Air ducts move gas-air mixtures, and also supply clean air in a given direction. They differ in shape and cross-sectional dimensions, length, material, installation methods and nature of operation. Often, different types of air ducts are combined into a single ventilation scheme, creating branches, bends and sleeves. Rigid and semi-rigid metal ventilation pipes are most in demand in industrial and civil construction.
Duct classification
galvanized steel
air ducts Air ducts are installed in ventilation systems of various characteristics. Therefore, there are many types of air ducts, they are combined into subgroups according to the following qualities:
sectional shape (can be square, oval, round, rectangular);
cross-section diameter (there is a standard set of diameters for different types of cross-sections; air ducts of any diameters are produced by special order for ventilation);
material (sheet metal, plastic, metal-plastic);
construction (longitudinal seam or spiral wound);
rigidity;
fastening method (with or without flanges);
type of fastening (bends, tees, turns).
Selection of air ducts
Sectional shape
air ducts and fittings for different sections
The most popular sectional shapes used in the construction of ventilation are rectangular and round. In some cases, it is possible to install only flat air ducts for ventilation. They are made from round pipes compressed into an oval using special equipment.
The production of round air ducts is cheaper, they use less material and the technology itself is simpler. For example, for the manufacture of a metal rectangular duct, 25% more metal will be used than for a round duct for ventilation of the same size and throughput. This is explained by the fact that a rectangular pipe is assembled from several patterns.
Pros of round ducts:
excellent tightness;
high aerodynamic properties (there are no obstacles to the passage of air);
quiet work;
easy to install;
weigh less than rectangular.
The main advantage of rectangular (flat) ventilation ducts over round ones is that they fit more easily into confined spaces. Therefore, with lower aerodynamic qualities and noisier operation, rectangular air ducts are often installed in offices, country cottages, that is, in relatively small objects.
The listed advantages put round air ducts in the first place in industrial ventilation. Some manufacturers claim that it is more profitable to install 2 round ducts in the ventilation in parallel than 1 rectangular or flat one. This statement is true for a direct ventilation network. With many branches, approximately one third of the line area is occupied by fittings, which are quite costly.
Air duct cross-sectional diameter
an adult can easily fit into an industrial duct
The dimensions of the ventilation ducts depend on the design values of the flow rate. So, for residential premises, the speed is limited to 4 m / s. Otherwise, the hum will disturb people.
If the speed of movement is known, then the cross-sectional area is determined by the formula:
Smin = 0.9 * L,
here: L is the air flow rate in cubic meters per hour, Smin is the minimum cross-sectional area of the air duct in sq. centimeters.
According to the regulatory requirements set out in VSN 353-86 and SNiP 41-01-2003, round galvanized ventilation ducts are produced in the following diameters in mm: 100, 125, 160,140, 200, 180, 225, 250 to 2000 mm. The dimensions of the cross-section of rectangular air ducts are also regulated: 100 - 3200 mm.
Design
longitudinal seam air duct of square cross-section
Structurally, air ducts are folded or longitudinal seam, spiral-welded and spiral-wound.
Straight-seam air ducts are also called industrial, they are made from a single piece of metal sheet 1 - 2.5 meters long. Used steel sheets with a thickness of 0.5 mm - 1.2 mm. The seam connection strengthens the rigidity of the galvanized ventilation duct, therefore it is more often placed at the bend.
Spiral-wound (lock) pipes are made from a metal strip (strip) up to 1 mm thick. The width of the strip is no more than 13 cm, the length can be any. The tape is folded in two ways: in a tape or in a ring. The second method of manufacturing is more expensive, but stainless steel ventilation ducts are much higher in quality.
Spiral-welded galvanized air ducts for ventilation are made from templates with a width of up to 0.75 m and a sheet thickness of 0.75 - 2.2 mm. The edges of the pattern are overlapped and welded. The result is a strong, tight seam.
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Materials
Galvanized steel ventilation ducts are used to transport air of standard humidity, heated to no more than +80 degrees, without admixtures of active substances. Zinc protects steel from oxidation, increasing the service life by several decades, significantly increasing the price of the pipe. Fungus does not develop in galvanized air ducts, so they are good for working in high humidity conditions (canteens and restaurants, swimming pools, showers).
Stainless steel ventilation ducts transport air masses heated to +500 degrees. For industrial air ducts use fine-fiber, heat-resistant steel, resistant to aggressive substances. The wall thickness can be up to 1.2 mm. Corrosion-proof air ducts are expensive, but the most durable ventilation is collected from them. Most often they are installed in industries associated with the release of heat, radiation, and abrasive particles.
