Drying compressed air with refrigerated dryers

May 2005

Refrigerated dryers are the most common and practical method of drying compressed air in industrial applications. Unfortunately, they are often the most misunderstood component in a compressed air system. The drying of compressed air is a relatively simple process, but before diving into the mysteries of the dryer, some of the reasons for having them in the first place will be looked at.

Typical compressed air systemJust about every manufacturing process these days uses compressed air. It is a reliable, easily controlled energy source that is used to operate some of the most costly and critical machinery and equipment in industrial, medical, petrochemical, and mining applications.However, the process of compressing air actually makes it unsuitable for most processes.During compression, atmospheric air becomes so wet that, on delivery, it could destroy the very equipment it is meant to operate. Not only does the air exit a compressor wet, it contains a range of contaminants including oil, dust, and fumes. The act of compression concentrates whatever goes into the compressor intake.Where the water comes fromAll ambient air contains some amount of moisture. Geography plays a big part - consider Durban in summer compared to up on the Reef in the dry season. The relative humidity (RH) we hear on the weather forecast tells us the amount of water vapour as a percentage that the air is holding, compared to the maximum amount the air can hold at that temperature. For example, 60% RH at 20°C means that the air is holding 60% of the water vapour it can potentially hold at that temperature. At 100% RH, the air can no longer hold the moisture as vapour which then presents itself as dew, visible mist or fog. The temperature that results in the vapour condensing into visible moisture is known as the 'atmospheric dew point'. More on this later.Not only do geographic factors play a part in the likely moisture content of the ambient air but so do specific locations within an industrial environment. The local relative humidity near a cooling tower or in a building with little ventilation and lots of people is likely to be much higher than outside in the fresh air. The big factor though is temperature.The higher the temperature, the more vapour the air can hold and vice-versa. Expand the air and it can hold a greater amount of vapour; compress the air and you reduce the amount it can hold.If an air compressor draws 8 m³ of air into the intake, the parcel of air gets reduced to a volume of 1 m³ at 700 kPa. However, the amount of water that is now in this small compressed package of air passing down the air line is the same amount that was drawn in. The difference is more moisture per cubic metre of air. If the air drawn into the compressor is reduced to an eighth of its original volume, then its vapour holding capacity is also reduced to one eighth of the ambient capacity.

Where moisture comes fromA real exampleIf a compressor, let us say a typical 30 kW model, draws in air at 20°C, that has a relative humidity of 60%, and compresses it to 700 kPa, it will pump approximately 20 litres of water into the airline in an eight-hour shift. In a year that is 4800 litres.While contemplating how much of a swimming pool that will fill, consider that a 30 kW compressor is a relatively small unit. If you are a plant manager with a couple of 150 kW compressors running the same duty you can expect about 650 litres a day passing through your pipes. That is 156 000 litres in a year.Removing the waterEarlier we discussed temperature as being a factor in how much water air can hold. One fact that is common among all compressors, regardless of the method of construction, is heat. Heat of compression along with heat from other frictional loads keeps the water in a vapour state. That is, until it travels down to your machine, cooling all the way, and finally condenses in the expensive machine or process. Better to remove the heat at the right time, in a controlled way, and condense the vapour in the line. Once the water vapour is condensed, it is far easier and cheaper to remove.Most air compressors used in industry today are rotary screw types, with piston compressors being used for smaller applications or specialist high-pressure work. Rotary screw compressors nearly always have a built-in compressed air 'aftercooler'. Without an after cooler, the air exiting a rotary screw compressor is in the order of 70-80°C.A correctly sized air cooled aftercooler will reduce the compressed air temperature to about 10°C above ambient. The resultant temperature drop causes some of our vapour to condense - and as the compressed air's ability to hold water vapour is an eighth of what it was before it was compressed, any temperature reduction results in the saturated air releasing the water vapour as liquid condensate. Most aftercoolers have a drain mechanism to drain off this water. If the aftercooler is doing its job, it can remove approximately 65% of the moisture.Removal of the remaining 35% of moistureA refrigerated air dryer works by simply removing heat from compressed air. By lowering the compressed air temperature below the ambient through refrigeration, the vapour is condensed and drained away. The air, which is now cooled to about 2°C must then be re-heated, otherwise cold air lines running around the plant would themselves have moisture condensing on the outside of the pipe. The air is reheated using the heat of the incoming air and raises the temperature to just above ambient. This leaves the air, from the point of view of 'dryness', suitable for most industrial applications in South African climates.

