In vacuum filters, the driving force for filtration results from the application of suction on the filtrate side of the medium. Although the theoretical pressure drop available for vacuum filtration is 100 kPa, =1 Kg/cm2 in practice it is often limited to 70 or 80 kPa or even less. principle

In applications where the fraction of fine particles in the solids of the feed slurry is low, a simple and relatively cheap vacuum filter can yield cakes with moisture contents comparable to those discharged by pressure filters. Further­more, this category includes the only truly continuous filters built in large sizes that can provide for washing, drying and other process requirements.

Vacuum filters are available in a variety of types, and are usually classified as either batch-operated or continuous.

One important distinguishing feature is the position of the filtration area with respect to gravity.

A number of vacuum filter types use a horizontal filtering surface with the cake forming on top. This arrangement offers the following advantages:

1 Gravity settling can take place before the vacuum is applied. In many cases, this may prevent excessive blinding of the cloth due to action of a pre-coat formed by the coarser particles.

2 Heavy or coarse materials can be filtered because if they settle out from the feed they do so onto the filter surface.

3 Fine particle penetration through the medium can be tolerated because the initial filtrate can be recycled back onto the belt.

4 Top-feed filters are ideal for cake washing, cake dewatering and other process operations such as leaching.

5 A high degree of control can be exercised over cake formation. Allowances can be made for changed feeds and/or different cake quality requirements. This is particularly true of the horizontal belt vacuum filters. With these units the relative proportions of the belt allocated to filtration, washing, drying, etc., as well as the belt speed and vacuum quality, can be easily altered to suit process changes.

There are, however, two major drawbacks:

1 Such filters usually require large floor areas. 2 Their capital cost is high.

Saving in floor area as well as in installed cost can be made by using a filter with vertical or other non-horizontal filtration surfaces but at the cost of losing most if not all of the five advantages of horizontal filters listed above.

one of the most important and widely used vacuum filters is Nutsche Filter