Let me ask you few Questions first before dig into vapour pressure. How do you define boiling? How it is different from evaporation and at what point the boiling arise?
Some of you might be confusing with the term boiling and evaporation. Many speculations would be done so far among the students according to their level of understanding or may be some misleading articles. But let me clear you one thing that these phenomenon are not same. So, lets try to understand the fundamental concept behind all these questions.
Vapour Pressure
Vapour pressure is very useful term throughout the thermal engineering (thermodynamics, fluid mechanics, power plant, refrigeration and air conditioning etc.). Also, we have seen the use of vapour pressure in the cavitation phenomenon both in mechanical as well as in the civil engineering.
In mechanical engineering the cavitation is unwanted phenomenon in centrifugal pumps, reaction turbines and venturimeter. In civil engineering it is unwanted in oil and water supply lines.
The various questions which we come across in our day to day life:
v How to calculate vapour pressure?
v How the vapour pressure plays the major role in avoiding cavitation?
v Is Vapour pressure depends on type of solution (i.e. type of intermolecular bonding)?
v How the vapour pressure varies with height?
v How the vapour pressure is varies with temperature?
v What is the effect of atmospheric pressure?
v What is the use of vapour pressure?
Concept of vapour pressure
consider a closed vessel have certain level of water at a particular temperature T as shown in figure container A, the air present between the free surface and top of the vessel has certain tendency to absorb moisture (i.e. water molecule) it characterises that the air present is unsaturated as shown in figure container B. After certain point of time a stage comes when air becomes saturated (air cannot hold further more molecules of water vapour).
At this particular condition evaporation and condensation equilibrium sets up i.e. the rate of evaporation from water free surface is exactly equal to the rate of condensation from saturated air present above the free surface and the absolute pressure exerted by only those water vapours (pressure of air particle is not considered) present at the equilibrium is known as vapour pressure at that particular temperature T as shown in figure container C, for example in container C there is 4 molecules which are evaporating the liquid surface and exactly the 4 molecules of water vapour is condensing back to the water.
How the vapour pressure plays a major role in avoiding the cavitation:
During the designing phase of any meter/device take care must have done to avoid cavitation because if the cavitation cannot prevent it will damage the whole system. Due to the cavitation the overall efficiency of the system greatly reduced such that we cannot reach even break even point. Hence it is our first and foremost priority that the cavitation cannot occur during the flow process.
(fig source - Encyclopedia Britannica)
Cavitation occurs in the flow system if the absolute pressure becomes equal to or less than the vapour pressure. Cavitation can be avoided by keeping the absolute pressure in the flowing fluid greater than the vapour pressure corresponding to the surrounding temperature.
Factors affecting the vapour pressure:
- Altitude/Height: Vapour pressure always decreases with respect to height because of the less air present in the atmosphere hence the evaporation and condensation equilibrium would achieve at faster rate. For example water.
- Temperature: If the temperature of surrounding air is increases the air becomes unsaturated i.e. the solubility of water present in air will increase (capacity to hold water vapour is increases) and hence the vapour pressure always increase with respect to temperature. we can see the variation in given table.
Temp. in degree Celsius
|  Pv (vapour pressure) in kpa |
20
|
2.34
|
30
|
4.25
|
40
|
7.38
|
- Type of Solution: If the liquid present is having strong cohesive force (force of attraction between same nature of fluids) than it results in less vapour pressure as compared to weaker cohesive force media. For example vapour pressure of water at 25 degree Celsius is 3.17 kpa and for mercury it is nearly 0.246 kpa that is why the mercury is used in thermometers.
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