Many buildings and industrial processes require specific relative humidity operating ranges. These relative humidity operating ranges typically require dehumidification. In the article “Manufacturing the Right Moisture” (Engineered Systems, April, 2007), I discussed how to calculate the amount of moisture that needed to be removed, recommended space relative humidity, and provided a brief discussion on available dehumidification technologies. This article will provide dehumidification industry standards and offer in-depth discussion on available dehumidification technologies.
Two standards define moisture-removal efficiency requirements. The Air-Conditioning, Heating & Refrigeration Institute (AHRI) Standard 920, “Performance Rating of DX Dedicated Outdoor Air System Units,” developed two calculations for moisture removal efficiency. The moisture removal efficiency (MRE) is pounds of removed moisture divided by energy used to remove the energy (LBw/KWHR). The second calculation is integrated seasonal moisture removal efficiency (ISMRE), which measures the moisture removal efficiency at different operating conditions. The equation is ISMRE = (0.12 x A) + (0.28 x B) + (0.36 x C) + (0.24 x D). The specific operating conditions are provided in AHRI 920. The ASHRAE 90.1 Standard committee adopted AHRI 920 and established minimum moisture removal efficiencies (MRE) as part of section 6.4.1. The required minimum MRE is provided in tables 6.8.1-14, 6.8.1-15, and 6.8.1-16. The equipment manufacturer is responsible for providing the moisture-removal-efficiency information.
COOLING COIL DEHUMIDIFICATION
The lower the air temperature, the less moisture content air can retain. The air moisture content reduces when cooled below its dew point (Figure 1). Looking at Figure 1, the entering air is 74.7°F dry bulb (DB) and 57.8° wet bulb (WB). Cooling the air below 60° will cause moisture to condense out of the air. As the air temperature drops further below the dew point, the air has more moisture removal.
FIGURE 1. Air moisture content reduces when it’s cooled below its dew point.
- Performance Capability — Cooling coil dehumidification systems can provide dry air moisture content with dew points down to 40°. The cooling coil dehumidification system can provide moisture content control of +/- 1% relative humidity (RH).
- System Description — Typical cooling-coil dehumidification system components (Figure 2) include a return/outside air mix box, a MERV-8 prefilter, an optional MERV 11-14 secondary-filter, a preheat coil, a cooling coil, a process air fan, and a reheat coil. The cooling coil can be DX refrigerant or chilled water. The reheat coil can be DX refrigerant (wrap-around), hot water, or steam. Wrap-around heat pipes are available that precool the air before entering the cooling coil and reheat the air after leaving the cooling coil.
- System Operation — The process air fan and compressor (if it’s a DX system) are activated. The cooling coil reduces the incoming process air temperature below its dew point, causing moisture to condense out of the air. Depending upon the space cooling requirements, a reheat coil may be needed to heat the air to the required discharge air temperature. For systems utilizing a heat pipe wrap-around system, the upstream heat-pipe coil cools the incoming air. Then, the cooling coil cools and dehumidifies the air. After the cooling coil, the air is reheated as it goes through the downstream heat-pipe coil.
- System Operation Modification — Modifying the cooling coil discharge air temperature will change the process air moisture content.
- Available Equipment Sizes — Cooling coils are available in many sizes. A cooling coil dehumidifier has a typical airflow modulation range between 50%-100% of the maximum design airflow.
- System Advantages — The cooling coil dehumidification system has a number of advantages, including:
- It is efficient at moisture removal for dew points above 40°;
- Typically, the cooling coil used for dehumidification is the same air-handling-unit cooling coil used for general cooling, and no additional equipment or equipment space is needed;
- It’s the lowest capital and operating cost of the three dehumidification technologies; and
- It has the flexibility of using different cooling sources (chilled water, DX) and energy sources (waste heat, hot water, gas, steam) for reheating.
FIGURE 2. The typical cooling-coil humidification system components.
