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Suffering with wet compressed air.

 

Firstly, letís be clear air compressors do not produce water!

The amount of water that a compressor inhales is a result of its normal operation and the amount of water is defined by:

A. Ambient conditions i.e. temperature and humidity.

B. The size or the capacity of the compressor in use.

Removing this issue is best dealt with at source as otherwise moisture problems may occur in different positions/locations within the compressed air pipework due to seasonal temperature changes, pipes being in/outdoors etc.

All air compressors operate at high temperature normally 80° C to 130° C and so the moisture inhaled with the ambient air following compression is hot and usually gaseous at that time. When the temperature of the compressed air falls, the cooling of the air results in condensation and that is why large deposits of condensate can usually be found in the air receiver.

The hot water vapour cannot be removed using conventional filters as the gaseous water vapour will travel through the filter media in the same way as the air or any other gas does

Ways and devices to remove water from compressed air systems.

After coolers.

After-coolers are normally fitted as standard to all but the most economical piston compressors. An after-cooler is a simple heat exchanger normally cooled by air from the compressor flywheel in the case of piston compressors or by the cooling air flow or separate fan when employed with a packaged rotary screw or vane compressor. This devise is very effective in reducing the temperature of the compressed air which may result in a large amount of moisture (up to 60-70%) being condensed allowing itís removal from the air receivers drain valve.

Dryers: - Refrigeration, Adsorption and Membrane.

Refrigeration dryers.

This device is used in around 70-80 % of industrial applications to provide dry compressed air usually in systems where the compressed air network is not exposed to low temperature and in non- critical applications. Refrigerated dryers are powered by a standard 230 V electrical supply except in the case of very large dryers that may require a three phase power supply.

As the name implies this devise uses refrigeration technology to reduce the temperature of the compressed air (pdp +3° C to +5° C) at this temperature and in normal ambient conditions around 99% of the moisture is condensed allowing its removal from the condenser within the dryer. Once the condensation is removed the compressed air is re-heated using the heat removed from the compressed air and thus arrives to normal ambient temperature so that no condensation is formed on the exterior of pipes and pneumatic devices..


Adsorption dryers.

Adsorption dyers are used in more critical application in the food, medical, pharmaceutical and other sectors. In simple terms they employ two towers or chambers filled with a desiccant material (silica, activated alumina or molecular sieve). Adsorption dryers are normally designed to provide compressed air with a dew point between -20° C and -70° C and are designed to remove essentially 100% of the moisture.

The desiccant is material (granules/pellets) that has a specific porosity corresponding to the molecular size of the components that form water which means that these can be separated or trapped allowing the nitrogen and the other constituent gases of air to pass through to the network.

The reason for two towers is that only one tower is used to dry the air whilst the other tower must be purged of moisture after becoming saturated. The purging process is performed using some of the dry compressed air that has been created or by an alternative source of dry or heated air (from a blower for instance).


Membrane dryers.

Membrane dryers are normally assembled in a tube stack and essentially consist of a bundle of small tubes/capillaries. These tubes are structures with a permeable material that allows certain molecules to pass through and others to remain, in this way the moisture can be separated, collected and ejected via drain valve. Membrane dryers normally employ a continuous purge of compressed air to operate and are designed to operate within a dew point of -10° C to -30° C normally.

Things to consider when selecting a compressed air Dryer - Which type?

You will need to consider the following points when selecting a suitable system;

How much compressed air (volume l/min, M3/hr, cfm etc.) is to be dried and at what pressure. This is normally defined by the size and type of the compressor(s) but if a dryer is being used for an individual process then the volume air flow required for that application should be established in order to correctly size the dryer.

All dryer specifications will be shown in terms of volume air flow, by the pressure and by the dew point that the unit is designed to operate at. The pressure and approach temperature of the compressed air will affect the capacity and performance of the dryer so it is important to consider all of these factors. Also do not be tempted to oversize the dryer particularly in the case of adsorption dryers as this will result in greater compressed air (purge) losses and increased costs.

What air quality is needed?

This is the most important consideration.

For most industrial applications using air tools, pneumatic controls, paint spraying and cleaning etc. a Refrigeration dryers. is normally perfect, especially if the compressed air network is not outdoors or exposed to very cold conditions.

The most critical applications can be satisfied with an Adsorption dryers. and these can be specified to offer various dew points according to the application from high end manufacturing through to the most stringent food grade, pharmaceutical and medical applications. The very low dew point means that the compressed air quality is not affected by exposure to lower temperature.

Membrane dryers. offer usually lower costs whilst achieving a low dew point (not as low as an adsorption dryer though). The membrane dyer uses a significant amount of air in purging and this should be considered when choosing this type of dryer.

Pros and cons!

Refrigeration dryers.

Pro Cons
  • Simple and reliable
  • Suitable for most industrial applications
  • Low power consumption
  • Low investment cost
  • Very little compressed air loss
  • Easy to install and low maintenance
  • Should not be used for more critical applications as the lowest PDP achievable is+3° C
  • Should not be used when the compressed air network is exposed to low temperature i.e. below 3° C

Adsorption dryers.

Pro Cons
  • Can be used for the most critical applications with dew point down to -70°C
  • Reliable and simple to install.
  • Low power consumption
  • Heatless types use some of the compressed air for the purge operation (apx 15%)
  • Adsorption media will require replacing periodically
  • Higher investment cost

Membrane dryer

Pros

Compact design

Low dew points down to -30íC (-40íC)

Lower investment compared to an adsorption dryer

Low maintenance

 

Cons

Membrane tubes/stack can be damaged beyond use if the compressed air is contaminated (oil)

Purges losses are significant 15% or more

Not suitable for the most critical air quality applications.

 

So what can I do if I do not use a compressed air dryer.

As stated earlier the amount of moisture delivered in to your system has little to do with the air compressor and is a natural environmental element, so donít blame the compressor!

Things you can do that may help

Keep your compressor clean, well maintained and well ventilated- a cooler operation and cooler delivered compressed air will result in more moisture being collected in the air receiver which can be easily removed.

Fit an automatic drain (timer or sensor/zero loss) to the air receiver or make sure the air receiver is drained daily.

When installing pipework, install the pipe run so that it leans slightly to a Ďlow pointí where you can connect a ĎTí piece pointing down ward with an extension down to ground level or just above where a manual or automatic drain can be installed.

Make sure your compressed air outlets are fed from the top of a tube not the bottom!

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