Boiler Natural Circulation Ratio & Head Calculator
Determine thermosiphon gravity driving pressure, circulation ratio ($CR = dot{m}_w / dot{m}_s$), riser void fraction, and waterwall DNB margin.
Boiler Loop Geometry & Pressure
Thermosiphon Head & Void Fraction
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Natural Circulation Watertube Boilers
Natural circulation boilers rely on natural gravity thermosiphon convection without circulating pumps. Large unheated downcomer pipes bring cold, dense water from the steam drum down to bottom furnace ring headers. As water rises through furnace waterwall tubes, intense radiant heat boils water into a bubbly two-phase steam-water mixture.
Thermosiphon Head & The 185 Bar Upper Limit
The driving pressure difference is purely gravitational: $$\Delta P_{driving} = g H \left(\rho_{downcomer} - \bar{\rho}_{riser}\right)$$ As drum pressure rises toward the critical pressure ($220.6\text{ bar}$), water density decreases while steam density increases. Above $185\text{ bar}$ ($2700\text{ psi}$), the density difference collapses, requiring forced circulation pumps or Once-Through supercritical designs.