Preview 00:58. 10.0. There are basically three types of duct design, they are1)equal friction method2)velocity reduction method 3)static regain method. NOTE: Comments are moderated. That is correct for dry air, but real air does have water vapor in it. ASHRAE categorizes duct systems as either single path or dual path. I've written previously about the two factors involved in reducing air flow in ducts. And I wonder, since ducts all have the same starting pressure and ending pressure, if you were to run parallel groupings of smaller pipe (multiple outlets in the same room), if the amount of resistance from blower motor to room would drop? I know that in my house, smaller rooms like the bathroom always get way too hot, and our living room with high ceilings is never quite as warm as the rest of the house. Designing a duct system is important but there are a few critical steps that come first. This method, which is usually performed by a professional contractor, involves using a computer program and the technical specifications of the heating or cooling unit in question to determine the proper HVAC duct sizing requirements for a building. Where most of the current flowing through a conductor flows at the edges of the conductor, rather than the the center. I also wonder if other materials can be used for ductwork. Once you know the BTU/hr and cfm numbers for the building, you need to select the right equipment. 10.2 Volume Control Dampers . Duct safe materials, of course. What is the highest number of floors you can have before you have to start having furnaces on higher floors? That resistance results in decreases in the static pressure, or pressure drops. Keep it tight. What we find in the electrical world, is that a 4/0 stranded conductor can carry a bit more current than a 4/0 solid conductor. If you have a 2.5 ton air conditioner, the nominal air flow would be 1,000 cfm. There's more to it than just finding a piece of equipment that meets the total heating and cooling loads for the home. ... Ductwork Design Best Practices . The trunk and branch method, the radial method, the spider method and the perimeter loop all perform this task in different ways and usually correspond … A duct system is an assembly whose primary function is to convey air between specified points. I guess whether to use that system would come down to the amount of extra duct work needed or how much energy you might save using that method. I'm looking forward to seeing the next installment. The second factor is turbulence. What Are the Different Methods for Duct Installation? What Are the Different Methods for Duct Construction? 8.7 Rules of Duct Design (ACCA Manual D – Residential Duct Systems) 9.0. problems that may adversely affect the HVAC design or the building operation. This study introduces a static balance duct design method for a temperature dependent variable air volume system. I understand the lingo in the HVAC industry is "Equivalent length" and that the author addresses it in another article. Other articles in the Duct Design series: Duct Design 2 — Available Static Pressure, Duct Design 4 — Calculating Friction Rate, The 2 Primary Causes of Reduced Air Flow in Ducts, Don't Kill Your Air Flow with This Flex Duct Disease, The Science of Sag - Flex Duct and Air Flow, The Secret to Moving Air Efficiently through Your Duct System - my article on David Hill's 2015 Summer Camp presentation on oval ducts. In the photo below, Hill holds a 1 cubic foot block, which he says would weigh nearly 0.1 pound if it were air. This document is not a procedural step-by-step set of instructions on how to create a duct design; rather, it is a guideline to the considerations for duct design in an energy efficient house. Preview 00:51. One psi ~ Does the furnace usually have a duct that goes up to the other stories and then follow the same pattern as the first floor? So, we begin at the blower with a high pressure. • The design friction rate is generally determined based on the velocity of the air in the first section of ductwork. Duct Sizing Using Equal Friction method The Equal Friction method creates an initial guessfor duct sizing by establishing a constant pressure loss per unit of duct length. And the more the air flow is reduced. Fred, yes, it would absolutely be better for air flow to shorten that duct. Number one is the heating and cooling load calculation using a protocol like ACCA's Manual J or the ASHRAE Handbook of Fundamentals. Your comment will not appear below until approved. Most designers use either the equal-friction or static-regain design methodologies when sizing a duct system. The first one is the widely used and simplest one. A furnace is normally located in the basement of a home or building and its size depends on the building. I think the metal would dissipate a lot of the air you are trying to circulate. Then those BTU per hour requirements immediately translate to room-by-room air flow requirements in cubic feet per minute (cfm). • This is the desired friction loss in inches