Publish Time: 2021-11-30 Origin: Site
The laboratory fume hood looks inconspicuous, but it is one of the essential basic equipment in every laboratory involving chemical operations. Among the various performances of the fume hood, the most important one is undoubtedly the safety feature. The safety features of traditional full-row fume hoods mainly include air isolation features such as airtightness and backflow prevention, chemical resistance such as no damage and discoloration, electrical safety features such as short-circuit prevention and leakage prevention, and anti-pollution features such as no residual dead angle and easy cleaning. etc. Ductless fume hoods, as a specific category of fume hoods, also include the reliability of the filtration system.
Essentially, a fume hood is a device that isolates the air in the cabinet from the indoor air. Therefore, the airflow isolation performance of the fume hood is the most important performance of the fume hood, which is mainly manifested in airtightness and backflow resistance.
The airtightness of the fume hood is easier to understand. A fume hood with poor airtightness will cause the gas in the cabinet to leak into the room, thereby reducing the safety performance of the fume hood. A qualified laboratory ventilation system not only requires good airtightness of the fume hood itself, but also requires firmness and reliability at all connection points, eliminating all potential leakage risks.
The backflow resistance of the fume hood is also extremely important, because the air flow in the fume hood is very complicated and closely related to the structure of the fume hood. If it is not handled well, it will form dead air flow and difficult to control turbulence and turbulence. When designing a fume hood, it is necessary to establish a reliable airflow model based on a large amount of experimental data, set up diversion devices in a targeted manner, and pay special attention to the smoothing effect of the airflow at the entrance.
In the use of fume hoods, it is easy to cause a misunderstanding that the greater the airflow, the higher the safety of the fume hood. In fact, as the airflow increases, the rebound effect and vortex effect of the airflow also increase, and the possibility of the gas in the cabinet escaping from the fume hood also increases, and the safety of the fume hood will decrease instead. Therefore, standards such as SEFA all have a recommended range of surface wind speed for fume hoods, and generally the recommended surface wind speed is between 0.3-0.6 m/s.
Compared with the traditional full row fume hood, the air flow stroke of the ductless fume hood is very short and there is no external connection at all, and its tightness can be extremely guaranteed. Excellent sealing performance is one of the most prominent advantages of ductless fume hoods in terms of gas isolation.
In the ductless fume hood, there are two types of airflow design according to the amount of harmful gas to be processed. For occasions where the gas processing volume is small, the upper-discharge airflow exhibits a good vertical laminar flow pattern, which will not form obvious dead angles and vortexes, and there will be no airflow backflow phenomenon. At this time, a vertical laminar flow design without baffles can be used, which has a simple structure and is easy to clean. A ductless fume hood with this structure is usually also called a ductless workbench or a ductless chemical workbench. If equipped with a HEPA filter, this ductless fume hood can also be used for weighing powder samples with a precision balance, sometimes called a ductless balance cabinet.
If the gas processing volume is large, the air flow inside the fume hood will no longer be a good vertical laminar flow. At this time, it is necessary to add a guide device in the cabinet to make the air flow distribution more uniform, and it is less likely to produce vortex and gas backflow. The fume hood of this structure is very similar to the traditional full row fume hood. In order to better meet the needs of different applications, in addition to the classic sliding window, the front window of the ductless fume hood also has a folding window that is more convenient to operate. The former is suitable for applications that require frequent adjustment of the window opening height, and the latter is suitable for most laboratory applications.
The air flow control of the ductless fume hood is very important, because the fan is built in the head of the fume hood, and the variable air volume (VAV) control can be easily realized without a special air valve. The wind speed of the airflow surface can be monitored in real time by a special sensor and displayed on the safety controller. When the air flow is abnormal, the safety controller will flash the warning light and send out an audible alarm signal in time to remind the user to check and maintain.
Through careful design of the cabinet structure and real-time monitoring of the airflow in the cabinet, the airflow of the ductless fume hood is not only smooth and smooth, but also has no leakage and no backflow, and the airflow safety is excellently guaranteed.