In industrial automation and control systems, the quality of compressed air plays a crucial role in ensuring reliable operation. Among various types of compressed air, instrument air refers to the purified, dry air supplied specifically to pneumatic instruments and control systems. This article explains the differences between plant air and instrument air, and provides a detailed overview of the most commonly used instrument air drying systems: refrigerated air dryers and desiccant air dryers.
1. Plant Air vs. Instrument Air
At the outlet of a typical air compressor, compressed air is usually divided into two branches:
Plant Air (Unpurified Air): Used for general factory operations such as material conveying, cleaning, and pneumatic tools. It is not dried or filtered extensively.
Instrument Air (Purified Air): Further dried and filtered to meet the requirements of precision control instruments, such as pneumatic actuators, explosion-proof pressurization cabinets, and valve positioners.
The key difference lies in the dew point—instrument air typically requires a much lower dew point to prevent condensation, corrosion, and malfunction in sensitive equipment.
2. Refrigerated Air Dryers
2.1 Working Principle
A refrigerated air dryer (commonly referred to as a “refrigeration dryer”) removes moisture from compressed air using the cooling principle, similar to how air conditioning systems work. The unit consists of two main systems:
Air System: Handles the flow of compressed air.
Refrigeration System: Uses refrigerant to cool down the compressed air.
2.2 Drying Process
The hot, humid compressed air enters the evaporator where it is cooled to near-freezing temperatures (typically 2–10°C).
Moisture in the air condenses into water droplets.
The condensate is discharged through an automatic drain valve.
The dried air is reheated slightly before exiting the dryer to prevent pipe sweating.
2.3 Key Features
Typical Pressure Dew Point: +2°C to +10°C
Advantages: Simple structure, low cost, low maintenance
Limitations: Not suitable for extremely low dew point applications or cold climates
3. Desiccant Air Dryers
Desiccant air dryers work by adsorbing water vapor onto the surface of porous materials called desiccants. These dryers are essential for processes requiring ultra-dry air, often achieving dew points as low as -40°C to -70°C.
3.1 Common Desiccants Used
Activated alumina
Silica gel
Molecular sieve
Moisture is temporarily held on the surface of the desiccant particles and later removed through a process called regeneration.
3.2 Regeneration Methods and Dryer Types
Desiccant dryers are categorized based on how the desiccant is regenerated:
A. Heatless Regeneration Desiccant Dryer
Regeneration Principle: Uses dry purge air (typically 15% of total flow) to regenerate the saturated desiccant under reduced pressure.
Cycle: Two towers (A and B) alternate between drying and regenerating.
Advantage: No external heating required
Disadvantage: High air consumption
B. Heated Regeneration Desiccant Dryer
Regeneration Principle: Uses electric or steam heating to raise the temperature of purge air, reducing purge air volume.
Variants:
Internally Heated: Uses dry air from the system.
Externally Heated: May use ambient air or separate heat source.
Advantage: Lower air loss
Disadvantage: Higher complexity and energy cost
C. Blower Purge (Heated Without Purge Loss)
Regeneration Principle: Ambient air is heated by a blower and used to regenerate the desiccant, avoiding the use of purge air from the system.
Advantage: Very low operational cost
Disadvantage: More complex setup
4. Application Scenarios Comparison
| Dryer Type | Dew Point | Energy Use | Maintenance | Best Use Case |
|---|---|---|---|---|
| Refrigerated Dryer | +2°C to +10°C | Low | Low | General plant air, standard pneumatics |
| Heatless Desiccant Dryer | -20°C to -40°C | Moderate (air loss) | Medium | Instrument air in dry zones |
| Heated Desiccant Dryer | -40°C to -70°C | High (power needed) | Medium-High | Precision instruments, cold areas |
| Blower Purge Desiccant Dryer | -40°C to -70°C | Low (no air loss) | High | Energy-sensitive continuous use |
5. Conclusion
Choosing the right instrument air drying system depends on specific process requirements, environmental conditions, and cost constraints. For standard industrial needs, refrigerated dryers provide sufficient performance at a lower cost. For critical applications requiring ultra-low dew points, desiccant dryers—especially heatless or heated types—are indispensable.
Understanding the characteristics and differences between these systems helps ensure instrument reliability, reduce maintenance downtime, and improve overall automation efficiency.