Compressed air dryer

Compressed air dryers are special types of filter systems that are specifically designed to remove the water that is inherent in compressed air. The compression of air raises its temperature and concentrates atmospheric contaminants, primarily water vapor, as resulting in air with elevated temperature and 100% relative humidity. As the compressed air cools down, water vapor condenses into the tank, pipes, hoses and tools connected downstream from the compressor which may be damaging. Therefore water vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive industrial processes.
Excessive liquid and condensing water in the air stream can be extremely damaging to equipment, tools and processes that rely on compressed air.
For example, water can

cause corrosion in the tank and piping made out of steel that may compromise its integrity
wash out lubricating oils from pneumatic tools
emulsify with the grease used in cylinders
clump blasting media and fog painted surfaces.

Therefore, it is desirable to remove condensing moisture from the air stream to prevent damage to equipment, air tools and processes.
Next to these damage mechanisms, in outdoor situations, water can accumulate and then freeze, leading to failure of components, e.g. braking systems.
There are various types of compressed air dryers. These dryers generally fall into two different categories: primary, which includes coalescing, refrigerated, and deliquescent; and secondary, which includes desiccant, absorption, and membrane. Their performance characteristics are typically defined by flow rate in standard cubic feet per minute and dew point expressed as a temperature.

Applications

Water contamination is an inherent byproduct of compressing atmospheric air since the atmospheric air contains a large amount of water existing in the gaseous form. Because of this phenomenon, compressed air systems usually benefit from the addition of a compressed air dryer located on the output of the air compressor and/or at various locations throughout the distribution system. In most cases, the output of the compressor is processed through a primary dryer or system dryer. In cases where higher quality air is required the output of the primary dryer is further processed through a secondary dryer or polishing dryer.

Characteristics

A regenerative desiccant dryer typically delivers a dew point of between −40 °C and −73 °C
A refrigerated dryer delivers a dew point not lower than approximately 2 °C
A deliquescent dryer delivers a dew point suppression that fluctuates with air temperature. Typically this suppression is 11 °C below the compressed air temperature.
Coalescing filters

Coalescing filters aren’t actually filters, rather they operate more as a consolidation element. The idea behind these devices is that the compressed air is forced through gaps or porosity within an otherwise solid element. These gaps and/or porosity are microscopic and small enough that water vapor wets to the internal surfaces. The liquid water that forms during the wetting process is forced through the media and drips down into a trap. The dry air travels up to and out of the discharge port.
There are two basic types of coalescing elements. The first type uses a cast material that is dominated with an internal microscopic lattice. The air is forced to flow through the lattice which, in turn, allows the water vapor to wet to the internal surfaces. The second type is generally referred to as a stacked plate element. In this case, fine discs are stacked with microscopic gaps between them. The air is forced to flow through the gaps which, in turn, allows the water vapor to wet to the internal surfaces.
In principle, a coalescing filter is an ideal way to separate water from the compressed air stream. Practically speaking, this is not the case. Coalescing elements are extremely sensitive to oil and particulate contamination and therefore would be better placed as a second stage dryer. However, the dew point performance of a coalescing filter places it in the primary category of compressed air dryers. In order to use coalescing filters as primary dryers, they are typically set-up in pairs. The first filter has an element with larger gaps that are designed to remove oil from the stream. The second filter uses a finer element that is intended to remove water vapor. Because of the sensitivity of the elements, coalescing filters are not particularly common. One area where these filters have found acceptance is with dental compressors. The way that dental compressors are designed and used makes a two-stage coalescing filter an almost ideal solution for water contamination in these systems.

