Author: Mycond Technical Department.
Desiccant dehumidifiers are an effective solution for humidity control in industrial and commercial spaces; however, their use creates a specific heat load that must be considered when designing cooling systems. In this article, we examine the methodology for calculating the heat load generated by adsorption dehumidifiers and explain the physical principles of converting latent heat into sensible heat that underlie this process.
1. Why it is important to calculate the heat load from a desiccant dehumidifier
The key difference between desiccant and condensation dehumidifiers lies in the principle of moisture removal. Condensation dehumidifiers remove moisture by cooling the air below the dew point, causing water vapor to condense and be removed. In contrast, desiccant dehumidifiers operate on the adsorption principle: they absorb moisture using a porous material (silica gel, molecular sieves, zeolites) WITHOUT cooling but, on the contrary, with heating of the air.
One of the most common design mistakes occurs when engineers try to extrapolate experience from designing condensation systems to desiccant ones. In condensation dehumidifiers, the temperature change of the process air is relatively small, whereas in desiccant dehumidifiers the temperature rise is significantly larger. This increase depends on the amount of moisture removed, the type of adsorbent used, and the regeneration mode, and must be accounted for in the design.
Ignoring this heat load can lead to serious consequences: overheating of the space, insufficient capacity of the air-conditioning system, and a significant increase in energy consumption. According to ASHRAE Standard 62.1, which regulates ventilation for acceptable indoor air quality, failure to account for the heat load can cause deviations from the design microclimate parameters by 15–30% depending on the dehumidification intensity.
2. Physical basis: conversion of latent heat into sensible heat
To understand the heat load from desiccant dehumidifiers, it is necessary to clearly distinguish between latent and sensible heat. Latent heat is the energy hidden in water vapor that does not change the air temperature but is spent during water evaporation and released during condensation. Sensible heat is the heat that directly changes the air temperature without changing its humidity ratio.
The adsorption process in a desiccant dehumidifier involves water molecules adhering to the porous structure of the adsorbent. This releases intermolecular bond energy when water transitions from the gaseous to the adsorbed state. This energy is the heat of adsorption, which is approximately 2400–2600 kJ/kg for silica gel, explained by the energetics of intermolecular interactions as bonds form between water molecules and active sites on the adsorbent surface.
The magnitude of the heat of adsorption is close to the heat of condensation of water (approximately 2500 kJ/kg under standard conditions), due to the similarity of the physical processes—in both cases, water molecules transition from the gaseous state to a more ordered one (liquid during condensation or adsorbed during adsorption).
On the Mollier psychrometric chart, the desiccant drying process is shown as a movement down and to the right: dry-bulb temperature increases while humidity ratio decreases. This is fundamentally different from condensation dehumidification, where the process moves down and to the left (simultaneous reduction in both temperature and humidity ratio).
3. Sources of heat load in a desiccant dehumidifier
The heat load in a desiccant dehumidifier is formed by four main sources:
1. Heat of adsorption – the primary source of heat load, released directly into the process air stream during adsorption of water vapor. The share of this heat in the overall balance depends on equipment design, the ratio between adsorption and regeneration sectors, and the quality of thermal insulation between these sectors.
2. Heat transfer from the regeneration sector – during regeneration, the adsorbent is heated to restore its adsorption capacity. The regeneration temperature depends on the type of desiccant: for silica gel it is lower (80–120°C) due to lower desorption energy; for molecular sieves it is higher (150–200°C) due to stronger bonds within the crystalline structure. Part of this heat is transferred through the rotor to the process air, even with purge zones separating the adsorption and regeneration sectors.
3. Mechanical heat – arises from rotor rotation and fan operation. Electrical energy consumed by motors is partially converted into heat through friction and electrical losses.
4. Cabinet losses – with insufficient insulation of the dehumidifier cabinet, part of the heat from the regeneration sector can be transferred through the equipment walls to the process stream or the surrounding environment.
Although the heat of adsorption is the dominant source, the overall heat load is determined by the combination of all factors, and the exact distribution among them depends on the design features of a specific dehumidifier.
4. Methodology based on moisture mass balance
For a preliminary calculation of the heat load from a desiccant dehumidifier, a method based on the moisture mass balance can be used. Consider the step-by-step algorithm:
Step 1: Determine the air parameters at the inlet and outlet of the dehumidifier (temperature and humidity ratio). These data can be obtained from a psychrometric chart or calculation tables corresponding to ISO 7726 standards for measuring microclimate parameters.
Step 2: Calculate the mass flow rate of dry air. If the volumetric flow rate is known, determine the mass flow via air density, which depends on temperature and pressure according to the ideal gas law.
