Minimum duct velocity

Minimum Duct Velocity. Once the types and locations of hoods, enclosures and booths have been established and the exhaust rates determined, it is necessary to design the ductwork to transfer the contaminated gas to the collection or cleaning device. An essential parameter required for this purpose is the minimum duct or conveying velocity, Vfmm, which is required to ... 

The solution to the above problem is to re-size the entire network, in particular branches I-V and II-V, with the aim of ensuring a suitable minimum transport velocity along each section of duct. 

When designing a local exhaust ventilation system for a process that generates dust particles, it is important to consider the minimum air velocity. The minimum air velocity is the velocity required to prevent settling of dust particles in the air ducts. The minimum velocity is a function of dust particle size and particle density. Listed in the table below are the minimum air velocities recommended for the transport of various types of particulate contaminants. 

Thus, the minimum gas velocity in the flow ducts and the minimum particle diameter are respectively given by... 

Type A1 (formerly, Type A) These Class II BSCs maintain a minimum inflow velocity of 75 ft/min, have HEPA-filtered, down-flow air that is a portion of the mixed down-flow and inflow air from a common plenum, may exhaust HEPA-Altered air back into the laboratory or to the environment through an exhaust canopy, and may have positive-pressure contaminated ducts and plenums that are not surrounded by negative-pressure plenums. They are not suitable for use with volatile toxic chemicals and volatile radionucleotides. 

Type B1 These Class II BSCs maintain a minimum inflow velocity of 100 ft/min, have HEPA-filtered down-flow air composed largely of uncontaminated, recirculated inflow air, exhaust most of the contaminated down-flow air through a dedicated duct exhausted to the atmosphere after passing it through a HEPA filter, and have aU contaminated ducts and plenums under negative pressure or surrounded by negative-pressure ducts and plenums. If these cabinets are used for work involving minute quantities of volatile toxic chemicals and trace amounts of radionucleotides, the work must be done in the directly exhausted portion of the cabinet. 

Observation of operating conveyors indicates that the solids are rarely uniformly dispersed in the gas phase. With infrequent exceptions, the particles move in a laminar pattern, following a streamline along the duct wall where the flow velocity is at a minimum. Complete or even partial diffusion in the gas phase is rarely experienced even with low-specific-gravity particles. Air velocities may approach 20 to 30 m/s. It is doubtful, however, that even finer and lighter materials reach more than 80 percent of this speed, while heavier and larger... 

Class 1IB3 A Class IIB3 BSC (Fig. 10.98) is a ducted Type A cabinet having a minimum inward air velocity of 0.5 m s E All positive-pre.ssure contaminated plenums within the cabinet are surrounded by a negative air pressure plenum. Thus, leakage in a contaminated plenum will be into the cabinet and not into the environment. 

Kintner et al (K7) and Damon et al. (Dl) have discussed photographic techniques applicable to the study of bubbles and drops. Sometimes it is desirable to hold a bubble or drop stationary, to study internal or external flow patterns and transfer processes. To prevent the particle from migrating to the wall, it is desirable to establish a minimum in the velocity profile at the position where the particle is to reside, and various techniques have been devised (D4, FI, Gl, Pll, M15, R15, S20) to do this. Vertical wandering of such particles may occur (W7), and may be reduced by using a duct tapered so that the area decreases towards the top (D4). Acoustic levitation of liquid drops may also be used (A3). 

At specified mass flow rate and inlet conditions Py and V), Eq. (6.68) predicts a relation between the area ratio A2IAX and the pressure ratio P-JPy when isentropic flow prevails. It turns out that, as the pressure falls, the cross section at first narrows, reaches a minimum at which the velocity becomes sonic then the cross section increases and the velocity becomes supersonic. In a duct of constant cross section, the velocity remains sonic at and below a critical pressure ratio given by... 

The EPA Method 1 (see Appendix B for a complete list of EPA Methods) recommends that both sample and velocity measurements be made at least 8 internal diameters downstream of any flow disturbance (e.g., elbow, duct contraction, or expansion) and at least 2 internal diameters upstream from any flow disfurbance. If this is not possible, then measurements may be made as close as 2 internal diameters downstream and 0.5 internal diameters upstream of disturbances, but more measurements across the duct cross section are needed. Figure 7.5 shows the recommended minimum number of traverse (sampling) points for measuring either the gas sample composition or the gas velocity where no particulates are in the gas stream. The figure shows that at least 8 or 9 measurement points are recommended for internal stack diameters between 12 and 24 in. where the measurement location is at least 8 internal diameters downstream and 2 internal diameters upstream from any upsfream disturbances. Por internal stack diameters greater than 24 in. in internal diameter, at least 12 sampling points are recommended if fhe measuremenf location is af leasf 7... 

Premix duct based on a lean premixed prevaporised (LPP) concept with a venturi configuration, which has been optimized in order to obtain a good velocity profile at the inlet to the catalytic section. Fuel is injected upstream of the venturi minimum section with injectors angled relative to the airflow. Mixing is achieved with an inlet radial/axial turbulence generator in the venturi inlet. 

Commercial OTEC plants sited in the tropical oceans will each generate power in the range of 100 00 MWe (net) and will use modular construction. An OTEC plant will require a total water flow of about 3 m /sec MWe (net). Thus, commercial plants of 100-400 MWe will have total flow rates of 300-1200 m /sec. Design studies indicate that plants composed of 50- to 60-MWe modules will be optimum and that the flow velocity in the water ducts will be limited to about 2.5 m/sec to avoid excessive drag losses. Therefore, the minimum total duct area will be 60-75 m /module. 

The laboratory shall be equipped with a fume hood. The fume hood should meet any specific safety requirements mandated by the nature of the research program. A discussion of hood design parameters will be found in a later section, but for high hazard use the interior of the hood and the exhaust duct should be chosen for maximum resistance to the reagents used the blower should either be explosion-proof or, as a minimum, have non-sparking fan blades the hood should be equipped with a velocity sensor and alarm should the face velocity fall below a safe limit the interior hghts should be explosion-proof, and all electrical outlets and controls should be external to the unit. It may be desirable to equip the unit with an internal automatic fire suppression system. 

The velocity of the air in the ducts must be kept hic i, a minimum of about 1,000 m/min. The fan is sized on the basis of the volume of air required and the pressure drop through the system. The fan is installed on the clean air side of the cyclone or filter bags. 

Figure 11.5 Location of velocity measurements in ducts, a. Log linear rule for posifions of fraverse points for a Pitot survey on three different diameters of a circular ducf b. Log Tchebycheff Rule for position of measuring points and traverse lines for Pifof survey of a recfangular ducf. Note Minimum ratio of ducf diameter to Pitot static tube diameter is 32 BS 1042 Part 2.1. Source Reprinted from Daly BB, Woods Practical Guide to Fan Engineering, Woods of Colchester, 3rd ed., 231, 1992. By kind permission of Flakt Woods Ltd 2004.

Adhesion of Particles to Bottom of Air Duct. Dust particles will not fall to the bottom of a duct (and hence there will be no dust adhesion) if the vertical pulsating velocity Vy of the air flow is greater than the terminal (free-fall) velocity of the particles in air, i.e., if Vy > Uff. If we know Vy and its relation to the flow velocity, we can calculate the minimum air-flow velocity to prevent the settling of dust. Ryzhenko [242] found that for particles with diameters smaller than 10 ixm in moving air, the allowable velocities in air ducts with circular, rectangular, or trapezoidal sections, i.e., Vq, v, and are expressed by the formula... 

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