Compressed Air Dryers: Will -70 deg C Pressure Dew Point Truly Makes A Difference Than -40 deg C?

Compressed air dryers, regardless of type, are rated using pressure dew point or PDP. PDP is the temperature to which compressed air can be cooled without condensation of the water vapor initially contained in the air. Since it depends upon the final compressed air pressure, its value varies. If pressure drops, the PDP drops with it.

But the questions that really matter to engineers, especially to the users, are: How much water must be removed from compressed air? What PDP must be specified and what type of dryer must be chosen?

The answers to these questions depend on the area of application of compressed air. If the application is for pneumatic tools, spray painting, or conveyance air, then DIN ISO 8573-1 recommends Class 4 with a maximum residual water content of 5.953 grams of water per cubic meter of compressed which is equivalent to 3 deg C PDP. A conventional refrigerant for these applications might be selected. However, DIN ISO 8573-1 gives a wide Class 1 to 3 recommendation for applications such as breathing air, breweries, dairies, instrument air, and pharmaceutical industry. This is equivalent to -20 deg C to -70 deg C PDP, the latter being Class 1. This wide gap is a challenge to engineers because a -70 deg C PDP regenerative drier is perhaps a few thousands of dollars more expensive than a -20 deg C PDP one. Most novice and conservative engineers will probably select the lowest recommended PDP for safety and hence will select an expensive regenerative dryer, perhaps heated and blower purged. On the other hand, more experienced engineers will probably pick a relatively economical -40 PDP regenerative dryer with dew point analyzer or monitor for the same application. These engineers likely know that the moisture in air might exceed what a -40 DPD dryer can handle especially in cold environments. Why still go with the higher PDP? The answer is pretty much straightforward. As the PDP goes very low the decline in the maximum moisture that the air can handle is relatively small. A rule of thumb is for every 20 deg F (11 deg C) drop, the moisture drops by a half. This means that if the moisture content at -40 deg C PDP is 0.117 grams per cubic meter, then at -51 dec C PDP, the moisture will be around 0.058 grams per cubic meter and so on. This leads to the fact that the moisture content in air will never reach zero no matter how low the PDP becomes. Hence, to these engineers the small difference in the moisture content between -40 deg C and -70 deg C PDP is not worthy of the additional thousands of dollars in initial and operating costs the lower PDP dryer entails.