Lesson 6 Clothes washing machines Part 2

The washing machine program

All the important parts of the clothes washer are electrically controlled, including the inner drum, the valves, the pump, and the heating element. The programmer is like the conductor of an orchestra, switching these things on and off in a sensible sequence that goes something like this:

You put your clothes in the machine and detergent either in the machine itself or in a tray up above.

You set the program you want and switch on the power.

The programmer opens the water valves so hot and cold water enter the machine and fill up the outer and inner drums. The water usually enters at the top and trickles down through the detergent tray, washing any soap there into the machine.

The programmer switches off the water valves.

The thermostat measures the temperature of the incoming water. If it's too cold, the programmer switches on the heating element. This works just like an electric kettle or water boiler.

When the water is hot enough, the programmer makes the inner drum rotate back and forth, sloshing the clothes through the soapy water. The detergent pulls the dirt from your clothes and traps it in the water. The programmer opens a valve so the water drains from both drums. Then it switches on the pump to help empty the water away.

The programmer opens the water valves again so clean water enters the drums.

The programmer makes the inner drum rotate back and forth so the clean water rinses the clothes. It empties both drums and repeats this process several times to get rid of all the soap.

When the clothes are rinsed, the programmer makes the inner drum rotate at really high speed—around 80 mph (130 km/h). The clothes are flung against the outside edge of the inner drum, but the water they contain is small enough to pass through the drum's tiny holes into the outer drum.

Spinning gets your clothes dry using the same idea as a centrifuge. The pump removes any remaining water from the outer drum and the wash cycle comes to an end.

You take your clothes out and marvel at how clean they are!

Why do washing machines need so many programs?

Your machine doesn't know what you put into it and can't automatically tell how carefully to wash something like a delicate woollen jumper—because it doesn't know that's what it's got to do! The only things under its control are the amount and temperature of the water, the speed of the spin, the number of times the drum oscillates, the number of rinses, and so on. No-one wants to wash clothes in a scientific way: "I think I need 5.42 litres of water at exactly 42°C, I'll need to wash for exactly 34 minutes, and I'll need 200 spin revolutions when I'm done." That would give us literally an infinite number of possibilities, which is too muсh. Recognizing this, machine engineers try to make life easy by offering a few preset programs: each one uses a slightly different combination of these variables so it washes safely within the tolerance of different fabrics.

Why does that matter? All fabrics are different. A fabric like wool is immensely strong but has two big drawbacks (from the point of getting it clean): it's extremely hygroscopic (absorbs huge amounts of water) and loses its elasticity as the temperature increases. So if you're designing a washing machine to wash woollens, that's your starting point: don't allow the wool to become too hot (because the fibers will degrade and stretch too much) and don't agitate it excessively because it will stretch and not return to shape. With sturdier fabrics like denim, you can afford to bash them about in the drum much more—indeed, you must do so, because you need the agitation to get the detergent deep into the fibers and break up the dirt (and, of course, clothes made from denim are more likely to get dirtier than more delicate fabrics such as cashmere jumpers, which people treat more carefully).

Each program you find marked on a clothes washing machine is a best guess by the engineers as to how much agitation a particular garment/fabric is likely to need and how much it can put up with without getting damaged. If you were handwashing in a sink, you'd make those judgements instinctively, balancing the need to get your garment clean with the need to protect it from damage. While your brain/hands would do that without thinking, the washing machine does it with a certain wash temperature, so many agitations, so many spins, and a certain spin speed.