An engineering method for calculating the thermal load of desiccant dehumidifiers on the air-conditioning system

Author: Mycond Technical Department.

Desiccant (adsorption) air dehumidifiers are increasingly used in projects with heightened requirements for humidity control, especially for precision manufacturing, pharmaceuticals, the food industry and process rooms. However, their use creates an additional thermal load that is often underestimated by designers, which can lead to serious errors when calculating air-conditioning and cooling systems.

Why it is important to calculate the thermal load from a desiccant dehumidifier

The main difference between desiccant and condensing dehumidifiers lies in the principle of moisture removal. Condensing dehumidifiers work by cooling the air below its dew point, causing moisture to condense. This process usually does not cause a significant increase in air temperature. By contrast, desiccant dehumidifiers remove moisture through adsorption on the surface of the desiccant, which is accompanied by heat release and an increase in air temperature.

A typical design error is that engineers often extrapolate their experience with condensing systems to desiccant ones, failing to consider that the air temperature after an adsorption dehumidifier can rise by 10–20°C. This value is not fixed and depends on many factors: the amount of moisture removed, the type of adsorbent used, the regeneration regime and the design features of the equipment.

Neglecting this additional thermal load leads to serious consequences:

• Overheating of the space, which can negatively affect technological processes
• Insufficient capacity of the air-conditioning system to compensate for the thermal load
• Increased energy consumption due to inefficient use of equipment
• Inability to achieve the design indoor environmental parameters

Physical basis: conversion of latent heat into sensible heat

To understand the process, the terms must be clearly defined:

• Latent heat — the energy hidden in water vapour that does not cause a change in air temperature. This energy is consumed during evaporation and released during condensation.
• Sensible heat — the energy that directly changes the air temperature without changing its moisture content.

The adsorption of water vapour on the desiccant surface (silica gel, zeolites, molecular sieves) is accompanied by the release of adsorption heat. During this process, water molecules adhere to the porous structure of the adsorbent, transitioning from the gaseous state to the adsorbed state. This transition is accompanied by the release of intermolecular bond energy.

The heat of adsorption of water on silica gel is approximately 2400–2600 kJ/kg. This value is close to the heat of condensation of water (approximately 2500 kJ/kg at 20°C), which is explained by the similarity of the physical processes — in both cases there is a change in the state of water molecules from gaseous to a more bound state, accompanied by energy release.

From the standpoint of the Mollier psychrometric chart, the adsorption dehumidification process is represented by a line that goes downwards to the right: the moisture content of the air decreases while the dry-bulb temperature increases. This differs significantly from the condensing dehumidification process, which is represented by a line downwards to the left (both temperature and moisture content decrease).

Sources of thermal load in a desiccant dehumidifier

In a desiccant dehumidifier, four main sources of thermal load can be identified:

1. Adsorption heat: released directly into the process air stream during adsorption of water vapour on the desiccant surface. This is the main contribution to the overall thermal load, the magnitude of which depends on equipment design, the ratio of adsorption and regeneration sectors, and the quality of thermal insulation.
2. Heat transfer from the regeneration sector: the adsorbent is heated to restore its adsorption capacity (regeneration). The regeneration temperature depends on the type of desiccant — silica gel requires less heat (typically 80–120°C) due to lower desorption energy, whereas molecular sieves require higher temperatures (120–180°C) due to stronger bonds in the crystalline structure. Part of this heat is transferred to the process air through the rotor, even with purge zones.
3. Mechanical heat: arises from rotor rotation and fan operation, where electrical energy is partially converted into heat.
4. Heat losses through the casing: particularly significant when the dehumidifier casing is inadequately insulated.

Thus, although the main source of thermal load is adsorption heat, the overall load is determined by the combination of all four factors and can vary significantly depending on the design features and operating parameters of the dehumidifier.

Calculation method via moisture mass balance

Calculating the thermal load via the moisture mass balance is a relatively simple method for a preliminary assessment. It is based on determining the amount of moisture removed and the corresponding release of adsorption heat. Below is a step-by-step calculation algorithm:

Step 1: Determine the air parameters at the dehumidifier inlet and outlet (temperature and moisture content) using a psychrometric chart or calculation tables in accordance with ISO 7726 and ASHRAE Standard 55.

Step 2: Calculate the mass flow rate of dry air. If a volumetric flow rate is given, determine the mass flow via the air density, which depends on temperature and pressure.

