Document 675gp0ODnR3nENzQQ8nnzGq1

$ 156 CHAPTER 11 1960 Guide forces of wind and temperature head that create flow through other types of openings. The capacity of a ven tilator depends upon four things; (1) its location on the roof; (2) the resistance it and the ductwork offer to air flow; (3) the height of draft; and (4) the efficiency of the ventilator in utilising the kinetic energy of the wind for inducing flow by centrifugal or ejector action. For maTimnm flow induction, a ventilator should be located on that part of the roof where it will receive the full wind without interference. If ventilators are installed within the suction region created by the wind pasting over the building, or in a light court, or on a low building be tween two high buildings, their performance will be seriously influenced. Their normal ejector action, if any, may be completely lost. The base of the ventilator should be of a taperreone dpign to produce the effect of a bell-mouth nozzle whose coefficient of flow is considerably higher than that of a square-entrance orifice. If a grille is provided at the base, or if the base or structural members present obstructions, additional resistance is introduced, and the base opening should be increased in size accordingly. Air inlet openings located at lower levels in the building, should be at least equal to, and preferably larger than, the combined throat areas of all roof ventilators. The air dis charge by a roof ventilator depends on wind velocity and temperature difference, and, in general, its performance will be the same as any monitor opening located in the same (dace but, due to the four capacity factors already mentioned, no simple formula can be devised for expressing ventilator capacity. Roof ventilators may be classified as stationary, pivoting, or nariiUftting, and rotating. Generally, these have a round throat, but the continuous-ridge ventilator would fall in the stationary classification. When selecting roof ventilators, some attention should be given to ruggedness of construction, storm-proofing features, dampers and damper-operating mechanisms, possibility of noise, original cost, and main tenance. Natural ventilation units may be used to supplement power-driven supply fans, and under favorable weather conditions it may be possible to stop the power-driven units. Units are not subject to code tests for ratings. Generally, they must be selected from manufacturers' tables. It is, therefore, very important to consider the reliability of the ratings used. Controls Gravity ventilators may have dampers controlled by hand, thermostat, or wind velocity, in combination with a fan. The thermostat station may- be located anywhere in the building, or it may be located within the ventilator itself. The purpose of wind velocity control is to obtain a definite volume of exhaust regardless of the natural forces, the fan motor being energized when the natural exhaust capacity falls below a certain minimum, and again shut off when the wind velocity rises to the point where this minimum volume can be supplied by natural forces. Slacks Stacks or vertical flues are really chimneys which function through the effects of the wind and temperature difference, like the roof ventilator, the stack outlet should be located so that the wind may act upon it from any direction. With little or no wind, the chimney effect depends entirely on temperature difference to produce a removal of air from the rooms where the inlet openings are located. GENERAL VENTILATION RULES A few of the important considerations, in addition to those already outlined, are; 1. Inlet openings in the building 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. Outdoor air will then be supplied to the zone to be ventilated. 2. Inlet openings should not be obstructed by buildings, trees, ign boards, etc., outdoors, nor by partitions indoors. 3. Greatest flow per square foot of total opening is obtained by using inlet and outlet openings of nearly equal areas. 4. 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 group ing of ventilation openings, can be readily arranged to take full advantage of the force of the wind. Where the wind's direction is quite variable, the openings should be arranged in side wails and monitors so that, as far as possible, there will - be approximately equal areas on all sides- Thus, no matter what the wind's direction, there will always be some openings directly exposed to the pressure force, and others to a suction force, and effective movement through the building will be assured. 5. 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. 6. 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 the' difference might be. 7. In order that the force of temperature difference may operate to maximum advantage, the vertical distance between inlet and outlet openings should be as great as possible. Open ings in the vicinity of the neutral zone are least effective for ventilation. 8. 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. 9. 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, The apcfiiaQ H should qnel oa-fcdf the feast dimeoo'on of (fa floe. Hear? msuJatioa of ft*e iev4 deck fs essonHat. fig. 5 .... Recommended Type of Cover for Wooden Outlet Rue 1 Infiltration and Ventilation to provide for the most active and satisfactory, removal of the heat and gas-laden air. 10. In cn it is impossible to locate furnaces in the wind ward end. part of the building in which they are to be located mould oe built higher than the rest, so that the wind, in splashing therefrom, will create a suction. The addi tional height also increases the effect of temperature difference to cooperate with the wind. 