This article shows how engineers used a supply air dew point of 45°F on an HVAC system retrofit with active desiccant dehumidification on a high school HVAC system retrofit project, resulting in significant energy savings for the district. The design and installation addressed concerns associated with chronic high indoor humidity at a high school in the hot and humid southern valley of Texas.
Using dedicated outdoor air systems (DOAS) within the HVAC market as the primary means of moisture removal has consistently gained a higher market share worldwide. Providing a comfortable and healthy indoor environment has been a difficult task for many commercial applications where ventilation rates are high, such as hospitals, schools, theaters, retail stores, hotels, restaurants, nursing homes, and office buildings.
Humidity control plays an important role in establishing and maintaining a comfortable indoor environment.1 Using a DOAS in series or parallel with non-DOAS HVAC systems offers cost-effective humidity control when compared to dehumidification schemes using cooling coils and reheat. And it provides the capability to meet the outside air ventilation rates as per ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality. In most applications, using DOAS in parallel with non-DOAS HVAC systems is the preferred method of application.
Fresh Air Ventilation- Introducing pretreated fresh air into a building can improve the IAQ and eliminate many problems associated with poor ventilation and lack of fresh air. Bayer notes that IAQ improves when using active humidity control and continuous ventilation in schools2 to meet the requirements of Standard 62.1. In a study of 10 schools in Georgia, Bayer noted that of the five schools having HVAC systems without DOAS, none supplied outside air at the ASHRAE recommended 15 cfm (7.08 L/s) per person.
The schools with desiccant DOAS dehumidification were delivering as much as three times more outside air, while maintaining equal or better control of the indoor relative humidity than the systems without DOAS. The average total volatile organic compound (TVOC) concentrations tended to be lower in schools with dehumidified air. The school showing the highest air exchange rate used a rotary desiccant system, and had the lowest carbon dioxide, TVOC, airborne microbial concentrations, and the lowest average indoor relative humidity.
In Phase II of the same project, Fischer and Bayer stated that increasing the air ventilation rate from 5 cfm to only 8 cfm (2.36 L/s to 3.78 L/s) per student challenged the ability of the non-dedicated outdoor air systems to maintain the space relative humidity below the ASHRAE and American Conference of Governmental Industrial Hygienists’ recommended 60% level. Increasing the ventilation rate of the non-dedicated outdoor air systems to the recommended 15 cfm (7.08 L/s) per student allowed the space relative humidities to routinely exceed 70%.
These data explained why all of the non-DOAS HVAC system schools were designed and/or operated with only 6 cfm (2.83 L/s) per student of outdoor air or less. The decreased ventilation rates were in direct response to the performance limitations of the non-DOAS cooling equipment and contributed to the poor IAQ within the schools. Schools served by the non-DOAS HVAC systems experienced absenteeism at a 9% greater rate than those served by the desiccant-type dedicated outdoor air systems. Increased absentee rates can have significant negative economic impacts on a school or school system.
Packaged, non-DOAS HVAC equipment is not designed to handle the continuous supply of outdoor air necessary to comply with Standard 62.1. As a result, these schools are likely to experience IAQ problems. Higher ventilation rates as specified by Standard 62.1 translate into greater cooling loads for non-DOAS equipment, specifically, greater latent loads during cooling seasons when indoor relative humidities must be controlled to inhibit growth of microorganisms that may result in health problems or damaged building materials.5 Non-DOAS HVAC systems cannot adequately dehumidify the air in warm and humid climates. In a non-DOAS reheat system used for dehumidification, previously cooled air is heated and then introduced into the interior of a building.
The air is first cooled to 55°F (13°C) or lower to remove the latent moisture load. This cold air is then reheated to satisfy the relative humidity and temperature requirements of the indoor space. Unless equipped with an energy recovery system, reheat systems used for dehumidification incur a quadruple penalty: (1) the first cost of the cooling generation plant, associated auxiliaries, and electrical service is increased by the amount of reheat and added cooling load; (2) the reheat coil first-cost premium includes increased electrical service and/or heating distribution piping; (3) the owner pays the annual operating cost for the extra sensible cooling of the air, and (4) then pays the annual operating cost of reheating the air.
To comprehend the advantages of enhanced dehumidification systems, it is vital that owners and HVAC system designers understand this quadruple penalty associated with the use of reheat systems. Reheat should not be the sole or first-in-control sequence means of dehumidification.8 Buildings in the humid south should be pressurized to minimize infiltration of moist outside air and HVAC system design should incorporate dehumidification that maintains the space in the 45% to 55% relative humidity range during the entire cooling season.
