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?

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).

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.

The applied methods of air distribution are characterized depending on the direction of movement of the air supplied to the premises: "from top to bottom", if the supply occurs in the upper zone, and removal – in the working; "from top to top", if the supply and removal of air occur in the upper zone of the premises, as a rule, this method ensures the movement of the supplied flows above the working level zones, and in the working area, the set air velocity is created by the reverse flow; "side up", etc. When choosing the method of air distribution, the direction of heat flows and the areas of the premises characterized by the maximum intake of heat or moisture are taken into account.

If it’s necessary to provide increased air velocity in air conditioning systems with an increased value of the height of the working area of the premises (hP. Z > 2 m) it’s possible to provide air distribution through conical nozzles.

The aerodynamic coefficient of conical nozzles m varies in a wide range, and its values depend on the angle of taper of the nozzle. Conical nozzles are characterized by the highest (maximum possible) values of the aerodynamic coefficient. As the taper angle increases, the value of m also increases. At the same time, the maximum value of m = 7.7 … 7.8. Simultaneously with an increase in m, the coefficient of local resistance also changes significantly. Figure 5.1 shows a diagram of a cylindrical nozzle with preload.

Supply grilles of various designs, ceiling lampshades and diffusers are also widely used, capable of working as supply and exhaust devices.

In rooms of small height (up to 5 m), air distribution can be carried out through perforated panels mounted in the ceiling. In this case, the normative mobility of air in the working area is achieved with a large multiplicity of air exchange. Perforation holes are made with a diameter of 2 to 10 mm, the area of the living section of the panel does not exceed 10% of the total area of the panel.

In air distributors-lamps, air is released (removed) through fluorescent lamps mounted in the suspended ceiling structure. Such a solution in the case of air extraction reduces the heat input from the lighting into the room, since part of this heat is carried away together with the exhaust air, moreover, due to the cooling of the lamps, it increases illumination.

A sufficiently satisfactory air distribution can be achieved by using perforated air ducts. The holes in the duct of variable cross-section length are located in its lower part.