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dampers i, i are mounted on the wall, as shown. The hot and cold air are mixed in an enlargement j of the ducts, the temperature of the air supplied to the various rooms being controlled by thermostats, in accordance with the operation of which the mixing dampers are actuated by their motors. The mixed air is distributed by single galvanized-iron ducts arranged, for instance, as shown in Fig. 2, where the fan a is placed within a sheet-metal, fresh-air room or chamber, drawing its supply of fresh air over a duplex tempering coil b and thence forcing it over a duplex main heater c, c' into the plenum chamber d, from which separate single ducts e, e lead to the various rooms. By means of doors in the metal casings, access is had to the main heaters, which are arranged side by side in the same plane.
Double-Duct System.-A common arrangement of apparatus for the double-duct system of distribution is composed of a blower driven by an upright engine, and forcing the air through a top horizontal discharge outlet between the pipes of the heater and thence to the main galvanized-iron air ducts. A portion of the cold fresh air is usually by-passed over the top of the heater into the cold-air duct. The quantity of air delivered to the cold-air duct depends on the position of the baffle-plate damper. Both ducts are carried to the base of the vertical flues, where the mixing takes place.
Relative Position of Fan and Heater.-The relative position of fan and heater determines whether the apparatus is of what is known as the blow-through or draw-through type, the adoption of either arrangement being dependent on the space available for the installation of the apparatus, on the size of the latter, and on the personal preference of the designer.
AIR SUPPLY FOR BUILDINGS.
Amount of Air Required.-The amount of air commonly supplied to each occupant, in various types of buildings, is given in the accompanying table.. Some states, notably Massachusetts, have passed laws relating to the ventilation of school buildings and providing for a minimum per capita supply per minute in such buildings.
The Massachusetts law demanding, in schools, an air supply of 30 cu. ft. per occupant per min. may readily be met. This volume has become the commonly accepted allowance for such buildings in communities where the importance of ventilation is properly appreciated. The per capita space in churches is generally 300 cu. ft. or more. Schools have about 200 cu. ft., and ordinary halls frequently have only 100 cu. ft., or even less.
To secure the air supply stated in the preceding table in halls having 100 cu. ft. of space per occupant, an air change once in about 5 min. would be necessary, whereas the same allowance per capita in churches would give a 15-min. air change.
Temperature of Air Supply.-In factories and buildings where heating and not ventilation is the main item, the air is commonly changed every 15 to 20 min., the inlet temperature being roughly 130° to 140°. In ordinary schools, with a frequent air change, say every 6 or 8 min., the room may be kept at 70°, with a temperature at inlet of less than 100°, even in severe weather. The heat given off by the occupants of a room has a decided effect on the temperature, so much, i fact, that in theaters with little or no exposure the air must frequently be admitted 5o or 10° lower than the temperature at which it is desired to keep the room.
In determining the proper temperature of the air supply,
consideration must be given to the fact that an adult gives off per hour about 400 B. T. U.; a gas burner about 4,000 B. T. U.; an incandescent electric light about 1,600 B. T. U. The animal heat production of 80 persons is therefore, approximately, equivalent to 1 boiler H. P.
Splitting Air-Currents.-The proper manner of dividing a main current into three portions so as to supply branches or independent air flues is shown in Fig. 1. If the partitions A and B were absent, the greater part of the current would continue its motion until it was deflected by the side C. In that case, the flue D would receive a larger proportion of the current than either of the others, and the supply to the flue F would be quite insufficient.
The shoulders a, a against which the air impinges ought to be beveled off, instead of being left square, as in the figure. A proper supply of air to each flue may be insured by adjusting the partitions A and B in such a manner that they will
intercept the desired proportions of the main current before any change is made in its direction.
Fig. 2 shows the proper method of splitting up a main aircurrent into several smaller ones, and of subdividing these so as to supply a large number of vertical ducts. This illustration shows a centrifugal fan a set in a fan room that is formed by air-tight partitions p, p. The fan wheel is driven by a belt from the engine b. The air supply to the fan is taken in through the windows, as shown."
The fan forces the air into the main duct c, where it is split into three currents c1, d, and d1. The current d suppres two vertical flues e and f; di supplies g and h. The current c1 is then split into two unequal currents do and d; the latter supplies the flues and m, while the former supplies the flues i, j, and k.
Capacity of Disk Fans.-The fan capacities given in manufacturers catalogues may be attainable where fans are delivering freely, with no resistance due to the passage of air through ducts, registers, or heaters, or caused by wind pressur Such ratings, if attainable with free delivery, must generally be reduced at least 50 per cent. when the fans are connected with heaters and ducts; under this condition the catalogue horsepower must be about doubled.
The results of tests made on three well-known propeller type fans, each arranged to discharge through a pipe 90 in. long and of the same diameter as the fan, showed that V = .95 D3 R, where I' volume in cu. ft. per min., D = fan diameter in ft., R revolutions per min. This formula is based on free delivery, but when a disk fan is used to force air through a system of ducts, or through a heater, the formula used in computing the table "Approximate Air Delivery of Disk Fans" should be used. This formula is
V = .6 D3 R
The letters have the same meaning as in the previous formula. The horsepower given in the table is sufficient under the working conditions there stated.
A velocity of 1,000 ft. per min. in a duct of the size of the fan should be considered a maximum in heating and ventilating work when using disk or propeller fans. The delivery for speeds not given in the table will be proportional to the revolutions per minute.
Capacity of Cone Fans.-The capacities of cone fans that are given in the appended table are safe ones to figure on under ordinary working conditions with fans drawing air through heaters and forcing it through ducts and flues. Under favorable conditions, fans of this type will deliver at least 10 per cent. more air than is stated in the table. The maximum number of revolutions at which it is customary to run such fans in connection with heating and ventilating systems is such as to secure a peripheral speed of about 4,500 ft. per min. A tip speed of 3,700 ft. per min. is a fair average. Many engineers favor the Briggs type of cone fan encased in a steel plate housing, using two fans set back to back, with a plate of iron between, taking in air on both sides of the casing. The principal objection to this style of fan is its expensive construction.