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. That would also lower the amount of pressure the blower would have to surpass to move the air. What Are the Different Methods for Ductwork Installation? Planning (laying out), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design." Use the actual heat, cooling or air quality requirements for the... 2. The purpose of Residential Ductwork Design is to properly distribute the airflow, produced by your heating/cooling system, to your house. A furnace is normally located in the basement of a home or building and its size depends on the building. That resistance results in decreases in the static pressure, or pressure drops. Actually, we're both right, but you're more right. Duct Sizing Examples • This is the desired friction loss in inches w.g. The various texts suggest three methods: • Constant friction gradient • Constant velocity, and • Static pressure regain A pragmatic solution to optimise the conflicting elements in duct design, the late W R (Roy) Ahern, offered the following approach(1)for the sizing of low-pressure air conditioning ductwork. Fred, yes, it would absolutely be better for air flow to shorten that duct. Efficient ductwork design is created to distribute air correctly through the home. per 100 ft. of equivalent length (in. In this formation, several branches of ductwork stem off directly from the furnace and individually reach every room. Remodeled home,Attic mounted HVAC unit, Return box mounted approx 18" from side of AHU end, has 20" flex duct attached and run in a big loop (10-15ft) around to the end of the AHU. Your comment will not appear below until approved. Preview 01:54. Tsal developed a life-cycle cost-based duct design method called the T-method. 9.3 Pressure Balancing . Ducts are looped together, which cause the air to self-balance and minimize the pressure loss across the duct network. 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. Hey Allison, So, I don't see these as insurmountable obstacles. 1 and the charts below. Well, actually, if you remember your introductory physics class, you know that's not quite true. It simplifies complex information for the layperson (or, in my case, the architect). The procedures of residential HVAC design (see Figure 1) are covered in detail by a series of 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 to a particular building type. 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 . And the more the air flow is reduced." The following duct sizes are based on a fraction drop of .10 inches per 100 feet of lineal duct. We're following the Manual D protocol for duct design, a standard developed by the Air Conditioning Contractors of America (ACCA). Ideally, you have the manufacturer's performance data tables to help you get it right. Similar to a car on a road....the bigger the curve, the faster you can take it. One is friction. ASHRAE categorizes duct systems as either single path or dual path. 9.2 Central Return . The actual number is 0.0807 lb at standard temperature and pressure. 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. Larger volumes or higher static pressures should be … Select suitable velocities for main and branch ducts from the table below. This question is for testing whether or not you are a human visitor and to prevent automated spam submissions. The first method of HVAC duct sizing is what is called a Manual J. 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. By the time the air comes out of the supply vents, that pressure has dropped to zero (relative to room pressure). Number one is the heating and cooling load calculation using a protocol like ACCA's Manual J or the ASHRAE Handbook of Fundamentals. I thought that you never ever want to install anything on the end of the main plenum, even a flex duct system, because it will not allow or will decrease back pressure and therefore decrease airflow in the system? The fourth edition of HVAC Systems Duct Design presents the basic methods and procedures required to design HVAC air distribution systems. Today we use all that to find out how big the ducts need to be. And by code, I can use different material that burns (like wood) as long as I have 1/2 inch of gypsum board covering it. w.g./100 ft. E.L.) of duct. Equal friction method: (Medium to large sized commercial installations) 3. The Velocity Reduction Method can be used when sizing air ducts. The Initial Duct Sizing, Pressure Augmentation and Size Augmentation (IPS) method has the potential to reduce ducting system life-cycle costs and the time it takes to design them. 1) Velocity reduction method : In this method the duct designed in such a way that the velocity decreases as flow proceeds. 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. HVAC Duct Design Steps. If you have a 2.5 ton air conditioner, the nominal air flow would be 1,000 cfm. Once you know the BTU/hr and cfm numbers for the building, you need to select the right equipment. Duct safe materials, of course. One psi ~ This is a device that allows the user to input certain calculations about the areas where HVAC ducts will be installed, such as air volume and air velocity, and uses that information to output proper supply and return duct … This insures that the air flows through the system at the proper rate. DUCT DESIGN METHODS There are mainly three methods which are commonly used for duct design. The rougher the surface, the more it slows down the air. How do I Choose the Best Ductwork Design? 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. You've got to make sure you adjust for the indoor and outdoor design conditions of the home. The base sheet should be prepared (Chapter 11) as soon as the architect™s floor plan is received. The pressure drops are calculated for this velocities for respective branches and main duct. 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 … And because it is a better insulator, there is less sound produced within the pipe than there is in typical sheet metal duct. You've got to know how much heating and cooling you need for each room (in BTU/hr). How Do I Determine the Correct Duct Size? Bryan Adams, P.E. and That means the blower has to push about 81 pounds of air through the system each minute. There are basically three types of duct design, they are1)equal friction method2)velocity reduction method 3)static regain method. 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. This generally arises when you move air through fittings, or when you turn the air. As the air moves through a duct, it interacts with the surfaces. This method works for smaller homes and buildings, but can be inefficient for larger buildings because of the amount of ductwork needed. Air conditioning, while its unit is located outside of a building, also passes through this unit in order to incorporate the same ducts. 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. Robert Welner, P.E. Preliminary Design Œ Decisions are made about the types of systems to be used, building zoning, equipment locations, and routing of ductwork. Does the furnace usually have a duct that goes up to the other stories and then follow the same pattern as the first floor? 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. 10.0. A duct system is also called ductwork. 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. In caustic environments, cpvc pipe is used. I have noticed similar behavior with water. This manual provides the HVAC system designer detailed information on duct design and U.S. and metric units are provided in all examples, calculations, and tables. [Duct (flow). While installing cable in a duct and manhole system, one has to worry about the friction built up around corners as well as the increased pulling tension it takes pull the cable through the pipe due to the corners (gotta make sure that one does not stretch the limits of tensile strength on the conductor or insulation). ACCA's Manual S protocol helps you do that. Universal Method. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort. Minnesota "Excellent and was able to receive helpful guidance via a simple phone call." Today I'll start with what you do in the HVAC design process before you get to the duct design phase (Manual D) as well as the physics of air flow when it's constrained by ducts. Why? A lot of poor air flow problems in ducts are on the return side so anything you can do there to fix it should result in noticeable improvement. Number one is the heating and cooling load calculation using a protocol like ACCA's Manual J or the ASHRAE Handbook of Fundamentals. RETURN DUCT SYSTEMS . 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. Since water vapor is lighter than dry air, it brings down the average density. The spider style of duct design is similar to the radial configuration, but slightly more efficient. In the next article, I'll talk more about those pressure drops and how they determine the available static pressure, which then leads to the total effective length of our duct system. They do make a low smoke version for use in confined spaces like electrical vaults and substation basements. 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