Airflow disturbances

Typically, the share of the static pressure across the inlets, p, is selected to be between 0.1 and 0.4. This allows one to keep a low velocity of airflow through inlets so as not to disturb thermal plumes above heat sources. 

It the main reason for the stratification strategy is contaminant control in [he occupied zone, the same strategy should be applied in winter conditions, too. Thus, the selected hearing method has to fulfill two requirements to siip-pttrt the creation of the vertical stratification and not to create disturbing airflows into the hall. In this case one option would be the floor heating method see Fig. 8.16c. Additionally, one should consider the prevention of boundary layer flows along the outer walls using, for example, passive methods. -... 

The effect of the disturbance of the supply airflow on the plumes is presented in Fig. 8.39. The heat removal effectiveness j and contaminant removal effectiveness are presented as functions of the penetration factor P of the plume, which is the ratio of penetrated plume airflow rate to the whole plume airflow rate. The total heat load is 60 W m"- and the power of one hear source is 500 W. The supply airflow rate is 8 L S m -. 

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. 

Disturbances to the airflow into and in a fume cupboard can be caused by... 

Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet,. should be relatively low with a uniform velocity but high enough so that it is not disturbed by the. surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker. 

Supply Air When designing workbenches, it is essential that the supply air face area be large enough to cover the contained area. Therefore it is important to have some indication of the operator s range of movements for all intended operations. Moreover, for efficient protection the supply airflow must be adequate to get a stable flow field that will not be affected by ambient disturbances. In industrial applications the suitable mean supply air velocities are typically between 0.2 and 0.45 m s h Low velocities should be used when the distance between the supply air unit and the operator is small or for cool supply air. High velocities are applicable at greater distances and in hot environments, with thermal comfort being considered. 

Both types of noncontact Doppler measurements are suitable for nearly all hydrodynamic and aerodynamic velocity measurement applications. When locating other anemometers, the structures on which they are mounted are likely to disturb the airflow. A rectangular building will disturb airflow up to an elevation of about twice its height above grade, six times its height leeward, and twice its height in the windward direchon. 

Many operations within the laboratory start with weighing. It is difficult to undertake precise weighing in conventional laboratory fume hoods, as the airflow and vibration inside an operating fume hood often disturb the balance. In order to accommodate the need for an engineered control at this scale of operation, dust control systems have evolved known as ventilated weighing safety enclosures or powder weighing hoods. Key characteristics of a ventilated weighing safety enclosure include ... 

When the box is used for productions of radiopharmaceuticals incorporating a-or P-emitting radionuclides, closed box units without any lead coating may be sufficient. When handling radionuclides with mixed emitting properties, a possibility is to concentrate the shielding to critical parts of the process. This can be done by use of local shielding inside the production unit. However, for aseptic production, one must keep in mind a potential disturbance of the airflow inside the box. 

FQ series mine pneumatic fan is developed by China University of Mining and Technology, the working medium and the power of the motor are compressed air. Output shaft are installed at the both ends of fan, fan blade is installed in the front end and linen wind wheel driven by the fan airflow is installed in the back, and there has an incomplete circular air outlet in the linen wind wheel. Thus, blade and line wind wheel would be rotated by the fan when the fan is working, and a certain frequent and disturbed jet which can reach the range of 8 meters to 15 meters would be generated in a conical area, so it is suitable for blowing dead space of ventilation, especially for harmful gas accumulation in upper comer. 

The air floater dryers are also similar to those described in the paper-drying section. Hot air is impinged to the web from blow boxes above and below, and the web floats supported by the airflow. The low sheet tension ensnies a greater ability to tolerate disturbances or sheet defects, and the quality of pulp is less affected. Pulp dryers also consist of stacked drying decks, as shown in Figure 39.16. Pulp dryers are larger than paper dryers as pulp is air dried from the press section to the cutter. 

With Scandinavian stacking practice, the end zones of the stack dry faster than the central, fully filled part (Salin and Ohman, 1998). The lower air velocity in the ends is more than compensated by higher heat-transfer coefficients associated with the flow disturbance and smaller wood volume. (There is a smaller decrease in temperature and increase in humidity along the stack in the airflow direction.) In general, it is expected that the local transfer coefficients diminish with distance in the airflow direction due to a thickening of the boundary layer. This variation and the downwind accumulation of moisture in the airstream result in the maximum possible evaporation rate dwindling with distance from the air inlet to the air outlet from the stack. 

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