Handbook of air conditioning and refrigeration / Shan K

17.50 CHAPTER SEVENTEEN
ASHRAE/IESNA Standard 90.1-1999 also mandates that:
● Metal round and flat oval ducts should have a leaking class C L3, and metal rectangular ducts, rectangular
fibrous ducts, and round flexible ducts have a C L6.
● Ducts that are designed to operate at a static pressure exceeding 3 in. WC (750 Pa) shall be leak
tested according to industry accepted test procedures. Representative sections totaling no less
than 25 percent of total installed duct area for the designed pressure class shall be tested.
● The maximum permitted duct leakage , in cfm/100 ft2 (L/s�m2 V˙
L
), shall be calculated from
Eq. (17.73).
Shapes and Material of Air Ducts
Ductwork Installation
Fire Protection
When a designer chooses the shape (round, rectangular, or flat oval duct) or material (galvanized
sheet, aluminum, fiberglass, or other materials) of an air duct, the choices depend mainly on the space
available, noise, cost, local customs and union agreements, experience, quality, and the requirements
of the project. In many high-rise commercial buildings, factory-fabricated round ducts and sometimes
flat oval ducts with spiral seams are used because they have fewer sound problems, lower air leakage,
and many configurations of wyes and tees available for easier pressure balance. Round ducts also have
the advantage of high breakout transmission loss at low frequencies (see Chap. 19).
For ducts running inside the air conditioned space in industrial applications, metal rectangular
ducts are often chosen for their large cross-sectional areas and convenient fabrication. Round ducts
are often used for more demanding projects. In projects designed for lower cost, adequate duct insulation,
and sound attenuation, fiberglass ducts may sometimes be the optimum selection.
Ductwork installation, workmanship, materials, and methods must be monitored at all stages of the
design and construction process to ensure that they meet the design intent.
The design of air duct systems must meet the requirements of National Fire Codes NFPA 90A,
Standards for the Installation of Air Conditioning and Ventilating Systems, Warm Air Heating and
Air Conditioning Systems, and Blower and Exhaust Systems as well as local codes. Refer to these
standards for details. The following are some of the requirements:
● The duct material discussed in Sec. 17.3 must be made of class 0 or class 1 material. Also the
duct coverings and linings—including adhesive, insulation, banding, coating, and film covering
the outside surface and material lining the inside surface of the duct—must have a flame spread
rating not over 25 and a smoke development rating not over 50 except for ducts outside buildings.
Supply ducts that are completely encased in a concrete floor slab not less than 2 in. (50 mm) thick
need not meet the class 0 or class 1 requirement.
● Vertical ducts more than two stories high must be constructed of masonry, concrete, or clay tile.
● When ducts pass through the floors of buildings, the vertical openings must be enclosed with partitions
and walls with a fire protection rating of not less than 1 h in buildings less than four stories
high and greater than 2 h in buildings four stories and higher.
● Clearances between the ducts and combustible construction and material must be made as specified
in NFPA 90A.
● The opening through a fire wall by the duct system must be protected by (1) a fire damper closing
automatically within the fire wall and having a fire protection rating of not less than 3 h or (2) fire
doors on the two sides of the fire wall. A service opening must be provided in ducts adjacent to