Pressure loss of bag duct in uniform air supply

Bag air duct, also known as fabric fiber air duct or bag type air supply duct, etc., is an air supply end device made of special fabric that integrates air transmission and distribution. Fresh air passes through fiber pores or small holes opened. Diffusion from the inside of the cavity into the room. Compared with the conventional air supply system, the bag air duct has the characteristics of easy cleaning, large air outlet area, light weight, simple construction and short cycle. At the same time, it can also solve the problems of cold air and surface condensation that are prone to occur in low-temperature air-conditioning systems. 1) The 'self-rectification' effect of the permeable bag air duct on the air is basically independent of the air supply volume, but the surface air velocity increases with the increase of the air supply volume, and there is an approximate linear relationship between the two. 2) With the increase of air supply volume, the total air pressure and static pressure value in the cavity of the permeable cloth bag air duct gradually increase, the static pressure increase gradient and dynamic pressure decay gradient become larger and larger, and the static pressure recovery phenomenon at the end becomes more and more serious. obvious. There are three factors that affect the pressure in the bag air duct: dynamic pressure, static pressure and pressure loss. The dynamic pressure is converted into static pressure along the length direction in the bag air duct, also known as static pressure recovery. The pressure loss along the way and the local resistance loss caused by the connecting piece are referred to as pressure loss for short. Therefore, the average static pressure in the pipe can be considered to be composed of inlet static pressure, static pressure recovery and pressure loss. Since the pressure loss is proportional to the wind speed, when the wind speed along the length of the pipe becomes smaller and smaller, the resistance loss also decreases. At the same time, there is a local resistance loss in the standard parts of the air duct and the air outlet. In the bag air duct, the straight pipe is the main component, the tee, the elbow and the reducer are less in the system, and the resistance loss along the way is the main component. The inlet static pressure is generally provided by the fan. Generally speaking, the standard of the inlet static pressure should be ≥125Pa, generally not more than 700Pa. The frictional resistance coefficient is an indeterminate value, which is related to the flow state of the air in the air duct and the roughness of the air duct wall. The pressure calculated in the air distribution duct can be used to overcome the drag loss along the path and the local loss and achieve the ideal airflow distribution. The drag loss is usually negligible. In theory, the air distribution pipe can be regarded as a static pressure box, but the static pressure in the pipe cannot be consistent. The dynamic pressure at the inlet does not change, and its static pressure is the inlet static pressure. The dynamic pressure is gradually converted into static pressure along the length of the pipe, and the static pressure reaches the maximum value at the end, which is the vector sum of the inlet static pressure, the static pressure recovery and the resistance loss. Terminal static pressure u003d inlet static pressure + static pressure recovery - pressure loss The average pressure in the pipe is the average of the inlet static pressure and the terminal static pressure. To sum up, when the air is uniformly transported in the bag duct, the air flow continuously decreases along the whole system, and the pressure loss gradually decreases with the decrease of the wind speed, thus significantly reducing the actual total pressure loss caused by friction, Therefore, in most cases, the pressure loss in the fiber duct is less than 1/10. When there is a local resistance loss, such as near elbows, at the outlet of fittings and main supply pipes, and at long straight pipes (greater than 30m), the pressure loss cannot be ignored. !

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