Step 3: Determine the amount of moisture removed. The mass of removed moisture is defined as the product of the dry-air mass flow rate and the difference in moisture content at the inlet and outlet of the dehumidifier.

Step 4: Calculate the heat of adsorption. The adsorption heat is found by multiplying the mass of removed moisture by the specific heat of adsorption. For silica gel, the specific heat of adsorption is approximately 2400–2600 kJ/kg due to the intermolecular bonding energy during adsorption of water molecules. For molecular sieves, this value may be higher (2600–3000 kJ/kg) due to stronger bonds in their crystalline structure.

Step 5: Determine the temperature rise. The temperature rise is the ratio of the adsorption heat to the product of the air mass flow rate and the specific heat capacity of air.

Step 6: Determine the actual outlet temperature taking all heat sources into account. Additional components of thermal load from regeneration, mechanical heat and heat losses are assessed based on design features or data from the equipment manufacturer.

It is important to note that this method is simplified and intended for preliminary assessments. Accurate thermal load calculation requires detailed manufacturer data or process modelling in accordance with EN 308 and EN 13053.

Calculation method via the change in air enthalpy

The method of calculating via the change in air enthalpy is more accurate, as it automatically accounts for changes in both temperature and moisture content. The enthalpy of moist air is the sum of the enthalpy of dry air and the enthalpy of the water vapour contained within it.

The cooling thermal load is defined as the product of the air mass flow rate and the difference between the enthalpy after the dehumidifier and the target enthalpy required for supply to the space.

The air enthalpy at the outlet of the dehumidifier includes the enthalpy of the inlet air plus the heat of adsorption of the removed moisture distributed across the air mass:

Q(cooling) = G(air) × [h(after dehumidifier) - h(target)]

Consider a numerical example:

• Air parameters at the dehumidifier inlet: temperature 25°C, relative humidity 60% (moisture content approximately 11.9 g/kg, enthalpy 55.7 kJ/kg)
• Air flow rate: 5000 m³/h (approximately 1.7 kg/s under standard conditions)
• Required outlet moisture content: 6 g/kg

Amount of moisture removed: 1.7 kg/s × (11.9 − 6) g/kg = 10.03 g/s

Adsorption heat: 10.03 g/s × 2500 kJ/kg = 25.1 kW

Distributing this heat across the air mass increases the enthalpy after the dehumidifier by 25.1 kW / 1.7 kg/s = 14.8 kJ/kg

Thus, the enthalpy after the dehumidifier: 55.7 + 14.8 = 70.5 kJ/kg

If the target enthalpy for supply to the space is 50 kJ/kg, then the cooling thermal load is: 1.7 kg/s × (70.5 − 50) kJ/kg = 34.9 kW

Note that these figures are illustrative and in a real project are determined based on actual operating conditions, room parameters and equipment characteristics. They cannot be transferred to other sites without recalculation in accordance with the methodology described in EN 16798 and ASHRAE Standard 62.1.

Influence of design and operating parameters

The thermal load from a desiccant dehumidifier significantly depends on a number of design and operating parameters:

1. Ratio of adsorption to regeneration sector area. A larger regeneration area increases heat transfer to the process stream, but improves adsorbent recovery. The optimal ratio is defined by a balance between dehumidification efficiency and thermal load, and depends on the specific application.
2. Regeneration air temperature. A higher temperature accelerates moisture desorption from the adsorbent but increases heat transfer to the process stream. For silica gel, a regeneration temperature of 80–120°C is usually sufficient due to lower desorption energy, whereas molecular sieves require 120–180°C due to stronger water-molecule bonds within their structure.
3. Rotor rotational speed. Affects the contact time of air with the adsorbent and the efficiency of heat and mass transfer. The optimal speed depends on the type of desiccant, air parameters and the desired level of dehumidification.
4. Degree of adsorbent saturation. A more saturated adsorbent is less effective at removing moisture and heats up less, as the adsorption process slows down.
5. Type of desiccant. Different adsorbents have different heats of adsorption: silica gel — 2400–2600 kJ/kg due to relatively weak adsorption forces, molecular sieves — up to 3000 kJ/kg due to stronger ionic and covalent bonds in their crystalline structure.
6. Presence of cooling sectors. Some dehumidifier designs include cooling sectors for the rotor, allowing the adsorbent temperature to be lowered before contact with the process air, thereby reducing the thermal load.

It is important to understand that all these parameters are interrelated, and their influence cannot be expressed by simple coefficients. Accurate assessment of the thermal load requires detailed equipment specifications from the manufacturer or specialised modelling in accordance with ISO 16818 and EN 13779.

