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So. Humidification is used to improve comfort, control static electricity, and maintain relative-humidity (RH) levels. Applications include health-care facilities, computer rooms, museums, refrigeration warehouses, and food-processing plants.
As diverse as the areas they serve are humidification units themselves, utilizing various processes and methods of moisture delivery.
This article will discuss humidification processes, available technologies, and factors to consider when evaluating competing products.
HUMIDIFICATION PROCESSES
Basically, there are two processes of humidification: adiabatic and isothermal.
In the adiabatic process, the energy required for evaporation is obtained from ambient air. This process is characterized by constant enthalpy. While enthalpy remains constant, the moisture content of the air increases, and the temperature decreases.
The isothermal process, on the other hand, involves the addition of energy for the evaporation of water. Isothermal steam humidifiers increase the enthalpy of air, but do not change air temperature.
ADIABATIC HUMIDIFIERS
In adiabatic humidifiers, water can be circulated and returned to the reservoir or sprayed and not returned.
Some adiabatic humidifiers use mechanical energy to generate fog or mist of water particles. These particles absorb heat from air and evaporate. This process adds moisture to air and lowers its temperature, thus, providing a cooling effect. This effect must be considered in the determination of the heating capacity of a system.
Adiabatic humidifiers can be classified into one of two groups, based on their method of introducing water into air:
• Atomizing, which fracture water into particles 1 to 200 microns in size and discharge the particles directly into the air stream, where they evaporate.
• Evaporative, which evaporate water before introducing it into the air stream.
Atomizing type
Ultrasonic. In ultrasonic humidifiers, electronic oscillation is converted to mechanical oscillation using a piezo disk immersed in a reservoir of mineralfree water. The mechanical oscillation is directed at the surface of the water, where it creates—at very high frequencies—a fine mist of 1-micron water droplets.
Immersion-type germicidal ultraviolet (UV) lamps can be mounted inside the stainless-steel water tanks of humidifiers. The tanks drain automatically when not in operation.
Advantages of ultrasonic humidifiers include:
• Lower energy consumption than with electrical steam humidifiers.
• Lower system cost than with electrical steam humidifiers.
• Additional cooling effect.
• Low-temperature humidification.
Disadvantages of ultrasonic humidifiers include manufacturer-recommended use of mineral-free, deionized, or reverse-osmosis treated water.
Ultrasonic humidifiers are well-suited for dataprocessing centers, cleanrooms for electronics and pharmaceutical manufacturing, and telecommunications centers. They also work well in the refrigerated storage of fruit and vegetables, food processing and storage, the cultivation of mushrooms, plant nurseries, orchid culturing, wine cellars, the storage of cut flowers, and other industrial applications.
Centrifugal. Centrifugal humidifiers draw water from the reservoir onto large rotating discs. Centrifugal force accelerates the water across the discs and throws it against an atomizing screen. Water is fractured into non-wetting particles of 5 to 10 microns and discharged directly into the air, where it evaporates, creating an even distribution of droplet-free mist throughout the room.
For easy maintenance and cleaning, air used by a centrifugal unit is drawn through a built-in filter, which protects working parts from dust and dirt in the air.
Advantages of centrifugal humidifiers include:
• Lower energy consumption than with electrical steam humidifiers.
• Lower system cost than with electrical steam humidifiers.
• Additional cooling effect.
• Work with reverse-osmosis or demineralized water, as well as potable water.
• Charcoal filters.
• Safety overflow or level control.
• Automatic flushing device.
Disadvantages of centrifugal humidifiers include a lack of provisions for UV and other germicidal devices that would prevent algae formation in the reservoir.
Centrifugal humidifiers work well in the lower capacity ranges (several pounds per hour) and may be used locally to store and protect perishables and raw and finished goods. Also, they are well-suited for use on a room-by-room basis in small laboratories, tobacco humidors, and greenhouses.
Compressed air/water nozzle. A pneumatic atomizing system that uses only air under pressure and draws water from open, non-pressurized tanks is called a gravity system. It is susceptible to algae and bacteria growth in the tanks. A system that uses both water and air under pressure typically is called “dual pneumatic,” as it utilizes dual pneumatic atomizing heads. Minimum water and air pressure, consumption, and droplet size vary from manufacturer to manufacturer.
Advantages of compressed-air/water-nozzle humidifiers include:
• Lower energy consumption than with electrical steam humidifiers.
• Lower system cost than with electrical steam humidifiers.
• Additional cooling effect.
• Reduced maintenance with use of reverseosmosis or demineralized water with all-stainlesssteel construction.
• Use of control cabinets.
• Fail-safe shutdown upon loss of air.
• Various control options.
• Connection to building-automation system.
Disadvantages and limitations of compressedair/ water-nozzle humidifiers include:
• The relatively high cost of oil-less compressors and water pumps.
• The energy consumed by the compressors and pumps.
• Mineral fallout, or “dusting.”
• Sensitivity to distance for proper evaporation.
• Water-quality limitations.
• Noise.
• Sensitivity to dirty, unfiltered, and oily air.
Compressed-air/water-nozzle humidifiers work well in greenhouses, cold storage, woodworking, the printing and paper industry, textile-manufacturing areas, cleanrooms, and other industrial applications in which a large amount of humidification is required.
Evaporative type
Because no droplets of water enter the air stream, minerals remain in evaporative humidifiers and must be removed periodically by cleaning, blowdown, and continuous bleed-off. On the positive side, no mineral fallout or “dusting” is experienced with this type of humidifier.
Wetted media. Among the most popular materials for evaporative cooling and humidification are specially impregnated and corrugated cellulose sheets with different bonded flute angles. This design yields a cooling/humidification pad with high evaporative efficiency, but relatively low pressure drop.
A water-distribution system allows uniform supply of water to the cooling pads to minimize dry spots.
Advantages of wetted-media humidifiers include:
• Lower energy consumption than with electrical steam humidifiers.
• Lower system cost than with electrical steam humidifiers.
• Additional cooling effect.
• No mineral carryover.
• Low scaling.
• Use of potable water.
Disadvantages of wetted-media humidifiers include:
• Potential microbial contamination from water being drawn from an open tank.
• High water consumption because of continuous bleed-off and periodic blowdown.
• Additional air-pressure drop from evaporative media being mounted in an air stream.
Air washers. Air washers pass air over a continuous film of water. Because high levels of humidification cannot be achieved with this method, it is less popular than the wetted-media approach.
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