The basics of humidification systems

In a previous post on virus and humidification, I promised to discuss more about humidification systems.  In this post, I am going to discuss the basics of humidification systems.  Humidity is a measure of the moisture in the air.  Technically, the humidity that everyone is usually talking about, including the weather guys is relative humidity.  Every so often, the term absolute humidity sneaks out of the scientific and engineering circles into the public.  This is a different measurement of moisture in the air; but that is a subject for another day when I pretty much run out of other topics—and I do have a list.

Humidification systems are simply devices for putting moisture into the air.  In our area, we only want to put moisture in the air during the winter.  In the summer, we are more concerned with getting out of the air.  The reason that people want to humidify air is that the air in our homes is extremely dry during the winter.  Even though outdoor air during the winter may have what seems like a normal humidity level, when that air comes indoors and is heated, the relative humidity levels drop to levels seen in a desert—somewhere around 10%.

For homes, a homeowner has really two basic choices in humidification systems:  central and portable.  A central humidification system supplies water to the air going into or coming from the air handler.  The humidified air is then distributed to all areas of the home.

Central humidification systems typically are recognized as a plastic box mounted on a return air duct near the air handler as seen in the photo below.  However, I recently saw on system mounted on the supply air duct, which may have been the work of an installation instruction-challenged installer.

A typical central humidification system

Inside the box is a web-like media through which air flows.  The air that flows through this media usually comes from the supply air duct just downstream from the air handler (the duct coming into the side of the humidifier in the above photo).  A humidistat connected to a sensor normally in the return air duct upstream of the humidification system (the little white box above the humidifier in the above photo) operates an electronic water valve that controls the water flowing onto the media (the valve can be seen alongside the humidifier in the photo above).  The humidistat is a control system for sensing moisture in the air similar to the way a thermostat senses temperature.  The water valve is connected to the home’s water supply.

When the humidistat senses that the moisture level of the air is too low, it opens the valve to let water flow over the media.  When the humidistat is satisfied, it closes the valve.  Air flowing through the media is supposed to pick up the moisture through evaporation and carry it into the return air stream.  Water flowing across the media that is not absorbed by the air is supposed to flow into an overflow that has a drain line to a nearby drain (the black tubing below the humidifier in the photo above).  In a few cases, the humidistat is located in a living area of the home, usually next to the thermostat.  Some humidistats are incorporated into the thermostat.

Most people will be more familiar with the portable humidification system, particularly since they are sold pretty much everywhere.  These units are self-contained humidifiers that have a water reservoir and a means for converting the water into a mist.  Some units also have a small fan to power air through the unit.  Portable humidifiers have three methods for turning the water into a mist.

The first method for turning water into a mist is a nebulizer that is simply a vibrating plate onto which the water dribbles.  The vibration breaks the water into a mist that is usually carried into the air by a fan-powered air stream.  These units are pretty simple devices that use the electric current to generate the vibrations in the plate.

The second method for turning water into a mist is a rotating disk that slings water at a comb-like device, which is supposed to break the water up into a mist.  The spinning disk operates both as a kind of pump to pull water up out of the reservoir and a means of slinging the water at the comb and a fan to push the mist out of the unit.  This method tends to generate larger water droplets than the other methods, which can lead to water falling on surfaces around the unit.  But, they are also commonly the cheapest units.

The third method for turning water into a mist is the old tried and true method from many of us boomers child-hoods is steam generation.  These humidifiers have a electrical resistance element that heats up when electricity runs through it, similar to the elements on an electric stove.  Water in contact with the heated element boils and is converted into steam.  The steam literally is forced out of the unit because of the energy it absorbs as it turns to steam.  This method of humidification is also used in most commercial and higher-end home humidification systems.

So, in summary, humidifiers convert water into water vapor through three methods; but all of them ultimately work through evaporation.  Central humidification systems use direct evaporation of the moisture into the air as it passes through a wet media, similar to the way that we evaporate water from our skin.  The other methods generate a mist with the intent to get it into the room air where the water will evaporate into the surrounding air.  For these mist generators, the smaller the size of the droplets in the mist, the better the water evaporates.  That doesn’t mean that central systems are more effective humidifiers than the others.  Steam systems are actually the most effective method, but take more energy to do the humidification.

I will end this part of the discussion there.  In the next post on humidification, I will discuss the pros and cons of central and portable humidification systems.

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One Response to “The basics of humidification systems”

  1. Tweets that mention The basics of humidification systems « Criterium-Cincinnati Engineers -- Topsy.com Says:

    […] This post was mentioned on Twitter by Matthew Klein, Matthew Klein. Matthew Klein said: The basics of humidification systems: http://wp.me/pHkGv-1F […]

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