Views: 8 Author: China xicheng Publish Time: 2021-05-28 Origin: China xicheng
The laboratory ventilation system is one of the largest and most influential systems in the entire laboratory design and construction process. The design of its ventilation system has an important impact on the comfort of the laboratory environment, the health of the experimental staff, the operation and maintenance of the experimental equipment, etc., and is related to the investment cost, energy consumption and subsequent operating costs of the laboratory.
The purpose of the design of the laboratory ventilation system is to provide a safe and comfortable working environment for the staff and reduce the possibility of exposure of the staff to dangerous air. Since toxic and harmful gases may be generated during the operation of the experiment, in order to ensure the safety of the experimenters, such experiments that may generate toxic and harmful gases are required to be carried out in the fume cupboard or biological safety cabinet as much as possible, and cannot be carried out in the fume cupboard. Local exhaust facilities, such as atomic absorption hoods, universal exhaust hoods, etc., must be set up. At the same time, the laboratory must have a comprehensive ventilation (or auxiliary exhaust) system to meet the laboratory's minimum ventilation requirements. The design goal of the ventilation system is to make the laboratory have good ventilation effect, low noise, easy operation, and energy saving. The indoor pressure difference and temperature and humidity can maintain the comfort of the human body.
Laboratory ventilation systems generally have the form of constant air volume system, dual air volume system, and variable air volume system:
1. Constant air volume exhaust system: The exhaust fan adopts a single-speed fixed frequency fan, and the exhaust air volume is basically constant. The system cannot adjust the air volume according to the number of fume hoods used and the opening height of the fume hood doors. The advantages of this system are direct design, low investment, and simple control. The disadvantages are poor flexibility, high noise, and difficulty in ensuring the wind speed of the fume cupboard. Some toxic and harmful gases will escape from the fume cupboard (the surface wind speed is too small or too large. It can cause gas to escape from the fume cupboard), and the operating cost is high (the power consumption of the exhaust fan is maximized, and the heat energy consumption discharged by the exhaust fan is maximized).
2. Dual air volume system: the exhaust fan adopts a two-speed fan, which switches between high-low (or maximum-minimum) between two predetermined air volume values, which is an improvement over the constant air volume system, but not fundamentally The shortcomings of changing the constant air volume system are not widely used at present.
3. Variable air volume system: The exhaust fan adopts a frequency conversion fan, and a pressure sensor is set on the main air duct of the exhaust or supplementary air. The static pressure controller PID is used to control the frequency converter to adjust the fan speed to achieve the purpose of constant static pressure or dynamic pressure to adjust the air volume. The high-performance airflow controller can be adjusted quickly and stably within a large flow rate change range. In the variable air volume system, the fume cupboard generally adopts the face wind speed control. A human sensor is used in front of the fume cupboard to detect the presence of personnel to control the wind speed of the cabinet door. When the operator is working in front of the fume cupboard, the fume cupboard The surface wind speed is set to work mode. At this time, the wind speed of the fume cupboard is controlled at 0.5m/s. When the operator leaves, the wind speed of the fume cupboard is set to the standby mode, and the wind speed of the fume cupboard is controlled at this time. 0.3m/s, complying with the requirements of JG/T222-2007 specification of "Laboratory Variable Air Volume Exhaust Cabinet". In addition, the room should be equipped with auxiliary exhaust air. When the exhaust air volume of the fume cupboard does not meet the minimum exhaust air volume requirements of the laboratory, the auxiliary exhaust air will be automatically turned on to ensure that the minimum exhaust air volume of the laboratory meets the standard requirements. The advantages of the automatic variable air volume control system include: significant energy saving, high safety, and easy adaptation to changes in air volume.
When the exhaust air volume of the fume cupboard does not meet the minimum exhaust air volume requirements of the laboratory, the auxiliary exhaust air is automatically opened to ensure the minimum air exhaust volume in the laboratory. The number of air changes in the exhaust air volume is in accordance with the function and requirements of the room.
In order to ensure that there is no air flow backtracking, the laboratory will generally set a higher exhaust air volume and a long continuous exhaust time (usually the exhaust system is turned on for 7*24 hours), so the negative pressure in the laboratory is too large The situation is very common. In order to balance the indoor and outdoor pressure difference and avoid excessive negative pressure, the laboratory needs to plan a laboratory air supply system during the design to maintain the set pressure in the room, and the negative pressure prevents toxic gases, particles and chemicals during the experiment. The harmful gases volatilized during the storage of the product spill to other spaces. The positive pressure laboratory is not affected by external air pollution, and the air exchange rate in the laboratory can be effectively controlled. The indoor air should be fully replaced to ensure its freshness.
1. Compensation air volume calculation
There is a corresponding calculation formula for the setting of the supplemental air volume, which should be calculated and determined according to the laboratory exhaust air volume and the set negative pressure value (relative to the adjacent room, corridor or atmosphere). The supplemental air volume is calculated according to the following formula: Lb=Lp-Ly .
