Structure And Use
Older fume hoods were mostly made of wood, but modern ones are made of epoxy-coated steel. Common cabinet widths are 1000, 1200, 1500, 1800 and 2000 mm, with a depth of 700-900 mm and a height of 1900-2400 mm. It can be used by 1-3 people at the same time. Fume hoods are generally installed against the wall. In the middle of the front is a transparent door (mostly glass) that can be moved up and down, and the opening height is generally 100-600 mm. Behind the door is the working surface for experiments, with connections for water pipes, sewers, power supplies, vacuum pumps, and gas pipelines, etc., and there are lamps with protective covers on them. After the air is drawn away by the exhaust fan at the front and upper part of the cabinet, it is led to another place through pipes (called full ventilation), or it is filtered and recirculated (it becomes ductless ventilation). Most fume hoods will issue an alarm when (1) the exhaust air volume is too small (2) the front glass door is opened too far to remind the operator to pay attention. The internal exhaust fan speed and lights have switches that can be adjusted. When using, stand or sit in front of the cabinet, lower the glass door as much as possible, and put your hand into the cabinet through the door to conduct experiments. Since the exhaust fan draws air inward through the open door, the harmful gas will not escape in large quantities under normal circumstances.
Ventilation Principle
The fume hood with ductless ventilation does not require external piping, does not pollute the external environment, and has little effect on the laboratory temperature. However, the disadvantage is that the filter material must be replaced regularly, the experimenter may be exposed to harmful gases more than using an extraction fume hood, and because the multi-installed exhaust fan is very close to the experimenter, it will cause noise. This type of fume hood has the following filtration procedures: filter dust before using foam plastic, filter with activated carbon, and absorb many chemicals. But ammonia and carbon monoxide require other special filters to remove them. The pre-filtered material can be used for an average of 6 months, and the activated carbon layer can be used for an average of 2 years, but the specific life depends on the usage. A fully ventilated fume hood draws the air inside the hood to another place, where it is vented to the atmosphere after proper treatment. This type of fume hood can remove harmful gases in the laboratory more effectively than ductless ventilation, with low noise and simple maintenance. The disadvantage is that a dedicated exhaust duct needs to be installed, which may prevent the laboratory from maintaining a constant temperature when the indoor and outdoor temperatures are different, and it is not good for environmental hygiene.
Other Configuration
Some special fume hoods have water washes in the vent ducts or inside the hood to remove accumulated chemicals. Some will also have a vacuum inside, which avoids the need for a glove box, but still handles chemicals that react violently to air and moisture.
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