Integration of the dehumidifier into the ventilation and air-conditioning system

There are two main options for integrating a desiccant dehumidifier into a ventilation and air-conditioning system, each with its own characteristics in terms of thermal load:

IF the dehumidifier is located after the cooler:

• The air is already partially dehumidified by condensation on the cooler surface
• The load on the adsorbent is reduced, which decreases adsorption heat
• The post-dehumidification temperature is higher, so an additional cooling stage is required
• Advantages: reduced load on the dehumidifier, lower energy consumption for regeneration
• Disadvantages: a more complex scheme, additional equipment required

The thermal load in this case is determined by the product of the air mass flow rate and the difference in enthalpy between the air after the dehumidifier and the target enthalpy for supply to the space.

IF the dehumidifier is located before the cooler:

• The dehumidifier operates with warm, humid air
• The entire temperature increase is compensated by the subsequent cooler
• The cooler capacity must be significantly higher
• Advantages: a simple scheme, the entire increase is compensated by a single cooler
• Disadvantages: higher cooling capacity required, greater load on the adsorbent

The thermal load is defined as the sum of the cooling and dehumidification load of the inlet air plus the additional load from adsorption heat.

The choice of the optimal integration scheme depends on many factors: target air parameters, energy efficiency, project budget and available space for equipment. The decision should be based on a techno-economic comparison of the options for the specific project, not on universal rules.

Typical engineering errors and misconceptions

When designing systems with desiccant dehumidifiers, the following engineering errors are often encountered:

1. Assuming an isoenthalpic process. Some engineers mistakenly believe that dehumidification occurs without a change in enthalpy. This leads to an underestimation of the cooling load by 20–40%, depending on the amount of moisture removed. The more moisture removed, the more adsorption heat is released and the greater the error when ignoring the enthalpy increase.
2. Using empirical formulae for condensing dehumidifiers. The temperature rise in condensing dehumidifiers is 2–3°C due to heat from the compressor, whereas in desiccant dehumidifiers the rise can be 10–20°C due to adsorption heat. The difference is explained by the fundamental distinction in physical processes: in condensing systems the main process is cooling, while in desiccant systems it is adsorption with heat release.
3. Ignoring the impact of regeneration air. Heat transfer from the hot regeneration sector can add 10–30% to the total thermal load, depending on regeneration temperature and rotor design. The higher the regeneration temperature and the poorer the insulation between sectors, the more heat is transferred.
4. Incorrect assessment of air parameters after the dehumidifier. Designers often underestimate the temperature or overestimate the moisture content at the dehumidifier outlet, leading to discrepancies between actual and design parameters.
5. Lack of compensation in the heat balance. The thermal load from a desiccant dehumidifier can be 15–50% of the total cooling load, depending on the amount of moisture removed and the overall heat balance of the space.
6. Using catalogue data without clarifying test conditions. Dehumidifier specifications in catalogues are usually given for standard conditions, which may differ significantly from the design conditions.

The correct approach for each of these points is described in the corresponding sections of this article: for understanding the conversion of latent heat into sensible heat see Section 2, for calculation methods — Sections 4 and 5, for considering design features — Section 6.

Limits of applicability of the methodology and special cases

The described methods for calculating thermal load have certain limitations and features of application:

1. Temperature limits. At low temperatures (below 5–10°C) the diffusion of water molecules slows down, reducing adsorption efficiency. At high temperatures (above 35–40°C) adsorption capacity decreases due to thermodynamic regularities — increased thermal motion of molecules hinders their adsorption. Exact threshold temperatures depend on the adsorbent type: silica gel is more effective at lower temperatures, while molecular sieves work well over a wider range.
2. Humidity limits. At very low relative humidity (below 20%) dehumidification efficiency drops due to the reduced vapour pressure gradient between the air and the adsorbent surface. At extremely high humidity (around 90–100%) condensation may occur within the adsorption layer, changing the nature of heat exchange.
3. Systems with partial regeneration. If adsorbent regeneration is incomplete, residual moisture accumulates, altering the system’s heat balance. Such regimes are often applied to save energy but complicate calculations.
4. Systems with integrated cooling. Some dehumidifier designs include built-in coolers, which change internal heat flows not accounted for by the standard methodology.
5. Liquid desiccant systems. These systems use liquid hygroscopic solutions instead of solid adsorbents, fundamentally changing the physics of the process. The heat of adsorption in such systems is removed directly by cooling the liquid, rather than via the air stream.