In the formula, Lb is the supplemental air volume, m3/h; Lp is the exhaust air volume, m3/h; Ly is the air volume required to maintain negative pressure, also called residual air volume, m3/h; Ly can ventilate the room according to the negative pressure value The relationship of the frequency can be determined, and it can also be calculated according to the gap method. According to the pressure difference function of the room, the proportion of the supplementary air volume ranges from 50% to 90%, and the proportion of the supplementary air volume for rooms with positive pressure requirements is not less than 110%.
2. Ways to make up the wind
There are generally two methods of supplementing air in the laboratory, namely natural supplementation and mechanical supplementation.
Natural supplementary air does not install a supplementary fan. The outdoor fresh air is sucked into the room through filters and pipes through the pressure difference between the inside and outside of the laboratory to achieve the effect of supplementing air. Because the fresh air is forced into the negative pressure room by the outdoor positive pressure, Therefore, the natural supplemental air can make the airflow reach a dynamic balance, while also ensuring that the laboratory airflow is stable and always in a negative pressure state. The advantage of natural supplemental air is that it does not need to install supplementary fans, which saves the initial investment and electricity costs for supplementary fan operation; the disadvantage is that the area of the air inlet shaft or the air inlet louver needs to be expanded in advance during the design and decoration, which requires the use of building area or Affect the effect of the building facade, followed by the natural wind that cannot be heated or cooled without passing through the supplementary fan.
The mechanical supplemental air is equipped with induced draft fans to form an ideal air flow organization. After the outdoor fresh air is centrally processed, it is sent into the room by the supplementary fan through the pipe. The mechanical supplemental air can be pre-processed such as heating or cooling treatment and filtering, and adopts a constant air volume. Supplementary air or variable air volume supplementary air system generally corresponds to the exhaust system, that is, the exhaust air is a constant air volume system, the supplementary air is also a constant air volume system, the exhaust air is a variable air volume system, and the supplementary air is also a variable air volume system.
3. Replenishment location
There are generally no restrictions on the location of indoor air supply. It can be directly added in the fume cupboard or in front of the cabinet door, or in the room, but different air supply locations have different effects. In the fume cupboard Or the advantage of the air supply in front of the cabinet door is that there is no need to heat or cool the fresh air, which can save energy and cost; the disadvantage is that the fresh air will affect the experimental data during the experiment in the fume hood, and at the same time damage the comfort of the operator’s working environment. This kind of supplementary air location is not suitable for exhaust systems with face wind speed requirements. In contrast, when the air supply location is set in the room, the advantage is that the working environment of the operator is basically not affected, and it has no effect on the wind speed of the exhaust cabinet and the airflow in the cabinet; the disadvantage is that the fresh air generally needs to be heated or The energy cost of cooling treatment is higher than that of the first type of air supply location.
One type of fume hood variable air volume controller is to measure the wind speed of the cabinet door. By comparing with the set wind speed, adjust the opening of the air valve to reach the set value. The air valve generally adopts a butterfly valve. Its disadvantages are: accuracy is affected by the location and number of measurement points It is easier to be affected by the installation position and internal and external air flow (turbulent flow). The measured wind speed needs to be converted into an electrical signal to adjust the air valve through a sensor. The system response speed is slightly slower. The control signal response time: <3 seconds, the surface wind speed control accuracy Generally within ±20%.
Another type of fume cupboard variable air volume controller, the current mainstream use is the Venturi air volume controller. By measuring the position of the sliding door, the opening degree of the sliding door is measured by the displacement sensor and transmitted to the controller. The controller is based on "flow = average The calculated air volume is displayed in the "flow velocity × area" to change the valve opening size in real time to adjust the exhaust air volume. The system has good anti-interference performance and stable control. Theoretically, it is an open-loop control. In the application of reagents, an anemometer or other feedback device is required to ensure the accuracy and reliability of the control. The opening degree of the adjusting air valve has a precision that is not affected by the presence or absence of people in front of the cabinet, the air volume adjustment process is rapid and accurate, and the response time is short. It can achieve a high adjustment ratio (1:17~20) that is impossible with the traditional variable air volume valve. The pipeline static pressure is independent (150~750PA), the response time is fast (less than 1 second), and the air volume control is accurate to ±5% of the air flow control signal.
