What air dryer do I need for my compressor?

Every air compressor makes water. Compressing atmospheric air means compressing the moisture in it too. An air dryer stops that water reaching your tools, pipework and product. This guide explains how to choose the right one.

Why water forms in compressed air

Air always carries water vapour. To reach 7 bar, a compressor squeezes roughly eight volumes of air into one, and the vapour goes with it. Hot air leaving the compressor can hold it for a while, but as it cools in the aftercooler, receiver and pipework, the vapour condenses into liquid.

To give a sense of scale: on a mild UK day of around 15°C and 75% relative humidity, each cubic metre of air carries roughly 10 grams of water. A compressor delivering 3 m³/min draws in about 180 cubic metres an hour, so somewhere near 1.7 litres of water an hour goes in with it, or well over 10 litres in an eight hour shift. Warm, humid summer days give more.

Much of it drops out in the aftercooler and receiver. The rest leaves as saturated air and condenses further down the line. Left untreated it causes:

• Corrosion and scale inside steel pipework
• Lubrication washed out of air tools, cylinders and valves
• Spoiled paint and powder coat finishes
• Frozen lines and valves in cold weather
• Bacterial growth where air touches food, drink or medical products

Pressure dewpoint explained

Dryer performance is measured by pressure dewpoint (PDP): the temperature at which water vapour in the compressed air starts to condense, measured at line pressure. The lower the pressure dewpoint, the drier the air. Atmospheric dewpoint is a different measurement that looks much lower for the same air, so always compare dryers on PDP.

The practical rule is simple: the pressure dewpoint must be below the lowest temperature the air will meet anywhere in your system. If the pipework never drops below 10°C, a dewpoint of +3°C keeps it dry. If a pipe crosses a yard in January, it will not.

ISO 8573-1 moisture classes in plain terms

ISO 8573-1 is the international standard for compressed air purity. It grades solid particles, water and oil, written as three numbers in that order: “1:4:1” means particle class 1, water class 4 and oil class 1. A lower number means cleaner air. For water vapour, the 2010 edition sets these classes by pressure dewpoint:

Water class	Pressure dewpoint
Class 1	minus 70°C or lower
Class 2	minus 40°C or lower
Class 3	minus 20°C or lower
Class 4	+3°C or lower
Class 5	+7°C or lower
Class 6	+10°C or lower

The main types of compressed air dryer

Refrigerant dryers

A refrigerant dryer works like a fridge. It chills the air to a few degrees above freezing, drains off the condensed water, then warms the air back up. A typical pressure dewpoint is around +3°C, which is water class 4.

They are simple, reliable and cheap to run, so they are the standard choice for general factory and workshop air. Their limit is physics: they cannot dry below freezing without freezing the water inside them.

Desiccant dryers

A desiccant dryer passes air through a bed of adsorbent material that attracts water vapour. Most have twin towers: one dries while the other regenerates. Standard designs reach a pressure dewpoint of minus 40°C, with minus 70°C available where needed.

The trade off is running cost. Heatless designs use a share of the dried air to purge the regenerating tower, and your compressor has to make that air. Heated and blower purge designs cut that loss but cost more to buy. The desiccant also needs replacing periodically.

Membrane dryers

Membrane dryers use bundles of hollow fibres. Water vapour passes through the fibre walls and is carried away by a small sweep of dry air. They need no electricity but suit low flows, so they are mostly used at the point of use on a single machine or instrument.

Choosing a dryer by application

General workshop and factory air

For air tools, cylinders, valves and blow guns inside a heated building, a refrigerant dryer with good filtration is normally right.

Spray painting and finishing

Water and oil ruin finishes. A refrigerant dryer with high efficiency oil removal filters is the minimum, and many finishers add a point of use desiccant or membrane dryer at the booth. Air supplied to masks for breathing has its own separate legal requirements.

Food and drink

The usual UK reference is the BCAS Food Grade Compressed Air Code of Practice. Where air contacts food, or could, it equates to water class 2, a pressure dewpoint of minus 40°C, which means a desiccant dryer. Where air never touches the product, class 4 (+3°C) is generally accepted.

Pharmaceutical and laboratory

These sites work to their own quality specification, which in practice almost always means a desiccant dryer at minus 40°C or minus 70°C.

Outdoor pipework in winter

Pipes that run outside or through unheated buildings can drop below freezing in a British winter. A refrigerant dryer at +3°C cannot protect them. Use a desiccant dryer for the whole system, or at least for the branch serving the cold sections.

Sizing a dryer: the correction factors

Dryers are rated at reference conditions shown on the data sheet: a set working pressure, inlet temperature and ambient temperature. Your site will rarely match exactly, so manufacturers publish correction factors. In plain terms:

• Inlet temperature: hotter air carries more water, so a hot compressor room means a larger dryer.
• Ambient temperature: a refrigerant dryer rejects heat into the room. The warmer the room, the less it can handle.
• Working pressure: at lower pressure the same free air takes up more volume, so capacity falls. Higher pressure gives a little extra.
• Required dewpoint: asking for a lower dewpoint than the rating reduces usable capacity.

Size the dryer on the full output of the compressor feeding it, not your average demand, and allow some margin if another compressor may be added later.

Filtration before and after the dryer

A dryer removes water vapour. It is not designed for liquid water, oil aerosol or dirt. A coalescing filter upstream keeps a refrigerant dryer’s heat exchanger clean and stops oil coating and ruining desiccant. Desiccant dryers also need a dust filter afterwards to catch fine desiccant particles.

Reliable condensate drains on the receiver, filters and dryer are essential. Condensate from oil lubricated compressors contains oil, so it should pass through an oil water separator rather than straight to drain. Our compressed air treatment range covers dryers, filters, drains and separators.

Filter elements, condensers, valves and desiccant all wear or clog over time. We stock air dryer spares and can look after your dryer and filters as part of your air compressor servicing.

Frequently asked questions

My receiver has an automatic drain. Do I still need a dryer?

Usually, yes. The drain only removes water that has already condensed. Air leaving the receiver is still saturated and will drop more water as it cools in the pipework.

I have a refrigerant dryer but still find water in the lines. Why?

Common causes are an undersized dryer, a blocked condenser, a failed drain or a hot compressor room. Pipework colder than the dryer’s dewpoint will also collect water.

Should the dryer go before or after the receiver?

In most installations, after. The receiver cools the air, drops out liquid water and smooths the flow, giving the dryer a steadier load. There are exceptions, so discuss your layout with an engineer.

J&J Air Systems has supplied and serviced compressed air systems from High Wycombe since 1963. We are ISO 9001 and ISO 14001 certified, BCAS members and an authorised CompAir and Gardner Denver distributor, covering London and the Home Counties. Not sure which dryer you need? Get in touch.