Ac Ductwork Supplies Series

What is duct and types of duct
In a HVAC system, ductworks and insulation is one of the essential part to consider. A proper ductwork and insulation will save users lots of costs for electricity and maintenance. Ducts are used to distribute air for various air conditioning systems and ventilation systems. Many people are confused about the duct types. Actually, ducts used for HVAC systems and home ventilation systems are different in many aspects. So today we’ll take a look at different types of ducts.

Flexible Ducts
Flexible ducts are one of the most common used ducts in an HVAC system or in a home ventilation system. They are relatively cheap, light in weight, and easy to install. So it’s not necessary to use a CNC duct fabrication machine. They are often insulated with fiberglass insulations when applied for air conditioning. However, flexible ducts which have no insulations are only used for ventilation such as toilet exhaust.

Flexible ducts are common in use for their low prices and easy installation. However, the main disadvantage of flexible ducts is high pressure drop and low airflow. Because flexible ducts are flexible, they are prone to sharp bends. If a flexible duct is bent too much, it can block the airflow entirely. Hence, it is advisable that a flexible duct should not be more than 2.5 meters in length.






Rigid Ducts
The second commonly used type is rigid ducts, which are also known as sheet metal ducts. Rigid ducts often are made of sheet metals which are usually galvanized steels. Rigid ducts can be made in square, rectangular or round shapes. A CNC duct fabrication machine is needed to speed up the fabrication process.

Square rigid ducts are more economical to produce than both rectangular and round rigid ducts. Because the total surface area needed lesser. Therefore, the material used is lesser. In HVAC, main ducts and branch ducts are usually rigid ducts. They are more likely in a rectangular shape rather than in a square shape. Even though the cost is higher. Round ducts are more preferred when installed below the ceiling especially when the interior aesthetic requirement is high.

If rigid ducts are used for air conditioning, they must be insulated to prevent condensation. People use either fiberglass insulations or cross-linked polyethylene for insulations. If it is the former, the outer appearance will be in silver color. Otherwise, they may look grey in color.





Pre-Insulated Ducts
Pre-insulated ducts are made of either polyurethane (PU) or polyisocyanurate (PIR) material. They have been launched in the market for years, but the application rate is rather low. Because the material cost is higher than the other types.

However, pre-insulated ducts have many benefits that other materials don’t have. Firstly, they have a very high efficiency. Pre-insulated ducts are joined by glues and clips, sort of like a slot-in. Such a jointing method has a much lower leakage rate than both slip & drive joint and TDC joint.

Secondly, pre-insulated ducts are pre-insulated and hence, they do not require further insulations on-site which enable them to be installed very close to the soffit slab or even flush with the soffit slab. So, pre-insulated ducts occupy less space than rigid ducts.

Last but not least, pre-insulated ducts are much lighter than traditional galvanized iron rigid ducts. Thus, the installation process is faster. If you use a CNC duct fabrication machine to do the measuring and cutting, it will be even faster. There are more and more HVAC contractors willing to use pre-insulated ducts for their projects. For big HVAC contractors, they often have many projects on-going, to speed up the fabrication process, they often buy CNC duct fabrication machines to replace man cutting. A CNC duct fabrication machine is fully automatic, which cuts phenolic boards fast and precisely. The application of CNC duct fabrication machines has revolutionized the duct industry since its launch.


In conclusion, if your budget is enough, we suggest you to use pre-insulated ducts for better efficiency, fire-proof, and easier fabrication. If you budget is limited, then use rigid ducts as a second choice. If it’s for home ventilation, and you have very small budgets, then use flexible ducts might also work.

HVAC air duct installation operation process
First, find out the elevation of the air ductaccording to the design requirements and with reference to the civil construction datum line.


Second, making hangers and high hanging points.
1. Setting of hanging point:
(1) Embedded parts method:
①Pre-embedded in the early stage: Generally, the pre-embedded parts are firmly fixed on the steel bars of the civil structure by the reserved personnel according to the coordinate position of the drawing and the spacing between the supports and hangers.
②Pre-embedding in the later stage: Embed the bracket on the brick wall: Calculate the distance from the surface of the bracket section steel to the ground according to the elevation of the air duct, find the correct installation position, and make a square hole of the required size. The inside and outside of the hole should be the same size, and the depth should be 30-50mm greater than the depth of the bracket buried in the wall. After the hole is made, wet the hole with water and flush out the brick chips inside the hole. Then fill a part of 1:2 cement mortar in the wall hole, and bury the bracket, and the burying depth is generally 150-200mm. Use a spirit level to level the bracket, adjust the depth of burial, continue to fill the mortar, and properly fill some water-soaked stones and broken bricks to facilitate fixing the bracket. When filling in cement mortar, it should be slightly lower than the wall, so that the wall can be decorated by civil works.
Embed the hanger under the floor: After determining the position of the elevator, use an impact drill to drill a hole in the floor, and then tick a groove on the ground according to the required size. Insert the hanger into the groove and fill the groove with cement mortar.
(2) Expansion bolt method: It is characterized by flexible, accurate and fast construction.


