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# 120125 duct sizing

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• Existing Duct Sizing Methods /The traditional duct sizing &lt;ductworkdesignfundamentals.htm&gt; methods are Equal Friction and Static Regain [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;]. Both methods were developed as expedient practical procedures and neither addresses optimization. Available computer programs are simply automated versions of these manual procedures. / /The *Equal Friction* method creates an &quot;initial guess&quot; for duct sizing by establishing a constant pressure loss per unit of duct length. A number of sources recommend using 25 Pa (0.1 in. WG) pressure loss per 30 m (100 ft) total length. This length is selected for the &quot;critical path,&quot; which is the longest branch in an air distribution system. It is assumed that the longest run will have the highest sum of total pressure loss. However, the longest run is not necessarily the run with the greatest friction loss, however, because shorter runs may have more elbows, fittings, and other flow restrictions. The procedure for using the Equal Friction method for duct design, including system balancing, Follows… /The *Static Regain* method of duct sizing &lt;ductworkdesignfundamentals.htm&gt; is based on Bernoulli&apos;s equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs. This is used as the major principle for sizing the ducts so that the increase in static pressure at each branch offsets the friction loss in the succeeding section of the duct. The static pressure should then be the same before each terminal and at each branch. This method provides a convenient means of designing a long duct run with several take offs so that the same static pressure exists at the entrance to each branch, outlet, or terminal take off. The Static Regain method applies to supply systems only. This method is also based on an arbitrary parameter, which is the velocity for the root section. _The ASHRAE 2001 Fundamentals Handbook_, Chapter 34, Table 10 [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;] gives the suggested range of velocities based on &quot;engineering experience.&quot; When energy cost is high and installed ductwork cost is low, a lower initial air velocity is most economical. For lower energy costs and high duct costs, higher air velocity is most economical. / /Like the Equal Friction method, the Static Regain method requires iterations. The major difference between the Static Regain and Equal Friction methods is that one uses the ratio of pressure loss to the length, and, in the other, the succeeding cross section is selected as a function of previously established air velocities at junctions:/ /(Pressure loss)1-2 = [(Velocity)12 - (Velocity)22] x (Density) / 2/ /Both methods are based on an initial guess./ /The Static Regain method has been shown to have a number of deficiencies [Tsal and Behls, 1988 &lt;references.distributionsystems.htm&gt;]. The method has been partially modified [Brooks,1995 &lt;references.distributionsystems.htm&gt;] to compensate for some of these problems. / /Popular traditional duct design methods, including Equal Friction and Static Regain [ASHRAE, 1997 &lt;references.distributionsystems.htm&gt;], provide engineers with design tools. However, these methods involve some engineering judgment and extensive manual recalculations, so air distribution systems designed by different engineers for identical situations will turn out to have different fans, duct sizes, costs, and overall system energy demands. / /Tsal and Behls (1986)/ &lt;references.distributionsystems.htm&gt;/conducted a comprehensive analysis of existing duct-sizing methods . This analysis shows that these methods, after a number of iterations, can select cross sections that deliver the designed amount of flow to terminals; these methods cannot, however, select the most economically efficient design.
• Existing Duct Sizing Methods /The traditional duct sizing &lt;ductworkdesignfundamentals.htm&gt; methods are Equal Friction and Static Regain [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;]. Both methods were developed as expedient practical procedures and neither addresses optimization. Available computer programs are simply automated versions of these manual procedures. / /The *Equal Friction* method creates an &quot;initial guess&quot; for duct sizing by establishing a constant pressure loss per unit of duct length. A number of sources recommend using 25 Pa (0.1 in. WG) pressure loss per 30 m (100 ft) total length. This length is selected for the &quot;critical path,&quot; which is the longest branch in an air distribution system. It is assumed that the longest run will have the highest sum of total pressure loss. However, the longest run is not necessarily the run with the greatest friction loss, however, because shorter runs may have more elbows, fittings, and other flow restrictions. The procedure for using the Equal Friction method for duct design, including system balancing, Follows… /The *Static Regain* method of duct sizing &lt;ductworkdesignfundamentals.htm&gt; is based on Bernoulli&apos;s equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs. This is used as the major principle for sizing the ducts so that the increase in static pressure at each branch offsets the friction loss in the succeeding section of the duct. The static pressure should then be the same before each terminal and at each branch. This method provides a convenient means of designing a long duct run with several take offs so that the same static pressure exists at the entrance to each branch, outlet, or terminal take off. The Static Regain method applies to supply systems only. This method is also based on an arbitrary parameter, which is the velocity for the root section. _The ASHRAE 2001 Fundamentals Handbook_, Chapter 34, Table 10 [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;] gives the suggested range of velocities based on &quot;engineering experience.