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CBW Machines (Tunnel Washers)

Background history of development of continuous processing

Initial development of the commercial washing machine led to a side loading machine i.e. loading into the side of the cage. Little significant further progress occurred from the late 1800s to the mid 1900s until the front loader (i.e. loading through the end or front of the cage) was developed. These machines quickly became popular and replaced the side loader. During this period, a German engineer Dr Schultsman, developed the principle of contra-flow and his company Peonsgen Schultsman, known as P & S produced the first continuous processing machine - the "Carousel" - by arranging a group of machines on a circular or carousel foundation with the machines moving away from the loading stage and returning for unloading after completing the circle. The system was organised to make the liquor transfer from machine to machine in a direction opposite to the movement of the machines, hence contra flow.

The development of the Washer-Extractor in the late 1950s brought the front loader with extraction facilities firmly back as the popular machine for washing and extraction, and temporarily gained ascendancy over the contra flow type machines which still required a separate system of extraction.

However, development of the continuous washing system continued and a new design provided a system of continuous processing through loading, washing, water extraction and tumbling. Problems existed in the early designs due to uneven residence times of work in both washing and tumbling. This was overcome through time by the development of a membrane press and the division of the washing machine into compartments hence the current name of Continuous Batch Washing (CBW) machine or Tunnel Washer. This type of system has not made a major penetration into the laundry market for the processing of large quantities of linen. The batch wash extractor however is retained for specialised processes and the production of small units. Current developments of CBW's are leading to more flexible processing being possible which formerly could only be done by washer-extractors. In this specialised area and in future, it is possible we may see a laundry operating without washer-extractors.

Principle of continuous processing

In general, terms the interior of a tunnel cage is designed as an Archimedean Screw, each section holding a batch of work. Normal wash movement of the screw retains the work in its position but a 360º forward turn of the screw moves the work forward to the next stage of processing.

The tube or tunnel cage is therefore divided into processing sections which conform to pre-wash, hot-wash and rinse zones, the work moving from pre-wash through the machine and out after rinsing.

Movement of work is from the load end of the machine to the discharge point.

Water flow is from the discharge point to the load end as show in diagram 1.

The principle of contra flow can clearly be seen - the work moving against the flow of water.

During its passage through the machine, wash chemicals, bleaches, etc are added to the water flow at specific points to carry out the functions of soil stain removal, etc. Steam also is added at specific locations to raise the temperature to required levels in accordance with processing requirements.

Contra-flow therefore could be regarded as two opposing forces, i.e. work in one direction, water, heat and chemicals in the opposite direction. The ideal equilibrium for the two forces is in the centre of the hot wash section. If the forces are unbalanced so that work can be overcome water flow, the result will be poor soil removal and rinsing. The opposite where water is the stronger flow will result again in poor washing as the wash chemicals will be prematurely flushed to drain and there will be a waste of steam (see diagram 2).

Diagram 2

Washing process balance

Balance in the washing cycle is obtained by:

  • Correct water settings in relation to work flow;
  • Maintenance of the required work flow by constant load weights;
  • Correct additions of chemicals to provide the required chemical levels; and
  • Correct settings of the thermostats for the required temperatures.

The following outlines each of these areas in more detail showing what factors are important towards achieving balance.

Water - measured using a flow meter (litres/ hour or m³/ hour)

NOTE 1000 litres = 1 m³

  1. Set the flow meter in accordance with manufacturers recommendations, e.g. 8 litres/ kg 
  2. Calculate the number of machine cycles/ hour. E.g., a 10 section machines with a section time of 2.7 minutes.

Therefore tune if 1 machine cycle = 27 minutes.

Therefore number of machine cycles/ hour = 60÷27 = 2.2

  1. Assessed work load = 35kg / stage = 350 kg
  2. Therefore work load/ hour = 350 x 2.2 = 770 kg
  3. Volume of water required/ hour = 770 x 8 litres = 6150 litres/ hour

Workflow - this is the number of loads per hour at the loading rate achieved. A tunnel washer with a section load capacity of 36 kg states the theoretical capacity. True capacity depends on the accurate maintenance of the chosen load weights.

An under load of 3kg per section would, on the example of 10 sections at 2.7 minutes per section, lose 2574 kg of production in 39 hours. To make this up would require an additional 3.5 hours of work at the load rate of 33kg.

A further problem connected with workflow is the maintenance of the number of loads per hour. The theoretical calculation given above is rarely if ever achieved due to the delays which occur between transfers. For example, the calculation above where the machine is being used on a 2.7 minute section time can produce, in theory 22 loads/ section/ hour. However, allowing for transfers a good result would probably be 20 loads and may at times drop to 18 - 19 depending on other factors which may hold up the production line. Work flow should therefore be calculated on actual production rather than theory.

Chemical additions - these have to be measured by titration of wash liquor samples. Take into account the wave effect of concentration. This means that immediately after injection the titration is at its maximum and weakens through transfer as the cycle progresses to the next transfer.

Temperature - the required temperatures for the sections are usually controlled by thermostats. Not every section has a steam entry point as heat is transferred with both water and work flow. Hence, if water flow is incorrect, temperature correction often requires use of greater quantities of seam than is normally necessary.

Finally, in the assessment it is necessary to record all relevant information in a form that is understandable. The best method is in a visual chart form known as an activity diagram (See Diagram 3)

Preparation of the chart requires recordings of temperatures and alkalinities. These should be taken together for each section of the machine at the same time, e.g. halfway through time cycle of each section.

The ideal result will provide two curves broadly superimposing on one another with the peaks occurring at mid point of the hot wash. The alkalinity should fall after rinsing to a satisfactory level.

Diagram 3

This information courtesy of Division of Building Technology, CSIR.