Cross flow, airflow

Horizontal blades installed at air inlets on towers to control splash-out of water and promote uniform airflow, esp. cross-flow towers. 

Equation (5.48) is an experimentally determined equation that includes a value for f. The f factor or equivalent DPA factor here accounts for the friction loss of airflow through the fin tubes in cross-flow. This equation is emperical and is limited to the specific experimental conditions for which it is derived. It is reasonable to use Eq. (5.48) for most air-cooler design and rating applications. For other applications, however, such as airflow in an HVAC duct, it is not feasible or applicable. Another experimental equation is required for the HVAC-specific applications. 

The cross-flow tower manufacturer may effectively reduce the tower characteristic at very low approaches by increasing the air quantity to give a lower L/G ratio. The increase in airflow is not necessarily achieved by increasing the air velocity but primarily by lengthening the tower to increase the airflow cross-sectional area. It appears then that the cross-flow fill can be made progressively longer in the direction perpendicular to the airflow and shorter in the direction of the airflow until it almost loses its inherent potential-difference disadvantage. However, as this is done, fan power consumption increases. 

The operational aspects of maintenance and caUbration for safety cabinets are almost the same as for cross flow laminar airflow units (see Sect. 28.3.5). Additionally the inflow as protective barrier has to be measured. 

The influence of room transverse cross-section configuration on airflow patterns created by air jets supplied through round nozzles in proximity to the ceiling was studied by Baharev and Troyanovsky and Nielsen (see Fig. 7.37). Based on experimental data, they concluded that when the room width B is less than 3.5H, the jet attaches to the ceiling and spreads, filling the whole width of the room in the manner of a linear jet. The reverse flow develops under the jet. When B > 4H, the reverse flow also develops along the jet sides. Baharev and Troyanovsky indicated that air temperature and velocity distribution in the occupied zone is more uniform when the jet develops in the upper zone and the occupied zone is ventilated by the reverse flow. Thus, they proposed limiting room width to 3-3.5H,. 

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. 

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... 

Class U The Class U (Types A, Bl, B2, and biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection, in addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it... 

Class HA in a Class IIA BSC, an internal blower (Fig. 10.9,St draws sui-ficient room air into the front grill to maintain a minimum calculated measured average velocity of at least 0.37 m s at the opening of the cabinet. The supply air flows through a HEPA filter and provides particulate-free air to the work surface. Laminar airflow reduces turbulence m the work zone and niim-mizes the potential for cross-contamination. 

The ventilation model is a simple flow network with one zone and the different openings modeled as airflow links from the hall to outside Fig. 11.52). For the flow through the roof hood, two additional nodes were considered between the different cross-sections through which the air flows (Fig, 11.53). 

Scale Up of Process. The scale up of fluidized bed coating processes has received little attention in the literature. Current practices in the pharmaceutical industry are reviewed by Mehta (1988). The basic approach described by Mehta (1988) is to scale the airflow and liquid spray rates based on the cross-sectional area for gas flow. This seems reasonable except for the fact that in the scaling of the equipment, the height of the bed increases with increasing batch size. For this reason, a time scale factor is also required. 

Airflow Bate This is often the most difficult to measure. Fan curves are often available for blowers but are not always reliable. A small pitot tube can be used (see Sec. 22, Waste Management, in this Handbook) to measure local velocity. The best location to use a pitot tube is in a straight section of pipe. Measurements at multiple positions in the cross section of the pipe or duct are advisable, particularly in laminar flow or near elbows and other flow disruptions. 

Unidirectional airflow is a rectified airflow over the entire cross-sectional area of a clean zone with a steady velocity and approximately parallel streamlines (see also turbulent flow). (Modern standards no longer refer to laminar flow, but have adopted the term unidirectional airflow.)... 

Airflow visualization (To verify required airflow patterns) All classes 24 months Tests to demonstrate airflows from clean to dirty areas do not cause cross-contamination uniformly from laminar flow units. Demonstrated by actual or videotaped smoke tests. In accordance with ISO 14644-3 Annex B7 ... 

A cross section of the sensor element shows the heating zone where the temperature is controlled to be about 180 K above ambient temperature. Without air flow the temperature is identical upstream and downstream of the diaphragm. With air flow the upstream side of the diaphragm is cooled. Temperature resistors upstream and downstream of the diaphragm detect the resulting temperature difference, which is used as an airflow signal... 

This example shows that, for the assumption of perfect mixing of benzene into the shop air, it is quite straightforward to compute the required dilution air to meet the industrial hygiene standard. We also see that this is an impossibly large airflow rate. If we divide the above flow rate by the cross-sectional area of the shop (4 m x 4 m), we find... 

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