Wet Bulb Temperature

Detailed Explanation

Wet bulb temperature (Twb or WBT) is measured by wrapping a wet wick around the thermometer bulb and exposing it to an air stream. Evaporation of water from the wick draws heat from the thermometer, causing the temperature to drop. The equilibrium temperature reached is the wet bulb temperature.

Wet bulb temperature is always equal to or lower than dry bulb temperature. The difference between the two (wet bulb depression) indicates how dry the air is: at 100% RH the difference is zero (no evaporation occurs), while at very dry conditions like 20% RH the difference can reach 10–12°C.

Wet bulb temperature represents the lowest temperature that air can reach through evaporative cooling. For this reason, it is the fundamental parameter in sizing evaporative cooling systems for desert climates, dry industrial process facilities, and data center indirect coolers. It also determines the theoretical lower limit of condenser water temperature in cooling tower design through the outdoor wet bulb temperature.

Calculation

Stull approximate equation:

Twb = T × atan[0.151977 × (RH + 8.313659)^0.5] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 × RH^1.5 × atan(0.023101 × RH) − 4.686035

T: dry bulb temperature (°C) RH: relative humidity (%) Twb: wet bulb temperature (°C)

Validity: ±0.3°C accuracy between −20°C and +50°C

Practical check: on the psychrometric chart, find the intersection of the dry bulb vertical line with the RH curve, then read the wet bulb diagonal line.

Practical Example

A textile weaving facility is experiencing yarn breakage due to low humidity. Ambient conditions: 30°C dry bulb, 25% RH. Using the Stull equation, wet bulb ≈ 16.5°C.

When selecting an evaporative humidification system: theoretically, the system can cool and humidify the air down to 16.5°C. In practice, with 80–85% efficiency, it drops to approximately 19.5°C. Supply temperature:

Tsupply = 30 − 0.85 × (30 − 16.5) = 18.5°C

At the same time, ambient relative humidity rises from 25% to approximately 75%. The yarn breakage problem is resolved, and ambient temperature also drops without additional cooling load. This is the dual benefit of evaporative cooling: humidification and cooling — but it works only in dry climates.

Engineering Note

Considerations when using wet bulb temperature in design:

• Economic limit for evaporative cooling: if outdoor wet bulb temperature is below 22°C, it can be an alternative to mechanical cooling; above this value, efficiency drops. • In Türkiye geography: Central Anatolia (Ankara, Konya, Kayseri) summer WBT 18–22°C → evaporative cooling is suitable. Mediterranean/Aegean coast (İzmir, Antalya) WBT 24–26°C → insufficient, mechanical cooling is required. • In cooling tower design, the approach (condenser outlet temperature − outdoor WBT) is typically 4–7°C; a lower approach means a larger (and more expensive) tower. • When taking field measurements with a sling psychrometer or aspirated psychrometer, minimum 3 m/s air velocity, clean wick water, and keeping the wick saturated are critical.

The NKT humidification system selection tools automatically calculate adiabatic cooling potential from the outdoor WBT input.