This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The general arrangement of a galvanized iron casing and mixing damper is shown in Fig. 7. The cold-air duct is brought along the basement ceiling from the inlet window and connects with the cold-air chamber beneath the heater. The entering air passes up between the sections and rises through the register above, as shown by the arrows. When the mixing damper is in its lowest position all air reaching the register must pass through the heater, but if the damper is raised to the position shown, part of the air will pass by without going through the heater and the mixture entering through the register will be at a lower temperature than before. By changing the position of the damper the proportions of warm and cold air delivered to the room can be varied, thus regulating the temperature without diminishing, to any great extent, the quantity of air delivered. The objection to this form of damper is that there is a tendency for the air to enter the room before it is thoroughly mixed, that is, a stream of warm air will rise through one half of the register while cold air enters through the other. This is especially true if the connection between the damper and register is short. Fig. 8 shows a similar heater and mixing damper, with brick casing. Cold air is admitted to the large chamber below the heater and rises through the sections to the register as before. The action of the mixing damper is the same as already described. Several flues or registers may be connected with a stack of this form, each connection having its own mixing damper.
The arrangement shown in Fig. 9 is somewhat different and overcomes the objection noted in connection with Fig. 7 by substituting another. The mixing damper in this case is placed at the other end of the heater. When it is in its highest position all of the air must pass through the heater before reaching the register, but when partially lowered a part of the air passes over the heater and the result is a mixture of cold and warm air, in proportions depending upon the position of the dumper. As the layer of warm air in this case is below the cold air, it tends to rise through it, and a more thorough mixture is obtained than is possible with the damper shown in Fig. 8. One quite serious objection however to this form of damper is illustrated in Fig. 10. When the damper is nearly closed so that the greater part of the air enters above the heater, it has a tendency to fall between the sections, as shown by the arrows, and becoming heated rises again, so that it is impossible to deliver air to a room below a certain temperature. This peculiar action increases as the quantity of air admitted below the heater is diminished. When the inlet register is placed in the wall at some distance above the floor, as in schoolhouse work, a thorough mixture of air can be obtained by placing the heater so that the current of warm air will pass up the front of the flue and be discharged into the room through the lower part of the register. This is shown quite clearly in Fig. 11 where the current of warm air is represented by crooked arrows and the cold air by straight arrows. The two currents pass up the flue separately, but as soon as they are discharged through the register the warm air tends to rise, and the cold air to fall, with the result of a more or less complete mixture as shown.
It is often desirable to warm a room at times when ventilation is not necessary, as in the case of living rooms during the night, or for quick warming in the morning. A register and damper for air rotation should be provided in this case. Fig. 12 shows an arrangement for this purpose. When the damper is in the position shown, air will be taken from the room above and be warmed over and over, but by raising the damper, the supply will be taken from outside. Special care should be taken to make all mixing dampers tight against air leakage, else their advantages will be lost. They should work easily and close tightly against flanges covered with felt. They may be operated from the rooms above by means of chains passing over guide pulleys; special attachments should be provided for holding in any desired position.
American Radiator Company.
METHOD OF INDIRECT WARMING AND VENTILATION, SHOWING ROTARY CIRCULATION OF AIR.
American Radiator Company.