There are many ways to humidify in today’s market. This paper describes the primary methodologies and provides guidance on choosing the most appropriate methodology for a given application.

Isothermal vs. Adiabatic HVAC Humidification

The goal of any humidification system is to convert liquid water from a make-up water source into water vapor within the conditioned space. There are essentially two methods to achieve this – isothermal and adiabatic. Regardless of the method, the latent heat of vaporization of water (970 BTU/# at atmospheric pressure) must be overcome to cause the liquid water to vaporize. The essential difference is that isothermal humidification systems apply latent heat of vaporization via an external energy source before the water is introduced into the air. In contrast, adiabatic systems allow liquid water droplets to “steal” the heat from the surrounding air.

Isothermal

Electricity or natural gas are the most common sources of energy used to vaporize water for humidification. Typically, a steam generator will produce steam very near atmospheric pressure. That steam will either be injected into a duct via tubing and a steam distribution array or injected directly into a space via a fan mounted atop the steam generator. The term isothermal is used to describe the fact that the temperature of the surrounding air (space or duct) remains constant. Although, this is not precisely true, as the injected steam slightly elevates the air temperature. In the case of a 100% outside air system, the humidification load is relatively high, and a 5-7°F duct temperature rise will be experienced. Much smaller temperature rises will occur in a recirculated air system with modest quantities of outside air.

Typical methods of isothermal humidification include:

• Immersed electrode – Electrical current is passed directly through the water to elevate its temperature to boiling.
• Immersed element – A heating element is used to bring the water to a boil.
• Natural gas – A very low-pressure combustion boiler utilizes natural gas as its energy source.

Adiabatic

Adiabatic humidification systems turn liquid water into fine droplets that are injected into the space or duct. The fine droplets absorb heat from the surrounding air and vaporize. Temperature depression in an adiabatic system is significant. In a 100% outside air system, 15°F to 25°F is common. The temperature depression will be smaller in recirculated systems with modest quantities of outside air. The psychrometric process is shown below:

Whenever an application requires simultaneous cooling and humidification, the temperature depression in adiabatic systems can be very valuable as a free cooling source. Examples include data centers and high-heat gain industrial processes.

In other applications, most notably 100% OA, there is no opportunity for free cooling. The adiabatic temperature depression must be overcome by additional energy input into the pre-heat coil. In this case, there is no significant difference in energy consumption between a system that uses isothermal humidification vs a system that uses adiabatic humidification.

Typical methods of adiabatic humidification include:

• Ultrasonic – high-frequency vibrations cause very fine water droplets to become airborne
• High-pressure spray – Water is forced through small orifices at high pressure, resulting in an airborne mist.
• Compressed air atomization – Compressed air and liquid water are forced through small orifices to create airborne droplets again.

Your Accuspec representative is well-equipped to provide proper selection guidance for your HVAC humidification needs. Please call us at 203-261-8100 or email us with any questions. We look forward to serving you!