Direct airflow method

The particular feature of Gonidur sheets is the specific hole shape which creates a directional airflow to help in discharging the product and to influence the retention time in the fluid bed. The special method of manufacturing Gonidur sheets enables finishing of fine perforations in sheets with an initial thickness many times over the hole width. Perforations of only 100 pm in an initial sheet thickness of 0.7 mm are possible. With holes this small 1 m of plate may comprise several hundred thousand individual orifices. 

Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. Fhe exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. lliis method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved. 

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). 

The conventional top spray method shown in Figure 2 has been used for layering and coaling for decades. It evolved from the fluidized bed dryers commercialized more than 40 years ago. The substrate is placed in the product container, which is typically an unbaffled. inverted, truncated cone with a fine retention screen and an air or gas distribution plate at its base. Perforated plates such as a Conidur or Gill plate may also achieve air distribution and product retention. These types of plates may have directed holes for guiding the airflow horizontally in the product container (for side discharge as an example). Process air is drawn through the distribution plate and into the product. 

Figure 6 Schematic diagram of the method of collection of aerosol particles by an impactor. Solid arrows show direction of movement of airflow. Large particles impact on the collection surface, while small particles pass around it.

The pressure cascade regime and the direction of airflow should be appropriate to the product and processing method used. 

Acetone is analyzed by GC-FID. A Car-bowax 20 M, Carbopack, fused-silica capillary column, or any equivalent column may be suitable. It may be analyzed by GC/MS using a purge and trap technique or by direct injection. Characteristic ions are 43 and 58 (electron-impact ionization) (U.S. EPA 1986, Method 8240). Acetone in air is adsorbed over coconut shell charcoal, desorbed with CS2, and analyzed by GC-FID using a 10% SP-2100 or DB-1 fused-silica capillary column (NIOSH 1984, Method 1300). Airflow rate 10 to 200mL/min volume 0.5-3 L of air. Trace acetone in water can be determined by the fast HPLC method discussed in Chapter 5. 

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