Cooling-based vs Desiccant: which dehumidification method to choose for your project

Author: Mycond Technical Department

Air humidity control is one of the most complex and important engineering tasks across many areas: from ensuring a comfortable microclimate in residential and commercial spaces to creating special conditions for industrial processes, museums, laboratories, and pharmaceutical manufacturing. Excess humidity can cause a number of problems: from mold formation and corrosion to disruption of technological processes and product spoilage. For effective air dehumidification, engineers use two fundamentally different methods, each with its own advantages, limitations, and optimal application areas.

In this article, we will take a detailed look at two key air dehumidification methods: cooling-based dehumidification and desiccant dehumidification. These methods are based on different physical principles and have fundamentally different characteristics, which determines their suitability for different projects and operating conditions.

Introduction: Air dehumidification as an engineering task

Air dehumidification is the process of removing excess moisture to achieve the desired relative humidity level or dew point. Dew point temperature is the temperature at which air becomes saturated with water vapor and condensation begins. This parameter is often key when designing dehumidification systems, as it directly indicates the absolute moisture content in the air.

There are two fundamental approaches to air dehumidification:

• Cooling-based (condensation method): based on cooling the air below its dew point, which leads to moisture condensation. This method uses refrigeration cycle principles to reduce air temperature.
• Desiccant (adsorption method): uses special materials with low vapor pressure at the surface (desiccants) that attract water molecules from the air due to differences in partial pressures. This method does not depend on air temperature and can deliver extremely low dew points.

The choice of the optimal dehumidification method depends on many factors: required dew point, operating temperature range, availability of energy sources, initial and operating costs, as well as the project’s specific requirements.

Cooling-based air dehumidification

The cooling-based dehumidification method is the most common and is used in most residential and commercial dehumidifiers. This method is based on the physical principle that cold air can hold less moisture than warm air.

Physics of the dehumidification process

The cooling-based dehumidification process can be explained by the following steps:

1. Air cooling: Moist air passes through a cold heat exchanger (evaporator).
2. Reaching the dew point: When the air temperature drops to the dew point, its relative humidity reaches 100%.
3. Moisture condensation: With further cooling, excess moisture condenses on the cold surfaces of the heat exchanger.
4. Condensate removal: The collected water is drained away.
5. Reheating (optional): The dehumidified air is often reheated before being supplied to the space to reduce its relative humidity and ensure a comfortable temperature.

Types of refrigeration dehumidification systems

There are three main types of refrigeration systems for air dehumidification:

• Direct expansion (DX) systems: Use a refrigerant that circulates through a compressor, condenser, and evaporator. These systems are most common in residential and commercial dehumidifiers.
• Chilled liquid systems (chilled water or glycol): Use chilled water or glycol to cool the air. Such systems are often used in large industrial or commercial installations where centralized cooling is already available.
• Dehumidification-reheat systems: Include additional reheating of dehumidified air, often using condenser heat to improve energy efficiency. This approach enables deeper dehumidification without excessive space cooling.

Advantages of cooling systems

• High energy efficiency at high humidity (COP 2.0-4.5)
• Simultaneous air cooling and dehumidification
• Proven, widely available technology
• Relatively low initial cost
• Simple control and operation

Critical limitations of cooling systems

• Minimum achievable dew point limited to +4...+7°C due to the risk of condensate freezing on the evaporator
• Problems at low outdoor temperatures (condensation dehumidifiers are inefficient in winter)
• Outlet air is near saturation (100% RH) and often requires reheating
• Reduced efficiency at part load
• Environmental concerns related to refrigerants

Desiccant air dehumidification

The adsorption method (desiccant dehumidification) represents a fundamentally different approach to air dehumidification, which does not depend on temperature and can provide extremely low dew points.

Physics of moisture adsorption

Desiccants are materials that have low vapor pressure at their surface. When moist air contacts such materials, water molecules are attracted from the air to the desiccant due to differences in partial pressures. This process occurs regardless of air temperature and can continue until equilibrium is reached between the vapor pressure in the air and at the desiccant surface.

It is important to understand that the moisture adsorption process is accompanied by the release of heat (heat of adsorption), which leads to an increase in the temperature of the dehumidified air.

Rotary dehumidifier: operating cycle

The most common type of desiccant dehumidifier is the rotary adsorption dehumidifier. Its operation is based on the following cycle:

1. Adsorption: Moist air passes through a sector of the rotor (70-75% of the area) containing desiccant. Moisture from the air is adsorbed by the desiccant, and the dehumidified air is heated due to the release of the heat of adsorption.
2. Regeneration: A smaller part of the rotor (25-30%) simultaneously passes through the regeneration zone, where hot air at 120-250°C removes the accumulated moisture from the desiccant.
3. Rotation: The rotor rotates slowly (usually 8-10 revolutions per hour), ensuring continuous system operation.

Types of desiccants

Different types of desiccants are used in adsorption dehumidifiers, each with its own characteristics:

• Silica gel: The most common desiccant type with an adsorption capacity of 10-40% of its own weight. Silica gel is effective at relative humidity of 20-70% and is the basis of most commercial rotary dehumidifiers.
• Molecular sieves: Provide extremely low dew points (down to -40°C and below) and are particularly effective at low relative humidity. Molecular sieves have an orderly pore structure that allows selective adsorption of water molecules.
• Lithium chloride: Has an extremely high adsorption capacity (up to 1000% of its own weight) and is effective at high relative humidity. However, this material can be corrosive and requires special handling.

