Wet bulb temperature
Also known as: WB, wet-bulb temperature
Short definitionThe reading of a thermometer whose bulb is wrapped in a wet wick, held in an air stream. It is the lowest temperature water can reach by evaporating, and the drier the air, the further it falls below the ordinary temperature.
Water evaporating from a wet surface cools that surface. If the air is saturated there is no evaporation and the wet bulb reads the same as the dry bulb; the drier the air, the wider the gap. In 35 °C air the wet bulb is about 21–22 °C at 30% humidity and about 28 °C at 60%. It is not the same as the dew point; in unsaturated air the dew point is lower.
In a project you meet it when choosing the condenser type. An air-cooled condenser depends on the dry bulb; in 35 °C air it condenses at 45–50 °C. An evaporative condenser approaches the wet bulb and brings condensing down to 32–35 °C at a 22 °C wet bulb. The gain is large in a dry climate and narrow on a humid coast. Selection uses the local design value; looking at dry bulb alone leads to high pressure trips on humid days.