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CBW Machine Design

Up to this point the information provided has been rather simplistic, i.e. two contrasting flows which require balancing. However, as one would expect, reality is not that simple.

It is necessary now to look at design configuration to achieve the various stages of a wash process. In tunnel washing it is unusually broken down into the three stages of Pre-wash, Hot wash, Rinse and if required, special treatments such as starching.

A washing machine usually consists of an inner wash cage surrounded by an outer casing. However, Tunnel washers have three different designs.

Case types

Single skin - describes a machine having only a single revolving cylinder. The inside of the cylinder contains the water and the work with the drive and rotation of the cylinder being applied directly to the outside of the drum.

Single/ double skin - describes a machine constructed basically from a single skin cylinder but in places having an outer casing around the cylinder.

Double skin - the term used to describe a machine with an inner washing cylinder surrounded by an outer casing.

Size of drum

Drums (cylinders) are normally of large diameter, varying between 1500mm to 1780mm for the 36-kg designs and between 1760mm to 2000mm for the 50kg designs. From this it can be seen that there are two main sizes used, either a 36kg or 50kg load factor. However, development of machines for specific functions has resulted in other 'one off' sized machines becoming available.

Wash action

The type of wash action in general is considerably different to that of a washer-extractor. Typically, a washer-extractor has a loading factor of 12-18 litres/ kg and a rotation speed of 33rpm which provides a 'G' Force of approximately 1.7, ideal for rotary washing machines.

Tunnel Washers in general have rotational speeds of 9-11 rpm with consequent very low 'G' forces. Moreover, in many designs the rotation during washing is incomplete and is known as a 'Rock' or 'Rocking' action as the cage usually oscillates between 180º minimum to 330º maximum. The cages are in most instances fitted with beaters or lifters but these are more closely spaced and smaller than those in conventional wash cages. They are also confined to those areas of the cage in which the work moves during washing. The wash action provided is that of rolling and rubbing with some compression of the work as the work rolls over. As can be judged by the cage diameter and the load weight per section, the loading factor is very low typically 31 to 50 litres/ kg.

Stages

A cylinder is divided into stages or sections to provide batch separation. These are usually either a 36kg size or 50kg size. The cylinder is also divided into pre-wash, hot wash and rinse zones, by various means depending on the type of the machine. To allow a sensible process to be designed for a tunnel washer, a minimum number of stages is required, usually no less than seven. There is also a maximum size, which for 36-kg machines is in the order of 21 stages and for 50kg machines in the order of 15 stages.

Basic design

In order to conserve water, steam and chemicals, which is an important aspect of the concept of contra flow, the system makes maximum use of the water by means of an internal recovery system. With the recovery of water, the means are provided to save both steam and chemicals although the method of recovery in terms of engineering design differs from machine to machine.

There is in principle a common approach as shown in diagram 4 .

Fresh water flows through a flow meter into Section 11 which contra flows to Section 8 where it overflows the weir to the centre recovery tank. At this point the flow splits and approximately 1/3 is returned through a flow meter to Section 7, the remaining 2/3 transfer to the pre-wash wet-out tank.

Water from the Press is recovered and returned to the Pre-wash Wet-Out Tank.

The hot wash which contra flows from Section 7 to Section 3 overflows the weir in Section 3 to main drain.

Occasionally a weir is fitted within the pre-wash section, but more frequently the design allows the pre-wash to transfer with the work from Section 2 to 3 where it overflows to drain with the hot wash liquor.

Diagram 5 illustrates a more complex 9 stage machine in which water flow procedures can be varied.

Dip levels

As opposed to a batch washer-extractor where the dip levels can be at any chosen height, tunnel water levels are controlled by weirs which are usually set by the manufacturer.

Their weirs which can be internal or external, control the level for each processing zone, therefore a flow of water in excess of that required to maintain the correct level will be wasted as the increased flow will cause overflows from the centre recovery tank or wet out tank as well as from the hot wash to drain. On the other hand, a flow below that required will eventually lower the level of water in the machine to a point where there is insufficient to enable work to transfer since water assists in the mechanical energy of transfer. In the first instance, a waste of resources and an unbalanced process will result and in the second, apart from poor wash performance, the high probability that machines will block or rope through improper transfer of work.

Drive systems

Drive systems will depend on the configuration and type of machine but generally are as follows:

  • A. Single skin machines

    The cylinders of single skin machines rest on friction wheels which are electrically driven through a reduction gearbox. The friction wheels have a long life, are easy to replace if necessary and provide a good positive drive to the cylinder.

  • B. Single/ double skin machines

    Generally, these are driven on the single skin sections only, in the same way as single skin machines, however, an alternative is a toothed wheel surrounding the single skin section which is driven by a silent chain from a sprocket on a gear box which is electrically driven.

  • C. Double skin machines
  • A number of different designs exist for double skin machines.

  • Individual motors for each section where the drive is by a motored gear wheel directly onto a toothed wheel surrounding the cylinder.
  • Individual motors for each section with a sprocket wheel driving a toothed wheel via a chain.
  • Two drive motors one at each end of the machine with sprocket driving a toothed wheel by a chain.

  • This information courtesy of Division of Building Technology, CSIR.

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