Figure 13-3. - Forming rectangular fiber glass ducts from duct board.
metal ducts. Therefore, fabric joints (usually constructed of heavy canvas) are used to join the branch ducts to the plenum.
There are numerous factors to be considered when sizing duct systems. These factors cause you to make modifications and adjustments through- out the planning and installation process to develop an efficient working system. First, you must calculate the air volume required for heating and for cooling the required space. This will assist you in determining the necessary duct size, fan size, fan speed, and so forth, that is needed to circulate the conditioned air. While determining the heating and the cooling factors, you should think in terms of air circulation throughout the building and in each individual room or space. Remember, air movement is determined by the type of return airflow that you use.
Four other important duct system components are diffusers, grilles, registers, and dampers. Each of these components has a direct correlation between functional design, amount of air accommodated, and the air movement pattern.
The elbows within the duct system are a major source of airflow restriction. Whenever possible, you can gain efficiency by installing long sweeping elbows. Short 90-degree elbows should be used sparingly on long duct runs. However, they can be used very effectively with a minimum of air turbulence and airflow restriction when installed just before diffusers, grilles, and registers.
Your final duct calculations involve taking unit pressure drops and total pressure drops throughout the system. Some of the major contributing factors to these pressure drops are as follows:
Length of duct
Duct material and interior finish
Changes in duct size
Number of elbowsContinue Reading