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Every day in Britain alone, 150 million cups of tea are made
from the teabags. small paperbags packed with enough tea to
give one cup when steeped in boiling water. The net-like filterpaper
that forms the bag has holes big enough to let boiling water
in, but small enough not tolet any leaves escape. It is also
strong enough not to break in a high-speed tea-packing machine
or during handing in use, when dry or wet.
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No ordinary paper could meet these
exacting demands. Teabag paper is made from two strong fibres,nianila
henip, a long natural fibre (used to make rope) for strength,
and thermoplastic fibres, to sual the bags.The two fibres are
not woven together, they are laid down as a watery mixture in
two separate layern. They formpaper when the water drains away
and the damp web remaining is squeezed dry through rollers.
This gives thepaper an irregular. web-like structure will pores
varying in size.
The paper goes through the tea-packing machine in a sandwich
of two strips and the machine measure out the amount of tea
on the lower strip. The thermoplastic: fibres melt to form the
bond, which stays strong whenit solidifies again on cooling.
Its melting point is higher than 212 F (100 C), so the bag will
not come appearwhen boiling water is poured over it. |
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When you boil potatoes in a saucepan, they take 20-30 minutes
to cook. But boil themin a pressure cooker, and then they
will be ready in 4-5 minutes. Why ?In a saucepan, water boils
at 2120F (1000C). no matter how much you heat the water, the
temperature can never go any higher. It just turns into steamBut
a pressure cooker has a sealed lid, so steam produced when
the water boilsbuilds up inside. As the pressure rises, so
does the boiling point of the water. The cooking atemperature
is therefore increased, which reduces the time needed to cook
the flooc The domestic pressure cooker evolved from a steam
digester’ patented in Britain by a French physicist,
Denis Papin, in 1679. A typical modern cooker operates at
an extrapressure of about 151b. Per square inch (1kg per square
cm), or at about twice normal airpressure. So the water boils
at 2520F (1220C).The pressure cooker has a saucepan-like base
and a domed lid. A rubber gasketbetween the two ensures a
pressure-tight sea. In the centre of the lid is a vent in
whicha weight is placed. The weight seals the vent, but lifts
when the steam inside teaches therequired pressure. Rings
can be added or removed, giving the cook a range of temperatures.There
is also a safety plug in the cover, which releases the pressure
if the weight fails to rise
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People who live in parts of the country founded on limestone
rock, and who havelocal water piped into their homes, end up
with some of the rock in their kettles.
When rainwater percolates through a landscape that contains
a lot ofcalcium, it slowly dissolved away some of the mineral.
When the water is boiled,this comes out of the solution and
sticks to the sides of the kettle as lime scale,or kettle fur. |
Water that is laden with either
calcium or magnesium from the soil, is described as hard’.
You cannotget much of a lather when you wash in it with soap.
Instead of lathering, the water reacts with the soap chemicalsto
form an insoluble scum.
Lime-scale stains also occur on baths and lavatories and around
the outlets of taps.The lime scale can be removed with proprietary
declares. A common type uses a concentrated solutionof formic
acid.
The acid dissolves the lime scale, making it fizz as carbon
dioxide gas comes off.The lack of a good lather in hard water
is less of a problem than it used to be because modern detergentsform
no scum.
In some boilers and hot-water systems hardness can be more than
a nuisance. The lime scale clogsup pipes and reduces the water
flow. In boilers the scale forms a barrier that prevents the
efficient transfer ofthe heat. leading to much higher heating
bills. So, particularly in industrial plants, water needs to
be softened before it enters hot-water systems.
Many waterworks remove hardness by chemical methods, such as
treatment with slaked lime and sodaash, before pumping the water
to houses and factories. 45 |
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