Last month, we analyzed the operational costs of an AHU with steam humidification and economizer controls. As expected, the results showed us that the two really don't mix, at least for the example presented. Given the assumptions in that particular example, the only economizer option that resulted in a cost reduction was the waterside economizer because waterside systems don't impact the amount of outside air being conditioned. Does that mean that airside economizers should not be used if humidification is required? Perhaps not. Lets take a look at another option - an adiabatic humidifier.

With steam humidification, we saw that the increase in outside air by the economizer controls resulted in a significant increase in humidification load. All of the cooling cost savings were wiped out by the added cost to generate steam.

Adiabatic humidifiers (e.g., evaporative, wetted media, ultrasonic, etc.) exchange sensible heat of air with the latent heat of water to accomplish evaporation at constant enthalpy. The only energy consumption associated with these humidifiers would be the small amount of electricity required to deliver the moisture to the airstream.

For example, ultrasonic humidifiers generate a fine mist through the high frequency vibration of an electrical transducer. They typically draw about 24 W (82 Btu/hr) per lb of water atomized. That's about 7% of energy required to generate steam (about 1,050 Btu/lb).

Another added benefit (or expense) of this type of humidifier is the resulting "free cooling". Refer to a psychrometric chart and you can see that as moisture is added to an airstream at constant enthalpy, the dry bulb temperature decreases. The result is a significant drop in air temperature while the total heat (enthalpy) remains the same. This phenomenon will result in further cooling cost reduction. But at the same time, heating costs could potentially go up, especially in colder climates with large minimum outside air requirements.

Because of the adiabatic process, the temperature of the air entering the humidifier must be warm enough to absorb the moisture required to achieve the desired humidity level. Therefore the entering air sometimes needs to be preheated, even beyond the desired supply air temperature setpoint. Also, the controls can be somewhat more complicated in comparison to the steam system. Dewpoint control is often necessary.

Figure 2 makes a comparison between the two humidification systems with enthalpy economizer controls and a minimum OA setting of 25%. The adiabatic humidifier in this example is an ultrasonic unit operating at 24 W/lb. As you can see, the total cost comparison isn't even close primarily because of the energy required to generate steam. Also note that due to the colder Minneapolis climate, there was actually a preheat load with the ultrasonic system.

As discussed above, this load represents the heat required for the entering air to absorb moisture adiabatically. As shown in Figure 3, there is actually a difference in water consumption as well. This is another benefit from the adiabatic process discussed above. When the economizer dampers are in the modulating mode, less outdoor air is required with the adiabatic unit.

So what's the bottom line? Adiabitic humidification has some obvious significant operational cost advantages over steam humidification where airside economizer systems are concerned. However, the advantages diminish as the minimum OA setting increases, because of the added heat energy required.

In fact, the operational cost comparison between the two systems can be close to a toss up in 100% outdoor makeup air systems. A more detailed description of the two types of humidifiers is made in the article, "Humidification Side by Side" (Engineered Systems, April, 2002). ES EDITOR'S NOTE: The tables associated with this article do not translate to the Internet. To view the images, please refer to this month's print version of ES.