Fuel Oil Heating for Atomizing Viscosity

Heavy fuel oil will not atomize unless it is thin enough, and thin enough is a viscosity number rather than a temperature. Viscosity falls very steeply with heat and the relationship is log-log linear -- log10(log10(v + 0.7)) against log10(absolute temperature) -- so two points off the oil's data sheet define the line and the temperature for any target viscosity follows from it. That is what lets a plant set the heater for the grade actually delivered rather than for the grade the setpoint was chosen for, which matters because grade designations cover a wide range: a No. 6 running 7,000 SSU at 100 degF and 340 at 180 wants about 212 degF to reach 150 SSU, while a lighter delivery running 4,000 SSU at 100 degF and 200 at 180 reaches the same target at 191 degF -- twenty-one degrees lower. Run the lighter oil at the heavier oil's setpoint and it is hotter than it needs to be, which risks vapour lock in the line; run the heavier oil at the lighter one's 191 degF and it arrives at the burner near 251 SSU, well outside the atomizing band. Storage and pumping have their own, much looser limit -- around 4,000 SSU -- reached at a far lower temperature, and confusing the two is how an oil system ends up designed to pump oil it cannot burn. The tank heater keeps the oil movable; a separate final heater at the burner brings it to atomizing viscosity, and they are two setpoints for two different jobs. The symptom of getting it wrong is visible from the stack. Oil too viscous atomizes into large droplets that do not burn completely: smoke, soot, unburned carbon, fouled tubes, and in the worst case an uncontrolled fire in the furnace. Oil too hot can vaporize in the line and starve the burner, so the target is a band rather than a floor. This is an interpolation from two data points the user supplies and it requires the actual oil's data; a table value for a grade can be far from a specific delivery, which is the whole reason to run it per delivery. It does not select a burner, size the heater or the piping, or evaluate the atomizing steam or air requirement, and it does not address the flash point, which limits how hot oil may safely be heated and which the fire code and the oil's own data sheet govern. It does not address water and sediment in the oil, which cause more burner trouble than viscosity does, or sulphur, ash, and the emissions consequences of the fuel, and it does not evaluate combustion, excess air, or stack condition. The burner manufacturer's atomizing viscosity requirement, the oil supplier's data sheet, the adopted fire and mechanical codes, and the jurisdiction's boiler inspector govern.

ASTM D341 (Walther): log10(log10(v + 0.7)) = A - B log10(T absolute), fitted through two viscosity-temperature points from the oil's data sheet; the temperature for any target viscosity follows by interpolation on that line.

The ASTM D341 viscosity-temperature relation by name. Typical atomizing viscosity is about 100 to 150 SSU and typical pumping limits about 4,000 SSU; both are entered rather than assumed. The burner manufacturer's atomizing viscosity requirement, the oil supplier's data sheet, the adopted fire and mechanical codes, and the jurisdiction's boiler inspector govern.

A two-point interpolation on data the user reads off the oil's own sheet; no viscosity chart is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: v1_ssu, t1_f, v2_ssu, t2_f, target_ssu, pumping_limit_ssu, check_temp_f, slope_b, temp_for_target_f, temp_for_pumping_f, viscosity_at_check_ssu, setpoint_spread_f

• The oil's own data sheet is required a table value for a grade can be far from a deliveryASTM D341 practice
• The target is a band, not a floor oil too hot can vaporize in the line and starve the burnerburner manufacturer practice
• Flash point is not evaluated it limits how hot oil may safely be heatedthe adopted fire code and the oil data sheet