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## Condensation and Boiling Heat Transfer

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**Condensation and BoilingHeat 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**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.**Condensation**• In the remaining lecture we now focus on indirect contact filmwise condensation**Mass flow of condensate**Heat transfer at wall**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**For condensation of refrigerants at low vapor velocities**inside horizontal tubes For higher flow rates**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 1**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)