Another way of proportioning the air-flow in cases of this kind is to divide the hot-air chamber above the heater into sections, by means of galvanized-iron partitions, giving to each room its proper share of heating surface. If the cold-air supply is made sufficiently large, this arrangement is preferable to using adjusting dampers as described above. The partitions should be carried down the full depth of the heater between the sections, to secure the best results. The arrangement shown in Fig. 62 is somewhat different, and overcomes the objection noted in connection with Fig. 60, by stituting 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 damper. 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. 60. One quite serious objection, however, to this form of damper, is illustrated in Fig. 63. 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 perature. This peculiar action creases as the quantity of air ted below the heater is diminished. When the inlet register is placed in the wall ai some distance above the floor, as in schoolhouse work, a thorough mixture of air can be obtained by plac ing 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. 64, where the cur rent of warm air is represented by crooked arrows, and the cold air by straight ar rows. The two currents pass up the flue separate ly; but as soon as they are dis charged 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 is this case. Fig. 65 shows an arrange ment 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.
Warm-Air Flues. The required size of the warm-air flue between the heater and the register, depends first upon the difference in tem perature between the air in the flue and that of the room, and second, upon the height of the flue. In dwelling-houses, where the con
ditions are practically constant, it is customary to allow 2 square inches area for each square foot of radiation when the room is on the first floor, and 11 square inches for the second and third floors. In the case of hospitals, where a greater volume of air is required, these figures may be increased to 3 square inches for the first floor wards, and 2 square inches for those on the upper floors.
In schoolhouse work, it is more usual to calculate the size of flue from an assumed velocity of air-flow through it. This will vary greatly according to the outside temperature and the prevailing wind conditions. The following figures may be taken as average velocities obtained in practice, and may be used as a basis for calculating the required flue areas for the different stories of a school building: 1st floor, 280 feet per minute.
2nd " , 340 " " 3rd " , 400 " " It These velocities will be increased somewhat in cold and windy weather and will be reduced when the atmosphere is mild and damp.
Having assumed these velocities, and knowing the number of cubic feet of air to be delivered to the room per minute, we have only to divide this quanity by the assumed velocity, to obtain the required flue area in square feet.
Example. A schoolroom on the second floor is to have an air-supply of 2,000 cubic feet per minute. What will be the required flue area? 2000 _ 340 — 5.8 + sq. feet.
The velocities would be higher in the coldest weather, and dampers should be placed in the flues for throttling the air-supply when nec essary.
Cold-Mr Ducts. The cold-air ducts supplying heaters should be planned in a manner similar to that dbscribed for furnace heating. The air-inlet should be on the north or west side of the building; but this of course is not always possible. The method of having a large trunk line or duct with inlets on two or more sides of the building, should be carried out when possible. A cold-air room with large inlet windows, and ducts connecting with the heaters, makes a good arrangement for schoolhouse work. The inlet windows in this case should be provided with check-valves to prevent any outward flow of air. A detail of this arrangement is shown in Fig. 66.
This consists of a boxing around the window, extending from the floor to the ceiling. The front is sloped as shown, and is closed from the ceiling to a point below the bottom of the window. The remainder is open, and covered with a wire netting of about i-inch mesh; to this are fastened flaps or checks of gossamer cloth about 6 inches in width. These are hemmed on both edges and a stout wire is run through the upper hem which is fastened to the netting by means of small copper or soft iron wire. The checks allow the air to flow inward but close when there is any tendency for the current to reverse.
The area of the cold-air duct for any heater should be about three-fourths the total area of the warm-air ducts leading from it.