Walk-in Fume Hood (2)

(1) Release function: it should have a mechanism to dilute the harmful gas generated from the inner fume hood by absorbing the gas outside the fume hood and then exhaust the poisonous gases out of the room.

(2) Non-reversal function: the air flow generated by the exhaust fan inside the fume hood should have the function to expel the harmful gases of the fume hood and not reversing the harmful gas from the inside fume hood into the indoor. In order to ensure the implementation of this function, the best way is to connect a fume hood and a ventilating fan with a single pipe, if the fume hood and  the ventilating fan cannot be connected by a single pipe, it can be connected in parallel but only of the same room on the same floor, ventilating fan should install at the end of the pipe as far as possible (or at the rooftop).

(3) Isolation function: the fume hood should be separated by a no sliding glass window in front of the fume hood.

(4) Supplementary function: there should be a passage or an alternative device that can takes inside air out of the fume hood while discharging harmful gases.

(5) Wind speed control function: In order to prevent harmful gas escape from the walk-in fume hood, the inhalation speed of the harmful gas must be as quick as possible. The factors that determine the inhalation speed of the walk-in fume hood are: the heat and the number of ventilation produced during the course of the experiment. Foremost among them are the content of the experiment and the nature of the noxious materials. According to regular regulations, generally nonpoisonous pollutants commonly are 0.25-0.38 m/s, poisonous or dangerous pollutants are 0.4-0.5 m/s, highly toxic or slightly radioactive pollutants are 0.5-0.6 m/s, gas substance pollutants are 0.5 m/s, granular materials are 1 m/s. In order to ensure such a wind speed, exhaust fan should have a necessary static pressure, which is the frictional resistance when air pass through the ventilation pipeline. When checking wind speed, the noise issue should also be noticed, the air flow in the pipe is limited to 7-10 m, when air flow speed exceed 10 m, noise will thus be produced, laboratory (indoor background noise level) noise is usually limited to 70 dBA, pipeline laying area's increasing will reduce wind speed, then reduce noise, in view of the financing and construction issue of the pipeline, pipeline and exhaust fan power must be selected carefully.

(6) Functions of resistance to heat to acidic and to alkali properties: the walk-in fume hood should install electric furnace inside, some experiment will produce a large amount of toxic and harmful gases, such as acid and alkali, these substances are highly corrosive.  The table-board, the lining board, the side board and the selected water mouth and the nozzle of the fume hood should all have the anti-corrosion function. Acid, nitric acid, hydrofluoric acid and other strong acids used during the experiment of the semiconductor industry also require alkali resistant function of the overall material of the fume hood, it should be made of stainless steel or PVC materials.
◆Before the experiment begins, it is necessary to confirm that the walk-in fume hood is in operation condition before carry out the experimental operation.
◆Before ending the experiment, the fume hood cannot be turned off to discharge the residual gas in the pipe unless it has running at least 5 minutes .A ventilation time delay device to ensure that the fume hood’s operation will be delayed.
◆During the experiment, do not put any equipment within 150mm of the glass window. Large experimental equipment should have sufficient space in order not to affect the flow of air. The front window should be turned off wherever possible.