Basic Requirements For Good Washing

The loading factor of washing machines

A high level of standardisation and efficiency cannot be attained unless machines are loaded to the correct degree. Overloading results in inadequate cleansing and makes efficient rinsing difficult while under loading means loss in production and causes greater wear on the fabrics. For cotton work, the most satisfactory loading factor is 18 litres of cage space per kg of washing load and for woollens 25 litres per kg. Polyester/cotton blends are usually best loaded at 22-25 litres per kg. The load in the machine absorbs an appreciable quantity of liquor; for 400kg dry weight of cotton sheets, for example, about 100kg of water is retained by the work at each run out, and if the overloading is considerable, an unduly large proportion of the wash liquor is held by the load causing an unnecessarily large quantity of the dirty liquor to be retained at the run out and carried over to the next stage of the process.

When washing processes appropriate for different types of work are designed, different alkalis and detergents are used for different fibre types/blends in most processes, the concentrations of detergents are different at different stages of the process. It is therefore important that the proper types and quantities of detergents are used to ensure good results and economy of materials.

The chief cleansing agents used in laundry work are detergents and alkalis and in order to get good results with economy of materials, it is necessary that quantities should be carefully controlled. The addition of a scoopful of several handfuls of 'dry' materials to a machine is not a very satisfactory procedure. The method is very wasteful as more materials than necessary are often used, and additional time is required for them to dissolve in the water in the machine. It saves time and trouble to keep stock solutions of soap, alkali and bleach, the materials most frequently used in the laundry. By this means it is possible to measure accurately the quantities used and when added to the machine, the solutions are ready to function when the mixing time is completed. In order to make up a solution of known concentration it is necessary to calculate the volume of the tank which is to hold it, and the amount of liquid it will contain.

Alternatives to stock solutions are now available, the major one being 'liquid feed injection'. Such systems have many advantages over standard stock solution methods but control is still essential to ensure their efficient and economic operation.

For the following reasons, it is important to use the correct amount of water in the machine at any particular stage of the washing process:

To control the amount of mechanical rubbing to which the load is subjected.

Generally, low dips increase the amount of rubbing, thus increasing the wear on the fabrics, but even so, they do not always give a greater degree of dirt removal. The dip is not the same throughout all the washes of a white work process, as greater rubbing is required towards the end to remove resistant soiling, and therefore somewhat lower dips are used in the later washes than in the earlier ones.

For woollens and some other fibres, where it is essential to avoid mechanical action as far as possible, high dips are used at all stages to minimise felting.

The quantities of detergents and alkalis and other possible additives to be added, are calculated for known quantities of water and load weight, and if the dip is not adjusted correctly the solution in the machine will not be of the required concentration. This is particularly important when bleach is used, as a solution more concentrated than necessary can have serious weakening effect on fabrics.

The removal of wash materials and dirt from the machine and therefore from the load, depends on the proper adjustment of the dips at the different stages of rinsing. With a low dip level a smaller proportion of the dirty liquor is removed from the machine than with a high one, and therefore the quantity of dirt carried over into the next wash is greater.

Note: The difference between standing dip and running dip should be noted. Standing dip refers to the depth of liquor in the machine when the machine is in motion. For a given quantity of liquor, the running dip is less than the standing, and care should be taken to avoid confusion when designing or monitoring a process. Usually most processes quote running dips although on the continent it is sometimes the practice to quote water volume to unit load weight ratios.

The control of the temperature of wash liquors is necessary because different fabrics must be washed at different temperatures. With soiled white work a high temperature in the final wash is essential to ensure adequate cleansing, but high temperatures might damage woollen, coloured or heat sensitive articles and if a high temperature were used in the initial stages of a white work process certain stains might become set and very difficult to remove. It is therefore important that the appropriate temperature is used for any given process or any particular stage within it and the directions given should always be carefully followed. The control of temperature is also very important when using hypochlorite bleach, as serious damage to the fabrics can occur if the proper temperature is exceeded. Thermal disinfection is totally dependant on the correct temperature control and adequate standards of disinfection can only be achieved by careful and exact temperature control within the washing machine.