Third, Install the hanger.
1. Find the placement position of the boom according to the center line of the air duct. The single boom is on the center line of the air duct, and the double boom can be installed symmetrically according to the screw hole spacing of the tray or the center line of the air duct.
2. The boom can be welded or hung on the hanger according to the form of the hanger. Anti-rust paint should be applied after welding.
3. When the air duct is long and it is necessary to install a row of brackets, you can install both ends first, and then use the brackets at both ends as the benchmark to find the elevation of the middle bracket by pulling the wire method for installation.
4. When installing the riser pipe clamp, the top pipe fitting should be fixed first, and then use a wire hammer to hang the line at the center, and the lower pipe clamp can be fixed according to the line.




Fourth, spacing of air duct support and hanger.
1. If there is no design requirement for the distance between the air duct support and hanger, the non-insulated air duct should meet the requirements in the following table. When there is no design requirement for the distance between the insulation duct, support and hanger, it shall be implemented as required. The distance between the support and hanger of the spiral duct can be appropriately increased.
2. The support and hanger shall not be installed in the tuyere, valve inspection hole, etc., so as not to hinder the operation. The hanger must not be hung directly on the flange.
3. The thermal insulation air duct should not be in direct contact with the support and hanger. It should be padded with a strong thermal insulation material, whose thickness is the same as that of the thermal insulation, to prevent the occurrence of "cold bridges".
4. The polyvinyl chloride air duct also adopts the support and hanging brackets like the steel plate air duct, but generally the hanger is the mainstay, and the bracket spacing is made and installed according to the requirements of the table below.

Fifth, arrangement and installation of air ducts
1. Flange connection:
(1) In order to ensure the tightness of the flange interface, there should be gaskets between the flanges. In the case of no special requirements, flange gaskets are selected according to the table below.
(2) How to use the gasket sealing tape:
① Clean up the foreign matter and accumulated water on the surface of the air duct flange and wipe it clean.
②Attach the tape from the corner of the flange, and the end of the tape should be slightly longer than the flange.
③ Paste it evenly and evenly along the flange, and press it firmly by hand during the pasting process, it must not fall off, the interface should be tight, and no part should protrude into the air duct. , cut off the excess.
④ Peel off the release paper.
(3) When the flanges are connected, it is required to specify the gasket material, align the two flanges, thread the bolts and put on the nuts, (note: the nuts should be on the same side), do not tighten temporarily until all the bolts are put on , and then tighten the bolts. When pressing the air duct, the flanges at both ends shall prevail. Pull the wire to check whether the connection of the air duct is straight.
(4) Matters needing attention when connecting gaskets and flanges.
① Correctly select the litter and avoid using the wrong litter.
②The flange surface should be clean and free of foreign matter.
③The flange gasket cannot be squeezed or shoved into the pipe, otherwise it will increase the flow resistance and increase the dust accumulation in the pipe.
④After the flange is connected, it is strictly forbidden to fill the gap of the flange with gasket.
⑤ The nuts of the connecting flange should be on the same side.
⑥ Stainless steel flange connection bolts should be made of stainless steel bolts of the same material.
⑦ Galvanized bolts should be used for flange connection of aluminum plate air ducts, and galvanized washers should be placed on both sides of the flange.
⑧ Galvanized bolts are used for the connection between the PVC air duct flange and the FRP flange.
2. The arrangement of air ducts cannot be connected by flanges
①Hoop connection: It is mainly used for the connection of steel plate round air ducts and spiral air ducts. First, roll out the ribs at both ends of each pipe section, and shrink one end into a small opening. When installing, insert the small opening into the large opening according to the direction of airflow, and use steel hoops on the outside to tightly connect the drum hoop at both ends.
②Plug-in connection: mainly used for rectangular or circular air duct connection. First make the connecting pipe, then insert the air ducts on both sides, and then fix it tightly with rivets.
③ Insertion type connection: mainly used for rectangular air duct connection. Insert different forms of cuttings into both ends of the duct and compact. Its shape and takeover method are shown in the figure.
④Hose type connection: It is mainly used for the connection of air ducts and components (such as radiators, side air outlets of static pressure boxes, etc.). When installing, the two ends of the hose are put on the outside of the connected pipe, and then the hose is fastened with a special soft clip.




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Phenolic ductwork represents a significant advancement in the construction and industrial sectors due to its unique properties. This composite material, which combines phenolic foam insulation with a phenolic resin jacket, offers superior thermal and chemical resistance compared to traditional duct materials. The insulated design enhances insulation efficiency, particularly in harsh environments where thermal integrity is crucial. Phenolic ductwork also provides long-lasting performance due to its resistance to moisture, chemicals, and microbiological growth, reducing the need for frequent maintenance. In industrial settings, its applications range from petrochemical plants to food processing facilities, where it handles extreme conditions and maintains hygiene standards. Integrating phenolic ductwork with smart building technologies and renewable energy systems further amplifies its benefits, offering enhanced environmental control and energy efficiency.

