What causes natural draft in a boiler furnace?

Natural draft in a boiler furnace is caused by the principle of buoyancy, which is influenced by the density of gases. When hot flue gases rise, they are less dense than the cooler outside air. The warmer gases expand and rise through the stack, creating a difference in pressure that draws in cooler air from the surrounding environment to replace them. This movement creates a natural draft.

The correct response highlights that the gases in the stack have less mass than an equal column of outside air. This delineation in mass causes the hot gases to move upward, allowing cooler, denser air to flow into the furnace and maintain the combustion process. Understanding this dynamic is essential in ensuring efficient operation in boiler systems.