The duration of any washing process must be controlled so that on the one hand sufficient time is allowed for adequate cleansing, and on the other, the process is not unduly prolonged, since this results in soil-redeposition, unnecessary wear on the textile and waste of time with consequent loss of production. In the case of coloured items, fading may be increased by a process which is unnecessarily long. The time taken for dirty liquor to drain from a machine should not be excessive, but as the size and construction of machines vary it is not possible to be  specific about the required time. In all cases adequate time should be allowed for the drain period to ensure maximum removal of dirty liquor from the machine, as otherwise an unduly high proportion of dirt remains in the load which is then carried over to the next wash or rinse.

Sufficient time to allow complete mixing of reagents/load/water must be allowed at the beginning of each wash. This is known as "mixing time".

Washing Requirements

Water is referred to elsewhere as the miracle solvent. This it indeed is, but water on its own is an inefficient washing medium because strange as it may seem, it is not very good at making things wet, if water is poured into a glass then emptied out again, the inside of the glass will be quite unevenly wetted. The water has shrunk right back from some areas and left them dry. This example demonstrates the phenomenon known as surface tension. The main function of a detergent is to reduce this tension and to allow water to wet thoroughly.

Tension exists in the surface of a body of water for the following reason. Water consists of innumerable molecules  which are strongly attracted operate in all directions, and the water is held in a state of equilibrium. At the surface of the water, however, there are no forces acting from outside. All the forces are inward and equilibrium is produced only when molecules at the surface are drawn in towards the body of water, reducing the surface area to a minimum. That is why the drops left at the sides of a glass after it has been emptied tend to become spherical in shape, a sphere having the smallest surface area for a given volume. The force which attracts molecules inwards is balanced by a tension in the surface which prevents further shrinking. This later force is called surface tension. In the case of washing, this tension prevents the water from coming into intimate contact with either the surface of the material or the dirt to be removed from it. To solve the problem and alter the condition of the water's surface, something must be found which is sufficiently attracted to water to dissolve in it, but is also to some extent repelled by it.

Detergents whether soapy or non-soapy, solve the problem because they have a molecular structure consisting of a hydrophilic ('water-loving') head and a hydrophobic tails of the detergent molecules are pushed out between the water molecules, breaking the bonds between them and so reducing tension. In the case of a drop of water the spherical structure will collapse, since the equilibrium has been destroyed. As it collapses the surface area increases, pushing out more detergent molecules. So the water spreads and wets the surface.

As well as solving this basic problem of reducing surface tension, detergents perform other very important functions in washing.

Neutralising

Most soil is acidic in nature and before any washing takes place this condition should be neutralised. If the load which is on the acid side is not neutralised, the acidic materials react with soap to form acid soaps, which have little or no cleaning value.

To offset this, alkalis of various kinds are used, these are generally much cheaper than soap. Soap alone may be used, but it's expensive.

Water soluble materials are easily taken care of, but the proteins and albuminous material which make up the greater part of the stains are more difficult to remove.

There are often many saponifiable materials in soiling. Saponification is the reaction of an alkali with greasy, fatty materials present in the load such as kitchen grease, to form soap.

Among the oily types of soil, there are likely to be mineral oils and greases, such as lubricating oils and medicinal oils which will not saponify. Emulsification helps get rid of this type of soil. Emulsification is the action of an alkali or detergent which breaks up the globules of grease and oil into very small particles. Emulsification keeps them separated so that they do not reform into larger units.

The most difficult soil a laundry has to remove from fabrics is solid dirt, which includes not only carbon but dust and earth such as clay. The material is firmly held on the fabric and often penetrates deeply into it. Water alone will not remove such soil, but a long soaking in soap and water gradually removes and suspends it. However, this action is very slow. It is speeded up by the mechanical action and use of alkalis. Much dirt is in the form of tight clusters of particles, and as these clusters are removed from the cloth in their original form, they are likely to be redeposited. To prevent this, a binder which acts as a deflocculating agent should be used. Thus the tight clusters are broken up into individual particles.

We see from the forgoing that if soil particles are removed from the fibre, they must be prevented from re-depositing onto the fabric. The action of holding these particles in suspension is called "suspending power", and the action of preventing redeposition on  the fabric after these particles have been removed is called "prevention of redeposition". Soil redeposition is referred to as "greying".

