Commercial Duct Design Principles Methods Applications ENGINEERING TRAINING Technical Development Program TOP-30 791-48 (Supersedes 7200-25) (Supersedes 791-125)This Technical Development Program (TDP) is designed to stand alone as a module of training. If desired, this text may be used in conjunetion with the PowerPoint presentation TDP-39PP (Cat. No 793-446) and the Duct Calculator (Cat, No. 794-036) or with Carrier's Duct Design software program, When properly combined with other TDP modules, this training program is one component of a coordinated course of study of HVAC system design. A systems approach to learning about our business makes the most sense of any because both engineers and designers deal with equipment in the context of operating systems within a building. Please consult the Packaged Training Programs Catalog to see how Commercial Duet Design fits within a total curriculum of system design. Training Program Objectives This TDP will provide the student with the skills and knowledge necessary to: Understand fundamental principles and theory of airflow through duct systems. Calculate air velocity in round and rectangular duets. Size supply and return air duct systems using the equal friction and static regain methods. Calculate the approximate total duct system static pressure losses requited in order to size a fan system, Calculate friction losses of new and existing duct systems. Verify that an existing duct system is adequate to handle an increased airflow quantity after system upgrades. ‘Those who wish to build their knowledge in air system duct design should consider the following related publications for their library. Carrier Systems Design Manual if 2 (510-308) — Air Distribution TDP-11 (791-418), Duct Sizing Using The Duct Calculator 200-11 (794-036), Duct Calculator 7200-07 (791-007), Air Side '7200-24B (791-224), Room Air Distribution Catrier Duct Design Software Important — We have dozens of engineering and system design training materials which cover theory through application. For a FREE Packaged Training Programs Catalog, call (800) 962-9212COMMERCIAL DUCT DESIGN TABLE OF CONTENTS INTRODUCTION ,. DUCT DESIGN CRITERIA. CLASSIFICATIONS OF DUCT SYSTEMS...... DUCT DIMENSIONS... os FUNDAMENTALS OF AIR DUCT DESIGN Fan and Air Distribution System Components... ‘The Law of Conservation of Energy Air Flow through Terminals. Friction Losses in Ducts. Example 1 — Using the Friction Loss Chart. Velocity Pressure Conversion Line... Fitting Losses... Example 2 — Fitting Loss Calculatio Friction Losses in Straight Duct... Example 3 ~ Material Roughness Correction Round vs, Rectangular Duct of Equal Friction Rate. Round Ducts of Equal Velocity and Equal Area... Example 4 ~ Converting from Round to Equivalent Rectangular Ducts. Other Pressure Losses Fan Outlet Static Pressure Loss or Gain System Effect. om Wind and Stack Effects... STEPS REQUIRED TO DESIGN AN AIR DUCT SYSTEM DUCT SIZING METHODS... EQUAL FRICTION DUCT DESIGN PROCEDURE... MODIFIED EQUAL FRICTION DUCT DESIGN PROCEDURE. STATIC REGAIN DUCT DESIGN PROCEDURE..... QUIZ I INTRODUCTION QUIZ Il- DUCT SIZING... WORK SESSION I. Equal Friction Duet Design.WORK SESSION IL.. Static Regain Duct Design .. ‘APPENDIX. enn Chart | ~ Frietion Loss Chart for Round Duct... Table 1— Circular Equivalent Diameter, Equivalent Area, and Duct Clas of Retangulat Duets for Equal Friction... A ‘Table 2— Recommended Maximum Duct Velocities for Low Velocity Systems (FPM) Table 3 ~ Velocity Pressures... ‘Table 4 — Duct Roughness Multipliers. Table 14 - SMACNA Fitting Loss Tables.. Duct Design Sizing Workshec... Equal Friction Example Worksheet. Static Regain Example Worksheet. Quiz I Solutions. Quiz MI Solutions ‘Work Session I Solution... ‘Work Session II Solution. REFERENCES:INTRODUCTION This program covers the fundamental principals of ir system duct design, primarily for commercial building applications. The two most popular duct design methods—equal friction and static regain—are covered in detail, Although there are other duct sizing methods used (velocity reduction, T-method, extended plenum, constant velocity, etc.) these are not widely used by designers and are beyond the scope of this training module. The reader should refer to other publications for information on these design methods. It is recognized that the use of computer-aided uct design is becoming more and more popular; however, itis important that the student learn the manual method of duet design before using a duct design software program. This will provide the knowledge necessary to recognize possible design errors and understand the effects of various design decisions. A duct design should only be performed after the space cooling and heating loads have been calculated. Once the space loads have been determined, the air distribution system should be designed. This includes locating the supply and return grilles and registers to provide adequate room ait distribution to the spaces. Load estimating and room air distribution principles are covered in other related publications, After determining the layout (location) of the supply and return air grilles, the designer must determine the required sizes of supply and return, ductwork that will deliver the correct amount of air to the spaces. Onee the duct sizes and transition fittings are known, the designer can determine the total pressure that the fan must overcome to deliver the required airflow. Ductwork must be sized properly to ensure that the air system can be balanced to deliver the required air volume to each space, An oversized uot system will be difficult to balance and will drive up the installed cost of the system, An undersized duct system will create higher than necessary air pressure drops, generate unwanted noise, and will not deliver the required airflow quantities, Quizzes and work sessions are included after each section of this program to assist the student in evaluating his or her understanding of these fundamental principles. DUCT DESIGN CRITERIA Several factors must be considered when designing a duct system. Generally, in order of importance, they are as follows: * Space availability «First cost * Air friction loss © Room sound level * Duet heat gain and leakage loss The sizing criteria will often be defined by the space available to run the ductwork. Ceiling plenums, duct chases, and obstructions such as walls and beams often dictate that a certain size duct be used, regardless of whether or not it is the best size from a first cost or air friction loss perspective. First cost is often important; however, first cost mostly impacts the type of materials used to construct the ductwork, rather than the size of ducts. Air friction loss is affected by the duet sizes as well as the material and fittings used, For instance, round galvanized sheet metal has the lowest friction loss per linear foot and flexible ductwork has the highest friction loss per linear ‘Commercial Duct Designfoot. Also, the quantity of fittings has a direct effect on the overall air pressure drop of a duct system, The number of fittings should be minimized to minimize the duct system pressure drop and installed cost. ‘An undersized duct system, that is, one with high velocity, creates noise that is often ‘objectionable to the occupants. Poorly selected or installed transitions and fittings also create turbulence, which creates noise and air pressure drops and should be avoided. Poorly insulated or large ductwork that runs through very warm or very cold areas can incur ‘a heat gain or loss that effectively reduces the capacity of the cooling and heating equipment, and will likely result in occupant discomfort and higher operating costs. Leaky ducts have the same energy-wasting effect, and may create odors and stained ceiling tiles if duct insulation becomes wet from the formation of condensation at the leak points. CLASSIFICATIONS OF DUCT SYSTEMS ‘According to SMACNA (Sheet Metal And Air Conditioning Contractors National Association, Inc.) standards, duct systems should be classified with a numerical pressure class as shown in Table 1. ‘These pressure-velocity relationships have replaced the older terminology, including terms such as low pressure, high pressure, low velocity, and high velocity. These older terms are rather vague and have been replaced with static pressure classification values. Note in Table 1 that pressure classes from ¥4" to 3" are designated as either positive (+) or negative (-) pressure. Pressure classes 4" and above are for positive pressure systems only. ~~ Static Premure | resureRangs | Max. Velety Cin, wag) (in. wg) (ft/min. tn" O" to 4" 2,000. a | >and wo | 2.500 | Sia pw | 2.500 or [ aan pw" | 000 ru “>3" and up to 4" 4,000 se [andy = 0 [ >and up o 10 | > * Determined by designer ‘Table 1 ~ Pressure-Velocity Classifications Static pressure classifications are much more usefull than terms such as low velocity because they may be used fo establish the required duct construction materials and reinforcing. The designer and sheet metal contractor may use these values to establish the required metal gauge and maximum duet dimensions allowable to prevent failure, Therefore, for instance, if someone refers to a SMACNA Pressure Class 4 system, according to the chart, the pressure is greater than 3.0 in, w.g. and up to 4.0 in. w.g with a maximum velocity of 4,000 ft/min (fpm). Over-designing a duet system by using a heavier than necessary gauge sheet metal will incur additional cost; therefore the reader should refer to. SMACNA guidelines for determining required metal gauges and reinforcing. DUCT DIMENSIONS Ductwork is specified and constructed to whole inch increments, not fractions. Rectangular ductwork can be constructed to “odd” number dimensions (7", 9", 11", ete.), but generally most fabricators prefer to manufacture fittings in even number dimensions 10 minimize inventory requirements, ‘Commercial Duet DesignRound duct, especially flexible-type, is readily available in both even and odd dimensions: When looking at a set of plans, there are generally several views available: a plan view, an elevation view, and a section view. In plan view, which is the most common duct drawing, the standard convention for representing duct dimensions is to always list the first number of the duct dimension as the visible dimension, In other words, if a duct is indicated as 36" x 16", the 36" dimension would be visible and the 16" dimension would not be shown. FUNDAMENTALS OF AIR DUCT DESIGN Fan and Air Distribution System Components The primary purpose of an air duct system is to deliver the proper amount of conditioned air from the air handler to the conditioned space, and then to return the air from the space back to the air handler, as shown in Figure 1. Supply Fan —— =r aL} Not all air duct systems have both a supply and return duct, For instance, an exhaust system has only a return duct, and a hospital operating room may use all (100%) outside ventilation ait, in which case there would not be a return air duct required. In addition, there are circumstances where it is advantageous to utilize the plenum space above the ceiling for return airflow. This is illustrated in the next section, A typical air system consists of a fan, supply ait ductwork, transition fittings, discharge grilles, return air grilles, retum air ductwork, cooling and heating coil, and filter. The air in the room is essontially “still air” (15 ~ 50 fpm) and is typically at atmospheric pressure (zero gauge pressure). ‘The fan draws air through the retum grille, return duct, coil, and fan due to the negative pressure created by the fan, The fan pushes the air through the supply duct, transition fittings, and discharge grilles by creating a positive Pressure at the fan outlet. The total pressure difference between the inlet pressure of the fan and the outlet pressure is referred to as the fan total pressure. ‘The Law of Conservation of Energy Objects may contain either potential energy or kinetic energy. Potential energy is derived from the object’s relative position, that is, its location, when compared to a reference position, For instance, an automobile parked at the top of a hill contains potential energy due to its mass and the force of gravity, which try to make the car roll downhill. The brakes produce friction, which is greater than the force of gravity. If you release the brakes, the car will roll downhill and the potential energy will be converted into Kinetic energy. The word kinetic means motion, so the faster the car rolls, the more kinetic energy it has, and the less potential energy it contains (Figure 2), A typical fan wheel, driven by an electric motor, creates pressure and flow because the rotating blades on the impeller impart kinetic energy to the air by increasing its velocity, The air leaving the fan contains air pressure (energy) in two different forms: © Static pressure (Sp) © Velocity pressure (V,) Commercial Duct DesignPOTENTIAL ENERGY vs. KINETIC ENERGY Potential Energy (Static Pressure) Kinetic Energy (Velocity Pressure) 2 As the high-velocity air exits the fan, the total pressure consists mostly of velocity pressure (Figure 3), This velocity pressure begins to convert into static pressure in the first few feet of supply air duct, Both velocity pressure and static pressure exist throughout the entire air system. Whenever the duct changes cross sectional flow area, there is a conversion of yelocity pressure into static pressure. Velocity Pressure O jason Static pressure is a force that is exerted against the sides of the duct wall equally in all directions (Figure 4), Static pressure is essentially the potential energy component in the air. ’ Velocity Pressure 4 An inflated balloon contains all static pressure If you release an inflated balloon, the static pressure inside creates a flow of air out of the balloon, which is actually converting the static pressure into velocity pressure, The sum of the static pressure and velocity pressure values is called the foral pressure and is represented by the following equation: ip + Vp Daniel Bernoulli, a 16th century Swiss mathematician, physician, and physicist, developed a concept now known as Bemoulli’s Law. Bemoulli’s Law simply states that: whenever there is a change in velocity there is a corresponding and inverse change in static pressure. In other words, when velocity increases, static pressure decreases by the same amount, causing the total pressure to remain constant. Likewise, a decrease in velocity causes an increase in static pressure. This increase in static pressure is also referred to as static regain Total pressure, however, always decreases in the direction of airflow due to friction losses in the uct, This principle is illustrated in Figure 5. ‘Commercial Duet DesignFor example, let’s assume that the area of duct section © is 1,0 ft” and that the area of section Disoon According to the laws of physics and fluid flow, the velocity (V) of a substance, in this case air, is equal to the flow quantity (Q) divided by the cross-sectional area (4) of the flow conduit, in ‘our case the duict. This can be stated as follows: For airflow, Q is cubic feet per minute (CFM), V is velocity (ft/min.), and 4 is ft. Referring to Figure 5, if we assume that the airflow rate is 1,000 CFM, we can calculate the velocity in both sections of duct as follows: Vie £000 ft/min / 1.0 f= 1,000 ft/min, V2 = 1,000 ft/min / 0.6 f= 1,667 ft/min, Velocity pressure (Vp) for air at standard conditions (density = 0,075 Ibp/ft') may be calculated by the following equation: ry Pete 7 ( DOs “ A 10000 000 Ree x rN nods ooo Cen SST XY) 000 ‘000 } SAAR soo 3000 5000 = 1000 og 8 000 sooo & 3 a 8 300 100 2 1000 1000 80 700 00 00 400 409 00 300 \80 180 wo |- oo ‘0 0 so +0 0 meee cece it 40 Let it EEN i op 03 04 0 06 08 al QS 02 03 04 0506 a8 10 15 20 30 40 S060 FRICTION LOSS (IN. WG PER 100 FT OF EQUIVALENT LENGTH) ‘Commercial Duct Design 8However, our friction rate has increased to 0.15 in, w.g./100 ft. E.L. As a designer you will have to decide if this higher friction rate is acceptable. For short duct lengths it is probably not a significant factor, When designing duct systems, however, it is best to stay as close as possible to the initial design friction rate, in our ease 0.10 in. w.g/100 ft. E.L. Velocity Pressure Conversion Line On the friction loss chart (Figure 9) there is a line running diagonally from left to right labeled Velocity Pressure Conversion Line. The velocity pressure may be obtained by reading vertically upward from the intersection of the conversion line and the desired velocity. This line is actually a plot of the velocity pressure (Vp) equation and is used to calculate the total friction loss of a particular section of duct. Fitting Losses There are two commonly used methods for determining the friction losses due to duct fittings (elbows, transitions, etc.): the equivalent length method and the total pressure method The focus of this publication will be on the total pressure method. The equivalent length method is discussed briefly; however, you should refer to other related publications for additional information, Equivalent length (E.L.) is a concept used when referring to components in the duct system other than straight sections of duct, such as elbows, ‘transitions, and fittings. The component friction loss is converted into the same (equivalent) loss as a section of straight duct, For instance, a rectangular elbow may have an equivalent length of 18 feet of straight duct. Let’s assume that the design friction rate for the system is 0.12 in. w.g/100 ft. ELL. By converting the friction losses of these components into an equivalent Tength of straight duct, you can add them directly to the actual length of straight duct to obtain the total equivalent duct length. To calculate the total friction loss of the duet system, including fittings, multiply the friction rate times the total duct length (f x L) as ated in Figure 10, 15 12in. wo 00 8 ‘Total Length = 18° + 8+ 18°=47 20.121, wa /100" x47 = 0.089 in. wo. The equivalent length method is widely used for residential and light commercial applications However, for larger, more extensive commercial duct systems, the total pressure method is recommended, The total pressure method gets its name from the fact that you are calculating the total pressure change across the fitting, As duct sizes change, throughout the system there is an exchange between static pressure and velocity pressure. ‘These pressure losses are referred to as dynamic Josses, and are caused by either a change in flow direction (as with an elbow) or a change in cross sectional flow area (as with a transition), The dynamic loss coefficient, abbreviated C, is a dimensionless number representing the number of velocity (pressure) heads lost through the duct fitting, These values have been determined through laboratory testing and are published by ASHRAE and SMACNA. A partial listing of fitting loss coefficients is included in the Appendix. Commercial Duct DesignTo arrive at the total pressure of a fitting loss, simply multiply the loss coefficient, C, by the velocity pressure at the fitting as follows: Tp=Cx¥, Where: Tp= Total pressure loss (in. w.g.) C= Fitting loss coefficient ¥, = Velocity pressure (in. w-g.) The velocity and corresponding velocity pressure may be obtained using the equations as introduced in the previous section. Example 2~ Fitting Loss Calculation Note: The Fitting Loss Table is partially reproduced in the Appendix with the permission of SMACNA. Calculate the total pressure loss of the fitting indicated in Figure 1 (diverging, symmetrical, rectangular, 90° wye fitting). wx" Ayy = 0.69.89 (4, =500 CFM ZA x10" c= 1.3959 fh 0, = 1,000 CF 10x10" ay = 0.69 54 ft (yy =500 CFM “ To calculate the velocity pressure for any fitting in SMACNA Table 14, pay particular attention to the notes. For most converging-type fittings, use the downstream velocity pressure. For most diverging fittings, you always use the upstream velocity pressure (see Figure 12 for examples of converging and diverging fittings). CONVERGING FLOW DIVERGING FLOW 2 Cross sectional areas and flows are indicated. The C value is obtained by referring to the fitting loss coefficient table included in the ‘Appendix under a Type “Y” fitting (Table 14- 14). For a @ = 90° wye fitting, the loss coefficient is read as 1,0, Since the flow through each branch is the same, the pressure losses are also identical. First, solve for the velocity: V=QIA; 1» = Von = 500/0,69 = 725 fpm and Ve = 1,000/1.39 = 719.4 fpm Now solve for the velocity pressure: Vpo= (We/4,005) = (719.4/4,005)° = 0.032 in, w.2. Finally, we solve for the fitting total pressure loss: Tp=Cx Ve=1.0 x 0.032 =0,032 in, w.g. ‘Commercial Duet DesignFriction Losses in Straight Duct Duct friction loss is affected by the following: * Airvelocity @ Duct size and shape * Duct material roughness factor © Duct length ing any one of these variables will affect the air friction loss. Other synonymous terms are often used when referring to friction loss in air systems such as “air pressure drop”, “static * and “coil pressure drop”. In most cases these terms are all used with reference to units of inches of water gauge (in. w.g.). Duct material roughness refers to the inside surface of the duct material, The rougher the surface, the higher the friction loss. Most duct, sizing tables use the roughness factor for smooth, galvanized sheet metal as the reference value, For other duct construction materials such as duct board, flexible duct or duct liner, a multiplier is used to correct for the higher value. A duct roughness multiplier table is included in the Appendix. Example 3 ~ Material Roughness Correction What is the correction factor for converting from smooth, galvanized sheet metal to rigid fiberglass duct board for a supply air system? Using the Duct Roughness Multiplier Table 4 in the Appendix, the correction factor is read as 1.32, In other words the friction rate for rigid duct board is 32% higher than smooth sheet metal duct per given length. This correction factor is multiplied by the design friction rate prior to selecting the required round duct size. For instance, in the previous Example | for the 18 in, duct, the friction rate was 0.10 in, w.g/100' E.L. For rigid duet board the design friction rate is 0.132 in, w.g./100 fl. E.L, (1.32 x 0.10), If you desire (0 use a design friction rate of 0.10 in. w.g/100 ft. E.L., and you are using rigid duct board, then you must determine the required friction rate for the smooth, round duct that is equivalent to a friction loss of 0.10 in. w.