The drum deck is sectionalized into individual division strips to control vacuum without losses. Each section is an integral vacuum zone that prevents the leaking of vacuum from one zone to the next. Each section has its own filter fabric, support grid, and filtrate piping often positioned in leading and lagging parts of each section. This design promotes high hydraulic rates out of the filter.

The filtrate pipes welded to each section are then turned through the inside of the drum to the the non-drive side drum trunnion called the filtrate end. The filtrate pipes are welded in an equally spaced pattern to a pipe plate. The pipe plate is in effect a manifold for all of the filtrate pipes.

• The drum deck, equipped with replaceable filter grids, supports the filter medium (cloth made from fabric or metal). A vacuum pump provides the necessary gas stream to accomplish dewatering. The vacuum sucks the liquor through the filter medium and forms a filter cake out.
• Trevor Sparks, George Chase, in Filters and Filtration Handbook (Sixth Edition), 2016. 4.4.2.2 Rotary vacuum disc filter. The vacuum disc filter operates a similar way to the vacuum drum filter – filtration elements (sectors), covered in a filter cloth, pass into a slurry trough, vacuum is applied to the sector, a cake forms on its surface, it emerges from the slurry and the cake is dried.

Precoated rotary vacuum drum filters (Fig. 6) are used by filtering a slurry of filter aid and water first, then subsequent product filtration. Difficult filtering materials, which have a tendency to blind, are removed with a doctor blade.

Rotary Vacuum Filter Application

Mounted tight against the rotating pipe plate is a stationary rotary valve that has a sacrificial wear plate which takes the friction between the pipe plate and the valve body. The valve body not only directs the liquid filtrate away from the drum. It also allows for vacuum control in the sections of the drum, to segregate cake formation from cake drying from cake washing to cake discharging. This vacuum control is called the “timing” of the vacuum with the use of adjustable bridge blocks within the valve’s internal cavity. The location of the bridge blocks allows control of vacuum where it is most useful and then, with certain designs, to cut off the vacuum when the cake must be discharged.

To segregate cake formation from cake drying, the drum is commonly submerged from 10% up to 37.5% in the slurry. During cake formation, the vacuum applied deposits suspended slurry solids on top of the filter fabric. During cake drying, the vacuum continues to be applied to the filter cake. The cake now creates a pressure differential that dries the cake to low moistures. During cake discharge, the vacuum is released, and the cake is discharged several different ways: belt discharge, scraper with air blow-back discharge, precoat knife discharge, roll discharge, string discharge.

In the vat is an oscillating agitator that sweeps the bottom of the vat to uniformly suspend and distribute.

Rotary Vacuum Filter Specifications

Filtrate discharging from the rotary valve flows to a vacuum receiver tank. In this tank, the air-filtrate velocity is significantly reduced to allow for separation. The air discharges to the vacuum pump through the top outlet of the tank while the filtrate discharges to a self-priming pump located at the bottom outlet of the tank.

The drum filter commonly utilizes VFD drives on the drum, on the discharge roll, and on the agitator. The production rate is directly determined by the drum speed (measured in “MPR” or minutes per revolution) and drum submergence (or vat level). However, the discharge roll and the agitator can indirectly help or hinder the rate.

Operating trade-offs often pose dilemmas for operators. High drum submergence will generally yield thick cakes. However, the moisture content may be high due to the shorter dry time. When the low cake moisture is required, then the drum submergence is reduced to offer more dry time. But then the cake thickness will be low resulting in a lower production rate.

Form time versus dry time can be precisely managed with the timing position of the bridge blocks internal to the rotary valve. High vacuum levels during cake formation increase cake thickness and yield. Compressible cakes, however, may have a high pressure differential which will reduce the production rate. If cake washing is required, the chances of cake cracking from premature dewatering is often by the vacuum level. If cake washing is critical, the wash solute will diffuse through the filter cake if the cake is thin and the vacuum flow is high.

The filter fabric selection is an important consideration. The trade-offs for operators are chemical and temperature compatibility, fine particle capture for high filtrate clarity, low blinding tendency, durability with abrasive solids, cleanability, and of course cost. This also applies to filter aids such as diatomite, perlite, cellulose, fly ash, and some process derivatives.

Rotary Vacuum Filter

Rotary Vacuum Filter Price

An Auto-Vac® utilizes a liquid ring vacuum pump that pulls water through a precoat drum filter while sucking moisture out of the suspended solids. The inside of the drum is hollow; the outside consists of a wedge wire screen covered by a polypropylene cloth. The drum rotates in a trough [filter pan] where one-third is submerged in liquid and two-thirds are exposed to ambient air. When activated, and before filtration, the drum surface is coated with a filter aid 'cake' of Diatomaceous Earth (DE) or Perlite. This consumable filter media is porous, allowing air and water to pass through while capturing micron size particles.
Once a full cake is built on the drum, the wastewater is pumped into the pan at a controlled rate. The vacuum sucks the water [liquid] through the drum and pumps it away. Suspended solids, precipitated metals, or de-emulsified fat, oil and grease larger than one-micron will accumulate on the filter cake surface. These solids will build with each revolution of the drum until the vacuum cannot draw any more water through. At this point, a stepping motor activates a variable speed knife blade that removes the top layer of solids with lathe-like precision. The knife also shaves off a slight amount of filter aid cake as it advances toward the drum; leaving a clean layer of media to grab more solids.
The drum size, filtration rate, dryness of solids and filter aid consumption varies with the characteristics of the incoming liquid. Contact ALAR to help determine the Auto-Vac® that best suits your application.