Hopper vacuum

Oxidation] temperature too hot/screw speed too low/[residence time too long] / oxygen present/nitrogen purge ineffective/antioxident stabilizer ineffective/[trap-ped.air] /hopper vacuum inadequate. 

This continuous votator process was first used for margarine production by Girdler USA in 1938, introduced in the UK during the 2nd World War and in the rest of Europe in the 1950s. In some factories, the semi-continuous cooling drum/hopper/vacuum kneader process is still used, but almost exclusively for special-purpose margarines. 

Vacuum casting n. A method used for casting fluid thermosetting resins to avoid inclusions of air bubbles. The mold is placed in a vacuum chamber and filled with resin from an external hopper. Vacuum is applied to pull out bubbles, held until they have all risen to the surface, then released. Curing follows. 

One method to overcome this air entrapment problem is to use a vacuum feed hopper. In principle, this is quite simple however, in practice, a vacuum feed hopper is not a trivial matter. The first problem to occur is how to load the hopper without losing vacuum. This has led to the development of double hopper vacuum systems, in which material is loaded into a top hopper and the air is removed before the material is dumped in the main hopper (see Fig. 3.14). 

For fine powders that tend to bridge or stick and are of low bulk density, some form of forced feed, such as the tapered screw feeder shown in Figure 9, must be used to deaerate, precompact, and pressurize the feed into the nip. Large machines are available with up to five screw feeders to spread the flow across the roUs, and vacuum hoppers are also used to remove air when densifying low density feeds. 

Table 21-28 gives dimensions of hopper cars and other cars typically used in the chemical industiy. Vacuum-pressure systems are used most frequently for unloading covered hopper cars. For certain free-flowing materials, in both covered and open-top hopper cars, shakeout devices are useful. 

Because of the railroad-car shortage that has persisted for many years, boxcars are often used for bulk materials. Lined with suitable materials to prevent contamination and with special bulkheads at each door, these cars are acceptable substitutes for covered hopper cars even though unloading is more difficult. Vacuum conveying wands are used to pick up the material, as are front-end-loader-type vehicles. 

Figure 4-6 A combination vacuum-pressure system. A vacuum is used to withdraw material from a hopper car and positive pressure is used to transport it to storage silos.

Against this, it may be necessary with a batch vacuum filter to provide holding vessels for feed, and possibly filtrate and wash. The discharge of solids is intermittent and this may lead to complications in feeding the solids to a continuous dryer or to some other continuous process, and cake hoppers and a feeding arrangement may have to be provided. 

Fig. 7 Extruder and extrusion. 1 Drive unit, 2 barrel inlet, 3 temperature control, 4 feed hopper and dosing unit, 5 vacuum vent, 6 barrel, 7 screw. (Reproduced with permission from Reifenhauser)...

Thermoforming station (pocket depth, pocket rupture) Forming head pressure and pressure distribution Cooling water supply for heated and cooled tools Vacuum exhaust system Empty hopper detector Fill control system... 

The equipment feed design consisted of a funnel-shaped powder hopper (with no vacuum deaeration system) that was located directly above the rolls. The powder hopper was retrofitted with vacuum deaeration capability. A high compression feed screw, fitted inside the funnel hopper, fed the powder directly into knurled rolls. The compaction trials were conducted with and without vacuum deaeration. The compact was carefully collected directly on a 10-mesh screen. Powder particles, that were not compacted (i.e., those particles which were not attached to the compact, for example, fines bypassing roll compaction and the non-adhering compacted powder particles) were weighed and separated. The compact was not milled. The parameters are noted in Table 4. 

The mechanical and electrostatic ash samples were obtained by emptying the specified vacuum-operated hoppers just prior to the start of the run. When the test was completed, a cap plug was unscrewed from the bottom of the hopper permitting the hot ash to flow out. A 10-gal sample of each of these ashes was collected and split, maintaining a 16-oz sample of each ash. With the amount of ash involved, it was impossible to obtain a representative sample of the total amount of ash collected in the hopper during the test period. 

---