¢/100" E.L. To arrive at this value, simply divide the desired friction rate by the correction factor as follows: (0.10 in. w.g./100 ft. B.L,) / (1.32) = 0.076 in. w.g./100 ft. ELL. Now you use this new friction rate for sizing all duct sections that are to be made from rigid duct board. In effect, you are increasing the size of the duct to account for the higher friction loss per unit length, Round vs. Rectangular Duct of Equal Friction Rate Ata given velocity, round duets have the lowest static pressure drop per linear foot of any duct shape. Figure 13 indicates the relationship between duct shape and frietion loss, Notice the ratio of perimeter to area is the lowest for a round duct, Less duct surface area translates into less air ftiction loss. As you increase the aspect ratio (longest dimension divided by the shortest dimension) the ratio of perimeter to area increases proportionally. The last example illustrates a duct with an aspect ratio of 9:1. This is a highly undesirable shape for an air duct, as the friction loss is nearly twice that of a square duct of the same cross-sectional area. In addition, ducts with large aspect ratios have more heat gain than smaller aspect ratio duets. ul Commercial Duct Designfata Fin equa ais Pete 5 toArea | Round Duct | 15,090 cr 40.7 in. } 9.032 VO65 fH. | 1.18: 40.7 in. 07 in, 100 ft it lade ven | ra | asain, | aecvtoon aa RE hae ran | 14st | sare, | onsitoon ain feng, _joone ssn. | t67 | aren, | sisnvtoon i gu donne | mon | azar | aes | worn 43 Large aspect ratio ducts are more difficult to reinforce structurally and may exhibit what is commonly referred to as “tin canning”. Tin canning occurs when the fan tums on and off and the duet walls actually flex in and out due to the air pressure changes inside the duct, causing a loud, disturbing, thumping sound. Since it hhappens each time the fan tus on or off, you can imagine what the occupant’s reaction will be As a general rule of thumb, duct aspect ratios should be limited to no more than 4:1. Round ducts are often used for branch duets off rectangular main ducts. Round duet unfortunately requires a larger height clearance when compared to rectangular duet. This is illustrated in Figure 13, with a 3 ft. x 3 fl. square duct having a cross sectional area of 9.0 fi’, The equivalent round duct diameter for the same area is 39.4 in, This means that an additional 3.4 in, of height is required when using a round duct for this particular example. Saving space is one reason many designers use rectangular trunk ducts Round Duets of Equal Velocity and Equal Area Duct systems are generally sized first as round ducts, Then they are converted to rectangular sizes of equivalent friction rate, Most friction charts are constructed for round duct (see the friction loss chart in the Appendix). You cannot simply calculate the area of the circle (round uct), then use a rectangular duct with the same cross-sectional area. Doing so would create a duct with a higher pressure drop than the round duct of the same area. This goes back to the ratio of perimeter to area shown in Figure 13. The velocity in a rectangular duct with equivalent friction rate will be less than the velocity in the round duct, This is necessary so that the pressure losses for the two ducts are equal To accomplish this conversion from round duct to an equivalent rectangular duct, refer to Table 1 in the Appendix. Commercial Duct Design 2Example 4 — Converting From Round to Equivalem Rectangular Ducts Convert a 22 in. round duct to an equivalent rectangular duct with a maximum aspect ratio of 3:1. In Table 1 of the Appendix you will find the rectangular duct dimensions listed along the top and left side scales, Now the trick is to find a combination of dimensions that meet the criteria stated above; that is, a maximum aspect ratio of 3:1 and a 22 in, round duct. Refer to Figure 14. Note there are ‘two numbers under each column, area (sq. ft.) and diameter (in.). Scanning down the first column (6 in. side) you are looking for a diameter of 22 in. I's not under the first column, as the largest diameter is a 16.5 in. duct. Now look under the 8 in. column, There is a 22 in, diameter duct listed for the 64 in, side duct. However, the 8:1 aspect ratio (64/8) violates the 3:1 aspect ratio limit, Continue over to the 10 in. column, Read down till you see the 21,9 in, diameter, This equates to a 46 in, duct, which makes our aspect ratio 4.6:1, still oo large. Read over to the 12 in, column, then read down to the 21.9 in, diameter corresponding to a 36 in. duct. ‘That works, as the aspect ratio is 3:1 (36/12). There are other combinations of values that also work, such as 14" x 30", 16" x 26", 18" x 24" and 20" x 20", Designers generally attempt to use the larger dimension for the width and the smaller dimension for the height. This is due to the fact that available space to run duct above most ceilings is usually in short supply. Also, try to maintain one of the duct dimensions (either height or width) whenever possible when transitioning fiom one duct size to the next, This saves money during fabrication by simplifying the transition, Other Pressure Losses There are additional factors that affect duct system pressure losses that relate to the physical characteristics of the fan inlet and outlet conditions. Fan Outlet Static Pressure Loss or Gain Fan outlet static pressure loss or gain must be included in determining the required fan static pressure, The SMACNA fitting loss. tables included in the Appendix contain loss coefficients that account for various orientations, such as converging or diverging flow at the fan discharge. ‘Commercial Duet DesignOF RECTANGULAR DUCTS FOR EQUAL FRICTION. CIRCULAR EQUIVALENT DIAMETER,* EQUIVALENT AREA AND DUCT CLASS} Commercial Duct DesignSystem Effect A fan/duct system with an improper fan outlet ot inlet connection will result in a reduced airflow quantity, less than the manufacturer's published ratings. Fans are generally rated, designed and tested with open inlets and with a section of straight duct connected to the outlet of the fan, In reality most installations do not have open inlets or adequate lengths of straight duct attached 10 the fin outlet. This condition is commonly referred to as system effect (see Figure 15), System effect is a de-rating factor used to predict the reduction in actual fan performance caused by these physical limitations placed on the fan system. When there is less than 100% effective length of straight duct directly off the fan outlet, like an abrupt transition or elbow, system effect should be considered, The effective duct length depends on the particular characteristics of the fan design, but in general is a length of duct whereby the velocity of the air flowing inside the duct reaches a uniform velocity profile, This is generally defined as a length of 2-1/2 duct diameters for ducts with velocities of 2,500 fpm or less. LAST AREA, PiscHance: Buet \ } Se FAN HOUSING er 15% CENTRIFUGAL. 100% EFFECTIVE DUCT LENGTH TO CALCULATE 100% EFFECTIVE DUGT LENGTH, AS- SUME A MINIMUM OF 2 DUCT DIAMETERS FOR 2600 FPM Of LESS.ADD 1 OUCT DIAMETER FOR EACH AGDL TIONAL. 1000 FPM, EXAMPLE: 5000 FPM = 5 EQUIVALENT DUCT DIAMETERS. IF DUCT IS RECTANGULAR WITH SIDE CIMENSIONS a AND b, THE EQUIVALENT DUCT DIAMETER IS EQUAL TO Commercial Duet Design‘A considerable amount of subjective judgment must be applied when working with system effect factors, as there may be a wide variation in different manufacturer’s fan designs. It is important to check for system effect in the total pressure analysis of the fun, The reader should refer to ASHRAE and SMACNA publications for additional information on system effect Wind and Stack Effects The effects of wind blowing directly into building air intakes or exhausts should be considered, Intakes and exhausts should always be located away from prevailing winds. Infiltration also has a negative effect by imposing an additional load on the cooling or heating system since unconditioned outside air is, introduced directly into the spaces. In tall buildings or in instances where the outside temperature is colder than the inside temperature, air tends (o move from the lower areas to the higher areas. Natural paths are via elevator shafts, equipment chases, and stairwells. This movement, of air ig referred to as normal stack effect. During the summer, or when the outside temperature is ‘warmer than the inside temperature, the opposite effect can occur, referred to as reverse stack effect. Since cool air is heavier than warm air, the building air tends to drop. You may have experienced this phenomenon when exiting an elevator of a tall building. SMACNA and ASHRAE publications list various formulae and have examples of how to calculate stack effect. It is mentioned here only to bring it to the reader's attention. This concludes the introduction and the fundamentals of airflow through ducts. It is recommended that you complete Quiz. I located at the back of this book in order to test your understanding of these important concepts. Commercial Duct DesignSTEPS REQUIRED TO DESIGN AN AIR DUCT SYSTEM Before we begin sizing ductwork, a few preliminary tasks are required, Figure 16 indicates the steps required to design an air duct system. AIR SYSTEM DESIGN STEPS @ Dotormine Number of Zones @® Perform Cooling & Heating Load Estimate @ Detrmine Rone Aiflow Guanes (CF) @® Select Duct Materia! (Sheet Metal, Fiborglass, Flex) © Loca Gils & Reis and Lay Out Ductwork ® Summarize Duet Airflow Quantities (CFM) ® Size Ductwork From Fan Out to Extremities: @® Calculate Air System Pressure Lossas @® Selact and Adjust Fan Step 1: Zone Selection Determine the number of unique temperature control zones that will be required. Perimeter and core areas should be separated into zones depending on variations in internal loads and building exposure (N, 8, E, or W). Further sub- zoning into additional control zones may be accomplished if necessary. Executive offices, conference rooms and computer rooms are all examples of sub-zoning that may be accomplished. Step 2: Cooling and Heating Load Estimate ‘The importance of an accurate cooling and heating load estimate cannot be over- emphasized. Computer software