For many buildings, the dehumidification capacity of conventional air conditioning equipment is inadequate to keep indoor relative humidity under control. Humidity must be kept low enough to prevent microbial growth: less than 60% rh. Conventional equipment removes moisture from the air only as a byproduct of the cooling process – essentially, dehumidification is a side effect. System designs based on standard equipment and practices are no longer necessarily able to meet the dehumidification requirements of today’s buildings, nor are they the most economical.
Certain building and system characteristics can warn of possible humidity problems during the design phase. These may include any of the following commonly found specifications, when combined with a conventional HVAC system design.
Outside air fraction greater than 20%
Constant uncontrolled outside air flow
High efficiency lighting system
High R-value roof and windows, or a shaded site
Complex footprint with many wall and roof joints
Numerous wall and roof penetrations
Large exhaust fans or exhaust hoods
Equipment oversized for rapid cool-down
Variable or unpredictable occupancy
Suspended ceiling space used as return air plenum
Conventional system design practices reflect the fact that dehumidification is a secondary process when conventional equipment is used. Since much of the U.S. is outside of the hot & humid climate region, this has become a standard design parameter upon which equipment selection specifications and streamlined design procedures are based, and it works well in temperate dry climates.
Two primary developments in building design are changing the approach to dehumidification in hot & humid climates. First, the increase in outside airflow rates, especially in systems that maintain a constant outside air volume, has resulted in excessive indoor humidity and fungal growth in many buildings. Second, the increased efficiency of lighting systems and thermal performance of windows and roofs has resulted in a decrease in the load sensible heat ratio, especially at part load conditions, beyond what conventional equipment can handle.
CONTROLLING AIR QUALITY
Insofar as humidity above 60% rh can lead to fungal growth in buildings, which in turn leads to serious air quality, health, and legal concerns; excessive moisture in the air can be considered a pollutant. Before dehumidification equipment is added to the HVAC system, it is far more cost effective to reduce the latent-cooling load. In some cases, this will allow conventional systems to perform adequately. If dehumidification equipment is needed, its expense will be minimized.
Air-tightening and positive pressurization will preclude uncontrolled infiltration of humid, unfiltered outside air. Fresh air systems that control outside airflow based on occupant needs and air quality will help to reduce space relative humidity. Analysis procedures for implementing these measures are relatively new; often, specialized assistance is helpful during the design and construction phases. Moisture form indoor sources, such as cooking, showers, and carpet cleaning should be contained and exhausted directly to the outdoors, or minimized if possible.
Given the proclivity for microbial contamination in hot & humid climates, filtration is especially important. Filtration of outside air separately, combination filter-grilles, anti-microbial filters, and other high performance filtration components reduce spore and particulate levels, and help protect coils and ducts from infestation. If specific indoor air pollutants can be identified for a particular building, filtration of these pollutants can reduce the flow of outside air needed for dilution.
Currently, there are about a dozen types of commercially available dehumidification equipment and technologies.
1. ADP Controls (Apparatus Dew Point)
Conventional air conditioning equipment cools air to within a few degrees of 55°F. The conventional 55°F standard evolved from the need to prevent condensation on ductwork and diffusers, and from the desire to maximize coil temperatures in the days when chillers and compressors were far less efficient than current equipment. Colder air temperatures can greatly increase dehumidification capacity. System design with ADP is more complex and less forgiving of design and control inaccuracies. Both first cost and energy costs can be less than that of conventional systems.
Reheat is the most energy intensive method of increasing dehumidification capability. Essentially, reheat coils add heat to the space to cause the cooling coil to cycle longer and remove more moisture – heating and cooling run at the same time. First cost is low, but energy costs can be double or triple that of other dehumidification methods.
3. Subcooling/Reheat Coils
A more efficient type of reheat which uses recovered heat from the compressor instead of new energy. A re-heat coil downstream of the cooling coil subcools the refrigerant, which increases the moisture removal capacity of the coil. Energy costs can still be significantly higher than that of other dehumidification methods.
4. Heat Pipe Wrap Around Coils
This is a pair of coils, linked with a series of heat pipes. One coil is placed upstream of the cooling coil to pre-cool coil entering air. The other coil is placed downstream of the cooling coil to provide reheat. Dehumidification capacity can be as much as double that of the cooling coil alone. Energy efficiency is virtually unaffected, with less than a 5% loss in efficiency due to the added air pressure drop.
5. Fan Cycling Controls
If the fan continues to blow air through the cooling coil after the compressor cycles off, the cold, wet coil quickly becomes a warm, wet coil and water on the coil is re-evaporated into the space. Cycling the fan with the compressor has been shown to reduce humidity by as much as 10% rh, and save energy. Not desirable if constant airflow is needed for ventilation.
