Evaporation from Water Surfaces

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

The evaporation of water from a water surface, as an open tank, a swimming pool or similar, depends the temperature in the water and the temperature in the air, the actual humidity of the air and the velocity of the air above the 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 in saturated air at the same temperature as the water surface (kg/kg)  (kg H2O in kg Dry Air)

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

The heat supplied can be calculated as:

q = hwe gs        (2)


q = heat supplied (kJ/s, kW)

hwe = evaporation heat of water (2270 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 air is 0.0098kg/kg - check the Mollier diagram.

For a 25 m x 20 m swimming pool and 0.5 m/s velocity of air 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

The heat supply required to maintain the temperature can be calculated as:

q = (2270 kJ/kg) (0.049 kg/s)

= 111.2 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 outside operation time. Very effective and commonly used

Note! During operation time the activity in a swimming pool may increase the evaporation and the need of heat supply dramatically.

To reduce the energy consumption and to avoid moisture damages in the building constructions, its more and more common to use heat recycling devices with heat pumps to move latent heat from the air to the water in the swimming pool.

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 - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more

Related Documents

  • Sizing Swimming Pool Heaters - Calculating outdoor swimming pool heaters
  • Air Required to Remove Moisture - Required air flow to remove moisture from a room
  • Humidity Ratio of Air - Humidity ratio (GPP) is the ratio between actual 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 may be expressed as the moisture holding capacity of the air and the evaporation capacity from a water surface to the 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 DiagramĀ  - 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

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