Selection and Precautions of Laboratory Fume Hoods
Building a modern biochemical laboratory is a comprehensive system project. While equipping it with various instruments, equipment, and supporting facilities, one must consider requirements such as power supply, water supply, drainage, air delivery, exhaust, purification, waste discharge, etc., while also taking into account the safety of personnel, objects, and the surrounding environment, comfort regarding noise, odors, and visual environment, operability and functionality of instruments, as well as the convenience of information processing. Therefore, a modern biochemical laboratory must have optimal design and high-quality equipment to meet these needs.
In modern laboratory equipment, there are fume hoods, central lab benches, side benches, chemical cabinets, vessel cabinets, gas cylinder cabinets, etc. Among them, the fume hood plays an extremely important role in biochemical laboratory equipment and is an indispensable device. Thus, selecting a fume hood is an important issue in laboratory construction that must be given sufficient attention.
Main Functions of Fume Hoods
The most important function of a fume hood is its exhaust function. In chemical laboratories, harmful gases, odors, moisture, flammable, explosive, and corrosive substances are produced during experimental operations. To protect the safety of users and prevent contamination from experiments spreading to the laboratory, a fume hood should be used near the pollution source. In the past, fewer fume hoods were used, only for particularly harmful and dangerous gases or experiments producing large amounts of heat. The fume hood only served as an auxiliary function to the lab bench. In recent years, to improve the experimental environment, experiments conducted on lab benches have gradually been moved to within fume hoods, requiring them to possess the most suitable functions for equipment use. Especially in many newly built laboratories that require air conditioning, the number of fume hoods used should be incorporated into the planning of the air conditioning system during the preliminary design stage of the building. Since fume hoods occupy a very important position in biochemical laboratories, considering the improvement of the laboratory environment, improvement of labor hygiene conditions, and enhancement of work efficiency, the number of fume hoods used has rapidly increased. Consequently, ventilation ducts, piping, wiring, exhaust, etc., have all become important topics in laboratory construction.
The primary purpose of using a fume hood is to expel harmful gases generated during experiments and protect the health of experimenters. This means that a fume hood must have high safety and superior operational performance, requiring it to possess the following functions:
(1) Release Function: It should be capable of diluting harmful gases generated inside the fume hood by drawing in external air and then discharging them outside.
(2) Non-backflow Function: It should inhibit harmful gases from flowing back from inside the fume hood into the room. To ensure this function, the best method is to connect one fume hood to one ventilator through a single pipe. If it cannot be connected via a single pipe, it is limited to being connected within the same floor and room. The ventilator should be installed as close to the end of the pipe as possible (or on the roof).
(3) Isolation Function: A non-sliding glass sash should be used at the front of the fume hood to separate the interior and exterior of the fume hood.
(4) Supplement Function: It should have channels or alternative devices to draw in air from outside the fume hood when harmful gases are being expelled.
(5) Wind Speed Control Function: To prevent harmful gases from escaping from inside the fume hood, there needs to be a certain intake speed. Factors determining the intake speed of the air entering the fume hood include the heat generated by the experimental content and its relationship with air changes. The main factor is the nature of the experimental content and the harmful substances. Generally, the intake speed for non-toxic pollutants is 0.25-0.38 m/s, for toxic or hazardous substances it is 0.4-0.5 m/s, for highly toxic or slightly radioactive substances it is 0.5-0.6 m/s, for gaseous substances it is 0.5 m/s, and for particulate matter it is 1 m/s. To ensure such wind speeds, the exhaust fan should have necessary static pressure, i.e., the friction resistance of air passing through the ventilation duct. When determining the wind speed, noise issues must also be considered. The limit for air flow within pipes is 7-10 m/s; exceeding 10 m/s will generate noise. Typically, the indoor background noise level is 70 dB(A). Increasing the cross-sectional area of the pipe can reduce wind speed and thus reduce noise. Considering the cost and construction of the pipes, careful selection of the pipes and exhaust fan power is necessary.
(6) Heat Resistance and Acid-Alkali Corrosion Resistance Function: Some fume hoods need to accommodate electric furnaces, and some experiments produce large amounts of acidic and alkaline gases with strong corrosiveness. The countertop, lining board, side panels, faucets, and gas nozzles of the fume hood should all have corrosion-resistant properties. In the semiconductor industry or experiments involving strong acids like sulfuric acid, nitric acid, and hydrofluoric acid, the overall material of the fume hood must be resistant to acids and alkalis, made from stainless steel or PVC materials.
2 Selection and Precautions of Laboratory Fume Hoods Types of Fume Hoods
Fume hoods can be classified according to their exhaust methods into three types: upper exhaust, lower exhaust, and simultaneous upper and lower exhaust. To ensure uniform airflow in the working area, cold-process fume hoods should use lower exhaust, hot-process fume hoods should use upper exhaust, and for processes with unstable heat generation, exhaust ports can be set both above and below, adjusting the proportion of upper and lower exhaust based on the internal heat generation of the fume hood to achieve uniform airflow.
