Simple estimation and measurement of indoor and outdoor wind pressure difference in fresh air system

Simple estimation and measurement of indoor and outdoor wind pressure difference in fresh air system

At present, most of the fresh air systems we use follow the design philosophy of large air volume and slight positive pressure, but many people are also confused about this. How to detect the pressure in the room on site? If the fresh air system compares air volume to the swing power of a weapon, then wind pressure is the sharpness of the weapon. After the fresh air system is turned on, is the indoor pressure positive or negative? How big is the pressure difference? How to measure? Here is a simple and easy measurement method for your reference only.

　　 Wind pressure is when the wind moves forward at a certain speed and encounters an obstruction, it will generate pressure on the obstruction, and the wind is formed by air flowing from a place with high air pressure to a place with low air pressure. From the formation of wind, we can see that wind and pressure are inseparable, and pressure generates wind. Since wind pressure generates wind, we have an easy way to measure pressure using wind.

　　 Using this formula, we can use an anemometer to estimate the pressure difference between indoor and outdoor. The reason why it is estimated is because the wind speed may have a large difference in different positions and the average value is difficult to calculate. Because of the different external conditions, the roughness of the wind propagation medium is different. The wind speed measured at different distances will not be the same.

　　 We regard the room to be measured as a static pressure box. After the fresh air system is turned on, the pressure of the indoor gas when the system introduces fresh air is greater than the same standard atmospheric pressure, and the room is in a positive pressure state, otherwise, it is a negative pressure state.

1. Measuring position and measuring point

　　 To measure the true pressure value of the gas in the room, in addition to the correct use of pressure measuring instruments, a reasonable choice of measurement location and reduction of airflow disturbance have a great impact on the measurement results. When the test site is difficult to meet the requirements, in order to reduce the error, the measurement points can be appropriately increased. Of course, since it's a simple test, it's a rough number, so you can't be serious.

We can choose a door far away from the fresh air outlet or other available gaps, use a breeze meter to measure the wind speed of the holes (such as door mirror holes) and the upper and lower gaps of the door, and use multiple points to measure the average value and the wind passing area. , Calculate the approximate indoor pressure, and use its value to evaluate whether the indoor fresh air system can achieve the expected effect.

　　 When selecting a measuring point, the convenience and safety of the measuring operation should also be considered.

　　 2. Test hole and measuring point

　　 Due to the unevenness of the velocity distribution, the pressure distribution is also uneven. Therefore, it is necessary to measure multiple points on the same section, and then find the average value of the section.

　　The commonly used pitot tube and pressure gauge can be used, but it is difficult to read the accurate pressure difference.

　　The commonly used direct-reading anemometers are thermal bulb thermoelectric anemometers and impeller anemometers.

　　 The measurement of crevice wind speed generally uses uniform movement method, fixed-point measurement method, etc.

　　 For example, for an exhaust hood with a gap area of ​​less than 0.3m2, the anemometer can be moved slowly and uniformly along the entire gap. The anemometer must not leave the measurement plane when moving. The result measured at this time is the average wind speed of the gap. Take the average of multiple measurements.

　　 The average wind speed of the slit is calculated by the arithmetic average.