Step 3: Determine the amount of moisture removed. The mass of removed moisture is the product of the dry air mass flow rate and the difference in humidity ratio at the inlet and outlet of the dehumidifier.
Step 4: Calculate the heat of adsorption. The heat of adsorption is determined by multiplying the mass of removed moisture by the specific heat of adsorption. For silica gel this value is approximately 2400–2600 kJ/kg due to the energy of intermolecular bonds during water adsorption. For molecular sieves the values are higher (up to 3000–3200 kJ/kg) due to stronger bonds in their crystalline structure.
Step 5: Determine the temperature rise. The temperature increase is the ratio of the heat of adsorption to the product of the air mass flow rate and the specific heat capacity of air (approximately 1.005 kJ/(kg·K) under standard conditions).
Step 6: Determine the actual outlet temperature considering all heat sources. Additional contributions from regeneration, mechanical heat, and losses are evaluated based on equipment design features or provided by the manufacturer.
It is important to understand that this is a simplified method for preliminary estimates. Accurate calculation requires manufacturer data or detailed modeling of heat transfer processes in accordance with EN 308, which regulates test methods for heat exchangers.
5. Methodology based on change in air enthalpy
Calculating the heat load via the change in air enthalpy is a more accurate approach, as it automatically accounts for both temperature and humidity ratio changes in a single parameter. The enthalpy of moist air is the sum of the enthalpies of dry air and the water vapor contained in it.
The air enthalpy after passing through the dehumidifier includes the enthalpy of the inlet air plus the heat of adsorption of the removed moisture. This corresponds to the thermodynamic law of energy conservation as regulated by ISO 16818 on energy efficiency in buildings.
The heat load on the cooling system is determined as the product of the air mass flow rate and the difference between the air enthalpy after the dehumidifier and the target enthalpy required for supply to the space.
For illustration, consider a numerical example: when dehumidifying air from a humidity ratio of 14 g/kg to 7 g/kg with a mass flow rate of 1 kg/s of dry air, the amount of moisture removed is 0.007 kg/s. With a specific heat of adsorption of 2500 kJ/kg (for silica gel), the heat load will be approximately 17.5 kW. Note that these figures are illustrative and, in a real project, are determined based on actual operating conditions, space parameters, and equipment characteristics. These data cannot be directly transferred to other projects without recalculation.
6. Influence of design and operating parameters
The magnitude of the heat load from a desiccant dehumidifier strongly depends on a number of design and operating parameters:
Factor 1: Ratio of adsorption and regeneration sector areas. A larger regeneration sector area increases the efficiency of adsorbent recovery but simultaneously increases heat transfer to the process air stream. The optimal ratio is determined individually for each dehumidifier type based on the balance of efficiency and energy consumption.
Factor 2: Regeneration air temperature. Higher temperature accelerates moisture desorption from the adsorbent but increases heat transfer to the process stream. Silica gel requires lower regeneration temperatures (80–120°C) due to lower desorption energy, while molecular sieves require higher temperatures (150–200°C) due to stronger bonds in their structure.
Factor 3: Rotor speed. Affects the contact time between the adsorbent and the air and the intensity of heat transfer. At excessively high rotor speeds, heat transfer from the regeneration sector may increase; at excessively low speeds, adsorption efficiency decreases.
Factor 4: Degree of adsorbent saturation. A more saturated adsorbent has lower moisture uptake efficiency and releases less heat because the adsorption process slows down. This corresponds to thermodynamic principles regulated by ISO 9346 on moisture transfer in building materials.
Factor 5: Type of desiccant. Different adsorbents have different specific heats of adsorption. For silica gel it is 2400–2600 kJ/kg due to the nature of surface bond energies; for molecular sieves it is higher due to stronger bonds in their crystalline structure.
Factor 6: Presence of cooling sectors. Additional cooling sectors can reduce the heat load on the air-conditioning system by removing part of the heat from the rotor before the air is supplied to the space.
All these parameters are interrelated, and their impact cannot be expressed by simple coefficients. Accurate determination of the heat load requires detailed manufacturer specifications or comprehensive process modeling.
7. 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 heat load specifics:
IF the dehumidifier is installed after the cooling coil: In this case, the air is already partially dehumidified by condensation on the cooling coil, so the load on the adsorbent is lower. However, the air temperature after the dehumidifier will be higher, which requires an additional cooling stage. The advantages of this configuration are reduced load on the dehumidifier and extended adsorbent service life. The disadvantages are a more complex scheme and the need for additional cooling equipment.
The heat load in such a system is determined as the sum of the load for pre-cooling and the additional cooling after the dehumidifier, taking into account the heat of adsorption in accordance with EN 15251 requirements for indoor environmental parameters.
