The problem is in the air – quite literally: When atmospheric air cools down, as is the case after compression in a compressor, water vapour precipitates as condensate. Under reference conditions (+20 °C ambient temperature, 70 % relative humidity and 1 barabs), a compressor with a free air delivery of 5 m³/min will “produce” approximately 30 litres of condensate per eight hour shift. This condensate has to be removed from the air system in order to prevent potential damage and avoid costly production downtime. Cost-effective and environmentally-friendly compressed air drying is therefore a key component of application-tailored compressed air treatment.
1. A practical example
If a fluid- / oil-cooled rotary screw compressor draws in 10 m³ of air per minute at 20°C at atmospheric pressure and with a relative humidity of 60%, this air will contain approximately 100 g of water vapour. If this air is compressed to an absolute pressure of 10 bar at a compression ratio of 1:10, then this is referred to as 1 working cubic metre. However, at a temperature of 80°C after compression, the air is capable of absorbing up to 290 g of water per cubic metre. As only approx. 100 g is available, the air is very dry with a relative humidity of approximately 35%, so that no condensate can form. The temperature of the air is then reduced from 80 to approx. 30°C in the compressor’s aftercooler.
At this temperature, a cubic metre of air can absorb only about 30 g of water. As a result, an excess of approx. 70 g/min occurs, which condenses and is then separated. This means that approximately 35 litres of condensate accumulate during an eight hour working shift. A further 6 litres are separated each shift if using a downstream refrigeration dryer. The air is initially cooled down to +3°C in these dryers and is then later rewarmed to ambient temperature. This leads to a water vapour saturation deficit of approximately 20 % and therefore to drier, better quality compressed air.
2. Causes of humidity
Our ambient air always, to a lesser or greater extent, contains a certain amount of water. The actual amount of moisture depends on the temperature of the air. For example, air saturated to 100% with water vapour at a temperature of +25°C holds almost 23 g of water per cubic metre.
3. Accumulation of condensate
Condensate forms if the volume of the air is reduced and the temperature of the air is reduced at the same time. Therefore, the capacity of the air to absorb water is reduced. This is precisely what happens in the airend and in the aftercooler of a compressor.
4. Important terms – A brief explanation
a/ Absolute air humidity
Absolute air humidity is the water content of the air, given in g/m³.
b/ Relative air humidity (Hrel)
Relative air humidity is the ratio of the current absolute humidity to the highest possible absolute humidity, or saturation point (100 % Hrel). This is variable according to temperature; warm air can hold more water vapour than cold air.
c/ Atmospheric dew point
The atmospheric dew point is the temperature at which the air reaches 100 % humidity saturation (Hrel) at atmospheric pressure (ambient conditions).
d/ Pressure dew point
The pressure dew point (PDP) is the temperature at which compressed air reaches its humidity saturation point (100 % Hrel ) under its absolute pressure. This means, in the above case, that air subjected to a pressure of 10 bar (a) with a pressure dew point of +3 °C has an absolute humidity of 6 g per working cubic metre. To clarify – if the cubic metre mentioned is expanded from 10 bar (a) to atmospheric prespressure then its volume multiplies by 10 times. The water vapour component of 6 g remains unchanged, but is now distributed over 10 times the volume. This means that every cubic metre of free air now contains only 0.6 g of water vapour, which corresponds to an atmospheric dew point of -24°C.
5. Efficient and environmentally- friendly compressed air drying with a refrigeration or desiccant dryer?
New environmental legislation concerning refrigerants cannot change the fact that desiccant dryers do not provide an alternative to refrigeration dryers, neither from an economical nor from an environmental point of view. Refrigeration dryers consume only 3% of the power that the compressor needs to produce the compressed air; desiccant dryers, on the other hand, require 10 to 25 percent, or more. For this reason, refrigeration dryers should always be used wherever possible. The use of a desiccant dryer only makes sense if an extremely dry air quality with a pressure dew point down to -20, -40 or -70 °C is required.
Over the course of a working day, compressed air systems often experience considerable fluctuations in compressed air demand. Similar also occurs over the course of a year as a result of large fluctuations in temperature. Therefore, compressed air dryers should be designed to handle the least favourable operating conditions that may occur, for example: lowest pressure, maximum compressed air consumption, as well as maximum ambient and compressed air inlet temperatures. This requirement used to be solved simply by continuous dryer operation, which – especially in partial load operation – led to considerable energy wastage. Modern refrigeration dryers with efficient cycling control, on the other hand, ensure consistent air quality and are able to adapt their energy usage according to changing operating conditions.
Consequently, they are able to achieve average annual energy savings of more than 50%.
It is particularly important to use energy-efficient technology to reach pressure dew points in the minus range, as the desiccant dryers required to achieve this level of performance have a very high energy demand.
However, using a cost-effective and energy-efficient combination process, such as with the HYBRITEC system, it has been possible to significantly reduce energy consumption. The system comprises both a refrigeration dryer and a desiccant dryer. The refrigeration dryer first brings the inflowing compressed air to a pressure dew point of +3 °C efficiently and cost-effectively. Having been pre-dried, the air then passes into the desiccant dryer, which subsequently requires considerably less energy to dry the air further to a pressure dew point of -70 °C.