# Evaporation from Water Surfaces

## Evaporation of water from water surfaces - like swimming pools or open tanks - depends on water temperature, air temperature, air humidity and air velocity above the water surface - online calculator

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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}=ΘA (x_{s}- x) / 3600(1)

or

g_{h}=ΘA (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}= humidity ratio 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 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 (2257 kJ/kg)

### Example - Evaporated Water from a Swimming Pool

For a swimming pool with water temperature *25 ^{o}C* the saturation humidity ratio is

*0.02 kg/kg*. With an air temperature of

*25*and

^{o}C*50%*relative humidity - the humidity ratio in the air is

*0.0098kg/kg*- Mollier diagram.

For a *25 m x 20 m* swimming pool and *0.5 m/s* air velocity above the surface - the evaporation can be calculated as

g_{s}=( 25 + 19 (0.5 m/s)) ((25 m) (20 m)) ((0.02 kg/kg) - (0.0098kg/kg)) / 3600

= 0.049 kg/s

Heat supply required to maintain the temperature of the water can be calculated as

q =(2257 kJ/kg) (0.049 kg/s)

=110.6 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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