According to ASHRAE, what are the two classifications of humidifiers?

• A. Mechanical and chemical
• B. Adiabatic and isothermal
• C. Passive and active
• D. Electric and steam

Answer: (B)

Humidification is categorized by how heat behaves when moisture is added to the air. In adiabatic humidification, moisture enters the air mainly by evaporation without significant heat exchange with the surroundings. As water evaporates, it absorbs latent heat from the air, so the air tends to cool slightly while its humidity rises. In isothermal humidification, moisture is added in a way that keeps the air temperature essentially unchanged, typically by introducing vapor at the air’s temperature or by providing heat as needed to maintain the same dry-bulb temperature while increasing humidity.

This is why the two classifications recognized by ASHRAE are adiabatic and isothermal. The other pairings describe different aspects (like how the device is powered or whether it’s steam-based) rather than how heat interacts with the air during humidification.