Document pmBRw3Jq24xpovBjgoqo2wLgB

American Society of Heating and Ventilating Engineers Guide, 1934 generous allowance because either a direct wind of 5 mph or an average temperature difference of 10 deg acting alone will more than suffice to carry away the heat, and when the two forces' are acting together, the system may have an excess capacity of 25 per cent to 50 per cent, especially if the outlets are made up partially of roof ventilators which employ the force of the wind for producing a suction effect. On the other hand, the wind may at times come from an unfavorable direction, or its velocity may fall below 5 mph or the building construction may not permit a full 2400 sq ft of inlet window area and an equal amount of monitor or roof ventilator outlet area. In case the two sets of openings are not equal, their effectiveness is reduced. From this example it must be apparent that while formulas may furnish a reliable guide, the final solution of a problem of natural venti lation requires a common sense analysis of local conditions to supplement and to modify the dictates of the formulas. ''}' GENERAL RULES A few of the important requirements in addition to those already outlined are: 1. Inlet openings should be well distributed, and should be located on the windward side near the bottom, while outlet openings are located on the leeward side near the top.' Outside air will then be supplied to the zone of occupancy. 2. Direct short circuits between openings on two sides at a high level may clear the air at that level without producing any appreciable ventilation at the level of occupancy. 3. Roof ventilators should be located 20 to 40 ft apart each way and preferably on the ridge of the roof. The closer spacings are used when ventilating rooms with low ceilings. 4. Greatest flow per square foot of total opening is obtained by using.inlet and outlet openings of nearly equal areas. 5.. In an industrial building where furnaces, that give off heat and fumes, are to be installed, it is better to locate them in the end of the building exposed to the prevailing wind. The strong suction effect of the wind at the roof near the windward end will then cooperate with temperature difference, to provide for the most active and satisfactory removal of the heat and gas laden air. 6. In case it is impossible to locate furnaces in the windward end, that part of the building in which they are to be located should be built higher than the rest, so that the wind, in splashing therefrom will create a suction. The additional height also increases the effect of temperature difference to cooperate with the wind. 7. In the use of monitors, windows on the windward side should usually be kept closed, since, if they are open, the inflow tendency of the wind counteracts the outflow tendency of temperature difference. Openings on the leeward side of the monitor result in cooperation of wind and temperature difference. 8. In order that the force of temperature difference may.operate to maximum advan tage, the vertical distance between inlet and outlet openings should be as great as possible. Openings in the vicinity of the neutral zone are less effective for ventilation^ 9. In order that temperature difference may produce a motive force, there must be vertical distance between openings. That is, if there are a number of openings available in a 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 be. .. 10. In the design of window ventilated buildings, where the direction of the wind is quite constant and dependable, the orientation of the building together with amount 66 Chapter 4--Natural Ventilation , _.nuoine of ventilation openings can be readily arranged to take full advantage of fnrce 0fthe wind. On the other hand, where the direction of the wind is quite tD dahle it may be stated as a general principle that windows should be arranged in bewails 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 * nosed to the pressure force of the wind, and others opposed to a suction force, and effective movement through the building will be assured. 11 The intensity of suction or the vacuum produced by the jump of the wind is reatest just back of the building face. The area of suction does not vary with the wind velocity, but the flow due to suction is directly proportional to wind velocity. 12 Openings much larger than the calculated areas are sometimes desirable, especially when'changes in occupancy are possible, or to provide for extremely hot days. In the former case free openings should be located at the level of occupancy for psychological reasons. 13 Special consideration should' be given to thespossibility of sidewall or monitor windows being closed on account of weather conditions. Such possibilities favor roof ventilators and specially designed storm-proof inlets. MEASUREMENT OF NATURAL VENTILATION The determination of the performance of any ventilating system involves measurements which are not easy to make. The difficulties are increased in the case of natural ventilation, since the motive forces and the air velocities are very small. The measurements necessary for giving the capacity of a system are (1) velocity of the wind, (2) velocity of the air through inlet and outlet openings, (3) outdoor air temperature, and (4) average indoor air temperature. Measuring Wind Velocity. The cup-type of anemometer as used for Weather Bureau observations is sufficiently accurate for this measure ment. Some more accurate instruments as well as direct-reading types have been developed for airport service, but for ventilation work it is the average wind velocity over a long period which determines the capacity of the system. Hence-the use of the Weather Bureau instrument, with an observation period of one hour or. more, is satisfactory. If observations of wind direction are required, these should be taken by observing a sensitive weather vane at frequent intervals (about every 5 minutes) during the same period. Velocity of Air Through Openings. The vane type anemometer is the most practical instrument for this measurement. Use a small (4 in.) low-speed anemometer, and correct all readings according to a recent calibration. Mount the anemometer in a strap iron clamp with a long handle for convenience. Divide each opening into 5 in. squares (by string or wire) and hold the anemometer in the center of each square for a definite period of from 15 to 30 seconds. Record the result of the traverse as soon as completed and start another one im mediately. A series of traverses over a period of one hour, or the full period covered by the wind velocity observations with a fairly steady wind, may be considered a satisfactory test for that wind velocity: It is preferable to Have an anemometer observer at each opening. If the opening is covered by a grille or register, use_the proper correction factors (see Chapter 40). Outdoor Temperature. It is easy to make an error of T to 5 deg in 67