&quot; When energy cost is high and installed ductwork cost is low, a lower initial air velocity is most economical. For lower energy costs and high duct costs, higher air velocity is most economical. / /Like the Equal Friction method, the Static Regain method requires iterations. The major difference between the Static Regain and Equal Friction methods is that one uses the ratio of pressure loss to the length, and, in the other, the succeeding cross section is selected as a function of previously established air velocities at junctions:/ /(Pressure loss)1-2 = [(Velocity)12 - (Velocity)22] x (Density) / 2/ /Both methods are based on an initial guess./ /The Static Regain method has been shown to have a number of deficiencies [Tsal and Behls, 1988 &lt;references.distributionsystems.htm&gt;]. The method has been partially modified [Brooks,1995 &lt;references.distributionsystems.htm&gt;] to compensate for some of these problems. / /Popular traditional duct design methods, including Equal Friction and Static Regain [ASHRAE, 1997 &lt;references.distributionsystems.htm&gt;], provide engineers with design tools. However, these methods involve some engineering judgment and extensive manual recalculations, so air distribution systems designed by different engineers for identical situations will turn out to have different fans, duct sizes, costs, and overall system energy demands. / /Tsal and Behls (1986)/ &lt;references.distributionsystems.htm&gt;/conducted a comprehensive analysis of existing duct-sizing methods . This analysis shows that these methods, after a number of iterations, can select cross sections that deliver the designed amount of flow to terminals; these methods cannot, however, select the most economically efficient design.
• Existing Duct Sizing Methods /The traditional duct sizing &lt;ductworkdesignfundamentals.htm&gt; methods are Equal Friction and Static Regain [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;]. Both methods were developed as expedient practical procedures and neither addresses optimization. Available computer programs are simply automated versions of these manual procedures. / /The *Equal Friction* method creates an &quot;initial guess&quot; for duct sizing by establishing a constant pressure loss per unit of duct length. A number of sources recommend using 25 Pa (0.1 in. WG) pressure loss per 30 m (100 ft) total length. This length is selected for the &quot;critical path,&quot; which is the longest branch in an air distribution system. It is assumed that the longest run will have the highest sum of total pressure loss. However, the longest run is not necessarily the run with the greatest friction loss, however, because shorter runs may have more elbows, fittings, and other flow restrictions. The procedure for using the Equal Friction method for duct design, including system balancing, Follows… /The *Static Regain* method of duct sizing &lt;ductworkdesignfundamentals.htm&gt; is based on Bernoulli&apos;s equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs. This is used as the major principle for sizing the ducts so that the increase in static pressure at each branch offsets the friction loss in the succeeding section of the duct. The static pressure should then be the same before each terminal and at each branch. This method provides a convenient means of designing a long duct run with several take offs so that the same static pressure exists at the entrance to each branch, outlet, or terminal take off. The Static Regain method applies to supply systems only. This method is also based on an arbitrary parameter, which is the velocity for the root section. _The ASHRAE 2001 Fundamentals Handbook_, Chapter 34, Table 10 [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;] gives the suggested range of velocities based on &quot;engineering experience.&quot; When energy cost is high and installed ductwork cost is low, a lower initial air velocity is most economical. For lower energy costs and high duct costs, higher air velocity is most economical. / /Like the Equal Friction method, the Static Regain method requires iterations. The major difference between the Static Regain and Equal Friction methods is that one uses the ratio of pressure loss to the length, and, in the other, the succeeding cross section is selected as a function of previously established air velocities at junctions:/ /(Pressure loss)1-2 = [(Velocity)12 - (Velocity)22] x (Density) / 2/ /Both methods are based on an initial guess./ /The Static Regain method has been shown to have a number of deficiencies [Tsal and Behls, 1988 &lt;references.distributionsystems.htm&gt;]. The method has been partially modified [Brooks,1995 &lt;references.distributionsystems.htm&gt;] to compensate for some of these problems. / /Popular traditional duct design methods, including Equal Friction and Static Regain [ASHRAE, 1997 &lt;references.distributionsystems.htm&gt;], provide engineers with design tools. However, these methods involve some engineering judgment and extensive manual recalculations, so air distribution systems designed by different engineers for identical situations will turn out to have different fans, duct sizes, costs, and overall system energy demands. / /Tsal and Behls (1986)/ &lt;references.distributionsystems.htm&gt;/conducted a comprehensive analysis of existing duct-sizing methods . This analysis shows that these methods, after a number of iterations, can select cross sections that deliver the designed amount of flow to terminals; these methods cannot, however, select the most economically efficient design.
