(The following is taken from HVAC Duct Construction Standards, Metal and Flexible, published by the Sheet Metal and Air Conditioning Contractors' National Association.)
A duct system is an assembly whose primary function is to convey air between specified points. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) categorizes duct systems as either single path or dual path. Systems should be designed using accepted engineering practice and data such as that in the four ASHRAE handbooks and the SMACNA HVAC Duct Systems Design manual.
A duct system may contain ducts under positive and negative pressure. Air velocities will vary within the system. At coils and filters, the velocity may vary from below 1,000 feet per minute to over 3,000 fpm. Velocity in duct mains and braches can be at constant (high or low) or varying levels. With the many available systems-sizing methods (equal friction, static regain, velocity reduction, total pressure) and system types, performance cannot be economically optimized unless the designer selects construction details appropriate for the given pressure and velocity.
Generally speaking, duct strength, deflection and leakage are more functions of pressure than of velocity. In conventional systems, noise, vibration and friction loss are more related to velocity than to pressure.
Because total pressure is less downstream than upstream, a duct-construction pressure classification equal to the fan-outlet pressure (or to the fan total static pressure rating) cannot economically be imposed on the entire duct system.
Pressure in ducts near room air terminals is nearly always below one-half water gauge.
For a clear interpretation of requirements for ducts and for economical attainment of performance objectives, it is essential that contract plans identify the portion of each duct system to be constructed for a particular pressure classification or that the entire system be assigned a pressure classification.
General performance requirements for all air ductsIn fulfilling the function of moving air, the duct assembly must satisfy certain fundamental performance criteria. Elements of the assembly are sheets, reinforcements, seams and joints. Theoretical and/or practical limits for the following criteria must be considered for the duct assembly and its elements.
1. Dimensional stability (shape deformation and strength).
2. Containment of the air being conveyed (leakage control).
3. Vibration (fatigue and appearance).
4. Noise (generation, transmission or attenuation).
5. Exposure (to damage, weather, temperature extremes, flexure cycles, wind corrosive atmospheres, biological contamination, flow interruption or reversal, underground or other encasement conditions, combustion or other in-service conditions).
6. Support (alignment and position retention).
7. Seismic restraint.
8. Thermal conductivity (heat gain or loss and condensation control).
In establishing limitations for these factors, consideration must be given to effects of the pressure differential across the duct wall, airflow friction losses, air velocities, infiltration or exfiltration, as well as the inherent strength characteristics of the duct components.
Construction methods that economically achieve the predicted and desired performance must be determined and specified. To the extent that functional requirements for ducts are not identified by testing or rating criteria, the construction details here represent acceptable practice in the industry, except in special service conditions.
Where other construction details are needed to meet the special needs of a particular system design, the designer should comply with appropriate construction standards.
Pressure-velocity classificationThe terms "low" and "high" as applied to velocity and pressure are vague and are no longer used. The designer must select a numerical static-pressure class or classes that satisfy the requirements of the particular system.
The construction described in this manual is related to heating, cooling, ventilating and air-conditioning systems. Although some detail and discussions of hood exhaust and dishwasher exhaust is included, systems carrying particulate, corrosive fumes, flammable vapors or systems serving industrial processes are not covered.
Reinforcement arrangementsThe basic elements of duct construction consist of duct wall(s), transverse joints and reinforcements at, or between, joints and supports. All of these form an integrated combination for each pressure class and duct size. Each size in a pressure class has a minimum duct-wall thickness and a minimum specification for joints, reinforcements, etc. An element from a higher-pressure class or larger duct size may be substituted in a construction of a lower pressure class or smaller duct size.
This is generally acceptable because the substituted element will exceed the minimum requirements. However, using some overdesigned elements does not justify underdesigning other elements in the composite assembly, unless the overall resulting construction can be shown to meet the minimum standards.
For example, substituting a stronger reinforcement member does not necessarily permit a larger reinforcement interval; the minimum requirements for each element in the system must continue to be met. For certain duct widths and reinforcement intervals, duct-wall deflection is not affected by the strength and rigidity of joints or reinforcements.
The designer must apply construction standards appropriate for the requirements and scope of each project. Fabricators and installers must select features from the joint, seam, reinforcement and support options that will result in a composite assembly that will conform to the performance criteria identified in this manual.
Experience in construction is valuable; no book can provide all the detail and knowledge necessary to select, fabricate and install a workable assembly. Careless selection and poor workmanship weaken construction integrity. However, the contractor's obligation to make suitable selections does not mean the contractor must make up for the designer who writes a negligent specification.