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Desiccant Systems


Commercial cooling equipment dehumidifies air by dropping its temperature below the dew point, causing water to condense on the coil. From there it is drained from the unit. Most commercial space is maintained at ~50% humidity so circulating 44F water through the conventional cooling coils will usually do. But what if the space requires 15-20% humidity, say to handle some humidity-sensitive material? Achieving that humidity in the space would require a much colder coil temperature. Needless to say, after the water was removed, the air would have to be reheated. This is the domain of desiccant based systems. Desiccant systems remove moisture directly from the air without cooling it. In fact, they usually end up heating the air.

Consequently, desiccant based systems tend to be used in a series with conventional cooling equipment. The common design approach uses desiccant to remove the moisture (i.e. latent load) while conventional cooling removes the sensible load. As in almost any technical area, there are several alternatives. The two most common commercial designs uses a honeycomb wheel impregnated with a solid crystalline material (such as lithium chloride or silica gel) or a liquid spray into the air stream.

Both capture the moisture in the air as it passes through. Another alternative to these desiccant concepts worthy of side-by-side comparison is the heat pipe, which is much simpler and may provide the same performance.

Desiccant systems are widely used in low humidity applications such as pharmaceutical powder production and packaging. They have also been used in supermarkets where lower space humidity has been shown to improve the energy performance of case work, increase occupant comfort and may even improve sales. The two key questions are : Are they cost effective to install? Do they represent the best system to install in a given application? Both of these questions should be addressed by a qualified professional.

If they fit and are correctly designed, desiccant systems can also produce these benefits:

1. Eliminate condensation on cooling coils and in drip pans, and reduce humidity levels in ducts. This will virtually eliminate the growth of mold, mildew, and bacteria. The combination can reduce maintenance and help avoid indoor air quality problems.

2. Lower humidity levels in occupied spaces provides equivalent comfort levels at higher ambient temperatures. This could allow chilled water set-points to be raised and there-by save energy and reduce system operating costs.

3. Reduce the mechanical cooling load which should permit the use of smaller chillers and possibly even smaller ducting in new construction. These construction cost offsets should be factored into any economic evaluation.


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