Glossary
Viscosity
Definition
Resistance of the fuel to flow, quoted in centistokes (cSt); it sets how much of the pump pressure is lost in the oil train.
Viscosity describes how easily the fuel flows. It is normally quoted as kinematic viscosity in centistokes (cSt) at a stated temperature, because heavy fuel oil at 20 °C can be hundreds of times more viscous than at 120 °C.
For an atomizer this matters twice. First, the pressure the nozzle actually sees is the pump pressure minus the losses in the line, and those losses grow with viscosity. Second, a more viscous fuel needs more energy to break into droplets, which shifts the spray angle and the droplet size.
Fluid Control Instruments Ltd publishes design values at a fuel viscosity of 5 cSt, and chose the internal geometry of the simplex atomizer specifically so that the flow rate depends as little as possible on oil temperature. When requesting a quotation, always state density, viscosity and operating temperature of the fuel.