Duct Systems I – Understanding Air Flow

Understanding duct system air flow can be simplified by focusing on the operating characteristics of the fan wheel and motor-drive independently.  That makes evaluating air flow capacity, fan operating characteristics and duct system air flow resistance impacts much easier.

Diagnostic mistakes can be eliminated as a result.

  • The relationship between air flow velocity, volume (CFM) and duct size and how each of these components can be used for diagnostic purposes is illustrated.
  • Differences between axial and centrifugal fan types, operating characteristics are evaluated.
  • The most common residential fan wheel, the forward-curved centrifugal fan is examined thoroughly to include fan total, velocity and static pressure ratings, performance curves, operating limitations and diagnostics.
  • System effect as it relates to duct configuration and fan performance is explored and clarified using duct system friction rate and duct fitting air flow characteristics.
  • Fan performance impacts as a result of changes in standard air conditons associated with temperature and altitude adjustment are explained.
  • Fan motor types and their unique performance characteristics are clarified in terms of impact on air flow, motor amperage and power consumption.
  • The three basic fan laws are highlighted in a practical manner that can be applied everyday for service and diagnostic purposes.
  • Duct system pressure regions, friction losses, dynamic losses and their impacts on fan capacity are reviewed.
  • Air distribution system total, static and velocity pressures and the interdependencies between these pressures are explored.
  • Proper use and application of air flow testing and diagnostic tools is shown in detail.
  • Turbulent and laminar air flow characteristics and how they affect both the air stream velocity profile and the ability to measure air flow accurately are also reviewed.

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