Several strategies for system design and controls provide real-life solutions, avoid mistakes, and minimize energy spent on overcooling and reheat.
The keys to efficient dehumidification include selecting the appropriate system for the application and controlling the system to minimize energy consumption. In most scenarios where excessive energy is wasted, it is not just the dehumidification process, but also the temperature control of the space that is performed inefficiently. This article focuses on dehumidification with mechanical cooling.
Relative Humidity: What is it?Relative humidity (rh) is a function of both the moisture in the air and the air temperature. It is equal to the amount of moisture in the air relative to the amount of moisture the air could hold if it were 100% saturated (e.g., fog). Air that has an rh of 50% has 50% of the moisture it can hold at that drybulb temperature. Therefore, rh can be lowered by either decreasing the moisture in the air, or raising the air temperature.
- Dewpoint: the temperature at which air is saturated with moisture, i.e., at 100% rh
- Drybulb temperature: sensible temperature that a thermometer reads
- Wetbulb temperature: temperature of a wet surface that is evaporating in equilibrium with the surrounding air in an adiabatic process
We need to control moisture and temperature. Typical parameters include controlling space temperatures between 73°F to 75°, and 50% rh. It just so happens that air with a dewpoint of 55° and a drybulb temperature of 75° has an rh of almost exactly 50%. Many air-handling systems with mechanical cooling (chilled water or direct expansion) are controlled to a discharge temperature of 55° or slightly lower.
Moisture SourcesFor most commercial applications, outside air for ventilation and the building occupants are the primary sources of moisture. Consider that a typical occupant in a commercial building, such as a school, office building, retail store, or health care facility, produces about as much heat as a 100W light bulb. About 40% of this is in the form of moisture released to the space, and the rest is sensible heat associated with temperature rise. A typical person will release about 0.14 lb/moisture/hr. A typical ventilation rate is 15 cfm per person. The moisture delivered by this flow of outside air is of course dependent upon the outdoor air conditions. Figure 1 shows the moisture coming from the ventilation air generally required for each person, at varying but typical outdoor air conditions in humid climates, which include states east of the Rocky Mountains. Moisture released by occupants may constitute about 50% or more of the total over the course of a cooling season.
Note also that ventilation air required for occupants is not necessarily the minimum airflow needed in the building. Often in commercial buildings, the amount of makeup air required for exhaust for restrooms or other spaces is greater than the ventilation air required for good air quality for occupants.
ReheatDehumidification via mechanical cooling occurs by driving the conditioned air temperature down to the desired dewpoint. When too much air at the desired dewpoint temperature is supplied to a space, the air must be reheated to maintain desired conditions. If the target is 50% rh, the space must be about 74° to 75° if the air-handling system is reducing air moisture to a 55° dewpoint. Air at 70° with a 55° dewpoint is out of range at 59% rh, “clammy,” and not acceptable. This is also close to 60% rh, which presents potential for mold growth. Heat from building envelope gains, internal gains, and/or energy sources like natural gas or electric resistance is needed.
Minimizing reheat energy is often the difference between an efficient system and an energy hog.
ControlsAs with any energy intensive system or process, controls are critical to efficient operation. This section provides control strategies to minimize energy consumption associated with dehumidification.
Dehumidification control strategy #1: Only cool to the desired dewpoint when necessary. On occasion when space rh must be controlled for comfort, manufacturing, or material preservation such as in libraries and museums, air handler discharge temperature is set to maintain a constant temperature that equals the dewpoint temperature associated with the desired rh. This can waste energy from unnecessary mechanical cooling, reheat, or both.
Air handler discharge should be reset as high as possible to maintain temperature in the warmest zone served, with an override for humidity control. Return air rh is typically monitored, or in the case of 100% outside air units, indoor sensors are monitored. Once the rh setpoint is exceeded, the system goes into dehumidification.
Dehumidification control strategy #2: Control cooling using variable volume to the maximum extent possible. Most energy codes (International Energy Conservation Code 2006) require that VAV systems be capable of reducing airflow to 0.4 cfm/sq ft, or 30% of design flow, or ASHRAE Standard 62 ventilation levels, whichever is greater. In many existing buildings, the minimum zone flow, or minimum VAV box position, is higher than some or all of these metrics.
Sensible cooling delivered to a space is proportional to the product of difference in space temperature and supply temperature, and airflow.
Q α (TSpace – TSupply) x cfmWhen cooling delivered by the supply air is greater than the load, the supply temperature must rise, or the airflow must be decreased. If flow is not low enough while maintaining a desired dewpoint temperature, reheat is required. Therefore, the minimum flow should be set by ASHRAE Standard 62, which covers both minimum ventilation levels for occupied spaces and exhaust for restrooms.
One way to accomplish this is to incorporate occupancy sensor control in series with CO2 demand-controlled ventilation. When a space is unoccupied, the minimum box position is set to 0 cfm. During normal business hours, the VAV box controls only temperature when these spaces are unoccupied. As the space becomes occupied, the box will open to the minimum required for the area component of ASHRAE Standard 62. As CO2 rises, indicating a greater rate of occupant pollutant production, the minimum box position increases.
In many cases, total system airflow can be reduced to almost as low as the required exhaust flow, essentially turning air-handling systems into 100% outside air units. Minimizing flow minimizes dehumidification energy both for cooling and any reheat, as well as fan energy.
Systems that are constant volume - such as dual-duct, multi-zone, or constant volume reheat - are likely to be near the end of their expected life and should be converted to VAV. Many times, this can be done by installing VAV boxes and new digital controls, with little ductwork modifications and relatively minor air handler upgrades.
