Humidity Control: A Function of Type That Depends on Needs
By Mark Lentz, P.E. President Lentz Engineering Sheboygan Falls, Wis. -- Consulting-Specifying Engineer, 4/4/2006 12:56:00 PM
Editor’s Note: For a detailed discussion of humidification, check out M/E Roundtable in the upcoming April print or online version of CSE.
Humidity control is strictly a function of humidifier type and depends on the needs of the process.
For example, duct-mounted steam and atomizing-type evaporative processes have the ability to try to put more moisture in the air than the air can accept. These types of systems need to be minimally controlled from return/exhaust air relative humidity through a supply air high limit control and an airflow switch to prevent operation when the airflow is off. A better solution is to use the return/exhaust air sensing as a sub-master input to the discharge control to get a short feedback time between control action and system response. This makes for more precise control over simple return/exhaust air control, which means that relative humidity swings will be smaller and shorter in duration. The discharge high limit protects against super-saturation of the supply air, which can result in condensate running out of the ceiling diffusers and mold growth in the ducts.
When using a saturated rigid media type evaporative process, the best control is through an apparatus dew-point control strategy. This is because the capacity of these devices cannot be modulated by means of a valve. The precision of control then becomes a function of media saturation effectiveness and discharge air temperature. If a wide range of control is desired, a bypass mechanism is best. However, precision control is definitely possible.
When the saturation effectiveness is high—98% or better—the moisture content of the air, or the dew-point temperature, so closely approximates the air temperature leaving the device that the two are essentially identical. By controlling the supply air temperature, one controls the absolute moisture content of the leaving air, which, in turn, makes space relative humidity a direct function of temperature. If the supply air temperature is maintained in a stable condition, the same occurs with the relative humidity within the space. In this way, the spaces are served by a single humidifier in a single air-handling unit with a nominal 55°F supply air temperature, and can be individually controlled with a precision of as little as 0.5% RH over a 20% RH—a 40% to 60% RH range—by controlling the space temperatures between 70°F and 80°F respectively. This technique works with both variable- and constant-volume strategies.
Evaporative processes have another control issue, and that is for microbial control. The scrubbing effect of these devices will strip contaminants from the air, and if there is a sump and recirculated water, these contaminants will collect there. This water needs to be continuously cleaned of these organisms, or they can take up residence and form the highly organized bacteria colonies known as bio-slimes within the device. While bio-slimes are typically innocuous when alive, when a die-off takes place, such as when the device is drained and the organisms are permitted to become desiccated, the residual biological matter will begin to decay and create an odor similar to dead fish. This is known as “swamp cooler odor.” This odor is in itself harmless, but it is noxious, and it is the primary reason evaporative humidifiers are not high on most engineers’ lists of favorite processes. The trick to preventing this nuisance and its associated complaints is to prevent the formation of bio-slimes in the system; we want to make this environment hostile to microbes so they do not set up house. To accomplish this, we want to keep as many of these organisms from entering the system as possible, deprive them of nutrients and disrupt their reproductive processes. This typically involves effective air filtration, daily desiccation cycles to kill off incipient colonies that might be forming, daily drain and purge cycles to flush out accumulated nutrients and bacteria and applying a biological control polishing process like low-level ozonation.
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