22 ANDRITZ


extremely well, and also mechanical limitations where performance is less than optimum. Building on technical strengths and overcoming limitations are what has motivated Andritz’s engineers to innovate new designs – such as the VST. With more than 500 units in


E


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 per cent). “How can we remove the


limitations in lower feed consistency applications – while keeping the inherent design strengths of the SCP?” That was the question that Andritz engineers answered when developing the developing the VST.


The idea behind the 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 using a rotating screw that tapers to increase the dewatering effect on the pulp. Feed consistencies are typically in the range of 5-10 per cent. 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 per cent are achieved. The horizontal design has two main limitations: firstly, at low feed consistencies (3-5 per cent) the holes of the screen baskets


July/August 2018


very item of production equipment in a paper mill has a range in which it performs


Pulp Paper & Logistics


Going vertical


Removing the limitations in lower feed consistency while retaining the strengths of a screw press were behind the latest innovation in dewatering. Peter Ortner reports


in the inlet zone tend to plug; and secondly, 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.


The impact of hydrostatic pressure The SCP is fed from the top through the headbox into the first dewatering section. Due to the


height difference between the top and bottom of the dewatering screen, hydrostatic pressure builds up in proportion to the diameter of the press (see Figure 1). For large screw presses, this hydrostatic pressure can reach between 0.10 and 0.15 bar.


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