Humidification and Dehumidification
Technologies And Strategies To Manipulate Psychrometric Effect
The increasing globalization of scientific research, advanced health care, and other specialty facilities is expanding the range of climates in which facilities with specialized HVAC needs are sited. At the same time, rising energy prices and the volatility of global energy markets have driven some rethinking of traditional means of humidification and dehumidification of buildings. This article examines a number of tools that may be used alone - or, in select cases, more effectively together - to reduce energy consumption, be it in Jeddah or Seattle, in Hyderabad or Hyde Park.
Humidification is typically required in dry climates year-round and during winter in cold climates. Facilities where humidification is either required or beneficial include hospitals, laboratories, certain types of process facilities, and museums. Humidification reduces microbial growth (the growth of microorganisms is minimized in the 50% rh range), and prevents static electricity buildup that occurs at low relative humidity levels, which can harm electronics.
Dehumidification is typically applied in warm humid climates, in summertime in most middle latitude areas, in certain function-specific facilities such as cold rooms, specialty research spaces that require dry air, and spaces that have high latent gains (e.g., indoor swimming pools). Dehumidification is applied for some of the same reasons as humidification, including reduction of microbial growth. Special process needs drive other uses, and of course some dehumidification is driven by the need to provide outside air to buildings during humid weather.
The ToolsAlthough the variety of tools available to humidify and/or dehumidify air in building HVAC systems is broad, the use of an isothermal humidifier for humidification (where required) and reliance on the cooling coil for dehumidification represent the default design approach in North America, and other technologies are often not considered. Given the now more widely varying demands of locale, and the imperative of optimizing functional efficiency, understanding the unique capabilities and capacities of each is essential to truly effective application, since one size most assuredly does not fit all.
Isothermal HumidifiersThese have been the most commonly used type of humidifier in HVAC systems. Isothermal humidifiers inject steam into the airstream, increasing the absolute humidity level in the air (i.e., moisture content), without significantly changing the air temperature. The source of the steam varies - it may be generated by an electric source, a gas-fired source, a direct plant steam source, or an indirect steam-to-steam exchange. The water source for the steam also varies. City water may be used, but softened water or pure water may be required based on water quality or process needs. Steam is typically injected directly into the airstream via a jacketed manifold type humidifier or a dispersion tube array.
Advantages of isothermal humidifiers include the ability to controllably approach complete saturation of the airstream, relatively short absorption distances, and minimal concerns over microbial growth. Disadvantages include the energy required to create steam. (In many systems, this energy would ultimately have to be added to the airstream. For instance, adiabatic humidifiers cool the air as they humidify, so winter humidification applications in 100% outside air buildings require the heating coil to provide all of the energy that would have been used to create steam in an isothermal humidifier.)
Adiabatic HumidifiersAdiabatic humidifiers are available in several forms. The category includes wetted pad type humidifiers, spray nozzle humidifiers (which may use either pressurized water or water and compressed air for atomization), and ultrasonic humidifiers. Adiabatic humidifiers evaporate moisture into the air by presenting a large water surface area to the airstream. This is achieved by either wetting a medium with a large surface area (wetted pad type) or by atomizing the water into very small droplets (spray nozzles or ultrasonic). The water source is often city water, although fouling may be reduced through the use of either softened or pure water. Ultrasonic humidifiers require pure water.
Advantages of adiabatic humidifiers include the low energy required to humidify the air (which is typically negated in winter 100% outside air applications as noted above) and the associated cooling effect, which can be used to great advantage in warm dry climates. Disadvantages include a somewhat increased microbial growth risk due to the ambient temperature water source, particularly in the wetted media type where water is essentially stagnant. Absorption distances also tend to be longer (for spray type) and approach to full saturation is generally more difficult.
Adiabatic humidifiers are best applied in dry climates, particularly if humidification is required during cooling season, and in systems with airside economizers, which can also benefit seasonally from the reduced energy use of adiabatic humidifiers.
Solid Desiccant DehumidifiersSolid desiccant dehumidifiers usually take the form of desiccant wheels, rotary heat exchangers either coated or impregnated with a desiccant material (typically silica gel or lithium chloride). Desiccant wheels are designed to remove moisture from the airstream, and operate nearly adiabatically, dependent on the heat of sorption of the desiccant and the mass of the matrix used to present the desiccant to the air. The path followed by the supply air on a psychrometric chart is nearly the opposite of the adiabatic humidifier, but the path tends to be slightly flatter due to the effects mentioned above.
Desiccant wheels are divided into a process side and a regeneration side. The process (supply) side air is dried by the desiccant material, while on the regeneration side, another airstream is preheated and runs in counterflow through the opposite side of the desiccant wheel, driving moisture back out of the wheel. The potential for drying of the supply side is determined by the relative humidity of the regeneration air. The hotter the regeneration air is heated, the lower the relative humidity and the greater the drying potential in the supply side. Outside air and building exhaust air are the most common sources of regeneration air. The heat source for regeneration is typically hot water, ideally from a waste source.
Advantages of desiccant dehumidifiers include the ability to dry air to very low dewpoints. The ability to dry using a heat source may also be an advantage, particularly where a waste heat source is available. Disadvantages include first cost and elimination of the sensible heat that must be removed in most cases (typically through the use of a supplemental rotary heat exchanger).
Desiccant wheels are best applied when required dewpoint temperature approaches 32°F, making it impractical to use cooling coils due to coil frost issues; when the cost of heat is low relative to electricity; or where very high levels of moisture reduction are necessary.
Liquid Desiccant DehumidifiersLiquid desiccant dehumidifiers operate in manner similar to solid desiccants but use a spray tower rather than a wheel to present the desiccant to the airstream. The process side air runs in counterflow with a dry desiccant spray (usually lithium chloride or lithium bromide), the desiccant is then regenerated by a heated airstream in a regeneration side spray tower.
