How fast does water evaporate?
Water evaporation depends on a large number of factors, namely:
- Relative Humidity of the air
- Temperature of the air and the water
- Surface area of the water
- Velocity of the wind/air over the water
This is why sprays evaporate faster (large surface area), clothes dry faster on windy days (higher velocity of air), you feel hotter on a humid day (your sweat does not evaporate as easily) and you will see more steam from a hot cup of coffee than cold one (higher temperature).
Evaporation occurs by some molecules getting a higher amount of energy than is needed to turn into a gas, by collission with other molecules. When some molecules in an object turn into a gas even when the temperature is below boiling point, this is known as evaporation.
A typical equation from ASHRAE handbooks says that the rate of evaporation in kg/s/m2 is:
(Pw-Pa)x(0.089 + 0.0782V)/Y
Where Pw is the pressure exerted by water at the temperature of the air, Pa is the pressure of the vapour in the air at the air temperature and pressure, V is the velocity of the air over the water and Y is the latent heat of vapourisation of water. Pressures should be in kPa
So, the speed of evporation could be practically anything depending on these conditions.
As an example, let's take a cup of tap water in a kitchen at room temperature (21C).
So, the water from the tap will be at around 5C, at this air temperature of 21C the vapour pressure of water is 0.02486Bar = 2.486kPa.
Typical room conditions are around 60% humidity, so 0.6x0.02486 is the pressure of the water vapour in the air which will be 1.4916kPa.
(Remember humidity = Ps/Pg for given temperature, in this case 21C.
There is no wind in the kitchen, so V=0m/s, and Y for water is given as 2272kJ/kg.
So, ((2.486-1.4916)x0.089 + (0x0.0782V))/2272 = 0.000038953kg/s/m2
A typical cup is around 15cm high and with a diameter of 8cm, giving a surface area of 3.14x0.04x0.04 (circular surface area = pi x(r squared) = 0.00502, and so every second, 0.00502 x 0.000038953kg/s evaporate. This gives 0.000195799g/s. At this rate, a typical cup of water with 753g of water in it (calculated from above dimensions, water = 1000kg/m3) would take 1069hours to completely evaporate, or 44 days.
pure water Discussion Dissolving something in water which does not evaporate easily will make the mixed solution less likely to evaporate than pure water. The more stuff you add to it, the less likely it will evaporate. Adding salt to water makes ions in solution and these ions reduce the vapor pressure of the solution compared to pure water. The lower the vapor pressure of a liquid or solution, the slower it will evaporate. This…