The Science of Comfort

Your Premiere Source For All Things HVAC

What is System Effect?

Posted by Price Industries on September 10, 2019 at 9:47 AM
Price Industries
In duct design and installation, system effect is the generation of higher than expected pressure drops through changes in duct direction or geometry. It can also be caused by improper installation of  fittings, which result in excessive, unanticipated turbulence in the airflow. In short, system effect is the installed pressure drop which is different from the design pressure drop.
 
Illustrate system effect with duct changing sizes and directions
 
System effect typically refers to fan installations, but can also be determined for silencers and any geometry or directional change with non-ideal inlet or outlet conditions. Components that commonly experience system effect include duct take-offs, terminal units, chilled beamsdiffusers, silencers, fans, and water coils.
 
Symptoms of system effect include excess static pressure drop, instability in air volumes through a device, excessive power consumption in a blower, lower fan air volumes, and higher levels of radiated and discharge sound.
 
Duct designers layout their ductwork in a way that minimizes the potential for system effect. However, as is often the case, the real world interferes. Obstructions such as fire suppression piping and  electrical conduits can prevent the duct from being installed as drawn. As a result, additional fittings such as elbows may be installed without the length of inlet or discharge duct to allow the air velocity to be equalized. 
 
When air experiences a change in direction, the air velocity profile in an elbow will shift (Figure 1). If another fitting is attached to the discharge of that elbow, the air velocity profile will not be fully formed. This will result in a higher pressure drop. To minimize this effect, turning vanes – as well as a length of straight ductwork – are often used to settle the air into a more ideal air velocity pattern.
 
Ideal and non-ideal velocity profiles in ductwork
 
Figure 1: Ideal and non-ideal air velocity profiles in ductwork.
 
A typical recommended minimum length of straight ductwork is three equivalent diameters. Depending upon the velocity in the duct, the length of straight ductwork may be longer. A great way to see the impact of system effect is to look at silencers, as the inlet and discharge conditions can have a significant impact in sound absorption.
 
A silencer should be tested to the ASTM Standard E477-13e1, which states that there shall be straight duct of no less than five equivalent duct diameters upstream of the silencer, and not less than ten duct diameters downstream of the silencer. In “real-life” applications these conditions are rarely possible, but three to four duct diameters on both sides of the silencer should be a minimum design goal. 
 
Diffusers and terminals can also be impacted by system effect. This results in higher sound levels and poor inlet discharge air patterns. Figure 2 shows the air velocity patterns in a diffuser with varying air inlet conditions.
 
basis of manufacturer's rating
 
Basis of manufacturer's rating
 
 
sound levels up to 12 dB higher with no equalizing grid
 
Sound levels up to 12 dB higher with no equalizing grid
 
 
sound levels same as manufacturer's rating with equalizing grid
 
Sound levels same as manufacturer's rating with equalizing grid
 
Figure 2: Various inlet conditions for a diffuser
 
When a designer uses acoustic data for a terminal that was tested according to ASHRAE Standard 130, and then adds a silencer that was tested according to ASTM E477, the results will not be what the math would suggest. This is because the inlet and discharge ductwork lengths are now different than what was tested. Figure 3 shows the predicted NC value of the installation is 24, while the actual NC value is 35. When attaching silencers to terminals it is best to use units that are designed and tested as an assembly, such as the Price SDVQ
 
single duct terminal unit with silencer
Figure 3: SDV (ASHRAE 130) + Silencer (ASTM E477) can be unpredictable
 
One thing to keep in mind is that static pressure drop is related to noise generation. The higher the turbulence due to pressure drop, the higher the noise generation.
 
For more information on this topic, the Price Engineering Handbook has several chapters that are relevant:
  • Chapter 8: Duct Design
  • Chapter 9: Mixing Ventilation
  • Chapter 10: Noise Control
  • Chapter 12: Terminals
  • Chapter 13: Fan Coils and Blower Coils

Access the Price Engineer's HVAC Handbook

Topics: System Effect, Static Pressure, Outlet Conditions, Pressure Drop, Velocity Profile, Inlet Conditions

At Price, when we say “the sky is the limit”, we actually mean it.

Our customers have come to count on a consistent and ever-increasing stream of new products and enhancements.  Price has a large team of experienced and talented engineers who are consistently working on breakthrough ideas.  We are not afraid to move into whole new product categories if we believe we have a contribution to make.

  • Research & Innovation
  • Technical Tours
  • Training Classes
  • Targeted Discussions
  • Mock-up Testing
  • Custom Solutions & Specials
  • Custom Finishes
  • Computational Fluid Dynamics
  • Virtual Reality

Subscribe to Email Updates

Recent Posts