IF the dehumidifier is installed before the cooling coil: In this case, the dehumidifier operates with warm, humid air, which increases the load on the adsorbent. The entire temperature rise created by the dehumidifier is compensated by the subsequent cooling coil, whose capacity must be correspondingly higher. The advantages of this scheme are simpler layout and the ability to compensate for the entire temperature rise with a single cooling coil. The disadvantages are higher cooling capacity and a greater load on the adsorbent.
The heat load is determined as the product of the air mass flow rate and the difference between the air enthalpy after the dehumidifier and the target enthalpy of the supply air to the space.
It is important to understand that the choice of the optimal configuration depends on the specific target indoor parameters, energy efficiency requirements, budget, and available equipment space. This choice should be based on a techno-economic comparison of options rather than universal rules.
8. Typical engineering mistakes and misconceptions
When designing systems with desiccant dehumidifiers, engineers often make a number of common mistakes that lead to incorrect assessment of the heat load:
Mistake 1: Assuming an isenthalpic process. It is often mistakenly assumed that the dehumidification process occurs without a change in air enthalpy, as in throttling. This leads to a significant underestimation of the heat load, the magnitude of which is proportional to the amount of moisture removed. When removing 1 g/kg of moisture, the underestimation is approximately 2.5 kW for each 1 kg/s of air flow, due to omission of the heat of adsorption. The correct approach is described in Sections 4 and 5.
Mistake 2: Using empirical formulas for condensation dehumidifiers. In condensation dehumidifiers, the temperature rise is only 2–3°C due to removal of both sensible and latent heat during condensation, whereas in desiccant dehumidifiers the temperature rise is much greater due to the release of intermolecular energy without simultaneous cooling. The correct calculation must consider the specifics of the desiccant process.
Mistake 3: Ignoring the effect of regeneration air. Neglecting heat transfer from the regeneration sector can lead to underestimating the total heat load by 10–30%, depending on the regeneration temperature and rotor design. At regeneration temperatures of 120–150°C, the contribution of heat transfer is particularly significant.
Mistake 4: Incorrect assessment of post-dehumidifier parameters. Calculations based on catalog data without accounting for actual operating conditions can lead to major errors. It is necessary to consider the actual inlet temperature and humidity in accordance with ISO 13790 requirements for the energy performance of buildings.
Mistake 5: Lack of compensation in the heat balance. Ignoring the heat load from the dehumidifier when calculating the overall heat balance of the space can result in insufficient air-conditioning capacity. This load can account for 10–40% of the total load on the cooling system, depending on the dehumidification intensity and other heat gains.
Mistake 6: Using catalog data without clarifying test conditions. Manufacturers often provide data obtained under certain standard conditions that may differ from real operating conditions. It is necessary to clarify test conditions and apply correction factors.
9. Limits of applicability and special cases
The presented methods for calculating heat load have certain limitations that must be considered:
Group 1: Temperature limits. At low temperatures (below 5–10°C), diffusion of water molecules into the porous structure of the adsorbent slows down, reducing adsorption efficiency. At high temperatures (above 35–45°C), adsorption capacity decreases due to thermodynamic regularities—thermal motion of molecules counteracts adsorption forces. Specific temperature limits depend on the type of adsorbent: silica gel works better at lower temperatures, while molecular sieves maintain efficiency over a wider range.
Group 2: Humidity limits. At very low humidity ratios (below 2–3 g/kg), dehumidification efficiency drops due to a decrease in the partial pressure of water vapor, which complicates adsorption according to Henry’s law. At very high humidity ratios (above 25–30 g/kg), capillary condensation may occur in the pores of the adsorbent, changing the nature of heat transfer.
Group 3: Systems with partial regeneration. In systems with incomplete adsorbent recovery, accumulation of residual moisture leads to changes in adsorption characteristics and the heat balance. Standard methods do not account for this factor, which can lead to errors in assessing the heat load.
Group 4: Systems with integrated cooling. Some dehumidifier designs include built-in cooling elements that alter the overall heat balance. Internal heat flows in such systems are not accounted for by standard methods.
Group 5: Liquid desiccant systems. In liquid desiccant systems, the physics of the process differs significantly from solid adsorbents. The moisture absorption process is accompanied by desiccant dissolution and changes in its concentration, requiring a completely different approach to calculating the heat load.
In all these cases, standard methods provide only an approximate estimate. Accurate determination of the heat load requires specialized analysis, detailed process modeling, or consultation with equipment manufacturers.
10. FAQ (Frequently asked questions)
Question 1: By how many degrees does the temperature increase after the dehumidifier?
