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23 31 00: HVAC Ducts and Casings

by Sheet Metal and Air-Conditioning Contractors' National Association (SMACNA)

Last updated: 06-03-2009

Introduction

Sheet metal work is one of the few remaining crafts that transform raw materials into finished products through skilled craftsmanship. Sheet metal workers cut, form, weld, solder, and assemble individual pieces to create final products and systems that serve all building types and functions. While many of the repetitious steps and processes have been automated, individual skill—a product of training and apprenticeship under master craftsmen—is the critical element. Sheet metal types range from coated and uncoated steel and aluminum sheet stock to copper and stainless steel, and the coatings vary from galvanized finishes to high-tech anodizing treatments. Like wood or stone, metal is a basic building material—like wood it has grain but, unlike stone, metals can be mixed to create totally unique metals. Applications range from architectural, which span decorative elements, to "invisible" roof flashing to duct systems, which are used in residential, commercial heating, air-conditioning, and ventilation systems (HVAC) to industrial ducts in manufacturing facilities. This particular section of the Whole Building Design Guide deals specifically with mechanical air distribution.

Description

Fig. 1. Rendering of a typical sheet metal shop

Fig. 1. Rendering of a turn of the century sheet metal shop

Today, ductwork is constructed to high standards from a broad range of materials. It is an integral component in the production of energy in power plants, the creation of chemicals in pharmaceutical manufacturing facilities, the protection of health in hospitals, and so on. Ductwork design is one of the key factors in the overall operation and maintenance of most facilities. The HVAC duct designer is faced with many considerations once the load calculations and the overall type of distribution system to be used has been determined. If the emphasis is on energy conservation, the designer must balance duct sizes between the spaces allocated and duct pressure losses which is directly related to operating cost. Other considerations are addressed below.

Materials

The different materials used for ducts systems can substantially affect the overall performance of the systems. The advantages of the various material choices should be evaluated and balanced with the limiting characteristics. Examples of materials used for duct construction include galvanized steel, black carbon steel, aluminum, stainless steel, copper, fiberglass reinforced plastic, polyvinyl steel, concrete, and fibrous glass.

Historically, the primary drivers of material choice were durability requirements and first cost. As sustainability issues, such as recycling, become larger influencers steel metal ducts are likely to receive even more emphasis. Metal shops are fastidious recyclers of scrap metal and nearly 64% of all scrap steel is recycled. Though not usually labeled as such, virtually all steel products made in the United States have some recycled content. While steel is the primary metal used in duct construction, other metals used by sheet metal workers are also actively recycled. The aluminum industry, for example, processed 4.3 million metric tons of scrap and recycled it into 3.4 million metric tons of aluminum. This is roughly 37 percent of the total U.S. aluminum production of 104 million metric tons. Other examples based on 1996 data for recycled content are: copper, 43%; lead, 55%; zinc, 19%; stainless steel, 50%.

Methods of Construction

The operating pressure and what is conveyed in the air stream are the primary parameters that determine a duct system's construction method and materials. Ductwork ranges from light gage galvanized steel for low pressure, 1/2 inch water gauge, residential and light commercial applications to thick-walled stainless steel for high negative pressure, minus 30 water gauge and more, to convey corrosive gases and abrasive materials.

Economics of Duct Systems

Typically, the most cost-effective design for the intended application decides the type of duct system. However, architectural duct systems are often times designed to provide a strikingly visual element to a building's overall design.

Duct System Layout

The overall design of the building and structural and mechanical system elements govern the ultimate layout of each duct system. Generally, ductwork is concealed above drop-down ceilings, in mechanical rooms, and in vertical chases that allow duct and other mechanical systems to pass unseen through wall divisions and between floors.

Fan Selection

The HVAC fan systems often represents one of the largest energy users in a commercial building. Close attention to the overall diversity of a building's load and the use of zone-controlled terminal boxes can result in substantial energy savings. In addition, variable-speed drives are beginning to be more widely applied by designers resulting in lower annual energy use since the system airflow can be precisely matched to the actual HVAC requirements.

Duct Leakage

The impact of duct leakage depends on duct location, length, design, and pressure. When duct is located in the conditioned space leakage is less of a factor than when duct is located outside of conditioned areas. SMACNA encourages designers to specify equipment leakage control—at the level they deem most appropriate for the application—and to rely on prescriptive sealing of ductwork as measures that will normally lead to effective control of leakage without the need of extensive leakage testing.

