Refrigerated dryers remove moisture by cooling compressed air. Consequently, many users believe that the lower the outlet air temperature of a refrigerated dryer, or the larger the temperature difference between its inlet and outlet, the better. But is this actually true?

To analyze this question, we first need to understand the working principle of refrigerated dryers.

Saturated compressed air at 40°C undergoes two cooling processes in the pre-cooler (A-A) and evaporator (A-R), and finally turns into saturated air at approximately 5°C, corresponding to a pressure dew point of 5°C. If the refrigerated dryer is not equipped with a pre-cooler (or the pre-cooler has insufficient heat exchange area), dry air at around 5°C will be directly delivered to the dryer outlet. Under such circumstances, the finished air discharged by the dryer is at low temperature and nearly saturated.

If a pre-cooler is properly installed, the dry compressed air of roughly 5°C coming out of the evaporator exchanges heat with incoming moist air inside the pre-cooler. After heat exchange, the temperature of dry air rises while the temperature of moist air drops. As the moist air cools down, part of the water vapor condenses and is drained. This heat exchange fully utilizes the cold energy carried by low-temperature dry air, reducing the energy consumption of the refrigeration compressor. Meanwhile, the dry air is reheated during the heat exchange, and its relative humidity gradually declines as temperature rises. Assuming the separator can fully separate and drain condensed water, the absolute water content of the reheated dry air remains unchanged — in other words, the pressure dew point stays at 5°C.

In both scenarios described above, the pressure dew point of the finished air from the refrigerated dryer is 5°C. Nevertheless, the outlet air temperature differs greatly: around 5°C in one case and approximately 32°C in the other. It proves that evaluating the water removal performance of a refrigerated dryer simply based on its outlet air temperature is not feasible.

In fact, apart from energy saving, the pre-cooler configured in the refrigerated dryer system can also effectively protect compressed air pipelines. Although the compressed air from the evaporator contains little moisture after water separation, its relative humidity reaches 100%. Higher relative humidity leads to greater risk of corrosion on pipelines. Rust corrosion can be ignored at low relative humidity. Once the relative humidity rises to the critical relative humidity of pipeline materials, the corrosion rate will surge sharply. The critical relative humidity for ordinary steel ranges from 50% to 70%.

By properly adding a pre-cooler in the design of refrigerated dryers, the load of the dryer is effectively reduced. Furthermore, the outlet air is reheated and its relative humidity drops below 50%, preventing corrosion of compressed air pipelines.

Therefore, during the operation of refrigerated dryers, blindly pursuing low outlet air temperature is unnecessary. Instead, attention should be paid to the evaporation temperature (which reflects the dew point performance of the dryer). On the premise of maintaining stable evaporation temperature, a smaller temperature difference between the inlet and outlet means more sufficient utilization of cooling capacity and lower relative humidity of outlet air. This not only enables energy saving of the refrigerated dryer, but also provides effective protection for the user’s gas transmission pipelines.