Laboratory Duct Construction

Ductwork systems serving laboratory or cleanroom applications have unique construction requirements that must be addressed by the design engineer. Most laboratories are usually put in the general category of chemical laboratories; however, there are many subclasses in this category, and other types of laboratories (e.g. radiological and biological) exist as well. Different types of laboratories have different requirements for duct systems.

The most critical portion of laboratory ductwork is the exhaust system. In addition to transporting air, the exhaust system must transfer chemical vapors, emitters, pathogens, condensates, and powders, among other substances. Laboratories often contain fume hoods or glove boxes in which hazardous materials are handled. The design engineer for a ductwork system needs an analysis of exhaust material from the laboratory. It is usually inadvisable to design a laboratory exhaust system to cover "what if" scenarios; the actual specifics of a laboratory's exhaust material will present enough of a challenge. If in the future new materials are brought into the laboratory, a new analysis will be required, which could result in an exhaust system retrofit.

Exhaust ducts are normally round and made from galvanized steel. Steel or zinc galvanizing coating may not be appropriate for all exhaust contaminants. Use of 304-stainless steel is appropriate for many special exhaust conditions; however, stainless steel is expensive. Other stainless steel alloys and several types of plastic are available. Sometimes a special interior coating will be appropriate for particular types of exhaust. Fiberglass duct, linings, and flexible ducting should not be used because they may add particles to the air stream. Duct linings can also fail and clog ductwork. Flexible ducts may have an irregular surface that contributes to higher pressure-drops. In all cases, designers should choose duct material based on a careful analysis of exhaust components.

In a multi-laboratory facility, segregated exhaust systems are often the most economical approach. If an analysis shows, for example, that only three out of several laboratories require a stainless steel duct system and one lab requires an exotic plastic duct liner while the rest of the labs could use a standard galvanized steel exhaust duct system, it would be cost effective to install segregated exhaust systems.

Construction materials are not the only special consideration in laboratory exchange systems. Other typical considerations are: the need for negative pressure or filtration at the source, ease of cleaning, the need for scrubbers or containment of liquids, and the need for back-up systems.

Duct tape should not be used to seal laboratory duct systems. Laboratory exhaust systems may need to be sealed to insure zero leakage, which requires welding the duct joints. In radiological lab exhausts, the welded ducts should be round and installed with the longitudinal seam within 45° of top center, to prevent radionuclides from accumulating at a weld bead.

Supply, return, and exhaust ductwork should comply, at a minimum, with "HVAC Duct Construction Standard - Metal and Flexible" (SMACNA, 1995) for all laboratory types. More stringent requirements for laboratory exhaust may be necessary in individual cases. For laboratory exhaust systems with negative pressures exceeding 1,500 Pa (6 in. WG), accepted "Industry Practice for Industrial Duct Construction" (SMACNA, 1975) should be used.

Sealing (ASHRAE, 1997) of the laboratory duct systems should always be to at least Class 3, which is very tight. Supply air ductwork in many laboratories (e.g. nuclear laboratories) should consider flexible duct connections to the outlet terminals only if a potential back flow of air would be acceptable to the duct material. In addition to specifying duct material and leakage class, the design engineer needs to specify duct shape, reinforcements, hangers, spacing, insulation, and required acoustical treatment.

Redundancy is usual where HVAC systems must remain in operation. Redundancy requires branching into parallel exhaust fans, filter trains, etc.

Branches require installation of isolation dampers to prevent air flowing backwards through various components, from fan back-draft dampers to bubble-tight butterfly dampers. Fire dampers should not be used in fume hood or exhaust systems. If a fire wall must be penetrated, the code allows the use of heavier steel duct material and substantial hangers so that the duct can withstand fire and remain in place.

In some radiological laboratory exhaust systems, a water spray arrangement with de-mister pads will need to be installed upstream of the final high-efficiency particulate air (HEPA) filter bank(s) to protect the filters from a fire upstream. A drain line should be installed to a tank that can safely hold additional material. The drain should be directed to a special holding tank for safe treatment and disposal of drain material. Additional requirements may imposed in response to DBA concerns, such as seismic or tornado threats.

Cleanrooms are laboratory-type rooms containing special processes that require a dust-free environment. The rating of a cleanroom defines the acceptable dust particle concentration. Cleanrooms usually have a laminar air flow, which can be vertically and downward or horizontal. Cleanroom air distribution systems handle the necessary large quantities of air by means of overhead and under-floor plenums.

HEPA filters located as close as possible to the supply outlets will minimize dust delivered to the cleanroom. Ducting between HEPA filters and supply outlets should be made of material that does not release microscopic particles in the air stream as it ages. Acrylic pre-painted aluminum is a good choice.

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