Document oeNzVOmZxjN4qKNwo9jmYBmB8

538 CHAPTER 29 1965 Guide And Data Book 30. Vane Ratio: The ratio of depth of vane to minimum width between two adjacent vanes. STANDARDS FOR SATISFACTORY CONDITIONS The object of air distribution in warm air heating, ventilat ing, and air-conditioning systems is to create the proper combination of temperature, humidity, and air motion in the occupied sone of the conditioned room (door to 6 ft above floor level). To obtain comfort conditions within this zone, standard limits have been established as acceptable effective temperature*. This term comprises air temperature, motion, humidity, and their physiological effect on the surface of the human body. Any variation from accepted standards of one of these elements may result in discomfort to the occupants. The same effect may be caused by lack of uniformity of con ditions within the space or by excessive fluctuation of-condi tions in the same part of the space. Such discomfort may arise due to excessive room air temperature variations (hori- f - zontaDy, vertically, or both), excessive air'motion (draft)/ failure to deliver or distribute the air according to the load requirements at the different locations, or too rapid fl.uctua- . tion of room temperature or air motion, (gusts). - The effect of air' motion has been'investigated at Case' * Institute of Technology.1 The comfort criteria used'by these investigators are of interest. Draft was defined as any localized feeling of coolness or warmth of any portion of the body, due to - both air movement and air temperature, with humidity and radi ation considered constant. The warmth or coolness of a draft was measured above or below.the controlled room condition of 76 P dry-bulb temperature at the centerof the room, 30 in. above the floor, and air moving at approximately 30 fptn. -, To define the difference At in effective temperature between any point in the occupied tone; and the control condition,' the investigators used the following equation proposed.by Ryd berg and Norback:* ' Ot ((, - 76) - 0.07(7, - 30) ' '-`(I) ioKeU ` I* ,-- local air stream dry-bulb temperature, Fahrenheit.7 V, -- local air stream velocity, feet per minute. Equation 1 takes into account the feeling of coolness pro duced by aix motion. Equation 1 shows that a one degree tem perature change is equal to a 15 fpm velocity change, and is used to establish the neutral line in Fig.-l. In summer, the local air-stream temperature t* is below the control tempera ture. Hence, both temperature and velocity terms'are nega tive when the velocity'V, is greater than 30 fpm, and both,of them add to the feeling of coolness. If, in winter, (* is,above the Conttol tomperature. any air velocity above 30.fpm:will subtract from the feeling of warmth produced by 1*. Therefore, it is usually, possible to have aero difference in effective tem perature between location x and the control point in'winter, but not in summer. - - r * Houghton1'presented data which make it pos&ble to inter pret statistically, the percentage of room occupants who will tolerate or object to a'given draff condition. Fig.l presents Houghton's data in the form used In. Reference 1. Although the percentage of room occupants who will object to certain conditions may. change, over a period of years, Fig. 1 gives valuable insight into the possible objectives of room air distribution. The data show that a person will tolerate higher velocities and lower temperatures at ankle level at neck leveL They also prove that conditions in the occupied zone are more critical at Beating level than they are n^r the floor. ` . With a one degree temperature depression, a velocity of 15 fpm produces a feeling of warmth, whereas, velocities `of 40 and 60 fpm would be acceptable to 90 and 80.percent of the occupants, respectively. Room air velocities from 20 to'50'fpm have been considered as a criterion for room air motion, how ever, Fig. 1 shows that evenhigher velocities may be accept able to a .large percentage of the occupants. ,t !T In the following section, it will be shown that, during heat* mg, a temperature gradient exists in a stagnant zone with air velocities generally between 15-20 fpm. Since the stagnant cone occurs from the floor or ankle level up, Fig. 1- shows that 80-90 percent of the occupants will accept an aoklo-to-dttmg- level gradient of 3.5 to 4 F deg during heating. The following section also indicates possible regions of high'room'dir've locities with various outlets and the later sections oh.jets give data to calculate the magnitude of the air .velocities.'!With this information, Fig. 1 permits an evaluation of the accepta bility of the air distribution. . ' Reference should be made to Chapter 7,. Physiological Principles, for information on effective temperature and com fort lines, and to Chapter 14, Sound Control, for acceptable room noise levels and noise generated by air outlets.. Material in' Chapters 21 and 24 to 28 in'the 1964 Gums'And`Data Book deals with the health, safety, and efficiency of workers, and with temperature and humidity requirements for.prod; ucts and manufacturing processes. " - ii'_ Space Air Distribution 539 PRINCIPLES OF AIR DISTRIBUTION Conditioned air is supplied to air:outlets at temperatures and velocities which differ greatly from those in the occupied, outlet designs,'outlets are often better described by .their, construction-features. (See Chapter 30,-Air Diffusing Equip-, ment.) ; > ` " ; zone of the room. Proper air distribution, therefore, calls-for. Outlet Performance (1) entrainment of room air by the primary air stream outside of the zone of occupancy in order that air motion and tem perature differences will be reduced to acceptable limits before In the outlet performance tests at the University-of Uli-nois,4 the investigators used five groups of outlets: the air enters the occupied zone, and (2) counteraction of the natural convection and radiation effects within the room. The complexity of practical room air distribution problems 1. Group, A. Outlets mounted in or near the ceiling and dis-j charging the air.horizontally. 2. Group B. Outlets mounted in or near the floor and dischaig-' mg the air vertically in a non-spreading jet. ' is due to innurnwrehlft variations in building construction, 3. Group C.` Outlets mounted in or near the floor and dis system, design, and operating requirements, and makes the exploration of the basic character of air. distribution impera tive. The theory of room air distribution is not complete but charging tne air in a vertical spreading jet. . ,-'j 4. Group D.1'Outlets mounted 'in or near the floor and dis- charging'the air.horizontally. 5. Group E.'Outlets mounted in or near the. ceiling and pre^ a considerable fund of knowledge supported by experimental jecting the primary air vertically. (This group was tested but hot guidance is available for the solution of many.air distribution reported in Reference 4.) . problems. . Analysis .of outlet, performance was based .'on primary air,, Many heating and cooling tests at the University of Illinois pattern, total air pattern,, stagnant air layer, natural con Engineering Experiment Station4 have resulted in the formu vection currents, return air pattern, and room air motion. lation of.certain principles of outlet performance, which pro^' Figs. 2 to 6 show the air motion characteristics of; the five vide an important guide for solving air distribution problems - : outlet groups. The general principles of air,'distribution em- involving (a) proper selection and application of supply and ' phBBtsed by these tests which should be kept in mind while return intakes, (b) selection of pertinent indices of air dis reviewing-Figs. 2 to 6 are: tribution, (c) elimination of trouble spots in tisting systems, 1. The primary air, shown by clear envelopes in Figs. 2 to 6, (d) comparison of air distribution field studies, (e) evaluation of other factors winch may affect air distribution, such as loading and blower operations, (0 evaluation of particular air from-the outlet down to a velocity of about 150 fpm can be treated analytically and shows the same characteristics for both heating and cooling.' 2. The total air, shown by diagonally lined envelopes in Figs. distribution devices prior'to installation, and (g) design of 2 to 6, is still influenced by the primary air and is relatively high supply outlets. '`velocity air, but less tH&n 150 ipm, with air temperatures gen . Types of Air Outlets. Supply outlets may be classified as sidewall, ceiling, baseboard, or floor outlets, according to . erally within 1 F deg of the room temperature. The total air. is , influenced by the environment, and, thus, tends to drop* during cooling tends to rise during heating. The total air is not sub- their location in the room. However, since there are numerous v ject to precise analytical treatment.