Evaporation from a Water Surface
Evaporation of water from a water surface  like a swimming pool or an open tank  depends on water temperature, air temperature, air humidity and air velocity above the water surface  online calculator.
Evaporation of water from a water surface  like an open tank, a swimming pool or similar  depends on water temperature, air temperature, air humidity and air velocity above the water surface.
The amount of evaporated water can be expressed as:
g _{ s } = Θ A (x _{ s }  x) / 3600 (1)
or
g _{ h } = Θ A (x _{ s }  x)
where
g _{ s } = amount of evaporated water per second (kg/s)
g _{ h } = amount of evaporated water per hour (kg/h)
Θ = ( 25 + 19 v ) = evaporation coefficient (kg/m^{2}h)
v = velocity of air above the water surface (m/s)
A = water surface area (m^{2})
x _{ s } = maximum humidity ratio of saturated air at the same temperature as the water surface (kg/kg) (kg H_{2}O in kg Dry Air)
x = humidity ratio air (kg/kg) (kg H_{2}O in kg Dry Air)
Note! The units for Θ don't match since the this is an empirical equation  a result of experience and experiments.
Required Heat Supply
Most of the heat or energy required for the evaporation is taken from the water itself. To maintain the water temperature  heat must be supplied to the water.
Required heat to cover evaporation can be calculated as
q = h _{ we } g _{ s } (2)
where
q = heat supplied (kJ/s (kW))
h _{ we } = evaporation heat of water (kJ/kg)
 1 kW = 3412 Btu/h
Example  Evaporated Water from a Swimming Pool
There is a 50 m x 20 m swimming pool with water temperature 20 ^{o}C. The maximum saturation humidity ratio in the air above the water surface is 0.014659 kg/kg. With air temperature 25 ^{o}C and 50% relative humidity the humidity ratio in the air is 0.0098 kg/kg  check Mollier diagram .
With air velocity above the water surface 0.5 m/s the evaporation coefficient can be calculated as
Θ = (25 + 19 (0.5 m/s))
= 34.5 kg/m^{2}h
The area of the swimming pool can be calculated as
A = (50 m) (20 m)
= 1000 m^{2}
The evaporation from the surface can be calculated as
g _{ s } = (34.5 kg/m^{2}h ) (1000 m^{2}) ((0.014659 kg/kg)  (0.0098kg/kg)) / 3600
= 0.047 kg/s
The evaporation heat (enthalpy) of water at temperature at 20 ^{o}C is 2454 kJ/kg . The heat supply required to maintain the temperature of the water in the swimming pool can be calculated as
q = (2454 kJ/kg) (0.047 kg/s)
= 115.3 kW
The energy loss and required heat supply can be reduced by
 reducing the air velocity above the water surface  limited effect
 reducing the size of the pool  not really practical
 reducing the water temperature  not a comfort solution
 reducing the air temperature  not a comfort solution
 increase the moisture content in the air  may increase the condensation and damage of the building constructions for indoor pools
 remove the wet surface  possible with plastic blankets on the water surface outside operation time. Very effective and commonly used
Note!  during operation time the activity in a swimming pool may increase the evaporation of water and the required heat supply dramatically.
To reduce the energy consumption and to avoid moisture damages in building constructions it is common to use heat recycling devices with heat pumps moving latent heat from the air to the water in the swimming pool.
Water Surface Evaporation Calculator
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