Lesson 5 - Convection SAQ

** The use of steam is the most common method for heating process fluids, usually indirectly via a heat exchanger or a jacket around a pipe or vessel. Describe the mechanisms by which steam can condense on cooler surfaces. What influences the mechanism that takes place? What are the relative film heat transfer coefficients that can be expected for each of these mechanisms? Explain why this is important for efficient heating. 

Response:

Condensation of steam (water vapour) occurs on surfaces of tubes that are cooler than the condensing temperature of the steam at the prevailing pressure. A vapour may condense on a cold surface in one of two ways: film and dropwise.

In film condensation, the liquid condensate forms a continuous layer of liquid that flows over the surface due to gravity. Film condensation occurs readily on smooth surfaces, which allows for a continuous stream of liquid.

Dropwise condensation, which is less common, is where the condensate forms at nucleation sites similar to the nucleation sites discussed during boiling. It most commonly occurs on rough surfaces. In dropwise condensation, droplets form but do not form a layer as readily as in film condensation.

The magnitude of the heat transfer coefficient for film condensation is dependent on the thickness of the layer of liquid between the condensing vapour and the wall of the tube (which limits effective heat transfer).

The average film heat transfer coefficient for dropwise condensation is 5-10 times that for film condensation (as no liquid film is in place).

A typical value for the convective film heat transfer coefficient for condensing steam is around 5,000 W m^-2 K^-1.

This has an obvious impact on the effectiveness of heat transfer per unit area in heat exchangers, hence heat exchanger sizing. You will learn more about this later in the Unit.