Understanding Effective Temperature and Air Velocity

Effective Temperature (ET) is a widely used index to assess thermal comfort. It considers the combined effect of air temperature (Dry Bulb Temperature or DBT), humidity (often represented by Wet Bulb Temperature or WBT), and air movement (air velocity) on how warm or cool a person feels.

The Dry Bulb Temperature (DBT) is the standard air temperature. The Wet Bulb Temperature (WBT) is the temperature read by a thermometer covered in a water-soaked cloth and exposed to moving air. The difference between DBT and WBT is related to the relative humidity of the air. When DBT is equal to WBT, it indicates that the air is saturated, meaning the relative humidity is 100%.

In this question, we are given that both DBT and WBT are 25°C. Since DBT = WBT, the relative humidity of the air is 100%. This means the air is saturated with water vapor.

Effect of Air Velocity on Effective Temperature

Air velocity plays a crucial role in heat transfer from the human body to the environment. The body loses heat primarily through convection and evaporation (sweating).

• Convection: Increased air velocity enhances the rate of convective heat transfer between the skin surface and the surrounding air. If the body surface is warmer than the air, increased air movement removes heat faster, making the person feel cooler.
• Evaporation: Increased air velocity also increases the rate of evaporation of sweat from the skin. Evaporation is a very effective cooling mechanism because it removes latent heat from the body.

However, the effectiveness of evaporative cooling depends heavily on the humidity of the air. When the air is saturated (100% relative humidity, as indicated by DBT = WBT), the air can hold no more water vapor. Evaporation from the skin becomes very difficult or stops entirely, even if the person is sweating. In this saturated condition, the primary mechanism affected by increased air velocity is convection.

Even though evaporation is severely limited at 100% RH, increased air velocity from 6 m/min to 20 m/min still improves comfort and heat dissipation by increasing convective heat loss and breaking up the insulating layer of still air around the body. This increased rate of heat removal from the body surface makes the person feel cooler. Feeling cooler corresponds to a lower Effective Temperature.

Therefore, when the air is saturated (DBT = WBT) at 25°C, increasing the air velocity from 6 m/min to 20 m/min will enhance the rate of heat loss from the body, leading to a decrease in the Effective Temperature.

Based on this analysis, increasing air velocity under saturated conditions at 25°C will cause the Effective Temperature to decrease.