In many cases, spaces that require significant exhaust including laboratories, kitchens, and locker rooms have their own dedicated makeup air units. These units are typically single-zone but they should also have variable volume capability. Single-zone systems are good candidates for VAV and they convert easily with a VFD. Temperature is controlled first by heating or cooling valve position, and then fan speed. This again minimizes cooling, reheat, and fan energy. Control sequences may need to be modified to provide better space air mixing if stratification becomes a problem, especially during the heating season, or if “dumping” occurs in cooling mode.
Dehumidification control strategy #3: Keep the building positively pressurized. When exhaust flow is greater than makeup air supplied, buildings are negatively pressurized and numerous problems can occur, including temperature and humidity control problems, as untreated air is drawn into the building. If a building is negatively pressurized overnight when most air handling systems are in unoccupied mode, there is a risk of freezing coils in air handlers or pipes in plenum spaces in northern climates.
Dehumidification control strategy #4: Shut down outside air when the building is unoccupied. This may sound like a no-brainer, but sometimes air conditioning is used in attempt to dry out a building when a space is unoccupied. School or university buildings that have floors waxed or carpet shampooed are good examples. The systems are placed in occupied mode to drive the temperature down to try to dry the spaces. The problem with this is that the source of moisture from outdoors is not shut down. Typically these buildings are not running boilers for reheat, and there are no internal heat gains from lighting, equipment, or people to provide a sensible cooling load. As a result, air conditioning coils are either running at relatively high temperature in the case of systems with chilled water, or direct expansion coils are cycling on and off, or using hot gas bypass to artificially load the unit. In either case, moisture is not being extracted to drive down the dewpoint, and in the case of hot gas bypass, the unit is wasting compressor energy.
With relatively high dewpoint temperatures that approach space drybulb temperature, humidity rises and the building gets clammy as described above. The dewpoint of the air in the building tracks the outside air dewpoint. In some cases, space temperatures are set lower to dry the air. This actually increases rh in many cases.
One strategy for drying out a building with minimal internal gains is to let nature provide reheat via skin heat gains. Again, higher temperatures tend to decrease rh, but in this case they also increase evaporation from the building materials. Increased evaporation, of course, tends to increase rh. Condition the air to the desired dewpoint periodically during the day, and let the building reheat on its own. Of course, this works best during hot weather that provides the most “reheat.”
SystemsAnother strategy for minimizing dehumidification energy is to select equipment and design air handling systems to provide built-in energy efficiency.
Dehumidification system design strategy #1: Dedicated outdoor air systems (DOAS). DOAS treat outdoor air independently from zone temperature control. The makeup air unit in the DOAS provides pre-treated ventilation to the temperature control zones. In the cooling season, dehumidification takes place in the DOAS. Zone temperature control is provided by fancoil units, heat pumps, or chilled beams and some type of zone heat such as fintube convection systems or radiant heat. The advantage to this design, when controlled effectively, is that it can minimize or eliminate the need for reheat.
However, the key word in the previous sentence is “can.” Oftentimes, the makeup air unit serving the DOAS is controlled to provide temperature-neutral air (70°) all year. This is a mistake and wastes energy most of the time in typical DOAS. Heating air to above 55° in the winter or reheating to above 55° after dehumidifying outdoor air in the summer often results in unnecessary simultaneous heating and cooling. For example, ventilation air may be driven down to maintain space rh, and the air is then reheated to a neutral 70°. This air then makes its way to mix with return air and in many cases must be cooled at the zone to maintain space temperature setpoints. Even if the reheat is free from some sort of heat recovery, unnecessary cooling energy is consumed at the zone. Figure 2 demonstrates wasted reheat and re-cooling associated with a DOAS makeup air unit with downstream cooling (Figure 2).
Dehumidification control strategy #5: Provide minimal temperature control with DOAS makeup air units. In the heating season, outdoor air should be heated to no more than 55°. In some cases, maintaining an even lower temperature saves energy by providing more free cooling to zones that need it, although drafts and condensation on ductwork in the building must be avoided. When dehumidification is required, drive the outdoor air temperature down to maintain desired rh and leave it there. Typically, when dehumidification is required, more zones being served by the DOAS require cooling rather than heating.
If heat recovery from the condenser of the makeup air unit or some other free heat source is available, and the BAS maintains status of heating and cooling in each zone, the discharge air temperature on the makeup air unit may be reset to minimize energy consumption. However, this is a complicated algorithm and it really only applies during light cooling loads. During periods of greater cooling loads, 55° air from the makeup air unit is not wasted because it is typically mixed with 70% to 90% return air, providing little cooling for the space. In other words, there is no overcooling and reheat.
Dehumidification system design strategy #2: Precool and reheat ventilation air with energy recovery. In some facilities such as laboratories, the need for makeup air is so great that reheat cannot be avoided. In these systems, an energy recovery unit with pre-cooling and reheating provided by energy recovery is a low energy-cost option. In dehumidification, outdoor air is precooled by exhaust air. The fresh air is then cooled to the desired dewpoint with mechanical cooling and then reheated again by the exhaust air. The exhaust air that gives up heat to the fresh air discharge is cooled in preparation to extract heat from the incoming ventilation air. Dry tempered air is delivered to the zones that are again conditioned using fancoil units, heat pumps, or chilled beams with some other source of heating (Figure 3).
Figure 3 provides a line diagram of an energy recovery unit that can be used for dehumidification. Air enters at 93°, 51% rh, and a dewpoint of 72°. It is dehumidified to a dewpoint of 54°, with a 65% reduction in cooling energy and 100% reduction in reheat. Note, however, that even at 67°, large amounts of this cool dry air provide substantial space cooling, and therefore some reheat may be inevitable.