Manual Q - Low Pressure, Low Velocity Duct System Design

Manual QManual Q - Low Pressure, Low Velocity Duct System Design

ACCA’s Manual Q: Low Pressure, Low Velocity, Duct System Design serves as a comprehensive guide for designing commercial, low velocity, low-pressure air distribution systems. This Manual provides time-saving, quick-fitting loss tables and a complete set of fitting loss tables compiled from the ASHRAE and SMACNA handbooks.

Additionally, Manual Q replaces all current and previous editions of ACCA Manual Q: Equipment Selection and System Design Procedures for Commercial Summer and Winter Air Conditioning.


Manual Q is divided into twenty-one informative sections, with an additional nine appendices for supplementary information.

Section 1 - Duct System Classification
Section 1 begins by classifying duct systems by function, path geometry, velocity range, pressure class, construction material, and class.

  • Classifications
    • Function
    • Paths
    • Velocity
    • Pressure
    • Materials
    • Shape

Section 2 - Duct System Requirements and Design Parameters
Section 2 covers the intricate system requirements, equipment requirements, and design parameters required for HVAC Duct System installations.

  • Classifications
  • Duct System Design Parameters
  • Architectural Considerations
  • Construction Costs and Operating Costs

Section 3 - Basics
Section 3 discusses some basic duct system principles and equations.

  • Flow and Pressure Equations
  • Duct Pressure Measurement Units
  • Duct Pressures - Total, Static and Velocity
  • Frictionless Regain and Conversion
  • Affect of Friction on Duct Pressure
  • Affect of Friction on Regain and Conversion
  • Pressure Measurement
  • Fan Performance Curves
  • Duct System Curves
  • Operating Point
  • Friction Charts and Duct Sizing Slide Rules
  • Correction for Duct Material
  • Altitude and Temperature Corrections
  • Duct Performance - Non-Standard Air
  • Fan Performance - Non-Standard Air
  • Operating Point - Non-Standard Air
  • Correction for Shape
  • Equal Friction Shape Conversion
  • Hydraulic Diameter

Section 4 - Duct System Pressure Changes
Section 4 goes over the duct system pressure changes, pressure loss, and pressure drops, and what component may be responsible for a pressure change.

  • Duct System Pressure Loss
  • Straight Section Pressure Drop
  • Fitting Pressure Drop
  • Fan Inlet and Outlet Pressure Drop
  • Airside Device Pressure Drop
  • Supply Outlet and Return Inlet Pressure Drop
  • Example of Duct System Pressure Changes
  • Duct System Total Pressure Drop
  • Duct System Static Pressure Drop

Section 5 - Fitting Losses
Section 5 details friction losses based on various aspects of the duct system.

  • Fitting Friction Loss
  • Fitting Dynamic Loss
  • Equivalent Length
  • General Comments on Fitting Design
  • Dynamic Loss Coefficient
  • Examples
    • Elbow Loss
    • Branch Take-off Fitting Loss
    • Diverging Flow Transitions
    • Converging Flow Transitions
    • Discharge into a Plenum
    • Exit from a Plenum
    • Offsets

Section 6 - Fan and Duct System Interface
Section 6 provides a general discussion of the “system effect” and its relation to various types of fan outlets and inlet configurations.

  • System Effect 
  • Fan Performance Tables
  • Field Performance
  • Effective Duct
  • Ideal Return
  • Loss Coefficients
  • Density Correction

Section 7 - Fan Outlet System Effects Losses
Section 7 details the system losses when a duct is not 100% effective.

  • System Effect at Fan Discharge
  • Blast Area and Outlet Area
  • Fan Discharge
    • Into Open Space
    • Into a Plenum
    • Into Elbow
    • Into an Outlet Diffuser
  • Fan Discharge Volume Control Dampers
  • Fan Discharge into Tees and Branch Take-offs
  • Air-Handler Cabinet Discharge Connections

Section 8 - Fan Inlet System Effect Losses
Section 8 discusses potential procedures that can be used to estimate how system factors affect pressure losses.

  • System Effect at Fan Inlet
  • Inlet Area
  • Non-ducted Inlets
  • Ducted Inlets
  • Fan Inlet Elbows
  • Documented Inlet Elbow Loss Coefficients
  • Undocumented Inlet Elbow Loss Coefficients
  • Fans Located in Plenums or Cabinets
  • Inlet Obstructions

Section 9 - Air Side Devices 
Section 9 categorizes various types of airside devices and provides information about their performance characteristics.

  • Pressure Loss Data
  • Velocity
  • Filters
  • Dampers
  • Louvers, Penthouses, and Hoods
  • Coils
  • Supply Air Outlets and Return Inlets
  • Air Distribution Devices
  • Humidifiers
  • Energy Recovery Equipment
  • Miscellaneous Equipment

Section 10 - Duct Sizing Methods
Section 10 goes over the various methods for duct sizing and fan selection.

