Document GR3qpxgJz0jZeg9qqppZjq2x

American Society of Heating and Ventilating Engineers Guide, 1930 Fig. 6. Relation between Total Heat Loss from the Human Body and Effective Temperature more sensible heat in (1-A) than in (1-B); and 67.8 per cent more latent heat or water vapor in (1-B) than in (1-A). In (1-A) 66.5 per cent of the total heat loss is sensible while in (1-B) only 42.5 per cent of the total loss is sensible. CONDITIONING AIR FOR HUMAN COMFORT There are four fundamental ways of producing effective cooling: (1) The dry-bulb temperature may be lowered by direct cooling or removal of heat. (2) The moisture content of the air may be reduced. (3) Air motion may produce effective cooling except for extremely severe con ditions. (4) Evaporation of water without addition or subtraction of Fig. 7. Relation between Sensible Heat Loss from Human Body and Dry-Bulb Temperature for Still and Moving Air 98 Chapter 3--Standards of Ventilation heat is accompanied by an increase in moisture content and a fall in dry-bulb temperature along the wet-bulb line resulting in effective cooling. Take as an example a condition of 92 deg. dry bulb and 40 per cent relative humidity having a wet-bulb temperature of .72.8 deg. and effec tive temperature of 81.1 deg. This condition can be made equivalent to 78 deg. effective temperature or it can be made to feel 3.1 effective deg. cooler by any one of the four fundamental changes mentioned. (1) By the removal of heat the dry bulb may be made to fall to 85.5 deg. (see Fig. 1) along the "90 grain moisture per pound of dry air" or 64.2 deg. dew-point line when the effective temperature will be 78 deg. (2) Without removal of sensible heat or lowering of the dry bulb, the moisture content may be.reduced from 90 to 44 grains per pound of dry air, when the effective temperature will be 78 deg. (3) A 500-ft. velocity (see Fig. 4) will change the still-air condition of 81.1 deg. effective temperature to 78 deg. effective temperature or will give 3.1 deg. effective temperature cooling. (4) Evaporation of water at room temperature without addition or removal of heat will cause the point on the chart, Fig. 1, indicated by our condition, to move along the wet-bulb line to the left thereby lowering the dry-bulb temperature and increasing the moisture content. The wet-bulb temperature will remain the same but the effective temperature will be lowered. By adding 14 grains of moisture without heat, the dry bulb will fall to 83.8 deg. and the effective temperature will fall to 78 deg. The best method of producing effective cooling to be employed in any particular case will depend upon accompanying circumstances and should be determined by a competent engineer. Generally, the removal of heat or water vapor or both, by direct cooling or dehumidifying is most effective. Effective cooling by air motion or evaporation of water is relatively much less expensive, but these methods of cooling are limited to certain conditions of temperature and humidity. Cooling by evapora tion of water is effective when the air is dry or when there is considerable difference between the wet and dry bulb temperature. Cooling by air motion is most effective at low temperatures. When the effective tem perature approaches that of the body little or no cooling results and for certain higher temperatures air motion will make an uncomfortable con dition even less bearable. For. moderately high temperatures greater effective cooling is ex perienced as the result of air motion at high humidities than at low humidities. This suggests a valuable'method of cooling by a combination of evaporation and air motion. Take, for example, a condition of 96 deg. dry bulb and 80 deg. wet bulb having an effective temperature of 85.7 deg. A 300-ft. air velocity will improve this condition by onjy 2.2 deg. Satura tion with water vapor will give a condition of 80.deg. dry bulb, 80 deg. wet bulb and 80 deg. effective temperature or 5.7 deg. effective tempera ture improvement. A 300-ft. air velocity with this new wet and dry.bulli) will give an effective temperature of 75.7 deg. or a total improvement of 10.0 deg. For cooling produced by velocities other than those given in Figs. 2, 3 and 4 the reader is referred to the Tables and Examples on page 737 of the November, 1926, Journal. Example 1.--Given dry-bulb and wet-bulb temperature of 75 and 68 deg. First: what is the effective temperature? Second: is this condition warmer or cooler than 80 deg. dry bulb and 60 deg. wet bulb? 99