Room air change rates

Effective air circulation is essential for safe laboratory operation. The rate of air circulation has recently been a subject of discussion not only with reference to potential energy efficiency but safety and efficacy as well.

The conventional, "national consensus standard" has been four to six outside air changes per hour recommended for a "safe" B-occupancy laboratory; in laboratories that routinely use more hazardous material, such as known carcinogens, 10 to 12 outside air changes per hour have been recommended. Some publications simply recommend four to 12 air changes per hour; such generalized recommendations represent engineering by "rule-of-thumb" at its worst. In health care facilities, air change rates are being discussed in relationship to CFM per patient. [Marshall, 1996]

Studies of laboratory facilities have demonstrated that the room air change rate had less effect on environmental conditions in the laboratories surveyed than did the room air diffusing system and other ventilation characteristics. More scientific data are needed to form a basis for laboratory air circulation guidelines because many conventional design parameters and recommendations are not directly related to microenvironmental (e.g., cage) conditions in the laboratory. [Zhang et al., 1992; McDiarmid, 1988]

For example, it has been demonstrated that air dilution or replacement does not protect personnel from exposure to concentrated bursts of aerosols in biological laboratories. In fact, Crane (1994) quotes Chatigny and West (1976), who say that, "increasing ventilation rates from six to 30 air changes per hour (ACH) has a minimal effect on aerosol concentration of microorganisms in the first few minutes after release." [Crane, 1994]

Other factors can have an impact on the determination of a lab's air change rate. Memarzadeh (1999) has shown that "controlling the humidity in animal rooms is more significant in managing the production of ammonia (from animal urine) than is the use of high air change rates." This allowed him to decrease "the air change rate from 15 to as low as 5, while improving the welfare of the animals." [Memarzadeh, 1999]

As described in Chapter 2, in California, laboratory-type facilities will usually fall into one of three California Uniform Building Code (CAL/UBC) classifications: B, H-8, or H-7. The Uniform Building Code (UBC), Uniform Fire Code (UFC), NFPA 45, and Uniform Mechanical Code (UMC) do not specify the air change rate of B (except in certain cases of flammable liquid storage) or H-7. The special California H-8 occupancy also does not specify an air change rate. In an H-6 occupancy (restrictive classification for large semiconductor fabrication facilities), an exhaust air-flow rate of one cfm (which may include recirculated air) per square foot (0.044 L/s/m2) of fabrication area is specified by the code. [Uniform Building Code, 1994]

A laboratory space with a 10-foot-high ceiling and a one cfm per square foot exhaust air-flow rate will result in six air changes per hour. However, if the ceiling height is greater, a ventilation rate based on air changes will result in air flows in excess of what even the H-6 occupancy requires and will consequently waste energy for less hazardous activities. Therefore, in the absence of better information, it is recommended that the air-flow rate through the laboratory space be set not at a particular air change rate but at one cfm per square foot (and not less than six air changes per hour in B laboratories where Class I, II, or III-A liquids are used) and for B, H-8, or H-7 occupancies. [California Uniform Building Code, 1994; California Uniform Fire Code, 1994]

The relatively new area of Computational Fluid Dynamics (CFD) may provide the energy engineer with information to help make laboratories safer and more energy efficient. Room geometry, HVAC system equipment, diffuser placement, and laboratory operational procedures all influence air movement around the fume hood sash opening and thus affect hood performance. The Computational Fluid Dynamics computer model is an attempt to simulate the interaction of all of these variables impacting hood contaminant performance. The National Institutes of Health, Division of Engineering Services, worked with Flomerics Ltd., the Building Services Research and Information Association (BSRIA), and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to develop Computational Fluid Dynamics-based (CFD-based) models that will predict the ventilation performance of different laboratory configurations, see Memarzadeh, 1996. . [Hayner, 1995]

More:

Computational fluid dynamics (CFD) research project

Animal facilities

Ventilation system overview

Ventilation rates


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