programs are readily available, inexpensive, easy-to-use, and represent the best method of performing an accurate cooling and heating load estimate. A computer-calculated load estimate projects a professional image of your company to your customers or clients. Whether it’s fact or fiction, @ computer printout is usually accepted at face value and lets the customer know that you take the extra steps necessary up-front to ensure that the job is done accurately and correctly. OF course, a computer program does not replace years of experience and should only be lered another tool in your toolbox. ‘An inaccurate load estimate results in oversized or undersized equipment, leading to an inadequate, poorly performing system and likely an unhappy customer. Step 3: Determine Room/Zone Airflow Quantities The results of a load estimate calculation include the airflow quantities required for conditioning each of the rooms or zones, The sum of these Zone airflow quantities is the total amount of air the fan must deliver, also referred to as the total supply air quantity. This value is used to size both the fan and the first section of supply duct off the fan, ‘The subject of load estimating includes a concept known as the block load. The block load is simply defined as the maximum cooling load for the entire building that occurs throughout the year. For most residential and light commercial buildings located in the Northen Hemisphere, this maximum load occurs late in the afternoon during the hottest summer month, typically 4:00 PM in July or August. In addition, computer programs generally calculate a room-by-room cooling load and coincident room airflow quantity. These room-by-room airflow quantities are used to size the zone supply diffusers and return grilles, as well as the branch ducts that serve them. The sum of these airflow quantities is used to calculate the main duct section sizes. Commercial Duet DesignLoud estimating and room air distribution principles are covered in greater detail in other related publications, The scope of this program will be limited to the task of duct sizing. Step 4: Select Duct Construction Materials After calculating the cooling and heating loads, the designer decides which materials and shape of duct to use. Common duct construction materials used in residential and light commercial applications include: # Fiberglass duct board © Galvanized sheet metal with either internal lining (Liner) or external insulation (wrap) © Flexible vinyl-coated with helical wire core Most commercial applications use galvanized sheet metal for the main duets and either flexible duct or sheet metal for the branch ducts. Flat oval spiral ductwork is used on high-pressure systems and will not be covered in this text. There is generally a trade-off between duct material and price. Fiberglass duct board is relatively inexpensive and easy to install; however, the pressure loss is approximately 30% higher than that of smooth galvanized sheet metal, Flexible (flex) duct is easy to install but exhibits a much greater (three times higher) pressure loss per linear foot as compared to smooth sheet metal, And that is when the flex duct is hung straight, If flexible duct is allowed to unnecessarily sag and bend, the pressure loss will be much greater. Poorly installed flexible duct is one of the most commonly encountered field problems and should be avoided. Duct insulation may be internal (also referred to as duct lining) or external (also referred to as duct wrap). Rectangular duct dimensions are expressed in outside dimensions, regardless of whether there is an intemal lining. Round duct dimensions are always assumed to be inside diameter (LD.). Internal fining is sometimes used in the first section of duct after the fan to attenuate noise generated by the fan, As mentioned before, internal lining has a higher pressure loss than smooth sheet metal and has been the subject of some academic discussion relating to indoor air quality (IAQ). Some in the scientific community claim that the porous nature of fiberglass duct liner creates an environment to harbor and incubate the growth of mold spores and other microorganisms. However, this is not generally a problem, provided adequate air filters are used {0 prevent the accumulation of dust and dirt on the internal duet wall surfaces. Some designers choose to externally insulate (wrap) duct surfaces instead of lining the interior, This prevents the fiberglass from coming into contact with the supply air stream and to facilitate cleaning of the duct wall surfaces. The designer should be aware that unlined ductwork has the potential to be noisier, and proper acoustic design measures should be taken to prevent this from occurring, One final comment about duct insulation; remember {0 allow extra room for the duct liner or insulation, Duets with a 1” intemal liner must be sized 2" larger on each dimension (height and width) to account for the liner. In other words, a 10" x 12" duet with internal liner must be fabricated 12" x 14" to accommodate the liner. Likewise, when using externally-wrapped duct be sure to allow for the extra height or width required. Step 5: Locate Gi Out Duct System Jes and Registers and Lay Laying out the duct is simply producing a sketch of where to route the duet from the fan to each of the zones, As the old saying goes, a straight line is the shortest distance between two points. Therefore, unnecessary tums and bends should be avoided. As a general rule, every elbow adds, the same air pressure drop as 15-20 feet of Commercial Duct Design 18straight duct. Try to allow at least a few feet of straight duct directly downstream of the fan before you make any turns or take-offs, The air in the duct is very turbulent in the first few feet of duct downstream of the fan. The air needs a few feet of straight duct to establish a uniform velocity profile so that all of the energy from the fan can be converted into pressure, Any energy not converted into pressure becomes turbulence and vibration, which will likely lead to a noisy system and customer complaints, As mentioned in the previous section, system effect will occur if a uniform velocity profile is not produced prior to an elbow, If it is necessary to have an elbow close to the fan, always turn the air in the same rotation direction as the fan Draw a preliminary single-line schematic of the duct system from the air handler(s) to each of the zones by the most efficient and economical route, avoiding unnecessary tums and bends. A semi-transparent paper such as “onion skin” or “velum” should be used to sketch the duct system over the architectural floor plan, Be sure to refer to reflected ceiling plans and structural drawings to avoid interference with plumbing, sprinklers, lighting, and structural beams. Label the duct sketch with numbers and/or letters identifying every point on the duct system. where there is either a change in flow quantity or a change in direction. This includes all transition fittings, elbows, branch take-offs and the connection to the air handler, Transfer these duct section labels to an appropriate worksheet (refer to the Duct Design Sizing Worksheet in the Appendis). Select supply outlets in each zone based on the zone airflow quantities calculated in the load estimate performed previously. Supply outlets should be selected based on their published and rated airflow quantity, pressure drop, and throw. ‘To minimize heat gain and loss, duets should not be located in extreme temperature areas such as hot attics whenever possible. If you must route duct through extreme temperature environments, be sure to use adequate insulation to minimize thermal losses. Don’t forget the return air duct system design. Return air grilles may be located in each zone or alternatively located in a central area such as a hallway or corridor. If you are locating the return air grille in a central area, be suze that the supply ait is able to escape areas with closed doors. Undercutting of doors or the use of transfer grilles may be necessary. A tightly sealed room will not receive adequate supply air. Some buildings with multiple floors use a ceiling plenum return for the return air, therefore a ducted return air system may not be required Generally, ceiling plenum retums are used only on intermediate floors of a multiple story building and not in top floor attic areas due to the heat gain from the roof (Figure 17). 19 Commercial Duet DesignFoot Heat Sa 7 vm %@ te Cling Plenum Return Ducted Retuin Air Supply and return outlets should be located to provide adequate room air distribution to the zones, Room air distribution is covered in another related publication. Step 6: Summarize Duct Airflow Quantities (CFMs) As discussed previously in Step 2, the zone-by- zone load calculation will yield the required airflow quantities, Beginning at each zone, working against the direction of airflow back toward the fan, sum the airflow quantities from each outlet to the branch duct sections then finally back to the main trunk duct, finishing at the fan outlet, This total airflow quantity should equal the amount calculated by the load estimate Finally we get to the fim part. The next task is to determine the required sizes of supply and return ductwork that will deliver the correct amount of air to the spaces, Beginning with the first section of duct off the fan, this section is sized based on an assumed initial velocity. Subsequent duct sections should be sized by the appropriate sizing method, from the fan out to the most distant points, The equal friction and static regain methods of duct design are illustrated in the following sections. Do not over-design the duct system by systematically downsizing each trunk duct section after each branch take-off. Often it is more cost effective to maintain a constant duct size for several branch runs. Duct transitions are expensive to fabricate and costly to install. Once the duet sizes and transition fittings are known, the designer can then determine the total pressure that the fan must overcome to deliver the required airflow. Transition fitting losses are covered later in a separate section of this manual. You should always use a Duct Design Sizing Worksheet, like the one found in the Appen to organize your calculations. Step 8: Caleulate Air System Pressure Losses Once the duct sections have been sized, you may summarize the air pressure losses from both the supply and return duct systems. This is often referred to as the external static pressure because these losses are “external” to the air conditioning unit. Manufacturers generally publish fan ratings and fan curves based on extemal static pressure; that is, internal losses due to coils, filters, ete. are accounted for in the cataloged fan ratings. ‘Commercial Duct DesignDetermine if calculated duet sizes will fit into the allowable space. If not, adjust sizes or aspect ratios to fit. Do not forget to allow space for