The Heat of Compression: Why Compressed Air Is Hot and Wet

Clamp the thermocouple's tip against the discharge pipe where it leaves the compressor pump with a hose clip — not with plastic tape, which softens on a pipe this hot — and start with the tank empty. Log the temperature every 30 seconds as the tank fills, with the tank pressure beside it. Aim the infrared thermometer at the cylinder head fins at the same moments. The discharge pipe will climb well past boiling on most small piston compressors as the pressure rises — nowhere near the notebook's adiabatic ceiling, because the cylinder loses heat through its fins, but enough to burn. That is why the pipe to the tank is metal and often finned, and why a plastic hose must never be fitted there. Do not touch the head or the discharge pipe while it runs, or for several minutes after.

The litres per minute printed on a compressor are usually displacement — the swept volume of the piston — not the air it actually delivers. The pump-up test measures the real figure, the **free air delivery**. Read the tank volume off its plate. Drain the tank, close the drain and all outlets, and start the compressor. Time how long the gauge takes to climb from 4 bar to 6 bar — somewhere in the middle, well away from the cut-out. Free air delivered = tank volume × (6 − 4) ÷ 1.013 ÷ time. A 24 L tank that takes 40 seconds to go from 4 to 6 bar gains 47 litres of free air: 71 L/min. Expect a figure well below the displacement on the label. The tank air is warm when you read it; for a closer figure, repeat after it has cooled and compare.

Note the room humidity on the hygrometer before a working session. At the end, with the tank still under a little pressure, put a measuring jug under the drain and open it slowly: water and rusty sludge come out first, then air. Weigh the water and compare with the notebook's estimate for your delivery, run time and humidity. It will be lower — some of the water left as mist with the air, some is still on the tank walls — but it will be the same order. Then lay the air system out to deal with it: the main line falling slightly away from the tank towards a low point with a drain, and each tool taken off the TOP of the main so water running along the bottom does not go down to the tool.

This is a practice rung: the physics underneath every compressor in this batch, measured on the one most makers own. It carries no patent; the relations are those of ideal-gas thermodynamics. What it explains in the machines before it: the **liquid-ring pump** stays cool because its ring swallows the heat as fast as it is made — close to the isothermal floor. The **oil-flooded screw** does the same with oil. The **scroll** and the dry **screw** compress with little cooling and run hot. And the **air-operated diaphragm pump** throws its compressed air away through the exhaust, cold, which is why it can ice up. **Honest limits of the numbers.** The temperatures are ideal adiabatic ceilings; real machines lose heat through their walls and run cooler. The condensate figures assume the air is cooled back to intake temperature — warm air in the tank holds more, and drops it later, downstream, in the pipes.