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Standard classrooms may have occupant levels anywhere from 32 square feet per person to 50 square feet per person. As a gauge of comparison, the typical commercial office environment averages over 140 square feet per person. Given so many individuals in a confined space, it is no wonder that schools are a major indoor air quality concern. In a classroom the major source of contaminants is usually the occupants. Bioeffluents are generated from the natural process of body respiration and increased metabolism. Although not likely health threatening, the existence of such can deter the learning experience. Additional gaseous contaminant generators include classroom items such as paper products, books, writing materials, clothing and in many cases, the computer equipment associated with today's modern classroom.
In instances where outside air is introduced to dilute indoor-generated contaminants, the designer must consider the quality of the outside air. Unacceptable levels of airborne pollutants may limit outside air introduction. The United States Environmental Protection Agency (US EPA) publishes the National Primary Ambient-Air Quality Standards (NAAQS). If any of the listed contaminants exceed published limitations then alternate air cleaning methods should be considered. The NAAQS considers sulfur dioxide, particulate, carbon monoxide, oxidants (ozone), nitrogen dioxide and lead. The designer should also consider other contaminants that may be introduced by the proximity of the emitter. This may include industrial facilities, sewage treatment facilities and in some cases active farmland.
If the outside air is unacceptable, then it should be cleaned to a level consistent with clean outside air before introduction into the facility1. To dilute indoor contaminants, a minimum of 15 cubic feet per minute (CFM) is prescribed. This figure is based upon a maximum of 50 persons per 1,000 square feet of space.
Even if outside air is used, air filtration must be applied to protect the HVAC equipment and assure clean coils for efficient heat transfer. The minimum efficiency for a classroom application should be a 25-30%2 ASHRAE dust spot efficiency filter as defined by ASHRAE Standard 52.1-1992, Gravimetric and Dust Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter. If central station air handlers are the base equipment of design, the addition of a second stage, having a minimum dust spot efficiency of 60-65%, by the same Standard, is recommended.
ASHRAE Standard 52.2 - Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size, allows the user to evaluate a filter using particle size versus filter efficiency. In this manner, a user can specify filter efficiency based upon the size of the offending contaminant. As an example, ASHRAE recommends a removal efficiency of 65% at 0.3-micron particle size for printer or copier rooms. This filter will be what is commonly referred to as a 95% ASHRAE grade filter or by ASHRAE Standard 52.2, with a Minimum Efficiency Reporting Value of 14.
The new Standard also allows the user to select a filter based upon the filter's minimum efficiency rather than average efficiency. Since this usually corresponds to a filter at its least efficient point, notably when clean and first installed in the system, the particle removal efficiency is from the point of the filter's installation and efficiency increases as the filter becomes dirty. An exception does exist for filters using electret media3.
In situations where outside air is unacceptable, a combination of particulate filtration and gaseous filtration may be applicable. Application of carbon for the adsorption of gaseous contaminants (VOC's and bio-effluents) may be used to simulate clean outside air. For further assistance in particulate and gaseous applications to reduce outside air please consult Farr's Filtration and Indoor Air Quality: A Two-Step Design Solution. This method applies the analytical (indoor air quality) procedure as defined in ASHRAE Standard 62-1989, Ventilation Standard for Acceptable Indoor Air Quality4.
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