A cowl is the hood of a
monk's cloak that obsures all of the head apart from the face.
A cowl is also the
cap or shape of the end of a
chimney, whose purpose is to keep out
rain and debris. This can be shaped like a monk's cowl (see below), but does not have to be.
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Some types of cowl need to be
swivelled to face
down wind, but this is not the case on a
ship where the cowls always point sternward. Buildings in
rural England, which were originally
oast houses or
maltings, have a vent with a
swivel cowl. This uses the same idea as a
windmill or a
weather cock to point the cowl down wind.
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Heating cowls
From the outside of a building, it is possible to identify the
fuel being used for
central
heating, from the shape of the chimney cowl. Different kinds of fuel generate different
volumes of
fumes,
temperature, and have different requirements for
ventilation.
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Oil heating cowl Gas heating cowl
A traditional
open fire does not need a cowl, because the fire generates and needs a huge
updraft. Any rain, leaves or bird's nest material will not be able to enter the chimney
if the fire is in use. When
fireplaces are removed from buildings, it is normal to cap the
chimneys with curved
ridge tiles. Passage of air is still needed for ventilation.
Kinetic cowls
Rotating cowls were something of a fad in the 1960s. I have childhood memories of being
fascinated by seeing these rotating chimneys. A combination of angled
flutes and
baffles
inside the chimney, deflects the airflow to turn the cowl.
The purpose of this is twofold: the cowl breaks the flow and prevents a huge smoke plume, and
also may take in some cold air to mix with the smoke. This releases the smoke more gently
into the atmosphere. The second purpose is aesthetic. The cowl gives a visual indication that
the chimney is in use.
In the United Kingdom, kinetic cowls fell into disuse when the clean air act abolished
open fires. Gas and oil fires are much more efficient, and do not generate enough updraft
to turn a cowl. Wood burning stoves and other modern solid fuel appliances are likewise
more efficient.
Kinetic cowls were also popular on factory chimneys. However, the introduction of heat
exchangers meant that much of the energy is recycled, instead of being dissipated or turning a cowl.
In addition, environmental legislation has required emissions to be filtered, and/or broken
down by a catalyst - again preventing use of a kinetic cowl.