1. Working principle of the wind speed control system of the fume hood
To determine a safe and effective surface wind speed, the dust collection capacity of the fume hood is a key issue for laboratory staff. Several factors related to the correct dust collection capacity are: face wind speed, cross ventilation and work practices. Research and field experience enable people to understand the effective face wind speed setting value of the fume hood. The general industry standard is 0.3m/s~0.6m/s. Now in many facilities, 0.5m/s is accepted as a safe operation standard. Studies have shown that the 0.5m/s face wind speed requirement is partly due to the presence and movement of operators. The unoccupied fume hood (that is, those fume hoods that are not used by anyone) can contain harmful gases when the surface wind speed is reduced to 0.3m/s. The occupied fume hood requires a higher flow rate to ensure the correct dust collection capacity of the fume hood. The movement of the operator has almost no effect on the dust collection capacity of the fume hood when the surface wind speed is 0.4m/s~0.5m/s. But below 0.4m/s, the disturbance will obviously cause the concentration of harmful gases to increase. And when there is no operator moving, the wind speed below 0.3m/s can also meet the requirements of the laboratory's general dust collection environment.
2. Air volume control method
At present, the control methods for air volume include surface wind speed method and displacement method.
The face wind speed method usually uses the wind speed sensor installed at the edge of the fume cupboard to measure the wind speed and transmit it to the controller. The required air volume is obtained through comparison and calculation, and then the air valve opening is changed to adjust the exhaust air volume to finally make it The face wind speed is maintained at a certain set value or a certain smaller range of 0.3m/s~0.6m/s (OSHA). Its characteristics are: the structure is simple and easy to install, and the control is relatively easy; the closed-loop feedback control guarantees the safety and energy-saving control effect; the face wind speed is replaced by a point wind speed, and the face wind speed of the real control object will have some deviations. It is more susceptible to the installation location and internal and external airflow (turbulent flow).
Displacement method, with the use of VAV variable air volume Venturi valve, the cabinet door opening is measured by the displacement sensor and transmitted to the controller. The controller changes the valve opening in real time according to the air volume calculated by the publicity of "flow = average flow rate x area" Degree to adjust the exhaust air volume. The system has good anti-interference performance and stable control. Theoretically, it is an open-loop control. In practical applications, an anemometer or other feedback device is required to ensure the accuracy and reliability of the control. The opening degree of the adjusting air valve has a precision that is not affected by the presence or absence of people in front of the cabinet, the air volume adjustment process is rapid and accurate, and the response time is short. It can achieve a high adjustment ratio (1:17~20) that is impossible with the traditional variable air volume valve. The pipeline static pressure is independent (150~750PA), the response time is fast (less than 1 second), and the air volume control is accurate to ±5% of the air flow control signal.
The equipment can also be equipped with an infrared human body monitor. The automatic control system can achieve the purpose of saving operating costs through different face wind speed settings. When an operator is working in front of the operating table, the face wind speed is controlled at a certain set value (such as 0.5m/ s), when no one is operating in front of the fume hood, the system will automatically switch to another set value (such as 0.3m/s). There will be a clear alarm when the position of the fume hood sliding door is too high. There will be an obvious alarm when a mechanical failure causes the face wind speed to be too high or too low. When an abnormal situation occurs, the face wind speed control system can turn on the accident exhaust mode, and open the air valve to the maximum opening degree, which is not controlled by the face wind speed setting value.
Some experimental equipment will produce exhaust gas during the working process, and some laboratory exhaust gas must be processed before it can be discharged. When the exhaust system is divided, this part of the exhaust air needs to be concentrated in one system for centralized processing. Which treatment method should be used should be determined according to the nature of the exhaust.
When designing the laboratory ventilation system, the exhaust air of the same or similar experimental nature should be installed in one system. If there is a possibility of burning, explosion or formation of more toxic harmful substances after mixing, a separate exhaust system is required. .
The biological safety cabinet should be single exhausted.
The design of the ventilation system used in the laboratory must first ensure the safety of the laboratory, and a certain number of air changes must be ensured; the design and system control of the negative pressure of the laboratory ventilation system are very important and must be resolved; the system must It can operate stably and reliably; the last is to reduce energy consumption to a minimum under the condition of ensuring a certain number of air changes and fresh air. To meet these requirements, ventilation equipment, especially fume hoods, must have certain standards and requirements.
When the laboratory exhaust volume is large, it is necessary to add an air supply system in the corridors, halls and other public areas in the laboratory ventilation system design to avoid difficulties in opening or closing doors caused by excessive negative pressure.
The ventilation system not only has special requirements for laboratory fume hoods, but also has certain requirements and standards for control systems and other equipment. The equipment model, ventilation system design and control system will all affect the pressure control and minimum ventilation control in the laboratory. The ventilation system design and control system are the most important. The unbalanced ventilation system will cause the exhaust and capture capacity of the fume hood to lose, the airflow will flow out of the laboratory, and the pressure in the building will be unstable.
The change of the air volume of the ventilation system cannot affect the reaction time due to the change of the static pressure in the air duct, thereby affecting the accuracy of the ventilation volume. Pressure-dependent systems will also affect the precise control of the system flow, making its response time slower, resulting in oscillating air supply and exhaust equipment, unstable speed, unbalanced air volume, and ultimately causing the control system to fail to adjust. All systems should be pressure-independent