Acoustic Considerations

Duct systems, unless properly designed, will act as large speaker tubes and transmit noise throughout the building. The direction of airflow has little effect on the transmission of noise. Adequate noise control in a duct system can be achieved if the designer understands basic noise control principles. Sometimes even the most careful design has unanticipated acoustical properties that must be dealt with to satisfy occupants. Unwanted sound-noise-can generally be mitigated if the technician troubleshooting the complaint has been trained to understand sound and vibration testing and principles.

Testing, Adjusting, and Balancing

Most of today's HVAC systems are being designed with individually-controlled zones to improve occupant comfort. At the same time as building envelopes are becoming tighter, the demand for fresh, filtered air is increasing. These opposing factors combine to result in constantly changing airflow requirements which results in more emphasis on proper HVAC system design, operation, and balancing. Testing and balancing can be particularly beneficial in older buildings, especially if there have been piecemeal upgrades and changes to the HVAC system or the building's interior layout.

Application

Each application drives the specific type of material and duct system design required. Several very general classifications are provided below as examples. SMACNA has over 25 technical manuals, some cover all or portions of all building types and the following groups were done for ease of use and simplification. Refer to the SMACNA website for a more detailed description of each manual.

Residential

Fig. 2. Stainless steel architectural duct

Fig. 2. Stainless steel architectural duct

Residential duct systems are generally low-pressure, 1/2 inch water gauge, and low velocity, less than 1,000 feet per minute in main ducts, and typically use light gage galvanized steel or fibrous duct. Steel ducts are joined by slips and drives, which are metal-to-metal joints and are not as airtight as duct section joining methods applied to commercial and industrial duct systems. Fibrous ducts are also used and are joined by outwardly-clinching staples and special tapes that are either pressure- or heat-sensitive. Examples of SMACNA manuals relevant to this type of duct system are:

Commercial

Light commercial/single-story duct systems use slightly heavier gauge sheet metal and the operating pressures and velocities begin to go up in an effort to move a larger volume of air. Fibrous ducts can be used in systems that operate up to 2 inches of water gauge and 2,400 feet per second. As the in-duct pressure elevates more effective joint construction methods are used to provide an effective seal at higher operating pressures.

Large commercial/multi-story duct systems use more ridge, air-tight construction standards and must also provide for protection in the event of fire. Higher velocities and pressures are used in the main duct systems to minimize the HVAC system physical space requirements. SMACNA's publications include design guidance for operating pressures up to 10 inches water gauge for commercial duct systems.

Examples of SMACNA publications relevant to the commercial sector include:

Industrial

Industrial duct systems are designed and constructed according to what is conveyed in the air stream and the operating temperatures and pressures, pressures that can range from plus or minus 30 inches of water gauge. There are five classifications: Class 1 for ventilation and gaseous emission; Class 2-4 for the conveyance of abrasives; and Class 5 for corrosives such as acid vapor. Examples of the primary materials used are galvanized steels for up to 400 degrees F, hot- and cold-rolled steels for up to 650 degrees F, and stainless steels are produced that can operate above 1,000 degrees F. Other specialized materials used in industrial applications, primarily for exhausting corrosive and toxic fumes, include thermoplastic (PVC) and Thermoset FRP.

Examples of SMACNA publications relevant to the industrial sector include:

Relevant Codes and Standards

Guide Specifications

Standards and Guidelines

  • National Fire Protection Association (NFPA)—The mission of the international nonprofit NFPA is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating scientifically-based consensus codes and standards, research, training, and education.
  • The International Code Council (ICC)—The ICC was established in 1994 as a nonprofit organization dedicated to developing a single set of comprehensive and coordinated national model construction codes.
  • Sheet Metal and Air-Conditioning Contractors' National Association (SMACNA)—Located in headquarters outside Washington, DC, the Sheet Metal and Air-Conditioning Contractors' National Association (SMACNA), an international association of union contractors, has 1,965 members in 99 chapters throughout the United States, Canada, Australia, and Brazil. The voluntary technical standards and manuals developed by SMACNA Contractors have found worldwide acceptance by the construction community, as well as foreign government agencies. ANSI, the American National Standards Institute, has accredited SMACNA as a standards-setting organization. SMACNA does not seek to enforce its standards or provide accreditation for compliance. SMACNA standards and manuals address all facets of the sheet metal industry, from duct construction and installation to air pollution control, from energy recovery to roofing.

Additional Resources

Trade Associations and Other Organizations