Chapter IV. Ducts and Fittings
4.1 Air ducts of ventilation systems
According to the type of section, the ducts are divided into round, square or rectangular. Circular air ducts are much stronger than rectangular ones, and their manufacture is less laborious.
However, circular air ducts often do not fit into the interior of public and civil buildings. In addition, in a confined space (under false ceilings, etc.), as a rule, only rectangular air ducts can be used.
According to the manufacturing method, the air ducts can be folded or welded. (Fig. 16.).
The first ones are connected with seam seams. The thickness of the metal for such ducts should not exceed 2.0 mm (if aluminum is used) and 1.0 mm (if corrosion-resistant steel is used).
The welded air ducts are connected by an overlap weld. The thickness of the metal with this type of connection is allowed in the range of 1.2 -3.0 mm.
Fig.16. Types of seam and welded joints of metal ducts:
1 – on a simple recumbent seam; 2 – on a double–cut seam; 3 – on an angular seam; 4 – on a transverse seam; 5 – on a seam with a latch; 6 – with a connecting bar; 7 – on zigs; 8 – butt; 9 – butt–to–butt with flanging; 10 – lap; 11 – corner
This type of duct is classified as dense.
All the various ventilation systems are assembled from only four standard parts.
The first is pipes with a length of 2000 mm or 2500 mm, depending on the metal used.
The second is from water, usually consisting of two or three segments and two glasses with an average radius of curvature equal to the diameter (Fig.17). They are used for general exchange ventilation systems.
Fig. 17. Outlet for air ducts
The third is rectangular bends, which are characterized by only two neck radii: 150 mm (for bends with a side less than 1000 mm) and 300 mm (with a larger size).
The fourth is the tees (branch nodes) of circular cross section. They exist in 3 versions:
– normalized, they are characterized by low coefficients of local resistance, but are laborious to manufacture;
– direct embedding;
– rectangular tees, they are usually equipped with one-sided unified transitions, which allows you to establish a constant ratio of the duct route from the wall along which it is laid.
According to the material used for the manufacture of air ducts, they are divided into several groups:
1. Folded air ducts made of thin-sheet galvanized steel, up to 1 mm thick (without painting).
2. Folding air ducts made of thin-sheet black steel, up to 1 mm thick (with subsequent painting inside and outside).
3. Welded air ducts made of sheet steel, 1.2- 3.0 mm thick (with subsequent painting with primer).
4. Seam and welded air ducts made of corrosion–resistant steel, thickness from 0.5 mm to 3 mm (usually grades X18N9T) – without painting.
5. Folding air ducts made of titanium (ρ = 4,500 kg/m3), which have the highest corrosion resistance when moving an aggressive medium.
6. Folding ducts made of metal plastic, clad on one or both sides with PVC or PVC film. With a one-sided coating, the film must be inside the duct, in contact with an aggressive environment.
The connection of individual parts of round air ducts to each other is performed by flanging bandages (with a diameter of up to 800 mm) or on flanges made of angular steel (with large diameters).
The connection of rectangular air ducts with a side less than 1600 mm is performed on profiled tires fastened with 4 bolts at the corners. If the side dimensions exceed 1600 mm, additional latches are used.
For conventional general exchange systems, it is preferable to use folded air ducts made of galvanized steel. Welded air ducts are installed with increased density requirements (smoke extraction shafts, air ducts passing through rooms with explosion hazard category A and B) and air movement with a temperature above 80 °C.
Flexible reinforced air ducts are widely used to avoid difficult fitting when connecting from mains to air distributors and grilles.
One of the latest solutions in the field of air distribution is textile air ducts. They are inflated by the flow of air and evenly distribute it along their entire length. In addition to noise reduction, textile ducts have the ability to trap all dust particles larger than 5 microns. They can be used in rooms with a high frequency of air exchange, without creating local areas with increased air mobility, as is the case with jet distribution.
This type of duct combines two devices – the duct itself and the air distributor.
There are several options for the operation of a textile duct. The air supply to the room can be carried out through its entire surface (in this case, airtight fabric channels made of 100% polyester are used) or through special holes made in the duct material (an impermeable fabric with perforated injector holes is used). Mixed versions are also used. The air flow velocity inside the airtight material does not exceed 0.01–0.5 m/sec, the air velocity coming out of the cracks is 4–10 m/sec, through perforated holes (injectors) is 7–13 m/sec.
