregularities on the heating
surface, including roughness and dirt, serve as
additional nucleation sites during boiling
23.
24. MAX heat flux
maximum heat flux found at point C
Ccr is a constant whose value depends on the heater
geometry.
27. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
29. Condensation
Condensation occurs when the temperature of a
vapor is reduced below its saturation temperature.
Ts > Tw
Vapor is going to condense on the surface
30. Types of condensation
Drop wise
condensat
ion
High rate
of heat
transfer
Film
condensat
ion
Low rate
of heat
transfer
U0 = 1/(Ao*((1/(h0 * Ao)) + Rw + (1/(hi * Ai)))
– Uo = Overall heat transfer coefficient
– hi= Tube side heat transfer coefficient
– ho = Condensation heat transfer coefficient
–Rw = Wall resistance
If the area of condensation decreases, coefficient will
increase, therefore drop wise condensation have more
condensation than film wise condensation
31. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
32. The rate of heat transfer
for the same temperature difference, drop
condensation get high heat flux more than the film
condensation.
Drop condensation is desirable but it difficult to
maintain.
33. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
Out lines
34. Film condensation on horizontal
tubes
A saturated vapor at temperature (
Ts ) condensing on the outside of a
horizontal tube maintained at ( Tw )
Liquid film has go down along the
circumference & finally falls off at
the bottom of the tube.
35. The average heat transfer coefficient
given by :
λ : The latent heat
ρ : Density
K : Thermal conductivity
μ: Kinematic viscosity
D : Diameter
37. • what is meant by Boiling
• Types of boiling
• Boiling curve
• Heat transfer correlations
• Condensation
• Types of condensation
• Rate of heat transfer
• Film condensation on horizontal tube
• problem as an example
Out lines
38. Problem on film condensation on
horizontal tubes :
Saurated R-22 vapor at 262 K condenses as a film
on a horizontal tube ( 22 mm OD ) whose outer wall
is at a temperature of 258 K
Calculate :-
1- The average heat transfer coefficient. “ h “
2-The film Reynolds number at the bottom of the tube
“Re “
39. We had to calculate the mean temprature first to get the
propertise of R-22 from the table at the mean temperature
MEAN FILM TEMP = ( Ts + Tw)/2
Mean Temp = (262 + 258 ) / 2 = 260 K
