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Sale/Auction: Kadant S23-2 Screw Press

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Make/Model:

 Kadant S23-2

Description:

Kadant S23-2 Tapered Screw Dewatering Press

Max HP: 15

Max Speed:  30 RPM

Serial: S23-2-002

Max 176 F / 80 C

SS with Motor and Gearbox 

Built: 04/2012 

Principle of operation 

The KADANT S23-2 Screw Press thickens pulp from a feed consistency of 3% to 4% to a discharge consistency of 30%. The pulp is fed into the inlet chamber and is conveyed to the discharge opening located at the bottom of the discharge chamber.

The screw shaft turns inside two cylindrical screen baskets. Each basket consists of a supporting backing shell with a drainage screen installed in each of the basket sections. The water drained from the pulp flows between the screen plate and the backing shell of the basket. The water is collected at the bottom of the basket and goes to a filtrate collector.

The volume available for the pulp decreases gradually as the pulp moves forward toward the discharge end. The pulp is therefore subjected to an increasing pressure as it advances in the press causing the water to drain through the perforated screen plate thus increasing the pulp consistency. The pulp is retained inside the press by the screens while the water passes through the openings of the screens. 

The last flight (spiral) of the screw ends before the plug sleeve which allows further thickening of the pulp and forming of a plug. This plug also serves to isolate the discharge chamber.

Table 1:           Parameters of operation

There are other factors that can have an effect in the performance of the press. A change in the type of wood / fiber, in the storing time or a modification of the process (chemistry) can influence the operation of the press. Short fiber buildups on the white water will also affect the capacity of the press.

3.5.1    Speed

The speed of the screw press (in RPM) is the parameter that has most influence on the capacity and the discharge consistency of the press.

With a mechanical drive (fix speed) it is possible to change the speed by modifying the gear ratio. For difficult applications, it is recommended to use a variable speed drive. The drive can be mechanical or hydraulic.

In general, the speed is adjusted to obtain the required discharge consistency. The speed must be reduced to increase the consistency. If the capacity of the press is not sufficient, then a compromise must be done between capacity and consistency.

3.5.2    Feed consistency

Depending of the type of pulp and the required performance, the comfortable span for the feed consistency is between 3% and 8%.

A low feed consistency will reduce the capacity of the press and the discharge consistency.

The easier the pulp is to drain, the lower the feed consistency can be for a certain production rate or discharge consistency.

For most applications, the fiber content of the filtrate is higher when the feed consistency is lower.

If possible, at start-ups, it is recommended to make a recirculation of the pulp until the consistency is stable before opening the feed valve. 

3.5.3    Feed pressure 

The feed pressure should never exceed 20 psi (140 kPa). The best results are obtained under 5 psi (35 kPa).

A too high feed pressure in most cases will push the fibers (particularly the short ones) through the screen openings. This will cause a high fiber loss in the filtrate. It is therefore very important to keep the feed pressure as low as possible for the required production rate. Remember that the pulp going to the filtrate must be subtracted from the production rate…

In certain cases, high feed pressure can plug the first screen and reduce drastically the capacity of the press.

In some case, the pulp goes directly to the discharge end without forming a plug on the throat of the press (see section 3.9.2). This can indicate that the feed pressure is too high, that the chemicals are unbalanced (pH) or that the press has reached its maximum capacity. In this case, the feed valve should be used to decrease the feed pressure (and at the same time the flow rate) until the press forms a plug. 

The feed pressure should be limited to a maximum of 10 psi (70 kPa). Above this value there should be an alarm. The opening of the feed valve should be reduced automatically when the feed pressure approaches 15 psi (100 kPa) to avoid losing the pulp in the throat of the press and to prevent sealing the screens. Above 20 psi (140 kPa), the feed valve should be closed completely to avoid damaging the press. 

3.5.4    Freeness (CSF)

The performance of the press is affected by the Freeness of the pulp. The higher the Freeness, the easier it will be to obtain the required discharge consistency. This allows the press to be run at a faster speed and to increase the capacity of the press. 

In general, the maximum speed of the press (and its capacity) reduces when the Freeness reduces. 

The quantity of fibers in the filtrate increases when the Freeness reduces. To compensate, the feed pressure can be reduced. This will in turn reduce the capacity of the press. 

3.5.5    Temperature of the pulp

Too low a pulp temperature can have a negative effect on the performance of the screw press. An increase of the pulp temperature will normally improve the drainage characteristics of the pulp. It is then possible to increase the capacity of the press as well as the discharge consistency.

Max Capacity is 25 TPD 

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