Evaporation occurs at the liquid-vapor interface when the vapor pressure is less than the saturation pressure, without bubble formation. Boiling occurs at the solid-liquid interface when the surface is above the liquid's saturation temperature, involving rapid bubble formation and detachment from the surface. The key difference is that boiling involves bubbles at the solid-liquid interface, while evaporation does not.

1. Difference B/W Evaporation &
Boiling
Evaporation
-occurs at liquid-vapor interface when
vapor pressure is less than saturation
pressure of the liquid at a given
temperature e.g. evaporation from water
at 20 C.
- No bubble formation or bubble
movement involved

3. Cont…..
 Boiling
- occurs at solid-liquid interface when
liquid is brought into contact with a
surface at temp Ts, above saturation temp
Tsat of the liquid
- It involves rapid bubble formation at solid-
surface that detach from surface and rise
to top surface

4. Boiling occurs when a liquid is brought into
contact with a surface at a temperature above
the saturation temp of the liquid

5. Laws Applicable to Boiling
Newton's law of cooling
q boiling=h(Ts-Tsat)=h ∆Texcess
∆Texcess =Ts-Tsat =Excess temperature
Ts=temperature of heating surface
Tsat= saturation temp of liquid

6. Classification of Boiling
1) Based on Bulk Fluid Motion
 a. Pool Boiling
 b. Flow Boiling
2) Based on Bulk liquid temperature
a. Sub-cooled Boiling
b. Saturated Boiling

7. Pool Boiling
 Boiling in absence of bulk fluid flow
 Fluid body is stationery
 Any possible fluid motion will be due to
natural convection currents
 E.g boiling of water in a pan on stove

8. Flow Boiling/Forced convection
boiling
 Boiling in presence of bulk fluid flow
 Fluid is forced to flow in a heated pipe or
over a surface by pump etc
 Convection effects will be present

9. Sub-cooled/Local Boiling
 Boiling is sub-cooled if temperature of
main body of fluid is below the saturation
temp Tsat (i.e. bulk of liquid is sub-cooled)
 It occurs at early stages of boiling
 Bubbles formation and disappearance
near hot surface
 Bubbles disappear as they transfer heat to
surrounding sub-cooled liquid
 Boiling is confined to locality of hot
surface so also called local boiling

10. Cont……
 Bubbles serve as energy movers and
transfer heat to fluid by condensing

11. Saturated/Bulk Boiling
 Boiling is saturated if temperature of main
body of fluid is equal to the saturation
temp Tsat (i.e. bulk of liquid is saturated)
 It occurs when entire liquid body reaches
saturation temperature
 Bubbles rise to the top

12. Boiling Curve for Pool Boiling
Four regimes/phases for pool boiling with
change in excess temperature are
1. Natural Convection Boiling
2. Nucleate Boiling
3. Transition Boiling
4. Film Boiling

13. Natural Convection Boiling
 Fluid motion in this regime is by natural
convection currents
 Heat transfer from heating surface to fluid
is by natural convection
 Liquid is slightly superheated

14. Nucleate Boiling
 Bubbles form at nucleation sites (rough surface)
 Bubbles form, travel & collapse in liquid
 Vacated space near heated surface is filled
by liquid
 Increased stirring & agitation
 Increased h and more heat flux
 Further rise in temp cause the formation
of more bubbles and these move to free
surface and break up and release vapor

15. Cont….
 At large values of ∆Texcess a larger fraction of
heater surface is covered with bubbles and
make difficult for liquid to reach heating surface
so max heat flux is reached called as
Critical/Maximum heat flux point c.
 Most desirable in industry because with small
∆Texcess, high heat transfer rates occur

16. Transition Boiling/Unstable regime
 At this phase larger fraction of heating surface
is covered with bubbles, so heat flux decrease
 vapor film reduces heat transfer b/c its k value
is less than liquid
 Partial nucleate and film boiling occur
 Usually this regime is avoided in industry

17. Film Boiling
 Heating surface is completely covered
with stable film, heat flux is minimum
 By increasing temp further, heat transfer
occur through vapor film by radiation
which is significant at high temperature

18. Boiling Regimes during mecthanol boiling on
steam heated copper tube
a) Nucleate Boiling
b) Transition Boiling
c) Film Boiling

20. Boiling Curve

21. Enhancement of heat transfer in
pool boiling
 In nucleate boiling rate of heat transfer
depends on active nucleation sites
 Increasing nucleation sites will increase heat
transfer
 Surface roughness and dirt increase heat
transfer
 Heat flux in nucleate boiling can be
increased by a factor of 10
 Coat surface with thin layer/Thermoexcell-E

22. Cont…..
Mechanical agitation and surface vibration
also increase heat transfer

23. Flow Boiling
 In pool boiling vapor bubbles rise due to
buoyancy forces, but in flow boiling, fluid
is forced to move by external source such
as pump as it undergoes a phase change
process
 Combined effects of pool boiling and
convection

24. Flow Boiling Types
 A) External Flow Boiling
If fluid is forced to move over a heated
surface (on external side)
 B) Internal Flow Boiling
If fluid is forced to move inside a
heated surface/tube (on internal side)

25. External Flow Boiling
It is similar with pool boiling, but added
fluid motion increases the nucleate boiling
heat flux and critical boiling heat flux
Velosity ∞ h ∞ q max

26. Internal Flow Boiling
 It is complicated as there is no space for
vapor for escape, so vapor and liquid flow
together, hence two-phase flow occurs.
 Different floe regimes are present
depending on relative amounts of vapor
and liquid present

27. Flow regimes in Internal Flow Boiling
FC= Total vapors
MF=liquid drops
suspended in
vapor
TF= complete dry
spot form on
tubes inner side
AF=core of flow
consist of vapors
and liquid flow in
annular space b/w
vapor and tube
SF=Bubbles grow
and coalesce into
slugs of vapor
BF= when bubbles
appear in liquid
FC=Total liquid

28. Flow regimes in Internal Flow Boiling

29. Applications of Boiling
-House hold refrigerator(refrigrant boiling)
-steam power plants boilers
-chemical industries boilers
-cooling of nuclear reactor by coolant
boiling
- industrial kettles
- electronic component cooling by boiling
of liquid in which these are placed
- Regenerative cooling of rocket motors

30. Condensation Heat Transfer
 Condensation occurs when temperature
of a vapor is reduced below its saturation
temperature.
 Vapor is contacted by a solid surface at a
temp well below vapor saturation temp
Condensation Types
 A) Film Condensation
B) Drop-wise Condensation

31. Film Condensation
 Condensate wet the surface and form a
liquid film on the surface which fall due to
gravity
 Thickness of film increases as it moves
down due to more condensation

32. Film Condensation

33. Drop-wise Condensation
 Condensed vapors form droplets on
surface instead of film, surface is covered
by drops
 Droplets slide down the surface, when
they reach a certain size

34. Drop-wise Condensation

35. Heat Exchanger
LMTD Definition
Log mean temp difference is defined as
that temperature difference which, if
constant, would give the same rate of heat
transfer as actually occurs under variable
conditions of temperature difference
Industrial Temp Control of
Heat Exchangers
E-2163 control