Radiators are made up of hollow sections of cast iron. The outer surfaces are so shaped as to give the greatest possible area or, as it is generally called, the greatest radiating surface. The castings for radiators are purposely made rough, and are often elaborately figured in pleasing designs, so as to present a larger radiating surface than would be the case if they were smoothly finished. The transfer of heat from hot metal surfaces to air is more efficient if the radiating surface is rough and the color is dark. Radiators are sometimes gilded for appearance, but practically they do not heat as well as if left ungilded. A cheap form of radiator for stores and shops is cast with innumerable projecting plugs or pins.
For large rooms a radiator may be made up of pieces of 1 in. or 1 1/4 in. pipe joined together by elbows and return bends. Such a radiator is called a box coil. A more common method of installing a direct radiation system is to run a group of 1 1/4 in. steam pipes along the side of a room and around the corner, by means of couplings to provide for expansion and contraction, and to connect the ends of the run into branch trees. The advantage of this arrangement is that it distributes the heat throughout the whole length of the room.
In a dwelling house the radiators are generally placed near the windows, since the cold air then reaches the radiators quickly. The direction of the flow of air along the floor is * towards the windows. Pipe coils in mills may be run along the walls on brackets under the windows. The cold air dropping downwards from the windows meets the current of warm air rising from the coil and is tempered and warmed at once. When it is desirable to have the hot-steam coils near the working space they are generally hung from the ceiling near the outer walls. This plan works well in a shop or mill where there are shafts with whirling pulleys and belts in constant motion. The air is churned by such motion and the heated air is brought downward and mixed with the cool. In office buildings and stores, coils placed near the ceiling are not effective, for there is nothing to cause circulation and the warm air naturally tends to remain at the top of the room.