Where the water comes from
Ordinary air contains water vapor. When a compressor squeezes that air into a smaller volume, the vapor becomes concentrated, and the air leaves hot and fully loaded with moisture. As it cools in the receiver and pipes, the air can no longer hold all that vapor, and liquid water condenses out.
The amount of moisture depends on the temperature and humidity of the intake air. A compressor drawing in warm, humid air will generate a lot more condensate than one drawing in cool, dry air. This is why problems tend to get worse in summer and in humid climates.
Why moisture hurts
Water in a compressed air system is not just a nuisance. It commonly leads to:
- Corrosion of pipes, receivers, valves and tools, with rust scale then carried downstream.
- Wear in pneumatic equipment, as water washes away lubricant from cylinders and tool motors.
- Sticking or failing valves, including solenoid valves.
- Frozen lines in cold areas, which block flow.
- Poor finishes in paint, coating and sandblasting work.
- Spoiled or contaminated products in food, pharmaceutical and electronics processes.
- Instrument errors, since many control devices run on air.
First steps before a dryer
A dryer is the main tool, but a good system removes bulk water earlier:
- Aftercooler. Cools the hot discharge air straight after compression, so a large portion of the water condenses and can be removed.
- Moisture separator. Catches the liquid droplets.
- Receiver with a drain. Provides more cooling and a place for water to settle.
- Drain traps. Automatic drains at low points and at the bottom of each separator and filter keep water from building up.
Even with these, the air is still saturated with vapor, and any further cooling in the pipes will form more liquid. That is where dryers come in.
Dew point
Dryer performance is described by the pressure dew point. It is the temperature at which the air, at line pressure, becomes saturated and begins to form liquid water. A lower dew point means drier air. If the pipes in the plant never get colder than the dew point, water should not condense inside them.
For that reason, the target dew point is chosen with the coldest temperature the air will encounter in mind, including outdoor piping or cold storage areas. Allow a safety margin.
Refrigerated dryers
A refrigerated dryer works like a small refrigeration system. The air is cooled in a heat exchanger to a temperature a few degrees above freezing, which makes most of the water vapor condense. The water is separated and drained, and the air is then reheated slightly so the outer pipe does not sweat.
Features:
- Reasonably low running cost and relatively simple operation.
- A typical dew point somewhat above freezing, so it suits general plant air in heated buildings.
- Not suitable where pipes pass through freezing areas, since the remaining moisture can freeze.
- Performance falls if the intake air or ambient temperature is very high, and heat exchangers need clean surfaces and free airflow.
Some designs adjust their cooling to follow demand, which can save energy at part load.
Desiccant dryers
A desiccant dryer passes the air over a drying material that adsorbs water vapor, a bit like the packets sold to keep products dry. Because the material holds water, it must be regenerated. Most industrial units use two towers: one dries the air while the other is being regenerated, then they swap.
Common regeneration methods:
- Heatless. A portion of dry compressed air is used to purge the wet tower. This is simple but uses a part of the compressed air.
- Heated. An electric heater or heated air helps drive off moisture, using less purge air.
- Blower or heat-of-compression styles. These use other sources of heat or air to regenerate the material.
Desiccant dryers can reach very low dew points, well below freezing, so they are used for outdoor lines, instrument air, painting, and sensitive processes. They usually cost more to buy and operate, and the drying material needs eventual replacement. The heatless type in particular may use significant air for purging, which matters for overall energy use.
Other types
Membrane dryers pass air through special fibers that let water vapor escape. They suit small, point-of-use needs. Deliquescent dryers use a material that dissolves as it absorbs water, and are less common in modern plants. Each has trade-offs in dew point, cost and maintenance.
Choosing a dryer
Questions that guide the choice include:
- What dew point does the process really need?
- How cold will the pipes and surroundings get?
- What flow and pressure must the dryer handle, including peak demand?
- What ambient and intake air temperatures apply?
- How much running cost and maintenance is acceptable?
Many plants dry the whole system to a moderate level with a refrigerated dryer, then add small desiccant units only where very dry air is essential. This avoids paying for very dry air everywhere.
Placement
Dryers are commonly installed after the wet receiver and before the dry receiver and filters. Ahead of a desiccant dryer, a coalescing filter is usually fitted to protect the drying material from oil. After it, a particulate filter catches dust released by the desiccant. Refer to the manufacturer’s installation diagram.
Upkeep
Keep the condensate drains working, clean or replace filters on schedule, keep heat exchanger surfaces and cooling fans clean, and replace desiccant at the interval the maker recommends. Watch dew point readings or indicators if the dryer has them. Rising pressure drop across a dryer or filter is a sign of clogging.
Compressed air carries stored energy, and servicing should follow lockout procedures and the equipment manual. This is general information and not engineering advice.