Environmental

What is wet bulb temperature?

wet bulb

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The temperature under relative humidity at 100% is called wet bulb temperature also means it is the lowest temperature the air can be cooled at a constant pressure. Generally speaking, wet bulb temperature is always higher than (or equal) dew point temperature. What is dew point temperature? Dew point temperature is a measure of how much water vapor in the air. The relationship between wet bulb temperature, dry bulb temperature and dew point temperature is shown in the following equation. Dry bulb temperature ≥ wet bulb temperature ≥ dew point temperature. By the way, you also can make psychrometer by yourself just need thermometer and wet cloth.


Psychrometric comparison table

  Relative humidity
Wet bulb 30% 40% 50% 60% 70% 80% 90% 100%

 

 

 

 

 

 

 

 Dry

 bulb

 

21℃ 11.59 13.14 14.61 16.02 17.35 18.62 19.84 21.00
22℃ 12.31 13.92 15.44 16.88 18.26 19.58 20.81 22.00
23℃ 13.02 14.86 16.26 17.75 19.16 20.50 21.77 23.00
24℃ 13.73 15.46 17.08 18.61 20.06 21.44 22.75 24.00
25℃ 14.44 16.22 17.90 19.48 20.97 22.38 23.73 25.00
26℃ 15.15 16.99 18.73 20.34 21.88 23.33 24.70 26.00
27℃ 15.83 17.76 19.55 21.21 22.79 24.26 25.67 27.00
28℃ 16.57 18.54 20.37 22.08 23.70 25.20 26.64 28.00
29℃ 17.28 19.31 21.20 22.95 24.61 26.15 27.61 29.00
30℃ 17.99 20.08 22.02 23.82 25.51 27.10 28.58 30.00
31℃ 18.70 20.85 22.84 24.69 26.42 28.04 29.56 31.00
32℃ 19.41 21.63 23.67 25.56 27.33 28.99 30.54 32.00
33℃ 20.12 22.40 24.50 26.43 28.24 29.93 31.51 33.00
34℃ 20.83 23.18 25.32 27.31 29.15 30.87 32.49 34.00
35℃ 21.54 23.95 26.15 28.18 30.06 31.82 33.46 35.00
36℃ 22.25 24.73 26.98 29.06 30.98 32.77 34.43 36.00
37℃ 22.96 25.51 27.82 29.93 31.89 33.72 35.40 37.00
38℃ 23.68 26.28 28.64 30.81 32.81 34.65 36.38 38.00
39℃ 24.39 27.07 29.49 31.79 33.72 35.61 37.36 39.00
40℃ 25.11 27.84 30.31 32.57 34.64 36.56 38.33 40.00

Cooling tower with wet bulb temperature

Why there has relationship between wet bulb temperature and cooling tower? Cooling tower major path of heat removal is evaporation. Wet bulb temperature is the lowest temperature the air can be cooled. Wet bulb temperature is not only effects the efficiency but also is the index of cooling tower operates efficiency. When discuss about cooling tower efficiency, we would consider the temperature difference between output water temperature and wet bulb temperature and the deviation of temperature is called approach temperature. Most air through heat exchange cannot reach to saturated status, so hot water are usually higher than wet bulb temperature for 3 to 6 degrees after cooling.


Impact of Climate on cooling towers in Singapore and Malaysia area

  Climate Average monthly temperature AAR RH WDIR Relationship to cooling tower

Malaysia

Tropical rainforest climate

23℃~33℃

2,000~2,500(mm)

Humid and hot

Jun - Sep:
southwest wind

Nov - Mar:
northeast wind

  • Damp climate is not good for evaporate.
  • Sea breeze would cause cooling tower shell corroded.

Singapore

Tropical rainforest climate

23℃~31℃

About 2,000(mm)

relative humidity 65%~70%, in early morning even can arrive 90%

Dec - Mar:
northwest wind (humid)

Jun - Sep:
southwest wind (dry)

  • Damp climate is not good for evaporate.
  • Sea breeze would cause cooling tower shell corroded.

Singapore and Malaysia climate are hot and rainy, the damp climate is not good for evaporate that would decrease the heat exchange efficiency. But when we discuss about cooling tower cooling capacity, not only climate type should be considered but also quality and scale need to be considered. In this case, enlarge the cooling tower size can ensure the capability of cooling performance. ( How to choose cooling tower? )


Cooling tower shell corrosion situation is more serious in coastal area. In Taiwan, many customers also have this problem. Finally, they almost gained the improvement by used Jin-Hui closed circuit cooling tower. ( Jin-Hui closed circuit cooling tower )

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