Advantages of desiccant systems

• Unlimited range of achievable dew points (down to -70°C)
• Efficient operation at any temperatures, including subzero
• Very dry outlet air (low relative humidity)
• Ability to operate with various energy sources (electricity, gas, steam, waste heat)
• Combines dehumidification with heating (during the cold season)
• High reliability and long service life (15-25 years)

Disadvantages of desiccant systems

• High thermal energy consumption for regeneration
• Elevated outlet air temperature (requires additional cooling for comfort)
• More complex control and operation
• Potential desiccant contamination
• Higher initial cost compared to cooling systems

Comparison of the two dehumidification methods

To ease the selection of the optimal dehumidification method, let’s look at a direct comparison of the main characteristics of cooling-based and desiccant systems:

Combined dehumidification systems

To achieve optimal energy efficiency and performance, in many cases it is advisable to combine both dehumidification methods. Let’s consider the three most common hybrid system configurations:

Pre-cooling before the desiccant

In this scheme, the air first passes through a refrigeration system that reduces its moisture content to a dew point of about +4°C, and is then directed to an adsorption dehumidifier to achieve the required low dew point. This approach significantly reduces the load on the desiccant system and cuts thermal energy consumption for regeneration by 30-50%.

Seasonal switchover

This strategy involves using different dehumidification methods depending on the season:

• Summer: predominantly cooling-based dehumidification, which simultaneously provides cooling and dehumidification.
• Winter: predominantly desiccant dehumidification, which provides simultaneous dehumidification and air heating.

This approach allows you to maximize the benefits of both methods according to seasonal needs.

Using waste heat

This scheme involves using waste heat from refrigeration plants for desiccant regeneration. This solution is especially effective in supermarkets, where there is a significant refrigeration load. This strategy can reduce the system’s total energy consumption by up to 40%.

Economic considerations

The economic feasibility of using one or another dehumidification method depends on the specific project. Let’s consider two illustrative examples:

Case 1: Residential basement

For a residential basement with high humidity and a moderate target dew point (15-18°C), the optimal solution is a condensation (cooling-based) dehumidifier. Reasons:

• Sufficient efficiency at temperatures above +15°C
• Lower initial costs
• Ease of operation and maintenance
• No need for extremely low dew points

Case 2: Pharmaceutical laboratory

For a pharmaceutical laboratory with dew point requirements below 0°C, the only viable solution is a desiccant or hybrid system. Reasons:

• Cooling systems physically cannot reach such low dew points due to condensate freezing
• The need to maintain low humidity stably regardless of outdoor conditions
• The high value of products and processes requires reliable humidity control

Decision-making flowchart

To simplify the process of choosing a dehumidification method, you can use the following decision logic:

1. If the target dew point is above +5°C and high outdoor humidity prevails — the optimal solution is cooling-based dehumidification, which will provide the highest energy efficiency.
2. If the target dew point is below +5°C and cheap thermal energy is available — the optimal solution is desiccant dehumidification, since cooling systems will not be able to achieve such low dew points.
3. If very low dew points with high energy efficiency are required — the best solution is a hybrid system with pre-cooling before the desiccant.

FAQ: Answers to common questions

Why is a condensation dehumidifier inefficient in winter?

At low temperatures, air contains less absolute moisture, so the condensation process becomes less effective. In addition, when the evaporator temperature drops below 0°C, the heat exchanger freezes, which requires periodic defrosting and reduces the overall system efficiency.

What is the minimum dew point for cooling systems?

The practical minimum dew point for cooling-based systems is around +4...+7°C. Further reduction is limited by the physics of the process—at lower evaporator temperatures, the condensate freezes, blocking the airflow and requiring defrost cycles.

When is desiccant dehumidification more economical?

Desiccant dehumidification is more economical in the following cases:

• When very low dew points are required (below +5°C)
• When cheap sources of thermal energy are available (waste heat, gas, steam)
• At low ambient temperatures
• When the value of the protected processes or products is high

Can both methods be combined?

Yes, combining methods often yields the best results. The most common approach is pre-cooling the air before feeding it to the desiccant dehumidifier. This reduces the load on the desiccant, cuts regeneration energy consumption by 30-50%, and allows achieving very low dew points with optimal operating costs.

How does temperature affect the choice of method?

Temperature critically affects the choice of dehumidification method:

• At high temperatures (above +25°C) and high humidity, cooling-based systems are usually more efficient.
• At low temperatures (below +15°C), the efficiency of cooling-based systems drops sharply, while desiccant systems retain efficiency regardless of temperature.
• At subzero temperatures, desiccant systems are the only viable solution.

Which industries require desiccant dehumidification?

Desiccant dehumidification is most often used in the following industries:

• Pharmaceutical manufacturing
• Electronics and semiconductor production
• Military and aerospace facilities
• Archives, museums, and valuables storage
• Production of hygroscopic materials and products
• Cold storage and freezers where frost formation must be prevented

Practical conclusions

Summarizing the comparison of dehumidification methods, the following practical conclusions can be drawn:

• Cooling-based dehumidification is optimal for moderate humidity requirements (dew point above +5°C) in conditions of high ambient temperature and humidity. It is a cost-effective solution for most residential and commercial spaces.
• Desiccant dehumidification is indispensable when very low humidity levels are required (dew point below +5°C), when operating at low temperatures, or when alternative thermal energy sources for regeneration are available.
• Hybrid systems provide the highest energy efficiency at very low dew points by combining the advantages of both methods.

The choice of the optimal dehumidification method for your project depends on many factors:

• The target dew point or relative humidity level
• The operating temperature range
• Available energy resources and their cost
• Initial and operating costs
• Reliability requirements and service life

Both air dehumidification technologies have their place and their optimal application areas. Understanding the physical principles, advantages, and limitations of each method enables engineers to make informed decisions and build energy-efficient, reliable humidity control systems for any application.