When organizing a ventilation system, it is necessary to calculate the allowable thickness of the ducts at which they operate in normal mode. Installation of an air box or pipe with walls of smaller thickness leads to disruptions in their operation. It is first necessary to understand the conditions under which the elements of ventilation systems are operated in apartment buildings and other facilities.
The main characteristics of the ducts
In accordance with the regulatory documents on ventilation, the technical characteristics of the used ducts must fit into the framework of the requirements. In this case, the quality of air purification in the serviced premises is considered satisfactory. These requirements relate to the shape, permissible sizes of the applied structures and the materials from which they are made.
According to the cross-sectional shape, all known types of ducts are divided into round and rectangular. According to the wall thickness of the steel billets, they fit into the following standard series: 1 mm, 1.2 mm, 1.5 mm and 2 mm. There are samples with high rates (3.0 and even 4.0 mm). One of the main characteristics of these elements is the area of the air channel, which is usually taken into account when choosing its shape.
Round products have a smaller wall thickness, since, ceteris paribus, they can withstand large pressures. Their cross-sectional area is 12% less than the same indicator for rectangular samples. When comparing the lengths of the sides, the difference increases to 40%. This allows, without loss of efficiency, instead of a single rectangular air exchange channel, to make a pair of round ones arranged in parallel.
Duct classification
In accordance with the set of rules of the joint venture, the air ducts used in these systems are divided according to the following indicators:
- evolving internal pressure limit;
- air velocity in the supply or drain channels.
In accordance with the first indicator, they are divided into ducts of low (less than 900 Pa), medium (900-2000 Pa) and high pressure (over 2000 Pa).
According to the speed of air movement in the channel are low-speed (less than 15 meters / sec) and high-speed (more than 15 meters / sec).
In small apartments, low pressure ventilation systems with relatively low flow rates are installed. In rooms of a larger area, and especially in high-rise buildings, pipe channels with high pressure and high permissible air velocities in air ducts according to SNiP are in demand.
The use of thin steel
Unscrupulous companies in the manufacture of air ducts use steel slightly thinner than the current standards in Russia require. As a result of technological deviations, the walls thin out by 0.5 or even 1 mm. These violations can be identified at the low cost of the proposed samples, the manufacturers of which are trying to maintain their position in the competition.
The use of thin metal ducts is a gross violation of construction technologies and a threat to the health of people at the facility. Ventilation mounted on the basis of such blanks will quickly become unusable or will be extremely inefficient.This is because strong air currents quickly destroy the thin walls of the structure and lead to a decrease in the outflow of contaminated masses.
Also, a violation of technology leads to the following undesirable consequences:
- decreased overall equipment performance;
- unjustified costs of paying for electricity;
- increase in noise and vibration;
- rapid corrosion of the surfaces of hoods and air ducts.
All these manifestations lead to the need to upgrade equipment, which will require additional costs or cause a downtime of the ventilation network.
Types of duct and pipe materials
When choosing a quality duct, in addition to the wall thickness, you will need to determine the metal that is used in the manufacture. The most common varieties of such materials:
- stainless steel;
- galvanized stainless steel;
- aluminum and its alloys;
- black hire.
The first two positions are most suitable for mass production of air ducts. According to their strength indicators, they satisfy the requirements of current standards and are suitable for operation in industrial ventilation networks. The air in the latter is saturated with harmful substances and industrial wastes, which, despite their destructive effect, do not harm a clean stainless steel. In accordance with GOST, galvanized steel ducts are only slightly inferior to the first of these materials.
The use of aluminum in the manufacture of ducts has its positive aspects. Due to the smoothness of the internal surfaces, the air flows in the duct practically do not encounter obstacles. The speed of their movement in regular and emergency situations increases significantly.
Features and Benefits of Black Steel
Materials are used when the air distilled through the channels has a temperature above 80 degrees. For their manufacture, as a rule, steel products of cold or hot type are selected. The pipes themselves in this case are made welded, for which the appropriate equipment is used. The wall thickness may vary depending on the category of the object on which they are used.
By design, welded black steel ducts are:
- straight;
- shaped;
- with non-standard contours.
Black steel differs from other materials in increased fire resistance. Its disadvantages include low corrosion resistance, which forces manufacturers to additionally treat the surface with a special primer.
Normalized duct sizes
According to current standards (SNiP for ventilation 2.04.05-91-2003), the wall thickness of steel pipes and ducts should be commensurate with their external dimensions. For greater clarity, the corresponding ratios are summarized in special tables.
Round ducts
Most characteristics of the duct depend on the shape of the cross section, since it sets the norms for the speed of air movement in the room and inside the ducts. This parameter determines the overall performance of the entire exhaust system (natural or forced) as a whole. If you evaluate the ducts from this position, a circular cross-section is most preferable. When it is used inside the pipes, almost no turbulence is formed, the friction forces on the inner surfaces of the walls are minimal.
Ceteris paribus, to obtain maximum efficiency of various types of hoods, a round shape of the duct is suitable.
Rectangular ducts
Sanitary standards and other permits are allowed to install rectangular pipes (ducts) made of stainless steel or black steel. Air doesn’t move along them as well as in the version with a circular cross-section, but in this case they proceed from the design features of the rooms - it’s not always convenient to mount pipes. This explains the frequent use of rectangular structures, which in their efficiency are significantly inferior to the first.
Disadvantages:
- Individual parts of the ducts are mated with flanges with gaskets, through which air leaks often occur.
- The flows of air masses in the internal spaces are distributed unevenly, creating turbulent zones.
- The noise is rising.
When designing systems using large-sized ducts, negative effects only increase, significantly reducing the efficiency of ventilation. In addition, it is necessary to invest additional funds to pay for the electricity spent to compensate for the deficiencies considered.
