Complete Guide to Compressed Air Dryers: Types, Working & Selection

ADSORPTIVE DESICCANT DRYER

Adsorptive desiccant driers adsorb moisture from compressed air as the compressed air passes through the desiccant. Adsorption is a physical process of condensing and holding moisture on the surface of a porous area of a hygroscopic or moisture-attracting substance.
Hygroscopic materials are solid substances that have the ability to attract moisture. These driers may use a single tower of desiccant where the desiccant is replaced when saturated, however, they normally consist of two towers through which air flow is alternated so that one tower is drying the compressed air while the other tower is being regenerated.

DEGREE OF DRYING

Adsorptive desiccant driers are normally designed to provide air at 40°F (-40°C) pressure dew point. Lower dew points can be supplied depending on type of desiccant and dryer design, reaching as low as -100°F (-73°C) at 100 psig (6.9 bar) pressure, or less than I part per million of water vapor (by volume).

ADSORPTIVE DESICCANT DRYER EQUIPMENT

• A single-bed desiccant dryer is the simplest type, where compressed air passes through one vessel and the desiccant absorbs moisture until it becomes saturated and needs replacement or regeneration by heating.
• Single-tower dryers are generally used only for small airflow or intermittent operations due to limited drying capacity.
• For most industrial applications, dual-tower desiccant dryers are preferred, allowing one tower to dry air while the other undergoes regeneration.
• Heatless (purge-type) dryers operate without heaters and regenerate the desiccant by reversing airflow and depressurizing the tower to release trapped moisture.
• These dryers use about 15–20% of dry compressed air for purging and typically work on short cycles, such as 5 minutes of drying and 5 minutes of regeneration.
• Heatless dryers are popular for small-capacity systems (1–100 scfm) and are especially useful in remote locations where electrical power is not available, as they can run entirely on compressed air.

Basic Working Principle

The operation of a refrigerated air dryer is similar to that of a refrigerator or air conditioner. In a direct-expansion system, high-pressure liquid refrigerant from the compressor flows into the evaporator through a control device. This device reduces the refrigerant pressure, causing it to boil and absorb heat from the compressed air. As heat is removed, the air cools and its moisture condenses into liquid, which is then separated and drained from the system.
The refrigerant, now in vapour form, moves back to the compressor where it is compressed into a hot, high-pressure gas. This gas enters the condenser, where it is cooled by air or water until it turns back into liquid. The liquid refrigerant then returns to the evaporator, and the cooling cycle continues.

Types of Refrigerated Dryers

Non-cycling direct-expansion dryers use a capillary tube or thermostatic expansion valve along with a hot gas bypass valve to regulate refrigerant flow. This helps maintain stable performance and prevents freezing during low-load or no-load conditions.
Cycling dryers, on the other hand, use a thermal mass such as glycol or an aluminium block to store cooling energy. During reduced load conditions, the stored cooling allows the compressor to switch off and restart only when needed, improving energy efficiency.

Note on Desiccant Systems

In contrast to refrigerated dryers, desiccant dryers use molecular sieves made of crystalline aluminium silicates to adsorb moisture. These materials can be regenerated by heating or purging with dry air, making them suitable for applications requiring extremely low dew points.