6. Face/Bypass Dampers
Another method of preventing air from blowing through a warm, wet coil. Dampers are used to divert air around the coil when it cycles off. Constant airflow can be maintained for delivery of ventilation air to the space.
7. Outdoor Air Pre-conditioners
Either a separate package unit, or a dedicated coil, is used to dehumidify outdoor air before it enters the primary air handler. Depending on the design of the pre-conditioner, energy costs can increase.
8. Desiccant Dehumidifiers
This equipment uses powerful heat activated desiccants to absorb water from the air. The units are capable of providing very dry air and can solve the most extreme humidity problems. Uses additional energy, but still less than reheat in a carefully designed system.
9. Enthalpy Wheels
Set up is similar to Heat Pipe Wrap Around Coils, with the added ability to transfer moisture around the cooling coil as well as heat. Can double or triple system dehumidification capacity with only a small decrease in energy efficiency. Requires more mechanical room space for wrap-around ductwork.
10. Energy Recovery Ventilators
These units capture the heat and humidity from incoming outdoor air, and transfer it to cool, dry exhaust air from the building exhaust fans. Reduces energy costs. Requires that outside air and exhaust air ductwork be proximate.
11. LPA (Liquid Pressure Amplification)
Allows for refrigerant subcooling on larger systems that would otherwise need head pressure control for proper TXV operation. The LPA system is based on a small pump placed in the refrigerant line ahead of the thermostatic expansion valve, as close to the upstream component as possible. The pump increases the pressure in the liquid line to prevent flash gas formation, and maintains the design pressure difference across the TXV. When used in conjunction with liquid line subcooling, the dehumidification capacity of the coil can be significantly increased.
12. DCV (Demand Controlled Ventilation)
Also called VOAV (Variable Outside Air Volume). Use of motorized air dampers in the fresh air intake ducts, in conjunction with carbon dioxide, occupancy, and/or other air quality sensors allows outside air flow to be minimized.
CASE STUDY: CORRECTIONS FACILITY
Selection of the best dehumidification equipment and system design is dependent on a long list of building characteristics and project constraints, as is the selection of any mechanical system component. As an example, consider a 228,000 square foot county corrections facility in Florida. The building is an addition to an existing 54,600 square foot jail, and is made up of 10 distinct buildings attached at various points to a central core building. Construction is block, with an insulated built-up roof and minimal window area.
The facility houses 934 inmates, an institutional kitchen and laundry, an infirmary, a courtroom suite, and offices for counselors, attorneys, administrators, and correctional officers. Total occupancy is 1,330 people.
The HVAC system consists of 76 packaged roof-top units ranging in size from 3 tons to 20 tons, with a total capacity of 690 tons. Design specifications called for 48,865 CFM of outdoor air, with outdoor air fractions of up to 42%. Total exhaust airflow from 42 exhaust fans was 44,430 CFM including 15,525 CFM of kitchen hood exhaust. Shortly after occupancy, humidity levels routinely exceeded 60% rh, with most areas experiencing levels as high as 75-90% rh. The following measures were implemented.
Source Control. This involved recalculating the outside air requirements based on actual occupancy, and using the ASHRAE "3-hour rule" which allows reduced ventilation for spaces occupied for less than three hours at a time. Outside airflow was reduced to 33,700 CFM.
Next, building pressurization was addressed. Even though the design called for 4,435 CFM of net positive airflow, measurements showed that this was not enough to maintain a positive pressure in this leaky building. Building pressures measured as low as –2.7 Pa. This building was sucking in humid unfiltered air. Analysis of building pressure requirements indicated a net positive airflow of 12,000 CFM is needed to maintain a building pressure of +2.5 Pa. Recalculation of kitchen and rest room exhaust requirements resulted in an increase of net positive airflow to 12,300 CFM, and a measured overall building pressure of +3.5 Pa.
Next, occupancy schedules were programmed into the DDC system so outdoor air dampers would close during unoccupied periods. Testing of the damper actuators revealed that they were not calibrated, so calibration was performed.
Finally, all units were fitted with 2-inch 60%-efficiency anti-microbial pleated filters.
Dehumidification Equipment. To provide an immediate solution to the initial high humidity concerns, the mechanical contractor installed a gas-fired hot water reheat system in order to provide active humidity control. Energy costs increased by $270,000 per year with the use of the reheat system, an increase of $1.18 per square foot. Source control and other energy efficiency measures produced immediate savings of $116,000 per year. The best option for this facility is heat-pipe wrap around coils. Fan cycling was enabled on a unit-by-unit basis during unoccupied periods to reduce re-evaporation of moisture from the cooling coil. Expected annual energy savings are $215,000 from a combination of source control, heat pipes, and control system changes.
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