23 - Air and water vapour mixtures

Abstract

The atmosphere consists of a mixture of dry air and water vapour. Air is itself a mixture of several elemental gases, mainly oxygen and nitrogen, but the proportions of these are consistent throughout the atmosphere and it is convenient to consider air as one gas. Water may be present in air in the liquid form, as rain or mist, or as a solid (snow, hail). However, in general ambient and indoor conditions the water present in the air will be in the vapour form, as superheated low-pressure steam. If air and water are present together in a confined space, a balance condition will be reached where the air has become saturated with water vapour. If the temperature of the mixture is known, then the pressure of the water vapour will be the pressure of steam at this temperature The specific enthalpy (or total heat) of the mixture can be taken from 0 K (– 273.15°C) or from any convenient arbitrary zero. Since most air-conditioning processes take place above the freezing point of water, and we are concerned mostly with differences rather than absolute values, this is commonly taken as 0°C, dry air. The human body takes in chemical energy as food and drink, and oxygen, and consumes these to provide the energy of the metabolism. Some mechanical work may be done, but the greater proportion is liberated as heat, at a rate between 90 W when resting and 440 W when doing heavy work.