COMMERCIAL DISTRIBUTION SYSTEMS Duct Design

COMMERCIAL DISTRIBUTION SYSTEMS

Duct Design

Level 1: Fundamentals

Table of Content

Introduction

Duct Design Criteria

Space Availability

Duct Terms

Installation Cost

Air Friction Loss

Noise Level

Heat Transfer and Leakage

Codes and Standards

Theory and Fundamentals

Law of Conservation of Energy – Bernoulli’s Law

Friction Loss in Ducts

Recommended Duct Velocities for Ductwork and HVAC Component

Effects of Shape

Surface Roughness of Ducts

Recommended Friction Rates

Fitting Dynamic Losses

Equivalent Length Method

Use of Fitting Loss Coefficients System

Duct Sizing Methods

Equal Friction

Modified Equal

Static Regain

Other Methods

Duct Design Process

Design Step 1: Determine Number of zone

Design Step 2: Perform Cooling and Heating Load Estimates

Design Step 3: Determine Space, Zone and Block Airflows

Design Step 4: Select Duct Material, Shape and Insulation

System Classification

What Can Be a Duct and What is it Made of?

How are Ducts put Together, Sealed and Insulated? 

Design Step 5: Lay Out Ductwork from AHU to Air Distribution Devices

Fit Trunk Duct to Building

Create a System Sizing Schematic

Design Step 6: Summarize Duct cfm and Label Duct Schematic

Use a Duct Sizing Worksheet

Design Step 7: Size Ductwork from Fan, Out to Extremities

Duct Sizing Using the Friction Chart

Example 1 – Using the Duct Friction Chart

Circular Equivalent Diameters of Rectangular Ducts

Example 2 – Converting From Round to Equivalent Rectangular Ducts

Duct Sizing Using Duct Calculators

Changing One Dimension at a Time

Extended Plenum

Return and Supply Air Plenums

Design Step 8: Calculate Air System Pressure Losses

Design Step 9: Select Fan and Adjust System Air flows

Introduction

This Technical Development Program (TDP) covers the fundamental principles of duct system design for commercial building applications. The most popular duct sizing method – equal friction – is covered in detail. Modified equal friction, incorporating many of the benefits of static regain, is presented in the related TDP-505, Duct Design, Level 2. Although many other duct sizing methods exist (e.g. velocity reduction, T-method, extended plenum, constant velocity,static regain), none are widely used by designers and are beyond the scope of this training module. The reader should refer to other publications for information on these sizing methods.It is recognized that the use of manual duct calculators is normal, and that computer-aided duct design is becoming more popular; however, it is important to learn the manual friction chart method of duct sizing that is the foundation of these other methods. This will provide the knowledge necessary to recognize possible design errors and understand the effects of various design decisions. Once properly covered, use of Carrier’s Duct Calculator for equal friction sizing will be presented.Proper duct design requires performing load estimates to determine the zone and space cfm that the duct system will distribute. Once the cfm has been determined, the duct system components can be laid out. This includes locating the supply and return diffusers and registers to provide adequate air distribution to the spaces. Load estimating and room air distribution principles are covered in detail in other related TDPs.This TDP will cover each duct design step in sufficient detail to permit the participant to layout and size ductwork into a coordinated system that is energy efficient and cost effective to fabricate, install and commission. The Level 2 Duct Design TDP will present many areas of design enhancement, such as SMACNA Duct Construction Standards, duct design code requirements,fitting selection using loss coefficients, avoiding acoustic issues, unique VAV system duct features, and using life cycle cost analysis as a design criteria.Level 1 Duct Design develops various aspects of sizing in detail because an oversized duct system will be difficult to balance and will increase the installed cost of the system. An under sized duct system will create higher than necessary air pressure drops, generate noise, and will not deliver the required airflow quantities.Work sessions are included as part of this program to assist the participant in evaluating his or her understanding of these fundamental principles and sizing parameters.

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