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Monday, April 9, 2018

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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