Dry Air and Cold Beer
An introduction to compressed air drying by David Ewan, Technical Services manager, BOGE UK
Air naturally contains moisture. Two factors to consider are how much moisture the air can potentially contain, and how much moisture the air actually contains. The capability of air to contain this moisture is dependent on the temperature and the pressure of the air. As air gets hotter, it can contain more moisture. As pressure increases, the ability of air to contain moisture decreases.
The ratio of how much moisture is contained in the air to how much moisture the air is capable of containing is called Relative Humidity (%rh).
When we compress air, the atmospheric air is drawn into the system at ambient temperature and pressure, with ambient relative humidity. During the compression process the air is heated to a high temperature which would allow it to potentially hold much more moisture, however, the pressure is also increased considerably, reducing the air’s ability to contain moisture. The compressor’s cooler then brings the air temperature down to only 10 to 15°C hotter than the starting temperature, with the overall result that the air can hold less moisture than when it started.
The Dew Point (°C) refers to the temperature we would have to cool the air down to in order to reduce the moisture capacity of the air down to a level equal to the actual moisture content of the air, or in simple terms, the temperature below which moisture would fall out of the air as liquid water.
The beer test
On a nice summer day, when the weather is warm and feels dry, the dew point is usually around 4-5°C. You can see this by taking a properly cooled bottle of beer from a fridge, or ordering a pint from the bar and seeing the condensation form on the outside of the glass. This is because the cold glass is cooling down the air touching it to a below the dew point, causing moisture to fall out of the air and form on the glass as liquid water.
If we are discussing air at anything other than ambient pressure, then dew point is referred to as Pressure Dew Point (°C).
If we come back to our compressed air system, after going through the compression process, and the cooler, our air will be at or near to, 100% relative humidity. That means it will be at its maximum capacity for holding moisture; if we draw air in at 10°C and it comes out of the compressor at 25°C, then the Pressure Dew Point will be about 25°C. If we cool the compressed air further, say by allowing it to dwell in a receiver momentarily, we will be reducing the temperature below that of the Pressure Dew Point meaning moisture will drop out of the air in the form of liquid water. The further we travel through the pipework from the compressor, the more likely we are to lose heat to the surroundings and reduce the temperature of the compressed air. Sections of external pipework exposed to low temperatures will also cool the compressed air further, resulting in liquid water forming in the pipework and carried along with the compressed air flow.
Preventing water build up
We can’t really control how much moisture is in the air that we draw into our compressors beyond simple things. This could be locating the compressor away from objects and areas such as cooling towers and water courses. So the best thing to do to prevent liquid water from building up in our compressed air system and ending up in our processes, is to control where the moisture drops out of the air.
Liquid water can be removed from the air flow by separators, and generally removed from pipework systems by correctly placed drains and traps.
Moisture can be removed from compressed air through the use of dryers in three different ways: Diffusion, Condensation, and Ab/Adsorption.
Dryers which operate using diffusion are called Membrane dryers. These work by moisture diffusing through membrane tubes and being carried away by a small amount of exhausted air which is reduced to a lower pressure so it can hold more moisture than the air we are trying to dry. These are normally used for point of use drying for critical processes that need dryer air than other items on site. They are not typically suited to drying the main volume of air for a site due to the air lost in purging.
Refrigeration dryers operate using the condensation method. The compressed air flow runs through the refrigeration unit and is cooled down to a low temperature, below that of the air’s pressure dew point, resulting in the moisture in the air condensing into liquid water to be removed by a drain. The outgoing cold, dry air is then warmed back up by the incoming wet air, with the double purpose of increasing the dry air temperature above the pressure dew point, and to pre cool the incoming wet air. Cleverly designed, energy efficient refrigerant dryers also use the cold condensed liquid water to cool the incoming wet air, saving on the energy required to run the refrigerant system. These are generally installed as a first step on most sites and are well suited to drying the full volume of air generated.
Absorption dryers work by passing the air through a bed of chemical drying agent which picks up the moisture from the air. Over time the drying agent loses its effectiveness and periodic replacement is required. These dryers are not common in mainstream applications as other methods provide better performance without the drawbacks associated with the corrosive drying agents.
Adsorption dryers work by passing air though a bed of porous desiccant, where adhesion causes the moisture to stick the desiccant at a molecular level (no chemical reactions). When the desiccant cannot take on any more moisture, it is regenerated using a small amount of the dry exit air, recirculated through the bed at low pressure to pick up large amounts of moisture before being vented off. Adsorption dryers will always have an online and offline bed to allow one to regenerate while the other dries the process air. Regeneration can be improved through the use of vacuum and heating systems to reach lower pressure dew points. Adsorption dryers are used when air required on site is required at a better level of dryness than would be achieved with a refrigerant dryer.
The dryness of compressed air is often specified as part of the class of compressed air (this can also include solid particulates and oil, but that’s a whole other conversation).
Compressed air quality is defined under ISO8573, with the different classes for moisture level as follows:
When we look at choosing a dryer for an application the dryers will generally be listed as meeting a specific class, with the different types of dryers falling into different classes depending on the technology and the duty (an Adsorption dryer might provide class 2 (from -40°C to -70°C) at one flow rate and pressure, and class three (+3°C to -20°C) under different conditions).
As a very rough rule of thumb, we can consider the following guidelines for the different technologies:
Membrane dryers vary performance based on how much of the compressed air is vented, from Class 6 at around 10% purge, to Class 3 at around 30% purge.
Refrigerant dryers can normally be expected to provide moisture removal to Class 5.
Adsorption dryers, when properly selected and operated can achieve Class 1 or better, but will normally be used to meet Class 2 standards.
Different types of dryers can be used in series to reduce the load and maintenance requirements of downstream dryers. It is common to install a refrigerant dryer before a desiccant dryer, as this will reduce the amount of moisture reaching the desiccant and so will reduce maintenance requirements and air lost in regeneration.
Dave Ewan, Technical Services Manager, Boge Compressors UK