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Fiber fabric air distributor is a new product and technology introduced from abroad. It is a special fiber woven into the replacement of the traditional air supply pipe, air valve, diffuser, insulation materials, etc. In the domestic market are called fabric air duct, cloth air duct, cloth duct and so on.
The air distribution pipe system also has pressure loss caused by friction resistance and local resistance along the pipe length. Because the pressure loss is directly proportional to the wind speed, when the wind speed along the pipe length becomes smaller and smaller, the resistance loss also decreases. At the same time, there are also local resistance losses in the standard parts of the duct and the air outlet. The air distribution pipe system is mainly straight pipe, and there are few tees, elbows and diameters in the system. Generally, the resistance loss is mainly along the path. The friction resistance along the path when the air flows in the pipe with the invariable cross-sectional shape is calculated as follows:
-- Friction resistance coefficient;
-- The average velocity of air in the duct, m/s;
-- Density of air, kg/m3;
-- Air duct length, m;
-- Diameter (inner diameter) of circular duct, M;
The friction resistance coefficient is an indefinite value, which is related to the flow state of air in the duct and the roughness of the duct wall.
According to the comprehensive study of fiber materials and air distribution duct system, the friction resistance coefficient is not more than 0.024(about 0.019 for iron duct). Since there are air supply holes in the direction of air distribution duct extension, the average wind speed in the tube is 1/2 of the inlet velocity of the air duct. It can be seen that the distance loss of the cloth duct duct is much smaller than that of the traditional iron sheet duct.
Calculation of partial pressure loss of components
When the air flow in the air distribution pipe passes through the elbow, reducer, tee and other components, the section or flow direction changes, and the same as the traditional air duct will produce corresponding local pressure loss:
Z: Local pressure loss (PA)
ξ: Local resistance coefficient (mainly measured by experiment, similar to that of traditional air duct)
ρ: air density (kg/m3)
V: wind speed (m/s)
In order to reduce the local loss of the air distribution pipe system, we usually carry out certain optimization design:
1. The pipe is selected based on a variety of factors to reduce the wind speed in the pipe as far as possible.
2. Optimize the design of special-shaped parts to avoid too rapid change of flow direction and too fast change of section.
Based on practical engineering experience, we summarize local resistance values of various fabric air duct components (wind speed =8m/s), as shown in the following table:
Elbow (curvature =1) constant diameter tee reducer (taper Angle 30 degrees) static pressure box
10 pa 12 pa 3 pa 46 pa
For example: a supermarket pressure loss calculation instructions
For the supermarket, AHU air conditioning box air volume is 36000CMH, select number AHU-14 air conditioning box system, head size is 2000*610mm, a total of 5 branch pipe, branch pipe diameter is 559mm. The longest unfavorable loop was selected as 25 m main pipe +20.6 m branch pipe.
1. Calculation of resistance loss along the path:
Supervisor :25 m, 2000 x 610mm, equivalent diameter
Branch pipe: 20.6m, 559mm
2. Calculation of local resistance loss:
Local loss of equal diameter tee is 12Pa, and for variable diameter tee, 20Pa.
The longest unfavourable loop pressure loss is 20+8.5+6=34.5Pa.
It can be seen that the resistance loss of the air duct system, especially the straight pipe system, is very small and generally will not exceed the value of the static pressure, so it can be ignored in the rough calculation.
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