Radiation of heat

Strahlungs-vermdgen, n. radiating power, -warms, /. heat of radiation radiant heat. 

The steam drum is maintained half-hill of water. The steam drum contains half water and half steam which are in equilibrium (both are at b.p. for a particular pressure). Water passes down the bottom of tubes forming the combustion chamber. The water which enters is at the boiling point but it requires latent heat to change to steam. The latent heat is supplied by heat of radiation from the flames. A steam-water mixture is thus returned to the drums. The steam is separated from the water. The water-tube boiler may be of horizontal straight tube design or bent tube design. 

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