Air conditioning

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Air conditioning At the present time increasing use is being made of air-conditioning plant in ships, theatres, concert halls and other buildings. Large fans draw air from outside and pass it through water sprays to wash out dust and other pollution. Afterwards it is heated and humidified to the correct value before being conveyed through large pipes […]

Humidity control

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Humidity control In certain manufacturing processes the relative humidity is an important factor. Cotton fibres, for example, must not become too dry, or they become brittle and difficulties arise through electrification by friction. For this reason the British cotton spinning industry came to be established in the damp climate of Lancashire on the west side of the Pennines. In […]

Wet- and dry-bulb hygrometer

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Wet- and dry-bulb hygrometer For meteorological purposes the relative humidity may be measured quickly with a reasonable degree of accuracy by means of the wet- and dry-bulb hygrometer (Fig. 20.6). This instrument has two thermometers mounted side by side, one of which has its bulb covered with a muslin wick dipping into a small vessel of water. Owing […]

Relative humidity

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Relative humidity On page 206 mention was made of the part played by perspiration in controlling bodily temperature. We shall now look into this a little more fully. The rate at which moisture evaporates from the skin depends on three factors, namely, the temperature, the amount of water vapour present in the atmosphere and the rate of movement of […]

Dew point

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Dew point Those who wear spectacles know that when they enter a warm room from the cold outside air a film of moisture is often deposited on the spectacle lenses. Similarly, a cold mirror may become dulled when brought into a warm atmosphere. In such cases the cold glass surface cools the air in its vicinity to […]

ATMOSPHERIC MOISTURE

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ATMOSPHERIC MOISTURE Owing to evaporation which goes on continuously from the sea and other water surfaces, the atmosphere always contains water vapour. The quantity of vapour which air can hold depends on the temperature. If warm air containing water vapour is cooled it can hold less vapour, so that below a certain temperature the excess vapour condenses out.

The pressure cooker

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The pressure cooker The time required for cooking vegetables and other food can be greatly reduced if the boiling point of the water is raised. This can be done by the use of a pressure cooker. The pressure cooker, or “digester” as it used to be called, dates from the seventeenth  century and was invented by a […]

Determination of boiling point

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Determination of boiling point We may use the fact that the saturated vapour pressure of a liquid at its boiling point is equal to atmospheric pressure as a means of finding the boiling point. A small glass U-tube, closed at one end and bent as shown in Fig. 20.4, is filled with clean mercury, with the exception of […]

Fresh water supply on ships. Desalination

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Fresh water supply on ships. Desalination Rapid distillation of sea-water under reduced pressure can be used to provide fresh water on steamships. The reduction in pressure in the condenser is brought about by a vacuum pump very similar in principle to a laboratory filter pump, except that it works by a steam jet instead of a water jet. […]

Boiling under reduced pressure

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Boiling under reduced pressure Water can be made to boil without heating it simply by reducing the atmospheric pressure above it to a value less than the saturated vapour pressure. This may be done with the aid of a filter pump. A stout, round-bottomed flask containing warm water is fitted with a two-holed rubber bung through which passes […]