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The pipeline design of the local exhaust system

in order to prevent the equipment from being used in the production process, but this is not to say that it is not recommended that everyone should set up and implement the harmful substances produced by the equipment to pollute the air in the workshop. The harmful substances are often captured locally through the exhaust hood or air intake, and are transported to the purification equipment for treatment with pipelines. After reaching the 742 ⑴ emission standard, they are reused or discharged into the atmosphere. This is the local exhaust of the equipment

when there are multiple such equipment in production, and the local exhaust air volume of each equipment does not need to be very large, for economic considerations, they are often connected into a whole with pipes to form a local exhaust system, and the whole system shares a purification equipment and fan

in the local exhaust system, in order to achieve the effect of trapping harmful substances, it is required that the local exhaust system can exhaust according to the local exhaust volume required by each equipment. The key to this lies in the design of pipeline system. When the local exhaust air volume of each equipment is equal, it is called uniform suction duct system. Considering the general situation, when the local exhaust air volume of each equipment is unequal, it is called quantitative air suction duct system

in the past, the design of uniform air suction pipe used to adopt the constant velocity design method or speed reduction trapezoidal design method of main pipe, and the resistance balance method of confluence TEE was used for quantitative air suction pipe. These methods were too rough to analyze the problem, and the air volume deviation was large after operation, which could not meet people's requirements for the environment. In this paper, the design method of local exhaust system duct makes a more detailed analysis of the energy relationship of the air flow in the exhaust duct, and can carry out a more accurate calculation. After operation, the deviation between the exhaust air volume of the equipment and the design value is very small. It is now introduced for your reference in engineering practice

Such as cardiac pacemakers, artificial blood vessels, etc.1 pipeline design principle of local exhaust system

one end of the suction branch of the local exhaust system is the suction port on the machine, and the other end is connected with the main pipe at the junction tee. The branch air flow and the main pipe air flow have a common point at the junction tee, which is called the confluence point of two air flows (as shown in Figure 1). The two air flows converge at the confluence point. Generally, the dynamic pressure is different, but its static pressure is equal. The static pressure at which the main air flow reaches the confluence point is called the residual static pressure, which is the actual static pressure at the confluence point. The static pressure difference between the suction port and the junction point is the power of branch suction. For the branch pipe with given shape and size, as long as the suction state and the static pressure at the junction of the confluence tee are constant, the suction air volume of the branch pipe is constant. After the status of the branch pipe suction and the shape and size of the branch pipe are determined, the static pressure of the branch pipe at the junction tee can be obtained according to the air volume or wind speed, which is called the calculated static pressure of the branch pipe

for the ith branch pipe, as long as the Bernoulli equation between the suction section and the junction point section of the confluence tee is listed, its calculated static pressure can be calculated. When the suction port is in the atmospheric state, there is

therefore, (1)

in the formula, it is the calculated static pressure of the branch pipe at the junction point of the confluence tee, (PA)

is the sum of local resistance coefficients of branch pipes (including suction ports)

is the friction resistance coefficient of branch pipe

is the length of branch pipe, (m)

is the inner diameter of branch pipe, (m)

is the dynamic pressure of branch pipe, (PA)

therefore, it is concluded that the absolute value of the calculated static pressure is equal to the sum of the total resistance and dynamic pressure of the branch pipe when the suction port is in the atmospheric state and the air volume is given

the shape, size and suction state of each branch pipe are the same. If the given suction air volume is also equal, the calculated static pressure of each branch pipe is equal. If the residual static pressure of the air flow in the main pipe reaching the confluence point is also equal, then the suction air volume of each branch pipe is equal, which is the so-called static pressure constant principle. If the residual static pressure of the air flow in the main pipe reaching the confluence point is not only equal, but also equal to the calculated static pressure of each branch pipe, then the suction air volume of each branch pipe is not only equal, but also can suction air according to the given air volume, which is the principle of uniform suction

generally speaking, for branch pipes of any shape and size and any given air volume, as long as the calculated static pressure of each branch pipe is calculated and the residual static pressure of the main pipe air flow to the confluence point is equal to the calculated static pressure of the branch pipe, the branch pipe can absorb air according to the given air volume, which is the principle of quantitative air suction. It has more universal significance for the design of air suction pipe

2 calculation of residual static pressure

as mentioned earlier, the residual static pressure at the confluence point of the main pipe gas flow to the confluence tee is the actual static pressure at that point. At the confluence tee, the two air streams converge at the confluence point. Generally, the dynamic pressure is unequal, but its static pressure is equal. Before the i-th confluence tee, the planned target speed of the main air flow is, the speed of the branch air flow is, and the static pressure at the confluence point is, so the total pressure of the two air flows is and respectively. For uniform suction pipes, the suction air volume of each branch pipe is the same and Q, so the air volume of the main pipe reaching the i-th junction tee is IQ, and the air volume of the branch pipe is Q. The total mechanical energy of the main air flow is IQ (), and the total mechanical energy of the branch air flow is Q ()

after the two streams reach the confluence point, they mix, exchange energy and momentum in the mixing process, and suffer resistance to cause mechanical energy loss. The amount of mechanical energy loss is related to the actual confluence velocity VI and the theoretical confluence velocity UI. The theoretical confluence velocity is the confluence velocity with the minimum loss of mechanical energy when two air streams are mixed. Set the inclination of branch pipe and main pipe connection as α， For the uniform air suction pipe, the theoretical convergence speed of the i-th confluence tee is:

the resistance of the confluence tee is divided into the direct local resistance and the branch local resistance. The direct local resistance causes the loss of mechanical energy during the confluence of the main pipe gas flow, and its value is; The local resistance of the branch pipe causes the loss of mechanical energy when the branch pipe air flow converges, and its value is; The mechanical energy after mixing is equal to the difference between the mechanical energy before mixing and the mechanical energy lost during mixing, that is, the energy relationship between the two air streams when the confluence tee is mixed is:

(2)

, in which, it is the static pressure at the confluence point of the ith confluence tee, (PA)

is the static pressure of the i-th confluence tee after the mixing of two air streams, (PA)

, are respectively the direct local resistance coefficient and branch local resistance coefficient corresponding to the actual confluence velocity VI, and there are:

α ≤ 45 ° UI ≥ VI

(3)

(4)

α ≤ 45 ° UI VI

(5)

(6)

will (3), (4

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