It is Russia that has nothing and Greece that has everything.
(A. Chekhov)

ell-type furnaces originated in Russia. They were developed in the 1920s for metallurgy by the Russian scientist Vladimir Yefimovich Grum-Grzhymaylo. His hydraulic theory of calculating flame furnaces has fully preserved its validity in relation to household stoves.

The operating principle of bell-type furnaces is as follows: hot smoke rises and fills out the bell closed at the top, forming what Grum-Grzhymaylo described as a “bagful of hot gasses”. Then smoke that is cooling down is ousted by hot smoke and descends to a point where it is removed via the smoke stack.

This means that each section of the furnace causes gases to move in their natural direction: currents of hot gas move upwards, and currents of cooling gas — downwards.

This principle can be graphically illustrated if you place a smoking cigarette under an upended glass. You will see hot smoke rise upwards, filling the volume of the glass. Upon reaching the upended bottom of the glass, the smoke cools down and starts to move downwards, leaving the glass.

For better heat exchange, the bell of the Herma furnace contains convective tubes and a small open stove for warming up food.