Booth Pressure Drop with a Two-Gun Demand
A two-booth shop runs a 7.5 kW screw compressor rated 900 L/min free air delivery, feeding a 500 L receiver and a refrigerated dryer rated 1,000 L/min. Two painters are spraying waterborne basecoat with HVLP guns at 2.4 bar at the gun, each consuming about 380 L/min. The line runs 25 m of 3/4-inch pipe with six elbows and a 0.5 bar regulator at each booth. The painters report the second booth drops to 1.9 bar whenever both guns trigger at once, causing dry spray and blotchy metallic. The shop wants to add a third booth next quarter. No leaks were found on a soap test, and the dryer dew point reads 3 degrees C.
1. Diagnose the most likely cause of the pressure drop and state whether adding a third booth is safe on the current air supply. Show your reasoning with the numbers given.
Read the worked answer
Combined demand is 2 x 380 = 760 L/min at the guns, which is below the 900 L/min FAD of the compressor, so the compressor itself is not obviously undersized. However, FAD is measured at the compressor outlet, not at the gun. The 25 m of 3/4-inch pipe plus six elbows introduces friction loss, and each 0.5 bar regulator consumes pressure to hold its setpoint. When both guns trigger, the instantaneous draw can exceed steady demand because HVLP guns spike on trigger pull. The 500 L receiver buffers only briefly: at a 140 L/min shortfall it would lose about 0.3 bar in under a minute, which matches the reported drop to 1.9 bar.
The practical diagnosis is marginal distribution capacity, not compressor failure. The fix order should be: (1) verify actual gun inlet pressure with a gauge at the gun, not at the regulator; (2) upsize the trunk line or add a dedicated loop to the second booth; (3) confirm the dryer is not adding restriction. Adding a third booth would push peak demand toward 1,140 L/min, well beyond the 900 L/min FAD and the 1,000 L/min dryer rating, so it is not safe without a larger compressor, receiver, and dryer, plus a redesigned ring main.
2. The shop proposes simply raising the compressor cut-out pressure from 8 to 10 bar to solve the problem. Evaluate this proposal.
Read the worked answer
Raising cut-out pressure increases the stored energy in the receiver and can mask the symptom at the gun, but it does not increase FAD. The compressor still delivers roughly 900 L/min; the higher pressure just means the receiver drains from a higher starting point. It also raises the load on the motor, increases heat and condensate in the lines, and can push the refrigerated dryer beyond its design inlet conditions, risking carryover moisture into the guns.
Higher line pressure also changes gun behaviour: if the regulator at the gun is holding 2.4 bar, the extra upstream pressure is mostly wasted as heat and friction. The better approach is to reduce pressure drop across the distribution system so the regulator sees a stable supply. A 10 bar system may also exceed the rated pressure of some quick couplers and filters, creating a safety issue. Raising pressure is a temporary workaround, not a fix, and it should not be used to justify the third booth.