Installation, starting and adjustment of ventilation and air conditioning systems A Guide
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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?
Chapter V. Ventilation Parts and Network Equipment
5.1.Ventilation grilles
The grilles can be supply and exhaust. Those and others are adjustable and unregulated; round, square, rectangular; metal (more often steel or aluminum) or plastic; with and without decorative design; of various colors and sizes; with the direction of the flow of supply (or with the intake of the removed) air in one, two, three or four sides. Depending on the design of the grid, compact, flat, incomplete fan or other types of jets are created.
The control devices of the supply grids are the following types of regulators: flow regulator, jet characteristics regulator, direction regulator.
If the air distributor is equipped with more than one of the above regulators, then the regulating devices are usually installed in the same order along the air flow.
Exhaust grilles can also have flow and direction regulators. Some grid designs are universal and are used in both supply and exhaust systems.
Grilles are installed more often on the walls above the serviced area. At the same time, they can be specially designed for installation in the ceiling (for exhaust, inflow or universal).
5.2. Air distributors
The supply of the air in ducted and ductless air distribution systems is carried out in the form of jets.
Table 5.1 shows the schemes of air distribution devices and their main characteristics: aerodynamic coefficient m, temperature coefficient n and local resistance coefficient ?. The coefficients m and n characterize the rate of attenuation of air velocity and temperature in the supply jet.Such indicators are necessary to calculate the speed and temperature of the air in the working area of the premises.
Table 5.1. Characteristics of air distribution devices
The choice of air supply methods depends on the height and purpose of the premises, the height and placement options of the equipment, as well as on the requirements for the uniformity of the distribution of air parameters.
Choosing the methods of air supply and removal, at the same time pre-select the supply and exhaust air distribution devices, their number and placement options relative to each other. The joint choice of air supply and removal methods is practically determined by the choice of the air distribution method in this room.