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

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.

swimming pool evaporation heat loss

The amount of evaporated water can be expressed as:

gs = Θ A (xs - x) / 3600         (1)


gh = Θ A (xs - x)


gs = amount of evaporated water per second (kg/s)

gh = amount of evaporated water per hour (kg/h)

Θ = (25 + 19 v) = evaporation coefficient (kg/m2h)

v = velocity of air above the water surface (m/s)

A = water surface area (m2)

xs = humidity ratio saturated air at the same temperature as the water surface (kg/kg)  (kg H2O in kg Dry Air)

x = humidity ratio air (kg/kg) (kg H2O 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 = hwe gs        (2)


q = heat supplied (kJ/s, kW)

hwe = evaporation heat of water (2257 kJ/kg)

Example - Evaporated Water from a Swimming Pool

For a swimming pool with water temperature 25oC the saturation humidity ratio is 0.02 kg/kg. With an air temperature of 25oC and 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

gs = ( 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

A - surface area (m2)

xs - humidity ratio in saturated air (kg/kg) (kg H2O in kg Dry Air)

x - humidity ratio in the air (kg/kg) (kg H2O in kg Dry Air)

v - velocity of air above the surface (m/s)

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content

Related Documents

  • Sizing Swimming Pool Heaters - Calculate outdoor swimming pool heaters
  • Required Air to Remove Moisture - Air flow required to remove vapor production in a room
  • Humidity Ratio of Air - The ratio between the mass of water vapor present in moist air - to the mass of dry air
  • Heat Loss from Open Water Tanks - Due to evaporation heat loss from open water tank as a swimming pools may be considerable
  • Drying Force of Air - The drying force of air can be expressed with moisture holding capacity of air and the evaporation capacity from water surface to air
  • Water - Thermal Properties - Thermal properties of water - density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more
  • Mollier Diagram¬† - The Mollier diagram is a graphic representation of the relationship between air temperature, moisture content and enthalpy - and is a basic design tool for building engineers and designers

Tag Search

  • en: evaporation water surface swimming pool
  • es: evaporaci√≥n superficial del agua de piscinas
  • de: Verdunstungswasseroberfl√§che Pool

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