• /The *Static Regain* method of duct sizing &lt;ductworkdesignfundamentals.htm&gt; is based on Bernoulli&apos;s equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs. This is used as the major principle for sizing the ducts so that the increase in static pressure at each branch offsets the friction loss in the succeeding section of the duct. The static pressure should then be the same before each terminal and at each branch. This method provides a convenient means of designing a long duct run with several take offs so that the same static pressure exists at the entrance to each branch, outlet, or terminal take off. The Static Regain method applies to supply systems only. This method is also based on an arbitrary parameter, which is the velocity for the root section. _The ASHRAE 2001 Fundamentals Handbook_, Chapter 34, Table 10 [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;] gives the suggested range of velocities based on &quot;engineering experience.&quot; When energy cost is high and installed ductwork cost is low, a lower initial air velocity is most economical. For lower energy costs and high duct costs, higher air velocity is most economical. / /Like the Equal Friction method, the Static Regain method requires iterations. The major difference between the Static Regain and Equal Friction methods is that one uses the ratio of pressure loss to the length, and, in the other, the succeeding cross section is selected as a function of previously established air velocities at junctions:/ /(Pressure loss)1-2 = [(Velocity)12 - (Velocity)22] x (Density) / 2/ /Both methods are based on an initial guess./ /The Static Regain method has been shown to have a number of deficiencies [Tsal and Behls, 1988 &lt;references.distributionsystems.htm&gt;]. The method has been partially modified [Brooks,1995 &lt;references.distributionsystems.htm&gt;] to compensate for some of these problems. / /Popular traditional duct design methods, including Equal Friction and Static Regain [ASHRAE, 1997 &lt;references.distributionsystems.htm&gt;], provide engineers with design tools. However, these methods involve some engineering judgment and extensive manual recalculations, so air distribution systems designed by different engineers for identical situations will turn out to have different fans, duct sizes, costs, and overall system energy demands. / /Tsal and Behls (1986)/ &lt;references.distributionsystems.htm&gt;/conducted a comprehensive analysis of existing duct-sizing methods . This analysis shows that these methods, after a number of iterations, can select cross sections that deliver the designed amount of flow to terminals; these methods cannot, however, select the most economically efficient design. /
• Existing Duct Sizing Methods /The traditional duct sizing &lt;ductworkdesignfundamentals.htm&gt; methods are Equal Friction and Static Regain [ASHRAE, 2001 &lt;references.distributionsystems.htm&gt;]. Both methods were developed as expedient practical procedures and neither addresses optimization. Available computer programs are simply automated versions of these manual procedures. / /The *Equal Friction* method creates an &quot;initial guess&quot; for duct sizing by establishing a constant pressure loss per unit of duct length. A number of sources recommend using 25 Pa (0.1 in. WG) pressure loss per 30 m (100 ft) total length. This length is selected for the &quot;critical path,&quot; which is the longest branch in an air distribution system. It is assumed that the longest run will have the highest sum of total pressure loss. However, the longest run is not necessarily the run with the greatest friction loss, however, because shorter runs may have more elbows, fittings, and other flow restrictions. The procedure for using the Equal Friction method for duct design, including system balancing, follows:/ /Step 1. Select the &quot;critical path&quot; as the longest branch between fan and terminal outlets./ /Step 2.Assign total pressure losses to each section of the &quot;critical path&quot; as the recommended pressure loss per unit of length multiplied by the actual section length./ /Step 3. Calculate cross sections for the &quot;critical path&quot; using previously assigned total pressure losses, and correct these if necessary in order to satisfy velocity and geometrical constraints. (Pressure loss in junctions cannot be calculated until branched cross sections are assigned. At this time, the pressure loss in junctions can be ignored; a constant pressure loss can be assumed for any junction, or the same cross sections can be used in branches as in trunk ducts./ /Step 4.Sum the pressure losses in the &quot;critical path&quot; and select a fan so that fan total pressure is close to the sum of total pressure losses in the critical path. This pressure is called the &quot;root pressure.