Desiccant dehumidifiers are different from cooling-based dehumidifiers. Instead of cooling the air to condense moisture, desiccants attract moisture by creating an area of low vapor pressure at the surface of the desiccant. The pressure exerted by the water in the air is higher, so water molecules move from the air to the desiccant, and the air is dehumidified. The most frequently used adsorption desiccant materials are silica gel, activated alumina, and molecular sieve. The silica gel desiccant is the most frequently used. For applications having dew points down to minus 30°, the silica gel and activated alumina are good selections. For dew points below minus 30°, the molecular sieve would be the best selection. The following discussion will provide the performance capability, system description, system operation, system operation modification, available equipment sizes, and system advantages for an adsorption dehumidification system:
- Performance Capability — Adsorbent dehumidification systems can provide the driest air of any moisture content with dew points from 55° down to minus 100°. The adsorption dehumidification system can provide moisture content control of +/- 2% to 10% Rh. For most applications with a dew point below 10°, fine control of moisture content is not typically required.
- System Description — Typical adsorption dehumidification system components (Figure 3) include a return/outside air mix box, a process MERV-8 prefilter, an optional MERV 11-14 secondary filter, a precooling coil, a face/bypass damper, a desiccant wheel, a desiccant/bypass mix box, a process air fan, an optional after-cooling coil, a reactivation MERV-8 prefilter, a reactivation heater, a reactivation fan, and an optional purge fan. The precooling coil and after-cooling coil can be cooled by chilled water or refrigerant. The reactivation heater can utilize steam, electric, a natural gas burner, or waste heat/hot water as a heat source.
- System Operation — The process air fan, precooling coil, desiccant wheel rotation, after-cooling coil, reactivation heater, reactivation fan, and purge fan (if the option is furnished) are activated. The desiccant wheel will rotate at five to 20 revolutions per hour. The system can operate without a precooling coil, but providing a precooling coil makes the desiccant system more efficient. The precooling coil can reduce the incoming process air temperature to 40°; typically, process air entering the desiccant wheel is between 45°-55°. After the precooling coil, the process airstream will either go through the desiccant wheel or bypass the desiccant wheel. The face/bypass damper will modulate to maintain the discharge process air moisture content. If the elevated temperature leaving the desiccant wheel is not desired, the process air is cooled to the discharge temperature set point using the after-cooling coil. The reactivation heater modulates to maintain its discharge temperature set point, with the reactivation fan moving the heated air through the wheel and discharging the cooler and wetter air to the atmosphere.
FIGURE 3. The typical adsorption dehumidification system components.
The precooling coil is an important component to the overall adsorption system efficiency. The colder the air is before entering the desiccant wheel, the more effective the desiccant wheel is at removing moisture. The required desiccant wheel reactivation temperature is based upon incoming process air conditions and the required process air discharge moisture content. Reactivation heater temperatures can range from 120°-350°, with the higher reactivation temperatures typically required for process air needing more moisture removal or lower dew points. Correspondingly, the process air temperatures discharging from the desiccant wheel can have a 5° to more than 40° temperature rise, which is dependent on how much moisture the wheel is removing from the process air.
As an option to reducing the dehumidification system’s total energy usage, use a purge fan with a purge section before and after the desiccant-wheel reactivation section (Figure 4). The purge fan blows air through the purge section on the desiccant wheel reactivation section outlet to reduce the desiccant wheel temperature before rotating into the process air section and blows the heated air through the purge section on the desiccant wheel reactivation section inlet to preheat the desiccant wheel before entering the reactivation section. Proper installation and operation can reduce energy usage by 30%.
FIGURE 4. As an option to reducing the dehumidification system’s total energy usage, use a purge fan with a purge section before and after the desiccant wheel reactivation section.
The adsorption dehumidification psychometric chart illustrates the process air conditions during the adsorption dehumidification process (Figure 5). The inlet air condition is 74.7° DB and 57.8° dew point (DP). As the process air flows through the adsorption dehumidifier, the precooling coil provides the first cooling and dehumidification. The precooling coil reduces the process air moisture content from 78 grains/lb. dry-air to 38 grains/lb. dry-air. The process air then goes through the desiccant wheel where its moisture content reduces to 18 grains/lb. dry-air. The example psychometric chart shows a process-air adsorption dehumidifier discharge condition of 52° DB and 39.5° WB (or 21.4° DP).
