Dishwashers
Last updated: June 13, 2007.
When you're feeling dirty, you jump in
the shower. When your pots
and pans are dirty, they take a shower too—in a magic machine called a
dishwasher. Scrubbing old food off dirty dishes is a tedious chore many
people love to hate. Thanks to an ingenious American called Josephine
Cochran (1839-1913), who invented the first automatic dishwasher in
1886, jobs like this can be a thing of the past. In Cochran's machine,
you
simply loaded your dirty crockery into baskets and the machine showered
it clean with jets of hot and soapy water. Dishwashers have changed
very little in the century since then—but how exactly do they work?
Have you ever stopped to think what goes on inside the machine after
you close the door? Let's take a closer look!
Photo: Inside a dishwasher. When the door closes,
pieces of rubber
around the frame ensure a tight seal so no water can escape.
Note the soap dispenser on the top of the open door.
Look no hands
When you wash your dishes by hand, the water and soap stays still in
the bowl or sink and you move the dishes around as you scrub them with
a cloth or a brush. In a dishwasher, the opposite happens: the dishes
and cutlery stay still in plastic baskets
while pressurized jets of hot water
shoot all around them. That means a dishwasher has to be sealed shut
all the time it's operating or the water would fire off in all
directions—all over your kitchen floor!
A dishwasher starts its cycle by taking in cold water from a hose
connected to the machine. Once a certain amount of water's sitting
inside the bottom of the machine, a heating element starts to warm it
up. The element is just a thick metal bar that gets hot when an electric current passes through it, and it
gradually heats the water during the first part of the wash cycle. An
electric pump takes the warming water and forces it up pipes in the
side of the machine, which are connected to two spinning paddles.
There's one paddle, made of metal, underneath the bottom rack of dishes
and another one, made of plastic, under the top rack.
Photos: This is the plastic paddle underneath the
top
rack of the dishwasher. In the left photo, which is taken from the
side, you can see where water enters the paddle
through a large pipe. The water squirts up from small holes in the top
of the paddle,
which you can see in the right photo.
When water enters
the paddles, it makes them spin around much like garden sprinklers. As
the paddles rotate, the water emerges from small holes in their
upper surface. The paddles make lots of hot spinning jets of water that
fire upward onto your dirty plates. (That's why it's best to arrange
your crockery so the dirty surfaces are facing downward.)
The bottom rack and the bottom paddle are nearer to the heating element
so the water is much hotter in
the lower part of the machine. (That's why you'll see some crockery
items marked "Top rack dishwasher safe"—which means it's alright to put
them in the upper, cooler part of the machine.) After the water hits
your dishes and plates, it falls back to the bottom of the machine,
where it's heated up once more by the element and pumped round again
for another cycle. Water pumps around the machine for half an hour or
so until all the dishes and plates are clean. A sieve at the bottom of
the machine catches any large bits of debris (to stop the machine
jamming up), while smaller bits (and food remains) simply flush down
the drain.
Photos: This is what the bottom of the machine
looks like when you remove the lower rack.
The curved dark grey loop on the left is the heating element.
You can see the metal paddle in the center. The holes in its upper
surface make very hot jets of water that fire up into the lower
rack. The drain holes are on the middle right.
Just beneath it is a compartment where you pour salt to make the
machine wash
properly.
What gets the dishes dry at the end of the cycle? The water that
washes them is so hot that it simply evaporates away in the heat.
Providing you don't overload your machine, the dishes should
dry naturally with no need for you to wipe them afterward.
Dishes that sparkle
The other essential ingredient in a dishwasher is a tablet of
detergent to help break down the grease and grime on your crockery. You
load up the detergent into a dispenser in the door and, when the
machine switches on, the dispenser flips open, dropping the detergent
into the hot water bath in the bottom of the machine. Dishwasher
detergents work much the same as clothes detergents; you can read all
about those in our article on detergents.
Photo: The detergent dispenser in the door of the
dishwasher. The spring catch at the bottom flips the door open when
the machine starts. The soap tablet drops down into the floor of the
machine and dissolves in the hot water.
