If the duct is of any considerable length or contains sharp bends, it should be made the full size of all the warm-air ducts. Adjusting dampers should be placed in the supply duct to each separate stack. If a trunk with two inlets is used, each inlet should be of size to furnish the full amount of air required, and should be pro vided with cloth checks for preventing an outward flow of air, as already described. The inlet windows should be provided with some form of damper or slide, outside of which should be placed a wire grating, backed by a netting of about i-inch mesh.
Very fair results may be obtained by simply letting the flues open into an unfinished attic, and depending upon leakage through the roof to carry away the foul air.
The sizes of flues may be made the reverse of the warm-air flues —that is, 11 square inches area per square foot of indirect radiation for rooms on the first floor, and 2 square inches for those on the second. This is because the velocity of flow will depend upon the height of flue, and will therefore be greater from the first floor. The flow of air through the vents will be slow at best, unless some means is provided for warming the air in the flue to a temperature above that of the room with which it connects.
The method of carrying up the outboard discharge beside a warm chimney is usually sufficient in dwelling-houses; but when it is desired to move larger quantities of air, a loop of steam pipe should be rtn inside the flue. This .hould be connected for drainage and air-venting as shown in Fig. 67. When vents are carried through the roof inde pendently, some form of protecting hood should be provided for keeping out the snow and rain.
A simple form is shown in Fig. 68. Flues carried outboard in this way should always be ex tended well above the ridges of adjacent roofs to prevent down drafts in windy weather.
For schoolhouse work we may assume average velocities through the vent flues, as follows: Where flue sizes are based on these velocities, it is well to guard against down drafts by placing an aspirating coil in the flue. A single row of pipes across the flue as shown in Fig. 69, is usually sufficient for this purpose when the flues are large and straight; otherwise, two rows should be provided. The slant height of the heater should be about twice the depth of the flue, so that the area between the pipes shall equal the free area of the flue.
Large vent flues of this kind should always be provided with dampers for closing at night, and for regulation during strong winds. Sometimes it is desired to move a given quantity of air through a flue which is already in place. Table XXIV shows what velocities may be obtained through flues of different heights, for varying ferences in temperature between the outside air and that in the flue.
Looking in Table XXIV, and following along the line opposite a 30-foot flue, we find that to obtain this velocity there must be a ence of 30 degrees between the air in the flue and the external air.
If the outside temperature is 60 degrees, then the air in the flue must be raised to 60 + 30 = 90 degrees. The air of the room being at 70 degrees, a rise of 20 degrees is necessary. So the problem resolves itself into the following: What amount of heating surface having an efficiency of 400 B. T. U. is necessary to raise 1,300 cubic feet of air per minute through 20 degrees? 1,300 cubic feet per minute = 1,300 X 60 = 78,000 per hour; and making use of our formula for "heat for ventilation," we have