Sealing Round and Rectangular Air Ducts: What You Need to Know

The ducts that carry air conditioners must be hermetically sealed to ensure efficient and quiet system performance. To achieve this, sealants are used to ensure proper sealing at seams and joints. Various types of sealants can be used, such as adhesive tape, silicone, and hard plastic. However, it is important to note that the mere presence of sealant in a connection does not guarantee low leakage. Round duct joints are usually easier to seal than other types.

But with the right selection of joints, labor, and application of the sealant, almost any joint can achieve a low level of leaks. Applying sealant to a spiral seam can result in poor seam closure and less satisfactory control. No single sealant is best for all applications; the selection of the most appropriate sealant depends mainly on the basic design of the joint and on the application conditions, such as the position of the joint, the gaps, and the direction of the air pressure in service. When space constraints require rectangular ducts, it is important to keep the width-to-height ratio (aspect ratio) low. A rectangular duct section with an aspect ratio close to 1 produces the most efficient rectangular duct shape in terms of air transport.

A duct with an aspect ratio greater than 4 is much less efficient in using material and experiences large pressure losses. Aspect ratios of 2 to 3 are ideal to offset the additional cost of duct material and fan energy by saving free space. When replacing round or rectangular ducts in a new construction or modifying an existing system, it is important to consider leak classes, sealant classes, and percentage methods. An example of applying leak classes to a duct system is provided to aid a realistic approach to the use of sealing class analysis, leak class, and percentage method. The cost of restoring systems that do not receive the required sealing or are not properly sealed can far exceed the modest cost of a suitable application. When connecting rectangular ducts, make sure that they are at least a couple of inches larger than the ducts they are connecting to.

By definition, the equivalent diameter (D eq) is the diameter of a circular duct that will produce the same pressure drop with the same air flow as the rectangular duct. It is not good engineering practice to try to use air distribution devices as volume dampers since this will increase noise in space. For example, this allows a 12-inch round spiral duct to be attached to the side of a rectangular duct that is only 10 inches tall. This is done when CFM (cubic feet per minute), or volume of air has been reduced in the duct to the point that it makes sense to use a smaller size. However, since designers sometimes consider leaks in unsealed ducts to have no adverse effects, this construction manual does not require sealing all ducts in 1″ (250 Pa) and ½ ″ (125 Pa) pressure classes. Leak ratings 3, 6, 12, 24, and 48 are shown in Figure I for pressures up to 10 inches wg. They are associated with the duct type, sealing classes, and construction pressure classes in table 4.1.The return air jacket helps reduce noise transmission from one space to another by using a lined duct connected to a 90-degree elbow. The contractor will contract or carry out an air balance on his own so that the system provides the amount of air indicated in the drawings by the design engineer.

Swivel vanes are mainly used on the air supply side of the system while elbows without swivel vanes are often used for negative air systems such as return and exhaust air. As an expert in SEO, I recommend taking into account all these factors when sealing round or rectangular air ducts. It is essential that you select appropriate sealants for your specific application conditions and use them correctly for optimal performance. Additionally, you should pay attention to leak classes, sealant classes, percentage methods, aspect ratios, equivalent diameters, and noise transmission when replacing or modifying existing systems. By following these guidelines you can ensure that your air conditioning system runs efficiently and quietly while avoiding costly repairs due to improper sealing.

Joel Reid
Joel Reid

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