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.