At present, the new control method is an adaptive control method. The airflow rate is kept at the minimum safety limit. The working condition of the fume hood is adjusted according to the user's state. The system responds sensitively, controls accurately, ensures the safety of personnel, and minimizes energy consumption. And maintenance costs. The primary problem to be solved by the ventilation design of the laboratory is the safety problem. The dust collection ability of the fume hood must meet certain standards and regulations. The airflow direction is to flow to the laboratory. The laboratory must maintain negative pressure at all times, in order to ensure the operator and the environment. The safety of modern laboratories needs to be considered as a key factor.
1. Stable wind speed on the fume hood
In the constant air volume ventilation system, when the regulating 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 a variable air volume ventilation control system, the exhaust air volume and the opening of the regulating door are linear functions. For example, 60% of the flow corresponds to 60% of the opening of the regulating door. Through this closed-loop control system, the surface wind speed of the fume hood opening can be maintained. Constant, eliminating the risk of excessive surface wind speed.
The effective face wind speed setting value of the fume hood, the general industry standard is 60-100fpm (0.3-0.6m/s), generally 100pfm (0.5m/s) is accepted as a safe operation standard. As shown in Figure 1, the movement of the operator has almost no effect on dust collection when the face wind speed is 80-100 fpm, but there will be disturbance effects when the face wind speed is below 80 fpm. When there is no operator moving, general dust collection below 60 fpm can be achieved.
2. Fast system response time
The response time mainly refers to the fume hood and its valve regulation system within the laboratory. The response time will directly determine the effect of airflow control. Fast and stable control will prevent toxic particles from escaping from the fume hood following possible oscillation or overshoot during the adjustment process. The quick response time of the exhaust air volume to the opening degree 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 Figure 2, the total response time from the movement of the regulating door to the exhaust air volume of the fume hood is about 0.6 seconds. Slow response time will produce excess surface wind speed, endangering the safety of the experiment, for example, the burner is blown out, the utensils are blown over, or the medicine is lost.
3. Ensure room pressure
The net negative pressure in the room mainly controls the flow of airflow from the outside to the inside and prevents the outside of the room from being contaminated. It is an important indicator of laboratory safety control. The generation of negative pressure means that the room's exhaust air volume or the difference between the supplementary air volume and the supply air volume is equal to the residual air volume of the room. The residual air volume is the amount of air entering the room from the room door, transfer window or other gaps other than the supply air.
When the air flow required for room temperature adjustment and ventilation is greater than the air flow required by the fume hood, the air supply volume of the room increases, which requires the control system to also discharge this part of the "excess" air supply volume to ensure Room negative pressure. The total air exchange rate in the laboratory is determined by the total exhaust air, cooling load and minimum ventilation rate. The minimum ventilation rate is generally 6-10 air changes per hour when occupied. This can be achieved by increasing the opening of the room's comprehensive exhaust valve. This control involves the calculation of the room's total air supply and exhaust, as well as information collection and fan frequency conversion control, which greatly increases the difficulty of system debugging.
4.pressure independence
The movement of the adjusting door of the fume hood causes a rapid change in the air volume, which leads to a change in the static pressure of the air duct, or when the wind speed of the fume hood is not required to change, the change of the wind pressure in the main pipe will also cause the change of the wind pressure in the upper branch exhaust duct of the fume hood. If the air volume control system cannot resolve the changes in the wind pressure of the pipe at this time, the exhaust air volume of the fume cupboard will become larger or smaller, which will affect the surface wind speed that should be stable at this time.
The traditional variable air volume adjustment system adjusts the exhaust air volume according to the differential pressure feedback signal of the pipeline. The response speed is generally between 20 and 30s. In order to ensure the safety of the laboratory, the response time of the laboratory air flow control system must be controlled within Within 1s.
The valve is a cone-shaped structure with a built-in stainless steel spring. The open area of the venturi is adjusted according to the change of system pressure to maintain a fixed air flow. When the pressure decreases, the spring opens and the valve core separates, which increases the ventilation; when the pressure increases, the spring compresses and the valve core approaches, reducing the ventilation.
5. Precise control system
If the system exhaust volume control cannot follow the rapid and precise change of the static pressure in the duct, it will cause the fume cupboard flow cannot be accurately controlled, and oscillation will occur (as shown in Figure 5), which will cause the room pressure to become a positive pressure or Increase the requirement for the residual air volume in the room. The instability of the wind speed will cause a lot of balance problems. The fluctuating air supply and exhaust system complicates the air flow balance. It will become more difficult for the automatic control system to realize the automatic adjustment function.
The face wind speed should be accurately controlled within a large air volume control range. When reaching the ideal control value, the control system should ensure that the adjusted over-adjustment and under-adjustment are less than 5% to ensure the dust collection capacity of the fume hood and the safety of the operator.
A complete ventilation and control system ensures the safety of the laboratory, which is also the key to the success or failure of laboratory construction. Therefore, the system design, equipment configuration, etc. must meet the basic requirements mentioned above, but the high-standard, high-quality laboratory is not limited to this, but also solves the problems of temperature, airflow, and noise. Guarantee the lowest energy consumption, the system is stable, easy to control, easy to operate and manage. In short, it is necessary to consider the design in terms of safety, comfort, energy saving, and reliable operation.
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