Dehumidification Enhancements for 100-Percent-Outside-Air Ahus
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Part 1 of 3 Dehumidification Enhancements for 100-Percent-Outside-Air AHUs Simplifying the decision-making process Author’s note: While this series of articles 100 is written from a 100-percent-outside-air standpoint, most of the dehumidification 50 40 35 55 technologies are applicable to systems that 50 40 45 50 handle both outside air and return air. 40 50 55 55 35 35 60 By DONALD P. GATLEY, PE 55 40 40 45 60 60 40 30 ver the last 10 years, there 35 65 30 has been a trend to provid- 50 35 ing outside air to buildings 40 65 60 70 using dedicated outside-air O 60 units. This approach decouples the 50 50 55 ventilation-air load from the air-condi- 60 tioning systems that handle the space- cooling and heating loads. In many cases, this results in less-complicated 60 65 70 units and simpler controls for the units 75 75 that handle the space-cooling and heating loads. Dedicated outside-air units result in better control of building FIGURE 1. Mean dew-point temperature isolines for August (1946 to 1965). pressurization, a more consistent intro- Source: Climatic Atlas of the United States. duction of required outside-air quanti- ties, and better management of water- ating conditions, which occur during tem, may allow the sequential use of vapor removal (often called “latent 97-plus percent of all operating hours. chilled water with the lowest-tempera- cooling”) from the outside air, which is Some designers extend this concept ture chilled water supplied to the dedi- especially important at part-load oper- and decouple both the outside-air load cated outside-air unit and then in series and the space latent load from space to the space air-conditioning units. A member of HPAC Engineering’s Editorial air-conditioning systems. In dedicated Eliminating or minimizing condensed Advisory Board, Donald P. Gatley, PE, is outside-air units, this is achieved by moisture at a space-conditioning unit’s president of Gatley & Associates Inc., an At- lowering the supply-air dew-point tem- cooling coils provides indoor-air-quality lanta-based consulting firm specializing in perature far enough below the desired benefits, reduces maintenance costs, building science; heating, ventilation, and air space dew-point temperature to offset and may eliminate the requirement for conditioning; chiller plants; cool storage; and space latent gains. This eliminates or double-wall casings in the unit. the prevention and correction of moisture prob- minimizes condensed moisture at most This is not a new concept. Willis H. lems in building envelopes and interior air. He if not all space air-conditioning units Carrier introduced the under-window can be reached at [email protected]. and in the case of a chilled-water sys- air-water induction air-conditioning sys- HPAC Heating/Piping/AirConditioning Engineering • September 2000 27 DEHUMIDIFICATION ENHANCEMENTS Dew Vapor pressure Humidity ratio Absolute humidity FIRST THINGS FIRST 3 3 point (F) (in. Hg) (mm Hg) (Gr/lbda) (lbswv/lbda) (Grains/ft ) (lb/ft ) A simple analysis using space and out- side-air dew-point temperatures is used Run 1 212 29.9 760.0 NA NA NA NA to determine when the dehumidification Run 2 110 2.6 66.0 416 0.05945 26.45 0.003778 of outside air is required. Dew-point tem- perature is a good indicator of the Run 3 104 2.2 55.4 344 0.04912 22.42 0.003203 amount of water vapor in the air. Run 4 100 1.9 49.1 302 0.04320 20.04 0.002863 What is the desired space dew-point temperature? Most air conditioning is de- Run 5 90 1.4 36.1 218 0.03120 15.01 0.002144 signed for space conditions of 75-F dry- Run 6 86 1.3 31.8 191 0.02733 13.32 0.001903 bulb temperature and 50-percent relative humidity (RH). This combination of Run 7 80 1.0 26.2 156 0.02234 11.10 0.001585 temperature and RH results in a dew- Run 8 70 0.7 18.8 111 0.01584 8.10 0.001156 point temperature of 55 F. Run 9 68 0.7 17.5 103 0.01476 7.59 0.001084 That said, in what part of the United States is the dehumidification of outside Run 10 60 0.5 13.3 78 0.01109 5.82 0.000832 air required? Basically, most of the coun- Run 11 50 0.4 9.2 54 0.00766 4.12 0.000589 try east of 100-degree west longitude. Fig- ure 1 is a map from the Climatic Atlas of Run 12 40 0.2 6.3 37 0.00522 2.88 0.000411 the United States1 showing mean dew- Run 13 32 0.2 4.6 27 0.00379 2.13 0.000304 point temperature isolines for August. Note that for most of the eastern half of Run 14 30 