either intemal duct Jiner or external duct insulation, Step 9: Select and Adjust Fan In Step 8 we summarized the static pressure losses for the duct system. To this amount we add any pressure losses due to filters, dampers and any other components in the duct system to arrive at a value referred to as the total static pressure. This total amount is the pressure that the fan must overcome to deliver the required amount of ait. This total amount is also used to select the required fan motor size. Once the fan is selected and installed, an air balance should be done. This involves measuring the fan speed (rpm) and total pressure drop across the fan as well as measuring and summarizing, the airflow at all of the diffusers. These measured values should be compared against the required values from the cooling and heating load estimate, Balancing dampers are adjusted to attempt to deliver the correct amount of air to cach zone. Often the actual airflow being delivered is less than or greater than required and the fun pulley (sheave) may require adjustment or resizing, DUCT SIZING METHODS The two most common methods of sizing subsequent duct sections in use today are: © Equal friction © Static regain While other duet sizing methods exist (such as the T-method, velocity reduction, total pressure, and constant velocity methods), they are not widely used and are not included in this text Readers interested in learning more about these other methods should consult ASHRAE, SMACNA, ACCA, or other related publications, With the equal friction method, as the name implies, ducts are sized for an equal (constant) friction loss per unit length. In its purest form, this uniform friction loss per linear foot of duct is held constant for the entire duct system. The equal friction method is the most widely used method for sizing lower pressure systems. This method automatically reduces the velocity of the air in the direction of flow. Therefore some “regain” of static pressure is created, however it occurs in unknown amounts and is not usually accounted for. Once initial sizes are calculated, the total pressure of all sections should be calculated and noted. Sections should then be resized to equalize the pressure at all junctions, This is demonstrated later in the example problem. ‘The equal friction method is generally used when sizing retum and exhaust systems, The primary disadvantages of the equal friction method are: There is difficulty in balancing branch flow rates, even with balancing dampers 2. It cannot ensure a uniform, constant inlet pressure to variable air volume devices and terminals. To overcome these disadvantages, modifications to the equal friction procedure have been made. This is sometimes referred to as the modified equal friction design procedure. This procedure is used whenever the duct system is non- symmetrical or for systems with both long and short duct runs. ‘The modified equal friction design procedure is described in detail later. The static regain method of duct sizing is a more complex and sophisticated method and may be used to size supply duct systems of any pressure 21 Commercial Duct Designor velocity, The advantages often outweigh the extra. work involved in performing the calculations, In this method, the velocities are systematically reduced, resulting in a conversion of the velocity pressure into static pressure, which ‘overcomes a portion of the ai tion loss in the next downstream section of duct. This phenomenon is not totally efficient, however. A regain efficiency of 75% is assumed for most average duet systems. This method is also advantageous because it creates a self balancing system, in that all gains and losses are proportional to the velocities. This makes it especially usefull for higher-velocity systems, where the turbulence and noise generated by air balancing dampers prohibits theit use. The main disadvantage of the static regain method is the larger duct sizes that result. This translates into higher first cost; however, the reduced static pressure on the fan often results in a lower fan operating cost over the life of the system Note: The SMACNA fitting loss coefficient tables included in the Appendix account for all static regains or losses that may occur. You should not add additional regain or losses 10 the system due to fittings or trans ms. EQUAL FRICTION DUCT DESIGN PROCEDURE This method assumes that the user has already completed the required steps as outlined in the previous “STEPS REQUIRED TO DESIGN AN AIR DUCT SYSTEM” section. Refer to Figure 18 for illustration ro 40" x 20’ Fan Outlet Supply Grille Loss = 0.10 in. wg. 18 The following design procedure is recommended ‘when using the equal friction method: The first section of duct downstream of the fan is not sized by any particular method (equal friction or static regain). Rather, it is sized based on an assumed initial velocity. Designers sometimes size the first section of duct to be the same size as the fan outlet connection, provided the outlet velocity is within an acceptable range. Remember to try to provide several feet of straight duct directly off the fan to prevent system effect from occurring. Recommended and maximum velocities are listed in Table 2 of the Appendix. Remember that the friction loss values that appear on the friction chart are for round, galvanized, smooth sheet metal duet. If you are using a duct material other than galvanized sheet ‘metal, you will need to correct the friction factor indicated. This is done by applying a material correction factor, which is listed in the Appendix. If you are not using round duct you must first size the system as round duct then convert the round sizes to equivalent rectangular dimensions by using the conversion table (Table 1) included in the Appendix Commercial Duct DesignThe use of flex duet should be minimized and used in no greater than 8-10 lengths since the equivalent pressure drop of flex duct is over three times that of galvanized sheet metal, In the example duct system shown in Figure 18, we have a 40" x 20" fan connection, Since most fan outlets are rectangular in shape, you will generally have to supply a transition from the fan outlet dimensions to the dimensions of the first duct section. Let’s assume that we will be using rectangular, smooth shect metal duct for this system. From Table 2 in the Appendix, for a general office we will use a maximum velocity of 1,500 fpm, which is the controlling factor for noise generation. The total airflow in section @ is 7,500 CEM (2,500 3). Since we now know the velocity and airflow we can solve for the area: A= O/V=7,500 CFM / 1,500 fpm = 5.0 f? Now enter Table I (Circular Equivalent Diameter) in the Appendix and locate a combination of duet dimensions with an area equal to or greater than 5.0 £°, From Table | we find that there are many possible combinations of duct dimensions with an area of 5.0 £, Since the fan outlet is 40" x 20", let’s check the velocity using the same size duct as the fan outlet. From Table I for a 40" x 20" duct, the equivalent area is 5.07 ’, which is adequate, Therefore, a transition from the fan to duct seetion @ is not required and there is no fan outlet gain or loss to consider. In Table 1, under each column of duct dimensions, there is a value labeled “Diam, in.”. This is the diameter for a round duct with an equivalent friction loss. For a 40" x 20" rectangular duct, the round equivalent is 30,5". Next, on the friction Joss chart, locate the 7,500 CEM fine on the vertical axis. Follow this line to the right until it intersects with the 30." round duct line. Now read vertically to determine the friction loss value, in our ease 0,085 in. w.g/100 ft. E.L. This means that for a flow rate of 7,500 CFM, for a 30.5" round duet, the friction loss for a 100 fl. length section will be 0.085 in, w.g. But section ® is only 40 fi. long. The corresponding friction loss for section ® will therefore be: 0.085 x (40/100) ~ 0.034 in, w.g. This corresponds to a velocity of approximately 1,500 fpm, which may be read from the friction chart. We will use this friction rate (0.085) to size the remaining duct sections, Using the Duct Design Sizing Worksheet included in the Appendix, enter the information ‘we know thus far The next element in the system is the outlet in section ® with an airflow rate of 2,500 CFM. The outlet loss is given as 0.10 in. wg. You should consult actual grille and register catalogs when designing duct systems to determine the appropriate friction losses. Add this known loss value to the worksheet. Next we come to duet run C-D. Since the airflow quantity is reduced significantly (7,500 — 2,500 = 5,000 CFM) we need to transition to a smaller duct. To determine the required rectangular dimensions of section @ we must first find the required round size then convert it to a rectangular equivalent. Refer to the Frietion Loss Chart and use an equal friction rate of 0.085 in, w.g/100 ft. E.L., Determine the intersection of the airllow quantity (5,000 CFM) with the friction rate. The intersection point falls very close to the 26” diameter line, Now refer to Table 1 and locate a combination of rectangular dimensions that are close to a 26" diameter. ‘There are several possibilities. As mentioned previously, it is generally a good idea to try to maintain at least one of the dimensions when transitioning duet sizes. This makes the fitting less difficult to fabricate and saves money too. Let’s maintain a constant duct height of 20" and vary the duct width. From Table 1 we will select a 28" x 20" size for section ®, This size is equivalent to a 25.7 round duet with a velocity of approximately 1,400 fpm and a friction rate of 0.095 in, w.g./100 f. ELL. 2 Commercial Duct DesignBefore we continue we must calculate the friction loss due to the transition between sections @ and @, Refer to the SMACNA friction loss tables, specifically Table 14-12, Fitting Type A, which is a converging rectangular to rectangular fitting, To determine the fitting loss coefficient, C, we must know the ratio of areas between the two duct sections along with the transition angle, @, Let’s assume that the transition angle is between {5-40°. The ratio of areas (A;/A) = (5.07/3.60) = 1.4, From the table for Fitting A, the corresponding loss coefficient is 0.05 (smallest value in table). To calculate the total loss we use equations 3 and 4 as follows: Vp= (V/4005)? = (1,520/4005) = 0.144 in. w.g, Tp =Cx Vp=0.08 x 0.144 = 0.007 in, w.g. For all practical purposes, this is an insignificant amount. Altematively, we could have read the velocity pressure direetly from the Friction Loss Chart or the Velocity Pressure Table (Table 3), You should note that the total pressure loss due to a fitting is much more influenced by the velocity rather than the fitting loss coefficient, C. Had the velocity in the previous example been say twice as high, the pressure loss would have increased by a factor of four times due to