Author: Mycond Technical Department
Controlling microclimate parameters in museums and archive spaces is a critical factor in preserving cultural heritage. Precise maintenance of temperature and humidity directly affects the rate of exhibit degradation and prevents mold growth, corrosion, and material deformation. Designing air dehumidification systems for such facilities requires a comprehensive engineering approach that accounts for building characteristics, types of exhibits, and room operating modes.
Regulatory requirements for the microclimate of museum and archive spaces
The temperature and humidity regime for museum and archive rooms is determined by the types of exhibits stored. International and national standards set the following ranges:
- Paper and documents: 18-22°C, 50-55% relative humidity (RH)
- Wood and furniture: 18-22°C, 45-55% RH
- Metal and weapons: 15-20°C, 35-45% RH
- Textiles and fabrics: 18-20°C, 50-55% RH
- Paintings on canvas: 18-22°C, 50-55% RH
- Photographs and films: 15-18°C, 30-40% RH
The general regulatory range for most materials is 40-55% RH; however, sensitive exhibits require narrower limits. Equally important is the restriction on permissible fluctuations: no more than 2-3°C and 5-7% RH per day to avoid thermal deformation. Seasonal changes must be gradual — no more than 3-5% RH per week.
The main regulatory basis for system design includes ISO 11799 (for archives), ASHRAE Chapter 24 (for museums), and EN 15757 (for cultural heritage sites), which define microclimate parameter requirements and methods of maintaining them.
Specifics of archive storage compared to exhibition halls
Archive storage rooms differ significantly from exhibition halls, which affects dehumidification system design. Archive operations typically involve infrequent personnel access, unlike exhibition halls with continuous visitor movement.
Archive temperature regimes are usually lower (15-18°C), helping slow material degradation. Relative humidity requirements for archival documents are often stricter (40-50% RH), with smaller allowable fluctuations (±3% RH) compared to exhibition halls.
A key feature of archives is the absence of moisture emissions from visitors, which simplifies moisture balance calculations. However, for archives, redundancy in climate control systems is crucial due to the high value of unique documents.
When choosing a dehumidification system for archives, note that in cold rooms (below 15°C) only adsorption systems are effective, as condensation dehumidifiers lose capacity under such conditions.
Components of the moisture balance of a museum space
To correctly size the dehumidification system, all components of the room’s moisture balance must be identified:
- Infiltration — moisture penetration through building envelopes, windows, doors, joints, and gaps. Especially critical for historic buildings. Calculated based on the difference in absolute humidity between outdoor and indoor air, multiplied by the air change rate.
- Moisture emissions from visitors — one adult emits 40-80 g/h of moisture, depending on activity level and room temperature. The calculation accounts for the number of people and their dwell time (typically 30-90 minutes).
- Moisture exchange with exhibits — hygroscopic materials (wood, paper, textiles) absorb or release moisture depending on changes in air relative humidity, creating a buffering effect.
- Humidity of supply air — depends on ventilation system parameters.
- Condensation on cold surfaces — may occur on display case glass, exterior walls, and uninsulated pipes if their temperature is below the dew point.
The algorithm for determining the total moisture load includes sequential calculation of all components and summing them. It is important to account for seasonal changes in moisture balance — in summer, excess moisture predominates due to infiltration; in winter, overdrying is possible.
Selecting the type of dehumidification system for museum environments
The main criteria for choosing between condensation and adsorption dehumidification systems are room temperature, target humidity, and energy efficiency.
Condensation dehumidification works by cooling air below the dew point, condensing moisture, and then reheating the air. Temperature limits of effectiveness: below 15°C, performance drops sharply, and below 5°C the heat exchanger may ice up. Advantages: high energy efficiency at moderate temperatures (COP 2-4), relatively low cost.
Adsorption dehumidification is based on moisture absorption by a desiccant followed by its regeneration with heated air. Use cases: cold archives (below 15°C), spaces with very low target humidity (below 35% RH). Advantage: stable performance regardless of temperature. Drawback: higher energy consumption (COP 0.5-1.5).
The choice between stand-alone dehumidifiers and a centralized system depends on room volume (threshold 500-1000 m³), the number of zones with different requirements, and service accessibility. Stand-alone systems offer easy installation, precise zoning, and redundancy, while centralized systems offer a single service point, heat recovery options, and integration with a Building Management System (BMS).
Calculating dehumidification system capacity
Dehumidification capacity is expressed in mass units — kg/h or L/day (1 L of water = 1 kg). The capacity formula is the sum of all moisture gains (infiltration + visitors + ventilation + other sources).
