Heat Loss from Open Water Tanks
Due to evaporation the heat loss from an open water tank like a swimming pool may be considerable.
The overall heat loss from an open water tank can be expressed as:
Q = Qevaporation fluid + Qradiation fluid + Qtransmission 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 (oF) Load Calculator! | Heat Loss from Liquid Surface (Btu/ft2hr) Load Calculator! | Heat Loss Through Tank Walls (Btu/ft2hr) Load Calculator! | |||||
|---|---|---|---|---|---|---|---|
| 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 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
Qtransmission through walls = (180 Btu/(ft2 hr)) (50 ft2)
= 9000 Btu/hr
Q = Q(evaporation fluid + radiation fluid) + Qtransmission 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 60oF 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