- System Operation Modification — There are two methods for controlling the content conditions of the delivered process air: modulate the face/bypass damper and change the reactivation heater temperature. Modulating the face/bypass damper will modify the delivered condition of the process air in a relatively short period of time. Due to the desiccant wheel rotating five to 20 revolutions per hour, when utilizing the reactivation heater temperature change method, the delivered condition of the process air will change slowly. In higher humidity environments, modulating the reactivation heater temperature at too low a temperature could cause damage to the desiccant wheel. The face/bypass damper modulation is the recommended method for modifying the operating conditions of the process air.
- Available Equipment Sizes — Adsorption dehumidifiers are available in sizes from 50-100,000 cfm. Adsorption dehumidifiers have an airflow modulation range between 50%-100% of maximum design airflow.
- System Advantages — The adsorption dehumidification system has a number of advantages; including:
- It can produce very low conditioned air moisture contents;
- It’s able to produce process air with dew points below freezing; and
- It has the flexibility of using many different energy sources (waste heat, hot water, gas, steam) for heating the reactivation air.
FIGURE 5. The adsorption dehumidification psychometric chart illustrates the process air conditions during the adsorption dehumidification process.
Absorption dehumidification systems operate on the principle of chemical absorption of water vapor (moisture) from the air. The most frequently used absorption desiccant chemical is lithium chloride in a brine solution. The following discussion will provide the performance capability, system description, system operation, system operation modification, available equipment sizes, and system advantages for an absorption dehumidification system:
- Performance Capability — The liquid lithium-chloride dehumidification system can maintain conditioned air RH between 18%-80%. The absorption dehumidification system can provide tight relative humidity control of +/- 1% RH.
- System Description — The liquid lithium-chloride dehumidification system has two main components: the conditioner and the regenerator (Figure 6). The conditioner components include a conditioned air fan, packing material, a sump, a distribution pump, a cooling plate and frame heat exchanger, circulation piping, spray nozzles, divert piping, a pump-in/pump-out interchanger, a flow meter, and a mist eliminator. The regenerator components include a scavenger air supply fan, packing material, a sump, a distribution pump, a heating plate and frame heat exchanger, circulation piping, and a mist eliminator. The common system components are the lithium-chloride brine and the controls.
- System Operation — The first operational step is to fill the conditioner and regenerator sumps with the lithium-chloride brine. The conditioned air moisture content requirement determines the required lithium-chloride percentage. If the conditioned air moisture content requirement is 18% RH, the maximum level of lithium-chloride concentration will be required in the brine. If the conditioned air Rh requirement is 50% Rh, the required lithium-chloride concentration is less. The typical lithium-chloride concentration ranges from 12%-40%.
FIGURE 6. The typical components within the conditioner of a liquid lithium-chloride dehumidification system.
The conditioned air fan, conditioner distribution pump, scavenger-air supply fan, and regenerator sump pump are activated. The conditioner cooling plate and frame heat exchanger chilled water or DX refrigerant flow modulates to maintain the conditioned air discharge-temperature set point. When the conditioner is operational, the conditioner distribution pump thrusts the liquid brine through the cooling plate and frame heat exchanger and to the spray nozzles on top of the packing material where it comes in direct contact with the conditioned airstream. The desiccant removes the moisture from the conditioned airstream, which increases the liquid volume. During system operation, the pump-in/pump-out interchanger conditioner pump-out valve will continuously modulate to maintain the conditioner sump level. When the pump-in/pump-out interchanger conditioner pump-out valve is modulating, the conditioner liquid brine is being pumped to both the conditioner spray heads and the regenerator sump.
