The essentials of disinfection and sterilization by steam at high and low pressures / by S. Delépine.

  • Delépine, S.
Date:
[1909]
Catalogue details

Licence: In copyright

Credit: The essentials of disinfection and sterilization by steam at high and low pressures / by S. Delépine. Source: Wellcome Collection.

    12/164 (page 4)
    7. Latent Heat or Heat of Vaporisation. \\ hen water is heated its temperature increases gradually until the boiling point is reached ; after this the temperature of the boiling fluid remains constant so long as the pressure remains the same. The heat conveyed to the boiling water is used up in causing the water to become vapour. The heat which is thus absorbed is known as latent heat or heat of vaporisation. The amount of heat which becomes latent when water is vaporised may be estimated as follows :— One part by weight of water heated from o° C. to ioo° C. requires as much heat as would raise loo parts of water i° C. ; one part by weight of water at ioo° C. converted into steam at 100° C. requires as much heat as would raise 537 parts of water 1° C. (Regnault). One great calorie is the amount of heat necessary to raise one kilogramme of water through 1° C., the 537 calories that are used in effecting the change of one kilogramme of water into steam become latent heat. (In British Thermal Units the quantity is given as 965.7 B.T.U., the British Thermal Unit being smaller than the calorie). When steam condenses again the latent heat of vaporisation is given off, and is communicated to the objects upon which conden- sation takes place. 8. Convection of Heat and Displacement of Air by Saturated Steam. Air is a bad conductor and conveyor of heat. Porous objects, which for the present purpose may be said to include objects such as mattresses containing many spaces full of air, are heated very slowly when exposed to the action of hot air or of superheated steam. By means of saturated steam, however, it is easy to heat objects such as mattresses in a verj^ short time. When saturated steam comes in contact with the surface of a cold, “ porous ” object, it condenses and imparts its latent heat to the superficial parts of that object, and at the same time a considerable displacement of air is produced by the shrinking which takes place when the steam condenses ; this allows the steam to penetrate further and to condense upon deeper layers. This double process goes on until the central parts of the object have reached the temperature of the steam owing to the replacement of air by steam. 9. Current Steam and Confined Steam. Whether steam is generated at high or low pressure, it may be produced so slowly that very little of it escapes from the boiler, or so rapidly that a more or less powerful current of steam is produced in the vessel. When such a current is produced the steam is called current steam. When steam is generated in a boiler, the outlet of which is guarded by a pressure valve through which very little steam is allowed to escape, the steam is said to be confined. Displacement of air and penetration of heat are more rapid when an abundant supply of steam is availalile than when the supply is limited. When steam is confined in a chamber, from which the air has not been removed by special means, the air esca])es very slowly, and the chamber remains filled with a mixture of air and steam for a considerable time.