Duct System Design Duct System Design Guide First Edition ©2003 McGill AirFlow Corporation McGill AirFlow Corporation One Mission Park Groveport, Ohio 43125 Notice: No part of this work may be reproduced or used in any form or by any means — graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems — without the written permission of McGill AirFlow Corporation. The performance data included in this Guide have been obtained from testing programs conducted in flow measurement laboratories and detailed in the reference test reports. The data are reprinted in this manual as a source of information for design engineers. McGill AirFlow Corporation assumes no responsibility for the performance of duct system components installed in the field. McGill AirFlow Corporation is a wholly owned subsidiary of United McGill Corporation. i Duct System Design Table of Contents Introduction ..............................................................................................................................vi Foreword......................................................................................................................vi An Overview of the Design Process.................................................................................vi How to Use the Duct System Design Notebook.................................................................viii Chapter 1: Airflow Fundamentals for Supply Duct Systems ...................................................1.1 1.1 Overview ..........................................................................................................1.1 1.2 Conservation of Mass.........................................................................................1.1 1.2.1 Continuity Equation ................................................................................1.1 1.2.2 Diverging Flows .....................................................................................1.3 1.3 Conservation of Energy ......................................................................................1.4 1.3.1 Static Pressure ........................................................................................1.5 1.3.2 Velocity Pressure.....................................................................................1.6 1.3.3 Total Pressure.........................................................................................1.7 1.4 Pressure Loss in Duct (Friction Loss)..................................................................1.7 1.4.1 Round Duct ............................................................................................1.8 1.4.2 Flat Oval Duct .......................................................................................1.10 1.4.3 Rectangular Duct ....................................................................................1.13 1.4.4 Acoustically Lined and Double-wall Duct ..................................................1.14 1.4.5 Nonstandard Conditions...........................................................................1.15 1.5 Pressure Loss in Supply Fittings ..........................................................................1.17 1.5.1 Loss Coefficients.....................................................................................1.18 1.5.2 Elbows...................................................................................................1.18 1.5.3 Diverging-flow Fittings: Branches ............................................................1.21 1.5.4 Diverging-flow Fittings: Straight-Throughs, Reducers, and Transitions............................................................................................1.28 1.5.5 Miscellaneous Fittings .............................................................................1.32 1.5.6 Nonstandard Conditions...........................................................................1.37 Chapter 2: Designing Supply Duct Systems..........................................................................2.1 2.1 Determination of Air Volume Requirements...........................................................2.1 2.2 Location of Duct Runs .......................................................................................2.1 2.3 Selection of a Design Method..............................................................................2.2 2.3.1 Equal Friction Design .............................................................................2.2 2.3.2 Constant Velocity Design .........................................................................2.3 2.3.3 Velocity Reduction Design........................................................................2.3 2.3.4 Static Regain Design ...............................................................................2.4 2.3.5 Total Pressure Design..............................................................................2.4 2.3.6 Which Design Method? ............................................................................2.4 2.4 Equal Friction Design .........................................................................................2.5 2.4.1 Introduction............................................................................................2.5 2.4.2 Duct Sizing.............................................................................................2.5 2.4.3 Determination of System Pressure .............................................................2.10 2.4.4 Excess Pressure.......................................................................................2.15 2.5 Static Regain Design ..........................................................................................2.16 2.5.1 Introduction............................................................................................2.16 2.5.2 Duct Sizing.............................................................................................2.16 2.5.3 Determination of System Pressure .............................................................2.20 2.5.4 Excess Pressure.......................................................................................2.20 i Duct System Design Chapter 3: Analyzing and Enhancing Supply Duct Systems 3.1 Analyzing a Preliminary Supply Design.................................................................3.1 3.2 Balancing Equal Friction Designs .........................................................................3.1 3.2.1 Balancing Dampers .................................................................................3.1 3.2.2 Orifice Plates .........................................................................................3.3 3.2.3 Enhanced Equal Friction Design ..............................................................3.4 3.3 Enhanced Static Regain Design............................................................................3.8 3.4 Fan Selection.....................................................................................................3.13 3.4.1 System Effect Performance Deficiencies ....................................................3.14 3.4.2 Duct Performance Deficiencies.................................................................3.15 3.4.3 Fan Pressures .........................................................................................3.16 3.5 Cost Optimization ..............................................................................................3.17 Chapter 4: Airflow Fundamentals for Exhaust Duct Systems 4.1 Overview ..........................................................................................................4.1 4.2 Conservation of Mass.........................................................................................4.1 4.2.1 Continuity Equation ................................................................................4.1 4.2.2 Converging Flows ...................................................................................4.1 4.3 Conservation of Energy ......................................................................................4.2 4.4 Pressure Losses.................................................................................................4.3 4.4.1 Pressure Loss in Duct (Friction Loss) ........................................................4.3 4.4.2 Pressure Loss in Return or Exhaust Fittings (Dynamic Losses) .....................4.3 4.4.3 Nonstandard Conditions for Dynamic Losses ..............................................4.10 Chapter 5: Designing Exhaust Duct Systems ........................................................................5.1 5.1 Defining the Application and Determining Parameters.............................................5.1 5.2 Determining Capture Velocities and Air Volume Requirements.................................5.1 5.3 Locating Duct Runs ...........................................................................................5.9 5.4 Determining Duct Sizes Based on Velocity Constraints...........................................5.9 5.5 Mixing of Two Air Streams ................................................................................5.11 5.6 Determining System Pressure Requirements .........................................................5.15 Chapter 6: Analyzing Exhaust Duct Systems.........................................................................6.1 6.1 Fitting Selection.................................................................................................6.1 6.2 Balancing the System .........................................................................................6.1