Cylindrical fabric ducts are the most common. They are convenient if you need to provide intensive air exchange, preventing drafts. Semicircular air ducts are used in rooms with low ceilings, for example, in catering establishments, non-food stores, hotels, etc. Air ducts in a quarter-section of a circle can also be used, installed around the perimeter of the room.
Installation of steel ducts in rooms with high ceilings is quite laborious and time-consuming. The use of textile air ducts greatly facilitates the work. Sections of air ducts (usually 5 m long) are connected to each other with the help of zippers. The air ducts are suspended using stretched cables or rails. In the latter case, the fastening is more rigid, and the duct retains its shape even without air supply. When condensation occurs, a nutrient medium is formed that promotes the development of microorganisms. Textile ducts can be easily dismantled, washed or cleaned. They are produced with a diameter of 100 to 1000 mm and a length of up to 100 m, as well as various densities that allow changing the air supply from 160 to 500 m3 / g * m2 at a static pressure inside 100 Pa.
When transporting hot gases with a temperature of more than 100 °C in ventilation systems, the thickness of the steel walls of the air ducts should be 1–2 mm. To transport air with an admixture of toxic gases and vapors, air ducts must be made of certain materials, depending on the composition of the gases (steel with a thickness of at least 0.7 mm, aluminum, vinyl, etc.).
In ventilation systems of residential, public and administrative buildings, as well as office and auxiliary premises of industrial enterprises, air ducts made of slag-alabaster, cinder-concrete, reinforced cement and plastic plates, bricks, concrete, etc
. are used for portable ventilation installations rubberized, as well as metal flexible hoses.
When laying rectangular vertical air ducts, they try to make the most of the internal walls, in which appropriate channels are left for this purpose. If this is not possible, side channels and shafts are mounted to the walls.
In addition, spiral-wound metal air ducts are used in construction practice, as well as plastic air ducts that resist corrosion (made of vinyl, polyisobutylene, etc.). Their disadvantage lies in the fact that they can only be used at a temperature of transported air no higher than 70 °C.
In addition to these modifications, the ducts can be flexible, semi-flexible, thermally insulated, and also acting as a muffler.
4.2. Metal air ducts
Most often, metal ducts are used in air conditioning systems, which have great strength and resistance to fire.
They are made of galvanized or stainless steel.
Spiral-lock round (with a folding connection). They are made of steel tape with an anti-corrosion coating 0.5–1 mm thick, about 130 mm wide. They have increased rigidity compared to straight-seam ducts, there is no length restriction, aesthetic appearance, high seam density. The disadvantages include the fact that the seam takes about 15% of the metal from which the duct is made.
Spiral-welded round. They are made of steel tape with an anti-corrosion coating 0.8–2.2 mm thick, about 400–750 mm wide. The joints are welded overlap. Spiral-wound air ducts have less metal consumption compared to spiral-lock air ducts, there is no length restriction, high seam density. However, they cannot be made from steel thinner than 0.8 mm.
Rectangular straight-folding air ducts. They are made of steel sheet, it is desirable to place the seam on the bend so that it serves as an additional stiffener. Such ducts are easier than round ones to place in a space with a limited height, they fit better into the interior of the building. Rectangular ducts have greater aerodynamic drag than circular ducts with similar performance.
Round straight-folding air ducts. According to the manufacturing method and material, they are similar to rectangular air ducts, they differ only in a circular cross section.
4.3. Metal-plastic air ducts
Metal-plastic air ducts consist of two layers of metal, between which foam plastic is laid. Usually such a construction is used: foamed plastic 2 cm thick, located between layers of corrugated aluminum 80 microns thick. They have a small mass with high strength. They have a smooth surface, aesthetic appearance, do not require additional thermal insulation.
4.4.Non-metallic air ducts
Non-metallic air ducts are made of synthetic materials: polyethylene, fiberglass, etc.
Polyethylene air ducts: are usually used in supply ventilation systems. When the fan is turned on, the duct (in the form of a sleeve) is inflated with air.
Fiberglass ducts: usually used to connect the fan and air distributors. They are stretched on a metal frame.
Vinyl plastic air ducts: used when moving air containing acid vapors, etc., causing corrosion of steel. The thickness of the vinyl plastic is 3–9 mm, connected by welding.
Non-metallic air ducts are resistant to corrosion, they can be bent in any plane and at any angle.
