Refrigerated air dryers are the most popular choice in removing condensate (water) from a compressed air system. Removing water from a compressed air system is crucial in the majority of applications as water can cause instrument/tool damage, product spoilage, pipe corrosion and higher maintenance costs.

The type of air dryer selected depends on the level of moisture required to be removed from the compressed air for each specific application. Selecting the right dryer is critical. However, the first step in condensate removal is the installation of an effective aftercooler and condensate separator. An aftercooler will not only remove approximately 65-75% of the moisture from compressed air it also lowers the temperature such that it is cool enough the dryer select. A coalescing filter prior to the dryer will remove contaminants which is crucial as an air dryer needs to be free from contaminants to be effective.

An aftercooler will cool compressed air to approximately 10°C above ambient temperature whilst a correctly sized refrigerated air dryer will reduce the temperature to between 1°C &4°C removing approximately 95% of the moisture contained in compressed air. At 95% condensate removal, most systems will not see any liquid downstream of a correctly sized refrigerated air dryer.

If a refrigerated air dryer is not sized correctly for a particular system then issues such as overheating, decreased efficiency and shut-down are common leading to moisture in your system and the dryer rendered ineffective.

To determine the correctly sized dryer for your application there are 5 variables that must be considered including:

1)    Flow Rate (Free Air Delivery (FAD))

The flow rate is determined by the FAD of your air compressor(s) to determine the minimum flow the refrigerated air dryer needs to be able to process. For example an 8 bar, 75kW KHE Series Rotary Screw Compressor has a FAD of 14.18m³/min.

2)    Operating Pressure

Refrigerated air dryers are usually specified at a 7 bar operating pressure. Therefore a compressor operating at 8 bar would require a correction factor to be applied. The correction factor table below indicates that for 8 bar operating pressure the dryer has a capacity of 1.06 times that of its capacity at 7 bar. .

bar

5

6

7

8

9

10

Correction Factor

0.85

0.93

1

1.06

1.11

1.15

 

3)    Maximum Ambient Temperature

The maximum ambient temperature of the environment the refrigerated dryer will be located must be considered in the calculation. It is important in Australia that dryers are selected to handle the ambient temperatures that we typically encounter during our summer months. An undersized dryer not selected to handle these temperatures may shut down and fail. Always check when purchasing an air dryer that it is rated to handle our ambient temperatures as some European models are designed for only 25°C conditions. A Southern Cross air dryers are always selected for a minimum ambient of 35°C. A correction factor would be applied for dryers operating at different capacities.

°C

25

30

35

40

42

45

Correction Factor

1.11

1.07

1

0.85

0.75

0.66

 

4)    Maximum Inlet Air Temperature

The inlet air temperature refers to the temperature of the compressed air as it enters the dryer. An effective aftercooler will reduce the temperature of the compressed air to around 10°C above ambient. An air dryer selected for an ambient temperature of 35°C will typically experience an inlet temperature of 45°C. If the inlet air temperature is above 45°C temperature another correction factor must be applied. .

°C

30

35

40

45

50

55

Correction Factor

1.43

1.30

1.20

1

0.87

0.68

 

5)    Pressure Dew Point

The pressure dew point is the point at which water vapour turns to liquid. A correctly sized dryer will lower the pressure dewpoint such that moisture is turned to liquid which can be easily removed. As a standard in Australia most refrigerated air dryers are selected to achieve a pressure dew pint of 3°C. In our example we will select our dryer at the nominated pressure dew point of 3°C. Once again, if a higher pressure dewpoint is OK, the dryer capacity can be adjusted.

°C

3

5

7

9

10

Correction Factor

1

1.09

1.18

1.30

1.33

 

To complete the calculation and get the ‘actual flow rate’ required by a refrigerated air dryer to suit our compressed air system we perform the calculation as follows:

FAD x Correction Factor (Working Pressure) x Correction Factor (Ambient Temperature) x Correction Factor (Inlet Temperature) x Correction Factor (Pressure Dew Point) = Actual Flow Rate of Dryer Required.

So the actual calculation would be:

14.18 x 1.06 x 1 x 1 x 1 = 15.03 m³/min.

This calculation means when selecting a refrigerated air dryer to suite our 75kW compressed air system the dryer needs to be able to process at least 15.03 m³/min of air flow to operate at full efficiency. So when selecting a model we would select the correct model designed to handle 15.03 m³/min or more to ensure a reliable source of dry compressed air.

If you would like more information on selecting and sizing a suitable refrigerated air dryer for your application don’t hesitate to contact us on 1300 098 901 or simply complete the ‘Enquiry Form’ on this page.