Compressor Too Small for Sander and Spray Gun
A shop has a single-phase compressor marked 2 hp, 50 litre receiver, maximum 8 bar, free air delivery 190 litres per minute at 6 bar. The cut-in is 6 bar and cut-out 8 bar. A technician wants to run a 150 mm dual-action sander rated at 350 litres per minute at 6 bar and a gravity-feed spray gun rated at 280 litres per minute at 3 bar. The air line is a 15 m hose with 8 mm internal diameter and two quick couplers. When the sander runs, it stalls within 20 seconds and the hose jumps. When the gun is used, the gauge at the gun reads 2 bar and the pattern is coarse and dry. The shop has a 25 m hose with 10 mm internal diameter and a spare 100 litre receiver. A full respray of a small hatchback is due next morning. Only 230 V single-phase power is available. The compressor has no aftercooler or dryer, and the shop is humid.
1. Diagnose why the sander stalls and the spray pattern is coarse and dry. Support your answer with the air-consumption figures.
Read the worked answer
The compressor is the limiting factor, not the tools. The sander needs 350 L/min at 6 bar, but the compressor delivers only 190 L/min; the 50 L receiver stores roughly 100 L of free air between 8 and 6 bar, so it is exhausted in about 17 seconds at that demand. The 20-second stall is therefore expected. The 8 mm hose and two couplers add pressure drop, which is why the hose jumps and the gun sees only 2 bar instead of 3 bar. The gun needs 280 L/min, still above the compressor output, so the coarse dry pattern is a low-volume and low-pressure symptom, not a gun defect. A 100 L auxiliary receiver adds buffer volume but does not increase FAD. The humid air with no dryer will also put water into the paint.
2. What configuration would you set up with the equipment available, and what would you refuse to promise the customer?
Read the worked answer
Use the 10 mm hose, as short as possible, with the 100 L receiver close to the gun and a filter-regulator at the gun inlet. Drain the compressor and receiver before the job and fit a coalescing filter and water trap. Set the gun regulator to 3 bar while the trigger is held and adjust the pattern on a test card. This configuration may allow a panel-by-panel blow-in, but it cannot support a continuous full respray: with both tools the demand exceeds supply. The honest judgement is to hire or borrow a compressor with FAD of at least 400 L/min at 6 bar, or use a lower-consumption LVLP gun and accept slower work. I would not promise a full respray with the existing compressor; I would schedule the sander work first, then allow recovery time, and test each stage.
3. List the checks you would make before starting the respray, including air quality and pressure measurement.
Read the worked answer
Before starting, calculate available FAD against the highest simultaneous demand; do not run the sander and gun together. Check the compressor duty cycle and motor supply; a 2 hp single-phase unit may not tolerate continuous running. Fit the 10 mm hose, remove unnecessary couplers, and use a dedicated regulator at the gun. Because humidity is high, install a dryer or at least a coalescing filter and drain traps. Test the gun pattern and pressure with the trigger held. If pressure at the gun falls below the data-sheet figure, stop and upgrade the air supply rather than thinning the paint or raising the regulator to compensate.