0.2 4.2 24 0.00346 1.95 0.000278 the U.S., the average August dew-point Run 15 20 0.1 2.6 15 0.00215 1.24 0.000177 temperature is above 60 F. The first rule in maintaining a com- Run 16 10 0.1 1.6 9 0.00132 0.78 0.000111 fortable, non-mold-producing environ- Run 17 0 0.0 1.0 6 0.00079 0.48 0.000068 ment in buildings is always introduce outside air into the space at a dew-point Run 18 -10 0.0 0.6 3 0.00046 0.28 0.000041 temperature lower than the room design Run 19 -20 0.0 0.3 2 0.00026 0.17 0.000024 dew-point temperature (about 55 F). A Run 20 -30 0.0 0.2 1 0.00015 0.09 0.000014 corollary is never allow outside air hav- ing dew-point temperatures above the Run 21 -40 0.0 0.1 1 0.00008 0.05 0.000007 room dew-point temperature to infiltrate the building. This is achieved by main- TABLE 1. Equivalent ways of expressing the quantity of water vapor in air, based on sea- taining each space in the building at a level altitude and a barometric pressure of 29.92-in. HG. slight positive pressure relative to the outside whenever the outside dew-point tem in 1937. This system supplied dehu- In the current era of fast-track design, temperature is above 55 F. This is not midified air from a central air-handling downsizing, and “commodity-priced” fee rocket science; however, these principles unit (AHU) to venturi-induction noz- structures, it should come as no surprise are violated in many designs and subse- zles in induction units under each win- that designers sometimes do not have quent construction and operations-and- dow. The slightly lower pressure in the the time or inclination to evaluate tech- maintenance practices. induction-nozzle area of the unit caused nology options for dedicated outside-air Contributing to designers’ difficulty room air to flow over a cooling-heating units. That many manufacturers have understanding systems and enhance- coil in the unit. The coil was supplied their own non-generic marketing name ments is the number of ways that meteo- with chilled water for cooling and hot for each of their equipment configura- rologists and dehumidification and air- water for heating. In the most popular tions or enhancements does not help the conditioning practitioners quantify version of this system, the central AHU designer—nor do case studies in industry water vapor associated with air. Table 1 supplied 100-percent outdoor air at a publications that compare an optimized lists equivalent ways of expressing the dew-point temperature several degrees system using the manufacturer’s tech- quantity of water vapor in air. below the design room temperature. nology with non-optimized “conven- Thus, the central AHU handled all of tional” equipment (e.g., an enhanced DESIGN SCOPE AND the outside-air cooling and dehumidifi- and optimized favored technology with TECHNOLOGY OPTIONS cation load and also the room-dehumid- 1950s conventional air conditioning Given the objective of introducing ification (latent) load. Another popular and electric reheat). outside air into a building at a dew- system used the same central dehumidi- This article simplifies the decision- point temperature of 55 F or lower, fication and outside-air concept, but making process and presents most of the what are the options? The sidebar on used under-window fan-coil units. technology choices. Page 32 lists applicable technologies. 28 September 2000 • HPAC Heating/Piping/AirConditioning Engineering Note that many optimum-choice de- input. Follow this with a package of pre- IS ENTHALPY RECOVERY humidification systems are a combi- liminary design sketches and specifica- FROM EXHAUST AIR nation of two or more of the technolo- tions sufficient to obtain rough price esti- ECONOMICAL? gies listed in the sidebar. Ordinary mates. Send the package along with The collection of exhaust air equal reheat with a cooling and dehumidify- climate, operating-hour, energy-cost, fi- to 100 percent of the outside air is ing coil, followed by hot-water, steam, nancial, and space-constraint data to re- rarely possible because some of the out- or electric reheat coils, rarely is the sponsible vendors representing other side air is used for building pressuriza- optimum choice. The optimum usu- technologies and challenge them to beat tion and some exhausts are too remote ally incorporates an exhaust-air en- the target technology. or contaminated for collection. From a thalpy-recovery wheel and/or a recu- The choice of a target technology in life-cycle standpoint, it usually is cost- perative enhancement.