All these special cases require specialised analysis, modelling or consultation with equipment manufacturers. Standard methods should be applied with caution, understanding their limitations and considering possible deviations from idealised models.

FAQ (Frequently Asked Questions)

By how many degrees does the temperature increase after the dehumidifier?

The temperature rise depends on the amount of moisture removed, the type of adsorbent and the regeneration regime. As a rough estimate: ΔT = m(removed moisture) × h(adsorption) / [m(air) × c(air)], where h(adsorption) is the specific heat of adsorption (2400–2600 kJ/kg for silica gel), and c(air) is the specific heat capacity of air (approximately 1 kJ/(kg·K)). This formula provides a rough estimate and is applicable only for preliminary calculations under standard conditions and moderate dehumidification.

Can I simply increase the air conditioner capacity to compensate for the thermal load?

Yes, this is necessary, but it has consequences. Increasing the air conditioner capacity raises both capital costs (larger equipment, larger ductwork) and operating costs (higher energy consumption). Alternatives can include: installing a pre-cooling system, using heat exchangers for heat recovery, applying dehumidifiers with cooling sectors, optimisation of operating regimes.

How can the thermal load from a desiccant dehumidifier be minimised?

Key measures include: using rotary dehumidifiers with purge sectors to reduce heat transfer from the regeneration sector; pre-cooling the air before the dehumidifier to reduce absolute humidity; using heat exchangers for heat recovery; optimising regeneration temperature (reducing it to the minimum necessary); using adsorbents with a lower heat of adsorption; implementing systems with variable rotor speed to adapt to the actual humidity load.

Does the calculation differ for silica gel and molecular sieves?

Yes, the heat of adsorption for silica gel (2400–2600 kJ/kg) and molecular sieves (up to 3000 kJ/kg) differs due to different physical and chemical surface properties. Molecular sieves form stronger bonds with water molecules due to the ionic nature of the interaction and their specific crystalline structure, which leads to a higher heat of adsorption and, consequently, a greater thermal load.

Which is better: the dehumidifier before or after the cooler?

There is no universal answer. The decision depends on the specific project and requirements. Placing the dehumidifier after the cooler reduces the load on the adsorbent but requires additional cooling. Placing the dehumidifier before the cooler simplifies the scheme but increases the cooler load. The choice should be based on a techno-economic analysis for the specific system.

Is a separate calculation required for each operating mode?

Yes, the thermal load changes significantly depending on the dehumidifier’s operating mode and environmental parameters. Calculations are needed for characteristic modes: peak load, typical operating mode, night mode, partial regeneration mode, etc. It is particularly important to analyse the most critical mode, which defines the maximum thermal load.

What is the accuracy of the methods provided?

Simplified methods yield an error within 10–20% under standard conditions. Accuracy depends on the quality of input data, consideration of all influencing factors and the specifics of the particular equipment. For critical design, it is recommended to use manufacturer test data or specialised modelling. It is also advisable to allow a reserve capacity of 10–15% for cooling equipment to compensate for possible deviations.

Conclusions

1. Desiccant dehumidifiers always increase air temperature due to the release of adsorption heat. This is a fundamental property of the physical process, which cannot be eliminated, only compensated.
2. The thermal load from a desiccant dehumidifier can be significant and account for 15–50% of the total cooling load, depending on the amount of moisture removed, the type of adsorbent and equipment design. Ignoring this load is a critical mistake.
3. The thermal load can be calculated by two main methods: via the moisture mass balance (for preliminary assessments) and via the change in enthalpy (for detailed design). Both methods must consider all heat sources.
4. The choice of system configuration (placement of the dehumidifier relative to the cooler) affects the distribution of thermal loads. The optimal solution is defined by analysis of the specific project; there is no universally best option.
5. Various measures exist to minimise the thermal load: from dehumidifier design features to optimisation of operating regimes. Each measure has its own benefits and costs and should be evaluated economically.
6. The accuracy of the thermal load calculation depends on the quality of input data. For critical projects, it is necessary to use equipment test data, modelling and to provide technical margins.
7. The methodology has limitations under extreme operating conditions, which require specialised analysis.

Proper consideration of the thermal load from a desiccant dehumidifier is a prerequisite for high-quality design of ventilation and air-conditioning systems. The engineer must master the calculation methodology, understand the physics of the process, use verified data and critically evaluate the results. Only such an approach will ensure efficient system operation and achievement of the design indoor climate parameters with optimal energy and economic performance.