Crossdraft

A laboratory study of surface-treatment tanks by Braconnier et al." showed the effects of cross-drafts and obstructions to airflow on capture efficiency. They found that, without obstructions, capture efficiency decreased with increasing cnrss-draft velocity but the importance of this effect depended on freeboard height. In their study, cross-draft direction was always perpendicular to the hood face and directed opposite to the hood suction flow. Follow cro.ss-draft velocities (less than 0.2 m s ), efficiency remained close to 1.0 for the three freeboard heights studied. With higher cross-draft velocities, efficiency decreased as freeboard height decreased. For example, when the crossdraft velocity was 0.55 m s , efficiency decreased from 0.90 to 0.86 to 0.67 as freeboard height decreased from 0.3 m to 0.15 m to 0.1 m, respectively. 

A similar effect was observed for changes in hood flow rate. With a fixed cross-draft velocity, capture efficiency decreased with decreasing hood flow rate. This effect was much more important when freeboard height was small. Their results showed that when hood flow rate was 1.5 m s m, efficiency remained close to 1.0 as long as the cross-draft velocin. was less than 0.45 in s. The most severe conditions tested were a hood flow rate equal to 0.33 m s" nr- and crossdraft velocity equal to 1.15 m s. Under these conditions, capture efficiency was equal to 0.83 for freeboard hei t equal to 0.3 m, but decreasing to 0.4 when freeboard height was decreased to 0.1 m. 

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. 

For this safety criterion, we consider the fact that as the velocity decreases with increasing distance from the surface of the tank, it will reach some critical velocity, at which the induced movement of air will be insufficient to overcome the effects of crossdrafts or the buoyancy velocity At this point, we must ensure that the concentration is at, or below, some critical allowable concentration, Qfj,. The values of the critical concentration and velocity will depend (tn particular circumstances, but it is worth noting that must be at least equal to I g in order to overcome the effects of buoyancy, and the appropriate value will depend on the crossdrafts, which typically vary between 0.05 m to 0.5 in s F For the sake of providing examples, we have chosen to be the maximum of the buoyancy velocity and the typical cross-draft velocity. For the critical concentration we have chosen two values, C = 0.05 and C = 0.10. The actual value used by a designer would depend on the toxicity of the contaminant in question. 

The capture velocity of a hood is defined as the air velocity created by the hood at the point of contaminant generation. The hood must generate a capture velocity sufficient to overcome opposing air currents and transport the contaminant to the hood. For enclosing hoods, capture velocity is the velocity at the hood opening. In this case, the velocity must be sufficient to keep the contaminant in the hood. In practice, hood shape and the influence of crossdrafts on the measured capture velocity have to be considered. All three velocity components should be measured and used to calculate the magnitude and direction of the total velocity. Other methods used, not as good as the previous one, are to measure the velocity with a directional velocity sensor towards the hood or to measure the net velocity by an omnidirectional velocity sensor. In the last method the main airflow direction should be viewed and evaluated by means of a smoke test (see Sections 10.2.1 and 10.2.2.1). 

Both the primary air flow and the fuel feed can have at least three possible directions through the fuel-bed system. The primary air directions are here referred to as updraft (upwards), downdraft (downwards) and crossdraft (sidewards) [19,35], whereas the fuel feed is referred to as overfeed, underfeed and crossfeed [9,10], Figure 24 illustrates different continuous conversion systems with varying direction of both primary air flow and fuel feed. 

Figure 24 Different types of conversion systems. For example, C- have a downdraft overfeed configuration, E - has a crossdraft-overfeed configuration, D- has updraft-crossfeed configuration. [10]...

The proceeding analysis and classification of conversion systems (fiiel-bed systems) in the next section will be confined to updraft fiiel-bed systems. The possible number of combinations, were all three air directions (up-, down- and crossdraft) considered, were too many to be included in the scope of this survey. Updraft fuel beds are chosen because they are the most common among PBC systems. 

The classification was made for updraft fuel beds only. However, it could have been made for downdraft and crossdraft as well. 

Wood Gasification in an Air-Blown, Crossdraft Gasifier-Combustor... 

Operating conditions for objects Crossdraft, Airflow velocities, f.p.m. ... 

Crossdrafts can be eliminated through proper design and such design should be sought. Crossdrafts in excess of 100 fpm (feet per minute) should not be permitted. 

Although the location of the fume hood is not under the control of the user, proper location is very important in both the efficiency and safety of the unit. The fume hood should be located out of the traffic patterns of the laboratory, away from ventilation crossdrafts, doors, and windows. Two fume hoods should not be facing one another across an aisle. Crossdrafts from ventilation and laboratory traffic can completely alter the airflow characteristics of a fume hood. 

If these conditions occur in an existing installation, take steps to avoid production of crossdrafts by restricting traffic during fume hood use, baffling air ducts (but not to the point where room air balance is completely upset), closing windows or doors, or taking whatever steps may be necessary to preserve the unit s airflow characteristics. 

Biomass is fed from the top of the reactor, while the airflow changes depending on the configuration downdraft, updraft, and crossdraft. 

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