When Should You Consider A Desiccant System? Part 2
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."
Some may also be hesitant to trust the information generated by DOE, because it is creating the data along with American Gas Cooling Center (AGCC), Gas Research Institute (GRI), as well as manufacturers of dehumidification equipment, such as Munters Cargocaire and SEMCO. All parties have a vested interest in seeing desiccant systems be successful.
One expert says the DOE sometimes adds to the confusion over desiccants, because their reports don't always make the distinction between active and passive desiccants. Also, in the past DOE was "seduced" by enthusiastic researchers and companies promoting heat-reactivated active desiccants for cooling rather than just for dehumidification. In theory it's possible, but it relies on evaporative cooling. Desiccants still don't have much cooling power, so the equipment and installation costs for desiccant-only cooling were outlandish.
This expert says that DOE has a more realistic approach these days: Use active desiccants for dehumidification alone and use the extra heat from distributed power generation to reactivate that desiccant (heat from BCHP - building cooling, heating, and power plants).
The Supermarket QuestionSo when can a commercial application benefit from an active, solid desiccant system? Most experts agree that many applications in which it's necessary to maintain less than a 50°F dewpoint could benefit. It's also best if there are large outside air requirements, because the outside air can carry a large moisture load. With those characteristics in mind, a refrigerated loading dock could be an excellent place to use an active solid desiccant system, as it complements the cooling capacity of the refrigeration systems and the dock cooling systems.
Another possible application could be a meat packing operation. Or electronics manufacturing, or anything connected with pharmaceutical or biomedical research or production. Alas, we digress. These are not run-of-the-mill commercial projects.
Supermarkets are a true commercial possibility. As Harriman says, "Everything you hear bad about desiccant systems is true, and everything you hear good about them is true." He says that it's true when an engineer says an active solid desiccant system in a supermarket makes absolutely no sense economically, as they cost more to install, they cost more to run, and there's gas involved, too. It's definitely possible to have no dehumidifier at all, just let a well-designed, cooling-based system take care of humidity control.
But on the other side of the coin is the engineer who says that by incorporating a desiccant system, he's saved $20,000 on the overall cost of the mechanical system and he's also going to save $20,000 to $30,000 every year in operating costs. It comes down to the way those systems are designed. "If you design a supermarket system for desiccants the way you would design a supermarket system with a cooling-based technology, the first engineer's assertion will be absolutely correct, it will make no sense economically," says Harriman.
However, if you take advantage of the characteristics of desiccant technology, such as reducing air flow by one-third, putting the warm air from a desiccant system into the cold aisle where it belongs, don't bother post-cooling it.
Furthermore, ignoring the fact that 30% to 50% of the evaporator load from the cases spills over into the aisles, then all of a sudden, desiccants can do what the second engineer says, asserts Harriman: They save money on the installation, and they save money every year compared to the first engineer's design. "Both of those engineers' systems work," says Harriman. "Product is sold, customers are happy, owners are happy. But they're both correct. And they're both wrong, too. So that is a classic example of the disputed ground between the two religions, if you will. Supermarkets are it."
Evaluate The ApplicationAll other commercial applications, such as retail spaces and office buildings are very much a question mark as to what is the best system to use - active, passive, or mechanical cooling-based dehumidification. What seems to be clear is that if there is a need for humidity control, then dehumidification is needed - whether that is an active desiccant system or a mechanical cooling-based unit.
Determining whether an application needs humidity control or humidity moderation will be the deciding factor in which system to apply. "If you need control, then that implies that you need a device that will respond to a humidistat. You've got to start there. Do you have a humidistat? If you don't have one, you don't need humidity control, you just need humidity moderation, in which case a passive desiccant system that reduces the load on the conventional equipment is an excellent solution for you," says Harriman.
Chuck Campbell, vice president and general manager, Munters Corp. - Dehumidification Division (Selma, TX) says that commercial applications such as restaurants, supermarkets, and ice rinks can often make use of wheel-based desiccant systems. "For office space, retail, and almost all commercial applications, wheel-based desiccants are the best approach to independently control temperature and humidity," says Campbell. "If makeup air or other factors are not significant, traditional air conditioning systems are sufficient.
But Campbell also notes that with the increase in makeup air requirements to comply with ASHRAE 62-89, desiccant systems are being more widely used. To provide systems for those applications, he notes that design guidelines must be obtained from the manufacturer, as manufacturers use different desiccant materials that vary in performance. (Two design resources include Munters' "The Desiccant Handbook," available by calling 800-843-5360 and the "Desicalc" program found at www.gri.org.).
To really determine whether or not an application can benefit from a desiccant system, Harriman suggests that engineers gather three key pieces of data:
- Have a clear understanding of the purpose of the humidity control project. Ask: What's the purpose of the project? Why does humidity need to be controlled? What happens if it's not controlled? What are the economic consequences or the comfort consequences? Work closely with the owner to find out the answers to these questions.
- Establish the control level that will achieve those purposes.
- Research the available utilities and what they all cost.
"If you've got those three things in your head before you start talking [to the manufacturers], you're not going to have any trouble when you try and make your decisions about desiccants or cooling or passive or active or liquid or solid. It will all fall out really quickly because, I guarantee, the manufacturers will be able to say yes or no in the first phone call. There will be much less confusion," says Harriman.
Next month's issue will look at how a properly designed and optimized mechanical cooling system may be all that's needed for proper dehumidification. ES
(All figures copyright 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permission, ASHRAE Journal, October 1999, vol. 41, no. 10.)