In the HVAC and Refrigeration field, humidification and dehumidification systems are used to transfer moisture to/from the air. These types of systems are sized based on the amount of moisture, measured in pounds of water per hour that is added or removed to the air.
Humidifiers are used to add moisture to air typically in order to achieve the best conditions for human occupancy. In dry areas, low humidity causes moisture to evaporate from people’s skin, creating the feeling that it is much colder than the dry bulb temperature indicates. Other times humidifiers are used to maintain best humidity levels for equipment or produce.
There are two main types of humidifiers, (1) Steam and (2) Evaporative humidifiers.
(1) Steam Humidifiers, also known as isothermal humidifiers, add moisture to air without the change in dry bulb temperature, hence isothermal humidifier. Steam is created through an external means like a gas fired boiler or electric boiler. Then the steam is typically directly injected into the air stream. It is common to assume that the temperature of the air will rise since steam is 212 F. However, it is important to think of steam as water vapor and as it is added to air, it will correspond to an upward movement on the psychrometric chart [Pt 1 to Pt 2].
(2) Evaporative Humidifiers, also known as adiabatic humidifiers, add moisture to air without a change in enthalpy, hence adiabatic humidifier. Evaporative humidifiers do not require an external energy source like Steam Humidifiers. Evaporative humidifiers work by blowing dry air over water or through water droplets. The energy to vaporize the water comes from the dry air. As the air releases heat to vaporize the water, the air also cools. On the psychrometric chart, adiabatic humidification is shown as an upward-left movement, along a constant enthalpy line. It is constant enthalpy because the enthalpy lost to sensible cooling is gained by latent heating [humidification].
Evaporative humidifiers operate on the same principle as air washers, evaporative coolers and cooling towers. These principles will be discussed in the Cooling Tower section. D
De-Humidifiers are used to remove moisture to air typically in order to achieve the best conditions for human occupancy. In humid areas, high humidity causes the feeling that it is much hotter than the dry bulb temperature indicates. Other times de-humidifiers are used to maintain best humidity levels for equipment or produce. De-humidifiers are especially important in preventing mold and mildew from forming.
There are two main types of de-humidifiers, (1) Condensing and (2) Desiccant de-humidifiers.
(1) Condensing de-humidifiers or cooling humidifiers work by decreasing the temperature of the incoming air so that it is unable to hold moisture, which causes condensation. A cooling coil acts a dehumidifier. In the Psychrometric chart below, hot, humid air enters the coil and leaves as cool air. The amount of water vapor removed from the air is shown in red. In some cases the air is reheated in order to lower the relative humidity and increase the dry bulb temperature.
(2) Desiccant de-humidifiers or chemical dehumidifiers use desiccants to adsorb water from air. As the air loses its water vapor, the heat from condensing the water vapor is gained by the air stream, which causes the air to increase its dry bulb temperature. A desiccant de-humidifier is shown as a downward-right movement, along the constant enthalpy line (adiabatic).