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.
| Water Temperature ( o F) | Heat Loss from Liquid Surface (Btu/ft 2 hr) | Heat Loss Through Tank Walls (Btu/ft 2 hr) | |||||
|---|---|---|---|---|---|---|---|
| Evaporation Loss | Radiation Loss | Total | Bare Steel No Insulation | Insulated (inches) | |||
| 1 | 2 | 3 | |||||
| 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 o F (15.6 o C) still ambient air
Example - Heat Loss from an open Water Tank
The heat loss from an open water tank with
- water temperature: 150 o F
- surface area: 10 ft 2
- un-insulated bare steel tank walls area: 50 ft 2
can be estimated with values from the table above as:
Q ( evaporation fluid + radiation fluid) = (1040 Btu/(ft 2 hr)) (10 ft 2 )
= 10400 Btu/hr
Q transmission through walls = (180 Btu/(ft 2 hr)) (50 ft 2 )
= 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 o F still ambient air.
Heat Loss from Water Surfaces Chart - Imperial Units
- T( o C) = 5/9[T( o F) - 32]
- 1 Btu/(ft 2 hr) = 3.1525 W/m 2
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