Key Components of Phenolic DuctworkPhenolic ductwork comprises several critical components, each designed to optimize its performance and durability:
- Phenolic Foam Insulation: This is composed of closed-cell structures, offering exceptionally low thermal conductivity. This minimizes heat loss or gain, reducing energy consumption and maintaining optimal temperatures within the duct system.
- Duct Material: Options include aluminum, steel, and composite materials. Aluminum is lightweight and easy to install, while steel ensures robustness and durability. Composite materials offer a balance of strength and lightweight properties.
- Seals and Gaskets: Thermal adhesive gaskets are essential for creating a tight seal and preventing air leaks, thus maximizing the insulating power of the phenolic foam. These gaskets enhance the overall efficiency and longevity of the ductwork.
- Connection Systems: Flexible joints and precision-fitted connections are necessary for seamless integration and to prevent air leaks. These systems ensure the integrity and performance of the ductwork, especially in complex building scenarios.
- Fire Suppression Ratings: UL-rated phenolic ductwork ensures compliance with fire safety standards, which is critical in safety-conscious environments such as hospitals and data centers.

Benefits of Phenolic DuctworkPhenolic ductwork offers several key benefits that make it an excellent choice for various applications:
- Excellent Thermal Efficiency: This material provides outstanding temperature stability, minimizing heat loss and gain. This translates to significant energy savings and helps maintain optimal temperatures within the duct system.
- Superior Durability: Made with resilient phenolic foam bonded to high-quality steel, phenolic ductwork is highly resistant to chemicals, moisture, and UV radiation. This ensures a longer lifespan and reduced need for replacement.
- Sound Insulation: Phenolic ductwork effectively reduces noise transmission, contributing to a quieter and more comfortable indoor environment, particularly in large facilities such as hospitals and educational institutions.
- Enhanced Fire Resistance: Phenolic ductwork meets stringent fire safety standards, providing an additional layer of security by preventing the spread of fire and smoke, ensuring the safety of occupants and property.
- Lower Maintenance: The long-term performance and inherent properties of phenolic ductwork mean fewer maintenance checks and lower overall maintenance costs, translating to a more efficient and cost-effective long-term solution.

Durability and Lifespan of Phenolic DuctworkPhenolic ductwork is characterized by its exceptional durability and prolonged lifespan, especially in environments with high temperatures and flame-resistant conditions such as industrial furnaces, automotive plants, and aerospace facilities. Its high-temperature stability and flame resistance contribute to its resilience under extreme conditions. In scenarios where traditional ductwork would face challenges, phenolic ductwork often outperforms, leading to reduced maintenance and replacement costs over time. For example, in automotive plants, phenolic ductwork has demonstrated a lifespan exceeding 20 years without degradation. Real-time monitoring systems, equipped with IoT sensors, can detect potential issues before they become critical, thereby extending the lifespan and reliability of the ductwork.

Installation and Maintenance of Phenolic DuctworkInstallation and maintenance of phenolic ductwork are critical for ensuring efficient and reliable HVAC systems, particularly in environments such as pharmaceutical facilities where precise temperature and humidity control are imperative. Phenolic ductwork's excellent thermal insulation, durability, and resistance to moisture and chemicals make it a preferred choice for high-performance applications. During installation, attention must be given to the thickness, density, and elongation of the material to optimize its insulation and load-bearing capabilities. Proper sealing techniques are essential to prevent leaks and maintain the integrity of the system. In pharmaceutical settings, real-time monitoring through IoT technologies, such as thermal, humidity, and pressure sensors, enables early detection of issues and supports predictive maintenance. These sensors, integrated with IoT platforms, allow for dynamic adjustments to HVAC settings based on occupancy and process requirements, optimizing energy consumption and ensuring consistent environmental conditions. Regular inspections and maintenance checks are crucial for identifying potential issues and preventing costly downtime, ultimately extending the lifespan of the ductwork and maintaining the overall efficiency of the HVAC system.

Cost Analysis of Phenolic DuctworkThe cost-effectiveness of phenolic ductwork can be evaluated by considering both the initial investment and the long-term savings. While phenolic ductwork initially carries a higher price tag due to its superior thermal and acoustic insulation qualities, it can lead to significant energy savings compared to traditional materials like aluminum, steel, and fiberglass. Over time, reduced energy consumption translates into lower utility bills and a more comfortable environment, making the overall cost more justifiable. In large-scale commercial and industrial applications, where energy efficiency is a critical factor, the initial cost is often offset by substantial long-term savings. Moreover, efficient manufacturing processes result in less waste and lowered labor costs, further contributing to cost-effectiveness. For residential applications, while the initial cost is generally higher, the long-term benefits of enhanced energy efficiency and durability make it a worthwhile investment. This analysis highlights that the use of phenolic ductwork not only provides cost savings through energy efficiency but also enhances the building's sustainability and indoor air quality.