Condensation and BoilingHeat Transfer

Source: • Vishwas V. Wadekar, HTFS, Aspen Technology• J.P. Holman

• boiling, condensation : high heat transfer rates• understand the processes to design the appropriate heat-transfer equipment

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 1800For 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 98C. 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 98C. (use condensate properties from Ex.1)