Air Velocity in Kitchen Exhaust Duct

Determining the reasonable air velocity in the pipeline is an important step in the system design by using the assumed wind speed method. After the air volume calculation, firstlyto calculate the air velocity in the pipeline, and then calculate the cross-sectional area of the pipeline according to the air volume and air velocity Finally,to determine the specific structural size.

(1) Factors for air velocity determination

Determining the reasonable air velocity of the air flow in the pipeline and leaving some room is the most basic method to calculate the cross-sectional area of the pipeline. The determination of the reasonable air velocity of the air flow is based on a variety of factors. Understanding these factors is conducive to the selection of the recommended air velocity.

The recommended air velocity, i.e. the economic flow rate, takes into account the operation cost and pipeline fabrication cost. If the air velocity is too small, the cross-sectional area of the pipeline shall be increased and the pipeline fabrication cost shall be increased. If the air velocity is too high, thefrictional drag of the pipeline shall be increased according to the square of the air velocity, and the energy consumption of the operating fan shall be greatly increased, which greatly increases the operation cost. According to equations (9-25) and (9-35) It can be seen that the pressure loss along the pipeline and local pressure loss are related to pT2 / 2, that is, they are directly proportional to the square of air velocity.

In order to illustrate the relationship between air velocity and pressure loss, we take250x200 steel plate rectangular air duct as an example, select different air velocity to see frictional drag(table 9-24).

Frictional Drag Changes in Various Air Velocity

Air velocity/(m/s) 2 4 6 8 10 12
Pipe friction(pa/m) 0.28 1.0 2.11 3.63 5.54 7.85

The local resistance of tuyere, elbow, reducer and soft joint is also directly proportional to the square of air velocity. It can be seen that at high air velocity, the system resistance will increase exponentially.

The determination of the maximum air velocity mainly considers the operating pressure loss and the allowable noise level of the surrounding environment. The requirements for noise in kitchen, open stall, restaurant and guest room are different. Reducing air velocity is an effective method to reduce noise. In order to illustrate the correlation, the air flow noise of the straight pipe with a cross-sectional area of 0.5 M2 under different air velocity is compared below (table 9-25).

Airflow Noise at Different Air Velocity

Air velocity/(m/s) 2 4 6 8 10 12
Airflow noise/dB 22 37 46 52 57 61

In the actual environment, due to the limitations of the structural environment, it is impossible to select the air velocity completely according to the ideal state, or it needs to be considered comprehensively in combination with the actual situation. This needs to be based on the structural environment and calculation analysis. Only when all kinds of analysis and calculation are scientific and reasonable, can the arrangement be scientific and reasonable. The pipeline has horizontal, vertical, inclined vertical and other installation directions, and the air flow in the pipeline also has upward, downward, horizontal, indoor and outdoor directions; There are differences in length and thickness between main pipeline and branch pipeline; The physical characteristics of oil fume, steam and fresh air are different; There are differences between positive pressure and negative pressure in the pipeline. For these differences, the design air velocity should be different. According to these differences, appropriate adjustment can be made to select a more reasonable air velocity.