Water in compressed air: There are three phases of water in compressed air being liquid water, aerosol (mist) and vapour (gas). The most noticeable and easily removable are water and aerosol, which can be removed by high efficiency filtration together with refrigeration dryers.
Water vapour is more difficult to remove and requires the use of a desiccant dryer together with high efficiency filtration.
Removing water and aerosol can be satisfactory for general industry where corrosion damage is not an issue. If there are concerns about corrosion damage inside the compressed pipe work, together with ensuing rust and debris which may damage and contaminate downstream equipment and products, then desiccant quality air should be considered.
The general measurement of air dryness is dewpoint, which can cause confusion when choosing the required dryer for the system, particularly in generally warm countries such as Australia. It should be noted that to stop corrosion (and rust, etc) air dryness should be 2% RH (relative humidity) or better. A dryness of 2% RH is equivalent to -30°C pressure dewpoint. It is also relatively easy to calculate how much more water (litres) can be removed from a compressed air pipe work by alternative dryers.
Typically, atmospheric air passing through a compressor is one eighth its previous volume, yet still contains the same amount of contaminants (water and particulate).
Increasing pressure would normally cause moisture to condense out of the air, however, due to frictional heat the temperature of the air rises during the compression process and increases the compressed air's ability to hold water vapour. The water content of compressed air can be decreased using dryers.
Dewpoint is the microprocessor below which water vapour will condense to liquid water at given conditions. Lowering the dewpoint effectively means the system can endure much lower temperatures before water droplets begin to condense. Essentially, for every 11°C drop in compressed air temperature, the moisture holding capacity of air is reduced by 50%.
Therefore, drying prevents liquid water forming downstream where it can contaminate or damage the system causing operating problems, costly maintenance, and repair expenses.
When selecting a dryer it is necessary to determine the most cost-effective system suitable for the application as compressed air dryers vary in relation to their dewpoint, initial cost and ongoing maintenance requirements. The factors to be considered include:
- Calculating the required dewpoint temperature, which needs to be below the lowest ambient temperature a compressed air system will be exposed to. Take into account the location of air lines ie, located in front of open doors or windows, throughout air conditioned or unheated areas, running underground or between buildings;
- Determining which type of dryers will produce the required dewpoint;
- Consider initial and operating costs. The lower the dewpoint, the more expensive the dryer is to purchase and operate.
After selecting the type of drying system required, determine the actual conditions under which the dryer will be operating.
- Max flow capacity (L/s);
- Max acceptable pressure dewpoint (°C);
- Minimum inlet air pressure (kPa);
- Maximum and minimum inlet air temperature (°C);
- Maximum ambient or cooling water temperature (°C);
- Maximum allowable pressure drop (kPa).
There are five main types of dryers suitable for compressed air systems and each will perform differently and will be suited to different applications.
The refrigeration dryer cools the incoming air condensing moisture out. The dried air is then re-heated by the incoming air in the air-to-air exchanger.
This method of drying is very popular as it produces dewpoints, which are adequate for most duties in an energy efficient and reliable manner. The extra cost will typically be 5% over delivering standard after-cooled air, which will deliver pressure dewpoints of +3°C and remove an additional 28% of the initial water content.
The use of a pre-filter before the dryer can not only protect the dryer from internal contamination (especially oil carryover) but also reduce the amount of liquid water (and particulate which can block drains) the dryers have to deal with.
Refrigeration dryers use technologies that encounter few problems if properly installed and maintained. The types of problems that can affect the performance and hence energy consumption include:
- Internal contamination affects dewpoint - pre-filtration is recommended;
- High compressor delivery temperatures;
- High ambient temperatures;
- Poor installation/ventilation;
- Faulty drain traps allowing liquids downstream of the dryer;
- Loss of refrigerant.
Desiccant dryers (-20 to 70°C dewpoint)
Desiccant dryers work by feeding compressed air through an alternating duty section while a non-duty section is used for regeneration. These units are designed to remove vapour phase moisture. There are several types of desiccant systems, the main two types being heated and heatless dryers.