4.5. Flexible air ducts
Flexible ducts of circular cross-section, lightweight, do not need special turns, as a result of which the duct has fewer connections, which simplifies installation. However, flexible ducts create a large aerodynamic drag, which may be excessive with an extended network, so they are often used as connecting pipes of small length. Flexible ducts are divided into:
• Flexible aluminum ducts without heat and noise insulation;
• Aluminum ducts with polymer coating without heat and noise insulation;
• Highly flexible polyvinyl chloride ducts;
• Highly flexible insulated air ducts consisting of two layers of polyvinyl chloride coated with polyamide fabric. A flexible steel wire spiral is located between the two layers.
• Semi-rigid aluminum ducts with thermal insulation;
• Flexible heat-insulated sound-absorbing air ducts. They consist of:
– Micro-perforated air duct made of high-strength metallized polymer;
– 25- 50 mm of thermal insulation layer with a density of 16 kg/m3;
– An outer coating made of aluminum seamlessly reinforced with fiber.
These ducts have excellent noise reduction performance in the low frequency range. To reduce the possible consequences of the use of mineral fibers on human health, the inner sleeve has minimal perforation, a special acoustic film is placed between the inner sleeve and the insulation layer. This reduces the probability of fiber entrainment at normal air flow speeds to almost a minimum.
Condensation formation, safety, noise, energy saving – these are the criteria that should be taken into account when choosing a material for thermal insulation of air ducts.
4.6. Thermal insulation
Thermal insulation of air ducts performs the following main functions:
• Prevention of condensation on both the inner and outer surfaces of the duct.
• Ensuring fire resistance to prevent the spread of fire in case of fire.
• Attenuation of noise and vibrations that occur during the movement of air through the duct.
• Reduction of heat transfer between the air flow in the duct and the environment.
For the organization of air ducts, materials with a zero (0) fire resistance class are used. If the channel has a multilayer lining, the fire resistance class "zero-one" (0–1) is allowed. This condition is met if all surfaces in the operating mode consist of non-combustible material with a thickness of at least 0.08 mm and if they provide continuous protection of the internal thermal insulation layer having a fire resistance class not higher than the first (1). Fasteners and connections whose length is not more than five times the diameter of the duct itself must be made of a material having a fire resistance class of "zero" (0), "zero-one" (0–1), "one-zero" (1–0), "one-one" (1–1) or "one" (1). Ducts of class "zero" (0) have an outer skin made of a material of fire resistance class not higher than the first (1).
Air treatment and air distribution systems create noise transmitted, among other things, through the duct system. It is possible to reduce noise if you maintain a low air velocity in the ducts, install damping devices at the point where the fan is connected to the duct, use an elastic suspension for the ducts, as well as damping gaskets at the points where the ducts intersect wall structures. The noise propagated through the air ducts can also be attenuated by the use of special silencers and a sound-proofing coating. Many thermal insulation materials have good sound insulation properties and can be used as both heat and sound insulation.
Thermal insulation of the duct can be performed from the inside or from the outside. When using thermal insulation inside the duct, it is necessary to increase the section of the duct to maintain the calculated carrying capacity at a given air velocity. In addition, the side of the thermal insulation in contact with the air flow should be smooth enough so as not to increase the resistance when air moves through the duct.
When installing silencers, the use of external thermal insulation is preferable. Also, the installation of external thermal insulation is the prevention of the occurrence of bacterial foci, the formation of dust and dirt deposits, due to which the thermal insulation material may begin to delaminate, release volatile substances and lose its qualities.
In addition, with external thermal insulation, the risk of fire spreading from room to room in the event of a fire is significantly reduced. The most important factor in the installation of thermal insulation is the prevention of thermal bridges that reduce the effectiveness of thermal insulation, as well as ensuring high vapor resistance. Heat transfer bridges can also occur in places where air ducts are attached to building structures.
The destruction of the thermal insulation material is prevented by:
• For internal thermal insulation – the use of composite materials, where thermal insulation is combined with a metal layer or film.
• For external thermal insulation – the use of neoprene sheathing, galvanized steel sheet or aluminum sheet.
Questions to Chapter 4
1. What are the disadvantages of circular ducts?
2. What are the advantages of rectangular ducts?
3. What material is used in the manufacture of air ducts for buildings and structures for general civil purposes?
4. What factors influence the choice of material for the manufacture of metal ducts?
5. How does the use of flexible air ducts affect the extended sections of the routes of ventilation systems?