Conventional Drum Washers

Rotary drum washers were the first pieces of continuous washing equipment to

be developed. They have undergone continuous improvement over the decades,

and today exist in a wide variety of designs. Conventional drum washers can be

categorized into vacuum washers and pressure washers.

5.6 Washing Equipment 547

Pulp of typically 0.5–1.8% consistency is fed to the washer vat. As the liquor

flows through the washer wire attached to the drum, the pulp mat is formed. The

pulp mat is lifted out of the vat with the rotation of the drum and enters the washing

zone, where several showers layer the wash liquor onto the mat. At the end of

the washing zone, the drainage may be supported by press rolls. Finally, the pulp

mat is released from the wire in the discharge zone and falls from the take-off

into the repulper, a shredding screw conveyor (Fig. 5.25).

Fig. 5.25 Impco Coru-Dek vacuum washer [33].

In a vacuum washer, the vacuum which causes the liquor to flow through the

pulp mat and washer wire is usually created by the downfalling filtrate in the drop

leg, a vertical pipe of adequate length. This set-up requires the vacuum washer to

be located at a higher level in the building. It also requires that the temperature of

the filtrate does not exceed about 80–85 °C, because above this temperature the

vapor pressure of water becomes a threat to the vacuum.

In pressure washers, the differential pressure across the pulp mat is created by

a gas pressure maintained in the hood by a fan. No drop leg is required, and pressure

washers can be operated much closer to the boiling point of the liquor than

vacuum washers. The pulp enters the vat, is taken up by the drum, and is then

washed in up to three countercurrent stages. After the pulp has passed the seal

roll, the take-off is provoked by the pressure inside the drum which blows the

pulp off the face wire. The encapsulated design minimizes emissions to the air.

548 5 Pulp Washing

Fig. 5.26 Two-stage Sunds Rauma pressure washer [34].

The washing efficiency on a conventional drum washer depends mainly on the

shower arrangement and on the washer’s capability to avoid rewetting of the pulp

mat with filtrate from the pockets of the drum before discharge.

As a rule of thumb, the outlet consistency is around 12%. However, it may fall

well below 10% for pulp with unfavorable drainage behavior, yet may be as high

as 18% for easy-draining pulp after press rolls. The E factors range from 1.5 and

3.0.

As a consequence of the design principle, conventional drum washers tend to

let air through the pulp mat towards the end of the washing zone. The entrained

air induces foaming in the filtrate tank, and would adversely affect drainage if

recycled back with dilution filtrate before the washer. In order to avoid the latter

situation, air must be given sufficient time to leave the filtrate in the filtrate tank,

and this is done by providing adequate retention time and surface area.

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