Document jgBQ3w6NoRMK2KrzQeZ7jRZmQ

! j,j. , ..sm ' American Society o/-Heating and-. Ventilating; Engineers Guidi These ventilators are expected to function by the force of difference alone, and have no external features designed tO w,, suction by- the action of the wind. The absence of a conspicuous ev,ateJ-J structure makes this type of ventilator- particularly suitable fnr - types of buildings: cerij! Selection of Roof Ventilators While the averag_e efficiency of on# e class of roof ventila.t.o_r.su will 1 ^ higher or lower than the average efficiency of another class, this does * in any way determine the capacity of individual ventilators, as ventila of the same class and, which from a casual observation appear to .v*. s__a__m___e_,___m_ _a__y_ i__h__a__v__e___e__n__t_i_r_e_il_y. difrfe__r__e__nj.t c_h__a__r_a__c_at.e__r_i_s__t_i_c__s_j, _d___u__e t, o t.h, e f.act tag' some of the fundamentals have been overlooked or changed in the other. The principles which should be followed in the selection of roof venti lators are: , . A--Stationary Ventilators. 1. A head sufficiently large to produce a large low-pressure area on' the side opposite the wind, and to give an area of outlet for the air leaving the head large enough to obviate undue resistance to flow? At the same time the head should not- be so large as to be unwieldy* in handling or to be structurally weak when erected. 2. A storm band on stationary non-siphoning ventilators should be. sufficiently wide and so placed as to prevent the entrance of: external air into the ventilator head. 3. If the ventilator is a siphoning type, additional outlet air space must be provided in the head in order not to restrict the air pas sage from the exhaust pipe. B--Rotary Ventilators. 1. A flaring outlet from a rotary ventilator will give a better exhaust than a straight outlet. - 2. Practically frictionless,and noiseless turning of the ventilator head, when the wind direction changes. The head should turn at very low wind, velocities. X, ... 3. Smallest possible change of direction of the air ascending from the building and least possible resistance to its egress by louvres or other obstructions at the outlet opening. C--All Ventilators. 1. Freest possible outlet for the air. from the building, with large areas and smallest possible change of direction of the air flow. 2. Freedom from down drafts and from entrance of rain or snow. 3. Freedom from being rendered inoperative by collection of snow or formation of ice on ventilator. 428 w.. ' CHAPTER 27-VENTU.ATION SYSTEMS B||^pfjihteievsaroieftyRoof faVcetonrtislaatoffrescting capacity makes it essential for the user gflyentilators to exercise great care in respect to the item of capacity. All ^^hpansons of capacity must be referred to a given dimension, namely Sthe throat area, corresponding to the nominal size of the ventilator. g&:Careful tests of ventilators of various types have been made by reliable Mj&estigators, and the reader is referred to their work for information on Jfeapacities See Transactions, A. S. H. V. E., Vol. 27,1921, p. 67; Vol. 28 1922, p. 189; and Vol. 29, 1923, p. 39. Also Bulletin No. 14, Engineering ^Experiment Station, Kansas State Agricultural College. Smfconservative figures for the best types of ventilators now on the market fmmder conditions of unrestricted flow of air to the ventilator, are given by fpthe equation: IgA. Q-Axfcx^X^-'o) -------- -- + 20 X V fifliwiVlierc Q * cubic feet of air exhausted per hour area at the throat of A square inches, through mounted a ventilator on a roof at having a free a height of H feet from the center of the ventilator outlet to the inlet opening of the i&i/ building, and with a wind velocity of V miles per hour, and average tern-- 7,' pera^ure li inside t0 outside. High class ventilators, for instance those of the ejector type, will, fyrdnder favorable conditions, discharge continuously 25 per cent more air !:>than these conservative figures indicate. Capacities are lower, on the .i+other hand, if ventilators of lower efficiency are used, or if the flow of air sfSinto or through the building is restricted, or if the ventilator is not exfefposed to the free sweep of the wind. Tests occasionally show consider- . -y-ably higher discharge rates over short periods of time. These abnormally $^;-high results are produced by the action of the wind upon certain openings the building; they are not due to the ventilator itself, and cannot be +ydepended upon for continuous ventilation if the direction of the wind changes. In the smaller sizes of ventilators (12 in. or less in throat ^diameter) the air discharge per square inch of cross-sectional area is gSfTeduced, on account of the frictional resistance and, in the rotary types, on account of reduction of free' area by the supports, bearings, etc. Example.--What is the capacity of an 18-in. ventilator, located 35 ft. above the inlet r.' openings, with 6 miles per hour wind velocity, 50 deg. fahr. outside temperature, 68 deg. fahr. inside temperature? Answer.--A = 0.7854 X (18)* = 255 sq. in. r 36 X 35 X (68 - 5o) = 50,000 cu. ft, per hour> Q = 255 X + 20 X 6 L' 6 + 6 average capacity under these conditions. The air supply required based on square feet of floor 429