The amount of moisture required to maintain a desired space relative humidity (rh) is directly related to the amount of outside air introduced. In a humidified building or space, moisture is constantly lost in the warm, moist air leaving the building either by natural or mechanical means. The exhaust air is replaced with cool, dry outdoor air, which requires moisture to maintain the rh setpoint. The humidification data in this report is the amount of moisture per cfm of outdoor air required to maintain a minimum 30% space rh at 70 degrees F. It is calculated hourly based on the differential between the outdoor air humidity ratio and the space humidity ratio at setpoint.
There are basically two types of processes that are commonly used today. Isothermal systems typically inject steam directly into the air and are most common in colder climates. Adiabatic systems like evaporative coolers, wetted media, and water atomization systems are sometimes used to cool as well as humidify and are popular in historically warm, dry climates.
The data applies to both types of systems since it represents water consumption and not energy. On the other hand, energy costs can vary widely depending on the type of system being used.
December 2001 Humidity Costs Way DownDecember 2001 was a relatively warm month for most of the country. All cold-weather indicators including heating degree days, outdoor air heating, economizer hours, and winter heat recovery savings were down in comparison to normal. So it is no surprise the data also shows that much less water was consumed by humidification systems in December.
The only reported city that experienced an increase for December was Salt Lake City. Figure 2 ranks the cities from the largest reduction in water consumption (Minneapolis). The magnitude of this reduction can be significant in humidified buildings with large amounts of outside air. If an hvac system in a Minneapolis building processes 50,000 cfm of outside air, and the building is maintained at 30% rh at 70 degrees on a 24-hr occupancy schedule, the reduction in water consumption for December was 50,000 x 0.325 = 16,250 gal. If steam is generated at $0.75/Therm and 80% boiler efficiency, the reduction in humidification energy cost would be 50,000 x $0.029 = $1,950.
Given this information, what if the utility bills show that the water consumption and heating bills were actually higher in December? Perhaps the heating and rh control systems need to be investigated, or the outside air intake damper settings are incorrect. It is also possible that some steam traps could be leaking. Another factor may be domestic water use. Good luck. But at least the weather can't be blamed this time! ES
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