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ISOTHERMAL HUMIDIFIERS

Utilizing a constant temperature, isothermal humidifiers use thermal energy to generate steam, which is introduced into the supply-air stream or directly into a space.

Isothermal humidifiers can be classified into one of two groups, based on the pressure of the vapor or steam and the method of steam generation and injection:

• Boiling-water-vapor injection, which inject a vapor at atmospheric pressure into an air stream.

• Direct-injection steam jacketed (double-wall distribution tube), which introduce 5- to 10-psig steam generated in a central boiler plant or “clean-steam” generator into an air stream.

Boiling-water-vapor-injection type

Electric. At least two types of electric humidifiers are available: electrode and heating element. With both, steam is generated from a wall, floor, or ceiling location and delivered to the supply-air stream via a dispersion assembly.

An advantage of boiling-water-vaporinjection- type humidifiers is that they introduce chemical-free vapor into air streams. Disadvantages of these units include:

• Additional space needs.

• The cost of the equipment.

• Wiring and controls.

• The need for additional water because of drainage.

• The additional cost of the emergency- power system.

• Sensitivity to water impurities.

With an electrode-type unit, the use of water with a high mineral content will result in the need to replace the canisters or clean them frequently, while the use of water with a low mineral content likely will cause the unit to malfunction, as it relies on water conductivity to operate.

Because current does not pass through water in heating-element-type units, purified water may be used to improve performance and greatly reduce maintenance.

Electric humidifiers primarily are used in existing facilities, where only a few are needed. However, they also may be considered where humidification requirements are limited and energy consumption is low.

Gas-fired. Because of an inability to vent the flue or provide combustion air, the use of gas-fired humidifiers often is limited to single-story facilities, where only a limited number of units need to be installed.

Steam-to-steam. In steam-to-steam humidification, steam is delivered from the central boiler plant to a wall-, floor-, or ceiling-mounted converter, where it passes through a heat exchanger, boiling makeup water and generating steam. That steam is conveyed via vapor piping to ductwork and dispersed into the air stream.

When potable water is used, minerals and other impurities are deposited as scale inside the water tank and on the heating coil, reducing the efficiency of  the humidifier. To minimize this problem, the use of automatic flush valves and softened or deionized water is recommended.

Disadvantages of steam-to-steam humidifiers include:

• Additional space needs.

• The cost of the equipment.

• The additional costs of makeupwater and drainage piping.

• The need for additional water because of drainage.

• Water-treatment equipment. If steam is available on site, advantages of steam-to-steam humidifiers are:

• Low energy cost.

• Humidification vapor as pure as the makeup water.

Direct-injection-steam-jacketed type A direct-injection-steam-jacketed humidifier injects live steam into the air for humidification by a distribution manifold. Steam flows through the outer jacket of the distribution manifold— keeping it hot to prevent condensate formation—into a condensate separator, through a control valve, and into the inner tube of the distribution manifold. From there, it is discharged through holes into the air stream.

Clean steam. An advantage of clean steam is that it is free of boiler-treatment chemicals. Disadvantages include the need for stainless-steel steam and condensate- system piping and components and the need to treat makeup water in reverse-osmosis or deionization equipment.

Central-plant steam. Compared with other vapor and steam systems, centralplant systems with direct-injectiontype humidifiers are the best controlled and have the lowest first and operational costs. These systems, however, have a major drawback: Boiler steam carries chemicals used for corrosion control. These volatile neutralizing amines pass through the humidifiers and into air supplies. Proper maintenance and monitoring procedures must be implemented to achieve low ambient-air concentration of amines.

HUMIDIFIER SELECTION A number of considerations go into humidifier selection:

• Technologies. The specifying engineer should be thoroughly familiar with all available humidification technologies and methods and understand performance characteristics, product features, utility requirements, and proper applications.

• Codes and standards. For a given application, code may prohibit the use of a particular type of humidifier. For example, some states allow only dry-steam humidifiers in health-care facilities, while other states make exceptions for computer rooms in these buildings.

• User preferences and requirements. For example, some large users have adopted Legionella-prevention policies that allow the installation of steam humidifiers only, thus prohibiting the use of adiabatic-type humidifiers.

• Humidifier capacity. Location, project type, and outdoor and indoor conditions should be analyzed to determine the capacity of a humidifier.

• Application. The application determines the humidification process and moisture-delivery method. (See Table 1 for recommended applications.)

• Availability of central steam. If steam humidification is required, but the central steam boiler is not available, then local electrical and electronic humidifiers should be considered.

• Chemicals in central-boiler steam. Chemicals in central-boiler steam may make steam unsuitable for direct injection. In such cases, steam-to-clean-steam converters must be considered.

• Relative cost of gas and electric power. The choice of gas or electric power should be based on a life-cycle-cost analysis.

• Maintenance. The intensity of maintenance should be considered in a lifecycle- cost analysis.

• Water quality. The use of potable or deionized, demineralized water should be verified with the equipment manufacturer. For deionized water, corrosionresistant stainless-steel components should be specified.

• Cost of water. When water carries dissolved minerals in excess of 120 ppm, a water-treatment system should be considered. Also, the most efficient humidifier should be selected to minimize water consumption.

• Availability of compressed air. When compressed air of a required pressure and purity is available, compressedair/ water-nozzle humidifiers should be considered.

• Availability of emergency power. In applications such as health care, humidification must be on emergency power.

• Cooling effect. The cooling effect of adiabatic-type humidifiers should be considered as heating-plant capacity is determined.

EVALUATING COMPETING PRODUCTS

Only humidifiers utilizing the same humidification process (adiabatic or isothermal) can be considered in an evaluation of equivalency. Characteristics that come into play in such a comparison include:

• Performance, including capacity,  efficiency, and dispersion distance.

• Quality of the distributed mist or fog, based on the size of the droplets.

• Controls (electric or pneumatic, modulating or on/off).

• Water consumption attributed to the drainage cycle. As local, self-contained steam humidifiers produce clean steam, minerals in the incoming water are left behind, increasing the water-conductivity level. To maintain the conductivity required for proper operation, the cylinder must be drained automatically. Drained boiling hot water is tempered before leaving the humidifier and being discharged into the sewer, as required by code.

• Steam, water, and air pressure and consumption.

• Materials of the humidifier components.

• Maintenance requirements.

• Installation requirements.

• Dripping potential. Atomizing heads in pneumatic systems using only air under pressure and drawing water from non-pressurized tanks often drip  at the end of the “on” cycle, while dual pneumatic atomizing heads using air and water under pressure tend to drip on “start-up.” A system should include controls to prevent dripping. Products without a non-drip feature should not be accepted.

• Dusting potential. Potable water used in atomizing humidifiers contains soluble and insoluble minerals and impurities. When an atomized water droplet evaporates in air, it leaves behind a white dust made up of these impurities. The amount of this dust depends on the quality of the potable water. If dusting is unacceptable, water must be purified. Other types of adiabatic humidifiers, such as wetted media, do not present a dusting problem.

• Disposability or reusability of components. Engineers should specify equipment with cleanable, reusable components.

CONCLUSION

The specifying engineer must learn about all available humidification technologies and base his or her specifications on the fundamentals of humidification theory, requirements of the project, the project budget, and costs of the products and utilities involved.