Plastic air ducts are also good for transporting active gas mixtures through ventilation. They are installed in pharmaceutical, chemical, food factories and laboratories. Usually, plastic ventilation ducts are made of PVC (modified polyvinyl chloride). It withstands contact with moisture, vapors of alkalis and acids. Plastic ventilation elements create sealed joints, they are lightweight and have a smooth inner surface. In supply ventilation systems, propylene air ducts are sometimes installed.
plastic
air ducts Metal-plastic air ducts include 2 layers of metal foil, sandwiched with foamed plastic. Air ducts made of metal-plastic do not need thermal insulation, are lightweight, very strong and look attractive. Compared to polypropylene ventilation ducts, they are quite expensive, so they are used to a limited extent.
Vinyl plastic air ducts are characterized by unsurpassed resistance to chemical elements. Like polypropylene ventilation ducts, they can be bent to any angle, they are strong and lightweight.
Fiberglass air ducts are used in the ventilation of chemical plants for the transfer of vapors of alkalis and acids, as well as internal ventilation of electroplating shops. Fiberglass pipes can be installed outside the building and are UV resistant unlike cheaper plastic ventilation ducts.
Rigidity
flexible vinyl duct
Generally, ventilation systems are equipped with rigid circular or rectangular ducts with additional thermal insulation with basalt wool. Such air ducts provide the lines with tightness and strength. Plastic pipes are produced on extruders, and metal pipes on roll forming machines. Rigid ventilation ducts are easy to install and have high air movement rates. When creating an extensive branched network of air ducts, it is necessary to calculate its total ventilation weight and select reinforced fasteners in advance.
Flexible air ducts for ventilation are made in the form of a corrugated hose. The air duct frame is a rigid steel wire, coiled and covered with laminated foil or polyester. Often the walls of flexible ventilation ducts are made multilayer. The advantage of coiled tubing is its unique ease of installation, repair and transport. The pipe can be bent in any direction, it is repeatedly compressed and stretched, new bends are easily attached to the finished system, it can withstand up to +140 degrees (foil), up to +90 polyamide.
A serious disadvantage of flexible air ducts is the corrugated inner surface. It obstructs the air, reduces its speed and causes additional noise.
Semi-rigid air ducts incorporate the best qualities of flexible and rigid pipes. They are flexible and at the same time very durable. Semi-rigid air ducts are produced from rolled metal strips (aluminum). Semi-rigid air ducts withstand up to +300 degrees, and steel air ducts up to +700, so they can be used for smoke removal systems.
Unlike flexible air ducts, semi-rigid air ducts stretch only once, after which they do not shrink. The presence of spiral seams also negatively affects aerodynamics, reducing the inner diameter of the ventilation duct. Therefore, in complex ventilation systems, semi-rigid air ducts are not used.
Methods and types of fasteners
flange for rectangular duct
For connection of ventilation ducts, flange and bandage (flangeless) fasteners are most often used. It is desirable that the ventilation system has as few duct connections as possible.
With the flange type, flanges are located at the ends of the air ducts and fittings, which are fastened together with rivets or self-tapping screws. Rivets are placed every 20 cm, in some cases, welding is used. The flanges are sealed with rubber gaskets, a tight connection of the ventilation ducts is created.
The flangeless method consists in the fact that a bandage of metal strips and a strip of thin metal is applied to the junction. This method is more economical, since less metal is spent, the installation of ventilation ducts is faster.
Air duct installation rules
air duct installation diagram with flange connection
Before installation work, the ventilation system is divided into enlarged blocks, the length of one unit cannot exceed 15 meters. The nodes are assembled according to the following algorithm:
Places of holes and fasteners on ventilation ducts and fittings are marked.
Holes are made.
Clamps are installed and bolted, all joints are sealed with special compounds or tape.
Shaped elements and ventilation ducts are mounted in enlarged units.
Secure the clamps and fasteners.
Raise the finished unit and hang it on the ready-made fasteners.
Attached to the previously installed section of the ventilation duct, the joints are sealed in diameter.
Installation of flexible and semi-rigid ventilation ducts is easier compared to rigid galvanized ones. The pipes are much lighter, turns and bends do not require special work, special attention should be paid to the connections of ventilation ducts, insulation and sealing of joints.
The flexible duct is fully stretched before installation;
Passage through walls is carried out only with the help of special adapters (sleeves);
The air duct must not come into contact with heating pipes;
When pulling the flexible air duct, it is necessary to observe the direction of air movement indicated on the pipe and packaging;
The bending radius of the flexible duct must be at least 2 diameters;
Foil tape, plastic clamps, hangers, clamps, etc. are used to connect the sections to each other. All joints are necessarily sealed;
The size of the ventilation duct must match the diameter of the clamp, if too small a clamp is selected, the inner section is squeezed;
The distance between the fasteners of the ventilation ducts can be 1 meter for horizontal placement and 1.8 m for vertical placement;
The permissible sag of the flexible pipe is 5 cm per meter of length.
While there are many advantages, flexible pipes are used to a limited extent in ventilation. For example, they are not suitable for vertical highways with a vertical drop of more than six meters.