&quot; At this step the root pressure is the same as the fan pressure. If the selected fan does not satisfy the pressure requirement, change the assigned pressure loss per unit length and repeat the process from Step 2./ /The engineer should achieve pressure balancing by selecting proper duct cross-sections rather than by using dampers. / /Note that during such a calculation process, the pressure loss in the &quot;critical path,&quot; which is already calculated, will change because of the change of cross sections in the branches of junctions. A major problem in this process is to satisfy the noise and geometry criteria. For example, a short section located close to the fan must be balanced with the long &quot;critical path.&quot; Often, this can only be done by dampening flow. However, this creates noise caused by high velocities in the damper. Occasionally, lowering fan pressure can prevent noise, but more often it indicates that the layout of the system must be modified. **/
• ### 120125 duct sizing

1. 1. Sharing our Guidance Duct System Sizing
2. 2. Duct System Sizing HVAC Design Sources ASHRAE – Fundamentals SMACNA – HVAC Systems Duct Design Carrier – System Design Manuals …and TES… 08/06/13 2
3. 3. Duct System Sizing HVAC Design Overview duct systems) HVAC Load Calculations Zoning Prelim equip layout and selection Diffuser/Grille Layouts Ductwork Layout Ductwork system design Fan/Equipment Selection 08/06/13 3
4. 4. Duct System Sizing Ductwork Design Basics HVAC duct systems deliver air from fan to diffusers, which distribute air to space Fan creates a pressure difference which moves air Duct layout/sizing is related to this pressure difference Goal: size ductwork to minimize pressure drops, while keeping costs installation/material costs down. 08/06/13 4
5. 5. Duct System Sizing Ductwork Design – System Pressure Total Pressure = Static Pressure + Velocity Pressure Static & velocity pressures vary throughout system depending on size of duct and airflow Total pressure decreases in system due to friction 08/06/13 5
6. 6. Duct System Sizing System Pressure Changes 08/06/13 6
7. 7. Duct System Sizing Fan Selection Type of application/system/fan Airflow (CFM) External Static Pressure Calculation Total Static Pressure (manuf driven, not typically used in selection) 08/06/13 7
8. 8. Duct System Sizing External Static Pressure Calculation Static pressure beyond HVAC equipment System static pressure determined from all friction losses Created by straight duct Elbow/fittings Duct Accessories 08/06/13 8
9. 9. Duct System Sizing Design Methods Equal Friction Static Regain Other (per SMACNA) Extended plenum Velocity T-method Total Pressure Constant Velocity 08/06/13 9
10. 10. Duct System Sizing Duct Sizing Methods Equal Friction Method Most common Method low pressure systems supply, exhaust & return air Friction loss per length of duct is determined to be a constant value. Ex: 0.1” of wc per 100’ of duct Total system losses of ductwork = (total equivalent duct system length ) * (friction loss) Equivalent length = all straight duct + equivalent lengths of elbow, fittings, etc. 08/06/13 10
11. 11. Duct System Sizing Duct Sizing Methods Static Regain Complicated design for systems with high pressure and high velocity systems Typically supply only 08/06/13 11
12. 12. Duct System Sizing Duct Sizing Methods Velocity Method Velocity assigned to each section of duct with a known airflow, area determined from that. Calculate friction loss from tables & charts based on now determined area and length of duct. 08/06/13 12
13. 13. Duct System Sizing Design Layout Clearances – Ceiling heights Floor to floor height Type of structure (beams, joists, etc) Coordination with other obstacles Piping Lights Other Ductwork Velocity and related noise 08/06/13 13
14. 14. Duct System Sizing Air Velocity Common Duct Velocities for standard low pressure/low velocity systems Supply/Return Shafts 2000 fpm max Supply/Return Mains 1500 fpm max Supply/Return Branch ducts 1200 fpm max Lower velocities may be required in noise sensitive areas. 08/06/13 14
15. 15. Duct System Sizing Air Velocity 08/06/13 15
16. 16. Duct System Sizing Duct Sizing Concepts Use even duct sizes (ex: 24/12) Branch ducts ideally should be 2” shorter than duct main (ex: 12” deep duct should have no more than 10” deep takeoff) Exception: some fittings available for same size takeoffs Internal duct insulation: 1” reduces 2” total of ductwork dimensions Coordinate exposed ductwork as it may be preferred to be round, spiral and/or painted by client or architect. Keep duct aspect ratios less than 4:1. (Square or Round is ideal.) 08/06/13 16
17. 17. X Duct System Sizing 08/06/13 17 Rectangular Duct Elbows
18. 18. X Duct System Sizing Turning Vanes 08/06/13 18
19. 19. X Duct System Sizing Elbows – Equivalent Duct Lengths 08/06/13 19
20. 20. Duct System Sizing CONCLUSION . . . THANK YOU.