Desiccant Dehumidification: Solid and Liquid Desiccants, Psychrometric Process

Effective humidity control is critical in many HVAC applications, ranging from indoor air quality enhancement to industrial process control. Traditional vapor-compression dehumidification methods can struggle in high humidity or low temperature scenarios due to freezing constraints or energy inefficiency. This is where desiccant dehumidification systems excel by leveraging materials with a strong affinity for water vapor to remove moisture from air streams. This deep dive examines the fundamental principles, types of desiccants (solid and liquid), psychrometric impacts, design procedures, and operational considerations essential for HVAC professionals.

Introduction to Desiccant Dehumidification

The core principle of desiccant dehumidification involves the use of hygroscopic materials called desiccants that absorb or adsorb moisture from air. These systems decouple latent and sensible cooling loads by selectively removing moisture without significant temperature reduction, providing moisture control beyond the limitations of mechanical cooling.

Two main types of desiccants are employed:

• Solid Desiccants — porous materials that adsorb water vapor on their surface or within microstructures.
• Liquid Desiccants — aqueous salt solutions that chemically bind water molecules by absorption.

This article explores each desiccant type's working principles, regeneration methods, psychrometric process interactions, system design, and practical guidelines for HVAC engineers.

Technical Background

Principles of Water Vapor Removal

Water vapor removal in desiccant systems occurs primarily via two mechanisms:

• Adsorption: Water molecules adhere to the surface of a solid desiccant.
• Absorption: Water molecules dissolve into a liquid desiccant.

Both processes reduce the absolute humidity (humidity ratio, \( \omega \)) of the air, which is defined as mass of water vapor per mass of dry air (kg/kg).

Psychrometric Process and Impact

The psychrometric properties of air relevant for dehumidification include dry-bulb temperature \( T_{db} \), wet-bulb temperature \( T_{wb} \), relative humidity \( \phi \), humidity ratio \( \omega \), enthalpy \( h \), and specific volume \( v \). The moisture removal impacts the air state by moving points horizontally on a psychrometric chart during desiccant regeneration and dehumidification cycles.

The fundamental equation governing humidity ratio is:

Where:

• \( P_v \) = partial pressure of water vapor (Pa)
• \( P_{atm} \) = atmospheric pressure (~101325 Pa at sea level)

Energy Balance in Desiccant Systems

Dehumidification is accompanied by a heat effect due to water vapor condensation or phase changes on the desiccant. The overall energy balance can be expressed as:

Where:

• \( Q \) = energy removed or supplied (kW)
• \( m_{dryair} \) = mass flow of dry air (kg/s)
• \( h_1, h_2 \) = specific enthalpy of air before and after dehumidification (kJ/kg)

Data Table: Common Desiccant Materials

Desiccant Type	Material	Water Capacity (wt%)	Regeneration Temp. (°C)	Typical Application
Solid	Silica Gel	20-40%	120-150°C	General dehumidification, HVAC wheels
Solid	Zeolite	10-30%	200-350°C	High temp process air drying
Liquid	Lithium Chloride Solution	30-50%	50-80°C	Liquid desiccant HVAC systems
Liquid	Calcium Chloride Solution	30-40%	40-80°C	Industrial dehumidification

Step-by-Step Design Procedures for Desiccant Dehumidification Systems

Step 1: Determine Moisture Load

Calculate the latent moisture to be removed from the air stream based on design conditions:

Where:

• \( \dot{V} \) = volumetric flow rate (m³/s)
• \( \rho_{air} \) = density of air (~1.2 kg/m³)
• \( \omega_{in} \) = inlet humidity ratio (kg/kg)
• \( \omega_{out} \) = desired outlet humidity ratio (kg/kg)

Worked Example:

An air handling unit processes 5,000 CFM (2.36 m³/s) with 80°F dry bulb and 60% RH. Desired outlet RH is 40% at 80°F. Calculate the moisture removal in kg/hr.

