Understanding the function of the laboratory is the determining factor for the selection and design of the ventilation system. Providing a safe environment for the staff is the primary goal. In the system design of the laboratory, the designer must thoroughly study all the factors and find the most suitable design. . The chemical laboratory consists of a wet chemical room, a heating room, a constant temperature and humidity room, a general test room, an injection room and an office room. Except for the environment of constant temperature and humidity room, other rooms only need temperature control, and there is no cleanliness requirement. ASHRAE stipulates that the laboratory air conditioning and ventilation design parameters include the following:
1) Indoor and outdoor temperature and humidity requirements;
2) Air quality;
3) Equipment and process heat load, including sensible heat and latent heat;
4) The expected increase in internal load;
5) Minimum number of air changes;
6) Intake and make up the wind;
7) Type of exhaust equipment;
8) Control and alarm;
9) It is possible to adjust the size and quantity of the fume hood;
10) Room pressure difference;
11) Backup of equipment and power supply.
1. Selection of the number of air changes
The "Chemical Heating, Ventilation and Air Conditioning Regulation Design Code" stipulates that the minimum air exchange rate of the laboratory room is generally 6 times/h to 8 times/h. ASHRAE stipulates that the overall number of air changes in the laboratory should be determined by the following air volume: the total air volume discharged from the local exhaust equipment or other room exhaust; the cooling air volume required to take away the heat load of the room; the minimum number of air changes required. Under the conditions of use, the minimum number of air changes in the laboratory should be maintained at 6 times/h~10 times/h.
Under normal circumstances >10 times/hour room air changes are considered appropriate. However, when there is a possibility of high thermal load analysis equipment in the laboratory, or a relatively large amount of local exhaust in the room, the ventilation volume may need to be increased accordingly. The wet chemical room has a fume hood, and the heating room has a large number of heating furnaces. The calculation method of the fume hood refers to the "Chemical Heating, Ventilation and Air-Conditioning Design Code" for light, moderate or dangerous hazardous substances. When the indoor ceiling is filled with air, the minimum suction surface of the operation port of the fume hood The speed is 0.5m/s. For the utilization rate of fume hoods, when the number of fume hoods is greater than 2, the simultaneous utilization rate should be 60%~70%. The heating furnace calculates the required exhaust air volume based on the heat balance law that maintains the heating temperature in the furnace. Through the above, the total safe ventilation volume can be calculated. In addition, the air-conditioning volume calculated by the load is compared with the minimum number of air changes 10 times, and the maximum of the three is taken.
2. Air supply and exhaust form
The "Chemical Heating, Ventilation and Air Conditioning Regulation Design Code" stipulates that when the exhaust air volume of the laboratory is large, an outdoor fresh air supply system should be installed and the fresh air load should be included.
The "Science Laboratory Building Design Code" stipulates that each exhaust device should be equipped with an independent exhaust system. All exhaust devices in the same laboratory should share an exhaust system. The laboratory that uses the exhaust system continuously during working hours should be equipped with an air supply system, and the air supply volume should be 70% of the exhaust air volume, and the air supply should be air purified according to the process requirements. For heating areas, the supply air should be heated in winter. The supply air flow should not disrupt the normal operation of the laboratory exhaust device.
ASHRAE stipulates that all gases exhausted from the chemical laboratory must be directly discharged outdoors and cannot be recycled. Therefore, unless the chemical laboratory also has cleanliness requirements, it is necessary to maintain its negative pressure relative to the adjacent area. Whether to choose a 100% fresh air supply system should be an important part of the laboratory risk assessment.
An independent exhaust system is set up between each unit of the laboratory, and the exhaust is installed on the roof. The wet chemical room and heating room must be treated with fresh air due to the production of toxic, corrosive, and high-temperature gases. For other general laboratory rooms for computer analysis and constant temperature and humidity rooms for material testing, a 100% brand new air supply system is not the only option. Because of the different process functions of the laboratory, fresh air ventilation or fresh air air treatment is not necessary. It can only be the first priority to meet the process. 100% of the fresh air is for the environment of the fume hood, and for the general laboratory circulating air treatment that can meet the requirements, 100% of the fresh air is not necessary. Moreover, in a fresh air-conditioning environment, energy consumption is very high.
3. Room pressure difference
The "Chemical Heating, Ventilation and Air Conditioning Regulation Design Code" stipulates that the laboratory should maintain a relatively negative pressure.
ASHRAE stipulates that all gases exhausted from the chemical laboratory must be directly discharged outdoors and cannot be recycled. Therefore, unless the chemical laboratory also has cleanliness requirements, it is necessary to maintain its negative pressure relative to the adjacent area.
This regulation actually depends on the specific implementation object. In this project, the constant temperature and humidity room needs strict temperature and humidity control range, and it should be designed as a positive pressure. Because if the design is negative pressure, the air in the adjacent area will enter, on the one hand, it may destroy the precision of temperature and humidity control; on the other hand, if the polluted air enters, it may also cause safety problems. For the wet chemical room and heating room, in order to prevent toxic, corrosive, high-temperature gas or volatiles from being emitted to the room, or even to other areas, it is necessary to design a negative pressure. The office area of the laboratory building should always maintain a positive pressure relative to the corridor and the laboratory. The airflow in the laboratory should flow from the low-risk area to the high-risk area, and finally be exhausted to the outdoors through various types of fume hoods or heating equipment.
4. Control system
The control should integrate the above items to meet the room pressure, pressure difference of each room, ventilation, temperature and humidity and various safety control requirements of the entire laboratory, while reducing energy consumption. There are often many chemical pollution sources that are not good for human health in the laboratory, especially harmful gases, and it is very important to eliminate them. But at the same time, energy is often consumed in large quantities. Therefore, the requirements of the laboratory's ventilation control system range from the early constant air volume, bistable, variable air volume system to the latest adaptive control system. The so-called safest and most comfortable environment and the most energy-efficient way is not to be too extravagant. The system responds quickly to ensure personal safety, and accurately controls the air supply and exhaust balance and indoor pressure with the highest accuracy to provide maximum stability. Try to reduce the user's initial investment, while reducing the user's costs in terms of operation, energy consumption and maintenance.
ASHRAE stipulates that laboratory control adjusts the temperature and humidity control of the equipment on the one hand; on the other hand, it monitors the safety facilities to protect the staff, and which system to use, as long as it is suitable for the current laboratory.
Whether the ventilation of the chemical laboratory adopts a constant air volume or a variable air volume control system depends on the comprehensive consideration of the initial investment and operating costs of the required functions by the designer, user and facility manager. The constant air volume (CAV) system is designed to provide total exhaust air flow for all fume hoods and heating furnaces, regardless of whether these fume hoods and heating furnaces are occupied or not, the total flow is kept constant. This method uses a mechanical limit stop to limit the valve opening, which may reduce the flow rate by as much as 40%. The variable air volume system (VAV) is a staggered design in which the system capacity is reduced by more than 10% or 20%.
The primary problem to be solved by the ventilation design of the laboratory is the safety problem, and it must also consider creating a comfortable working environment for the experimenters, solving the problems of temperature, airflow, and noise, while ensuring the lowest energy consumption, the system is stable, and easy to control. , Easy to operate and manage. In short, it is to design from the aspects of safety, comfort, energy saving, and reliable operation. Through this type of chemical laboratory involved in the number of air changes, the form of air supply and exhaust, room pressure difference, control system standards.