Lesser-Known Fact about Air Compressors: Why Desiccant Dryers Need to Be Used with Refrigerated Dryers?
Functions of Refrigerated Dryers
Based on refrigeration principles and refrigeration components, refrigerated dryers reduce the temperature of compressed air as much as possible. The lower the compressed air temperature, the more moisture condenses out, and the drier the compressed air becomes.
Why Is the Optimal Configuration to Operate Desiccant Dryers Together with Refrigerated Dryers?
(1) Combined Configuration for Graded Drying
When operating normally, refrigerated dryers that comply with ISO 8573‑1 (International Standard for Compressed Air Quality) deliver a pressure dew point ranging from 2°C to 10°C. A pressure dew point above 10°C fails to meet the ISO standard.
Qualified refrigerated dryers capable of reaching this moisture removal standard can eliminate 80%~90% of moisture in the compressed air system. Even low-performance refrigerated dryers can remove 20%~30% of moisture.
The refrigerated dryer performs pre-dehumidification and lowers the load on the desiccant dryer. In other words, the refrigerated dryer undertakes the primary task to remove bulk high-humidity moisture, and the desiccant dryer handles fine drying to capture residual low-concentration moisture. The division of labor between the two devices achieves optimal overall dehumidification performance.
Charts indicate the water content of compressed air under different pressures and temperatures: higher pressure and lower temperature correspond to lower moisture content.
The refrigerated dryer cools compressed air to condense moisture and produce relatively dry compressed air.
Even if compressed air is cooled to 0°C, trace moisture still remains. The air temperature cannot be reduced below 0°C, as condensed water will freeze, block compressed air pipelines and hinder smooth air flow.
For general industrial applications, a pressure dew point of 2~10°C satisfies ordinary working conditions. Nevertheless, for certain industries, production workflows, process requirements and precision equipment, the drying level corresponding to a 2~10°C pressure dew point is far insufficient.
(2) Energy Consumption Reduction
Most people believe adding a refrigerated dryer increases energy consumption and power costs. Since desiccant dryers can dry compressed air to a pressure dew point of -10°C ~ -70°C, why use refrigerated dryers that only achieve a pressure dew point of 2~10°C or worse?
Can we rely solely on desiccant dryers?
This viewpoint is not entirely wrong, but such a configuration is not ideal.
Assume an air compression system generates 1 kg of moisture every minute. The flow path of compressed air is as follows:
Air compressor discharge (approx. ambient temperature +10°C = 50°C) → Air receiver (stabilizes pressure and dissipates heat, temperature drops by 3°C to 47°C) → Refrigerated dryer (refrigeration system cools air from 47°C to 0°C). At 8 bar and 0°C, residual moisture in each cubic meter of compressed air falls to 0.00051 g (refer to the table).
The moisture undergoes temperature change from 47°C down to 0°C.
If there is no refrigerated dryer and all moisture must be adsorbed by the adsorbent inside the desiccant dryer: once the adsorbent becomes saturated with water, another tower’s adsorbent will take over the drying task. The saturated tower (Tower A) must undergo desorption, namely regenerating the adsorbent inside Tower A to restore its drying capacity. Only after the second tower (Tower B) becomes saturated can the system switch back to Tower A for adsorption.
Adsorbent regeneration requires heat, normally supplied by heaters. Part of the dried compressed air serves as a heat carrier to transfer heat into the saturated tower and evaporate the moisture trapped in the adsorbent. Many people overlook a key point: moisture adsorbed by the desiccant must be heated above 100°C for smooth evaporation. To heat the water above 100°C, the adsorbent material itself must first reach over 100°C (analogy: water inside a kettle can only reach 100°C when the kettle is heated above this temperature).
Simple comparison: Which consumes more energy? Condensing moisture by cooling it from 47°C to 0°C, or evaporating moisture by heating it from 47°C to above 100°C?
Moreover, desiccant dryers consume pre-dried compressed air as heat transfer medium, which wastes compressed air resources. If all moisture is directly adsorbed by desiccants, the required filling volume of adsorbent will rise significantly, leading to higher procurement costs for the desiccant dryer.
Conclusions
Equipping a refrigerated dryer reduces overall energy consumption instead of increasing it.
If a system uses only a desiccant dryer without a refrigerated dryer, yet the desiccant dryer has the same specifications as the model used in combination with a refrigerated dryer, risks exist. Without a refrigerated dryer and extra adsorbent filling, the adsorbent tends to saturate prematurely, shortening its service life, or the required drying standard cannot be achieved.