the “squared” relationship between pressure and velocity, Add these values to the worksheet. The last element in duct run C-D is the outlet, with a flow rate of 2,500 CFM. Again, this loss is given as 0.10 in. w.g. Add this value to the ‘worksheet. The final duct run is D-E. Similar to the previous section we have another transition since the airflow is reduced to 2,500 CFM. We will maintain our previous duct height of 20" and locate the width required. As before, we must determine the required round duct size and then convert it to an equivalent rectangular size. From the Friction Loss Chart find the round duct size corresponding to the friction rate of 0.09 in. w.g/100 ft. E.L, A 20" round duct meets our requirements for section ®@. Next, in Table 1 locate a round duet with a 20" diameter that corresponds to a duct height of 20", A 20" x 16" duct has an equivalent diameter of 19.5 in., a round duct velocity of 1,200 fpm, and a corresponding friction rate of 0.10 in. w.g/100 fi. E.L. Add these values to the worksheet. Now we may calculate the loss due to the transition between sections ® and @. This transition is the same as the one before. Since we now know the sizes of both sections, we may once again calculate the ratio of areas as follows: (AVA) = (3.60/2.07) = 1.7. The loss coefficient, C, is the same as our previous calculation, 0.05. ‘We are now ready to calculate the total friction loss of each duct run. From the Duct Design Worksheet we see that the total section loss for duct run A-C is 0.134 in, w.g. For duct run C-D the total friction loss is also 0.136 in. w.g. Duct run D-E also has a section loss of 0,136 in. w.g. The next step is to determine the duct run with the highest friction loss starting at the fan and proceeding all the way to the furthest duct ran, or the run with the most fittings. On large systems this is somewhat of a tedious task; however, for our simple system it is easy to see that duct rin A-E is the run with the highest friction loss. Summarize the total friction loss by adding the individual section losses beginning at the fan and ending at the outlet in section C-D, This. calculates to be 0.206 in. w.g. We will round this value to 0.21 in. w.g Comparing the section friction losses, they are within 0.05 in, w.g. of each other. Therefore air balancing will be easy to accomplish by the use of balancing dampers. Commercial Duet Design 24Remember to allow room for either internal duct, lining or external duct wrap (insulation) when designing duct systems. For example, if duct is wrapped with I" insulation and you only have 12" of clearance to run a duct, the largest duct height you can use is a 10" duct. Likewise, a 12" duct with a 1" lining is actually only a 10" duct when it comes to calculating flow area This completes our calculations for the equal friction method, MODIFIED EQUAL FRICTION DUCT DESIGN PROCEDURE This method was created to allow for easier air balancing of duct systems that have non- symmetrical layouts; i.e., some short duct runs along with some longer runs. The idea is that you size the main trunk duct using the equal friction method, then slightly downsize the branch runs that have low pressure drops. This minimizes the tequirement to “choke” off the airflow (in the shorter duct runs) with manual balancing dampers. The equal friction method, by definition, means that the friction loss per linear foot remains relatively constant for all duct sections, Using this method causes the shorter duct runs, as well as those closer to the fan, to receive more air than they are supposed to receive (because the total pressure drop in those sections is lower) and ‘causes the longer duct runs to receive less air, The designer will often place manual balancing dampers in each duct run to intentionally restrict the airflow in those sections and create an additional pressure loss such that the total pressure loss for the shorter duct sections will equal the pressure loss on the longer duct sections. With the modified equal friction method, the goal is to slightly downsize the branch runs closer to the fan or those sections with lower pressure drops. This has the same effect as closing a balancing damper and choking the flow. The advantage of this method is twofold: first, the use of smaller ducts saves money, and secondly the smaller ducts allow for easier balancing of the air flows. Keep in mind, however, that downsizing ducts creates higher velocities. You should not downsize to a point where it violates the recommended velocity range values. High velocities may create unwanted noise. Finally, if you do downsize duets slightly in the shorter runs you should still utilize balancing dampers at cach branch take-off to assist in air system balancing. The most precise duet design may theoretically work on paper and may not get installed exactly as designed. It may need some “tweaking” after start-up. Both the equal friction method and the modified equal friction method of duct sizing are demonstrated in the work session. STATIC REGAIN DUCT DESIGN PROCEDURE The static regain method of duet design is more complex and time-consuming that the equal friction method. Often a trial-and-error procedure is necessary. Therefore the use of a ‘computer program is highly recommended for designing large duct systems using the static regain method. When designing a duct system by the static regain method, the objective is to size the subsequent downstream duet sections by systematically reducing the velocity (and corresponding velocity pressure), which creates an increase in static pressure that offsets the friction Joss in the next duct section. As mentioned previously this conversion of velocity pressure to statie pressure is not completely efficient. Regain efficiencies range from 75% to 90% under ideal conditions, However, most designers use 75% as the average regain efficiency. 25 Commercial Duct DesignTo reiterate, the primary advantage of this ‘method of duct design is the fact that the system ‘will tend to stay in balance much better because the pressure changes in each section are directly proportional to the velocity changes. This makes, it especially useful for designing variable air volume (VAV) systems. The disadvantage is the fact that the ducts tend to be larger on the end sections, which adds to the installed cost, Let’s redesign the simple duet system shown in Figure 18 using the static regain procedure. Once again, the objective is 10 oversize the downstream duct sections to create a regain of static pressure that will help offset the friction losses in the next section. First, section ® will remain the same size as before, 40" x 20", but we will increase the sizes of sections @ and @ to reduce the velocity pressure and increase the available static pressure in those sections, Section @ was previously a 28" x 20" duct, Let’s increase the size to a 30" x 20" duct. From Table 1 we determine the equivalent round diameter is 26.7 in, From the Friction Loss Chart we read the friction rate as 0.08 in, w.g/100 ft. E.L. and the velocity as 1,320 fm. Add these values to the worksheet, Next we proceed to section ®. Previously it was sized as a 16" x 20" duct so let's upsize to an 18" x 20" duct. From Table 1 we read an effective round diameter of 20.7 in. From the Friction Loss Chart we read the friction rate as 0.075 in. w.g./100 fi. E.L. and a velocity of 1,070 fpm. Enter these values on the worksheet. Next we need fo account for the transition fitting, between sections ® and ®. The ratio of areas (AWA) is (3.89/2.34) = 1.6; therefore, the C-value is 0.05 as before. ‘The final element in the duct system is the outlet at the end of section ®. Add the information to the worksheet, ‘We are now ready to sum the pressure losses as we did before. The results indicate a total extemal static pressure of 0.19 in, wg, approximately 10% less than the equal friction ‘method, which was 0.21 in, wg To reiterate, the larger duct sections will be more expensive to fabricate and install, Also, lower velocities mean slightly higher duct heat guins, although in our simple example it would ‘de negligible. On large systems, you will have to calculate the fan energy savings and compare that to the higher initial installed cost. Since ‘most duct systems last at least 20 years and fans consume approximately 30% of the total building energy, on average, there may be a sound economic argument to be made for using the static regain duet design method. SMACNA and ASHRAE have developed the fitting loss coefficient tables included in the ‘Appendix, It is very important to understand that these fitting loss coefficient values include any static pressure gains or losses that may occur at various flow conditions. A loss coefficient with a negative number (-) indicates a regain, and a value with a positive number (+) represents a pressure loss, The designer should not use any additional regain values, other than the ones indicated in the fitting loss coefficient tables, when performing friction loss calculations. This completes the discussion of duct sizing methods, You should now complete Quiz II to test your knowledge of these concepts Commercial Duct Design 26WORK SESSION | EQUAL FRICTION DUCT DESIGN We will now put to use some of the skills that we have learned thus far to size a duct system using the equal friction method. ven: The fast food restaurant shown below will be served by three packaged rooftop units. For this example ‘we will only be designing the 12-1/2-ton duct system serving the South exposure seating area. Unit airflow rate: 5,000 CFM Supply duct connection size: 14" x 28" Supply diffuser pressure loss: 0.10 in. w.g Return duct connection size: 12"x 36" Return air grille pressure loss: 0.06 in. w.g. Task 1: Size the supply and return duet system using galvanized sheet metal duet with a 2" spray- coated liner on the main supply and a 1" spray-coated liner on the return duct, Use pre-insulated flex duct for the branch duct runs. Use a design friction rate of 0.10 in./100 ft. E.L. Task 2: Determine the total static pressure loss for both the supply and return duct systems. FAST-FOOD RESTAURANT 3 ‘Commercial Duct DesignWORK SESSION DUCT LAYOUT FAST FOOD RESTAURANT ; os | cls 1 | ef S ar ee a @ C2] 0D] rettentn Not To Scale ‘Commercial Duct Design 4WORK SESSION I! STATIC REGAIN DUCT DESIGN We will now work through a static regain example to illustrate the differences in the two methods, Refer to the example problem in Work Session I. Task 1: Size the supply and return duct system using the static regain method, Compare and contrast the results with the equal friction method used in Work Session I. 35 ‘Commercial Duct Designrz eS a SE APPENDIX, Chart 1 — Friction Loss Chart for Round Duct Table 1 — Circular Equivalent Diameter, Equivalent Area, and Duct Class of Rectangular Ducts for Equal Friction ‘Table 2 ~ Recommended Maximum Duct Velocities for Low Velocity Systems (FPM) ‘Table 3 — Velocity