The curtain or sheet of liquid that cascades over an internal weir plate. In boiler systems, nappes are engineered inside deaerators and feedwater heaters to maximize the surface area of the water. Breaking the water into a thin, falling nappe allows steam to heat the liquid rapidly and scrub out dissolved oxygen before the water is pumped into the boiler.
If the flow rate drops too low, the nappe breaks up and you lose your deaeration efficiency.
A ragged or uneven nappe usually means the weir is out of level or fouled with scale.
Related terms
Deaerator
A pressurized feedwater heater that strips dissolved oxygen and carbon dioxide out of boiler feedwater with steam.
Feedwater
The water pumped into a steam boiler to replace the water boiled off, a blend of condensate and treated makeup.
Oxygen scavenger
A chemical fed to boiler feedwater that reacts with and removes the last traces of dissolved oxygen.