Pressure dew point - PDP (measuring the level of 'dryness')As mentioned, the atmospheric dew point is the temperature at which ambient air will release vapour as condensate. We now know that pressure has an effect on the level of vapour contained in compressed air. As a result, the atmospheric dew point (or simply 'dew point') cannot be used to measure the dryness of compressed air. For this there is a different term: 'pressure dew point' (PDP). This is the temperature at which water vapour contained in compressed air at a particular pressure will condense to form liquid water. Most refrigerated dryers provide a PDP of between 2 and 8°C.From the above, we know that water will still condense at temperatures just below these figures.This is where the ambient temperatures come into the picture. Unless compressed air lines pass through - or end up in - areas where the ambient temperature is less than the PDP provided by the dryer, condensation should not re-occur.For example, if an air-line is passed through a cold storage plant at 0°C, before arriving at a palletising machine, there is likely to be some condensate dropping out when it is least wanted. Some sudden expansions of compressed air in part of a process can lower the air temperature enough to result in condensate.Various measures can be taken to provide air at a lower PDP than that provided by refrigerated dryers. SMC has considerable experience in the supply of twin tower desiccant dryers which can easily provide PDP of -40°C or greater. This is required for most precision instrumentation. This is achieved by adsorption via activated alumina and not refrigeration. Another method is via membrane dryers, which can provide high levels of dew point suppression. The degree of dryness required is a function of the process and geographical location.Hyflo offers all types of dryers from SMC - refrigerated, desiccant and membrane.The majority of applications in South Africa are generally working in ambient conditions of 15°C and over. Most applications can be catered for by a good aftercooler and refrigerated air dryer. As stated, the primary task of the dryer is to remove heat. As a result, ambient and compressed air inlet temperatures have a large bearing on dryer size.Dryers are initially sized around a particular known airflow, and then correction factors are applied for the known environmental conditions. A minimum of four things have to be known:1. The flow going through the dryer or the compressor type.2. The compressed air temperature going into the dryer.3. Ambient air temperature.4. Operating pressure.The flow through the dryer dictates the capacity of the dryer. The other three criteria affect the dryer size in relation to its 'nominal size' based on flow. The dryer should be conservatively sized to accept the highest anticipated flow at the lowest expected pressure and should be able to operate without overload on even the hottest days. A dryer receiving a compressed air supply at 45°C would need to be approximately 1,5 times larger than a unit with an inlet at 35°C.

Most compressed air dryers are rated in accordance with ISO Standard 7183. The figures on manufacturers' brochures are generally in accordance with this. That is, an operating pressure of 700 kPa, an air inlet temperature of 35°C and an ambient of 25°C. These figures are based on typical European conditions - corrections for local ambient conditions would need to be applied for most South African locations.

Typical temperatures in a 'normal compressed air system'Notice that relative humidity was not one of the selection criteria in sizing the dryer. The dryer does not actually 'dry' the air, it removes heat. The only factor that RH has is how much moisture will need to be drained away while the dryer is operating.AftercoolersWhat if an aftercooler cannot be fitted? Can the hot compressed air not just go straight into the refrigerated dryer? This is a problem that is faced often, especially with smaller compressors that do not have effective aftercoolers. An economic and space saving alternative to a separate aftercooler and dryer is a combined refrigerated dryer with in-built aftercooler (PFE Series). This allows a maximum compressed air inlet temperature of 90°C compared to 55°C for a standard unit (PDE Series).Other options available, particularly with large dryer units, include water-cooling rather than air-cooling. This has advantages in the ability to have more control over the heat exchange medium while also eliminating the need for potentially noisy fans and their associated hot air blast.A dryer is a necessityIn today's environment of high maintenance costs and the demands of current pneumatic equipment requiring good quality air, installing a dryer (and possibly an aftercooler) is a necessity rather than a luxury. Gone are the days of large brass valves, with large porting, basic seals and simple construction. Current electro-pneumatic equipment demands high quality, well filtered and oil free air. It is designed based on the basic assumption that it will at least be fed with dry air. The expectation of quality in manufacturing with corresponding increases in output has raised the stakes for all businesses. Downtime for the sake of wet, dirty air is inexcusable. Yet at least 60% of all pneumatic failures are due to water contamination.Most current pneumatic equipment is designed for oil-free operation. Water in the line flushes lubricants out of the system causing seal failure and corrosion. This could be as simple as a jammed spool or at worst, the failure of a valve or actuator spring in a safety-critical application.The obvious problems in paint spraying, liquid agitation, or cleaning - to name but a few - have been well documented.The correct use of an SMC refrigerated dryer will reduce the incidence of breakdowns in pneumatic tools and equipment, enabling maintenance resources to be redirected to more productive activities. Loss of production as a result of saturated compressed air will be a thing of the past. Factory output will increase while maintenance costs will be reduced.