This, of course, is provided by the rotation of the washer. Such things as change in cage speed, the load, weight, dips and the time of operation will increase or decrease the effectiveness of this action.

Temperature control is very important in good laundering especially for handling coloured work, synthetics, silks and woollens. Bleaches, setting of stains and fibre damage are also affected by temperature.

Accurate thermometers should be installed on all machines.

Water level (dip) gauges are very important. Every control formula is based on a given amount of water in the machine. If a wash formula calls for a water level of 10cm, but is actually 20cm, the concentration of supplies is totally altered. Usually the results are not what was expected. More water than necessary during the wash means a greater weight of wash products leading to wastage and expense. More heat energy will be required and in addition the mechanical effect will be reduced.

Technique Of Washing

There are six principal features of a washing process than can be varied by the process designer:

1. Loading factor
2. Dip
3. Temperature
4. Time (duration)
5. Mechanical action
6. Washing materials/rations

They can be varied between one process and another and with the exception of the first (loading factor) they can be varied between one stage and another of a single process. The effects of varying each of these features independently must be considered first, before considering how best to combine them in the design of an entire process.

Loading Factor (also referred to as the degree of loading)

This is defined as the unit weight of work related to the unit volume of the machine cage. It is usually expressed as weight/unit volume (e.g. kg/m³ where m³ of water = 1000 litres). If for example, a certain machine which has a cage volume of 800 litres is used to wash a 45kg load the loading factor is 18 l/kg. If a load of 15 kg is sometimes washed in the same machine, the loading factor is 53 l/kg.

In terms of their capacity (weight load) washing machines cover a very wide range from a few kg to something approaching 450kg. At the same time, weights of loads washed in one given machine may be varied within much narrower limits depending on the type of load, the weight of work available and the preference of the individual launderer, so a machine described as of 45kg capacity may not always be used to wash 45 kg of work. This would be the case with a load of average soiling where the launderer may load at 55kg. Just as in the case of heavy soiling he may load only 36kg. When washing special fabrics such as woollens or polyester/cotton, it is common practice to load the machine to 70% of the usual white work weight.

Once the loading factor has been chosen, the normal weight of load for a given machine is calculated on this basis. The machine is said to be overloaded if it is used for a heavier load, or under-loaded if the load is lighter than normal.

Overloading has several disadvantages unless there is some compensating feature. Firstly the amount of mechanical action is reduced because the load has less space in which to move within the cage. Thus the efficiency of soil removal is correspondingly decreased. Moreover, since the heavier load carries more soiling than a load of normal weight the liquors have to suspend a large amount of dirt. Finally, the efficiency of rinsing is reduced because the added water cannot readily penetrate the heavy load. One way of compensating for these disadvantages would be to lengthen the washes and rinses, but the heavier load would then require a longer time and there would be no net benefit in terms of increased output over the day. Nevertheless it is sometimes permissible to operate at a higher than normal loading factor.

If the work to be washed is only very lightly soiled, then increasing the load may not introduce more dirt than the liquors can deal with. This is true of some hospital linen. Machines of a very large cage diameter provide more mechanical action than smaller machines and can produce an adequate degree of soil removal even when fairly heavily loaded.

Just as overloading decreases the mechanical action imparted to the load, so under loading somewhat increases it, and the first  disadvantage of under loading is that it may increase the amount of wear on some fabrics. Moreover, an under loaded machine requires virtually the same amounts of heat, water and other washing materials as a correctly loaded machine, so under loading increases the consumption of all these supplies in relation to the weight of work washed. Here again, however, a lighter than normal loading factor can sometimes be justified, in this case if the work to be washed is exceptionally heavily soiled.

The disadvantages just described for both overloading and under loading are of importance only if they are practiced consistently. Occasional under loading or overloading is often unavoidable, depending on the amount of work or the consumption of materials. Table 9 illustrates the most commonly used loading factors established by trials in the UK.

Table 9:  Most commonly used loading factors

The term "dip" is used to denote the depth of the liquor in the cage of a washing machine, i.e. the depth measured from the bottom of the inner cage. References are made to a "dip" of 15cm or a 10cm dip, and so on. Two conditions of dip are possible: standing dip and running dip. A standing dip is that which is measured when the machine is not running and is only of interest when the cage is at rest. Dips measured when the cage is rotating are called running dips.