Answer: The temperature rise depends on the amount of moisture removed, the type of adsorbent, and the regeneration mode. As a rough estimate, the temperature increase can be evaluated by the formula: temperature rise = (amount of moisture removed × specific heat of adsorption) / (air mass flow rate × specific heat capacity of air). For example, when removing 1 g of moisture from 1 kg of air using silica gel (heat of adsorption 2500 kJ/kg), the temperature will increase by approximately 2.5°C. This is an empirical formula that provides a baseline estimate; its accuracy depends on the specific operating conditions.
Question 2: Can I simply increase the air-conditioner capacity to compensate for the heat load?
Answer: Yes, this is a necessary measure, but it has consequences. Increasing the air-conditioner capacity results in higher capital costs for equipment and higher operating electricity costs. Alternatives may include: using pre-cooling of air before the dehumidifier, installing heat recovery, using dehumidifiers with cooled rotor sectors, or selecting more energy-efficient cooling systems.
Question 3: How can the heat load from a desiccant dehumidifier be minimized?
Answer: Possible measures include: optimizing the regeneration mode to minimize heat transfer, installing purge sectors to reduce rotor temperature before contact with the process air, using heat recovery from the process air after the dehumidifier, applying pre-cooling, selecting adsorbents with lower heat of adsorption, and improving insulation between adsorption and regeneration sectors. The effectiveness of each measure depends on specific operating conditions and design capabilities.
Question 4: Does the calculation differ for silica gel and molecular sieves?
Answer: Yes, calculations differ due to different specific heats of adsorption. Molecular sieves have a higher heat of adsorption (up to 3000–3200 kJ/kg) compared to silica gel (2400–2600 kJ/kg) due to stronger bonds in their crystalline structure. This results in a greater temperature rise for the same amount of moisture removed. In addition, molecular sieves require higher regeneration temperatures, which can increase heat transfer from the regeneration sector.
Question 5: Which is better—the dehumidifier before or after the cooling coil?
Answer: There is no universal answer; the choice depends on the specific project conditions. Placing the dehumidifier after the cooling coil reduces the load on the adsorbent but requires additional cooling. Placing the dehumidifier before the cooling coil provides a simpler scheme but requires higher cooling capacity. The optimal option is determined based on a techno-economic analysis considering capital costs, operating costs, available space, and air parameter requirements.
Question 6: Is a separate calculation needed for each operating mode?
Answer: Yes, the heat load changes depending on the operating mode, as environmental conditions, dehumidification requirements, and other parameters vary. Calculations should be performed for characteristic modes: maximum load (usually in summer at high humidity), minimum load (usually in winter), and typical intermediate modes. For critical facilities, an analysis of the dynamic load variation throughout the year may be required.
Question 7: What is the accuracy of the calculations using the presented methods?
Answer: Simplified methods yield an error in the range of 10–25%, depending on how well actual operating conditions match the assumptions. The main sources of error are disregarding the nonlinearity of adsorption processes under different conditions, simplified consideration of heat transfer from the regeneration sector, and ignoring dynamic effects. For critical designs, it is recommended to use tested equipment data, results of detailed modeling, and to provide control capability in the cooling system to compensate for possible deviations.
11. Conclusions
Based on the analysis performed, we can formulate the following key conclusions:
1. Desiccant dehumidifiers always increase the process air temperature due to the heat of adsorption. This is a fundamental physical property of adsorption processes that cannot be eliminated, only compensated.
2. The heat load from desiccant dehumidifiers can represent a significant portion of the total load on the cooling system—from 10% to 40%, depending on the dehumidification intensity and other heat gains. Ignoring this load is a critical design error.
3. The heat load can be calculated by two main methods: via moisture mass balance (for preliminary estimates) and via change in air enthalpy (for detailed design). Both methods must account for all heat sources, including heat of adsorption, heat transfer from the regeneration sector, mechanical heat, and thermal losses. The enthalpy method yields an error within 5–10% provided the air parameters are determined correctly.
4. The choice of system configuration (placement of the dehumidifier relative to the cooling coil) affects the distribution of heat loads. The optimal solution is determined by a techno-economic analysis of the specific project; there is no universal option.
5. Various engineering measures are available to minimize the heat load, each with its own benefits and costs. Their economic feasibility is evaluated in the context of the specific project.
6. The accuracy of the heat load calculation depends on the quality of input data. For critical projects, it is recommended to use test data, modeling results, and to provide technical margins when selecting equipment.
7. The calculation methods have limitations under extreme operating conditions, for which specialized analysis is required.
Proper accounting of the heat load from desiccant dehumidifiers is a mandatory condition for high-quality design of ventilation and air-conditioning systems. An engineer must master the calculation methodology, understand the physical foundations of the processes, use verified data, and critically evaluate the obtained results to ensure optimal indoor conditions with minimal energy consumption.
