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Face velocity is the average amount of air that is pulled through the face of a hood. If these velocities are too low then chemical fumes can escape the hood and go back into the lab. If the velocities are too high, energy costs will be high as you exhaust large amounts of conditioned air from the room. In other words, if too much air is being pulled into the machine (high volumetric flow) it becomes extremely inefficient. If too little air is being pulled into the machine (low volumetric flow), then it can be dangerous for those around the equipment. Airspeed is everything when discussing face velocity. Today we are going to discuss fume hood face velocity requirements, so you can keep your equipment safe and efficient.
Average velocity is calculated by dividing the sash opening into an imaginary grid pattern. Each square of this grid should be approximately 1ftsq. Then take air measurements (velocity readings) at the center point in each of these imaginary squares. You can do this using a velometer, anemometer or similar device. Each one of these readings should be for approximately 10 seconds (the longer the better). Record the reading in a notepad and repeat until you have measured from all squares. Once complete analyze the measurements and look for outliers.
| Model Specification | WJ-1500A | WJ-1500B | WJ-1800A | WJ-1800B |
| External dimensions of equipment(mm) | 1500(W)*1205 (D) *2400 (H) | 1800(W)*1205 (D) *2400 (H) | ||
| Dimension of works pace (mm) | 1260(W1)*780(D1) *1100 (H1) | 1560(W1)*780(D1) *1100 (H1) | ||
| Panel material | 20+6mm thick butterfly ceramics | |||
| Material of internal lining board | 5mm thick ceramic fiber board | |||
| Diversion structure | Lower air return | |||
| Control system | Button control panel (LCD panel) | |||
| PH value control | The medium is alkaline water solution; manual monitoring, and manual control through acid pump and alkali pump. | |||
| Input power | Three-phase five-wire 380V/50A | |||
| Current for air fan | Not over 2.8A(380V or 220V can be directly connected) | |||
| Maximum load of socket | 12 KW(total of 4 sockets) | |||
| Water tap | 1 set (remote control valve + water nozzle) | No | 1 set (remote control valve + water nozzle) | No |
| Water discharge way | Magnetic chemical pump strong discharge | |||
| Using environment | For non-explosion indoor use, within 0-40 degrees Celsius. | |||
| Applicable fields | Inorganic chemistry experiment; Food, medicine, electronics, environment, metallurgy, mining, etc. | |||
| Ways of Purification | Spray sodium hydroxide solution, no less than 8 cubic meters/hour | Spray sodium hydroxide solution.no less than 12 cubic meters/ hour | ||
| Ways of surface air speed control | Manual control (through the electric air valve to adjust the exhaust air volume or adjust the height of the moving door) | |||
| Average surface air speed | 0.6-0.8 m/s Exhaust air volume: 1420-1890m3/h (when door height h =500mm) | 0.6-0.8 m/s Exhaust air volume: 1760-2340m3/h (when door height h =500mm) | ||
| Speed deviation of surface air | Not higher than 10% | |||
| The average intensity of illumination | Not less than 700 Lux; Standard white and uv-free yellow LED lamps; The illumination is adjustable. | |||
| Noise | Within 55 decibels | |||
| Flow display | White smoke can pass through the exhaust outlet, no overflow. | |||
| Safety inspection | No spikes, edges; Charged body and the exposed metal resistance is greater than 2 mQ; Under 1500V voltage, no breakdown or flashover occurred for 1min test. | |||
| Resistance of exhaust cabinet | Less than 160 pa | |||
| Power consumption | Less than 1.0kw/h (excluding power consumption of fans and external instruments) | Less than 1.2kw/h (excluding power consumption of fans and external instruments) | ||
| Water consumption | Less than 3.2L/ h | Less than 4.0L/ h | ||
| Performance of wind compensation | With a unique wind compensation structure, the volume of the wind will not cause turbulence in exhaust cabinet and will not directly blow to the staff (need to connect to the air compensation system of the laboratory) | |||
| Air volume regulating valve | 315mm diameter flanged type anti-corrosion electric air flow regulating valve (electric contact actuator) | |||
Variable air volume fume hood
Variable air volume (VAV) hoods are tested in a similar manner to CAVs for determining airflow face velocity. The hood is set to the operational sash opening position as specified in the contract documents or as agreed to between the owner/buyer and then the velocity is measured in the same way as in CAV fume hood.
The sash position should then be reduced to 50% of the specified opening and repeat airflow face velocity measurements/calculations as described above. The same procedure should also be carried out at 25% of the specified opening and fully open for reference and safety purposes.
Face Velocity too fast: Velocities greater than 0.8 m/s are likely to generate eddy currents around users standing in front of the cupboard and these are then able to draw contaminants out through the aperture, particularly during movement by the operator.
Face velocity too slow: In a standard fume hood it is unlikely that velocities below 0.4 m/s are able to arrest and contain contaminants within the enclosure, particularly where external air movements, due to movement of users or opening/closing of doors and windows, are likely to exceed the face velocity. In special situations, the fume hood may be designed specifically to operate at low face velocities.
• Hoods should be evaluated by the user before each use to ensure adequate face velocities and the absence of excessive turbulence.
• In case of exhaust system failure while using a hood, shut off all services and accessories and lower the sash completely. Leave the area immediately.6 Questions to Ask When Buying a Fume Hood:
-Which chemicals will you use within the hood?
-Is a ducted or ductless hood best suited to your needs and available space?
-Where will you place the fume hood in the lab? Consider workflows, access to external exhaust systems, and competing air patterns.
-What size fume hood will best suit your needs? Be sure to consider what (if any) equipment will be enclosed in the hood.
-Are any service fixtures or accessories such as airflow monitors, electrical outlets, water, or gas fixtures required?
-Are base cabinets for acid, solvent, or non-chemical storage required?
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