Variations in dip influence the amount of mechanical action imparted to the load, bearing in mind that the action of the machine is to lift the load repeatedly and drop it towards the bottom of the cage. In fact the falling load strikes the liquor rather than the cage itself, so the higher the dip the lower the degree of mechanical action. This depends to some extent on the degree of loading employed but generally higher dips create less mechanical movement of the load.

Dips also relate to the concentration of washing materials in the machine. Whilst the term dip relates to the depth of liquor in the inner cage, the gap between the inner cage and outer case varies from machine to machine. Thus it is necessary to know the total volume of water in the machine in order to establish correct and controllable wash material concentrations. See diagram 13.1.

To overcome this problem, many European countries use a volume/weight figure e.g. 4-1 15-1 usually expressed as litres per kg (load weight), this obviously means that the inner cage dip will vary from machine to machine but is easily controlled.

Diagram 13.1

Temperature

Many chemical reactions will not take place unless heat is applied and most can be accelerated by raising the temperature. To the extent that soil removal is achieved by chemical and physical action, there is no exception to this general rule. The higher the temperature of the solution up to a certain level, the more efficient the soil removal. The highest temperature which can be attained in a typical washing machine is 100ºC, the temperature at which the liquor boils, and it used to be thought that soil removal would be inadequate unless this temperature was reached.

For several years it has been known that the highest useful temperature for soil removal from most articles is 85ºC. The efficiency improves as the temperature is raised to this level but no further advantage is gained by exceeding this figure.

In the special circumstances of a hospital laundry another reason for increasing the temperature during the washing process is the need to ensure disinfection. Here again it used to be thought that boiling is essential for this purpose, but it is now accepted that somewhat lower temperatures are adequate, provided they are maintained for a sufficient length of time. At 65ºC disinfection is obtained in 10 minutes. It has to be remembered however, that the thermometer on a washing machine indicates only the temperature of the free liquor, and if the machine is being heated the actual temperature of the load will lag behind that displayed by the thermometer. The instrument  must therefore indicate 65ºC for at least 14 minutes, this time being extended to 18 minutes if the loading factor is high. Such times are considerably  longer than those necessary for soil removal, and would unduly prolong the washing process so reducing the output of work from the machine. It can be preferable to use a somewhat higher temperature. At 71ºC, 3 minutes is sufficient to ensure disinfection. The thermometer must show this temperature for a longer time (between 7 and 11 minutes) depending mainly on the loading factor. Some diseases can give rise to articles which could not be adequately disinfected in the washing conditions just described. This will of course be known to medical staff concerned and most probably this work would be sent for autoclaving, or it may be sent with special instructions that the washing process must include a period of at least 10 minutes at 93ºC.

Although fairly high temperatures are necessary for both soil removal and disinfection, it does not follow that every washing process should be operated at these temperatures from start to finish. Certain types of staining substances can be removed readily at low temperature, but become fixed and almost impossible to remove if subjected to high temperatures. The hottest part of the process must therefore be preceded by a sufficient period at lower temperature to ensure that these substances are removed early in the process.

Hospital laundry, apart from the need for disinfection, poses particular problems of staining due to the use of disinfectants at hospitals. Proteinaceous fluids (such as blood) and the use of chlor-hexidine products (such as Savlon or Dettol) iodine and other medical products can cause permanent stains if not properly handled during the washing process.

Proteinaceuos soiling and other staining substances on articles subjected to rapid exposure to wash temperatures above 40ºC can cause permanent staining. To avoid this, such articles should be separated and correctly pre-rinsed below 40ºC to remove the stains.

Articles that have been soaked in chloro-hexidine products require careful handling at laundry level. If chlorine based bleach products (Hypochlorite bleach) are used during laundering, the chlorine will cause the chloro-hexidine residue in the articles to set as permanent stains. If Hypochlorite bleach is used, it should never be used at temperatures above 60ºC as this will damage the fabric. An antichlor has to be added after the bleaching process to remove all traces of chlorine from the load as the heat in the drying process will accelerate damage to the fabric.

For the correct use of bleaches, starches, brightening agents, etc. reference should be made to the laundering section of the FCRA handbook.

