TechNews_ GOING VERTICAL-The latest innovation in dewatering
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GOING VERTICAL

The latest innovation in dewatering

VST manufactured in the ANDRITZ workshop in Graz, Austria.

Every piece of production equipment in a mill has a range where it performs extremely well, and also mechanical limitations where performance is less than optimum. Building on technical strengths and overcoming limitations are what motivates ANDRITZ engineers to innovate new designs – such as the VST.

With over 500 units in operation, the ANDRITZ Screw Press (SCP) is regarded as being top of the line. But, even a premiere product has practical limits. In the case of the SCP, the limitation could be observed in applications with lower feed consistencies (e.g., 3-6%).

THE IDEA BEHIND DEVELOPMENT OF THE VST

Simply described, the VST is a vertical configuration of the SCP, with some important differences. In an SCP, the pulp suspension is fed into the inlet housing of the unit and dewatered via a rotating screw that tapers to increase the dewatering effect on the pulp. Feed consistencies are typically in the range of 5-10%. The dewatered pulp is discharged by gravity from the outlet casing at the end of the rotating screw. Typically, outlet consistencies in the range of 26-30% are achieved.

The horizontal design has two main limitations: 1) at low feed consistencies (3-5%) the holes of the screen baskets in the inlet zone tend to plug; and 2) there tends to be uneven filling of the press across the complete area of the dewatering screen. These two limitations reduce the throughput and discharge dryness of the pulp. The VST was designed to overcome these limitations in specific applications.

TAKING IT VERTICAL

Often, the simplest ideas are the best. After evaluating several different design concepts to address these two limitations of the SCP in low feed consistency applications, the solution turned out to be a quite simple one: turn a horizontal SCP on its end and feed it from the top. In a vertical configuration, gravity works in a positive way: evenly distributing the pulp suspension across the full circumference of the dewatering screen and ensuring 100% filling.

In a VST, the pulp suspension is fed into the top of the machine and gravity transports the pulp downwards. The risk of plugging the inlet screen at low feed consistency applications is almost eliminated. Similar to the SCP, the VST has a conical shaft and decreasing pitch so the pulp is compressed and dewatered as it moves downward. The pulp suspension is automatically and consistently refilled into the area between screw flights. The rotating shaft moves the pressed cake downwards to the outlet casing.

A pneumatically controlled counter-pressure ring builds up the pulp plug quickly after startup and releases the pulp when the proper consistency is achieved. The ring pressure can be adjusted to fine-tune discharge dryness or can be quickly unloaded to avoid plugging. The result is that higher throughput can be achieved with the VST.

Figure 1: Dewatering physics of the Screw Press (SCP).

Figure 2: VST cross section and enhancement
of hydrostatic pressure.

MAIN BENEFITS OF THE VST:

  • Higher production – or higher output dryness at the same production – when compared with a traditional SCP of the same size
  • Highest dewatering efficiency (particularly for low feed consistencies)
  • 100% filling degree - screen baskets fully utilized to increase dryness and throughput
  • Gravity-aided filling – uniform dewatering from top to bottom
  • Uniform and consistent pulp discharge, which is beneficial to downstream processes
  • Small space requirement/footprint
  • Closed machine design
  • Telescoping filtrate housing for easy maintenance access to screw and screens in the high-pressure zone
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