Document wqep2bq8r5nN15mXOx75rjgDV

American Society of Heating and Ventilating Engineers Guide, 1936 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 and grouping of ventilation openings 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 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. 11. The intensity of suction or the vacuum produced by the jump of the wind is greatest 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 the possibility of sidewall or monitor windows being closed on account of weather conditions. Such possibilities favor roof ventilators and specially designed stormproof inlets. MEASUREMENT OF NATURAL AIR FLOW 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. Chapter 4--Natural Ventilation IJ a small (4 in.) low-speed anemometer, and correct all readings USd'ne to a recent calibration. Mount the anemometer in a strap iron afCo with a long handle for convenience. Divide each opening into ^ ' sauares (by string or wire) and hold the anemometer in the center of 5 "h souare for a definite period of from 15 to 30 seconds. Record the the traverse as soon as completed and start another one imrepdiately. A series of traverses over a period of one hour, or the full Hpriod covered by the wind velocity observations with a fairly steady wind may be considered a satisfactory test for that wind velocity. It is oreferable 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 43). Outdoor Temperature. It is easy to. make an error of 1 to 5 deg in observing the outdoor air temperature. An accurate thermometer, calibrated in 1 deg divisions should be used. The thermometer should be mounted in the shade at about mid-height of the building and not too near the building wall or adjacent to an air outlet. The heat from a wall or roof which has been exposed to the sun is easily transmitted to a thermometer, with resulting high readings. Average Indoor Temperature. It is important to note that the capacity of an opening (such as roof ventilator) does not depend on the difference in the temperatures measured adjacent to the opening. It depends rather on the difference between the average temperature of the column of air inside the building and that outside. Indoor temperatures should therefore be observed at various heights to secure a good average. DAIRY BARN VENTILATION4 A successful barn ventilating system is one which continuously supplies the proper amount of air required by the stock, with proper distribution and without drafts, and one which removes the excessive heat, moisture, and odors, and maintains the air at a proper temperature, relative humidity, and degree of cleanliness. Barn temperatures below freezing and above 80 F affect milk produc tion. Milk producing stock should be kept in a bam temperature be tween 45 and 50 F. Dry stock, at reduced feeding, may be kept in a barn 5 to 10 deg higher. Calf barns are generally kept at 60 F, while hospital and maternity barns usually have a temperature of 60 F or somewhat higher. The heat produced by a cow of an average weight of 1000 lb may be taken as 3000 Btu per hour. The average rate of moisture production by a cow giving 20 lb of milk per day is 15 lb of water per day, or 4375 grains per hour. To set a standard of permissible relative humidity for cowbarns is difficult. For 45 F an average relative humidity of 80 per cent is satisfactory; with 85 per cent as a limit. Where the barn volume is within the limit that can be heated by the stabled animals, the air supply need not be heated. The air should be Information on this subject refer to Technical Bulletin, U. S. Department of Agriculture (1930), by M. A. R. Kelley. Dairy Bam Ventilation, by F. L. Fairbanks (A.S.H.V.E. Transactions, Vo!. 34, 1928). Cow Barn Ventilation, by Alfred J. Offner (A.S.H.V.E. Transactions, Vot. 39,1933). I