Fume hoods can also be classified according to their intake methods into three types. Indoor air enters the cabinet, circulates within, and is then discharged outdoors, which is called full exhaust and is a widely applied type.
When a fume hood is placed in a room with heating or temperature-humidity control requirements, to save on heating and air conditioning energy consumption, outdoor makeup air is drawn in, circulated within the cabinet, and then discharged outdoors, which is called a makeup air fume hood.
Another type is the variable air volume control fume hood. Ordinary constant air volume systems require manual adjustment of fixed blade dampers to regulate the exhaust volume of the fume hood, reaching the desired face velocity when the damper is adjusted to a certain angle. Variable air volume control adjusts the air volume through sensors in the damper to reach the specified face velocity. Naturally, standard models are cost-effective, while variable air volume models are more expensive but suitable for high-precision situations.
Fume hoods can also be categorized based on usage status into integral, bottom-open, floor-standing, two-sided, three-glass, tabletop, linked styles, and specialized fume hoods designed for specific experimental needs such as radioactive experiments, synthesis experiments, and perchloric acid experiments.
Safety of Fume Hoods
Safety is the ultimate mission of fume hoods. Using a fume hood in the laboratory aims to ensure the safety of users and prevent environmental pollution.
Beijing Taiji Aofei Laboratory Equipment Co., Ltd. is one of the earliest professional companies in China to develop and produce fume hoods. Through years of experience and practice, combined with features of similar products domestically and internationally, Taiji Aofei has gradually formed a research force centered on fume hood technology for laboratory equipment. Taiji Aofei's fume hoods prioritize performance and functionality while giving great consideration to safety. These safety aspects mainly manifest in:
1. Utilizing a unique seam-style exhaust structure to effectively expel harmful gases.
2. Streamlined handles leave gaps between the handle and the glass, ensuring effective intake due to the rotational airflow on the surface.
3. Installing window drop prevention pins, so if the steel wire rope detaches and the glass window accidentally falls, the pin will catch it to prevent injury.
4. Using tempered glass for the windows, ensuring no injuries occur even if the glass breaks unexpectedly or explodes.
5. Ventilation holes are set on the top of the fume hood, allowing air to enter even when the glass window is closed, avoiding excessive negative pressure.
6. The shell of the fume hood is made of metal materials, fireproof and non-flammable. The interior uses flame-retardant or non-flammable materials like antibacterial boards or stainless steel plates, and the countertop uses solid core chemical-resistant boards or stainless steel plates for acid-alkali resistance, heat resistance, and flame retardancy.
7. The effective height of the glass window is 800mm, the interior cavity is 1200mm, and the countertop height is 800mm, conforming to ergonomics, facilitating operation, providing ample space for use, and enhancing safety.
8. Water taps and gas taps are equipped with remote operation handles outside the cabinet, making operation more convenient and safe than direct manual handling.
Safety must also be noted when using fume hoods:
• Before starting the experiment, confirm that the fume hood is operating before conducting any experimental operations.
• At least five minutes before the experiment ends, the ventilator should continue running to expel residual gases in the ducts. Consider installing a delayed exhaust timer to ensure the ventilator runs longer.
• During experiments, do not place any equipment within 150mm of the glass window. Large equipment should have sufficient space without affecting airflow, and the front window should be kept closed as much as possible.
Selection of Fume Hoods
When constructing a laboratory, choosing fume hoods and determining their installation positions requires selecting the type, material, shape, etc., based on the experimental content. The following factors should typically be considered:
• Chemicals Used: When using organic or other special reagents, fully consider their control wind speed.
• Heat Sources: When using equipment with heat sources, if the heat exceeds 2000 kcal/hour, consider the required ventilation volume to determine the ventilator's power.
• For experiments involving radioactive materials or perchloric acid: Specialized fume hoods must be selected, and the intake wind speed must be set greater than 0.5 m/s.
• When using large equipment for experiments: Consider the internal effective dimensions to leave necessary space for exhaust.
• Material: When using special acids in experiments, consider the fume hood's material, such as in the semiconductor industry, storage cabinets, or corrosion industries, where corrosion-resistant materials must be used.
• External Dimensions: Select the external dimensions based on experimental content; oversized volumes lead to waste, and undersized ones affect usability.
• Environmental Protection: Ensure the emitted harmful gases are below national environmental protection requirements. If exceeding national health standards, appropriate purification devices should be installed.
• Energy Saving: Try to consider energy savings; in heated or air-conditioned rooms, use makeup air fume hoods or choose fume hoods with air volume control. When selecting fans, determine their power based on needs, avoiding blindly increasing exhaust volume and pressure. Use frequency converters or variable-speed fans to reduce electricity consumption.
The installation location of fume hoods should avoid facing roads and areas with frequent pedestrian traffic, avoid blocking windows and ventilation lighting, avoid obstructing entrances and exits affecting door openings, avoid opposing placement or placing them in corner walls.