AIR VELOCITY and COMFORT

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Air velocity is caused by mainly different temperatures and pressures at different places. However, air can become mobile due to fans or such air circulating devices.

DRAUGHT (Draft) or very low velocity air movements are not always perceptible, as they do not cause any sensation of pain or pressure on skin. Draughts are more causative in small and enclosed spaces. Draughts occur due to temperature and air pressure thresholds near cracks and such leakage points in enclosed spaces. Draughts, however, help in convective heat exchange, evaporation and dilution of pollutants in air. Draughts cause localized cooling or heating of sensitive organs of our body. Such sensation on feet is a common experience in trains, buses, sofas, undersides of office tables, etc. Children and aged people with deficient blood circulation and body temperature regulation mechanisms, are readily affected by such currents.

BREEZE or low to medium velocity air movements generally affect only local areas. Breeze does not let airborne particulate matter to settle down. Skin sensation can be avoided by appropriate screening and deflection of the breeze. Since breeze causes effective pressure on skin, with very immediate and very perceptible change sensation.

WINDS are high air velocity movements of air affect large regions. Winds raise particulate matter in the air, cause rapid change in level of humidity and often cause discomfort due to high pressure sensation on the skin.

In hot and cold both types of climates people often close all the openings to reduce the air movements and thereby control the convective heat gain or loss. 

Turbulent air velocity is less comfortable than a Laminar air velocity. Turbulent air movement achieves a better mix of air whereas laminar helps in greater displacement of air mass. This is the reason why in hot arid climates small size openings are used to create turbulence or a viscous flow, and in hot humid climates the body level openings of horizontal nature create a laminar flow to displace the humidity.

Horizontal openings --Laminar flow

Air movement affects our body system very profoundly. Its effects are primarily sensed by the skin. Five types of sensations are involved with skin. The touch pressure (mechanico receptors), Cold and Warmth feeling (thermo receptors), Pain and Itch. Cold is a consequence of contraction of blood vessels and warmth is felt due to dilation of blood vessels; both are felt by the same receptors.

In certain acute work conditions like, mines, metal melting plants, textile plants, cold storage, the level of efficiency or productivity depends on the endurance level and adaptability of the body. A body may endure or adopt to certain abnormal conditions for a period of time but there may occur side effects. The side effects may be realized in a different form and at a different time.

Small windows -turbulent air movement

The perception or feel of air movement depends on several factors such as air velocity, fluctuations in air velocity, temperature of air, and personal factors like overall thermal sensation and activity level. Even for the same person, sensitivity to air movement may change from day to day.

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