The possible effect of heat on some dyes may have to be taken into account when designing processes for coloured work. The dyes on many coloured articles are fast at the temperature necessary for soil removal and disinfection, but others suffer fading and may do so even at lower temperatures. It is increasingly unlikely that the less fast coloured will be used in hospital departments, because of the accepted need to use disinfecting temperatures in washing but personal work from staff and patients must always be expected to include some coloured items on which dyes are not fast at 65ºC. Unfortunately there is no way of judging from the appearance of a colour whether the dye is fast or not, even if this were possible, and if the articles were separated according to their degrees of fastness, this would result in a number of very small loads which would be uneconomical and impractical to wash separately. The only practical alternative is to wash them altogether as a low temperature miscellaneous coloured load, in which some dyes may be fast and others not. A typical temperature used in a low temperature wash is 50ºC.

Another feature which may prevent the use of high temperature is the nature of the fibres. The heat sensitive character of some fibres may not allow this. If the temperature is too hot some man made fibres soften causing distortion and loss of shape of garments.

High temperatures must also be avoided on garments where special fabric finishes have to be retained. One example of this is the flame retardant finish imported to nightwear or protective work wear which may be affected by high temperatures.

Temperatures in rinsing need not, in general, be deliberately controlled. It is normal practice to use only cold water, but it does not follow that the rinses can properly be described as cold. The load and the liquor retains carry-over heat from the final wash, and this warms the water added for rinsing. This procedure is repeated from each rinse to the next, so the temperature falls in a step pattern: firstly if it is desired to bleach in a rinse the temperature must be controlled precisely for the reason already stated. The second exception is with washer-extractors. It is sometimes good practice to use a hot water final rinse in order to improve the hydroing efficiency.

The total duration of a washing process is made up of the times occupied by individual washes and rinses, the time taken to run in water for each stage and the times required to drain the machine at the end of each stage. Time is needed in a wash for soil removal to be achieved. However effective the cleansing properties may be, the dirt is not removed instantaneously. Rather it is removed gradually over a period of several minutes, but the rate at which it is removed is not constant. The rate is highest at the commencement of the wash and gradually becomes less at the time passes until dirt is being removed only very slowly.

However good the suspending power of a liquor may be it is never perfect. Redeposition of the removed dirt is taking place continuously, and when the rate of soil removal falls to a level not much higher than the rate of redeposition there is no point in continuing the wash. Dirt still remaining in the work is better left to be removed in a further wash. In practice it is found that about 6 minutes is the longest useful time for washing most rotary machines, but in many processes it is useful to extend the final wash as it is at this stage that the most resistant dirt has to be removed.

These times are appropriate for machines running at conventional cage speeds. As mentioned earlier some modern machines are designed to run at higher speeds and in these cases the wash times may be reduced. In all cases the times are sufficient only if the washing materials are already in solution when added to the machines. If dry materials are added, extra time must be allowed for them to dissolve in the wash liquor before they can be fully effective.

Rinsing is achieved, as already explained by diluting the 'carry over' from the final wash in order to reduce the concentration of washing materials and removed dirt. In many machines the method of "batch rinsing" is operated, whereby this procedure is repeated to give 3 separate rinses, or sometimes 4. In each rinse the added water is allowed to mix with the carry over, the machine is drained, more water is added and so on. Complete mixing of the carry over and the added water to produce a uniform concentration throughout all the liquor in the machine, takes about 3 minutes. It should be noted that the time for rinsing to be effective (about 3 minutes) is the same whether or not there is a low speed extract after the final wash and between rinses. The alternative to "batch rinsing" is "continuous rinsing" which has already been explained. Continuous rinsing can achieve the same efficiency as batch rinsing but will require different water quantities which will vary with time.

Another component of the total process time is the time required to mix water and materials into the machine for each stage of the process. This time is known as "mixing time" and must be included into the running time of the wash.

Finally the draining times contribute to the total process time. This is a recurring feature of the process, as the machine has to be drained at the end of every wash and with batch rinsing - every rinse. If the time is excessive the prolonging effect is considerable and clearly it is important that the draining times should be as short as possible. Most modern machines drain in 1 minute.

