Dryer types - where to start with selection

Published:  12 July, 2018

Whether designing new systems, or reviewing existing systems, the first step should be to define exactly what are the compressed air purity requirements. Ensuring you have the correct advice is key and the dryer is only part of the process. The British Compressed Air Society (BCAS) reports.

There is a very wide range of requirements for air quality, all of which can be met with the right equipment. However, unnecessary levels of treatment can significantly increase the associated energy costs.

There are many types of dryer available, with drying technologies often associated with the dewpoint they can deliver. Once the required pressure dewpoint (air quality) has been selected it is then a case of selecting the best technology to provide that dewpoint. All compressed air dryers have their own advantages and disadvantages and there is not currently one single technology that is suitable for every system and application.

Refrigerated Air Dryer Technologies

Refrigerated Air Dryers (+3°C / +7°C / +10 °C Pressure Dewpoints)

Refrigeration dryers cool the compressed air causing condensation of the water vapour into liquid water and then use a built-in water separator to remove the liquid. The dewpoint of a refrigeration dryer is dictated by its ability to cool the air to the desired dewpoint temperature and then the efficiency of the liquid removal separator. If the dewpoint temperature cannot always be achieved and / or the liquid separation is not always 100% efficient (for example in varying flow conditions) then the dewpoint cannot always be guaranteed.

Refrigeration Dryer Types

Refrigeration dryers consume energy through pressure drop, electrical energy for the refrigeration compressor and condenser fans or chillers and pumps on water cooled variants. Below is an overview of the 3 most common types of refrigeration dryers sold.

Direct Expansion Refrigeration Dryers

Direct expansion dryers are the most common type of refrigeration dryer. The refrigerant compressor is constantly running and of the 3 main types of refrigeration dryer, can be the costliest to run. Direct expansion dryer dewpoints are often more stable than for example those delivered by a thermal mass dryer.

Thermal Mass Refrigeration Dryers

Unlike direct expansion dryers that have a direct heat exchange between the air circuit and the refrigerant circuit, as their name suggests, thermal mass dryers have as large mass (typically a glycol/water tank or sand) which is cooled down by the refrigeration circuit. The heat exchange with the compressed air then takes place between the air circuit and the thermal mass.

The benefits of this is that once the thermal mass has been cooled down, the refrigeration compressor can be turned off to save energy.

Variable Speed Refrigeration Compressor

This type of refrigeration dryer uses invertor technology to vary the speed of the refrigeration compressor to try and match the cooling requirements of the dryer to the water vapour loading of the incoming compressed air. Whilst they can be more energy efficient than an equivalent direct expansion dryer, the cost of the inverter technology and complexity means they are often only available for larger compressed air flows and dewpoint.

Membrane Dryers

This type of dryer use hollow-fibre membranes which diffuse the moisture from the compressed air to atmosphere. To remove the permeated moisture from the membrane they require a constant flow of

compressed air called “sweep air”. To provide a constant pressure dewpoint in line with ISO8573-1 classifications can require the sweep air to be as much as 50% of the dryers rated capacity and are therefore not very efficient.

Adsorbent (Desiccant) Dryers

When lower dewpoints are needed than those which can be achieved from refrigerated air dryers, adsorbent (desiccant dryers) are used. The basic principle of an adsorbent dryer is to pass compressed air saturated with water vapour (not liquid or aerosols) over a bed of adsorbent desiccant material. The longer the air is in contact with the desiccant, the drier the air becomes . The desiccant material can only adsorb so much moisture before becoming saturated and therefore it must be regenerated if a constant dewpoint is to be supplied.

Adsorption dryers consume energy through pressure drop, and the methods used for regeneration of the desiccant material. The regeneration methods can consume energy in the form of purge air (taken from the process air), cooling air (again taken from the process air or from ambient air) and direct electrical requirements for heaters, blowers or vacuum pumps. There are different types of adsorption dryers sold.

This guide is designed to give a brief introduction to technology types and cannot replace the advice of a BCAS member to ensure the correct system is specified. Find a list of suppliers on the BCAS website: www.bcas.org.uk

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