Document ymKGk31Doz61pRkOmY6Jw5LrE

1 1 HEATING VENTILATING AIR CONDITIONING GUIDE 1940 CHAPTER 3. PHYSICAL AND PHYSIOLOCICAL PRINCIPLES f movement consistently increases evaporative cooling, and therefore heat tolerance, when relative humidity is high and air temperature low. When relative humidity is low and air temperature is high, however, an increase in air velocity from 17 to 30 or 100 fpm actually decreases heat tolerance. Under these conditions air temperature is above skin temperature and the increased demand for evaporative cooling exceeds the actual increase of evaporation due to the higher air movement. When an air velocity of 500 fpm is reached, the limits of the zone are broadened throughout its of the air current is 40 fpm and the temperature of the air current 2 F or more below the customary winter room temperature. Higher velocities are desirable in the summer time when the air temperature exceeds 80 F. Variations in air movement and temperature in different parts of occupied rooms are often indicative of relative air distribution. The work of the A.S.H.V.E. Research Laboratory indicates that an air movement between 15 and 25 fpm with a temperature variation of 3 F or less in different parts of a room, 36 in. above floor, represent satisfactory distribution. range. For very hot and dry environments, still air is more desirable than a slightly greater air movement but a high air movement is still THE FOUR VITAL FACTORS better. From the preceding discussion it is clear that thermal environment At low air temperatures the effect of increased air movement upon convection loss is a simple and direct one; but the exact quantitative influence of air movement upon the rate of cooling of a hot body has, until recently, been unknown. Studies at the John B. Pierce laboratory of Hygiene, as yet unpublished, have indicated that the cooling effect of air cannot properly be adjusted to the requirements of human health and comfort without control of all the four basic factors concerned. According to the recommendations of the Sub-Committee on the Hygiene of Environmental Conditions in the Dwelling62, it is of great actually increases as the square root of its velocity. Under the conditions Table 7. Corresponding Equivalent Air Temperatures Producing Equal Heat i Loss at 15 fpm for Various Air Temperatures and Air Movements Table 6. Percentage of Total Heat Loss Effected by the Three Chief Routes of Thermal Interchange Air Temperature.. Deg F 60 70 80 90 Observed Ambibnt Air Temperature Dec F Equivalent temperature at standard air move- ment op 15 fpm when observed air movement is _____________________________________________________________________ 15 fpm 100 fpm 250 fpm 500 fpm Air Movement__ fpm 15 100 250 15 100 250 15 100 250 15 100 250 Evaporation_____ per cent 18 lfr' IS 22 19 18 29 27 25 73 67 55 Radiation...._____ per cent 43 26 Convection............ per cent 39 58 19 40 26 66 38 55 17 37 24 65 34 49 17 14 10 58 13 23 13 32 of the experiments in question (semi-reclining lightly clothed subjects), the effect is represented by the formula: 95 95 95.0 95.7 96.0 90 90 88.5 87.7 87.0 85 85 81.7 79.5 78.0 80 80 75.2 71.6 69.0 75 75 68.7 . 63.5 60.3 70 70 62.2 55.5 51.2 65 65 55.5 47.5 42.5 60 60 48.9 39.5 33.6 55 55 42.3 31.8 24.7 50 50 35.7 23.5 15.8 -f = 0.413 VET * (2) where C = Convection loss in kilogram-calories per square meter of body surface per hour. A T -- Surface temperature of the body (clothing and exposed skin) minus air* temperature, degrees Fahrenheit. V = Velocity of air, feet per minute. The extent of the effect may be indicated by the computed data in Table 7. As emphasized in an earlier paragraph the problem of local drafts causing differential cooling of special areas of the body is one that must always be kept in mind. Experience, and recent field studies by theJ A.S.H.V.E. Research Laboratory61 place the desirable air movementbetween 15 and 25 fpm under ordinary room temperatures during the heating season. Objectionable drafts are likely to occur when the velocity importance in all research studies to make an accurate record of each of the four independent factors governing bodily heat exchanges, tempera ture, movement and humidity of the air, and mean radiant temperature of the surrounding surfaces. For this purpose the committee suggested in the interest of comparability the use of the following four types of instruments or others yielding similar data: 1. Silvered dry-bulb thermometers or hair-pin thermometers (Bargeboer). 2. Silvered dry Kata-thermometers or the hot wire anemometer. 3. Psychrometer, wet- and dry-bulb, whirling or ventilated. 4. Globe thermometer (Vernon) or the dry resultant thermometer (Missenard). Such instruments as these, when properly calibrated and their readings are compared, can be used for determining the four basic physical factors concerned separately or in certain combinations. The results of the four physical measurements thus determined can generally be translated into the terms of any special instrument combining two or more of them. nA.S.H.V.E. Research Report No. 1016--Classroom Drafts in Relation to Entering Air Stream Temperature, by F. C. Houghten, H. H. Trimble, Carl Gutberlet, and M. F. Lichtenfels (A.S.H.V.E. Transactions, Vol. 41,1935, p. 268). 74 ''Housing Commission of the League of Nations, adopted at Geneva, June 25, 1937. 75