• Convert volumetric flow: 5,000 CFM = 2.36 m³/s
• At 80°F, 60% RH, humidity ratio \( \omega_{in} \approx 0.0135 \) kg/kg (from psychrometric charts)
• At 80°F, 40% RH, \( \omega_{out} \approx 0.00916 \) kg/kg
• Use \( \rho_{air} = 1.2 \, kg/m^3 \)

\[ Q_{latent} = 2.36 \times 1.2 \times (0.0135 - 0.00916) = 2.36 \times 1.2 \times 0.00434 = 0.0123 \, kg/s \] Converting to kg/hr: \[ 0.0123 \times 3600 = 44.3\, kg/hr \]

Step 2: Select Desiccant Type

• For moderate temperatures and indoor HVAC, silica gel solid desiccant wheels are common.
• For continuous industrial applications with easy regeneration, liquid desiccant systems (e.g., lithium chloride) are used.

Step 3: Determine Regeneration Conditions

Efficient regeneration requires thermal energy to desorb moisture from the desiccant. Typical regeneration temperatures are 120-150°C for silica gel and lower for liquid desiccants.

Step 4: Size Desiccant Wheel or Liquid Solution System

Refer to manufacturer performance curves that specify moisture removal capacity per unit area or volume at given regeneration temperatures and air flow rates.

Worked Example (Solid Desiccant Wheel Sizing):

Given 44.3 kg/hr moisture removal, silica gel wheel capacity is approximately 0.3 kg of water/m² per cycle. If one cycle is 300 seconds, and air flow is 2.36 m³/s:

\[ \text{Required wheel area} = \frac{44.3\, kg/hr}{0.3\, kg/m^2 \times 12} \approx 12.3\, m^2 \] (Note: converting hour to 300-second cycles per hour = 12 cycles).

Step 5: Integrate Controls and Auxiliary Equipment

• Install temperature and humidity sensors pre- and post-dehumidification unit.
• Implement blower and regeneration air heater controls.
• Ensure integration with existing HVAC system for airflow balancing.

Selection and Sizing Guidance

When selecting desiccant dehumidification equipment, consider the following criteria:

• Humidity Reduction Required: Calculate latent load as described.
• Airflow Characteristics: Account for pressure drops and velocity to avoid desiccant erosion or drying inefficiency.
• Regeneration Energy Availability: Select desiccants and systems compatible with available heat sources (waste heat, solar, gas).
• Maintenance Accessibility: Choose solid wheels for low maintenance or liquid systems that require regular chemical checks.

Parameter	Solid Desiccant Wheel	Liquid Desiccant System
Typical Moisture Capacity (kg/m²/cycle)	0.2 – 0.4	Varies with solution concentration
Regeneration Temperature	120 – 180°C	50 – 80°C
Typical Air Flow Velocity	2.5 – 6.0 m/s	N/A - depends on heat exchanger design
Maintenance Frequency	Low, periodic replacement	Medium, chemical checks & handling
System Complexity	Moderate	High (pumps, heat exchangers)

Best Practices

• Pre-cool air if regeneration temperature is limited to improve capacity.
• Use multi-stage desiccant wheels or liquid absorption combined with evaporative cooling for energy optimization.
• Implement humidistat control to avoid over-drying and energy waste.
• Ensure desiccant material compatibility with ambient air contaminants.
• Monitor regeneration temperature closely to avoid thermal degradation of desiccant.
• Refer to related fundamentals in HVAC Psychrometrics and HVAC Load Calculations.

Troubleshooting Common Issues

Issue	Potential Causes	Recommended Action
Insufficient Dehumidification	Inadequate regeneration temperature Desiccant saturation or degradation Incorrect airflow or velocity	Increase regeneration heat supply Replace or reactivate desiccant Adjust blower speed to manufacturer's specs
High Pressure Drop	Accumulation of dust or contaminants Fouled desiccant material	Perform filtration and clean unit Replace desiccant media if fouled
Desiccant Wheel Excessive Wear	High airflow velocity Improper bearing lubrication	Reduce airflow velocity Regular lubrication and maintenance
Corrosion in Liquid Desiccant Loop	Improper material selection, leaks	Use corrosion-resistant × OK