Candidate Support Pack - Scottish Qualifications Authority

5.48
Time
Different parts of the wash process are given different amounts of time depending on what
is to be achieved. For example, it takes longer to remove soiling in the wash than it does
to remove residual detergent in the rinse. The amount of time given in the wash will
depend on the type of soiling and how much there is, the type of material being processed
(wool, cotton, polyester-cotton) and the degree of loading, but the average time in the
main wash zone will be between 7 and 15 minutes.
With very high loading levels the mixing time (that is, the amount of time it takes for the
water, temperature and chemicals to penetrate to the centre of the load) can be longer
than the time in the main wash zone. In conventional washing machines rinsing times will
usually be two or three rinses, each of 2–3 minutes duration once the dip has been
achieved and dilution of wash chemicals can take place.
This may vary if a cool-down is required for polyester-cotton blends. Because the wash
process in a CTW is a continuous process and the machine is made up from a number of
inter-connected compartments, when the work being processed is transferred from one
compartment to the next, the same thing happens simultaneously throughout the length of
the machine. The stage time for the pre-wash, the main wash and the rinses is determined
by the number of compartments in each zone.
For example, in say a 12 stage CTW, the work will transfer throughout the machine every
2½ mins, which means that it will take 12 x 2½ minutes — 30 minutes — for a load of
work to pass completely through the wash process in the machine.
Therefore, the process may be set up so that, say:
(a) Pre-wash/Wetting out — sections 1 and 2 = 2 x 2½ mins = 5 minutes
(b) Main wash stage — sections 3–8 = 6 x 2½ mins = 15 minutes
(c) Rinse stage — sections 9–12 = 4 x 2½ mins = 10 minutes
Total time: 30 mins
Temperature
The accurate measurement of temperature during the wash process is vital to allow the
correct chemical reactions to take place.
Some critical wash temperatures are:
38 ºC Protein stains (blood and perspiration) start to set above this temperature so
the best pre-wash zones operate below it.
40 ºC Optimum temperature for enzyme activity to digest protein stains. Some
enzymes are designed to work at higher temperatures.
60 ºC Maximum temperature for sodium hypochlorite use, to avoid fabric damage.
65 ºC Minimum temperature for thermal disinfection (10 minutes plus mixing time)
of polyester cotton; minimum temperature for effective use of hydrogen
peroxide.
GC8N 22 — Laundry Operations Level 2