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.

Cowl (koul), n. [AS�x3e; cuhle, cugle, cugele; cf. dial. G. kogel, gugel, OF. coule, goule; all fr. LL. cuculla, cucullus, fr. L. cucullus cap, hood; perh. akin to celare to conceal, cella cell. Cf. Cucullate.]

1.

A monk's hood; -- usually attached to the gown. The nname was also applied to the hood and garment together.

What differ more, you cry, than crown and cowl? Pope.

2.

A cowl-shaped cap, commonly turning with the wind, used to improve the draft of a chimney, ventilatingshaft, etc.

3.

A wire cap for the smokestack of a locomotive.

 

© Webster 1913.


Cowl, n. [Cf. OF. cuvele, cuvel, dim. of F. cuve tub, vat, fr. L. cupa. See Cup.]

A vessel carried on a pole between two persons, for conveyance of water.

Johnson.

 

© Webster 1913.

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