Each winter the ‘Dry January’ campaigners’ remit is to limit the country’s social liquid intake, but it is also the time when compressor users could be battling with a real problem that is quite the opposite – keeping any moisture, with the potential to freeze, out of the air supply and its point of use. A compressor user may think that their compressed air system is moisture-free but, when temperatures start to drop below 5˚C, this may no longer be the case.
Older dryers can sometimes fail or lose their gas and then no longer provide the required 3 degrees dewpoint (Class 4 of the ISO 8573.1 quality standard), which is the most common requirement for the UK’s weather conditions. From experience, such a failure may go unnoticed until the cold winter months when moisture drop-out leads to product spoilage.
To help put this challenge into perspective and ensure that your system stays dry and protected throughout the winter, Andy Lill, Atlas Copco's Business Development Manager – Quality Air explains why it pays to be familiar with the importance of air quality, the relevance of pressure dew point (PDP), and the role of the compressed air dryer.
The importance of air quality
Atmospheric air drawn into a compressor intake will contain particulate and water vapour. That is why the quality of compressed air is defined by the levels of different contaminants present in the airflow, which, to a large extent, dictate the air treatment needed to remove them. In particular, the presence of water in the air network when ambient temperatures plunge be can be a severe concern but a problem that, with the aid of appropriate water separators, filters, and dryers downstream, can be overcome.
Compressed air dewpoint
The higher the air temperature, the more moisture the air is capable of holding - its relative humidity. Dew point is the temperature at which the water vapour in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, thus forming more liquid.
But there is a significant difference between atmospheric dew point, the temperature at which the water vapour content of air reaches a saturation point without the influence of external pressure, and Pressure Dew Point (PDP). This is the fixed temperature of air and water vapour when subjected to a pressure exceeding the normal atmospheric pressure level. While one occurs naturally, the other is induced by an air compression system.
Reducing the temperature of saturated compressed air by 10°C will reduce the moisture content of a compressed air supply by approximately 50 per cent.
The role of the dryer
Removing water vapour from the system will automatically lower the pressure dewpoint of the compressed air and significantly reduce the chance of damaging effects of accumulated moisture on sensitive equipment, the air distribution network, tools, and end products. This precaution is particularly necessary in the depths of winter when downstream freezing of any moisture content in the system could result in costly consequences.
Even if the primary compressed air system is indoors, pipes may exit the heated space for some distance before re-entering another heated space. If condensate exists in the pipework, as it leaves a heated space, it will cool and potentially freeze when exposed to a colder ambient temperature. This is particularly true if the pipework has low points where liquid may be allowed to collect.
Adding trace heating elements may overcome this hazard. Still, the best way to limit this likelihood and to prevent downstream freeze-ups is to reduce the PDP to a level below the lowest temperature point that the compressed air system could drop to. That is the role of the dryer.
Dependent upon the application and the type of compressor system, there is a variety of different dryer technologies available to industry. The three main categories are Refrigerant, Desiccant or Membrane dryer, and each has different operating characteristics and degrees of dew point suppression. Dryer ratings usually are based on standard dryer inlet conditions. Deviations from these conditions, such as an increase in inlet temperature or a decrease in inlet pressure will reduce the dryer's rated capacity.
The refrigerant dryer
The refrigerant dryer, either freestanding or integrated within a full-feature compressor, is the most commonly used dryer within the industry, primarily because of its relatively low initial and operating costs. It is designed to deliver dewpoints specified to 3 levels detailed within ISO8573.1 Class 4 ≤ 3°C, Class 5 ≤ 7°C, Class 6 ≤ 10°C, of which Class 4 is the most commonly specified by UK manufacturers due to our potentially lower ambient temperatures in winter.
Most refrigerant dryers are equipped with a precooler/reheater that reheats the dried compressed air with an air-to-air heat exchanger using the hot incoming air. This Air to Air heat exchanger lowers the temperature of the incoming air before it passes through the refrigerant/thermal mass-to-air heat exchanger, reducing the heat load on the refrigerant system. Reheating the dried air prevents condensation on the outside of the compressed air piping in warm, humid environments.
But there are limitations. Although there have been many innovative developments in Class 4 refrigerant-type air dryers, including the latest, energy-saving VSD variants, they are not recommended for operation in sub-freezing ambient temperatures. The moisture in the compressed air can freeze and damage the dryer.
Typically, refrigerated dryers will only lower the pressure dew point to 3˚C meaning that if the compressed air temperature drops below this temperature, condensation will still occur. In these cold conditions, refrigerated compressed air dryers can become too cold and freeze internally causing severe pressure drop or, in the worst cases, stop airflow completely.
If these ambient conditions prevail, alternative dryer technologies, such as desiccant dryer designs capable of lowering the PDP to -40◦C are the recommended solution. But even if exposure to temperature extremes is an unlikely occurrence, refrigerant dryer users should be forewarned of a further challenge concerning refrigerant gases.
F-gas regulations
Compressor users with systems that include standalone or integrated refrigerant dryers need to be aware of the fundamental changes that have taken place with the use, availability and latest regulations concerning refrigerants, the F-gases.
As from January 2020, the F-gas Regulations banned the use of refrigerants with a high Global Warming Potential (GWP) above a rating of 2500 in all-new stationary refrigeration equipment. Furthermore, the operation and servicing of equipment, the handling of F-gases and their disposal are all now subject to strict regulations. For example, gases which are no longer supported cannot be replenished. Even if reclaimed gas is available, it could prove to be more costly than purchasing a new dryer. Even some dryers less than five years old could contain refrigerant that is difficult to source, thus making repairs, in the event of re-gassing, problematic to carry out.
The most typical dryer failure is when the gas leaks from the system. As far as compressed air refrigerant dryer users are concerned, not only will they have to adopt new disciplines for maintenance, servicing, record keeping and personnel competence issues, they also have to accept that in the event of equipment breakdown or leak it may not be possible to repair the unit or replace the original refrigerant.
In terms of service provision, it is now specified that contractors – both operatives and companies – must be certified to carry out tasks or services on certain types of refrigeration equipment, including installation, maintenance, servicing, leak checking, repair, decommissioning and disposal of old systems, and recovery of refrigerant gases. Be sure that your supplier is competent and qualified to do so.
In keeping with significant compressor dryer manufacturers, Atlas Copco is ahead of the game with its energy-efficient solutions such as its FX/FD low- maintenance dryer range which employs gases with a low global warming potential (GWP) such as the R10A4A refrigerant - and offers a 2-year warranty into the bargain.
Making sure it will be a Dry January
To make certain that your compressed air system is at a standard that is ready to cope with the possibility of severe winter conditions, it will pay to call in your supplier’s service engineering assistance and advice at the earliest opportunity.
A site visit to carry out a thorough but unintrusive installation check-up will answer such questions as ‘Do we need a different size or type of dryer to cope with installation changes’, or ‘Should it be a VSD version to reduce our energy costs?’ and, of prime importance, ‘Is there a threat of water in the system?’ a problem that can be readily identified by calling in an engineer with a portable hygrometer moisture checker to the site. Now is the time to take steps to protect your compressor system and make certain it is still delivering air of optimum quality.
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