Based on refrigeration principles and refrigeration components, refrigerated dryers reduce the temperature of compressed air as much as possible. The lower the compressed air temperature, the more moisture condenses out, and the drier the compressed air becomes.
Why Is the Optimal Configuration to Operate Desiccant Dryers Together with Refrigerated Dryers?
(1) Combined Configuration for Graded Drying
When operating normally, refrigerated dryers that comply with ISO 8573‑1 (International Standard for Compressed Air Quality) deliver a pressure dew point ranging from 2°C to 10°C. A pressure dew point above 10°C fails to meet the ISO standard.
Qualified refrigerated dryers capable of reaching this moisture removal standard can eliminate 80%~90% of moisture in the compressed air system. Even low-performance refrigerated dryers can remove 20%~30% of moisture.
The refrigerated dryer performs pre-dehumidification and lowers the load on the desiccant dryer. In other words, the refrigerated dryer undertakes the primary task to remove bulk high-humidity moisture, and the desiccant dryer handles fine drying to capture residual low-concentration moisture. The division of labor between the two devices achieves optimal overall dehumidification performance.
Charts indicate the water content of compressed air under different pressures and temperatures: higher pressure and lower temperature correspond to lower moisture content.
The refrigerated dryer cools compressed air to condense moisture and produce relatively dry compressed air.
Even if compressed air is cooled to 0°C, trace moisture still remains. The air temperature cannot be reduced below 0°C, as condensed water will freeze, block compressed air pipelines and hinder smooth air flow.
For general industrial applications, a pressure dew point of 2~10°C satisfies ordinary working conditions. Nevertheless, for certain industries, production workflows, process requirements and precision equipment, the drying level corresponding to a 2~10°C pressure dew point is far insufficient.
(2) Energy Consumption Reduction
Most people believe adding a refrigerated dryer increases energy consumption and power costs. Since desiccant dryers can dry compressed air to a pressure dew point of -10°C ~ -70°C, why use refrigerated dryers that only achieve a pressure dew point of 2~10°C or worse?
Can we rely solely on desiccant dryers?
This viewpoint is not entirely wrong, but such a configuration is not ideal.
Assume an air compression system generates 1 kg of moisture every minute. The flow path of compressed air is as follows:
Air compressor discharge (approx. ambient temperature +10°C = 50°C) → Air receiver (stabilizes pressure and dissipates heat, temperature drops by 3°C to 47°C) → Refrigerated dryer (refrigeration system cools air from 47°C to 0°C). At 8 bar and 0°C, residual moisture in each cubic meter of compressed air falls to 0.00051 g (refer to the table).
The moisture undergoes temperature change from 47°C down to 0°C.
If there is no refrigerated dryer and all moisture must be adsorbed by the adsorbent inside the desiccant dryer: once the adsorbent becomes saturated with water, another tower’s adsorbent will take over the drying task. The saturated tower (Tower A) must undergo desorption, namely regenerating the adsorbent inside Tower A to restore its drying capacity. Only after the second tower (Tower B) becomes saturated can the system switch back to Tower A for adsorption.
Adsorbent regeneration requires heat, normally supplied by heaters. Part of the dried compressed air serves as a heat carrier to transfer heat into the saturated tower and evaporate the moisture trapped in the adsorbent. Many people overlook a key point: moisture adsorbed by the desiccant must be heated above 100°C for smooth evaporation. To heat the water above 100°C, the adsorbent material itself must first reach over 100°C (analogy: water inside a kettle can only reach 100°C when the kettle is heated above this temperature).
Simple comparison: Which consumes more energy? Condensing moisture by cooling it from 47°C to 0°C, or evaporating moisture by heating it from 47°C to above 100°C?
Moreover, desiccant dryers consume pre-dried compressed air as heat transfer medium, which wastes compressed air resources. If all moisture is directly adsorbed by desiccants, the required filling volume of adsorbent will rise significantly, leading to higher procurement costs for the desiccant dryer.
Conclusions
Equipping a refrigerated dryer reduces overall energy consumption instead of increasing it.
If a system uses only a desiccant dryer without a refrigerated dryer, yet the desiccant dryer has the same specifications as the model used in combination with a refrigerated dryer, risks exist. Without a refrigerated dryer and extra adsorbent filling, the adsorbent tends to saturate prematurely, shortening its service life, or the required drying standard cannot be achieved.
