These are usually made up of 1 or 1 1/4 inch wrought iron pipe, and may be hung on the walls of a room by means of hook plates or suspended overhead on hangers and rolls.

Fig. 39 shows a common form for schoolhouse and similar work; this coil is usually made of l 1/4-inch pipe screwed into "headers" or "branch tees" at the ends, and is hung on the wall just below the windows. This is known as a "branch coil." Fig. 40 shows a "trombone coil," which is commonly used when the pipes cannot turn a corner, and where the entire coil must be placed upon one side of the room. Fig. 41 is called a "miter coil," and is used under the same conditions as a trombone coil if there is room for the vertical portion. This form is not as pleasing in appearance as either of the other two and is only found in factories or shops where looks are of minor importance.

Fig. 40.

Overhead coils are usually of the "miter" form laid on the side and suspended about a foot from the ceiling; they are less efficient than when placed nearer the floor, as the warm air stays at the ceiling and the lower part of the room is likely to remain cold. They are only used when wall coils or radiators would be in the way of fixtures or when they would come below the water line of the boiler if placed near the floor. A coil should never be made up as shown in Fig. 42, as unequal expansion of the pipes would cause strains which would soon result in leaky joints. When steam is first turned on a coil it usually passes through a portion of the pipes first and heats them while the others remain cold and full of air. Therefore the coil must always be made up in such a way that each pipe shall have a certain amount of spring and may expand independently without bringing undue strains upon the others. Circulation coils should incline about 1 inch in 20 feet toward the return end in order to secure proper drainage and quietness of operation.

Fig. 41.