Refrigerated dryers

Refrigerated dryers are the most common type of compressed air dryer. They remove water from the air stream by cooling the air to approximately 3 °C and effectively condensing out the moisture in a controlled environment. 3 °C is the realistic lower limit for a refrigerated dryer because a lower temperature runs the risk of freezing the separated water. They are typically specified as primary dryers and generally produce air quality that is appropriate for approximately 95% of all compressed air applications.
Refrigerated dryers employ two heat exchangers, one for air-to-air and one for air-to-refrigeration. However, there is also a single TRISAB heat exchanger that combines both functions. The compressors used in this type of dryer are usually of the hermetic type and the most common gas used is R-134a and R-410a for smaller air dryers up to 100 cfm. Older and larger dryers still use R-22 and R-404a refrigerants. The goal of having two heat exchangers is that the cold outgoing air cools down the hot incoming air and reduces the size of compressor required. At the same time the increase in the temperature of outgoing air prevents re-condensation.
Some manufacturers produce "cycling dryers". These store a cold mass that cools the air when the compressor is OFF. When the refrigeration compressor runs, the large mass takes much longer to cool, so the compressor runs longer, and stays OFF longer. These units operate at lower dew points, typically in the 1.5 °C - 4.5 °C range. When selected with the optional "cold coalescing filter", these units can deliver compressed air with lower dew points. Non-cycling dryers use a hot gas by pass valve to prevent the dryer from icing up.
Some manufacturers produce "cold coalescing filters" that are positioned inside of the air dryer at the point of the lowest air temperature.
Refrigerated dryers are generally manufactured in one of two different ways, Freon based units and Joule-Thomson based units.

Freon-based refrigerated compressed-air dryers

These dryers derive their cooling from a closed cycle refrigeration system based around one of three commercial refrigerants, R-22, R-134a or R410a. The refrigeration system these dryers use is similar to home and commercial air conditioning systems. The schematic shown to the right illustrates a typical Freon-based refrigerated compressed air dryer.
Freon-based refrigerated compressed air dryers generally consist of a heat exchanger that is similar to a water cooled after cooler. Instead of using water as the coolant, liquid CFC fills the shell of the heat exchanger. The liquid CFC is maintained at a pressure that allows it to boil at 3 °C. After the CFC boils, the vapor is drawn through the suction line into a compressor, which compresses the CFC to a high pressure and high temperature. The high pressure/temperature CFC is cooled in the condenser and relaxes into its liquid state. The liquid is reintroduced into the heat exchanger via the metering device and a closed refrigeration cycle is formed. When the compressed air passes through the heat exchanger, it is cooled to the temperature of the boiling CFC. As the compressed air is cooled, it loses its ability to retain moisture and the water vapor condenses onto the inside of the exchanger tube.
Variations on this basic design include units equipped with reheating exchangers, which are intended to improve efficiency. In these cases, the cooled compressed air is reheated by the incoming air.
Oil and water vapors in compressed air must have cooled to their dew point before the coalescing filter can operate effectively. Refrigerated compressed air dryers typically have a Remaining Oil Content of 6 mg/m3.
Refrigerated compressed air dryers with internal cold coalescing filters are rated to leave the remaining oil content as low as 0.008 mg/m3, which is far less than coalescing filters that are downstream of air dryers, because the cooled compressed air is reheated by the incoming air. Larger refrigerant dryers have air-to-air heat exchanger between warm incoming air and chilled outgoing air. Cooled oil and water fogs coalesce much better in a coalescing filter at the cold temperatures than one in the warmer oil and water vapors located downstream of the air dryer air-to-air heat exchanger.

Compressed Air Coalescing Filter Performance

Type	Location	PPM	mg/m3	Temperature, typical
Cold	Inside	0.006	<0.01	36 to 42 deg F	0.01
General Purpose	After	0.4	0.5	75 to 100 deg F	1.0
Standard	After	3.6	4.0	75 to 100 deg F	3.0

Coalescing Filters collect liquids and aerosols, not vapors, see the Mechanical Coalescers section at Coalescer. In the table above, the location means where the Coalescing Filter is relative to Refrigerated Air Dryer. 1 mg/m3 is a weight of oil in a volume of air and is approximately equal to 0.83 ppm by weight.
High temperature dryers are equipped with an additional pre-cooler that removes excess heat via a forced air system. These units are designed to allow excessively hot compressed air to be effectively dried. Compressed air temperatures in excess of 38 °C are very common in Southern climates, mining operations, steel mills, shipboard, etc. In areas and applications that demand operations in elevated ambient temperatures, high temperature dryers are a necessity.
Cycling dryers use a thermal mass, usually a tank of water, to store the energy produced by the refrigeration system. The temperature of the water controls the refrigeration system through a thermostat. The compressed air passes through the thermal mass via a water cooled heat exchanger. The value of this type of configuration is that they normally produce more consistent cooling results.