DOAS and the 45°F (7°C) Dew Point A separate and dedicated outside air pretreatment ventilation system may be the only reliable method of meeting Standard 62.1 and is also the simplest method. This separate dedicated outdoor air concept can be used to completely meet space latent loads, decoupling the space latent and sensible loads. The separation of the sensible and latent loads provides a mechanism for dehumidification when the building is in an unoccupied mode resulting in energy savings and low indoor vapor pressures to permit drying.
Designing the outside air system to deliver the required ventilation to each occupant requires a supply air dew-point temperature of about 45°F (7°C) to maintain a space dewpoint temperature around 52°F (11°C). To determine the supply air conditions for a dedicated outdoor air system working in parallel with distributed sensible cooling equipment, one should select an air dew-point temperature low enough to maintain a summer space relative humidity no greater than 40%, or a supply air dew-point temperature around 44°F (7°C).This results in the elimination of terminal reheat from the HVAC system and the ability to reduce the size of the cooling equipment due to the decrease in latent capacity required for non-dedicated outdoor air systems to dehumidify using subcooling and or reheat. Excess cooling capacity can be subtracted from the rest of the system, resulting in savings that may offset the cost of the pretreatment equipment. Reducing the latent cooling load burden of the refrigeration equipment results in a net increase in efficiency of the system, further offsetting initial purchase costs. The proper use of DOAS can result in improved indoor air quality with little or no increase in compressor size or annual energy consumption.
Two ways to remove moisture from the air for air-conditioning applications are by cooling the air to condense water vapor or by passing air over or through a desiccant medium, which removes moisture from the air through differences in vapor pressures. Some manufacturers use an energy conserving combination of cooling and desiccation by first passing the outside air through cooling coils and using the waste heat generated by the cooling coil compressors to warm the air necessary for desiccant drying. One manufacturer of a combination cooling coil/desiccant system captures and sanitizes the condensed water for drinking.
Using the 45°F (7°C) dew-point design criteria via DOAS significantly reduces the potential for microbial growth within the non-DOAS HVAC equipment, as the dedicated outdoor air system lowers the dew point of the air. Both cooling- and desiccant-type DOAS remove water from the airstream. Cooling-based dehumidification chills air below its dew-point temperature, resulting in moisture condensation on the nearest surface. Reheat may be necessary to increase the temperature for occupant comfort. Condensation within an HVAC system can result in microbial growth, equipment deterioration, and excess energy use and should be avoided in the design or retrofit stage.
For these reasons, many engineers specify desiccant or combination cooling coil/desiccant DOAS for air-conditioning applications. Since the mid- to late-1980s, desiccant-based cooling systems have found increased applications as humidity control devices as dedicated outside air pretreatment ventilation systems for non-industrial structures such as schools, homes, hospitals, and commercial buildings. The use of active desiccants enhanced the quality of the indoor air by helping to maintain comfort criteria (temperature, humidity, and ventilation),removing particulates and bioaerosols from the air, and removing chemical pollutants from the air. The application of desiccant dehumidifiers integrated with HVAC systems serves to precondition the outside ventilation air such that the latent load is removed. Some of the potential benefits of applying desiccant dehumidification to air-conditioning systems are humidity control, efficient latent load removal, and reduction in peak electric demands.
Other savings associated with desiccant dehumidification– HVAC system hybridization include
1) providing an enhanced occupant comfort with lower energy use;
2) providing improved humidity control resulting in sensible versus latent cooling;
3) reducing equipment expenditures by allowing the downsizing of the evaporator coil, condensing units, distribution plenums and terminal boxes, air handlers, reduced ductwork size and cross-section, and space used for mechanical equipment for comparable design loads
4) allowing independent temperature and humidity controls;
5) allowing higher temperature setpoints due to increased evaporation off the skin of building occupants
6) allowing for dehumidification and the complete shutdown of the sensible cooling equipment during unoccupied modes.
Desiccants have a natural affinity for removing moisture from air. As the desiccant removes water vapor from the air, the latent load is removed from air conditioning and the sensible load can be efficiently cooled mechanically to comfortable conditions. Solid desiccants take advantage of differences in vapor pressure to remove moisture from the air with energy required to heat regeneration air for removal of the adsorbed water from the desiccant medium. In many cases, the energy expenditures required for desorption can be offset by using waste heat from boilers, condensers, and other equipment. The honeycomb wheel-type desiccant is light, and its rotating mass is low compared to its high moisture removal capacity, resulting in an energy efficient dehumidification unit. The design is simple, reliable, and easy to maintain, and is the most widely installed of all desiccant dehumidifiers in ambient pressure applications like air conditioning.
