In addition to developing and issuing highly regarded industry standards, SMACNA’s Technical Services Department continuously fields questions about technical aspects of sheet metal construction and the application of specific SMACNA standards.
Some of the most frequently asked questions in the industrial sector are the use and application of the standard.
SMACNA addresses some of the most frequently asked questions below using SMACNA’s Rectangular Industrial Duct Construction Standards as a reference.
Technical inquiry regarding snow and ice loads:
My question relates to ductwork in terms of gage and the reinforcing required. The duct is HVAC duct and will be 304 stainless steel all welded construction; negative 1-in. pressure ductwork. The duct is up to 120 inches and over 120 inches to 160 inches. The duct is outdoors and subject to weather conditions.
Response from Technical Services:
Because the ductwork is outdoors and considering the region your project is in, we added a snow load of 40 lbs./ft2 and an ice load of 7.62 lbs./ft2. Together these loads add up to 47.6 lbs./ft2 or an equivalent static pressure of 9.2 in. wg., which is added to the duct’s “critical load.” Adding all dead and live loads, the duct’s critical load is 11.3 in. wg., which the duct must be constructed for. Being over 10 in. wg., SMACNA’s Rectangular Industrial Duct Construction Standards will be used for the construction solutions.
- -1.0 in wg.
- Class 1.
- Type 304 stainless steel.
- 120° F.
- Welded construction.
- External insulation 2-in., 6 lb. density.
- Cladding: 24-gage aluminum.
- Stiffeners: Fixed.
- Hanger spacing: 10 ft. max.
- Ice load: Zone 4, 1.6 in.
- Snow load: 40 lbs./square foot.
The following recommendations are per SMACNA’s Rectangular Industrial Duct Construction Standards:
- 16 gage.
- Duct to 120 in: Stiffeners (fixed): Top, sides, and bottom, R10 (See Rectangular Industrial Duct Construction Standard Table 7-D.), 3 ft. maximum spacing. If electing to use a single tie rod, can use R-9 stiffeners in lieu of R-10.
- Duct over 120 in to 160 in.: Stiffeners (fixed): Top, sides, and bottom, R12, 3 ft. maximum spacing. If electing to use a single tie rod, can use R-10 stiffeners in lieu of R-12.
- Be sure to include stiffeners at fittings as well as straight duct.
- Duct supports 10 ft. spacing max. Duct supports must be directly below stiffeners or use 12-gage reinforcement patches at all supports. (See Rectangular Industrial Duct Construction Standard Figure 4-9.)
Technical inquiry on the use of spiral duct in industrial applications:
Using the SMACNA Round Industrial Duct Construction Standards for Class 1 duct. Duct is -2 in. wg., stainless steel ductwork, and size 16 in. diameter and down. My question is, does anything prohibit the use of spiral ductwork for this application? Am I understanding the standard correctly that spiral can be used for this industrial application?
Response from Technical Services:
Following up to your technical inquiry, stainless steel spiral duct is covered in SMACNA’s Round Industrial Duct Construction Standards, Chapter 11. Stainless steel spiral may be used in Class 1, Class 2, and Class 5 applications and is subject to the following design limits.
- A design pressure within the range from negative 30 in. wg. to positive 50 in. wg. (7500 Pa to 12,500 Pa).
- Supported at intervals not exceeding 20 ft. (6100 mm).
- Design temperature not exceeding 250°F (121°C).
- Having a nominal diameter within the range of 4 to 96 in. (100 to 2440 mm).
- Minimum 22 gage.
- With diameter−to−thickness ratios of less than 1800 for all ducts.
- All ducts listed in the “bending" and “pressure tables" contained in other sections of Chapter 11 meet the minimum gage requirements described above.
- The tables are limited to indoor applications or outdoor applications where weather conditions are such that the effects of wind, ice, and snow may be neglected. (In designing systems for outdoor applications, where consideration must be given to the effects of wind, ice, and snow, the user is directed to Chapters 4, 5, 6, and 7 for guidance in manual calculation of their requirements).
Members and industry with technical questions are encouraged to submit their inquiries via SMACNA’s Technical Resource Inquiry web page, www.smacna.org/technical/make-a-technical-inquiry.