Liquid desiccant systems tend to be more expensive than solid desiccants but offer the benefit of enhanced microbial decontamination due to the bacteriacidal and virucidal nature of lithium chloride.
Advantages and disadvantages are similar to solid desiccant wheels. Liquid desiccants offer the advantage over solid wheels of slightly better isolation of supply and exhaust airstreams, since wheel carryover is not possible. Applications are also similar, but liquid desiccants may offer benefits where greater microbial decontamination is required or where the liquid desiccant’s capabilities closely match process requirements.
Enthalpy WheelsEnthalpy wheels are similar in nature to desiccant wheels - both are rotary heat exchangers either coated or impregnated with a desiccant material - but differ in their focus. While desiccant wheels manipulate the regeneration side airstream to achieve deep drying, enthalpy wheels focus on extraction of the available moisture, or lack thereof, in the building exhaust airstream. Enthalpy wheels are also intended to operate as heat exchangers. Accordingly, the sensible heat transfer capability of enthalpy wheels is high, whereas the heat transfer capability of desiccant wheels is low, since the transfer of sensible heat from the hot regeneration side air is an unwanted side effect.
Advantages of enthalpy wheels include the ability to simultaneously transfer both heat and moisture.
Enthalpy wheels are best applied in high outside air percentage humidified buildings in cold climates, where recovery of the energy invested in both heating and humidification is possible, and in high outside air percentage very humid climates where enthalpy wheels can recover both cooling and lack of dehumidification from the outgoing air.
Cooling CoilsAlthough dehumidification is typically not the primary purpose of a cooling coil, cooling coils act as summer dehumidifiers in most commercial buildings by limiting the dewpoint of the supply air to no more than the discharge air temperature of the air handling unit (typically 55°).
Advantages of cooling coils include simplicity and familiarity. Disadvantages include inability to dehumidify easily at or below 32° and inability to handle large loads without multiple coils.
Cooling coils are best applied where cooling loads are modest and where process requirements do not drive unusually low humidity.
Example 1The King Abdullah University of Science and Technology (KAUST) is sited in the high heat and humidity of Jeddah, in Saudi Arabia’s desert coastal climate. The requirement for 100% outside air in the laboratory facilities presented a real challenge given the design conditions. The logical approach of the enthalpy wheel (recommended by Gary Kuzma of HOK) can salvage a high percentage of the dehumidification and cooling that has been invested in the building exhaust air.
As the psychrometric chart shows, the burden on a cooling coil acting alone in an attempt to cool and dehumidify the incoming outside air would be extreme. The enthalpy wheel recaptures a high percentage of the cooling and dehumidification already performed on the exhaust air, leaving the cooling coil with a much more modest and manageable load. The reduction in cooling load handled by the cooling coil is 64% compared to a cooling coil operating alone, and the effectiveness of the enthalpy wheel was 81.4% (e.g., the wheel is able to transfer 81.4% of the enthalpy difference between the supply and exhaust air from the exhaust to the supply).
Example 2This biotech research facility is located in the mild marine climate of Seattle. The laboratory facility included a process space with very specific temperature and humidity requirements. The conditions presented a challenge because the desired humidity was just low enough that subcooling by a cooling coil would be difficult.
The solution was the use of a solid desiccant wheel to assist with moisture removal. Since the desiccant wheel’s ability to remove moisture was much greater that the required level of reduction, only a portion of the supply air was directed through the desiccant wheel. As seen on the psychrometric chart, by bypassing approximately 20% of the air the through a desiccant wheel, the total air absolute humidity was reduced from 51 grains/lb to approximately 42 grains/lb by mixing 7-grain desiccated air with the bypassed 51-grain air.
Example 3While the tools discussed here are often used alone, they may also be used in combination to achieve a particular psychrometric effect or to take advantage of available resources. An example is the Pennington Cycle. The Pennington Cycle is a 100% outside air cycle that incorporates a desiccant wheel, a sensible heat exchanger wheel, and an adiabatic humidifier. Supply side air is first dehumidified, then cooled sensibly in the heat exchanger, and finally humidified (and cooled) adiabatically by the adiabatic humidifier (evaporative cooler).
On the exhaust side, the air is first adiabatically humidified (and cooled) to enhance the cooling effect on the supply side air as the return passes the cool to the supply in the rotary heat exchanger. The exhaust air is then heated by a low grade heat source (often solar or waste heat), and dries (regenerates) the desiccant material.
As the psychrometric chart shows, the combination of desiccation, sensible cooling, and adiabatic humidification of the supply air creates a circuitous path around the psychrometric chart, but the beginning and end points are very close to those that a cooling coil alone could have achieved.
Ordinarily, this would seem to be an overly complex means to achieve cooling, but the cycle is intended to take advantage of waste heat (or solar heat) for cooling. By applying this particular series of components, waste heat is converted into cooling energy.
ConclusionBest available technologies for humidification and dehumidification vary according to system type, climate, process requirements, energy use expectations, space availability, and contamination concerns. The needs of the system, the efficiency of the technology, and the cost of available energy sources must be considered as systems are selected.
Some general conclusions:
- As reported elsewhere, adiabatic humidifiers may offer energy savings over isothermal humidifiers in systems with less than 50% outside air.
- Enthalpy wheels offer great benefits for pre-conditioning of outside air in hot humid climates (and the same holds true for high outside air percentage-humidified buildings in cold climates).
- Desiccant wheels and liquid desiccant systems provide deep drying capability in applications that require air at dew points less than 42° to 45°.
- Humidification and dehumidification technologies can be used in series (or parallel) to achieve a net effect equivalent to other technologies, with potential energy savings benefits.