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Determining Number And Size Of Equipment

Having established the maximum dry weight of articles to be processed per hour, a choice has to be made regarding the washing machines to be used.

The choice of washing machine(s) will, to a large extent, determine the size and layout of the laundry.

The choice of washing machine(s) will depend upon:

  • The dry weight of articles to be processed per hour
  • The total cost, both capital and running, of the equipment, energy (electricity & steam), water, and building

  • The availability and cost of labour.

Of the three generic types, side loaders are popular in rural areas for a number of reasons (see para 5.1.1), one being that they are locally manufactured. In urban areas the choice would generally be between washer-extractors (some of which are locally manufactured) and CBW machines which are all imported. (see para 5.1)

In developed countries washer-extractors or CBW machines are exclusively used because they are more cost efficient in terms of labour, water and energy. As stated previously when the load exceeds 600 kg/hour CBW machines should be seriously considered. It must be remembered that this is an arbitrary figure and cost comparisons must be made taking into account all the factors mentioned above.

When selecting equipment the following basic guidelines are considered important:

  • There should be generally be at least two of each type of equipment to allow for maintenance and breakdown. This may be difficult in a large production laundry with a CBW machine and a multi roller ironer.

  • Two smaller capacity washer-extractors are better for smaller laundries than one large washer because:

  • A smaller machine will take less time to accumulate a full load and small odd lots of laundry can be handled more efficiently.

  • Two machines give the flexibility to wash different types of items simultaneously

  • Small machines will impose less of a peak load on hot water and electrical systems.

  • Tumble dryers should have a 20-25% large drum capacity than the corresponding washers.

The methods for determining the number and size of washers and other related equipment are described below. CBW machines are not discussed in detail as this requires input from an experienced launderer to specify the wash process required within the machine to determine the number of sections, and engineering input regarding energy, water and maintenance factors. However, some guidance is given in Appendix C.

Washing machines

The key factors in determining the number and/or size of washing machines are the

  • Peak load expressed in kg/hour

  • Loading factor of the machine influenced by the fabric to be laundered and degree of soiling

  • Number of loads completed per hour

  • Efficient water pressure and temperature.

The methods for determining the peak load in kg/hour are discussed in Chapter 4. Note that many overseas equipment manufacturers refer to their model sizes in lb capacities. For example 35, 50 and 85 model machines refer to lb capacities and must be multiplied by 0,4536 to express the capacity in kg.

Common sizes are the following:

Locally available side loading machines are available in 20, 30, 50, 90 and 140 kg models.

The loading factor or degree of load (D.O.L.) relates the weight of fabric to the cubic capacity of the washing cylinder (cage) and strongly influences washing quality, mechanical action and mechanical damage if the machine is overloaded. This loading factor is also discussed in detail in Chapters 11-13 on the laundering process but is discussed here in terms of the sizing of the equipment.

In the past, the loading factor has been expressed as lbs/ft³. This is being superseded by either kg/ or gm/l (most commonly used in the UK) or a ratio expressed as kg/ l, for example 1:18. The reverse ratio l/kg is also used and may become the future standard. The loading factor equivalents are the following:

Lbs / ft³

G/l or kg/m³

Kg/l

L/kg

1

16

1:62

62

2

32

1:31

31

3

48

1:21

21

3.5

56

1:18

18

4

64

1:16

16

5

80

1:12

12

6

96

1:10

10

Note: The actual equivalent of lb/ft³ is often shown as an inverse ratio i.e. 1:62 means 1 kg per 62 litres cage volume. For the purpose of this report the loading factor will be expressed as l/kg (litres per kilogram).

Manufacturers often relate their machine size to the load in kg it can take over a one hour period. Two presumptions could be made. Firstly, that the loading factor is 10 or 11 l/kg and secondly, that two loads would be processed in one hour. On these assumptions a 100/kg machine would produce 2 x 50kg loads in one hour. In reality both the loading factor and the throughput could be grossly exaggerated. In determining machine size the cage volume must be calculated on the basis of an accepted loading factor for the fabric to be laundered and the degree of soiling. For medium soiled cotton work, the most satisfactory loading factor is 18 litres of cage volume to 1kg of cotton work. For woollens and polyester/cottons, loading factors of 25 and 22 l/kg respectively should be used. If machines having a large cage diameter in excess of one metre are envisaged, mechanical action on the washing would be greater thus allowing higher degree of loading and a loading factor for cotton fabrics of 11 to 12 l/kg could be used.

The machine cage volume should be specified by all manufacturers.

The maximum loading capacities should never be exceeded. Under loading on the other hand is wasteful in energy, water and chemicals and will result in a lower productivity. Another important aspect of under loading is that it can cause an imbalance in the washing machine with potential mechanical damage.

As stated previously, most hospital laundry is cotton and for planning purposes a loading factor of 18l/kg can be presumed. However, if poly-cotton sheets or heavy soiling of cotton fabrics is envisaged, the loading factor must be reduced to 22l/kg and therefore greater machine capacity is required.

The next important factor is to determine the number of loads per hour that can be achieved by the machine and the operator. Very lightly soiled linen will require a shorter total washing cycle than heavily soiled and /or infected linen. Manufacturers sometimes claim up to 2 loads per hour but these claims are often not obtainable in practice. For planning purposes rules of thumb have to be used. From discussions with the industry it appears that the loads per hour will vary from 1 to 1.4 per hour. The higher turnover can be achieved where the linen is lightly soiled such as in a hotel (in some instances there is no pre-wash of the sheets). If this is the case then up to 1.4 loads per hour can be achieved especially with small washer-extractor machines where the operator can load and unload rapidly. For hospitals and frail care units a complete cycle time between 1 and 1.3 loads/ hour should be used for washer-extractors. For side loaders requiring the removal of wet washing a cycle time of 1 hour per load should be used.

