Moisture in an air line causes failures

Where the water comes from, and what it does downstream.

Moisture in a compressed air line comes from the ordinary humidity already present in the air a compressor draws in, water vapour that the hot, freshly compressed air can hold easily at first and can no longer hold once it cools travelling through the pipework, so the excess condenses into liquid water inside a system nobody designed to carry a liquid.

Hot air holds water that cool air cannot

Warm air can hold more water vapour than cold air, which is why a hot summer day feels muggy while a cold winter one rarely does. Compressing air heats it substantially, so the air leaving a compressor is hot and, for its temperature, fully able to carry whatever humidity it drew in from the room. Compression also packs that humidity tightly, since a seventh of the volume at six bar now holds all the water vapour that seven volumes of room air carried. As the air travels on through metal pipework at the cooler temperature of the rest of the building, it loses heat, and the surplus water has nowhere left to stay as vapour and condenses as droplets inside the pipe.

The quantities are larger than most people expect. Room air at comfortable humidity carries roughly ten grams of water in every cubic metre, and after compression and cooling back to room temperature most of that has to come out as liquid. A modest compressor drawing in about a cubic metre of room air each minute can therefore drop something like three to four litres of water over an eight-hour shift, every drop of it starting as invisible vapour.

A bathroom mirror the length of the building

A hot shower fills a bathroom with warm, humid air, and none of that moisture shows as liquid while it stays warm and airborne. When the air touches the mirror's cooler surface it loses heat at that surface and leaves a fogged film of condensation exactly where it met something cold. A compressed air line does the same along its entire length, warm humid air meeting the cooler metal of the pipe wall and shedding moisture the whole way out from the compressor. The resulting water has no towel to wipe it away and collects wherever gravity and the pipe's layout let it settle.

The fog on a mirror is never even, thickest near the edges beside the cold tile and thinnest where warmth from the shower lingers. A pipe run shows the same unevenness for the same reason, condensing most heavily wherever the metal runs coldest, such as an exposed length against an outside wall or a run sitting in a draught.

Rust, stripped seals and frozen valves

Liquid water pooling inside the system corrodes steel pipework and fittings from the inside, a slow process invisible from outside until a fitting fails or a valve starts sticking on rust particles carried in the flow. Those particles travel wherever the air travels and lodge in the narrowest passage they meet, very often the fine orifice of a flow control valve or the tight clearance around a cylinder seal, so a rusting elbow in one place can jam a component a long way off with no obvious connection between the two.

Water also washes away the thin lubricating film that keeps a cylinder's seals moving smoothly, since water and oil do not mix and a slug of water can strip that film off exactly the surfaces depending on it. In cold enough surroundings the water can freeze inside a pipe or valve and block the flow completely. A machine that runs flawlessly all summer and jams on the first cold morning of the year has usually suffered no mechanical fault at all, having simply met the one condition under which its accumulated moisture could no longer be ignored.

Drains, dryers and how dry is dry enough

Well-built systems treat condensation as an expected, ongoing fact. Low points in the pipework are deliberately sloped and fitted with drains to collect the water before it reaches sensitive equipment, and dryers pull moisture out of the air before it leaves the compressor room. Leaving these out only moves the water downstream to wherever the pipe is coldest or lowest, which is often exactly the valve or cylinder a factory can least afford to lose.

How much drying a system needs depends on the job. A hand tool running in short bursts on a construction site tolerates a trace of water with barely any consequence. A sensitive instrument, a paint-spraying line, or any process where the air touches the product cannot, since the water ends up in the paint finish or in the delicate mechanisms earlier articles in this set described as needing clean, dry air. Opening a drain valve and finding it dry is reassuring, and finding it full is the early warning of a problem that is still building.