Transcript PRESENTATION NAME

Condensation and Boiling
Heat Transfer
• boiling, condensation : high heat transfer rates
• understand the processes to design the appropriate
heat-transfer equipment
Source:
• Vishwas V. Wadekar, HTFS, Aspen Technology
• J.P. Holman
Condensation Heat Transfer
Modes of condensation
• Dropwise/filmwise condensation
• Direct/Indirect/homogeneous condensation
Modes of condensation
In vertical flat plate, Tw < Tsat : condensate
will form at surface.
• Dropwise condensation: liquid does not wet
the surface, droplets are formed.
• Filmwise condensation: liquid wets the
surface, smooth film is formed. The surface is
blanked by the film, which grows in thickness
as it moves down the plate.
Filmwise Condensation
Dropwise Condensation
Homogeneous Condensation
Direct Contact Condensation
Condensation
• In the remaining lecture
we now focus on indirect
contact filmwise
condensation
General approach to condensation
General approach to condensation
Condensation on Flat Plate
Nusselt Analysis - Assumptions
Mass flow of condensate
Heat transfer at wall
Amount of condensate added between x and x+dx
Thus
Heat transfer coefficient
In term of Nusselt number
For vertical plates and cylinders and fluids with Pr > 0.5
and cT/hfg ≤ 1.0
For non-linear temperature profile
For laminar film condensation on horizontal tubes
To determine flow (laminar or turbulence) use
Renolds number
Critical Re is 1800
For Vertical plate of unit depth, P = 1
For Vertical tube, P = d
Relate mass flow with total heat transfer and heat
transfer coefficient
Using 20 % safety factor in design problems
For inclined surfaces
Condensation number (Co)
For condensation of refrigerants at low vapor
velocities inside horizontal tubes
For higher flow rates
Example 1
• A vertical square plate, 30 by 30 cm, is
exposed to steam at atmospheric pressure.
The plate temperature is 98C. Calculate the
heat transfer and the mass of steam
condensed per hour.
Example 2
• One hundred tubes of 1.27 cm diameter are
arranged in a square array and exposed to
atmospheric steam. Calculate the mass of
steam condensed per unit length of tubes for
a tube wall temperature of 98C. (use
condensate properties from Ex.1)