Boiling, Condensation and Evaporation Study Notes for Mechanical Engineering

By Vineet Vijay|Updated : August 21st, 2017


  • Boiling heat transfer is associated with the change in phase from liquid to vapour.
  • When a solid surface is exposed to a liquid and maintained above the saturation temperature fo the liquid, boiling occurs. High heat fluxes may be achieved in boiling phenomena.
  • The two basic types of boiling are pool boiling and flow boiling.
    • Pool boiling occurs on a heated surface submerged in a liquid pool 
    • Flow boiling occurs in a flowing stream.
    • The boiling of water in a kettle on a stove and the boiling of water in boiler tubes under pressurized conditions are examples of pool boiling and flow boiling, respectively.
  • The liquid and vapour flow associated with flow boiling is a case of two-phase flow. Generally boiling phenomena are studied with respect to the excess temperature, TW – TS, which is the difference between the temperature of the solid surface (TW) and the saturation temperature of the liquid (TS).
  • If the liquid temperature is below its saturation temperature, the boiling is known as the local or subcooled boiling. In this case, the bubbles formed from the solid surface collapse before reaching the liquid surface.
  • If the temperature of the liquid is equal to its saturation temperature, the bubbles rise to the liquid surface due to buoyancy and then burst. This type of boiling is called bulk or saturated boiling.

The mechanism of heat transfer can be divided into three distinct zones:

  • Free convection or pool boiling (Zone I)
  • Nucleate boiling (Zone II)
  • Film boiling (Zone III)


  • Zone I: Free convection.
    • In this zone, boiling takes place at an excess temperature of about 5ºC. The flow produced by free convection in the liquid is sufficient to transfer the heat from the surface.
    • The heat transfer rate in this zone can be calculated using free convection heat transfer relationships.
  •  Zone II: Nucleate boiling.
    • Due to an increase in the excess temperature, bubbles start to form at some locations on the heater surface and the formed bubbles leave the surface but collapse before reaching the liquid surface.
    • As the excess temperature is increased further, the bubble-generation rate also increases due to numerous nucleate locations formed on the solid surface. The bubbles in this region do not collapse inside the liquid but they rise to the liquid surface and then collapse.
    • In this region, the heat transfer rate will be very high with an increase in excess temperature. The heat transfer rate increases with temperature up to a point b and then drops.
    • The condition at b is known as the critical heat flux.
    • If boiling takes place beyond this point due to high temperature, the solid surface may get damaged or it may even melt. So, this point is also called the burnout point. Normally, the temperature of the solid surface must be maintained just below this temperature.
  • Zone III: Film boiling
    • Initially, increase in excess temperature accelerates the bubble-formation and the entire surface is covered with vapour film which prevents direct heat transfer to the fresh liquid.
    • Film resistance causes a reduction in heat transfer rate with increase in excess temperature, in this temperature range, the film is not stable.
    • The vapour film is stable beyond this range and the heat transfer rate to the liquid is further reduced. With further increase in surface temperature [i.e., (TS – TW) > 1000ºC], a major portion of the heat lost by the surface is due to thermal radiation, which increases the heat transfer rate from the surface. Most practical boiling equipment are designed based on nucleate boiling processes which have high heat transfer rate.


Film condensation on a vertical flat plate

  • The wall shown below in the fig. is exposed to a condensable vapour.
  • The condensate film is assumed to be fully developed laminar flow with zero interfacial shear and constant liquid properties.
  • It is also assumed that the vapour is saturated and the heat transfer through the condensate film occurs by condensation only and the temperature profile is assumed to be linear.

Fig. Condensation of film in laminar flow

  • The wall temperature is maintained at temperature Tw and the vapour temperature at the edge of the film is the saturation temperature Tv.
  • The condensate film thickness is represented by δx, a function of x. A fluid element of thickness dx was assumed with a unit width in the z-direction.
    The force balance on the element provides,

F1 = F2 - F3

  • In the subsequent sections of this module, the subscripts l and v will represent liquid and vapour phase.
    • Gravity force, F2 = ρlg (δx - y)dx; and
    • Buoyancy force, F3 = ρvg (δx - y)dx
    • Shear force, F1 = byjusexamprep


  • On integrating for the following boundary condition, u = 0 at y = 0; no slip condition

  • Above Equation shows the velocity profile in the condensate falling film.
  • The corresponding mass flow rate of the condensate for dy thickness and unit width of the film,

where dy is the length of the volume element at y distance.

  • The rate of condensation for  dx.1 (over element surface) area exposed to the vapour can be obtained from the rate of heat transfer through this area.
  • The rate of heat transfer

  • The thermal conductivity of the liquid is represented by kl. The above rate of heat transfer is due to the latent heat of condensation of the vapour. Thus,

  • The specific latent heat of condensation is represented by λ.

On solving eqs above for boundary layer conditions (x = 0; δx = 0)

The eq. 7.4 gives the local condensate film thickness at any location x.

  • If h is the film heat transfer coefficient for the condensate film, heat flux through the film at any location  is, 

We can also calculate the average heat transfer coefficient along the length of the surface,


  • Performance of steam heated tubular evaporators 
    • The performance of a steam heated tubular evaporator is evaluated by the capacity and the economy.
  • Capacity and economy
    • Capacity is defined as the no of kilograms of water vaporized per hour.
  • The economy is the number of kg of water vaporized per kg of steam fed to the unit.
  • Steam consumption is very important to know and can be estimated by the ratio of capacity divided by the economy.The steam consumption (in kg/h) is

Steam Consumption = Capacity / Economy

Single and multiple effect evaporators

  • In single effect evaporator, the steam is fed to the evaporator which condenses on the tube surface and the heat is transferred to the solution.
  • The saturated vapour comes out from the evaporator and this vapour either may be vented out or condensed. The concentrated solution is taken out from the evaporator.
  • In this process, the fresh steam is required for the second evaporator and at the same time, the vapour is not utilized.
  • Single effect evaporator does not utilize the steam efficiently.
  • The economy of the single effect evaporator is thus less than one and in many of the cases, the feed temperature remains below the boiling temperature of the solution. Therefore, a part of the heat is utilized to raise the feed temperature to its boiling point.

