The marks compressed air leaves on glass come from three things: water (rings), oil (greasy film, iridescence) and particles (specks, scratches). The remedy combines a refrigerated dryer, or a desiccant dryer if the network is cold, filtration of particles and oil at the compressor outlet, and fine filters as close as possible to the stations that blow onto the glass. A 4-in-1 compressor such as the K 110 already brings together the dryer and the basic filtration.
For the complete sizing of a glass workshop, see our page on the compressor for glass processing and glazing.
Which mark points to which cause?
Look at the mark in raking light, then wipe it with a dry cloth and with a cloth soaked in alcohol. The diagnosis is often immediate:
| What you see | Probable cause | Where to look |
|---|---|---|
| Whitish rings, dried droplets | condensed water | dryer missing, faulty or overloaded; blocked drains |
| Greasy film that smears with a dry cloth, iridescence | oil aerosols or vapour | saturated filters, worn air/oil separator, no activated carbon |
| Dark specks, small scratches | particles, rust, glass dust | old steel network, no particle filter |
| Round marks after handling | dirty or damp suction cup | venturi air, condition of the suction cups |
When the marks appear mainly in the morning or in winter, it is almost always water. When they appear after a long period without servicing, think oil.
Where does the water in compressed air come from?
Ambient air contains water vapour. The compressor concentrates it: at 8 bar, each cubic metre of compressed air contains the water from nine cubic metres of ambient air. As it cools in the receiver and the pipework, that water condenses.
To give an order of magnitude: a compressor delivering 1.7 m³/min over an 8-hour day draws in about 816 m³ of air. On a hot, humid day, that air can carry around 15 g of water per cubic metre, or more than 12 litres of water to drain away. Our condensate volume tool does the calculation for your site.
A refrigerated dryer cools the air to a pressure dew point of around 3 °C, condenses the water and drains it away. As long as the pipe stays warmer than that dew point, nothing more condenses. If the network crosses a cold building or runs outdoors in winter, you need to go lower: that is the job of the desiccant dryer. Our guide to pressure dew point explains these figures.
Where does the oil come from?
Our screw compressors are oil-lubricated. The oil cools and seals the screw, then an air/oil separator recovers it. A very small amount remains in the air as aerosols and vapour: the outlet filtration captures it.
For glass, think in stages:
- At the compressor outlet: an air/oil separator in good condition and coalescing filters. On the K series, they are built in.
- At the sensitive station: fine filters as close as possible to the nozzle or machine, to stop anything that may have formed in the network.
- For air that touches a face to be bonded or silvered: an activated carbon stage, which captures oil vapour.
The quality class under ISO 8573-1 is defined with our technicians, according to what the air touches. Our article on oil in compressed air explains the role of each filter.
Which stations should you watch first?
- Blow-off after washing. The air drives the wash water off the edges and faces: if it brings its own water or oil, the glass comes out marked.
- The insulating glass line. Whatever settles on the inner faces stays sealed between the panes.
- Venturi vacuum lifters. The air passes through the venturi right next to the suction cup; damp or oily air fouls the lip and marks the glass.
- The cutting table's air cushion, when it is fed from the network: it blows directly under the sheet.
- Blow guns. They are used everywhere, often at the end of a long hose that collects water.
A step-by-step treatment example
A glass workshop with a cutting table, two edging machines, a washer, vacuum lifters and blow guns uses 920 l/min before margins. With 25% for safety then 20% for expansion, as in our configurator, that gives 920 × 1.25 × 1.2 = 1,380 l/min.
- Compressor: K 110, 1.7 m³/min at 8 bar, with built-in refrigerated dryer and filtration.
- Network: pipes in a non-corroding material, sloping towards low points fitted with an automatic condensate drain, with take-offs from the top.
- Sensitive stations: fine filter and activated carbon on the washer's blow-off line and on the supply to the vacuum lifters.
- If the network runs into a cold building: a desiccant dryer on that branch, or for the whole site.
If the workshop grows with an insulating glass line, the K 150 (2.2 m³/min) keeps the same layout.
The mistakes that bring the marks back
- An undersized dryer. A dryer rated for 1 m³/min behind a compressor delivering 1.7 lets water through in the hottest hours.
- Filters that are never changed. A saturated element captures nothing more and pulls the pressure down. Change it at the interval the manufacturer gives.
- Compressor servicing put off. First service at 500 hours, then every 1,500 hours. An air/oil separator at the end of its life sends more oil on to the filters: see the air/oil separator.
- Blocked drains. A clogged drain fills the receiver with water, and the water ends up in the network. See water in compressed air.
Our technicians can also measure the quality of your air on site as part of a compressed air audit. To replace or extend your installation, start with the configurator or send a quote request.
For your trade: Compressor for glass processing, glaziers and double glazing.