Ashrae Duct Size Calculator [BEST] Free Download
Sumiko Fagnoni
The Duct Size Calculator is a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct vs. flexible duct. The calculator uses information from ASHRAE Research Project 1333, HVAC Duct Efficiency Measures, and was developed with funding support from ASHRAE and ADI. ASHRAE Technical Committee 5.2, Duct Design, sponsored the project.
The calculator includes fields for 4, 15 and 30 percent compression in flexible ducts. Van Rite notes that the calculations used to create these size references are based on straight line compression as performed in the laboratory on a flat surface. Field installed flexible ducts with bends, kinks and excessive lengths will have additional resistance, which will result in diminished airflow.
The cost of the calculator is $34, ASHRAE members ($40, non-members). To order, visit www.ashrae.org/bookstore or contact ASHRAE Customer Contact Center at 1-800-527-4723 (United States and Canada) or 404-636-8400 (worldwide) or fax 678-539-2129.
The Duct Size Calculator is a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct versus flexible duct. It includes sizing for metal ducts, and for flexible duct when compressed at 4%, 15%, and 30% straight line compression. The Duct Size Calculator is the result of collaboration between ASHRAE TC 5.2, Duct Design, and the Air Distribution Institute. Learn more
The calculator includes fields for 4, 15, and 30 percent compression in flexible ducts. Van Rite notes that the calculations used to create these size references are based on straight line compression as performed in the laboratory on a flat surface. Field installed flexible ducts with bends, kinks and excessive lengths will have additional resistance, which will result in diminished airflow.
The Duct Size Calculator is intended for use as a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct versus flexible duct. Includes sizing for metal ducts, and for flexible duct when compressed at 4%, 15%, and 30% straight line compression. Refer to the corresponding field on the wheel to read flexible duct size required to maintain equivalent airflow and same pressure loss for a given metal duct size. One side of the wheel uses Inch-Pound (I-P) units; the other side uses International System (SI) units.
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The next step is to use that friction rate and the air flow rate for each duct section in cubic feet per minute (cfm) to find the size necessary to move that amount of air. We do it with software, but duct calculators give the same information.
When sizing by the friction rate results in too high a velocity, we size by the velocity, which results in a larger duct. But larger ducts also result in less resistance, which means we may get too much air flow in that run. What do we do about that? Install balancing dampers.
There is also the question of register placement on the wall or ceiling. If it is just moving the register 8 or 10 feet from the center to the perimiter, while it may have an effect on the load, it is not likely to have an impact on the static pressure/duct sizing. The average 90 degree elbow has 3 times the impact on the friction rate at 20-35 EL (equivalent length) as an 8 foot straight line length extension. However, moving from wall to ceiling would likely add or remove a fitting, probably have different boot AND quite possibly a different register size. These changes can have significant impact.
The 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.
These requirements presume that the designers have prepared contract drawings showing the size and location of ductwork, including permissible fitting configurations. Where area change, direction change, divided flow, or united flow fittings other than those illustrated here are shown on the contract drawings, are not of proprietary manufacture, and are defined with friction loss coefficients in either the SMACNA HVAC Duct System Design manual or the ASHRAE Fundamentals Handbook chapter on duct design, such fittings shall be fabricated with materials, assembly techniques, and sealing provisions given here.
Ducts must be sufficiently airtight to ensure economical and quiet performance of the system. It must be recognized that airtightness in ducts cannot, and need not, be absolute (as it must be in a water piping system). Codes normally require that ducts be reasonably airtight. Concerns for energy conservation, humidity control, space temperature control, room air movement, ventilation, maintenance, etc., necessitate regulating leakage by prescriptive measures in construction standards. Leakage is largely a function of static pressure and the amount of leakage in a system is significantly related to system size. Adequate airtightness can normally be ensured by a) selecting a static pressure, construction class suitable for the operating condition, and b) sealing the ductwork properly.
If the maximum short side reinforcement spacing thus found exceeds a joint spacing that you are committed to, go to the column with the joint spacing to find the joint size. Even though the duct gage listed at this width-spacing cell may be less, the joint rating cannot be less than at this cell.
Sometimes, if a project calls for small amounts of ductwork in many size ranges or pressure classes, it may be more economical to select heavier constructions than are required, so that fewer variations are needed.
The T-1 drive slip connection provides sufficient rigidity to be treated as Class A, B or C reinforcement within the limits of Table 1-25. This gives the appearance of increasing the range of unreinforced duct sizes.
Fittings shall be reinforced like sections of straight duct. On size change fittings, the greater fitting dimension determines the duct gage. Where fitting curvature or internal member attachments provide equivalent rigidity, such features may be credited as reinforcement.
Holes made in the duct wall for tie rod passage shall be of minimum size and shall be sealed in accordance with the provisions of Sections 1.8 and 1.9. Except as limited by joint specifications and certain mandatory uses, tie rod alternatives are indicated in Tables 1-3 through 1-9 for reinforcement sizes listed to the right of duct wall thickness. G denotes the size with tie rod on 22 gage in H-22G nomenclature.
The traditional use of aluminum sheet two gages (Brown and Sharp schedule) heavier than standard galvanized sheet gage DOES NOT MEET THE REQUIREMENTS FOR EQUIVALENT STRENGTH AND RIGIDITY. The modulus of elasticity of aluminum is one-third that of steel and the yield strength is approximately one-half that of steel. Thus, aluminum has to be approximately 44% thicker. Table 1-21 gives the metal thickness conversion comparison. Tables 1-22 and 1-23 and notes explain how to adapt the steel duct reinforcement schedules to create comparable aluminum tables. However, tests have not been conducted on all of the indicated constructions to verify that deflections are the same as those of steel, to confirm that all construction will withstand a 50% overload, or to refine the fastener (screw, rivet, etc.) spacing, type, and size. Nevertheless, these provisions are more reliable than the tradition of simply increasing the duct gage by two size numbers. No failure at the rated pressure is anticipated, and none has been reported since this approach was introduced in 1976.
Transverse joints shall be selected and used that are consistent with the static pressure class, applicable sealing requirements, materials involved, duct support intervals, and other provisions for proper assembly of ductwork outlined in the construction standards. The precise type, size, location, and material of fastenings used in joint assemblies are sometimes left open to prudent judgment for the specific service. Notching, bending, folding, and fit-up tolerances shall be appropriate for the composite assembly. When there is a choice of materials and methods, do not use such latitude as to create a deficiency in the integrity of the ductwork.
Liner shall be folded and compressed in the corners of rectangular duct sections or shall be cut and fit to ensure butted edge overlapping. Longitudinal joints in duct the liner shall not occur except at the corners of ducts, unless the size of the duct and standard liner product dimensions make them necessary.
Reinforcement for flat sides of oval duct shall be of the same size and spacing interval as specified for rectangular duct or shall be provided to limit wall deflection to 3/4" (19 mm) and reinforcement deflection to 1/4" (6.4 mm).
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