This document discusses different types of condensation including filmwise condensation and dropwise condensation. Filmwise condensation occurs when a continuous film of condensate forms on a surface. Dropwise condensation occurs when droplets form instead of a continuous film, allowing for higher heat transfer rates. The document also provides equations to calculate heat transfer coefficients, mass flow rates, and film thickness for laminar condensation on vertical plates and horizontal tubes. Additionally, it describes how non-condensable gases can reduce heat transfer during condensation by forming a barrier between the vapor and condensing surface.

1. Unit-5 Boiling and Condensation
Prepared by:
Ankur Sachdeva
Assistant Professor, ME

2. Condensation
• The condensation is a phase change process from
vapour to liquid.
• It occurs when the vapour strikes a surface which
is at temperature (Ts) below the vapour
saturation temperature (Tsat), the vapour releases
its latent heat and immediately converts into
liquid phase.
• The condensation may occur in two possible ways
depending on the condition of the surface :
– Filmwise condensation, and
– Dropwise condensation

3. Types of Condensation

4. Filmwise Condensation
• If the condensation takes place
continuously over a surface cooled
by some process and the
condensate film covers entire
condensing surface and falls down
under the action of gravity, the
situation is called filmwise
condensation.
• The presence of condensate layer
acts as a resistance to heat transfer
between vapour and surface.
• This resistance increases with
condensate thickness, which
increases in the flow direction,
hence it is desirable to use short
vertical surfaces of horizontal
cylinders in situations involving film
condensation.

5. Filmwise Condensation
• The liquid film starts forming at the top of the
plate and flows downward under the influence of
gravity.
• The thickness of the film increases in the flow
direction x because of continued condensation at
the liquid–vapor interface.
• Heat in the amount hfg (the latent heat of
vaporization) is released during condensation and
is transferred through the film to the plate
surface at temperature Ts

6. Velocity and Temperature Profiles in
Filmwise Condensation

7. Dropwise Condensation
• In dropwise condensation, the
condensed vapor forms droplets on
the surface instead of a continuous
film.
• Dropwise condensation can occur
when the surface is non-wetting or
these droplets are taken away from
the surface by external flow or by
gravity.
• The vapour is in direct contact with
the surface over most of the area and
heat transfer rates are much higher
(more than 3 – 10 times higher) as
there is very little resistance for heat
flow between the vapour and the
surface.
• The droplets develop at nucleation
sites (points of surface imperfections
such as pit, scratch and cavities), and
grow in size as more vapour
condenses on its exposed surface

8. Dropwise Condensation
• Dropwise condensation, characterized by countless
droplets of varying diameters on the condensing
surface instead of a continuous liquid film, is one of
the most effective mechanisms of heat transfer, and
extremely large heat transfer coefficients can be
achieved with this mechanism .
• In dropwise condensation, the small droplets that
form at the nucleation sites on the surface grow as a
result of continued condensation, coalesce into large
droplets, and slide down when they reach a certain
size, clearing the surface and exposing it to vapor.
• There is no liquid film in this case to resist heat
transfer.
• As a result, with dropwise condensation, heat
transfer coefficients can be achieved that are more
than 10 times larger than those associated with film
condensation.
• Large heat transfer coefficients enable designers to
achieve a specified heat transfer rate with a smaller
surface area

9. Laminar Condensation on a Vertical
Flat Plate
Governing Equation

10. Heat Transfer Relations for
Laminar Condensation
Vertical Plate
• Mass flow rate of the condensate,
• Thickness of condensate film,
• Local Heat Transfer Coefficient, =
• Average Heat Transfer Coefficient,

11. Heat Transfer Relations for
Laminar Condensation
Horizontal Tube (Outside surface):
• Average Heat Transfer Coefficient
Horizontal Tube (Inside surface):
• Average Heat Transfer Coefficient
– (Low Vapour Velocity)

12. Heat Transfer Relations for
Laminar Condensation
Horizontal Tube (Inside surface):
• Average Heat Transfer Coefficient
– (High Vapour Velocity)
Mass Velocity of Vapour and Liquid

13. Effect of Non-Condensable Gases
• Most condensers used in steam power plants
operate at pressures well below the atmospheric
pressure (usually under 0.1 atm) to maximize
cycle thermal efficiency.
• Operation at such low pressures raises the
possibility of air (a non-condensable gas) leaking
into the condensers.
• Even small amounts of a non-condensable gas in
the vapor cause significant drops in heat transfer
coefficient during condensation

14. Effect of Non-Condensable Gases
• When the vapor mixed with a
non-condensable gas
condenses, only the non-
condensable gas remains in the
vicinity of the surface.
• This gas layer acts as a barrier
between the vapor and the
surface, and makes it difficult
for the vapor to reach the
surface.
• The vapor now must diffuse
through the non-condensable
gas first before reaching the
surface, and this reduces the
effectiveness of the
condensation process