introduction of condensation, what is it types etc. horizontal condenser, vertical condenser, process aplications, all examples related to the process,
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INTRODUCTION
Processes of heat transfer having phase change are complex than simple
exchange between fluids. It usually involves heat changes at constant or nearly
constant temperature.
When a vapor is exposed to a surface at a temperature below Tsat ,
condensation in the form of a liquid film or individual droplets occurs on the
surface.
HEAT TRANSFER FROM CONDENSING VAPORS:
Condensation of vapors on tube surfaces is cooler than condensing
temperature of vapor for those such as water, hydrocarbon, and volatile
substances are processed where latent heat of vaporization is removed for
condensation to occur. Heat transfer occurs between vapor and surface and
condensation occurs.
Condensing vapor consists of single substance, a mixture of condensable and
non condensable, or a mixture of two or more condensable vapors.
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Friction losses are very small hence condensation is a constant pressure
process.
The condensing temperature of a single pure substance depends on pressure
and hence the process is isothermal and condensate is a pure liquid. Mixed
vapors condense at constant pressure process over wide range of temperature
having variable condensate.
Examples of condensation are :
1) condensation of water from a mixture of steam and air.
2) Recovery of hydrocarbon solvents from air streams leaving
extractions/drying process.
Condensation occurs in two distinct mechanism at different rates:
1) Dropwise condensation
2) Film-type condensation
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TYPES OF CONDENSATION:
FILM-TYPE CONDENSATION:
Here the liquid condensate forms a film or a continuous layer, of liquid that flows
over the surface of tube under the action of gravity.
It is a layer of liquid interposed between the vapor and wall of tube that serves as
resistance to heat transfer and fixes the heat transfer coefficient.
DROP WISE CONDENSATION:
Dropwise condensation, characterized by countless droplets of varying diameters
on the condensing surface instead of a continuous liquid film, is one of the most
effective mechanisms of heat transfer, and extremely large heat transfer
coefficients can be achieved with this mechanism.
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The two mechanisms are different from each other and independent of the
quantity of vapor condensing:
Film wise condensation has greater rapidity at which the condensate forms on
the tube. Due to the resistance of the condensate film to heat passing through it
the heat transfer coefficient for drop wise condensation are 4-8 times more for
film wise condensation.
Drop wise condensation is seen in ethylene, glycol, glycerin, nitrobenzene,
isoheptane and some organic vapors. Liquid metals usually condense in drop
wise manner. Much of the experimental work is been on drop wise condensation
of steam and few conclusions are:
•Film type condensation of water occurs on tubes of common metals if both the
steam and tube are clean, in presence or absence of air or on rough or polished
surface.
•Drop wise condensation is only attainable when cooling surface is not wetted by
the liquid. Usually condensation of steam is contaminated by oil droplets.
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•The quantity of contaminant required to contaminate vapor is minute and
apparently a monomolecular film is necessary.
•Effective drop promoters are strongly adsorbed by the surface, and the
substances that merely prevent wetting are ineffective. Some promoters are
effective on some metals such as mercaptans on copper alloys.
•The average coefficient obtainable in pure wise condensation may be high as
115kW/m2 0C(20000 Btu/ft2. oF
9. PROCESS APPLICATIONS:
•Mainly in industries separation of liquid mixtures are carried out by distillation
where the compounds with lower boiling points are distilled off in pure
condition from those having higher boiling points. A mixture of solution having
several compounds exerts a partial pressure where the most volatile compounds
cannot be boiled of from the rest without carrying some higher boiling
compounds with it.
•If the vapor coming off is condensed, having lower boiling point than original
solution will indicate increase in the proportion of volatile compounds. By
successively boiling off part of a liquid mixture, condensing the vapor formed,
and boiling off a part of condensate, it is possible to obtain a nearly pure
quantity of the volatile compound by repetitions . Thus the separation by
distillation is accomplished by partial vaporization and subsequent
condensation.
•Continuous distillation requires the presence of liquid at all times on the plates,
so vapors of the less volatile compounds in the feed may be condensed and
carried downward. Always a volatile compound is used.
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In the power industry the term surface condenser is reserved for tubular
equipment which condenses steam from the exhaust of turbines and engines. The
turbine is designed to obtain the mechanical work from heat; the maximum
conversion is obtained in the turbine by maintaining a low-discharge temperature.
11. NUSSELTS THEORY:
In condensation on a vertical surface a film of condensate is formed and further
condensation and heat transfer to the surface occurs by conduction through the
film which is assumed to be laminar flow downward.
The thickness of this film greatly influences the rate of condensation, since the
heat accompanying the removal of vapors from the vapor phase encounters the
condensate film as a resistance.
The thickness of the film is the function of velocity of drainage which varies with
the deviation of the surface from the vertical position.
For vertical surfaces the thickness of the film increases from top to bottom. That
is why the condensing coefficient for a vapor condensing on vertical surface
decreases from top to bottom, and the height of the condenser shouldn’t be large
enough to attain large condensing coefficient.
For all liquids the viscosity decreases as the temperature increases, and the
condensing coefficient increases with the condensate temperature.
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Following assumptions are assumed for Nusselt’s equation:
•The heat delivered by the vapor is latent heat only.
•The flow is laminar for condensate film, and the heat is transferred through the
film by conduction.
•The thickness of the film at any point is a function of mean velocity of flow and
of the amount of condensate passing at that point.
•The velocity of the individual layers of the film is a function of the relation
between frictional shearing force and the weight of the film.
•Quantity of the condensate is proportional to the quantity of heat transferred,
related to the thickness of the film, temperature difference between vapor ,
surface.
•The condensate film is so thin that temperature gradient is linear.
•The temperature of the surface of the solid is constant. The curvature of the
film is neglected.
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CONDENSATION ON VERTICAL SURFACE:
The heat is assumed to flow through the condensate film solely by conduction,
and the local coefficient HX is therefore given by:
Where δ is local film thickness.
Film thickness is typically 2 to 3 orders of the magnitude smaller than the tube
diameter. Hence it can be found out for flow inside and outside tube, from the
equation for a flat plate which is:
Where ℾ is the condensate loading, the mass rate per unit length of periphery.
Substituting for δ gives the local heat-transfer coefficient, at a distance L from the
top of the vertical surface, the equation
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The average coefficient h for the entire tube is defines as:
Solving for ∆To gives :
Substituting ∆To into h gives:
Now integrating this equation gives:
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So equation for h becomes :
Thus the average coefficent for a vertical tube, provided that flow in the
condensate film is laminar, is 4/3 times the local coefficient at the bottom of the
tube.
Now arranging the equation gives,
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CONDENSATION ON HORIZONTAL TUBES:
For horizontal condensation we use equations corresponding for vertical
tubes, the following equations apply to single horizontal tube:
And,
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REFERENCES
•Process heat transfer by Donald kern
•Unit operations of chemical engineering by Mc Cabe and Smith
•Mass transfer operations by Treybal
•Condensation by Prabhal Thakur IIT Delhi, mechanical department
