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# 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.

Heat Loss from Open Water Tanks
Water Temperature ( oF)

Heat Loss from Liquid Surface
(Btu/ft2hr)
Heat Loss Through Tank Walls
(Btu/ft2hr)
No Insulation
Insulated (inches)
123
90 80 50 130 50 12 5 4
100 160 70 230 70 15 8 6
110 240 90 330 90 19 10 7
120 360 110 470 110 23 12 9
130 480 135 615 135 27 14 10
140 660 160 820 160 31 16 12
150 860 180 1040 180 34 18 13
160 1100 210 1310 210 38 21 15
170 1380 235 1615 235 42 23 16
180 1740 260 2000 260 46 25 17
190 2160 290 2450 290 50 27 19
200 2680 320 3000 320 53 29 20
210 3240 360 3600 360 57 31 22
• 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/(ft2hr)) (10 ft2)

= 10400 Btu/hr

Q transmission through walls = (180 Btu/(ft2hr)) (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 are for open tanks in 60 oF still ambient air.

### Heat Loss from Water Surfaces Chart - Imperial Units

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

## Related Topics

### • Heating Systems

Design of heating systems - capacities and design of boilers, pipelines, heat exchangers, expansion systems and more.

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### Cylindrical Tanks - Volumes

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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.

### Evaporative Cooling

Evaporative cooling tutorial.

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Heat loss from insulated and non insulated sheltered and exposed oil tanks and pipes.

### Oil Filled Tanks - Heat Loss

Heat loss from insulated and uninsulated, sheltered and exposed heated oil tanks.

### Oil Tanks - Heat Loss

Heat loss from lagged and unlagged, sheltered and exposed oil tanks.

### Pipes Submerged in Water - Heat Emission

Heat emision from steam or water heating pipes submerged in water - assisted (forced) or natural circulation.

### Rectangular Tanks - Volumes

Tank volume per foot depth.

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