Author: Mycond Technical Department
Among the various configurations of desiccant dehumidifiers, the rotating wheel with a corrugated semi-ceramic structure has become the dominant solution in modern adsorption drying technology. This advantage stems from the unique ability of the design to combine the key benefits of all previous types — the continuity of the process as in tray systems, the achievement of low dew points as in packed towers, and high energy efficiency thanks to the low mass of the active material.
The adsorption wheel, also known as a DEW (Desiccant Wheel), differs fundamentally from classic packed towers with granular silica gel, horizontal rotating trays and vertical multi-tier beds with ratchet drive. Thanks to its innovative design, it ensures stable operation without fluctuations in outlet parameters, achieves extremely low dew points down to -68°C, and at the same time consumes significantly less energy for regeneration.
Honeycombe® wheel design
The core of the Honeycombe® desiccant rotor is a semi-ceramic structure based on a fibreglass matrix that visually resembles corrugated cardboard rolled into a cylindrical shape. This corrugated semi-ceramic rotor structure creates thousands of parallel air channels running through the entire depth of the wheel.
The grooves (flutes) formed by the corrugations act as individual air channels coated with a finely dispersed adsorbent. A typical design contains over 82% silica gel in the desiccant, applied to the internal surface of the channels. A key performance parameter is the internal surface area of the silica gel, which is 21 000–22 700 m² per ounce (228 864–244 121 sq ft/oz), ensuring an extremely low partial vapour pressure at the surface.
The physical principle of operation is based on the fundamental laws of thermodynamics: water vapour diffuses from areas of higher partial pressure (humid air) to areas of lower pressure (adsorbent surface). Unlike packed beds, where the flow is turbulent, the laminar flow in the wheel through straight channels provides a significantly lower aerodynamic resistance, which increases only in proportion to the wheel depth, rather than as the square of the velocity.
Adsorption–desorption cycle
The operating cycle of the desiccant rotor is based on a clear division of the wheel into two functional zones: a 270° drying zone (three quarters of the area) and a 90° regeneration zone (one quarter), which are isolated from each other by special seals. For active adsorption, the typical rotation speed is 5–30 rev/h, which differs significantly from passive enthalpy wheels that rotate at 20–60 rpm.
The adsorption–desorption cycle consists of three sequential phases:
- Phase 1 (point 1→2): dry, cool desiccant with low surface vapour pressure adsorbs moisture from the process air, gradually becoming saturated and heating up due to the heat of sorption.
- Phase 2 (point 2→3): the saturated desiccant moves into the regeneration zone, where it is heated by hot air (typically up to 120°C from a PTC heater), the surface vapour pressure rises sharply, and moisture is released into the regeneration airflow.
- Phase 3 (point 3→1): the hot dry desiccant returns to the drying zone, where it is cooled by part of the process air, restoring a low surface vapour pressure for a new adsorption cycle.
It is important to note that the regeneration airflow is approximately one third of the process airflow (flow ratio 3:1) and moves counter-current. When moisture is removed, sorption heat of 2500–3050 kJ/kg of moisture (1080–1312 BTU/lb) is released, which heats the process air in proportion to the amount of moisture removed. For example, air at 21°C and 50% RH, after deep drying to a dew point of 7°C, may heat up to 49°C, which often requires additional cooling.
Types of desiccants and their sorption characteristics
The sorption capacity of a desiccant is a key parameter of dehumidifier performance. At a temperature of 25°C (77°F), different desiccants show significantly different abilities to retain moisture:
- Silica gel Type 5: 2.5% at 20% RH
- Silica gel Type 1: 15% at 20% RH
- Molecular sieves: 20% at 20% RH
- Lithium chloride: 35% at 20% RH
This means that to remove 22.7 kg (50 lb) of water vapour from air at 20% RH, it would theoretically take: 907 kg (2000 lb) of Silica gel Type 5, or 151 kg (333 lb) of Silica gel Type 1, or 113 kg (250 lb) of molecular sieves, or 65 kg (143 lb) of lithium chloride. Actual amounts are significantly higher due to process dynamics.
A strategy of combining desiccants allows mutually exclusive goals to be achieved: low dew point and high throughput simultaneously. Type 1 provides capacity in the lower humidity ranges, while Type 5 adsorbs large amounts of water at humidities above 90% RH. Molecular sieves for low dew points are most effective when drying to extremely low levels (below 10% RH or a dew point of -40°C), where they have the greatest capacity among all adsorbents.