Pressures Table 4 Duct Roughness Multipliers Table 14—SMACNA Fitting Loss Tables Duct Design Sizing Worksheet Equal Friction Example Worksheet Static Regain Example Worksheet Quiz I Solutions Quiz Il Solutions Work Session I Solution ‘Work Session II Solution References ‘Commercial Duct Design 36SSS EE eee FRICTION LOSS CHART FOR ROUND DUCT loonco SA SO ols 93, 4 GBcs ow 19 ts 2 30 80 89855566 rooo0 [> A 7 70.000 0000 | 50000 40000 0009 20000 20000 15000, 1.000 000 000 +5000 000 3 4000 4000 = & 000 sooo © 5 500 vs00 2 500 200 00 800 300 300 200 200 160 180 s0 80 "ogo oF 506 08 1 OB Of 03 Ga asas as Wi to 40 Wo e040"? FRICTION LOSS (IN. W6 PER 100 FT OF EQUIVALENT LENGTH} CHART 1 37 Commercial Duct DesignTABLE 1—CIRCULAR EQUIVALENT DIAMETER,” EQUIVALENT AREA AND DUCT CLASS} ‘OF RECTANGULAR DUCTS FOR EQUAL FRICTION. = a 1° @ we 0 [ine Dicw |Are Diem [Aves Dion [Ang Dian [Area Diam [Ares Dion [Arce Diam [Area Dizm | Area Disem foe ae |e i ane Pia | Pia” | ath Pin” |e Pin |S Pin” [ay in” | eat Ti wo | ar ea| sea] os 108 ve | 298] mas or aw] roe tara 39 waa a7 fae asa [ias tea [ver as size lias ses [ia Ago] res a7a}tarss|aaa wr ois {tae rsa [re den[ tar ina|aor s9s|238 aor| 2s 219 von sar|tse59|ta2 ize [to 10a |aar2oa|asr 27 [zee aa | air 24 Py Vie tas [tao sean aie tsalzar aia [am aa lam 239/342 2s. En 1a aae iat va| ie tala ma |aas mar [sey ass [ass aa art ast 2 va ass [wigs wz [am —ivalaar via |aes azo [as aee aso 287 [aon ar x0 Ta jer |te2 qalam doa|aee 20 [306 a7 fsae” asa]asy ae7|aay a0 2 Tie jes [toscaha|aae doa|ze 97/325 tee] an colar ars[ass 209 ae [uss sae [ise ara 20s —malauy ara 296 203 ]348 ass]ae0 267] ear malear m7 ae fis ivy fies ira [ate toa taar —aiv]an a9 [sus ase] 400 ara| ase 20] 507 305 ae {iss iso fis raja a0a]are 22s |aar 245 [390 264/490 ras lane 2a| sar 31 tar 18a |2a8 207|aea 230 aaa a1 /ae7 rol as 288 | S07 sat a tae is |zes2is|n9e anal asr ase[aas ars[ ar a0a| sar oa Me [tae iso [iss ane [asa aes|aar 2s9|az1 20s ass 2x2 |4s0 so | 555 as qe fias tea |20r 9a [aa aro aaa aas|aan er] ae 287 (510 208/576 34a ae [ise tes|200 sean a23|aas 2e8/ aos w72|aes 292/530 912/597 a3 0 brs i99|aer727[aa6 asa|ars za [ ase 29a | sat 318/619 a5 ” aa m2 |291 asi|aarase|4a0 das |soo- dos | azz salons 360 se aus wos|2ee zal 261|4as 2s | sir 305| 390 329/604 bea se 230209 [noe 23s[ aaa 265/455 209 | sar 22 | ean 334/687 ma se aaa ait |aay 22 ]a94 aso | ase wea [sas azz | 026 39/ 706 30 ro aso at|aar 245/406 27a] aoe wal sas 322/050 345|726 ass o des 220 |saa 252/424 a79| 810 906/81 321 | ear 355/770 mr aaa aca] aan 207] sar ara | eae av9|7an aca] aa wr n ner 2es[ar) aoa| sas 323/600 Mal 7se or aa % top ara [ao o00| 56 s2a|eas sal 79s 382 aa ra ais wel sir sa Rajya 364 wolea afta a8 " say ns Ses] 7s 32 inal ors zs|t08 see 4 seu 30 sea | 7a7 30 as[tor ala ase 7 s26/eo1 ase]ain sas] oae ais|t04 wal nn7 460 oy S90 50/714 aca ea 392/970 au lioa as| 121 «72 10 yao 309/850 ms|omo aas[na 4ss| 29 ae 0% 740 a7aleg aos| toa aas[us wa] 190 aa 108 720 3s0|920 412/106 440/120 Wola we a tio ase|es0 ara] tos a7| ta a75| 198 903 We su aap via ass) a6 aes faa 51a 10 too wel is asolia wtlrea sts m4 wos as| 19 467/194 elise S24 we vipa #i[ne wales 9 wa ns 79/14 so3] ise 530 16 tae ans| 14s ste | 16a 54s wo 130 wea| a7 20] tas S50 Me ta walisa 529] 168356 “hese epdraln dane) Caled tom a = 1 ‘are rambled i. i.e a= a en rs Commercial Duct Design 38SEES eee TABLE T—CIRCULAR EQUIVALENT DIAMETER,* EQUIVALENT AREA AND DUCT CLASS} OF RECTANGULAR DUCTS FOR EQUAL FRICTION. (Cont.) ” 3 2 Ey * 36 = 7 08 [ares Diem [Area Dias [Aree Dior fara Dina [Arse Diam [Aree Diem [Area Diam [Ares Din sate Tin” [tae Sim” [tae in [eet Pin sett Tin” [rit “im [etn Sin” [Aare 24 [a70 362 26 [40s ara lam 254 soi lsar_sia|s79 mae |aas 398 [ase 50 ae [sas sta [sae aa fas 3a [706 360 [rsa a 36 [588 220/594 30/432 sus |ao 35 |ras sro |yas sez leas 394 a zaleae 042 /er 353 |ra0 a7 [per seo |nar et |een aod|oaa ane So [EE Relat ea [rm sea ry ee lease lata |vae ata] oan eae fos an 4a [eas sea] 703 59 |758 37.3 |arde gee [ass me |oa1 sii|seo waa|ton aae|ine cae aa [67s 382] 734 67 |70 301 [sso 395 [sor ana |ver az (toa aas|qoa aee|tia ee 46 [70s asa /70a azaleas sao leas wos aa [730 6s] 798 ana ser|eas ua ‘eafior wa[naae[ne aa eo}a gana ass|aa ae so | x50 389 wos|ser eo aslns wilma ia|i90 ase 32 |zar 36 |oas aia |ome ae ala. oalar aaa 07 sa ‘aloe woltea ae 4is|i6 ol na 2 [ine soa) ae aa|9o a7 [ror aa asa|ia a7a|i30 ass|ta7 sod [tas a1 so fea alors slios olna ase 3/109, oa) us srliss 533 os [om uslvoa wslna alia 72 a7 lia szo| ss sra| ies a0 oo [os aslto 7 na 4e|iz8 ssa sas|ts saws saa na jis aslns wolfe galiss 7 509 sos|vaa sa@ 7 {wa 43/0 golia ola sos 563 sra|i9a 395 eo fins asa a0 sou [1a a0 78 wa] ma ein fio aes sa sna 589 wor|na ea ns 9, 4 oo eal m1 637 as ay sna a w2|n0 «0 “6 Sa sso) 7a sol ies sas eas es\ ase 685 ms a 548 ssa|ia soa 37, ws|zs 7 1st sr ass sea|i97 os eral sy a7 [am 705 rr wea [ma ari fazo wefan as7) aan ars] 262 wo fiz 4s ot exo lar wes|242667| ae apa] 272 seilies“es[21¢ exo [m2 se|as2_oea| ae ora] 202 sealz0aot1[m2a ea |ts7 oso ase ana|a7a roa] 207 ss|208 o1t|m26 age |0s aro |a6a ps | 202 720/298 alas 0a7|m0 ao lr1 @o{ ae 70a| 207 r26| 305 ne [tse s52 we [a seo wa2_| ra 05} wo [as sea “ro [a3 640/28 was |as9 elas rtalara 735] aus ta [re sar] 206 ors | any ces ran 75 [26a ons|262__p20[ a0 7a| 320 “Cheam! dance Clad em as = 13 ‘Hore tn nab re dc Zen 39 Commercial Duct DesignTABLE 1—CIRCULAR EQUIVALENT DIAMETER,” EQUIVALENT AREA AND DUCT CLASS} ‘OF RECTANGULAR DUCTS FOR EQUAL FRICTION. (Cont.) str Salma ts [ina 75) Be Sing les \gea\ees|zaeirs ia ss fea|ann a] ee ste fe rar on mtr) Caled om = 13 Commercial Duct Design 40SSS ee TABLE 2—RECOMMENDED MAXIMUM DUCT VELOCITIES FOR LOW VELOCITY SYSTEMS (FPM) SanRGline Fete ‘CONTROLNG FACTORDUCT FRICTION APPLICATION NOISE GENERATION, Mein Duets is Bor Supply, ‘Return Supply [Return Roden a 0 200 7 609, “Apartments | Atal Bogrooms 000 100 1300 100 1000 Hospital Bedrooms | Frito Often Draton Boom 1200 2000 1500 1400 1200 Ketone 00 1300 00 1000 300 General Officer High Clot Retourons 7 Hie cl te 500 200 1300 1400 1200 Senks tee 00 2000 1800 1400 200 Indu 2500 3000 100 700 700 TABLE 3—VELOCITY PRESSURES vetocry | vetocty | vaoery | veocny | vaoery | vocry | vaocry | veocmy pressure | (r/min) | pressure | iei/min) | pressure | “faymint | raeseune | “spares ina) ‘in wa) ine we) ins wal) or 0 3 Tiss 3 7055 a3 02 ses 30 aio r 300 eo om ies ey ino 3150 a0 7 ry ey 20 i200 rao a0 as a5 2 B00 3280 va i200 6 980 2330 00 Tae 70 or 1080 270 ss0 tsa 2930 8 1130 200 590, 136 ‘000 9 1200 ao 340 v80 000 i 170 2070 3499 Nee 3190 a 2500 3590 1a 3190 12 2530 3880 in 3250 3 2500 3420 176 Siro a 2590 3470 va0 Smo ns 380 710 Kee 5630 6 7650 750 7 $490 7 2830 379 1 3550 ‘ zo ano 136 +5600 3 ino 3ae0 200 3560 20 70 3020, be sro 2 7300 Th60 208 970 2 2830 co an 3230 2 iso toe ‘eo aie a 280 vo8 eo 2m a a0 aa 23 24 26 2940 Te “a1 ae 2 ar 120 38 290 um us NOTES: T Bato for aandard ar (2992 mn Hg ana 70 FL 2. Dota derived tom the following eavoton (aa) MY a tm > Coos he = pres diferance termed “veloity head” in wo! 41 Commercial Duct DesignTable 4 — Duct Roughness Multipliers DUCT MATERIAL ROUGHNESS MULTIPLIERS For internal ductwork surfaces other than smooth sheet metal, multiply equivalent lengths by: MULTIPLIER SUPPLY RETURN DUCTWORK DESCRIPTION Rigid Fiberglass — Preformed Round Ducts — 1.0 Smooth Inside Rigid Fiberglass Duct Board 4.30 Duct Liner — Airside Has Smooth Facing 4.30 Material . “Flexible Metal Ductwork (Straight Installation) 1.6 Duct Liner — Airside Spray-Coated “Flexible, VinylCoated Ductwork with Helical Wire Core (Straight Installation) INSTRUCTIONS: Multiply the measured length of each section by the appropriate multiplier to calculate the total footage equivalent to the same friction loss in smooth sheet metal ductwork. *Flexible duct multipliers assume that the duct is installed fully extended. ‘Commercial Duct Design aSMACNA FITTING LOSS TABLES TABLE 14 Reprinted by permission of SMACNA. 8 ‘Commercial Duct Designia DUCT DESIGN TABLES AND CHARTS ‘Table 14-10 LOSS COEFFICIENTS, ELBOWS (Cont) Use the velocity pressure (V,) of the upstream section. Fiting loss (TP) = © x Vs D. Elbow, Rectangular, Mitered (15) CCoatcentC (Soe Note 2—Page 14.19) ww as [os [ors [40 | 18 | 20] 30 | «0 | so ao ze | 008 | 008 | 00s [oor | oo7 | 007 | 008 | 008 | 005 005 go: | ove |Oy7 | os7 [ose | ors | avs | oa | 079 | 12 ont as | oe | 037 | 030 foe | 033 | os | 028 | o27 | ozs Ope bo | deo |osa | os7 oss | 082 | 040 | o4s | 04s | ot 038 Bet | 097 | 073 | O67 | oss | v6 os? 42 fut | ta | 099 | 092 | 089 bas SN oo |S [Sy [a E. Elbow, Rectangular, Mitered with Converging or Diverging Flow (15) © (Sse Note 2—Page 14.18) ww - lw. oa [12 [1] 16 | 20 1a ta fa faa 44 | to | as | 090 | ob 44 [om ave | o72 | 066 v0 [069 | 06s | oo | oss F. Elbow, Rectangular, Smooth Radius without Vanes (15) CCoeticionts for 80" etbows: (See Note) Cooticient & (5 rw _ - 035 [os [076 30 [40 | $0 | 60 | 00 os [1s [14 | ia F vo fur par [az | az 875 | osr |os2 | 04a | O44 | O40 038 | o40 | o42 | 043 | 048 1 | o2r |o2s | os | 021 | ore ‘18 | 038 | 020 | 027 | oat 15 | 022 | 020 | one | o17 | 015 ox4 | os | 0.46 | 017 | 017 20_| 020 | 018 | ox | ons | 014 043 | ate | 084 | 04s | 035 Raw . M TT2]Ts]* [ses [ee] ©] «| ao ao | 326 [vas | aaa | so | vos | soe | vo | co | 97 | vee | vse | tae | 138 | to | 5] to os [a 207s _| 2) us. unite ForStandard Air: (Metric Units) = 8580V (R= 684 0V) eee = Get amt. ichos (rm) 1 dct eta, pm rs) For Rectangular Duets aN De ae ew ‘Commercial Duet Design 44‘Table 14-11 LOSS COEFFICIENTS, TRANSITIONS (Diverging Flow) Use the velocity pressure (V,) of the upstream section, Fitting loss (TP) = C « Vy, A. Transition, Round, Conical (15) US. Units (Met Units) R-assov (A, 66.4 Dy) nore: = Upstream Oiamete: ches (mm) = Upsteam Voc. pm t's) Costciont G (See Note 6) 2 R fa — — 1@ | 20 | 30 | ae [ or | a0 | 100 | 100 05 8 10 2 aia | 019 | a32 1 033 | 033 | 022 | 031 | 030 4 |ozs | 020 | ove | og | ass | 064 | ces | oes & [oar | aay | oa | oes | avr | on | os | ove 10 [029 | 038 | os | o76 | oa | 099 | oes | ona 21s ost | ose | oso | oa oe | oes | oon 2 2 [oor [ov | 023 | o2e oar | az | oa 4 Joss | ois | 035 | 055 | 089 | 099 | ose | oer & Jars | ozs | oss | os | a70 | or | on | oes 19 |.020 | 024 | o43 | o76 | 000 | om: | oat | oe 218 [02s | ozs | ose | 076 | oar | obr | os7 | oar Ox 10 2 [eos | oor | ov | oar | oar | oar | oar | oz 4 for | 024 | oss | os | ose | 058 | ose | oar 5 Jars | 029 | 0x6 | og | og | a7 | 070 | 07 x0 Jozi | 033 | ose | os | ove | 40 | oes | cas 216 |oz | ose | 06 | are | ors | oes | oer | ono 'B. Transition, Rectangular, Pyramidal (15) ts 4 Wren e = 100" Costicion¢ (See Note 6) Wa. we [wo [oe |e] @ | oo | ee | ier 2 038 | 022 | 02s | o29 | or | oa | 038 | 030 4 gas | cas | oso | 086 | oe: | 089 | oss | 009 5 oa | o47 | oss | oss | ov | ove | ore | O78 210 ose | 04a | oss | a7 | 000 | oar | oes | oe ~ Aten (Entering