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 44°F 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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