Heat Loss from Open Water Tanks

The overall heat loss from an open water tank can be expressed as:

Q = Q_{evaporation fluid} + Q_{radiation fluid} + Q_{transmission through walls}                                     (1)

The heat loss due to evaporation of water from a surface of an open tank is totally dominant at higher water temperatures. In general heat loss through insulated walls can be almost neglected. Major heat loss can be avoided by covering the surface.

Note! - take care with the values below - they are rough estimates and depends on air velocity, air turbulence and air moisture over the surface, water stir and more. How fast a cup of coffee or similar cools down outside with some wind compared inside illustrates the power of these parameters and the roughness of the estimates below.

• the values above are for 60 ^oF (15.6 ^oC) still ambient air

Example - Heat Loss from an open Water Tank

The heat loss from an open water tank with

• water temperature: 150 ^oF
• surface area: 10 ft²
• un-insulated bare steel tank walls area: 50 ft²

can be estimated with values from the table above as:

Q_{(evaporation fluid + radiation fluid)} = (1040 Btu/(ft² hr)) (10 ft²)

           = 10400 Btu/hr

Q_{transmission through walls} = (180 Btu/(ft² hr)) (50 ft²)

          = 9000 Btu/hr

Q = Q_{(evaporation fluid + radiation fluid)} + Q_{transmission through walls}

         = (10400 Btu/hr) + (9000 Btu/hr)

         = 19400 Btu/hr

Heat Loss from Water Surfaces Chart - Metric Units

• Heat loss from Water Surfaces - Metric Units (pdf)

Heat loss are for open tanks in 60^oF still ambient air.

Heat Loss from Water Surfaces Chart - Imperial Units

• Heat loss from Water Surfaces - Imperial Units (pdf)

• T(^oC) = 5/9[T(^oF) - 32]
• 1 Btu/(ft²hr) = 3.1525 W/m²

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm