Taking Notes: Desiccant Dehumidification
Whether your client has a school or office building, desiccant dehumidification can successfully control the project’s humidity, save energy, and remove pollutants. This primer empowers you to teach your clients how desiccants work, while other tips and applications can help your future projects more than make the grade.
Many engineers have been intimidated by desiccant dehumidification (DD), and for good reason - it’s more complicated than relying on mechanical condensation alone. Desiccants like lithium chloride (LiCl) are made of tiny salts with vast inner surfaces called pores, making DD sound more like a chemistry class than an engineering project. Fear not: Steve and Charlie will uncomplicate all this desiccant science so that you’ll be able to rock it on your next project. Be sure to take notes so you can advance into desiccant guruness*.
IN THE BEGINNING THERE WAS...Most HVAC systems are designed to maintain 72°F and 55% rh “neutral” air by cooling coil dehumidification in order to hit a 55° dewpoint. If you want to dry to 45° rh or below within humid climate zones, you’ll need a supplemental technology to remove the extra latent load. Many engineers design 55° dewpoint supply air, yet they also show 55° dewpoint air at 72°/55% rh returning from the space in order to size the energy recovery wheel performance.
ENERGY SAVINGSTactfully dialing desiccant dehumidification into your new or retrofit project can significantly reduce both the tonnage required and system run times. Many engineers stop short of specifying the ideal desiccant system by only using a 55% rh setting as the target. Going to a lower rh has many comfort and energy benefits, making desiccant dehumidification systems a critical part of any green/energy efficient project.
DESICCANT DEHUMIDIFICATION MADE EASYASHRAE provides a nice review of DD in its 2008 HVAC Systems and Equipment and 2009 Fundamentals handbooks. The main benefit of DD systems is that they can snatch up to 100% of your outside air humidity (latent) load in one pass. Used within a dedicated outdoor air system (DOAS), DD can dry the entire ventilation latent cooling load before it sneaks into a building to create chaos. Since the majority of a latent load is from OA, this deep air drying can end up handling the building’s entire latent load, resulting in dry coils. (For more on dry coils, see Welty’s May 2010 article in ES).
EDUCATION'S GIFT TO MOLD: SUMMER BREAKEvery mold loves summer break, especially when humans turn off the power and shutter their schools to “save energy.” Unfortunately, humid air infiltration doesn’t take the summer off. This creates the perfect humidity and condensation storm for mold to exponentially multiply within the closed schools. Schools in humid summertime areas can have indoor humidity conditions of 80% rh or higher. Since mold grows above 60% rh, and the perfect conditions for mold soup are created at 80%, it’s no wonder that schools are a mess by the time September rolls around. (See Welty’s February 2010 article in ES along with the ASHRAE article, “Report card on Humidity”4.)
BACK TO SCHOOL FOR YOUUnderstanding how desiccants work empowers you to be able to talk with your clients about DD. At the micro level, all desiccant salts are made up lots of nooks and crannies (called micro and macro pores) which, when highly magnified, look like an English muffin’s insides. To get scientific, both desiccants and activated carbon (AC) surfaces are made of billions of fractals making them miniature moisture grabbing machines. Both desiccants and activated carbon have tremendous surface areas in relation to their size, only instead of primarily grabbing airborne chemicals (VOCS) like AC, desiccants nab humidity. One gram of LiCl desiccant has over 97,000 sq ft, which equals 1.7 football fields! With all that moisture sucking surface area, desiccants easily grab water vapor just like a Velcro® wall traps Nerf® balls.
TO DRY OR FIRE, THAT IS THE QUESTIONOnce the desiccant adsorbs moisture, it needs to blow it off like a boring lecture, and you have two choices: low rh air or heat. Heat is efficient because it evaporates moisture off the desiccant pores just like a drier does for your clothes. Dry air with low humidity can also regenerate the desiccant and is less complicated than using heat. Both methods desorb the captured humidity and define the two major desiccant dehumidification options: passive and active heat.
PASSIVE DESICCANT DEHUMIDIFICATIONPassive dehumidification uses desiccant wheels to dehumidify either outside air or return air. “Passive” merely means that no active heat source is required to regenerate the desiccant. Passive desiccant wheels use type III desiccant, and what differentiates type I-V desiccants is their “isotherm” curve. (Figure 1).
Type III desiccants are unique as they have a simple slope throughout their performance window. Each point on the curve shows how that type of desiccant reacts to a specific vapor pressure differential or a humidity differential across the desiccant’s surface. Passive wheels perform based on an rh differential across the wheel. In contrast, total energy recovery wheels or active desiccant wheels operate based on a vapor pressure differential and use molecular sieves and silica gel desiccants to accomplish dehumidification.
Based on the passive wheel’s depth, rotational speed, and rh differential across the rotor from the outside air inlet after the cooling coil to the return air inlet, 7 to 15 grains of additional moisture can be removed from the passing air. A cooling coil discharge temperature of 52 d.b./52 w.b. equals 57.8 gr/lb, so if the return air inlet entering rh is 50% or less, then the passive wheel can remove up to 15 grains of moisture by using a 200 mm deep configuration (Figure 2).
When desiccants adsorb moisture it creates heat and is known as an exothermic reaction. This raises the air’s drybulb temperature between 3° to 6° after the desiccant wheel and is proportional to the amount of moisture removed. Using the above conditions with 52 db/52 wb (100% rh air) coming off the cooling coil and a return air rh condition of 50% or less, a supply air condition of 58° at 42.8 gr/lb or 44.0 dewpoint can be achieved. Nice job.
ARE ALL WHEELS CREATED EQUAL?Choosing the correct wheel is the first critical step, since not all passive wheels are created equal. Passive wheels with an aluminum substrate coated with a type III desiccant can create some issues. Aluminum substrate wheels transfer a lot of heat, so you’ll have to add post cooling, which then adds additional energy load to the system and also lowers your energy efficiency grade. The wheel speed must modulate up and down to maintain the supply air drybulb condition due to the aluminum substrate acting as a sensible heat transfer median. This means that the grains of moisture removed will always be changing with space dewpoint fluctuations, which annoys building occupants, who annoy the building engineers, who then call and annoy you.
One alternative is synthetic (in-situ) desiccant wheels, used without an aluminum substrate and operated at a constant speed for maximum moisture removal. With an engineered synthetic rotor wheel, over 80% of the rotor mass acts as a desiccant, which blows away desiccant-coated aluminum wheels. With much less desiccant surface area, and therefore less total capacity, aluminum wheels have much lower dehumidification capacity. Synthetic desiccant wheels have greater exposed desiccant, which is like having a larger swimming pool: it allows you to remove more moisture per rotation like capturing more water in your pool.
Don’t make the mistake of locating the fan in position 2 because then the fan and motor heat will be added directly to the cooling coil load (not so good), and the passive desiccant performance will not be enhanced. By mounting the supply fan in position 1, it also allows you to make use of a “winter” bypass, thereby shunting the pressure drop of the passive wheel and the cooling coil during winter operation when the outside air dewpoint is below the space’s dewpoint setpoint. Now that’s beautiful energy efficiency music to any client’s ears. One last thing: make sure your fan is properly selected for the summer total static pressure and the winter total static pressure conditions.
You’ll score points by dropping in an enthalpy wheel upstream of the passive wheel (as long as there is a return air path with sufficient airflow), and for more points, add a recirculation damper for unoccupied mode or dehumidification mode. On your next school project, design the DOAS to have a decoupled DX system from the building’s chilled water system. If the DOAS uses DX in lieu of chilled water during unoccupied hours, then:
- The outside air damper can go to the minimum position;
- The recirculation damper can open; and
- The DOAS can over-dehumidify the space, allowing the building to handle excess moisture during the next operating day before hitting the humidity setpoint.
ACTIVE DESICCANT DEHUMIDIFICATIONActive desiccant wheels require a heat source on the regeneration or “regen” side of the wheel in order to drive out the water vapor trapped in the desiccant pores. Since these pores did such a fine job of adsorbing the vapor while air moved through the process air side of the wheel, the heater will cook off the trapped vapor using the northwest part of the system (Figure 5).
