Explanation:

On a psychrometric chart, a vertical downward line represents dehumidification.

Dehumidification is the process of removing moisture or humidity from the air. It is commonly used in air conditioning systems to control the humidity levels in a space.

On a psychrometric chart, the vertical axis represents humidity ratio or specific humidity, which is the amount of moisture in the air per unit mass of dry air. The horizontal axis represents dry bulb temperature, which is the temperature of the air without considering the moisture content.

When a vertical downward line is drawn on the psychrometric chart, it represents a process where the air is being dehumidified. This means that the moisture content of the air is being reduced while the temperature remains constant.

Dehumidification can occur through various processes, such as cooling, condensation, or absorption. In air conditioning systems, dehumidification is often achieved by cooling the air below its dew point temperature. As the air is cooled, its moisture content condenses into water droplets, which can then be removed from the air.

By following the vertical downward line on the psychrometric chart, one can determine how the air properties change during the dehumidification process. The specific humidity decreases while the dry bulb temperature remains constant.

In summary, on a psychrometric chart, a vertical downward line represents dehumidification, which is the process of removing moisture or humidity from the air. This is commonly achieved through cooling the air below its dew point temperature.