How do I size a compressed air dryer?

From desiccant to refrigeration and everything in between - there are a number of compressed air dryer technologies available. The type of compressed air dryer you require will no doubt depend on the level of air quality your compressed air application demands. (You can learn more about how to choose a suitable dryer technology to meet your requirements by reading our blog post ‘How to select the right compressed air dryer system'). However, once you have selected the appropriate compressed air dryer technology - how do you know what size dryer to select for optimum air quality and efficiency? After all, an undersized compressed air dryer can lead to poor air quality and an oversized dryer can waste electricity creating unnecessary energy costs.

Compressed air dryers are usually rated to achieve a specific moisture level for a certain volume of air flow. This nominal flow rating is typically based on a set of standard conditions. In reality individual application conditions may change from day to day and will rarely be the same as the standard conditions.
In sizing a dryer it is therefore important to understand how temperature and pressure affect water content in air. The water vapour content of air will vary depending on the temperature e.g. if the temperature increases, then the air’s ability to hold water increases. Pressure is the opposite. The water vapour content of air changes in reverse with pressure e.g. if pressure increases, more moisture is squeezed out.
Because of these two relationships, compressed air dryers have correction factors (supplied by the manufacturer) to help determine how much air a dryer can actually handle for specific ambient conditions.
It is recommended that you always select the worst case scenario when picking which correction factor to use. The worst case scenario will usually be during the hotter and more humid summer months. 

As an example here are some of the correction factors that Kaeser supplies for one of its compressed air dryer ranges.

Let’s use this information to size a refrigeration dryer; 

In this example we require a total flow of 5.5 m³/min of compressed air that needs drying. Purely using the size chart on the product brochure (see below example from Kaeser dryer product range), you may well select the TD 51 refrigeration dryer, which as below has a flow rate of 5.65 m³/min (at standard conditions). 

This may work well during the cooler months. However now factor in the warmer weather. In our example, the pressure at the dryer inlet is 8 barg, and during the summer months the ambient temperature where the dryer is situated is 35^oC, with the temperature of the compressed air at the dryer inlet at 40^oC. 

Now take the correction factors for the Pressure, inlet temperature and ambient temperature and multiply them all together, eg: 1.04 x 0.97 x 0.83 = 0.837 This means that at the above conditions, the dryer will only have 83.7 % (4.37 m³/min) of its nominal capacity to dry the air to 3°C pressure dew point. 

To calculate the right sized dryer, you therefore need to divide the total flow by the total correction factor: 5.5 m³/min / 0.837 = 6.57 m³/min (at standard conditions). Now, you can look at the dryers’ stated capacity on the dryer brochure and see which one will dry 6.57 m³/min effectively. Based on this, the TD 61 (7 m³/min) would be well sized to meet the demand during the worst case periods.

Can we help you size your compressed air dryer? Just phone 0800 447 820 or fill in the form below and we will contact you.