1. Stable ventilation speed of ventilation cabinet
In a constant air volume ventilation system, when the control door is lowered, excess surface wind speed will be generated, which will cause eddy current interference, affect the dust collection capacity of the fume hood and release toxic particles. In the ventilation control system with variable air volume, the relationship between the exhaust air volume and the opening of the regulating door is a linear function. For example, 60% of the flow corresponds to 60% of the opening of the regulating door.
通风柜有效的面风速设定值，通用的工业标准是60—100fpm（0.3—0.6m/s），一般100pfm(0.5m/s)被接受作为安全运行标准。图1所示，操作人员的移动在面风速80—100fpm时对集尘几乎没有影响，但在80 fpm以下时会有扰动影响。在没有操作人员移动时，60 fpm 以下一般性集尘可以实现。
The general industrial standard is 60-100fpm (0.3-0.6m/s), and the general 100pfm (0.5m/s) is accepted as the safe operation standard. As shown in Figure 1, the movement of the operator has little effect on dust collection at the surface wind speed of 80-100 fpm, but disturbance effect below 80 fpm. The general dust collection below 60 FPM can be achieved without moving the operator.
Two. Fast system response time.
The response time mainly refers to the fume hood and its valve regulating system in the laboratory. The response time will directly determine the effect of airflow control. Rapid and stable control prevents toxic particles from oscillating or overflushing out of the hood during the adjustment process. The quick response time of the exhaust air volume to the opening of the regulating door must reach its command value within 1 second after the regulating door is in place to effectively ensure the dust collection capacity of the fume hood. In Fig. 2, the total response time from the adjusting door to the exhaust volume of the hood is about 0.6 seconds. Slow response time will produce excess surface wind speed, endangering the safety of the experiment, such as burners blown out, containers blown over, or drug losses.
Three, ensure room pressure.
The net negative pressure of a room mainly controls the flow of air from outside to inside and prevents the outside of the room from being polluted. It is an important index of laboratory safety control. The negative pressure is generated by the difference between the discharge volume of a room or the volume of air supplied and the amount of air supplied to it, equal to the amount of air remaining in the room. The air volume is the amount of air that enters the room from outside the door, through the window or other gaps.
当房间温度调节与通风所需气流量大于通风柜所需气流量的时候，房间的送风量增加了，这就要求控制系统还必须把这部分“过剩”的送风量排出去，以保证房间负压。实验室总的空气交换率由总的排风、制冷负荷和低通风率决定。小通风速率一般为占用时每小时 6–10 次换气。这可通过增大房间综合排风阀门的开度来实现，该控制涉及房间总送风量和排风量的计算，以及信息采集、风机变频的控制等，系统调试的难度大为增加。
When the room temperature regulation and ventilation required air flow is greater than the required air flow of the fume hood, the room air supply increased, which requires the control system must also discharge this part of the "excess" air supply to ensure room negative pressure. The total air exchange rate in the laboratory is determined by total exhaust, refrigeration load and minimum ventilation rate. The minimum ventilation rate is usually 6 to 10 times of ventilation per hour. This can be achieved by increasing the opening of the room exhaust valve. This control involves the calculation of the total air supply and exhaust volume, as well as information acquisition, fan frequency conversion control and so on. The difficulty of system debugging is greatly increased.
Four, pressure independent
The change of wind pressure in the main pipe will also cause the change of wind pressure in the upper branch of exhaust duct if the air volume control system can not resolve the change of wind pressure in the duct. The exhaust volume of the cabinet is larger or smaller, which affects the surface wind speed which is stable at this time.
The traditional variable air volume control system is based on the pressure difference feedback signal of the pipeline to adjust the exhaust air volume. The response speed is generally between 20 and 30 seconds. In order to ensure the safety of the laboratory, the response time of the laboratory air flow control system must be controlled within 1 seconds.
The valve is a spine-shaped structure with a built-in stainless steel spring that adjusts the area venturi opens to maintain a fixed air flow rate according to changes in system pressure. When the pressure decreases, the spring opens, the valve core separates, and the ventilation rate increases; when the pressure increases, the spring compresses and the valve core approaches, thus reducing the ventilation rate.
Five. Precise control system.
If the exhaust air volume control of the system can not follow the change of the static pressure in the air duct quickly and accurately, the flow of the exhaust hood can not be accurately controlled, and the oscillation (Fig. 5) will cause the room pressure to become positive pressure or increase the room residual air volume requirements. The instability of wind speed will cause a lot of balance problems. The fluctuating air supply and exhaust system will complicate the air balance. It will be more difficult for the automatic control system to realize the automatic adjustment function.
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