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 an increase in occupancy may occur, or when extremely hot days may be anticipated. In the former case, free openings should be located at the level of occupancy for psychological reasons. 13. In single story industrial buildings, particularly those covering large areas, natural ventilation must be accomplished S' .airing air in and out of the roof openings. Openings in ; pressure zones can be used for inflow, and openings in the suction zone, or openings in zones of less pressure, can be used for outflow. The ventilation is accomplished by the manipulation of openings to get air flow through the zones to be ventilated. VENTILATION OF ANIMAL SHELTERS1* Animal shelters require ventilation to remove moisture, odors and, in the case of dairy stables, excess heat. Outlets. Outlet flues for natural draft systems should be round or approximately square. A thermal resistance (l/U) of not less than two is required in their side walls. They should extend at least two feet above the highest part of the roof. Only one outlet is recommended.for each room or pen. The use of several outlet flues may result in ex cessive up-drafts in some flues, and down-drafts in others. If flues have roofs or covers, these should be high enough to provide unobstructed openings on all sides, equal in height to one-half the least dimension of the flue,-Fig. 5. A level, heavily insulated ceiling under the flue roof, and over the entire area of the flue, is important. Inlets. Inlets should direct the incoming air vertically upward so as to avoid drafts on the animals, and to insure immediate mixing of incoming air with the room air. A rea sonably uniform distribution around the stable or pen is de sirable. Inlet flues, that deliver air close to the side walls, stimulate convection currents, which is desirable. When so placed, they also tend to bathe the ride walls with cool air, thus reducing temperature difference between the inride and outride of the wall. From the standpoint of air movement, insulation of inlet flues is not important. Condensation is, however, likely to occur on them, unless the thermal resistance of their walls is at least two. Controls or throttling devices in inlet flues are seldom required. If used, they are best applied to the inlets and limited to the ride of the building faring prevailing winter winds. They should be so made that an opening, at least one inch wide by the width of the flue, will always remain open. Amounts of heat and water produced by livestock vary not only with the different kinds of animals, but also with age, weight, feed consumption, and production. These facts, and the vagaries of the weather, make exact calculations impossible. The following practical recommendations are based on numerous, carefully checked observations. It is derirable to keep the relative humidity of livestock shelters below 85 percent. Temperatures may be as indicated id the discussion for each kind of animal 157 Dairy Slables The most commonly accepted temperatures for dairy stables, where cows are confined in stanchions or tie stalls, are from 45 to 55 F. These temperatures are readily main tained in winter weather by the body heat of the herd in well constructed, well stocked, and well ventilated stables. Stable volume in excess of 600 cu ft, and exposed wall area in excess of 130 sq ft per 1000 lb animal weight are, in general, undesirable. Side walls should have an overall thermal resistance of from 2 to 5, depending on the temperature zone and wind exposure. Thermal resistance of the ceiling should be 50 percent greater than that of the ride walls. In stables of this size and so insulated, a ventilation rate of 3200 to 3800 cfb per (1000 lb of animal) usually insures good conditions. The following recommendations do not apply to so-called pen stables or loafing hams in which there is a thick manure and bedding pack on the floor, and in which doors are normally kept open. Outlets. One flue will serve a stable 200 ft long. In stables over 120 ft long, the flue should be about midway between the ends or, if the stable is L-ehaped, near the angle. In shorter stables, it may be at any convenient location. The exhaust point in the stable should be not more than 18 in. above the floor. This permits removal of only the coolest air, and prevents rapid fluctuations in stable tem perature. A baric rule for finding the cross-section of the outlet flue is 176JV Vh m takers A, = area of the outlet flue, square inches. N = weight of animal population, thousands of pounds. h = vertical distance from top of inlet Sues to top of out let flue, feet. For large flues the flue area obtained from tire baric formula may well be reduced according to the chart, Fig. 6, because of a decrease in friction. Example t: Assume a stable in which the vertical height from the top of the inlets to the top of the outlet flue is 325 ft, and in which 38 cows, averaging 1300 lb, will be housed. Determine required rise of outlet flue. Solution: From Equation 6 176 X (38 X 1-300) A, Vas 1525 sq. in. From Pig. 6 the factor to be applied to this area is 945 fig. 6.... Modificotion of Rue Area for Outlets Exceeding 1000 Sq In.