If compressed air freezes at your car wash, water is still in the air and condenses wherever the pipes run through the cold. A refrigerated dryer is enough when the whole network stays in a heated room. As soon as a pipe runs outdoors, under a canopy or into an unheated gantry, you need a desiccant dryer, whose dew point goes well below zero.
Why does the air freeze at a car wash?
Every compressor produces water. The air it draws in carries vapour; once compressed and cooled, that vapour condenses. At a car wash, the air often leaves the plant room to reach the jet wash bays, the service points and the gantry. In winter it crosses areas a few degrees below zero. The water left in the air settles in the low points, then freezes.
The symptoms are well known: a bay that no longer foams, a programme valve stuck open, gantry brushes that no longer press on the car, a tyre inflator spitting water. Our page on compressors for car washes describes the full installation. Here we deal with frost.
The dew point rule
The pressure dew point is the temperature at which the water in compressed air starts to condense. The rule fits on one line: it must be lower than the coldest temperature the air will meet.
- A refrigerated dryer cools the air to a few degrees above zero. It cannot go lower, or the water would freeze in its own heat exchanger.
- A desiccant dryer passes the air through a desiccant. Its dew point goes well below zero.
A refrigerated dryer therefore cannot protect a pipe that drops below zero. This is the most common reason why car washes freeze even though they have a dryer.
Choosing step by step
Step 1: follow the air. Start at the compressor and follow every pipe to the bays, the gantry and the service points. Note every section that leaves the heated room: wall crossings, trenches, runs under a canopy, the gantry supply.
Step 2: note the lowest temperature. For each exposed section, take the coldest winter temperature at your site. If even one section drops below zero, the site needs a negative dew point.
Step 3: size on the compressor flow. The dryer must handle at least the full compressor flow. Example with an S 75 screw compressor, 7.5 kW: 1.1 m³/min at 8 bar, or 1,100 L/min. The dryer must be rated for at least 1,100 L/min, corrected for the inlet and ambient temperatures given on its data sheet.
Step 4: add the purge air. A desiccant dryer uses part of the air to regenerate its desiccant. The dryer data sheet gives that share. Add it to the site demand before checking that the compressor keeps up.
Three common situations
The whole network stays in the heated room
The bays are fed by indoor pipes and the gantry stands in a heated hall. A refrigerated dryer is enough. With an S 55 or S 75, that is the refrigerated dryer for 5.5 and 7.5 kW compressors. The K 75 4-in-1 already has one built in.
Some pipes run outdoors or under a canopy
This is typical of self-service bays under an open canopy. You need a negative dew point: a desiccant dryer with filtration upstream. Our technicians work out the layout with you, including behind a 4-in-1 that already dries the air once.
Small site with a piston compressor
The principle is the same. The dryer goes after the receiver. The more often the receiver is drained, the less liquid water reaches the dryer.
Pitfalls to avoid
- A dryer in the cold. A refrigerated dryer goes in a ventilated, frost-free room. Placed outdoors, it freezes itself.
- Forgotten drains. A receiver full of water sends liquid water into the dryer. An automatic condensate drain on the receiver and on every low point solves this. Our article on water in compressed air shows where to drain.
- Condensate. It contains oil, because our compressors are oil-lubricated. Route it through an oil/water separator before discharge, in line with local regulations.
- Air class. To set the target dew point and filtration, use our ISO 8573-1 guide and talk to our technicians.
Our article which air dryer to choose compares every technology. For your site, send us a sketch of the pipework and your compressor model with a quote request: we will size the dryer with you.
For your trade: Compressor for car washes.