It should be realised that wash time is money, so wash cycle times should be kept to the effective minimum, thus allowing high production from the washing machines. Another reason for avoiding excessive times in the wash cycle is to prevent fabric damage caused by excessive mechanical action.

This is provided by vigorous movement of the load as the cage rotates, so that the fabrics rub against one another and fall to strike against the cylindrical wall of the cage. If the cage had no internal fitment it would merely slide past the load and produce little or no mechanical action, so it is fitted internally with "lifters", these are devices which lift part of the load at a time and allow it to drop. Rubbing and the impact as the work strikes the liquor at the bottom of the cage produces the necessary mechanical action. Several precautions can be taken to limit the mechanical action. Very high dips can be employed in the washes and rinses to reduce the drop. The loading factor is restricted to 25 l/kg to reduce the amount of rubbing action. The speed of rotation of the cage can be reduced, or, the speed remains unaltered but the motion is interrupted from time to time. The machine may be run for say 15 sections and then be stopped for the same or somewhat longer period and so on alternately. Some machines have a mechanism which affects this interruption automatically and which is sometimes called an "interrupter gear". To increase mechanical action would be reversal of the procedure described above.

Basic essential materials for adequate cleansing during a washing process are:

1. Water
2. Detergent
3. Alkalis

These substances can be varied to suit different types of soils and work to be processed but within the requirement of a specific wash process, the quantities of each must be well defined.

Detergents (as has been seen) can wet efficiently and are important in the removal of soiling matter. Alkalis whilst assisting the detergent are valuable in their own right having an effect on the removal of ingrained and greasy soil removal, especially by saponification.

The detergent/alkali ratio can be seen as important when designing a wash process since the requirements of each wash can be fulfilled e.g. high detergent/low alkali for early washes, low detergent/high alkali for later hotter washes. 

Detergent and alkali are often purchased as dry substances and the necessary quantities are usually specified by weight in the washing process. It is not convenient however to weigh each separate amount of detergent and alkali for each wash. This difficulty is overcome by preparing a stock solution for each. A stock solution being a solution of known concentration, such as 50 grams/ litre, so that any measured volume contains a known weight of substance in solution. Thus any required weight of detergent of alkali can be obtained merely by measuring the corresponding volume of solution.  Measurement is supplied and the materials are already in solution when added to the machine. A concentration of 50 grams/litre is commonly used i.e. 1 litre contains 50g of material so that if the washing process calls for, say 300g of detergent the quantity is simply translated into 6 litres of stock solution. The concentration chosen can be whatever is most convenient in the light of all prevailing circumstances.

With the use of stock solutions it is necessary to know the capacity of the storage tank and decide the concentration of the stock solution, it is then a simple matter to calculate the weight of washing material to be dissolved to prepare a full tank.

Stock solutions may be tested to check that they have been prepared properly or whether the concentration has changed since it was first prepared because of re-heating. Heating causes the evaporation of water from the solution, which thus becomes  more concentrated. If heating is by open-ended steam pipe the steam entering the tank condenses to form water which dilutes the solution.

Alternatives to stock solutions are now available, the most popular being 'liquid feed' injection. Concentrated, pre-formulated detergents, alkalis and bleaches are added to washing machines using peristaltic pumps, creating a convenient space saving situation.

Once the materials are in the washing machine further control is necessary to ensure that correct conditions are produced in the process.The quantity of detergent can be readily judged by the appearance of the lathers. In a typical wash a rich lather should form and should persist throughout the wash. A thin or non-existent lather may indicate lack of sufficient suspension power, with the risk of disposition of the removed dirt. This depends on the type of detergent used. The main objection to a very heavy lather is that it represents a waste of detergent and therefore money but it may also be detrimental in that it reduces the mechanical action imported to the load. A lighter lather should be observed on the first rinse. This indicates suspending power which is essential at the beginning of rinsing.

Unfortunately there is no simple visual indication of whether an appropriate amount of alkali is present in the wash. It is therefore necessary to test a sample of the wash liquor by titration. Alkali will be present in water used in the wash process and this alkalinity can be determined by testing the process water separately by the same method used for the wash liquor, the figure that remains is the "net alkalinity" of the liquor i.e. the alkalinity due to washing materials. This in turn is due partly top soap and partly to alkali. Soap contributes relatively little to the net alkalinity which is largely due to the alkali so a test to determine the net alkalinity of a wash liquor indicates mainly whether the amount of alkali in the liquor is appropriate.

