Cross-drafts

The value of the coefficient of turbulent diffusion, D, depends upon the air change rate in the ventilated space and the method of air supply. Studies by Posokhin show that approximate D values for locations outside supply air jets is equal to 0.025 m-/s. Air disturbance caused by operator or robot movement results in an increase in the D value of at least two times. Studies by Zhivov et al. showed that the D value is affected by the velocity and direction of cross-drafts against the hood face, and the presence of an operator e.g., for a cross-draft directed along the hood face with velocity u = 0.5 m/s with D = 0.15 m-/s (with the presence of an operator), an increase to = 1.0 m/s results in D = 0.3 m-/s. 

Experimental studies have shown that velocity distribution in the cross-section of the directing jet can be described by the same equation as those in the axisymmetric jet in a cross-draft,... 

Gendrikson, V. A., and Y. V. Ivanov. 1973. Some regularities of axisymmetric jets supplied at an angle to the cross-draft. In Proceedings of the Tashkent Polytechnical Institute, vol. 101, pp. 184-1.98. 

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. 

Cross-draft velocity was normalized by dividing the measured cross-draft ve-locit by the capture velocity calculated at the tatik centerline. Capture velocity at the tank centerline was calculated using Silverman s - centerline velocity (Eq. (JO.l)) for unflanged slot hoods. There was considerable scatter in the data, show ing chat cross-draft velocity alone is not responsible for low capture efficiency. 

FIGURE 10.9 Hood capture efficiency versus normalized cross-draft velocity, ... 

The supply airflow rate should approximately equal the exhaust flow rate. A minor difference between supply and exhaust flow rates should nor disturb the exhaust, since exhaust systems usually are operated with higher pressure differences than supply systems. If the exhaust flow rate is higher than the supply, it could result in lower efficiency due to lower exhaust flow rates and cross-drafts (see Disturbances). If the exhaust flow rate is lower than the supply flow rate, there may be fewer problems with exhaust efficiency, but this could result in a supply airflow field different from the designed one and thus result in different kinds of disturbances. 

Simple Evaluation A simple evaluation can be done by checking the airflow rate into the opening, presuming the source characteristics, the placing of the exhaust, and the other parameters (cross-draft, work routines, supply airflow rate, etc.) have not changed since the detailed evaluation was done. It is necessary to do the simple evaluation at the same time as the detailed evaluation. The flow... 

Superposition of Flows Potential flow solutions are also useful to illustrate the effect of cross-drafts on the efficiency of local exhaust hoods. In this way, an idealized uniform velocity field is superpositioned on the flow field of the exhaust opening. This is possible because Laplace s equation is a linear homogeneous differential equation. If a flow field is known to be the sum of two separate flow fields, one can combine the harmonic functions for each to describe the combined flow field. Therefore, if d)) and are each solutions to Laplace s equation, A2, where A and B are constants, is also a solution. For a two-dimensional or axisymmetric three-dimensional flow, the flow field can also be expressed in terms of the stream function. 

This solution gives unequivocally the effective control range of both unflanged and flanged openings when the exhaust flow rate and velocity of the idealized cross-draft are known. The distance from the hood opening to the dividing streamline for a hood in uniform flow perpendicular to its axis is thus... 

This type of dependence of capture efficiency on the exhaust flow rate and cross-draft velocity has also been seen by Fletcher and Johnson who determined the capture efficiency of a flanged square exhaust hood in a cross flow. 

The design of low hoods is much simpler, since the adverse effects of turbulent mixing and cross-drafts are much less important than for high hoods. Low hoods are much more likely to capture a high percentage of the heated air and contaminants than high hoods, so they should be used whenever possible. 

Low Receptor Hoods Low receptor hoods are much easier to design, since entrainment of air into the plume and the effects of turbulent cross-drafts are not significant problems. In this case, the diameter of the plume at the hood face, dp, is assumed to equal the diameter of the source, d,.. Accord-... 

Working locations between the contaminant source and the capture openings dramatically reduce the efficiency of the capture system and should therefore be avoided. If the hood is enclosed on three vertical sides the sensitivity to cross-draft is low. 

Control of contaminant at the face of a fume cupboard depends on the movement of air through the working aperture. The velocity must be sufficiently high to resist the effects of disturbances created by the operator, other personnel, equipment, cross-drafts, etc. It should not be so high, however, as to cause disturbances to equipment in the cupboard or to create eddies in the wake of the operator or from the cupboard itself that will adversely effect containment. 

Flynn et al." applied a finite element based numerical model to solve the problem of a push-pull flow with cross-drafts and demonstrate that the results show good agreement with experimental data. They note, however, that the numerical method is time consuming and therefore computationally expensive. 

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. 

FIGURE 10.75 Required initial kinematic momentum, f/p, as a function of the length of the tank, L, and the buoyancy velocity, v, when the critical contour criterion is applied with the critical concentration, C ,j, equal to 5% and the cross-drafts equal to 0.05 m s". ... 

These systems can be used for relatively small sources that generate a large amount of contaminants at not too high temperatures and velocities, and where the process is not influenced by cross-drafts. 

For a new process plant, calculations can be carried out using the heat release and plume flow rate equations outlined in Table 13.16 from a paper by Bender. For the theory to he valid, the hood must he more than two source diameters (or widths for line sources) above the source, and the temperature difference must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests. - If exhaust volume requirements must he established accurately, small scale modeling can he used to augment and calibrate the analytical approach. 

The use of canopy hoods or remote capture of fume is usually considered only after the rejection of source or local hood capture concepts. The common reasons for rejecting source or local hood capture are usually operating interference problems or layout constraints. In almost all cases, a canopy hood system represents an expensive fume collection approach from both capital and opetating cost considerations. Remote capture depends on buoyant ait curtents to carry the contaminated gas to a canopy hood. The rising fume on its way to the hood is often subjected to cross-drafts within the ptocess buildings or deflected away from the hood by objects such as cranes. For many of these canopy systems, the capture efficiency of fume may be as low as 30-50%. 

Goodfellow, H. D., and P. Safe. Analysis of Remote Receptor Hoods Under the Influence cd Cross Drafts. ASHRAE Winter Meeting, Chicago, IL, Jan. 28-Feb. 1, 1989. 

Cross-drafts The unwanted or wanted movement of air within a space, which may be natural or mechanical. See Cross ventilation. 

Special Chemical laboratory hoods will have an average inward face velocity of 100 linear feet per minute (lfpm) 20% with the velocity at any point not deviating from the average face velocity by more than 20%. Existing laboratory hoods will have an inward face velocity of 150 lfpm 20%. Laboratory hoods will be located such that cross drafts do not exceed 20% of the inward face velocity. A visual performance test using smoke producing devices will be performed in assessing the ability of the hood to contain Lewisite. 

For a cross draft tower, water loadings of 4 gpm per square foot are often used. 

---