Dehumidification: The Problem No One Wants To Talk About (April 2000)
Now it's necessary to think of using dehumidification for other applications, such as schools, office buildings, and theaters. One reason is that ASHRAE Standard 62-99, Ventilation for Acceptable Indoor Air Quality, calls for many types of buildings to increase ventilation rates. Some buildings have already been bringing in more air to try and prevent indoor air quality problems. What some seem to forget is that often this outside air is sticky and humid, which increases the latent load in the building, making occupants uncomfortable.
Numerous engineers believe that mechanical cooling is sufficient to take care of any problems that might arise, often an incorrect assumption. But for whatever reason, some engineers seem to shy away from dehumidification equipment, either because they don't know about all the options, or they don't want to take the time to solve the problems. With all that outdoor air being poured into buildings, though, it's imperative that engineers provide dehumidification solutions.
The Need to LearnProbably the main reason why more dehumidification equipment isn't specified is simply because many engineers don't know that much about it. And it can definitely seem a little overwhelming at first, as there are a number of different options available (e.g., enthalpy wheels, heat pipes, and desiccants). The best advice is to find a reputable manufacturer or manufacturer's representative who can describe which applications can benefit from which product.
Mike Hayes, sales engineer and manufacturer's representative, Engineered Air Systems, Atlanta, says he sees engineers all the time who are unfamiliar with dehumidification systems. At that point, Hayes works with the engineer on the design and tries to find out which product makes the most sense for the application.
"The engineers we're dealing with aren't that familiar with [dehumidification equipment]. We're always trying to raise the awareness for consulting engineers," says Hayes. One way he does that is by sitting down with the engineers and asking questions such as "How is the system going to work when it's 65ûF and raining?"
Once the engineer starts thinking about some of those questions, it often becomes apparent that some type of dehumidification equipment will be needed. The main problem, notes Hayes, is that many engineers have time and money constraints, so they're sometimes unwilling to even consider dehumidification equipment.
"Engineers often try and take a previous design and make it fit a current application. Many engineers succumb to those dollar or time pressures," says Hayes.
Mechanical Cooling is InsufficientThere are also those holdouts who believe that mechanical cooling will take care of any humidity problems in a building. Some engineers have even stated that they just add a little capacity to a system design in order to take care of any humidity problems.
That can be a disastrous solution, says Curtis Musall, regional sales manager, Munters Southeast, Norcross, GA. "Oversizing the cooling equipment actually makes the problem worse, because you will overcool the space. That makes the relative humidity go up higher, and then your cooling equipment will shut off, moisture will build up, and the cooling equipment will come on again," says Musall.
It's true that cooling equipment functions as a dehumidifier, condensing the moisture out of the air. However, to also lower the relative humidity, it would be necessary to provide reheat after the cooling coil, which usually results in increased utility bills. Basically, when you combine outside air and high moisture loads, mechanical cooling can't do it all.
Another factor to think about is the newer types of building materials and methods being used in new construction. "Building methods are a lot better, and buildings themselves are more sealed to the outside. If there's moisture buildup inside the building, there's a possibility you can actually start to deteriorate some of the building materials," notes Musall.
Hayes adds that more efficient lighting, windows, and building skin are definitely creating dehumidification problems. That's because the higher efficiency components bring down the sensible load on that building, but the latent load remains constant, because there's still the same amount of outside air coming in. "That's changed sensible heat ratios, which makes dehumidification more difficult for engineers to handle," says Hayes.
Schools and theaters are some of the applications that have found dehumidification equipment to be necessary with the increased ventilation rates prescribed by Standard 62-99. "Buildings like auditoriums, which have a high people occupancy, have not worked out so well since the change in the standard. I think some engineers have seen that they've had problems, and now they're going back to see why that is," says Hayes.
It's more difficult to get dehumidification equipment into office buildings and retail spaces, even though it may be necessary. That's because the owner of the building rarely occupies it, or else it's a developer who builds it and wants to put in the least expensive type of equipment. Another factor is that office buildings have less outside air per square footage, and people are usually spread out more than in a movie theater or school gymnasium. But untreated outside air can still cause problems, no matter what application you're talking about.
More Equipment - Less First CostSome engineers may not even consider dehumidification equipment, believing that it's too expensive for the building owner to swallow. Nothing could be further from the truth, according to Ben Tritt, P.E., vice president, Des Champs Laboratories, Natural Bridge Station, VA. "A lot of times you can reduce the sensible load by supplying slightly cooler and drier outdoor air. That allows you to downsize your conventional equipment, so that the cost of the installation, if it's properly engineered, can be about the same as just a straight conventional system that would be sized to handle the full outdoor air load."
Tritt calls this the "divide and conquer" scenario. He notes that systems in the past would cycle on and off in order to control temperature. Standard 62-99 requires a continuous amount of outdoor air being introduced to the building, but it's not possible for the system to cycle on and off and continue to bring in outdoor air. "So you have to do one of two things. One is you have to have equipment that will be dedicated to the outdoor air portion and then have separate equipment dedicated to the sensible conditioning, and that equipment can cycle on and off."
