RMP Lecture Notes

Condensation and Condensers

The primary function of a condenser is to remove latent heat, although it is sometimes necessary to remove sensible heat as well. Examples include:

distillation columns
reactors
steam heaters, reboilers
power plants
refrigeration systems

Condensers are typically multipass shell and tube exchangers with floating heads. The heat is removed by contacting vapor with a cold surface (the tube wall). The liquid then flows off the tube under the influence of gravity, collects, and flows out of the exchanger. In some cases, vapor flow rates may be high enough to sweep the liquid off the tubes. This is called vapor shear and is a concern when liquid is condensing inside a tube.

Condensing vapor may be a single component or a mixture, with or without the presence of noncondensibles. Usually, mixed vapors are condensed inside tubes, while single components are condensed on the outside of tubes.

Under similar conditions, horizontal tubes tend to have larger condensing heat transfer coefficients than vertical tubes (x5 for film type condensation). Vertical tubes are preferred when substantial subcooling of the condensate is required.

In calculations, it is common to assume the vapor-liquid interface is at thermodynamic equilibrium at the vapor temperature. Liquid adjacent to the cold surface is assumed to be at the surface temperature. It is also common to treat condensers as constant pressure systems, since the total friction losses through an exchanger are usually small.

Condensation Mechanisms

There are two main mechanisms of condensation:

If the condensate "wets" the surface, a film forms as the drops coalesce. The condensate forms a continuous layer that flows over the tube (gravity flow) in film type condensation. The primary heat transfer resistance is in the film.
If the condensate does not wet the surface, drops form at nucleation sites (pits, dust, etc.) and remain separated until carried away by gravity or vapor flow. Only then do they coalesce, prior to falling off the tube. This is dropwise condensation. Most of the tube surface remains uncovered by liquid, so there is little heat transfer resistance and very high transfer rates.

In both cases, nucleation is typically the rate limiting step, rather than heat transfer. Most industrial applications are based on film mechanisms, since it is tricky and expensive to build non-wetting surfaces.

After condensation, the liquid flows down the tube surface under the influence of gravity (unless vapor rates are high enough to produce vapor shear). The flow may be laminar or turbulent, depending on the fluid, rate of condensation, tube size, etc. The film tends to thicken as it flows to the bottom of the tube, and the weight of the fluid may cause ripples to form. These will cause deviations from pure laminar flow.

Superheated Vapors

Before a vapor can condense, any sensible heat must be removed. For steam, sensible heat is usually much less than latent and hence is sometimes considered negligible, but this is not true for all vapors.

Practically, one can assume the entire heat load (sensible and latent) is transferred across the condensing film resistance, but if superheat (or subcooling) is substantial, the calculation should be separated into parts -- a desuperheater (gas cooler) and condenser -- and the areas determined separately.

Noncondensibles

The presence of even small amounts of noncondensible gases drastically reduces heat transfer. It has been suggested that only 1-2% air in steam can reduce heat transfer by 75%. Since the condensing vapor in such systems must diffuse through a noncondensible gas to reach the cooling surface, full consideration requires modeling of both heat and mass transfer.

Vents are sometimes installed (as in old fashioned home radiators) to bleed noncondensibles from the system.

References:

Bell, K.J., Process Heat Transfer Notes, Oklahoma State University, 1982, Chapter IX.
Foust, A.S. et al., Principles of Unit Operations, 2nd Edition, John Wiley, 1980, pp. 291-94.
Levenspiel, O., Engineering Flow and Heat Exchange, Revised Edition, Plenum Press, 1998, pp. 179-80.
McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5th Edition), McGraw-Hill, 1993, pp. 374-85.
McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (6th Edition), McGraw-Hill, 2001, pp. 377-79, 387-89.

R.M. Price
Original: 12/16/99
Modified: 1/3/2000, 2/14/2000, 2/13/2002, 2/18/2003