Rumford fireplace

The Rumford fireplace is a tall, shallow fireplace designed by Sir Benjamin Thompson, Count Rumford, an Anglo-American physicist best known for his investigations of heat. Its shallow, angled sides are designed to reflect heat into the room, and streamlined throat minimizes turbulence, thereby carrying away smoke with little loss of heated room air.

Rumford applied his knowledge of heat to the improvement of fireplaces in the 1790s. He made them smaller and shallower with widely angled covings so they would radiate better. And he streamlined the throat, or in his words "rounded off the breast" so as to "remove those local hindrances which forcibly prevent the smoke from following its natural tendency to go up the chimney..."

Rumford wrote two papers detailing his improvements on fireplaces in 1796 and 1798. He was well known and widely read in his lifetime and almost immediately in the 1790s his "Rumford fireplace" became state-of-the-art worldwide. Subsequent testing of Rumford's designs has shown that their efficiency would qualify them as clean-burning stoves.

The Rumford fireplace created a sensation in London when he introduced the idea of restricting the chimney opening to increase the updraught. He and his workers changed fireplaces by inserting bricks into the hearth to make the side walls angled and added a choke to the chimney to create a circulation of air inside the chimney. In the unmodified chimney, smoke rose up the chimney propelled only by buoyancy -- the heated gases from the fireplace being lighter than the surrounding air. This was especially ineffective when the fire is first lit and the temperature and density of the smoke was closer to the ambient air. Thanks to the discontinuity produced by the brick "smoke shelf", the flow of smoke gases up the chimney became detached from the outside wall at the lip of the shelf. This set up a counter-circulation of outside air which flowed DOWN the back side of the chimney, while a mixture of outside air and smoke flowed up the opposite side. The circulation inside the chimney, and above the smoke shelf, created a dynamic pressure in which the smoke gases were driven up one side of the chimney and cold air was pulled down the other. The air mixed with the rising smoke and increased the combined flow rising up the flue. It produced a circulating air-smoke flow, driving the smoke up into the chimney rather than lingering and often choking the residents. Many fashionable London houses were modified to his instructions, and became smoke-free as well as more efficient. Thompson became a celebrity when news of his success became widespread. In an age when fires were the principal source of heat, this simple alteration in the design of fireplaces was copied widely.

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