DUCTS, FLUES, AND REGISTERS Areas of Ducts and Flues. The area of the cold-air connec tions with the benches or stacks of indirect radiators, are generally based on 1 to 1} square inches of area to each square foot of surface in the radiators.
The flues to the first floor should have 1i to 2 square inches area to each square foot of surface; those to the second floor, 1 If to 1 square inches; and those to floors above the second, 1 to 11 square inches per square foot of radiation.
The sides and back of warm-air flues in exposed walls should be protected from loss of heat by means of a nonconducting covering, preferably inch thick.
Flue Velocities. A fair allowance for flue velocities with indirect steam heating is 275 feet per minute for the first floor, 375 feet for the second, 425 feet for the third, and 475 for the fourth.
Registers. The net area of registers should be 10 to 25 per cent in excess of the area of the flue with which they are connected. The net area of a register is commonly taken as the gross area; that is, a 12 by 15-inch register would have a net area of 120 square inches.
Registers in shallow flues must either be of the convex pattern, or be set out on a moulding to avoid having the body project into the flue and cut off a portion of its area.
Aspirating Heaters and Coils. To cause a more rapid flow of air in ventilating flues in mild weather, steam coils or heaters are used. These should be placed as far below the top of the vent flue as possible, for the higher the column of heated air, the greater the chimney effect.
The smaller the flue in proportion to the volume of air to be handled, the larger should be the heater. If cast-iron indirect radiators are used, they may be rated to give off about 350 heat units per square foot per hour; coils may be rated to give off nearly double that number of heat units.
To illustrate how to compute the size of coil to be used, assume for example that 1,500 cubic feet per minute are to be removed through a ventilating flue, the air to be raised 10° in temperature. Then 1,500 cu. ft. per min. X 60 min. per hour X 10° rise in temp. 25.-1- sq. 55 heat units X 650 heat unite per hour per sq. ft. of coil ft. of coil required.
(The number 55 is the number of cubic feet of air at 70° that 1 heat unit will raise 1° F.) In order to work out important problems of this nature, it is necessary to consult a table giving flue velocities for different heights and for excesses of temperature of air in the flue over that out of doors. From such a table, knowing the height of the flue, its size, and the volume of air to be moved, it is readily seen how many degrees the air must be heated. The size of coil is then determined as above. The arrangement of an aspi rating heater in a flue is shown in Fig. 36.