There is a lot of buzz these days about desiccant dehumidification systems: Where should they be applied? How should they be specified and designed? In the first of two parts, the author finds that after talking with the experts, the answers to such questions can still be a little murky.

Many different types of desiccant systems await designers out there, and it can be very confusing to sort through them all. There are liquid systems, solid systems, commercial systems industrial systems, active systems, passive systems. You get the idea.

So when someone wants to talk about desiccant systems, you first need to narrow the conversation down to application and type if you want to be on the same page. Then you have to figure out how the desiccant system is to be used - are you interested in trying to cool with it, are you going to use it to help precondition the outside air, or do you just want to reduce the load on the mechanical cooling system?

With so many if's, and's, and but's, it is no wonder people are confused. Not to mention that there are very few sources that can help engineers sift through the information in order to make an informed decision. And those who can theoretically help are usually the manufacturers, who want to sell the equipment, or the gas cooling industry, which has a vested interest in creating a summer gas load (read: active desiccant systems) in buildings.

The big question mark for most engineers is when to use desiccant systems in commercial applications. For that reason, this article will focus primarily on active solid desiccant systems for commercial applications.

SEARCHING FOR INFORMATION

"I'd be really astonished if engineers weren't confused when it comes to desiccant systems," says Lew Harriman, director of research and development for Mason-Grant Consulting (Portsmouth, NH). He believes there are three fundamental reasons why there's so much confusion surrounding the desiccant systems:

* It has not been a part of engineering training until the last five years, so engineers have no basis in their educational experience to understand, much less judge, the different statements made by people in that area.

* In the last 10 years there has been a relatively wide variety of companies developing new products for new applications. These products are promoted fairly heavily, but that doesn't mean that they're suitable for all applications. It doesn't help that the vendors, in an effort to promote their products, may say that just about any application would work well for their product.

* The people who make enthalpy heat exchangers adopted the name "desiccant wheel." The enthalpy wheel exchanges moisture between two different airstreams and has desiccant in it, and while it looks similar to a true desiccant wheel, it does not have the same function.

So where is an engineer to go for the definitive, unbiased answer on how to design a desiccant system? There's no place right now, says Harriman. "The industry is still small enough so that the good data comes from people who all have a vested interest in its success. Engineers are often getting some truly crummy information from people who don't know what they're talking about, or more probably, they see information that has to do with one application of the technology and have been assured by somebody that it will work for another application when, in fact, it's a different thought process."

One consulting engineer, who wished to remain anonymous says that he'd like to learn how active solid desiccant systems can work for commercial applications, but he's frustrated by the lack of data. "Even the ASHRAE Fundamentals book really does not give enough information for engineers to be able to calculate the leaving drybulb temperature for a desiccant system. There is a graph in there that you can use, but the graph can mislead you substantially."

Harriman is currently writing the new ASHRAE design guide for humidity control, and he says the problem is that desiccant products are very much equipment and vendor specific, so it's hard to generalize the data to a point where it can be applied in every situation. Therefore, it will still be necessary to consult the vendor for the best data. But there are problems with that as well, as manufacturers aren't yet large enough to handle all requests for information.

Jim Sand, program director for the Department of Energy (DOE) desiccant program at Oak Ridge National Laboratory (Oak Ridge, TN) notes that he hopes when the lab is done with its research, engineers will be able to turn to DOE for the best information. But he also puts the onus back on the engineers, stating that most are so busy now they don't want to learn about a different kind of technology. "Engineers are 110% busy with what they have now, and they wonder why the heck they should learn about something that may or may not work when there are a catalog of things that they know do work, even though they might not be the most efficient."