Instrument air quality and the importance of ambient conditions

Fortunately, there are different air treatment methods and types of dryers, air filters and water separators available that can get the job done. They help you efficiently and effectively reduce the moisture content and achieve the required instrument air quality.

However, with so many options, it can seem like a daunting task to pick the right air treatment equipment while minimizing your energy consumption as well as keeping investment and operating costs down.  

To get this process started, you should know your requirements, i.e. how much air you need and what kind of quality it has to have. It should be noted that, even if no class is required for your application, you should still strongly consider air treatment because it will protect your system from water particles and corrosion.

If you are unsure about the former, you can talk to a specialist to discuss your demand and your current equipment. For the latter, you can consult the chart below, which displays the different air quality classes in accordance with ISO 8573-1 (ed. 2010). This standard shows the permissible level of contamination for solid particles, water and oil for the different classes.

While the contamination levels for solid particles and oil are self-explanatory, the one for water probably requires a further clarification for non-experts. In this case, the air quality is determined by the pressure dew point (PDP). The lower the PDP, the less moisture is in your system and the better your ISO 8573-1 class. 

The pressure dew point is the temperature at which water vapor condenses and turns into water at the current working pressure, i.e. it describes the water content in the compressed air. 

Knowing your PDP allows you to select your primary type of drying equipment and the optimal drying method. With some techniques and technologies, you will not be able to reach the lowest pressure dew points. However, that may also not be necessary, which is why it is so important to know your requirements and the control air quality you need. 

There is one other variable that is often overlooked: Your ambient conditions. 

First of all, the more you have to cool the air, the more condensate is accumulated. Therefore, if the ambient air that is being compressed is very warm, then you have to expend more energy and use better equipment to achieve the required PDP. 

You can also use your ambient conditions to determine the PDP you need (or to find the right solution if there is no defined PDP). 

For this, you should consult ANSI/ISA-7.0.01-1996 and use it as a guide. This is the globally recognized standard that governs different elements of instrument air quality. This includes the relationship between ambient conditions and the minimum required PDP for instrument air. 

Specifically, it states that the pressure dew point (as measured at the outlet of the dryer should be at least 10°C (18°F) below the lowest temperature that any part of the instrument system is exposed to. In addition, the PDP should not exceed 4°C (39°F) at line pressure.

The chart below illustrates the relationship between the ambient temperature and the PDP and can be used as a guide.  

As the chart above illustrates, one of the first things you should do is to ensure that the ambient temperature your compressed air equipment is exposed to is as low as possible. 

However, even if there is no specific PDP, it is important to remember that reducing moisture always benefits your system because it reduces corrosion and prevents the formation of the free water droplets that harm your compressed air equipment and your end products. 

Once you have done your part to find out your PDP, our experts can do theirs and advise you on which equipment and method to use in order to not only achieve your targeted instrument air quality but also how to do so efficiently.