Figure 6 shows a classic 3-to-1 configuration as one of the ways to configure an active desiccant system. Even active desiccant wheels have limitations on how much moisture can be adsorbed into their pore structures. In order to achieve even lower dewpoints, add in a precooling coil to remove as much moisture as possible. In a typical industrial design, the air exits the cooling coil from 50 to 60 gr/lb and the desiccant wheel removes the additional grains down to the setpoint required, assuming that sufficient regeneration temperature, wheel depth, and rotational speed are all provided to meet design specifications.
ENTHALPY WHEELSMany MEs know about enthalpy wheels, and you may have designed them into projects, but did you know that all enthalpy wheels are not created equal (just like their passive and active desiccant wheel country cousins)? Enthalpy wheels come in:
- Multiple depths
- Different desiccant coatings
- Various substrate materials
- Several flute geometries (the way that final honeycomb appears, which also dictates pressure drop and efficiency)
Attention class! Specification language that states that wheels are to be “rated in accordance with AHRI Standard 1060” does not mean they are AHRI certified. The specifications must state the products are to be AHRI certified and bear the AHRI seal.
Enthalpy wheel substrates range from paper to plastic to aluminum. Desiccant options include molecular sieve, silica gel, and oxidized aluminum. Make sure to specify a purge plate option because this flushes out even more contaminated air before the wheel rotates from the contaminated return air back into the outside airstream. A purge plate mounted on the air exiting side of the outside airstream accomplishes this, so consider sliding this in. Taking purge into account when performing fan selections helps to prevent you from being short on air during startup on both the supply and exhaust fans. Don’t forget to calculate seal leakage factors into the fan calculations as well.
STUDENTS: WHAT ARE ALL THESE ANGSTROMS DOING IN MY DESICCANT?One angstrom is 1/10,000,000,000 (one ten billionth) of a meter, whereas a micron is 1/1,000,000 (one millionth) of a meter. Most microscopic germs are measured in microns so angstroms are ridiculously small. Synthetic desiccants have molecular sieves with tiny orifices designed to capture moisture but prevent them from capturing VOC odors in order to control cross-stink* contamination.
For your next project or retrofit, these basic options are key to selecting a proper enthalpy wheel:
- AHRI certification
- Substrate material
- Desiccant material
- Seal efficiency
- Fan locations
- Wheel depth
- Bearing types
- Tube welded frame or folded galvanized
- Segmented wheels for extra rigidness, or a single piece to save a little money
THE LUNCH BELL IS ABOUT TO RING...Today, we’ve reviewed your choices of DD technologies and different applications. Terms like regeneration, exothermic, and micro and macro pores have become familiar, and now you know just how small an angstrom really is. DD is a highly energy-efficient technology in humid times of the year as well as a viable option to shutting off your cooling coils, which may have been the only option to keep your buildings dehumidified.
Dehumidification is critical for preventing and controlling mold growth, and that enhances the learning environment for students and staff. You may now go forth and design multiple DD systems to save your clients money, earn extra Energy Star points, and to even help save the planet. On that note, class dismissed. ES
(* just invented term)
Works Cited1. Ronald H Howell. “Effects Of Indoor Space Conditions On Refrigerated Display Case Performance.” ASHRAE - 596RP, November, 1991. Accessed May 16, 2011, http://rp.ashrae.biz/page/RP596.pdf.
2. Ronald H. Howell. “Effects Of Indoor Relative Humidity On Refrigerated Display Case Performance.” Department of Mechanical Engineering University of South Florida Tampa, Florida, USA. Accessed May 14,www.inive.org/members_area/medias/pdf/Inive/clima2000/1997/P275.pdf.
3. Southern Edison. “Refrigerated Display Case Performance Evaluation.” Accessed May 15 2011.http://asset.sce.com/Documents/Shared/BusRefrigeratedDisplayCaseEvaluation.pdf.
4. Fischer, J, C. Bayer. “Report Card on Humidity Control.” ASHRAE Journal. May, 2003: p30.