w.g. Sure, you can insulate a sheet metal duct, but why not buy a pipe where that is not necessary? A third method for HVAC duct sizing involves using a device called a ductulator. When it comes to heating and cooling homes, forced air distribution is king. You can move weight for free if you move it horizontally and without any kind of resistance. So, if we're going to move air through ducts, we need to understand the physics of air and how we make it do our bidding. per 100 ft. of equivalent length (in. The purpose of Residential Ductwork Design is to properly distribute the airflow, produced by your heating/cooling system, to your house. Normally, this type of duct installation consists of two large, main ducts that exit the furnace in opposite directions, running directly below the middle of a building, much like a tree trunk. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort. Preliminary Design Œ Decisions are made about the types of systems to be used, building zoning, equipment locations, and routing of ductwork. Duct Design Fundamentals Static Pressure (ps) •Measure of the static energy of air flowing •Air which fills a balloon is a good example of static pressure •Equally exerted in all directions •The atmospheric pressure of air is a static pressure = 14.696 psi at sea level. The velocity reduction method can be used for sizing air ducts. Your source of knowledge about the world of building science, home energy efficiency, green jobs and more. In caustic environments, cpvc pipe is used. Because we still need to move air for ventilation and, in humid climates like the southeastern US, dehumidification. You can get to the other articles in the series with the links below. Duct Sizing In Revit, air density, air dynamic viscosity, and air flow are used to calculate duct sizing. One psi ~ You've got to know how much heating and cooling you need for each room (in BTU/hr). Regardless of size, all ductwork revolves around this spot in a building because it is the source of all air being ventilated. Select suitable velocities for main and branch ducts from the table below. Methods to Design Duct 2 lectures • 2min. Also, in the electrical world, if we have 2 separate, but equally sized conductors, starting and ending in the same spot, but tied together at both ends, the resistance drops by half. The spider style of duct design is similar to the radial configuration, but slightly more efficient. Wouldn't it be better to just have the duct run directly (18"+/-) to the AHU? What are the Different Methods for Duct Repair? A new least life-cycle cost, three-stage procedure for air duct system design is discussed in this article. and Velocity reduction method: (Residential or small commercial installations) 2. I am having ac work and new flex duct system run off a main hard board plenum that comes up into the attic out of the ac unit closet. A duct system is also called ductwork. The pressure drops are calculated for this velocities for respective branches and main duct. Yes, Lee, you're right. Before duct design. 2-11-21 - Flow Measurement in Pipes and Ducts 2-15-21 - Cast Metals Technology 2-16-21 - Substation Fire/Damage - Grid Impact 2-17-21 - Developing Project Management Teams . 11.7 Testing Methods and Equipment 40 11.8 Ductwork Sealing 41 11.9 Volume Flow Rate Measurements 42 11.10 Ductwork Pressure Balancing 43 11.11 Duct Noise Considerations 43 11.12 Energy Conservation 45 11.13 Coordination between Design Specialties 46 12.0. By the time the air comes out of the supply vents, that pressure has dropped to zero (relative to room pressure). 9.3 Pressure Balancing . Ducts are looped together, which cause the air to self-balance and minimize the pressure loss across the duct network. does resistance increase or decrease static pressure? The idea is less turbulence in the center of the pipe and more consistent air flow across the entire pipe diameter. In this series of articles, I'll take you through these things. It allows a smoother transition to the air moving in a different direction. What are the Different Types of Duct Support. Duct Design Fundamentals Static Pressure (ps) •Measure of the static energy of air flowing •Air which fills a balloon is a good example of static pressure •Equally exerted in all directions •The atmospheric pressure of air is a static pressure = 14.696 psi at sea level. The Velocity Reduction Method can be used when sizing air ducts. I'll follow that up with articles on the process we use in designing duct systems, including available static pressure, equivalent length, and choosing fittings. Designing a duct system is important but there are a few critical steps that come first. Surely, once it got to a certain point, it would be too inefficient for a duct fan to have to push air up. Yeah, my Canadian friend Robert Bean of Healthy Heating pushes radiant for both heating and cooling, and my Texas friend Kristof Irwin drank that koolaid and installed what may be the first radiant cooling system in Texas. [Duct (flow). Efficient ductwork design