Fig. Single effect evaporator

Evaporators in Series

  • The saturated vapour coming out from the evaporator-1 is used as steam in the second evaporator and the partially concentrated solution works as a feed to the second evaporator.
  • This arrangement is known as Double effect evaporator in forward feed scheme.

Fig.Double effect evaporator with forward feed scheme

Note: The benefit of the use of multiple effect evaporators is that in this arrangement multiple reuse of heat supplied to the first effect is possible and results in improved steam economy.

Boiling Point Elevation 

  • The evaporators produce a concentrated solution having a substantially higher boiling point than that of the solvent (of the solution) at the prevailing pressure.
  • The increase in boiling point over that of water is known as boiling point elevation (BPE) of the solution. 
  • In order to get the real temperature difference (or driving force) between the steam temperature and the solution temperature, the BPE must be subtracted from the temperature drop.
  • The BPE may be predicted from the steam table (in case water is a solvent).
  • An empirical rule known as Dühring rule is suitable for estimating the BPE of strong solution.
  • The Dühring rule states that the boiling point of a given solution is a linear function of the boiling point of the pure water at the same pressure.
  • Therefore, if the boiling point of the solution is plotted against that of the water at the same pressure, a straight line results. Different lines are obtained at different concentrations. 


Method of feeding: Multiple effect evaporators

Forward Feed:  

  • The liquid feed is pumped into the first effect and the partially concentrated solution is sent to the second effect and so on.
  • The heating steam is also sent through the first effect to another effect. This particular strategy is known as a forward feed.
  • In the forward feed the concentration of the liquid increases from first effect to the subsequent effects till the last effect.
  • The forward feed requires a pump for feeding dilute solution to the first effect. The first effect is generally at atmospheric pressure and the subsequent effects are in decreasing pressure.
  • Thus, the liquid may move without the pump from one effect to another effect in the direction of decreasing pressure. However, to take out the concentrated liquid from the last effect may need a pump.

Backward Feed

  • The backward feed arrangement is a very common arrangement in which a dilute liquid is fed to the last effect and then pumped through the successive effects to the first effect.
  • The method requires additional pumps (generally one pump in between two effects).
  • Backward feed is advantageous but provides lower economy as compared to forward feed arrangement and gives higher capacity than the forward feed when the concentrated liquid is viscous because the viscous fluid is at a higher temperature being in the first effect. However, this arrangement provides lower economy as compared to forward feed arrangement.

Mixed Feed

  • The combination of forward-feed and backward-feed is known as Mixed feed arrangement.
  • In mixed feed, the dilute liquid enters in between effects, flows in the forward feed to the end of the effect and then pumped back to the first effect for final concentration. 

Parallel feed:

  • Another common evaporator arrangement, which is more common in crystallization is parallel feed where feed is admitted individually to all the effects..

Enthalpy Balance Single effect evaporator

  • The latent heat of condensation of the steam is transferred to the boiling solution through the heating surface in order to vaporize the water. Thus, two enthalpy balances are required one for the liquid and another for the steam.

The following assumptions are required, in order to make the enthalpy balance,

  • Flow of non-condensable is negligible
  • The superheat and sub-cooling of the condensable steam is negligible
  • No solid precipitates out from the concentrating solution

The enthalpy balance for the steam side is,

  • byjusexamprep byjusexamprep=  rate of heat transfer through heating surface from steam
  • flow rate of steam
  • λs = latent heat of condensation of steam
  • hs = specific enthalpy of steam
  • hc = specific enthalpy of condensate

Enthalpy balance for the liquid side is,


  • byjusexamprep= rate of heat transfer from heating surface to the liquid
  • hv = specific enthalpy of vapour
  • hcl = specific enthalpy of concentrated liquid
  • hfl = specific enthalpy of liquid feed
  • flow rate of liquid feed
  • rate of concentrated liquid

The enthalpy balance at the steam side and the liquid side will be same in the absence of any heat loss . Thus,

byjusexamprep= rate of heat transfer from heating surface to the liquid

hv = specific enthalpy of vapour

hcl = specific enthalpy of concentrated liquid

hfl = specific enthalpy of liquid feed flow rate of liquid feed rate of concentrated liquid

The area of heat transfer A can be calculated from

When ΔT = (Tb - Tc);                            
Tb = Saturated temperature of steam in the shell
Ts = Boiling point of the solution at the prevailing pressure
UD = Overall coefficient (dirty)

Multiple effect evaporators

  • The steam goes into I-effect and heat the solution by the latent heat of condensation. If the heat required to boil the feed is negligible, it follows that practically all this heat must appear as latent heat in the vapor that leaves the I-effect and enter into II-effect as steam. The temperature of the condensate leaving the II-effect will be very near the temperature T1 of the vapors from the boiling liquid in the I-effect. Thus, in steady state operation, all the heat that was expanded in creating vapor in the I-effect must be given by when this same vapor condenses in the II-effect and so on.

  • The heat delivered into the II-effect will be,

The  byjusexamprep

Similarly, for III-effect

It can be seen that the temperature drops in a multiple effect evaporator is approximately inversely proportional to the heat-transfer coefficient.

The total available temperature drop will be given by

Ts : Steam temp. (I-effect); Tv3 : Vapor temperature leaving III-effect 
BPE : boiling point elevation in the solution in various effect.


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