Basically, you divide the loads: Let the specialized dehumidification equipment take care of the latent load, and let the conventional equipment handle the sensible load. There are about nine different combinations of dehumidification equipment that can take care of the latent load.
For an office building, Tritt notes that the least expensive system will usually be used. That's often an enthalpy wheel followed by mechanical cooling. However, the enthalpy wheel requires return air to operate, so it's necessary to have some sort of path for the return air to get back up to the unit. If that return air isn't available, then the most common equipment is a heat exchanger surrounding the mechanical equipment. That allows for free precool and free reheat, which reduces the cooling load and prevents overcooling of the space.
"There's also a combination of those two where if you have return air, you can run it through the enthalpy wheel and precondition your outside air, then combine the heat exchanger with the mechanical cooling for the free precool and reheat. That's probably the minimum energy requirement for any dehumidification system," says Tritt.
If no return air is available, Tritt recommends feeding the conditioned outdoor air into the intake of the conventional equipment. "You're not introducing any more load and, in fact, you're going to make that equipment operate more efficiently and in a more stable manner, because you don't have that fluctuation of outdoor air conditions. That's the real problem with conventional equipment. It's just not meant to operate under a wide range of conditions, and outdoor air fluctuates greatly."
But in all these scenarios, Tritt notes that the most important factor is proper engineering. "If you start with the wrong viewpoint on how to do something, or if you're not familiar with all the options that are available and you go down the wrong path, then it'll be more expensive."
The right path, according to Tritt, involves working closely with a manufacturer. But be forewarned that many different manufacturers claim to be the answer for dehumidification problems, which can lead to confusion. Tritt stresses that it's important to not get locked into the first thing that comes along, because often times it's not the best thing.
Ultimately, however, engineers have to assume the responsibility for becoming educated and learning about all the different means available for dehumidifying outdoor air. It's a formidable task, but a necessary one, if you want to end up with truly satisfied customers.
Common - but preventable - mistakesSince dehumidification equipment is new to many engineers out there, it's probable that some mistakes will be made the first time it's used. Ben Tritt, P.E., notes that a common mistake is to calculate building loads based on the drybulb outdoor air design condition. "Engineers will just look at the temperature and say, 'I have to take that outdoor air and bring it down to 70ûF to get it to the return air condition, and then I can size the rest of my system from there.' That only takes into account about half the load of the outdoor air."
Another mistake, notes Tritt, can occur if an active desiccant system is used and the requirements for postcooling after the desiccant wheel are underestimated or overlooked. "You still have the sensible load in the space to overcome and introducing 120° or 125° air without compensating for it causes problems."
Oversizing the desiccant equipment is yet another problem, says Curtis Musall, regional sales manager, Munters Southeast. "Desiccants can remove almost three times as much water vapor as a typical mechanical system in a recirculating-type fashion. So if an engineer is used to specifying a 5,000 cfm mechanical-based system, then the desiccant system could be a third of the size. The other mistake is not using the full advantages of a desiccant system. The desiccant system can remove the latent load from other parts of the building, so you can downsize the refrigeration equipment."
Mike Hayes says one of the biggest mistakes he sees are engineers trying to use other types of equipment for dehumidification purposes, such as standalone energy recovery devices. Those, he stresses, do not work for dehumidification. Another common mistake he sees is failing to look at part-load conditions, especially on the start-up of a building. "In a lot of cases, your humidity in the space will be rather high at the beginning of your day on a peak humidity day, and now you've counted on all this dehumidification capacity out of this device, and it's not there. So one major mistake engineers make is they fail to look at start-up part-load conditions with a lot of these dehumidification devices."
Let's hear it for desiccants!According to the National Renewable Energy Laboratory (NREL), desiccant dehumidification systems have been traditionally used in industrial/institutional applications for improving process conditioning, for product quality, or for specific applications. Desiccant systems are now also being used to improve air quality, to reduce humidity, and increase comfort in supermarkets, fast food restaurants, retail stores, ice skating rinks, and medical facilities.
Businesses in the United States could save $750 million and offset the use of 500,000 barrels of imported oil every year by capitalizing on advances in desiccant cooling technology.
Desiccant cooling and dehumidification systems can efficiently handle the larger flow rates of outside air required by new indoor air quality standards established by ASHRAE 62-99. Depending on an installation's location, application, and energy cost, a payback period of 1 to 2 years is possible, but paybacks of 3 to 5 years are typical.
Because desiccant coolers are heat-driven, they may be powered by natural gas, solar thermal energy, or waste heat, thus lowering peak electric demand. Desiccant systems improve indoor air quality by lowering humidity, improving ventilation rates, and removing pollutants and odors.
It is estimated that desiccant systems can potentially save 400 trillion Btu of energy each year in U.S. buildings and can prevent the emission of more than 24 million tons of carbon dioxide (CO2) by 2010. Desiccant dehumidification could reduce total residential electricity demand by as much as 25% in humid regions.