# Evaporation from Water Surfaces

## The amount of evaporated water from a water surface - like a swimming pool or an open tank - depends on the temperature in the water - and the temperature, humidity and velocity of the air above the surface - online calculator

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The evaporation of water from a water surface, an open tank, a swimming pool or similar - depends the temperature in the water and the temperature, the actual humidity and the velocity of the air above the 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 in saturated air at the same temperature as the water surface (kg/kg)(kg H_{2}O in kg Dry Air)

x= humidity ratio in the 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 required for the evaporation is taken from the water itself. To maintain the water temperature heat must be supplied.

The required heat supplied to cover the 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

The evaporation 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 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 building constructions
- 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 that moves latent heat from the air to the water in the swimming pool.

### Surface Evaporation Calculator

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- en: evaporation water surface swimming pool
- es: evaporación superficial del agua de piscinas
- de: Verdunstungswasseroberfläche Pool