The Different Kinds of Heat.—Heat may be communicated by radiation, conduction, and convection. By radiation of heat is meant the process by which heat passes from a fire or other source of heat, through a vacuum, dry air or any other medium, without heating any of the media through which it passes, but only the bodies against which it finally impinges. The solid bodies (including ourselves) which are warmed by radiant heat, by a process of conduction then warm the surrounding air. This method is the nearest imitation of the natural warmth of the sun.
Conduction of Heat is the passage of heat from one particle to another, whether it be of a gas or solid. It is an extremely slow process when air is concerned, and may be practically ignored.
Convection of Heat is the process by which a gas or liquid actually carries the heat in itself from one part to another. The heated particles are relatively lighter, and ascend to the higher parts of a room, while colder and heavier particles descend, and are subjected to the same process. Heat can be carried by convection only by gases and liquids. It is quite possible, therefore, for a person to be cold in a room filled with warm air, if the walls, etc., are cold; and on the other hand, to feel comparatively warm in a room filled with cold air, if more heat is radiated from an open fire-place or the warm walls to his body than he radiates to his surroundings. The feeling of “draught” when sitting near a wall is sometimes caused by radiation of heat from the body to the colder wall. The ideal arrangement, were it practicable, would be to have cool air to breathe, but to be surrounded by warm walls, floors, and furniture. A room warmed by an open fire is more comfortable than a room warmed by hot air from a furnace, assuming the temperature of the air is the same in both instances, because the walls of the room are several degrees lower in temperature in the latter than in the former. For warming walls as well as the air high pressure steam pipes are more efficient than hot-water pipes. The great advantages of radiant heat are that—(1) it heats the body without appreciably heating the air; while at the same time (2) there is no possibility of impure gases being added to the air.
It has, however, considerable disadvantages. (1) It is costly, though its expense may be greatly diminished by a well-constructed fire-place. (2) It only acts on bodies near it to any useful extent. Its effect lessens as the square of the distance; thus, its warming effect at five feet distance, is twenty-five times less than at a distance of one foot. It is evident, therefore, that for long rooms, and for large assembly-rooms, a single source of radiant heat is quite inadequate. The immense loss of heat in our ordinary fire-places is slowly leading to their modification; and although it is probable that radiant heat will always be the favourite source of warmth in dwelling-houses, it will be used for larger buildings chiefly as an adjunct to convection of heat.
The different sources of heat are employed, either singly or combined, in the following methods of warming our dwellings and other buildings:—
- 1. Warming by the open grate.
- 2. Warming by closed stoves.
- 3. Warming by hot-water pipes.
- 4. Warming by steam in pipes.
- 5. Warming by hot air.
- 6. Warming by electricity.
Warming by the open Grate.—In the open fire-place radiation is the source of heat chiefly employed.
The position of the fire-place is important. It should not be on the external wall of the house, as thus a large proportion of heat is lost; but should be placed where the heat from the flue may be utilised in keeping up the temperature of the house.
The construction of a fire-place is commonly faulty in several respects. (1) The fire-place may be too far included in the wall, so that the heat at once passes up the chimney. (2) It may be composed chiefly of iron, which rapidly conducts away the heat, and does not furnish a surface for radiation. (3) The bars and bottom of the grate may be so arranged, that coal and cinders fall out in an incompletely burnt condition.
It has been estimated that with an ordinary fire-place, seven-eighths of the possible heat is lost, one-half being carried up the chimney with the smoke, one-quarter carried off in the ascending current of warm air, and one-eighth of the combustible matter remaining unconsumed, forming the solid matter of the smoke.