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Circular Air Ducts VS Rectangular Air Ducts

Circular Air Ducts VS Rectangular Air Ducts


Circular Air Ducts VS Rectangular Air Ducts

The air duct system is the most important part of air conditioning and ventilation engineering. Its function is to transmit the conditioned air to the terminal equipment as efficiently as possible according to the designed flow rate. Generally, there are three shapes of duct cross-section: rectangular, circular and oblate. The production of rectangular air ducts is usually done by riveting four steel plates. The circular air duct is made by winding a 137mm wide steel plate in a spiral forming machine. The flat circular air duct is relatively rare, and it is generally formed by extruding the circular air duct. Before 1960, most of the ventilation systems used rectangular air ducts due to the simple production process and small installation space. With the successful development of the large-scale spiral circular air duct forming machine, a large number of engineering examples have proved that the circular air duct is far better than the rectangular air duct in terms of economy and other engineering parameters.

The fiber fabric air duct that is widely used in the market is an airflow distribution system that integrates the functions of air outlet, air supply duct, static pressure box, thermal insulation material, air valve, etc. Its advantages such as precise and uniform air supply, light weight, fast installation, high aesthetics, antibacterial and mildew resistance have been recognized by users and used in a wide range. The shapes of fiber fabric air ducts are circular, semi-circular, 1/4 round, oval, and semi-oval to meet the needs of different building structures.

From the perspective of economic analysis, all the costs of the air duct system in a building over its entire service life can be divided into:

1. Initial investment: including design cost, material and installation cost, space cost, commissioning cost, etc.

2. Operating costs: including staff wages, energy, and maintenance costs.

3. Renewal cost: including maintenance and replacement of worn parts.

Foreign research results show that the circular duct system performs better than the rectangular duct system in many parameters. This article will focus on the economic comparison of the duct system.


1. Initial investment:

   Some of the reasons why a circular duct system requires a lower initial investment than a rectangular duct are as follows:

· Circular ducts are easier to manufacture and transport.

· The components and fittings of circular ducts are highly standardized.

· Structurally, circular ducts are more able to withstand pressure without deformation, while rectangular ducts require more reinforcement measures such as bolts, rivets, and support beams.

· Under the same hydraulic diameter, the amount of metal required to manufacture a circular air duct is smaller than that of a rectangular air duct. The larger the aspect ratio of the rectangular air duct, the higher the metal consumption.

· The hanger structure of circular air ducts is simpler than that of rectangular air ducts.

· The measurement points required to measure the air volume of circular air ducts are smaller than those of rectangular air ducts, so when balancing the air duct system, the commissioning cost will also be smaller than that of rectangular air ducts.

· The circular duct can significantly reduce the introduction of low-frequency noise into the room, so it can reduce the noise reduction equipment.


2. Operating costs:

   In general, the largest part of the operating cost of an air conditioning duct system is energy consumption. Energy costs include the energy used to heat or cool the air and deliver it to the end equipment. If the entire air duct system is well insulated, air leakage from the air duct becomes an important source of excess energy consumption.

   To avoid sick buildings, air ducts need to be cleaned regularly. Cleaning methods include dry (using a vacuum cleaner and brush) or wet (using a long mop). In both cases, circular ducts are simpler than rectangular ones.

Economic analysis of air duct system is a very difficult task. It has to take into account a variety of factors, and a duct system is likely to have a lifespan of more than ten years, in which case even a small improvement in design and quality can increase the profitability of the investment. From this point of view, circular ducts should be a more economical solution.

Finally, it should be pointed out that, due to the consideration of quietness and space, some parts of the air duct system related to large flow and large size, such as the fresh air intake and the outlet of the air handling device, are still recommended to use rectangular air ducts.

air duct

air conditioning duct


air outlet

insulation material


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