Powder application nozzles

Fluidized-bed opposed-jet mills Hosokawa Micron Powder Systems Div.) differ from the Majac mill in that powder is not fed into the jets, but the jets impinge into a chamber mich contains suspended powder. The powder is entrained into the jets. This ehminates wear on the nozzles, and reduces contamination. Otherwise, construction and appheations are similar to the Majac mill. The fluidized-bed level is maintained a few inches above the jets. The Fluidized-bed mill is available in 13 sizes with air volumes ranging from. 50 to 11,000 mVh. One application is for toner grinding. 

The sonic nozzle is capable of providing a constant gas flow irrespective of the change of downstream pressure. Its successful application is dependent on the maintenance of a high upstream pressure. In theory, the pressure ratio should be at least 2, but in practice it is advisable to keep the pressure ratio above 6. At present, the sonic nozzle technique is the only reliable method for controlling low gas flow (e.g. -0.01 cm3 h 1, which is required for krypton adsorption measurements on powders of low surface area). 

The Majac jet pulverizer Hosokawa Micron Powder Systems Div.) is an opposed-jet type with a mechanical classifier (Fig. 20-55). Fineness is controlled primarily by the classifier speed and the amount of fan air delivered to the cfassifier, but other effects can be achieved by variation of nozzle pressure, distance between the muzzles of the gun barrels, and position of the classifier disk. These pulverizers are available in 30 sizes, operated on quantities of compressed air ranging from approximately 0.6 to 13.0 mVmin (20 to 4500 ftV min). In most applications, the economics of the use of this type of jet pulverizer becomes attractive in the range of 98 percent through 200 mesh or finer. 

A plasma spray can therefore be used to apply sintered coatings to substrates which would normally be adversely affected by the sintering temperature of the coating. However, the powders carried by the plasma are exposed to greater thermal stress than the substrate, and some experimenting with specific powders is usually necessary to define the best application conditions. Where molybdenum disulphide in a metal, resin or ceramic binder is applied by this technique, the optimum conditions will usually be different for the two materials. This difficulty has been overcome by the use of two separate entry ports into the nozzle for the two components. The use of this technique has even been applied to molybdenum disulphide in a polyethylene binder. 

Wettable powders can be used with most types of application equipment. As long as the suspension is adequately maintained, they will not clog nozzles, provided strainers and nozzle screens are sufficiently large. WPs are abrasive, however, and may cause both pumps and nozzles to wear more quickly. 

Figure 12-94 > shows a countercurrent flow chamber with pressure nozzle atomization. This design is in limited use because it cannot produce heat-sensitive products. Detergent powder is the main application. 

At some height in the expansion chamber, a nozzle (F/8) is positioned for the application of binder solution. The liquid is sprayed at a controlled rate into the randomly moving powder. 

---