'No Purge' low energy use desiccant dryers have recently been introduced. These dryers work on similar principles to standard desiccant dryers, however, regeneration is carried out by heated ambient air being drawn over the non-duty section under vacuum.
All types usually require oil removal filters, water removal filters and dust removal filters (and an activated carbon absorber unit may also be used oil vapour and hydrocarbon odour removal).
Desiccant dryers are reliable, however, it is important that they are correctly sized, controlled and maintained, otherwise they have the potential to be high energy users.
It is important not to oversize desiccant dryers, particularly the heatless models, as the dryer has a fixed purge loss/purge cycle time.
Most dryers are now supplied with energy management control systems, such as dewpoint dependent switching. This makes desiccant dryers more efficient and, by using an energy management control system such as dewpoint switching, can lead to savings of up to 90%, depending on installation and usage.
These models have built-in dewpoint switching or in-bed sensing devices that alter the level of regeneration according to the load level.
Efficient operation of a desiccant dryer can also be hampered by:
- Poor cooling of inlet air;
- Poor pre-filtration causing liquid phase water and oil carryover;
- High peak loads causing desiccant bed fluidisation (twin tower type only);
- Faulty changeover valves causing continuous purge;
- Faulty controls causing poor or no regeneration of individual towers;
- Desiccant contamination by oil.
Deliquescent dryers (10 to 11°C dewpoint)
This is a simple form of chemical dryer in which the compressed air is passed over soluble material such as a bed of salt. The soluble material dissolves as it absorbs moisture.
These dryers are not regenerative. Deliquescent dryers to not lose any air volume (unless fitted with an automatic draining system - generally a timed drain) and have virtually no energy loss. The only extra energy consumption is required to overcome the pressure drop that occurs within the dryer.
Deliquescent dryers generally require oil/water removal filters and dust removal filters. While they are the least expensive dryer and are very energy efficient, deliquescent dryers can only produce dewpoints about 6°C below the inlet temperature. Deliquescent dryer efficiency is hampered by:
- The deliquescent material needs to be regularly replaced incurring higher labour and material costs;
- If the dissolved deliquescent material is not correctly drained it can cause pressure drop and blockage of the post-filter;
- Corrosion and Health and Safety issues also need to be considered when using this method.
These dryers diffuse the moisture from the compressed air to the atmosphere using hollow-fibre membranes. Membrane dryers are mostly used when low dewpoints are required in localised areas of a system. These dryers can prove very costly, particularly if operated at a light load.
The membranes are also susceptible to oil and dirt, which causes them to break down quickly.
As the structure is microscopic, it cannot be cleaned and has to be replaced. In an oil-free environment the membrane dryer should last for many years.
Membrane dryers cost as much to run as a heatless desiccant dryer without the advantage of a low pressure dewpoint. Certain types of membrane can reduce the oxygen content of the compressed air and, therefore, should not be used in breathing air applications.
Sorption dryers can only be used with an oil-free compressor. Compressed air travels via a sealed segment of a drum, which contains the drying medium. A very small motor slowly rotates the drum, drying the air. In the part of the drum not being used the drying medium is regenerated by hot air taken from a previous process, ie, by the compressor's waste heat.
The cost to provide air by this method is typically around 3% more than delivering after-cooled air. Limitations of sorption dryers are as follows:
- Units must be accurately matched to an individual compressor and cannot be shared by multiple compressors;
- The dewpoint output is directly related to the temperature of the cooling medium used;
- Sorption drum replacement is expensive.
Dewpoint issues
No matter which configuration of dryers is chosen there are some common problems that impact on their efficiency and energy costs. Often dryers have difficulty performing at expected efficiency due to poor ventilation and/or incorrect installation.
Furthermore, inlet temperatures are frequently elevated above those allowed for in the system design, causing poor dewpoints. A common cause of poor dewpoint is when different dryers are arranged in parallel - different makes of dryer, or dryers of different capacities - creating unbalanced flow between dryers.
The correct manifolding, piping and using either sonic nozzles or orifices can overcome this.
Reprinted with the permission of AMEI and the Australian Greenhouse Office.