sirsteam), Ay = Area (Loaving airstream) 45 Commercial Duct DesignTable 14-11 LOSS COEFFICIENTS, TRANSITIONS (Diverging Flow) (Cont.) Use the velocity pressure (V,) of the upstream section, Fitting loss (TP) = C x Vo G. Transition, Round to Rectangular (15) vaso 7 For Figures and 0, compute using the ‘Sauaters and id thacoaicer Grom “abe 14-18, Tanaon. Retangvax, rami. . Transition, Rectangular, Sides Straight (15) In . le | [= | | | 2 ooo] 012 | 020 | om | as | ose | oss ‘ oie | 02s | 042 | 060 | oes | o7 | 006 6 oi | oo | oa | oss | ove | om | om F. Transition, Symmetric at Fan With Duct Sides Straight (15) Coaticiont © (S00 Note 6) avA [15 | 20 | 25 | 30 | 38 [40 RIK 051010 A, a Fat ie ct (te trends op) ‘Table 14-15 LOSS COEFFICIENTS, ENTRIES Use the velocity pressure (V,) of the downstream section. Fitting loss (TP) = G % Vp ‘A. Duct Mounted in Wall, Round and Rectangular (15) i =o |osolosr [ose 092 | 10 | 10 @NE wm 2t[23 [28 (8% [23 [888 [33 [8m s annem Pee WESTER ISTE + Rectang = BEN by, Thick Edge (UD, > 0.05):C, = G + Gy Cs new oofiet of sng wih a Sreen or peroato pat tthe etrance i tom above abo i em Tabi 16-72 (screen) or Table 4-178 (pedrated plete) Note 8:4 = Area.Q ~ Alon V = Vaocty Commercial Duct Design 30‘(6% w) $S07 SUNSSSUd UNIS WARING WLOL ‘een Buu rp Jo nope pus suo}suoUIpsoueHe ave pave Stan KY 'S510N Tau] eax ens| Taens| (oa [TEA Ne wor] eee | I] 9] aa] we | | aT ss01 | ssorvonoos | emduvioay| iang”| seo1_| seo. | 967 250%] 22109 |¢u) Bue] -y809 | auneean|sonp puna mount | sno | Ime omerming | “Teo |‘ioteanba | puney | unouy |uonsis| uonses | new 507 | ‘aooren | u hiporn — 10 — 30a ‘sua 133HSNHOM ONIZIS NOISa LONG Commercial Duct DesignTO_] 48n-¥) 9507 sunssaud ous TyMKsLxa MOL Jo 19 ep sue. 8 oe potent ATE S310 f 5 2086 [or x =a av eare | ease] =e ao | e00o | Fa] wep ay aio | 0 | _* =e py woo | woo | ™ aa | ww | av woo | veo =[ pay y SES CLG Gal oo | ao [>= [| —-[eol-[-)->l-l-1l- 1 — la — [= | ae 2a00 | ooo | exer | voz | — |oa0| coro | — | os | — | — | oozt foowz)—[1[—| © | aa 000 | 0000 | gi%eet} — | — |occo] — | — | — | coo | aro | om [et — [1 [—|[ sea] aa eae [oo f—— | —]oaot—-]|—|-—)-)-)—]— lo —|—-|-[=[e 2a00_| e200 oaxea | ve | — [exo] sooo] — | os | — | — | oom Joos] — | 7 [=] 2 [ao woo 2000 [aux oa — jeooo}) — = — soo | ono | cost a 1 [— | ees] ao =—[-[eol-~- p> L- 1- lp] 1) los — [= |= [re ov ror | sos] — |v] s00| — | oF | — | = | oom foom[ — [a f=[ + [ov worp—]|=]e >= >= = 1 | | oon [oo] — [> [= [=a] ov EAT OTR TERT ERT) S| aE | —F_ || a) | a a STdonoee] ose’ | asen” | seer” |ssdy see sou0d |r ur] 00 |aecaishonp pmo aoa | 200 owes | “one Toms | ny | os | vont | eoroes]) “Len $e | Koon ure ea: | na 10 soe a sis —om 380 FanWe NOL WAOS “HWM 103row 4133S HOM ONIZIS NoIS3q L9G Commercial Duct Design21O__] seow-w) ssoraunssaud oums Tmwa1Ks MOL ean Bn ep ma psp say pup Se EY "S310N = wero | are 5 =e Sy woo | e00 =e asa] S000] x sap een] av waco | 200 aT oe} eee | = a5) sea] SV ce = av = amp earo_[ ore == eee] == | 52 woo | e200 | cana =e we foo] — [a [=| = | 2a sa00 3000 | atta = = czar |g] — | 1 | —[ =a] a0 wo} aro [=x == Foose} — [=] = |e] ao TOO] waa Poa KOE =e eet [oor] =e 7200 | 06 = coat eT — PT pp as vara | oro =e fore} — |=} = Pare ov ¥aao | vaso | OFF =r wont oor} = PT T=> 1 pov 7 0 | eaxor = Ss Zea faa} — | 1} = | ame ov Tae) ss | 7 BARS) wa Pa LT RT OE soon | seorvonios | adonoey “aun |e dey lane pono} way] saab | eta fun] somes | “ams ewemang | “paar | Sur ten erBor ‘tea’ | na 20a RS naisss Ton 300 SUAVENVOR Due 2M so3rOw 33HSNHOM ONIZIS NBIS3a LONG ‘Commercial Duct Design 3WORK SESSION I - SOLUTION It is recommended that you initially work through this example using the tables in the Appendix so that you gain an understanding of the design process. In the future you may wish to use a duct sizing rule or wheel-type device (See Duct Calculator reference) to simplify calculations and reduce the time required for sizing, Since all required information is given, enter the known values on the Duct Design Sizing Worksheet. The supply duct layout is symmetrical, meaning that we only have to size one half of the duet system, then we can use the same sizes for the other side. Note: Table references below refer to the tables in the Appendix. Supply duct: Step 1: The first thing you should do is label all duct sections and nodes. A node is @ point in the duct system where there is either a change in airflow quantity or @ change in airflow direction (examples include a fitting, elbow or a piece of equipment such as a diffuser or grille). A duct section begins and cends between node points, Use whatever labeling convention you want; however, it is suggested that you use a combination of letters and numbers so you don’t confuse them. In our example we will label nodes with letters and sections with numbers. Examining the duct system layout, we can see that the duct run with the greatest friction loss will likely be either from the fan to node “F” or from the fan to node “O”. Since the duct system is symmetrical, the friction losses should be identical for both of those duct runs, Let’s size the duct sections on duct run (fan-F) first, then come back and size the other sections afterward. Step 2: Choose an initial velocity from Table 2 in the Appendix. Since this type of environment is not likely to have stringent sound requirements, let’s select the initial velocity at 2,000 fpm corresponding to a cafeteria. The supply duct connection size is given as 14" x 28". Since we have a “tee” fitting directly below the unit that splits the airflow, we need to transition from the unit duct connection to more of a square shape since most “tees” are not built with an aspect ratio of 2.0. Let’s try a 24” x 24" transition, Remember, however, that there is 2” of duct lining in the supply duct, therefore all metal dimensions will need to be increased by 4" to accommodate the lining. After you select the appropriate airflow dimensions you should then add for the duct liner. Go ahead and enter the transition as the first element on the duct sizing worksheet. Now we need to determine the transition fitting loss, From Table 14-11 there is @ type “B” fitting that ‘meets our requirements, The table of C-values requires us to know the ratio of inlet and outlet areas. It is calculated as: (Ay/A) = [(24" x 24") / 14" x 28")] = 1.47, The smallest value of (Ay/A) in the table is 2.0, so let's use that and assume that the transition angle is 30°. Read the C-value as 0.25, Since itis a diverging-type fitting, we need the velocity pressure upstream of the fitting in order to arrive at the fitting loss. Find a 14” x 28" duct in Table 1 and read the diameter as 21.3. From the friction chart at 21.3" diameter read the velocity of approximately 2,050 fpm. The velocity pressure is: (2,050/4,005)* = 26. Add this value to the worksheet and multiply by the friction rate. Therefore, TP = C x Vp = 0.25 x 0.26 = 0.065 in. w.g. Add this value to the worksheet. Commercial Duct Design 60Step 3: Section | will be the first section of straight duct off the fan. The length is given as 2 ft. Since the problem states that we are using spray-coated duct lining, we need the material roughness factor for it, From Table 4 read this as 1.9. Let's check our 24" x 24” duct and verify our velocity. We now work backwards from the rectangular duct dimension table (Table 1) and determine the round duct equivalent. Find a 24" x 24” duct and read the equivalent round diameter as 26.2". Now, using the friction chart, find the intersection of the round diameter with the airflow quantity (5,000 CFM). Read the friction rate as 0.10 in. w.g. / 100 ft. E.L. and the round duct velocity as 1,400 fpm. Enter the dimensions for duct section 1 on the worksheet, Next we need to determine the section friction loss. Multiply the friction rate by the material correction factor and the length to determine the friction loss; 0.10 x 1.9 x 2/ 100 = 0,004 in. w.g. You can calculate the velocity pressure or read it from the friction chart. You will need this information later to calculate the fitting loss associated with the “tee”. The velocity pressure is (1,400/4005)" = 0.122 in. w.g. Step 4: Since the problem did not state which fitting types to use, we will have to select appropriate ones from Table 14. We have a symmetrical “tee” fitting that splits the air equally. From Table 14-10 we find a type “L” fitting, To arrive at the appropriate “C” value, we need to know the “W" and “I” values. Since these values are dependent on the size of the next duct sections (sections 2 and 8), and we haven't yet sized them, let's skip the “tee” fitting calculations for now. We'll come back to it after we size duct sections 2 and 8. Go ahead and list the “tee” fitting as the next element on the worksheet, Step 5: Let's proceed to section 2, which begins at node C and ends at node D. Now is a good time to sum the airflow values, starting at the terminals and working back toward the fan. Go ahead and write the CFM values on the duct layout diagram next to the section numbers. Section 2 has an airflow quantity of 2,500 CFM. Using the design friction rate of 0.10 in. w.g. / 100 fi. E.L. find the intersection point with the airflow rate, This corresponds to a round diameter of approximately 20 in. Next find a combination of rectangular duct sizes in Table 1 with a diameter of approximately 20 in, A 20" x 16" duct has an equivalent round diameter of 19.5". From the friction chart find the corresponding velocity of 1,200 fpm. Enter these values and complete the section friction loss calculations. Step 6: Now that we know the size of section 2 we can go back and finish the fitting loss calculations for the “tee” fitting. Refer to Step 4 above. The note for Fitting type “L” says to use a similar elbow. Let’s use a type “D”, which is a 90° mitered elbow without turning vanes. The H/W ratio is: 16/20 = 80. From the table read the C-value as 1.2. The fitting loss may now be calculated as: Tp = 1.2 x 0.122 = 0.146 in, w.g. Step 7: Next we come to a branch fitting with a round tap off'a rectangular main at node D, Referring to the fitting loss table, we find a type “I” fitting in Table 14-14, Notice the table is labeled “Branch Coefficient” which means the table includes C-values associated with the branch duct section only. There is a note that says, “For Main Loss Coefficient see Fitting 14-14”, Since we are first sizing the Jongest duct run (fan-F) we will need the “Main” loss coefficient now. Later we will need the “Branch” loss coefficient for calculating the branch duct fitting losses at nodes D, E, L, and P. Refer to fitting type “A” for the “Main” loss value, From the table we need the ratio of velocities Vs / Ve, however we don’t yet know the velocity of the next section (section 3) because we haven't sized it yet. Add the “tee” fitting to the worksheet and let’s skip the fitting loss calculations for now until we complete the straight duct section sizing. 61 ‘Commercial Duet Design