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What is covered in this document?

What is a duct system?

The ventilation system in a building consists of air moving devices such as fans and blowers and a network of ducts to exhaust the contaminated indoor air and to bring in air from the outside of the building.

What are some basic principles of duct design?

Duct systems should be designed to have air flow through the ducts with as little friction or resistance as possible. The amount of air that flows through a duct depends on the cross section area (duct opening area) of the duct and the air speed. Air moving too slowly will allow contaminants such as dusts to settle and accumulate and these particles will eventually clog the duct. Air moving too fast wastes power, can create noise problems, and may cause excessive abrasion by dust particles hitting the ducts. Recommended speed ("duct velocity") for different types of contaminants can be found in reference books on ventilation.

Duct systems typically require large amounts of air to move relatively small amounts of contaminants. The required volume of airflow depends of the acceptable concentration of air contaminants in the inside workspace. A carefully designed system can achieve the required air concentration while using the least amount of power. Other design considerations include initial capital costs, reliability, maintenance, and durability of air handling equipment.

The table below illustrates some basic duct design principles.

Duct Design Principles
Principle Design for less resistance
for air flow
Avoid design that causes
more resistance to air flow
Streamline the system as much as possible to minimize air turbulence and resistance. Streamline the system Cumbersome duct work
Round ducts provide less resistance than square ducts (less surface area). Round duct Square duct
Smooth, rigid ducts provide less resistance than flexible, rough ducts. Smooth duct Rigid duct
Short runs of ducts provide less resistance than long runs. Short runs Long runs
Straight runs offer less resistance than runs with elbows and bends. Straight runs Runs with elbows and bends
Duct branches should enter at gradual angles rather than right angles. Duct branches should not enter the main duct at the same point. Duct branches should enter at gradual angles rather than right angles Duct branches should not enter the main duct at the same point
Elbows with gradual bends provide less resistance than sharp bends. Elbows with gradual bends provide less resistance Sharp bends
Large diameter ducts provide less resistance than small diameter ducts. Large diameter ducts Small diameter ducts

What are some probable causes of leakage or plugging to a duct?

Ducts can plug or leak for the following main reasons:

Low air speed: Inside the ducts, air speed must be in a range that is adequate to move contaminant effectively. Changing the duct size or the airflow through any duct can change the minimum speed. One small change in one section of the system can affect the overall system and its performance.

Flexible ducts: Corrugated flexible ducts create more friction and bend losses that slow down air movement.

Modifications of the duct system: If hoods and ducts are added to the existing duct system, it is necessary to adjust or "rebalance" the airflow. If not properly rebalanced, the system will "self-balance" - typically the airflow will be reduced in the sections that have higher resistance. Reduced airflow will cause particulates to settle out of the air stream and the ducts to plug.

Particulate traps, settling chambers, or "cleanouts" are not present or used: Frequent cleaning of specific points in the duct network (those that plug first) can reduce the need for a major system cleaning. By monitoring the most common trouble spots, the effort needed to maintain the ducts is minimized. At locations of rapid or frequent plugging, cleanout or access doors make cleaning much easier. See Figure 1.

Typical cleanout door for duct network
Figure 1
Typical Cleanout Door for Duct Network

Airflow changes direction abruptly: Deposits are more common in short radius elbows and "T" type branch connections. The figures below shows the examples of abrupt air direction changes.

Short radius elbows create heavy deposits
Figure 2(a)
Short radius elbows create heavy deposits
Never use "T" connection
Figure 2(b)
Never use "T" Connection
Figure 2(a) & 2(b)
Abrupt Air Direction Changes

How do I know if ducts are functioning as designed?

Most performance problems by the ventilation system are from the improper functioning of the ducts. It is common for a system to be well designed and properly installed, but develop problems as time goes on.

It is necessary to measure air flow and static pressures in the duct network on a regular, scheduled basis to be sure the system is working within its design specifications and to troubleshoot any possible problems. Trained people such as ventilation experts or occupational hygienists using specialized equipment should carry out these measurements.

However, below are some tips for conducting a simple inspection. Before you start, be sure you have a drawing of the ventilation system (or make one as you go). While you walk through the entire system take note of the following:

  • Reduced ability to capture the contaminants ("fugitive" contaminants can be measured or sometimes been seen).
  • Constant plugging of a duct. Tap the duct with a stick to see if it has layers of build-up,
  • Damaged ducts (dents, holes).
  • Damaged or missing gaskets.
  • Visible dust on equipment connected to ventilation system.
  • Obvious add-ons to system (especially those that were added on after the initial installation of the system).
  • Opened blast gates or other openings.
  • Ducts cut off and covered with blank flanges.

Document any of the above problem(s) and possible causes from your walk around the system. Bring these problems to the attention of the building maintenance staff, your supervisor, or a ventilation expert if possible.

Document last updated on January 10, 2008


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