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# 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/m2h)

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

A = water surface area (m2)

x s = maximum humidity ratio of 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 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 oC. The maximum saturation humidity ratio in the air above the water surface is 0.014659 kg/kg. With air temperature 25 oC 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/m2h

The area of the swimming pool can be calculated as

A = (50 m) (20 m)

= 1000 m2

The evaporation from the surface can be calculated as

g s = (34.5 kg/m2h ) (1000 m2) ((0.014659 kg/kg) - (0.0098kg/kg)) / 3600

= 0.047 kg/s

The evaporation heat (enthalpy) of water at temperature at 20 oC 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.

## Related Topics

• ### Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

## Related Documents

• ### Air - Drying Force

The drying force of air depends on the air moisture holding capacity and the water surface to air evaporation capacity.
• ### Air - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.
• ### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.
• ### Area Units Converter

Convert between units of area.
• ### Cooling Tower Efficiency

Maximum cooling tower efficiency is limited by the cooling air wet-bulb temperature.
• ### Evaporative Cooling

Evaporative cooling tutorial.
• ### Heat Loss from Open Water Tanks

Due to evaporation the heat loss from an open water tank like a swimming pool may be considerable.
• ### Ice and Water - Melting Points vs. Pressure

Online calculator, figures and tables with melting points of ice to water at pressures ranging from 0 to 29000 psia (0 to 2000 bara). Temperature given as °C, °F, K and °R.
• ### Indoor Temperature and Humidity Loads

Calculate sensible and latent heat from persons, lights, electric equipment, machines, evaporation from water surfaces, polluting fluids and miscellaneous loads.
• ### Liquids - Latent Heat of Evaporation

Latent heat of vaporization for fluids like alcohol, ether, nitrogen, water and more.
• ### Liquids and Gases - Boiling Points

Boiling temperatures for common liquids and gases - acetone, butane, propane and more.
• ### Moist Air - Enthalpy

Sensible and latent heat of moist air.
• ### Moist Air - the 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.
• ### Properties of Saturated Steam - SI Units

Saturated Steam Table with steam properties as specific volume, density, specific enthalpy and specific entropy.
• ### Required Air Flow to Remove Moisture

Air flow required to remove vapor production from a room.
• ### Saturated Steam - Properties - Imperial Units

Steam table with sensible, latent and total heat, and specific volume at different gauge pressures and temperatures.
• ### Swimming Pool Heaters

Calculate outdoor swimming pool heaters.
• ### Water - Boiling Points at Higher Pressures

Online calculator, figures and tables showing boiling points of water at pressures ranging from 14.7 to 3200 psia (1 to 220 bara). Temperature given as °C, °F, K and °R.
• ### Water - Saturation Pressure vs. Temperature

Online calculator, figures and tables with water saturation (vapor) pressure at temperatures ranging 0 to 370 °C (32 to 700°F) - in Imperial and SI Units.
• ### Water - Thermophysical Properties

Thermal properties of water at different temperatures like density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more.

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## Citation

• The Engineering ToolBox (2004). Evaporation from a Water Surface. [online] Available at: https://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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