Turn-down ratio

The turn-down ratio (also modulation range, German Regelbereich) states how far a burner can be turned down from full load before the spray pattern degrades. It is written as a ratio against 1: a turn-down of 1:5 means the burner can run at one fifth of its maximum fuel output.

The limit is not set by the fuel valve but by the atomizer. A plain simplex atomizer loses its cone below roughly 5 bar and therefore reaches about 1:2. A spill-back atomizer keeps the pressure high and modulates the returned quantity instead, which gives 1:3 – 1:5. An air or steam atomizer decouples the atomizing energy from the fuel pressure altogether and reaches 1:5 – 1:10 with a constant spray angle.

Because the definition depends on what is still considered acceptable, always compare turn-down figures together with the spray angle tolerance and the emissions measured on the actual combustion head.