The factors required to determine the number of sections for a CBW machine are only briefly described in Appendix C of this report. The total process time for each batch of say 35kg of washing may take from 30 minutes to 45 minutes but this does not determine the production. The production of a CBW machine is determined by the cycle time which could be as rapid as a 35 kg batch every 2 minutes! CBW machines vary in their batch capacity from 20-60 kg/ batch.

In order to achieve a desired production time or throughput of washing, the water pressure must be adequate in order to fill the washer within a given period of time and similarly the ambient water temperature must be brought up to the desired wash temperature in a given period of time. Where the water is electrically heated in the washing machines this will generally require the installation and supply of a hot water storage system of supplying hot water in quantities suitable to the workload. The water can sometimes be heated at night using lower electricity rates and stored in insulated tanks for use during working hours. Where steam is used, the direct introduction of steam into the machines provides a rapid rise in the temperature of the water. Techniques to speed up this process, if necessary, should be discussed with the suppliers of equipment and the mechanical and electrical engineer.

In Table 6, three hypothetical examples of on-site laundries are shown, each producing the same dry weight of laundry per hour, but differing in the type and soiling of the laundry items from heavily soiled cotton overalls to lightly soiled poly-cotton sheets. This table shows the influence of the loading factor on the machine cage volume.

Table 6: Three hypothetical examples showing the influence of the loading factor on machine cage volume.

EXAMPLES

OF ON-SITE

LAUNDRIES

A

PEAK LOAD

Assume same peak load

Kg/ hour

B

LOADING

FACTOR

L/kg

C

MACHINE CAGE

VOLUME

A x B = C

Litres

High proportions of heavily soiled cotton overalls

100

25

2500

Mostly cotton items with 70% cotton flat work - light to medium soiling

100

18

1800

High proportion of lightly soiled poly/cotton sheets etc. not requiring a pre-wash

100

22

2200

From above it can be seen that for the same dry weight turnover of laundry per hour for the three examples the machine cage volumes vary from 1800 litres to 2500 litres.

Percentage breakdown of articles to be processed through equipment other than washers

Generally 70% of all cotton items laundered for a hospital require flat ironing. The capital cost of flat ironers and the running and labour costs may force hospitals and budget hotels to use poly-cotton sheets etc. and do no flat ironing. The matron of one frail care unit, where cotton sheets are used, is of the opinion that with the high turnover of sheets in frail care per day and the cost of ironing the sheets, they will have to consider not ironing the bottom sheet.

The percentage breakdown of articles requiring flat ironing, pressing and tumble drying is important in establishing the need and size of equipment other than the washer (see also para 1.5). The type of washer will also determine the need and size of hydro extractors and tumble dryers.

Hydro extractors

If side loaders are used, all the washing must pass through hydro extractors. If high spin washer-extractors are used, hydro extractors are generally not required (see para 5.2)

Up to four loads per hour can be processed through the hydro extractors which spin at 1400 rpm and with a DC injection current to break them. Machines that do not have these features have to be sized according to the manufacture's specifications.

If normal or medium spin washer-extractors are used for 150kg weight of washing then it would be prudent to install at least one 16kg hydro extractor or preferably two smaller models. They would be used essentially for items tumble drying (see paras 5.1.2 and 5.2).

Tumble dryers

Tumble dryers are generally manufactured with 25% more drum capacity than washers for the same dry weight of laundry. The provision of a larger cylinder allows for a better tumbling, airflow and drying operation. Over-sizing the tumble dryer is especially important in processing towels, poly-cottons and other non-press items.

The percentage of items requiring tumble-drying could vary considerably depending upon the type of washer used and the fabric to be laundered. For example, where all flatwork has to be preconditioned prior to ironing, all items could pass through the tumble dryer for partial or complete drying. In other instances, all flat work could go directly from high spin washer extractors to the ironers and only towels etc. would be tumble dried. If CBW machines are used then all items are tumble dried prior to ironing or folding. The number and size of the tumble driers required must be established by the launderer and the equipment supplier.

Flat ironers

The rate of processing laundry items through flat ironers depends to an extent upon the skill of the operator and other aspects discussed in para 5.5. If washer-extractors are used for small to medium sized laundries, particularly on-site laundries, the choice generally is for single heated roll ironers. Where large output are required, irrespective of the type of washers used, multi-roller heated bed (chest) ironers are generally used. For a large production in association with CBW machines, automatic feeders and folders could also be considered.

Both single and multi-roller ironers are available with automatic folding devices. As stated in para 6.2, in general 70% of all cotton items laundered for a hospital require flat ironing but this percentage may decrease as discussed previously.

After estimating the percentage of laundered items to be flat-ironed, choose the most appropriate type of machine(s) for the output required. Manufacturers specify the output of their machines in kg per hour. This is normally for a 100% ironer bed usage factor but a factor of 80% should be used for planning purposes.

Laundry presses

The rate of processing items through presses depends very much on the skill of the operator. Throughput of these machines must be discussed with the suppliers and the launderer.

In hospital laundries, the number of items requiring pressing is decreasing and wash and wear items (including cotton items) are being used (see para 5.6)


This information courtesy of Division of Building Technology, CSIR.


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