A net alkalinity of at least 1,2 g/l is necessary to ensure adequate soil removal for washing process for white work or fast coloureds. Even with careful control, however, variations in the actual alkalinity must be expected and it is preferable to aim at a somewhat high concentration, usually 1,4 g/l to allow a margin of safety to meet the occasions when the alkalinity is unexpectedly low. If the concentration is consistently low the efficiency of soil removal is likely to be inadequate. If the alkalinity is consistently high  not only is alkali and money being wasted but the difficulty of ensuring adequate rinsing is increased.

For classifications which might be adversely affected by excessive alkali much lower net alkalinites are necessary, in the order of 0,6 g/l or even less. The test to determine the alkalinity of a wash liquor can be applied also to rinse liquor. In practice there is little point in testing the liquor  during the early stages of rinsing but a test at the completion of rinsing can indicate how efficiently the load has been rinsed. Even better from this point of view is a test of the effluent when the machine is on extract, because this liquor is forced out from within the fabric, where it cannot be reached while the load is in the washing machine and may have a higher alkalinity than the free liquor which surrounds the load.

If too much alkali is left in the fabric at the conclusion of the washing process the fabric is likely to become yellowed when subjected to the high temperature of finishing on a calendar or press. This fault is called "galling" and is especially likely to occur if the total alkalinity of the liquor left in the load exceeds 0,28 g/l. The aim of rinsing is to reduce the net  (or added alkalinity) as much as possible.

It has already been stated that a washing process must remove dirt from the load, it must hold dirt in suspension and it must discharge the removed dirt into the drain. It has also been shown that requirements may be met by suitable variation of certain features of the process, notably the degree of loading, the dip, temperature, time, the amount of mechanical action and the detergent/alkali ratio. A careful consideration of the effects of these variations, will show, however, that they give rise to some apparently conflicting requirements. For example a low dip is necessary for maximum soil removal, but in order to discharge the dirt from the machine a higher dip is required. Again a low temperature must be maintained  to prevent the setting of certain stains, but good soil removal calls for the temperature to be raised. Finally for soil removal a low detergent/alkali ratio is necessary to ensure suspension of the removed soiling. Clearly it is impossible to incorporate all these features at the same time, at a low dip and high dip, at a low temperature and a high temperature and with the detergent/alkali ratio both low and high. For this reason it cannot be expected that a process which includes only one wash can ever by satisfactory in a conventional rotary machine unless perhaps the load is initially so very small. A process must therefore include at least 2 washes. Once this is accepted it is obvious that each wash can be of a different character from the other, and can be designed to perform a different function.

It has already been stated that the greater proportion of dirt is relatively easily removed therefore conditions at the first washing stage need to ensure the best loose soil removing properties. The detergent/alkali ratio must be high to provide a good wetting and suspending power and the dip must be high to ensure that a large proportion of the removed dirt is discharged to the drain. Also the temperature must be low to ensure that no stains are set.

By the time the final wash is reached, only the most resistant dirt should remain to be removed. Very good oily soil removing properties are required and are achieved by operating at lower dip levels, at high temperature and with a low detergent, high alkali ratio. These are not conditions which always ensure good suspending power or the discharge of a large proportion of dirt to the drain but this is of little consequence as there is likely to be only a small amount of this very resistant dirt.

For most of the work to be laundered 2 washes on these lines should be sufficient but there is often a small amount of heavily soiled work where it would be unwise to attempt to remove all the dirt in only 2 washes. The process for such work should therefore include another wash between the first and final washes described above. Clearly this additional wash would have to deal with dirt of a type which is not so easily removed as that encountered in the first wash. The conditions therefore would be such as will remove this type of soiling.

There may even be occasional loads which are so very heavily soiled that 4 stages would be advisable particularly where polyester/cellulose fabrics are concerned. This should seldom arise, but a similar pattern would be followed with a gradual change of conditions throughout the successive washes, the dips decreasing, the temperatures rising and the detergent/alkali ratio decreasing. These principles are referred to as graded principles and most satisfactory washing processes for use in conventional rotary machines are designed on them.