Document 10qpoj7gEnrend3x3v2k3kYda

146 CHAPTER 11 1959 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 th? ventilator in utilizing the kinetic energy of the wind for inHniring flow by centrifugal or ejector action. For maximum 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 passing over the building, or in a light court, or on a low building be tween two high buildings, their performance' will be seriously influenced. Thar normal ejector action, if any, may be completely lost. The base of the ventilator should be of a taper-cone design 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 tiie base or structural members present obstructions, ftdditional 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 camp plar-g but, due to the four capacity factors already mentioned, no ample formula can be devised for expressing ventilator capacity. Roof ventilators may be classified as stationary, pivoting, or oscillating, 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 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. 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, sign boards, etc., outdoors, nor by partitions indoor?. 3. Greatest flow per square foot of total opening is obtained by 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 orientatioo of the building, together with amount and group ing of ventilation openings, can be readily arranged to take fiill advantage of tee force of the wind. Where the wind's direction is quite variable, the openings should be arranged in side walls 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, *nd 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 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 produced by temperature difference, no matter how great the difference might be. 7. In oider that. the force of temperature difference may operate to maximum advantage, the vertical distance between inlet outlet openings should be as great as possible. Open ings in the vicinity of the neutral tone are least effective for ventilation. *8. In the use of monitors, windows on the windward ode 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. Stacks 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 Tlw opening H thovld qnaf sM-talf Iho (*<uf dtBtGftaoo of Hw flv*. Hoovy nsdofiM of Mm lnl deck it emntial. Fig. 5____Recommended Type of Cover for Wooden Outlet Flue ' Infiltration and Ventilation to provide for the most active and satisfactory removal of the heat and gas-laden air. 10. In case it is impossible to locate furnaces in the wind ward end. that part of the building in which they are to be located should be built higher tnn 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. 1116 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 by taking air in and out of the roof openings. Openings in the pressure cones can be used for inflow, and openings in the suction cone, or openings in cones of less pressure, <*>n be used for outflow. The ventilation is accomplished by the manipulation of openings to get air flow through the cones to be ventilated. VENTILATION OF ANIMAL SHRTERS11 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 (1/C/) of not less than two is required in their tide 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 excessive 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 tides, 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 tide walls, stimulate convection currents, which is desirable. When so placed, they also-tend to bathe the tide walls with cool air, thus reducing temperature difference between the inside and outside 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 tide of the building facing 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 algo 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 desirable to keep the relative humidity of livestock shelters below 85 percent. Temperatures may be as indicated in the discussion for each kind of animal. 147 Dairy Stables 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 tide walls. In stables of this size and so insulated, a ventilation rate of 3200 to 3800 cfh per (1000 lb of animal) usually insures good conditions. The following recommendations do not apply to so-called pen stables or loafing bams 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-shaped, 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 basic rule for finding the cross-section of the outlet flue is 176.Y Vh (6) where A, ~ area of the outlet flue, square inches. N " weight of animal population, thousands of pounds. h . =* vertical distance from top of inlet flues to top of out let flue, feet. For large flues the flue area obtained from the basic 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 32.5 ft, and in which 38 cows, averaging 1300 lb, will be housed. Determine required sue of outlet flue. Solution: From Equation 6 176 X (38 X 1-300) V325 1525 sq. in. From Fig. 6 the factor to be applied to this area is 94.5 Fig. 6 .... Modification of Flue Area for Outlets Exceeding 1000 Sq In.