How it Works

If steam is let into one end of a cylinder behind an air-tight but freely-moving piston, it will bombard the walls of the cylinder and the piston; and if the united push of the molecules on the one side of the latter is greater than the resistance on the other side opposing its motion, the piston must move. Having thus partly got their liberty, the molecules become less active and do not rush about so vigorously. The pressure on the piston decreases as it moves. But if the piston were driven back to its original position against the force of the steam, the molecular activity—that is, pressure—would be restored. We are here assuming that no heat has passed through the cylinder or piston and been radiated into the air; for any loss of heat means loss of energy, since heat is energy.

The combustion of fuel in a furnace causes the walls of the furnace to become hot, which means that the molecules of the substance forming the walls are thrown into violent agitation. If the walls are what are called “good conductors” of heat, they will transmit the agitation through them to any surrounding substance. In the case of the ordinary house stove, this is the air, which itself is agitated, or grows warm. A steam boiler has furnace walls surrounded by water, and its function is to transmit molecular movement (heat, or energy) through the furnace plates to the water until the point is reached when steam is generated. At atmospheric pressure—that is, if not confined in any way—steam would fill 1,610 times the space that its molecules occupied in their watery formation. If we seal up the boiler so that no escape is possible for the steam molecules, their motion becomes more and more rapid, and pressure is developed by their beating on the walls of the boiler. There is theoretically no limit to which the pressure may be raised, provided that sufficient fuel-combustion energy is transmitted to the vaporizing water.

To raise steam in large quantities we must employ a fuel that develops great heat in proportion to its weight, is readily procured, and is cheap. Coal fulfills all these conditions. Of the 800 million tons mined annually throughout the world, 400 million tons are burnt in the furnaces of steam boilers.