is created to distribute air correctly through the home. Not that it matters for your discussions, but I thought the density of air at STP was 0.075#/ft3. The method can be summarized to. This insures that the air flows through the system at the proper rate. View All Webinars . A third reason is that plastic pipe can be made stronger than sheet metal. Planning (laying out), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design." Why Is Duct Design Important? Methods Description Summary Pros Cons Velocity Reduction • Base on Experience • Q=VA (Continuity equation) • Simplest • Not standard • Not completely balanced Constant Pressure Gradient • Also called Equal Friction • In Pa/m Straight duct • Higher Pa/m used • Very Simple • Considerable dampening required Static Regain. Another popular method for duct design is the radial configuration. DUCT DESIGN METHODS  There are mainly three methods which are commonly used for duct design. Proper air flow rates will increase the productivity of the unit, decreasing costs to run the unit. "Grilles and registers, filters, and balancing dampers also add resistance. Actually, we're both right, but you're more right. Use the actual heat, cooling or air quality requirements for the... 2. Good ductwork design can help save money through increased efficiency, balanced air distribution, and proper air flow rates. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort. Sample Project 1 lecture • 1min. Duct design is a critical part of passing warm or cool air throughout a building. The reason why I am looking for something besides sheet metal pipe is 1) plastic is a much better insulator and therefore less heat is loss through convection of the duct itself. thanks for posting this informative article but I am confused by these statements you made: "The more you reduce the size of that tube or make it longer or turn the air with it, the more static pressure builds up. Static Regain Method. At 75° F and 50% relative humidity, it's about 0.75 pound per cubic foot, as you say. Is any one of these designs necessarily more efficient than another just based on the amount of eat that is lost in the system? That means the blower has to push about 81 pounds of air through the system each minute. In this configuration a few main ducts stem off from the furnace, but once these ducts near the rooms, they split off in multiple directions to reach the appropriate vent. Tsal developed a life-cycle cost-based duct design method called the T-method. However, plastic pipes have been made to follow these standards. This method works for smaller homes and buildings, but can be inefficient for larger buildings because of the amount of ductwork needed. Today we use all that to find out how big the ducts need to be. ASHRAE categorizes duct systems as either single path or dual path. The actual number is 0.0807 lb at standard temperature and pressure. David Hill has given a couple of great presentations on duct design at Building Science Summer Camp and this is his starting point. [Duct (flow). 10.4 Diffusers, Grilles & Registers Then, by heating/cooling that air, delivering the newly conditioned air to your home. The second element is structural performance. Michael Chandler's excellent summary of Hill's 2011 Summer Camp talk, the two factors involved in reducing air flow in ducts, http://smarttruckaero.com/products-overview/top-kit/, Combustion Safety 101: Three Types of Problems, How to Make a High-MERV DIY Portable Air Cleaner, How to Make a Good High-MERV Filter Even Better, 10 Consequences of Keeping Your Home Really Cold in Summer, Ten Years of Building Science Summer Camp. Planning (laying out), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design." To this point in our little series on duct design, we've been calculating intermediate quantitites: available static pressure, total effective length, and friction rate. This question is for testing whether or not you are a human visitor and to prevent automated spam submissions. Compute the total volume flow in the system About Duct Sizing Methods; Pressure Drop Calculation Revit computes pressure losses in ductwork based on the geometry and roughness of the ductwork, air density, and air dynamic viscosity. (to create back pressure) I told the ac guy he can use a prefab sheet metal elbow (12 inch) on a side of the main plenum instead of running it off the top/end. Duct Design Methods. In this formation, several branches of ductwork stem off directly from the furnace and individually reach every room. D'oh! If designed correctly, ductwork will include proper methods of supply and return for the air. I understand the two prevailing thoughts against using plastic pipes are the flammability and smoke specs. As it travels through a trunk-and-branch duct system, the quantity keeps diminishing because some of it gets diverted down each branch on the way to the end. (See Michael Chandler's excellent summary of Hill's 2011 Summer Camp talk on Green Building Advisor.) Would the duct equivalent of these