  • Equal Friction Method
  • Modified Equal Friction Method
  • Extended Plenum Method
  • Semi-extended Plenum Method
  • Static Regain Method
  • Constant Velocity Method
  • Velocity Reduction Method

Section 11 - Duct System Design Procedure
Section 11 discusses components to consider when beginning the duct design process.

  • Design CFM Values
  • Select and Locate Air Distribution Devices
  • Locate the Central Equipment
  • Preliminary Layout
  • Sizing Calculations
  • Calculate the System Total Pressure Loss
  • Select the Fan
  • Noise
  • Final Drawings and Specifications

Section 12 - Example, Constant Volume System
Section 12 showcases examples of Constant Volume Systems in a variety of different commercial buildings.

  • Design Friction Rate for the Supply Ducts
  • Supply Duct Runs
    • A1to J
    • A1 to M
    • A1 to P
  • Return Duct Friction Rate and Pressure Loss
  • Return Duct Runs
    • T to Z1
    • X to W
  • Summary

Section 13 - Examples of VAV Designs
Section 13 showcases examples of how to design an air distribution system for modulating and dumping VAV systems.

  • Modulating VAV Systems
  • Example Problem, Modulating VAV Systems
  • Dumping VAV Systems
  • Examples Problem, Dumping VAV Systems

Section 14 - Application of Selected Types of VAV Equipment
Section 14 discusses how to design systems that utilize self-powered VAV diffusers and fan-powered VAV boxes.

  • Self-Powered VAV Supply Diffusers
  • Examples, Self-Powered VAV Diffusers
  • Fan-Powered VAV Box
  • Examples, Fan-Powered VAV Box

Section 15 - VVT, Rooftop Multizone, Radial and Flexible Duct Systems
Section 15 goes over design considerations that are specific to VVT systems, rooftop multizone systems, radial systems, and flexible duct/junction box systems.

  • VVT Systems
  • Designing the CO/BP-VAV System
  • Rooftop Multizone System
  • Designing the Rooftop Multizone Duct System
  • Radial Duct Systems
  • Designing the Radial Duct System
  • Flexible Duct - Junction Box Systems
  • Designing the Flexible Duct System

Section 16 - Fans
Section 16 provides detailed information regarding the design, performance, and application of different types of centrifugal and axial fans.

  • Fans
  • Definitions
  • Fan Types
    • Centrifugal Fans
    • Axial Fans
  • Fan Laws
  • Fan Control
  • Fan Economics
  • Factors that Affect Fan Selection
  • Fan Specification
  • Series and Parallel Fans
  • Standards

Section 17 - Building Pressure
Section 17 goes into detail about building pressure and considerations during the design process.

  • Room Air Balance
  • Economizer Cycle
  • Single Fan System
  • Supply Fan and Return Fan System
  • Supply Fan and Exhaust Fan System
  • Intake Openings
  • Control Dampers
  • Mixing Boxes and Stratification

Section 18 - Exhaust Air Systems
Section 18 discusses three broad categories of exhaust air systems, including human comfort, health and safety, and material handling systems.

  • Comfort Conditioning Exhaust Requirements
  • Exhaust System Applications
  • Codes and Regulations
  • Air Balance
  • Make-up Air
  • Toilet, Locker, and Shower Exhaust Systems
  • Kitchen Exhaust Systems
  • Non-ducted Exhaust Systems
  • Industrial Exhaust Systems

Section 19 - Provisions for Testing and Balancing the System
Section 19 details procedures and provisions for testing and balancing newly installed systems.

  • Design and Installation Affect the At&B Work
  • Balancing Dampers Required
  • Make Provisions for Performing the Balancing Work
  • Document the Design

Section 20 - Duct Insulation and Duct Leakage
Section 20 talks about the dangers and preventional measures against duct leakage.

  • Recommended Insulation Levels
  • Effective R-Values for Various Materials
  • Estimating Duct Wall Conduction Losses
  • Vapor Barries
  • Duct Sealing Requirements
  • Duct Leakage

Section 21 - Noise
Section 21 discusses noise and what it means for the HVAC system

  • Frequency
  • Sound Pressure
  • Decibel
  • Sound Power
  • Frequency Spectrum
  • Octave Bands
  • Loudness Chart
  • Noise Criteria Rating
  • Acceptable NC for Various Applications
  • Sound Meters
  • Equipment Performance Data
  • Sones and Phones
  • Attenuation
  • Regenerated Noise
  • Room Absorption Effect
  • Breakout Noise
  • Transmission and Transmission Loss
  • Crossstalk
  • Vibration Isolation
  • Designing for Noise Control

Appendix 1 - Equations

Appendix 2 - Friction Charts

Appendix 3 - Charts and Design Data

Appendix 4 - Device Pressure Drop and Performance Data

Appendix 5 - System Effect Loss Coefficients

Appendix 6 - Fitting Loss Coefficients

Appendix 7 - Drawing Standards

Appendix 8 - Guidelines and Checklists

Appendix 9 - “Quick" Fitting Tables


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