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How To Select A Compressed Air Dryer

How to select a compressed air dryer

To achieve reliability when using compressed air in any air driven equipment, you will quickly come to the realisation that the use of a compressed air dryer is “the solution. Compressed air without a dryer and filter bank, will be wet, oily, contaminated and will cause hours of costly and frustrating downtime.

What is a Refrigeration Air Dryer?

It’s a box that contains heat exchangers, a Freon cooling system and often a pair of externally mounted filters before and after the air dryer.

The air dryers function in life is to reduce the incoming warm wet compressed air temperature to a temperature of 3 °C. At °C the water and oils will turn from a gas / vapour into a condensed liquid. Liquids can be trapped and ejected to a waste water treatment system, when water and oils are in vapour form (hot) they cannot be trapped and ejected.

The water is entrained from the atmosphere at the intake of the compressor itself. Oils are carried into the airline from the air compressor.

This liquid waste is drained out of the dryer by automatic tied or level controlled drains. This waste is toxic to our river systems, please get a proper waste separation system that will allow you to eject clean water in the drains legally and it retains the oil waste for proper disposal.

Will any compressed air dryer do?

No, not quite. Dryers and the filters are sized for a published thermal load.

What is a thermal load? It’s a combination of:
* Air volume in m 3 or Cubic Feet per Minute
* The air pressure entering the dryer normally shown in Bar G or PSI
* The temperature of the compressed air entering the dryer
* The targeted dew point required to be achieved by the dryer

Dryers are designed to handle a published thermal load, the heat exchanger, condenser and Freon compressor are a thermally matched set. They are designed to handle a published airflow at 7 bar gauge and 25 °C ambient air with a 35° air inlet to the dryer at a dew point of 3 °C. This is the European standard. Compressed air dryers manufactured in the East or America may have different design conditions.

What are the causes of thermal overload?

* Too much compressed air flowing through a dryer
* Ambient temperature too high, or lack of cooling air or water on the dryers condenser coil
* The inlet air temperature too high

Look carefully at your potential suppliers’ brochure, it should show a number of tables that relate to the variables above and it will show you how to de-rate a dryer for our hot African climates. If the tables are not shown, request the tables from your air dryer supplier! The chances of getting a dryer to work in Africa without de-rating flow rates down from our cooler cousins catalogues in the Northern Hemisphere are next to zero on a hot summers day.

Failure to de-rate a dryer, especially in the hotter areas of our country will lead to hours of frustration and downtime. The dryer will trip and will not work or it will break components within the air dryer. That’s not why you wanted to buy a dryer, now was it?

The hotter the workshop or compressor room is, the bigger and more expensive the dryer will be. The cooler environment, the smaller the dryer will be and the price will go down.

How do I know what Air Volume I have?

There are three ways to establish air volume.

* Ask the compressor supplier for the FAD volume from your compressor at the working pressure that you plan to work at.
* Or, measure the flow with a thermal mass meter. This can be done, but you need to contact a specialist supplier for this, or perform a timed pump up test on your air receiver! If you want to do this, call us for the formula!
* Estimate the air volume based on the kW’s fitted to the air compressors main motor.

See the tables below, it shows the amount of compressed air that is delivered from the two different types of compressors based on the kW’s fitted. It is a general guide, but will suffice for 95% of the applications.

Who needs Filters?

Well, actually, most people. The elements will periodically need changing, yes, it will cost money, but the improvement in compressed air quality is immense.

Filters remove the contamination generated by the compressor and released by old pipe systems. The pre-filter will remove oils and particles, draining liquids to waste and retaining particles from the incoming air from the compressor before the automatic drain inside the dryer is jammed with airborne rubbish.

A dryer with a jammed drain is a water generator, not a water remover! The final filter will remove the final trace of oil and filter down to at least 0.01 micron and 0.01 mg/m3 of oil. If you have really great eyes, you can only see 40 microns at best. So this level of filtration is very fine. If necessary, for demanding applications, you can add an activated carbon filter to “remove” the final traces of oil vapours and smells. Activated carbon filters remove down to 0.003 mg/m3. Filters are also sized on airflow and pressure, again read the sales literature and select the correct filter for your flow and pressure ratings.

Does the Lowest Cost Win?

Well on rare occasions maybe! Like most things in life, you get what you pay for. Buying lower-priced dryers may lead to problems later. It’s best to go to the established leaders in the air treatment industry, they have superior knowledge and aftercare skills. Air treatment skills and aftercare knowledge, by the way, is not necessarily found in a compressor supply chain.

Strangely enough compressed air treatment companies will have more in-depth knowledge of the compressor air treatment, than the guys in the compressor industry. Ut’s two different trades… Would you buy a smart new luxury car from your local caravan dealer?

Having upset a few friends and maybe some enemies for this somewhat humorous view of air dryer selections, they may not wish me well.

The truth is that many companies get selections wrong, or fail to apply them and don’t understand the impact on the refrigeration in the air drying process. The result of getting it wrong is huge, and when it’s wrong, it’s wrong, there is no magic trick to rectify an undersized air dryer or filter. You have to get it right the first time.

Look at the selection guide, it refers to our model numbers operating at different site temperature conditions. See how the flow volume drops when temperatures increase! It is very marked.



Please note these flow rates refer to a 7 bar working pressure, if the pressure is lower, a larger dryer will be required. If it’s operating above 7 bar you will achieve a slightly higher flow rate through these dryers. Talk To Us fi you need help, it’s cheaper to talk and ask questions than making a wrong air dryer selection!!

This is a “rough guide” for the different air compressor capacities based on the kW’s fitted at 7 bar G (100 psi) outputs.

Artic Driers International PTY ltd ®    October 2019   ©

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