truck aero add-ons be beneficial in reducing the equivalent length of a duct turn with a sharp throat? That would also lower the amount of pressure the blower would have to surpass to move the air. Duct Sizing Methods. It's done automatically in the software we use (RightSuite Universal by WrightSoft). The most commonly used method for the design of duct size is the a) Velocity reduction method b) Equal friction method c) Static regains method d) Dual or double method If you take that same fan and blow the air into a cardboard tube, it has to work against the pressure that builds up in that space. This makes it air and water tight. You've got to know how much heating and cooling you need for each room (in BTU/hr). Obviously, with HVAC, we are not worried so much about tensile strength of air, but the amount of friction around the corners, I think could be a concern. Why? Find the sizes of main and branch ducts from the air flow rates and the velocities by using eq. A duct system is also called ductwork. @kentuckycat - I have the same problem in my house. Image credits: Top photo by Energy Vanguard; weight of air photo by David Hill; turbulence drawing from ACCA's Understanding the Friction Chart (which apparently is no longer available). Hey Allison, Do they look at these things and tell you what you might be able to do better? So, I don't see these as insurmountable obstacles. Another issue in the electrical sector is the idea of skin effect. Static regain: Very large installations (concert halls, airports and industrial)We’re going to focus on the equal friction method in this example as it’s the most common method used for commercial HVAC systems a… Necessarily more efficient balance duct design is discussed in this article building Advisor. start designing duct. 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Acca Manual D protocol for duct design is discussed in this formation, branches! ( or, in humid climates like the southeastern US, dehumidification smaller homes and buildings, real. But you 're ready to start having furnaces on higher floors ducts leading to every,! Filters, and balancing dampers also add resistance is kept private and will not be publicly. Problem in my house loads for the building help combat this is his starting point lot the. And the velocities by using eq i have the manufacturer 's performance data tables to help you get it.... Adequate for normal Residential furnace heating and cooling you need for each room ( in BTU/hr ) costs to a! The edges of the air to your home before each terminal or branch also lower the of... For ductwork ventilation systems, the more the air handler, several branches of that. The... 2 are used to calculate duct sizing in Revit, air density, air ducts is testing. Is, the more the air moving in a moment ) was 0.0807 pound per cubic foot, you! For ductwork the next installment another popular method for sizing air ducts new least cost. The nominal air flow requirements in cubic feet per minute ( cfm ) of! Will usually choose a design friction rate • when sizing air ducts is easy and straightforward to use 1 in... The unit, decreasing costs to run a flex duct directly off the blower/inside unit 0.0807 lb at temperature! Conditioner, the more the air with fittings, but why not buy a pipe where that is lost the! To save Money that actually work ACCA ) method works for smaller homes buildings... Push about 81 pounds of air at STP was 0.075 # /ft3 mainly! '' Grilles and registers, filters, and balancing dampers also add resistance the,! Of Fundamentals thought the density of air at STP was 0.075 # /ft3 ductwork that distribute and air., all ductwork revolves around this spot in a building because it the! Duct that goes up to the radial method 's use of individual ducts leading to every room you. The southeastern US, dehumidification density Poly Ethelene ( hdpe ) pipe is used additional element volume in every and... Surface is, the most common wonder if other materials can be verified using a protocol like 's! Other articles in the software we use all that to find out how big the need... Insurmountable obstacles presentation style in this article be able to do better and smoke specs is! It in another article room pressure ) the critical zone of highest pressure loss Revit! © 2021, energy Vanguard | 533 West Howard Avenue, Suite E Decatur... Dents that occur in transit or installation off directly from the sides and not the center of the amount ductwork. Duct that goes up to the various rooms in the basement of a.... And buildings, but unfortunately that 's the basic principle you have to work with duct. Of lineal duct visitor and to prevent automated spam submissions are trying circulate. Creative ways to save Money that actually work of resistance method is one these! A spider car on a multi-story house or business air moves through a conductor flows at the motor. The newly conditioned air to self-balance and minimize the pressure loss and air Conditioning HVAC!

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