Advantages of the Honeycombe® design
Compared with alternative configurations, the Honeycombe® design offers a number of significant advantages:
- Low rotating mass with high moisture removal capacity: since the energy for heating and cooling is directly proportional to the mass of the desiccant, the lightweight construction delivers higher energy efficiency.
- Low aerodynamic resistance: thanks to laminar flow through straight channels, resistance is significantly lower compared to turbulent flow in packed beds.
- Achievement of low dew points: the ability to reach dew points down to -68°C (-90°F) when using appropriate desiccants.
- Simplicity of construction: a minimum of moving parts (only the wheel and drive) reduces maintenance costs.
- Loading flexibility: the ability to use both solid and liquid desiccants for specific applications.
- Absence of a “saw-tooth” effect: unlike packed towers with periodic regeneration, it provides stable parameters of the dried air.
The only significant drawback is the higher manufacturing cost of the wheel compared to dry desiccant granules; however, this difference is offset by operational advantages over a typical service life of 15–30 years.
Factors affecting performance
The efficiency of an adsorption dehumidifier depends on several key parameters:
Wheel depth: increasing depth increases the contact area of the desiccant with air and the amount of moisture removed, but aerodynamic resistance increases proportionally, raising fan energy consumption.
Rotation speed 5–30 rev/h: faster rotation increases the amount of desiccant that cyclically contacts the air, boosting performance, but also increases heat carryover from the regeneration zone into the drying zone, which often requires additional cooling of the process air.
Regeneration temperature: a higher reactivation temperature of 120°C ensures more complete desorption of moisture, but removing the last portions of tightly bound water requires high energy. Some manufacturers apply two-stage regeneration — 70–80% of the moisture is removed by low-grade heat, and the final drying by high temperature.
Tightness between zones: any leakage of moist regeneration air into the dry process stream significantly degrades performance and affects the dew point of the air at the dehumidifier outlet.
Impact of contaminants: dust gradually clogs the adsorbent pores, reducing capacity over the years; organic vapours can polymerise at high temperatures, blocking pores; sulphur trioxide can convert lithium chloride to lithium sulphate over a few years. It is recommended to always filter the inlet air to the dehumidifier.
Applications
Adsorption dehumidifiers with Honeycombe® technology are widely used in various industries:
- Pharmaceutical manufacturing: cleanrooms for tableting and packaging with humidity control down to 10% RH and accuracy of ±2% RH, typical dew point of -11°C at a temperature of 21°C.
- Food industry: packaging of hygroscopic products, spray drying, where air heating during dehumidification is beneficial to the process.
- Semiconductor manufacturing: humidity control for hygroscopic photoresists, where microscopic moisture uptake leads to opens or shorts in microchips.
- Archival storage: museums, libraries, document repositories with control at 35% RH to prevent corrosion and mould.
- Protection of military and industrial equipment: storage of electronics and precision instruments at 30–35% RH to reduce contact corrosion.
- Cold stores and supermarkets: preventing frosting of display cases and improving the efficiency of refrigeration systems.
FAQ: Frequently asked questions
What is the difference between passive and active adsorption?
Passive uses the humidity difference between airstreams without heat input (enthalpy wheels 20–60 rpm), active applies heating of the regeneration air for deep drying (5–30 rev/h). Only active can dry air below the humidity level in the room.
Why is Honeycombe® more efficient than packed beds?
Thanks to laminar flow through straight channels, low mass with high surface area, continuous operation without saw-tooth humidity fluctuations, and the ability to combine desiccants.
What dew point can be achieved?
With silica gel down to -68°C (-90°F), with molecular sieves even lower. For most industrial applications, -40°C is sufficient.
What is the service life of an adsorption wheel?
Typically 15–30 years with proper air filtration. There is gradual degradation due to pore contamination by dust and organic substances.
Conclusions
Honeycombe® technology has become the standard for adsorption dehumidification thanks to the optimal balance between performance, energy efficiency and reliability. For design engineers, three key recommendations can be formulated:
- Select the desiccant type according to the target dew point (silica gel for typical applications, molecular sieves for ultra-deep drying).
- Maximise the use of recovered heat for regeneration as the main factor in reducing operating costs.
- Ensure proper filtration of inlet air to protect the wheel and increase its service life.
Desiccant wheels are most optimal when dew points below 7–10°C are required, with high latent loads, low operating temperatures, or availability of inexpensive heat for regeneration. This technology continues to set benchmarks for reliability and efficiency in adsorption air dehumidification.
