Face velocities

Face velocity, m /min Octave band center frequency, H2 ... 

Plutonium solutions that have a low activity (<3.7 x 10 Bq (1 mCi) or 10 mg of Pu) and that do not produce aerosols can be handled safely by a trained radiochemist in a laboratory fume hood with face velocity 125—150 linear feet per minute (38—45 m/min). Larger amounts of solutions, solutions that may produce aerosols, and plutonium compounds that are not air-sensitive are handled in glove boxes that ate maintained at a slight negative pressure, ca 0.1 kPa (0.001 atm, more precisely measured as 1.0—1.2 cm (0.35—0.50 in.) differential pressure on a water column) with respect to the surrounding laboratory pressure (176,179—181). This air is exhausted through high efficiency particulate (HEPA) filters. 

Vf Face velocity of a fluid approaching a bank of finned tubes m/s ft/h... 

The standard air face velocity (F ) is the velocity of standard air passing through the tube bundle and generally ranges from 1.5 to 3.6... 

Gas Pressure Drops The filtration, or superficial face, velocities used in fabric filters are generally in the range of 0.3 to 3 iTi/min (1 to 10 ft/min), depending on the types of fabric, fabric supports, and... 

These heaters are avaifable with rotors up to 6 m (20 ft) in diameter. Gas temperatures up to 1255 K (1800°F) can be accommodated. Gas face velocity is usually around 2.5 m/s (500 ft/min). The rotor height depends on service, efficiency, and operating conditions but usually is between 0.2 and 0.91 m (8 and 36 in). Rotors are driven by small motors with rotor speed up to 20 r/min. Heater effectiveness can be as high as 85 to 90 percent neat recovery. Lungstrom-type heaters are used in power-plant boilers and also in the process industries for heat recoveiy and for air-conditioning and building heating. 

Figure 10.16 shows the calculated velocity contours as a fraction of the average velocity in the channel (average face velocity). In addition, velocities ob- rained from the line sink model are plotted. It can be seen that, compared to the line sink model, the calculated contours are displaced somewhat in the positive direction, with the greatest relative difference near the exhaust opening and with decreasing relative difference as the dimensionless distance v/VX increases. 

V = centerline velocity, Vii = hood face velocity (Q/A), z = distance ftom hood face along centerline /length of one side... 

V = centerline velocity, V( - hood face velocity Q / (Z. W)), W — hood width, L — hood length, X - distance from hood face along centerline... 

The theoretical distance to the dividing streamline, X, is a function of hood dimensions, hood face velocity, distance parallel to the hood face, and crossdraft velocity, and is calculated from the equations for flanged elliptical openings in Section 10.2.2.2. 

Variable Air Volume Fume Cupboards This type of cupboard incorporates a variable air volume (VAV) controller that regulates the amount of air exhausted from the cupboard such that the face velocity remains essentially constant irrespective of the sash position. A sensor detects either the sash position, the pressure differential l>etween the fume cupboard interior and the room, or the vekxity at some point in the cupboard. This information is used to control either the exhaust fan speed or the position of a control damper. The supply air volume flow rate into the laboratory or workspace should also be regulated. It should be remembered that with the sash in the closed position the amount of air to dilute contaminants in both the fume cupboard and the laboratory is reduced and that there could, for example, be difficulty in reducing contaminant levels below the lower exphasive level. 

The recommended value of the face velocity has Ireen the subject of much debate and variation over the years. The recommended values have also varied widely with use. For example, CIBSE recommends values as low as 0.2 and 0.25 m s for fume cupboards for teaching and research purposes, respectively, and as high as 2.0 m for radioactive work dependii on grade. Middleton quotes the optimum range for the face velocity of laboratory fume cupboards to lie between... 

Face Velocity Measurements Although it is generally accepted that face velocity is not sufficient to specify or describe fume cupboard performance, it is a relatively easily made measurement that is readily understood and widely-quoted. Low face velocities make a fume cupboard sensitive to outside disturbances (for example drafts) whereas excessively high velocities can cause eddies in the wakes of operators and under sash handles which can lead to contaminant being drawn out of the cupboard. 

The average face velocity at all openings into a gas cabinet should be at least 1 m s with a minimum velocity at any point of 0.76 m s h These... 

Saunders recommends the auxiliary air have a uniform velocity distribution across the discharge area ( 20%). The discharge area must be of a size that the air velocity does not exceed the face velocity of the fume cupboard by more than a factor of 1.2. 

For some hood types, measurements usually seen as indirect method, are used to measure the hood s performance to determine regulatory compliance. For example, regulations specify minimum and maximum face velocities for laboratory fume hoods and static pressure (negative) inside enclosed hoods. Continuously monitoring instruments can be connected to alarms that sound when the measurement is outside the specified limits. 

Face velocity The average velocity across an opening or item of equipment, such as a hood, fume cupboard, heating or cooling coil, or filter. 

Filter medium face velocity The volume flow rate divided by the effective area of the filter element. 

The felted fabrics are generally used for maximum recovery of product and are used at high face velocity for airflow-to-cloth-area ratio. The felt promotes the greatest dust collection surface. 

The woven fabrics have various yarn patterns for different spacings between the yarn fabrics (Table 4-14). There is a wide variety of choices for not only the materials of construction but the tightness of weave and the size of the yarn. All of these factors along with the others noted earlier, make the selection of bag fabric an art that requires manufacturer s and plant s actual field tests. Woven fabrics have a low ratio of weave openings for yarn area and generally have a limited face velocity for air flow of about 1.5 to 3.0 cu ft/min/sq ft [47]. 

Pressure loss for inlet louvers based on two face velocity heads and 0.075 Ib/ft air is given as 0.02 in. water for 400 ft/min face velocity to 0.32 in. water for 1,600 fpm, varying slightly less than a straight line [19]. 

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