When the regenerator is operational, the regenerator distribution pump propels the liquid brine through the heating plate/frame heat exchanger and to spray nozzles on top of the packing material where it comes in direct contact with the scavenger airstream. The regenerator heating plate and frame heat exchanger hot water or steam flow modulates to maintain the regenerator-sump liquid-brine level, which removes the same amount of moisture collected by the conditioner. When the pump-out valve is modulating toward the open position, the flow meter and control valve modulates to control the concentrated liquid brine transfer from the regenerator to the conditioner.
The absorption psychometric chart illustrates the air conditions during the absorption dehumidification process (Figure 7). The entering air condition is 74.7° DB and 57.8° DP. The conditioned air is cooled and dehumidified as it flows through the absorption dehumidifier. The example psychometric chart shows a conditioned air absorption-dehumidifier discharge condition of 54° DB and 48° WB.
FIGURE 7. This absorption psychometric chart illustrates the air conditions during the absorption dehumidification process.
- System Operation Modification — If the conditioned air operating conditions require modification, it’s necessary to know whether or not the absorption dehumidifier provides all the system cooling requirements. If the absorption dehumidification system provides all the system cooling, it will be necessary to add or remove lithium-chloride brine from the sumps until reaching the required lithium-chloride brine concentration. If the absorption dehumidification system does not provide all the system cooling, modifying the conditioned air discharge temperature set point will modify the condition air operating point.
- Available Equipment Sizes — Absorption conditioners are available in sizes from 1,500-84,000 cfm. The conditioner airflow modulation is between 50%-100% of maximum design airflow. The conditioner has a mist eliminator that utilizes air impingement through its media to remove the liquid lithium-chloride spray. When the conditioner airflow is below 50% of design airflow, the lithium-chloride spray removal efficiency is reduced due to lower air velocity through the mist eliminator, resulting in the lithium chloride spray being carried into the system.
- System Advantages — The absorption dehumidification system has a number of advantages; including:
- It has a tight conditioned air Rh level;
- It features an effective biocide that captures and neutralizes airborne pathogens;
- It can operate at outside air temperatures down to minus 60° DB; and
- It utilizes lower temperature heating source (heating water, steam).
The cooling coil, adsorption, and absorption dehumidification systems utilize different technologies for removing air moisture content. Each dehumidification system has a specific temperature and Rh range it must stay within to effectively maintain process air conditions at. Looking at Figure 8, all three dehumidification technologies are effective with process air required dew points between 40°-55° and relative humidities between 18%-80%. Above 55° DP, cooling coil and absorption can maintain the required conditions. Below 40° DP, adsorption and absorption can maintain the required conditions. For process air conditions below 20% Rh, the adsorption dehumidification system is among the only technologies that can meet these conditions.
FIGURE 8. All three dehumidification technologies are effective with process air required dew points between 40° and 55°F and relative humidities between 18% and 80%.
There are a series of dehumidification systems that utilize desiccants with specific performance characteristics. A future article may present these series dehumidification systems.
With AHRI 920 providing a method for calculating moisture removal efficiency and ASHRAE 90.1’s adoption of AHRI 920’s MRE with specific efficiency guidelines for DX dedicated outside air systems, anticipate ASHRAE 90.1 extending the moisture-removal efficiency requirement to other dehumidification technologies in the future. It’s important to perform a detailed technical analysis to determine which dehumidification technology will provide the best performance for a specific application when using a DX dedicated outside air system.
1. ASHRAE 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings.
2. AHRI 920 - Performance Rating of DX Dedicated Outdoor Air System Units.
3. “Munter’s Dehumidification Handbook,” Second Edition, and consultations with Christopher Jensen, Product Manager.
4. “Alfa Laval Kathabar Liquid Desiccant Engineering Reference Guide” and consultations with Mike Harvey, Kathabar Product Manager.
5. Dedicated Outdoor Air Systems: AHRI 920, ISMRE & ASHRAE 90.1 by Peter Teige of Munters, October 12, 2016.
6. “Manufacturing the Right Moisture” by Vincent Sakraida, April 1, 2007, Engineered Systems Magazine.