Document 5LwKROd7xpr5ZapLwGj9ZpOx4

American Society 0/ Heating and Ventilating Engineers Guide, ig3l Chapter 26, and the number of the renewals of air contents per ho given in Chapter 4. Ur To obtain effective, uniform ventilation and avoid local drafts ventilators should not be placed more than 30 ft. apart; 20 ft. apart * good average. It is best, although not absolutely necessary, to locate th ventilator at the ridge of the roof, unless the building exceeds 40 ft width, in which case two rows of smaller ventilators should be used Where the building to be ventilated is surrounded by higher building which obstruct air currents, it is desirable to extend the ventilators abov the buildings by mounting them on stacks. e Example.--A foundry building is 40 ft. wide, 200 ft. long, with an average height 1 40 ft.; the ventilators are to be mounted at the ridge of the roof, at a height ofjs u above the floor. What number and size of ventilators.are required? n' Answer.--In this case, ventilation is especially necessary in summer. The air in th . building should not be over 10 deg. fahr. warmer than the outside air. The wind velocity may be as low as 4 miles per hour. Spacing the ventilators, tentatively, 25 fti apart 8 ventilators would be required. Under average conditions, !!) air renewals per.hourare sufficient. If the foundry is small and cramped, and pouring takes place over a large section of the floor space, 15 or more air renewals per hour may be needed. On the basis of 10 renewals per hour, the capacity of each ventilator must be: Q ~ --10 --X (--2--0--0--f-t-.---Xg-4--0--f-t-.---X---4--0---f-t--.-) = 400,^000 cu. ft. .per h. our p6 XThe discharge per square inch of throat area under these conditions is; Jv -- - L T6 +T4THl^l. nro^r hIk- ^- + 20 X 4 mi- per hr- * 166 cu. ft. of air per hour. The required throat area per ventilator is 400,000 . 165 " 2420st,lnif there is no resistance and no wind pressure. The diameter is a/-2^22. = 55.5 in. Standard sizes are 54 in. and 60 ir If 0.7854 Either eight--54-in. or else seven--60-in. ventilators could be used, spaced respective!' 25 ft. or 28 ft, apart. The foregoing is based on the use of. high class ventilators. If ventila tors of lower efficiency are used, or it the air flow into the building is restricted (as in winter) larger ventilators may be required. General Rules for Natural Ventilation Systems Although it is almost out of the question to attempt to predetermine accurately the pressure difference from fundamental data of wind velocity, wind direction, temperature difference, building dimensions anddisposition of openings, in most cases merely a general knowledge of how the forces of wind and temperature difference act, what their maximum magnitudes are, and how they are disposed in and about a building, .will be helpful in planning ventilation. On this basis, the following simple rules are suggested; 430 Chapter 27--Ventilation Systems Sad- industrial building where furnaces', that, give off heat and fumes, are to be is better to locate them in the end of the building exposed to the prevailing strong suction effect of the wind at the roof near the windward end will then temperature difference, to provide for the most active and satisfactory the heat and gas laden air. 1 rase it is impossible to locate furnaces in the windward .end, that part of the ^2.1" w[,;ch they are to be located should be built higher than the rest, so that building,' splashing therefrom will create a suction. The additional height also effect of temperature difference to cooperate with the wind, . jij. use 0f monitors, windows on the windward side should usually be kept " 3-1 since y tf,ey are open, the inflow tendency of the wind counteracts the outflow ydeea, " temperature difference. Openings on the leeward side of the monitor result ^cooperation of wind and temperature difference. A In order that the force of temperature difference may operate to maximum advan- the vertical distance between inlet and outlet openings should be as great as fage^e Qpenings in the vicinity of the neutral zone are less effective for ventilation. 5 In order that temperature difference may produce a motive force, there must be n ivai distance between openings. That is, if there are a number of openings available & building, but all are at the same level, there will be no motive head produced by temperature difference, no matter how great that difference might t>e. ,u g jn the design of window ventilated buildings, where the direction of the wind is 'ouhe constant and dependable, the orientation of the building together with amount feand grouping of ventilation opening can be readily arranged to take full advantage of -the.force of the wind. On the other hand, where the direction of the wind is quite J variable, it may be stated as a general principle that windows should be arranged in sidewalls and monitors so that there will be approximately equal area on all sides. Thus, no matter what the wind's direction, there will always be some openings directly exposed to the pressure force of the wind, and others opposed to a suction force, and effective movement through the building will be assured. MECHANICAL VENTILATION SYSTEMS The principal advantages of mechanical systems of ventilation are: 1. Positive circulation of air at any desired temperature and volume to all parts of the building. 2. Practical uniformity of ventilation regardless of indoor and outdoor temperatures and atmospheric conditions, inasmuch as a positive movement of the air is possible through all ducts, flues and registers at any desired pressure. , ; COMBINED AND SPLIT SYSTEMS Central supply systems of mechanical ventilation may operate inde pendently, or in conjunction with the heating of the building. In some cities, for instance, the schools for many years have been heated and ventilated satisfactorily with combined heating and ventilating, no heaters being placed in the classrooms. Temperature is controlled by the auto matic varying in proportions of the cool air and of the warm air. The heating, ventilation and cooling of many large banks and theaters is accomplished with combined heating and ventilating systems. With this system it is necessary to operate the fans in order to warm the building and to control the conditions. There are obvious advantages in such an arrangement for certain buildings. It tends to insure proper maintenance and intelligent operation. When the heating is accomplished with radiators, heat conductors or local heaters, separate from the ventilation, the scheme is called a split system. This type of mechanical ventilation is used for many schools. 431