Wednesday, September 14, 2011


               At any particular temperature air is able to hold a certain maximum amount of water vapour in clear solution with out the air becoming misty or foggy . Excess of water vapour over this maximum or saturation amount instantly causes a fine dew to form. The maximum amount of water vapour that can he held without dew formation is usually stated in terms of grains of water per cubic foot of air this being the absolute bumidity . For example air at 70 deg . F .can hold 8 grains of water per cubic foot at saturation point . At higher temperatures than 70 deg. F . air can hold larger weights of water for example at 83 deg .F. 12 grains of water per cubic foot are required for saturation.

                           Saturated conditions are very rarely approached in practical textile operation except perhaps in dye houses. Normally the water content of air is well below saturation point . The ratio of the actual amount of vapour present to the saturation value expressed as a percentage gives the relative humidity usually shortened to .R..H. For example if the air of a room at 70deg. F . Holds only 4 grains of water per cubic foot (the absolute humidity at this temperature being 8 grains ) its relative humidity 4*100/8 or 50% R.H.
              In practical textile work the actual weight of water present per cubic foot os of much less importance than the relative humidity It would clearly be possible to have two rooms one at 70 deg. F. and one at 83 deg. F.both containing 8 grains of water per cubic foot. The relative humidities however would be 100% and 66% respectively each cubic foot of air at 83 deg. G . would be able to take up a further 4 grains of water during such an operation as drying wet wool whereas the air at 70 deg. could take up no water and could hage no drying effect.
               A point of great importance in textile testing is that the percentage of moisture in a textile fibre which has been conditioned in an atmosphere and is in equilibrium with it deepens on the relative humidity and not on the absolute humidity . Thus wool exposed to 50% R.H in a room at 70% deg. F. .would contain just as much moisture as if exposed to 50% R. H in a room at 83 deg .F though the air in th elater room would contain twice as great a weight of actual water per cubic foot as the air in the room at 70 deg. F .

               The relative humidity of air which is not subject to control varies under uncontrolled conditions very greatly as the following figures relating to common English conditions will show . On a cold frosty day in winter with temperature at treeing point the absolute humidity of the air is only 1/2 grain per cubic foot and 95% R.H. is easily attained if this air be drawn into a spinning shed would become about 6% On a hot summer day after heavy thunder rain a mill temperature of 83 % deg. F. may be reached with a very moist atmosphere containing 10 grains of water per cubic foot and a relative humidity of 80% .
              The natural regain of wool in an atmosphere of 80% R.H. is almost five times as great as in an atmosphere of 6 % R. H. wool in the same spinning shed could vary in its moisture content by 500%.