Airless Spray Tip Size, Fan Width, Output, and Coverage Rate

The number is two facts in three digits and both matter. FAN WIDTH is the first digit doubled, measured at 12 inches from the surface, and it should be matched to the work: a wide fan on a narrow surface wastes most of the material and a narrow fan on a wall is slow. ORIFICE is the last two digits in thousandths, and it sets flow, which goes as the SQUARE of the orifice -- a .021 tip passes nearly twice what a .015 does at the same pressure. Orifice also has to be matched to the coating's viscosity, which is why a lacquer tip will not spray block filler. Pressure is the weaker lever because it enters as a square root, so raising it does far less to flow than changing the tip does, and it is the wrong knob to reach for. The output that turns all this into a decision is travel speed. A 517 tip at 2,000 psi laying 6 wet mils flows about 0.40 gpm, which covers 106 square feet a minute -- that is why airless exists -- and demands a gun speed of 128 feet a minute, about a foot and a half a second, which is brisk but entirely normal. Now try to build the same 6 mils with a .021 tip: flow goes to 0.61 gpm, coverage to 162 square feet a minute, and the required travel speed to 195 feet a minute, which is faster than most people can move a gun smoothly. That tip wants a heavier film or a wider fan, and forcing it produces runs. Flow and geometry only, from a calibration against a tip of known output. Real flow depends on the coating's viscosity and temperature, on pump condition and tip wear -- a worn tip passes more material through a wider, softer fan, which is how a job quietly starts running heavy -- and on the specific pump's performance at pressure. Transfer efficiency is not modeled and it is not high: overspray, bounce-back, and the light edges of the fan pattern all mean the material leaving the tip exceeds the material on the wall, and the fifty percent overlap the trade uses is what evens the pattern out. The coating manufacturer's recommended tip range and wet film thickness, the sprayer manufacturer's data, and the safety requirements for high-pressure airless equipment -- which injects fluid through skin -- govern.

fan width in = floor(tip / 100) x 2; orifice in = (tip mod 100) / 1000; flow = ref flow x (orifice / ref orifice)^2 x sqrt(psi / ref psi); coverage rate = flow x 1604.1667 / wet mils; travel speed = coverage rate / (fan width / 12).

The airless tip-number convention (fan width at 12 in is the first digit doubled, orifice is the last two digits in thousandths), by name; flow scaled from a calibration tip as the square of the orifice ratio times the square root of the pressure ratio, and coverage from the exact 1,604.17 sq ft per gallon at one mil. Flow and geometry only. The coating manufacturer's recommended tip range, the sprayer manufacturer's data, and airless injection-injury safety requirements govern.

The tip-number convention is public trade practice and the coverage constant is exact arithmetic (231 cubic in per gallon / 0.001 in / 144); the calibration tip's flow is a user-entered manufacturer figure.

Estimate. AHJ and licensed professional govern.

Field names used by the API: tip_number, pressure_psi, wet_film_mils, ref_orifice_in, ref_flow_gpm, ref_pressure_psi, fan_width_in, orifice_in, flow_gpm, coverage_rate_sqft_min, travel_speed_fpm

• Orifice squared, pressure square-rooted the tip is the strong lever and pressure is the weak oneairless spray practice
• Travel speed is the real constraint a tip that demands more speed than a person can deliver produces runsairless spray practice
• Tip wear runs a job heavy a worn tip passes more material through a wider, softer fansprayer manufacturer data