When calculating, consider the system’s operating mode: continuous (24/7) for archives or intermittent (during museum hours). It is recommended to use a capacity safety factor of 1.15-1.25 to compensate for unforeseen factors, load unevenness, and gradual equipment performance degradation over time.
Verification algorithm using a psychrometric chart:
- Determine the initial air state point (temperature, humidity)
- Find the final state point after dehumidification (target humidity)
- Check whether the difference in absolute humidity matches the calculated capacity
- Ensure that target parameters are achievable at the given temperature
Detailed numerical example: for an exhibition hall with a volume of 500 m³, temperature of 20°C, target humidity of 50% RH, and outdoor conditions of 26°C/70% RH in summer, with an air change rate of 0.5 h⁻¹, an absolute humidity difference of 7.7 g/kg, and 50 people present simultaneously for 1 hour (with a specific moisture emission of 60 g/h per person), the resulting capacity with a safety factor will be approximately 4.5 kg/h.
Heat balance of the room during dehumidifier operation
Dehumidifier operation significantly affects the room’s heat balance. Main heat gain components:
- Heat of condensation of moisture (2500 kJ/kg or 0.7 kWh/kg)
- Heat from the compressor (for condensation dehumidifiers)
- Heat from the heater (for adsorption dehumidifiers)
- Heat gain from visitors (80-120 W per person)
- Heat from lighting and other electrical appliances
- Heat gains through building envelopes
Total heat load during intensive summer dehumidification can reach 5-10 kW for a medium-sized hall, requiring integration with the air-conditioning system. Uncoordinated operation of dehumidification and air conditioning leads to energy losses: the dehumidifier heats the air while the air conditioner cools it, resulting in double energy consumption.
To determine the heat balance, calculate all heat gain components and compare the total load with the cooling capacity of the air-conditioning system.
Equipment placement and air distribution
Proper equipment placement and air distribution are critical for effective dehumidification. Key requirements for dehumidifier installation include ensuring free air circulation, service accessibility, and minimizing noise for visitors.
The distance from walls and obstructions should be at least 0.5-1.0 m to ensure air access to the intake. Stand-alone units are typically floor-mounted, while centralized system equipment is installed under the ceiling.
For effective system operation, ensure uniform distribution of dehumidified air and avoid stagnation zones in the room. Typical placement errors include installing the dehumidifier in a corner without adequate air circulation or behind a partition that blocks airflow.
Temperature and humidity sensors should be located at exhibit level (1.0-1.5 m above the floor) in a zone with stable parameters, away from doors and windows. For large spaces, install at least one sensor per 100-150 m², and add extra control points in critical storage areas.
Control and monitoring systems for microclimate parameters
Museum and archive spaces require high-precision temperature and humidity sensors with ±2% RH accuracy, a wide measurement range, and calibration stability. Recommended sensor calibration interval is annually, and for critical applications, verification with reference instruments every 6 months.
Modern monitoring systems collect and archive data at 10-30 minute intervals and store history for years, enabling trend analysis and optimization of operating modes.
Dehumidification control algorithms can be simple (a hysteresis controller with a 3-5% RH band) or advanced (PID control to maintain parameters within ±1-2% RH). Integration with a Building Management System (BMS) provides remote monitoring, alarm notifications, and trend analysis.
Data visualization as temperature and humidity graphs over a day, week, or season makes it possible to detect anomalies and respond promptly to deviations. Alarm systems notify about parameter excursions, equipment failures, or condensate tank overflow.
Operating modes and seasonal adjustment
Effective operation of the dehumidification system requires adapting modes to seasonal changes. In summer, intensive dehumidification is needed due to high outdoor humidity, potentially requiring continuous 24/7 operation. In winter, with low outdoor humidity, dehumidification intensity may be reduced or humidification may even be necessary.
In shoulder seasons (spring, autumn) loads are variable, requiring flexible capacity control. For exhibition halls, dehumidification intensity can be reduced at night or on weekends when there are no visitors, while maintaining parameter stability.
With changes in visitor numbers, the system should automatically increase dehumidification intensity during peak hours. Gradual changes to setpoints when switching seasons (no more than 3-5% RH per week) prevent exhibit deformation.
Periodic maintenance includes monthly filter cleaning, quarterly compressor inspection, and desiccant replacement every 2-5 years for adsorption systems.
Energy efficiency of dehumidification systems for museums
Energy efficiency is an important criterion when choosing a dehumidification system. The specific energy consumption of condensation dehumidifiers is typically 0.3-0.6 kWh/kg of removed moisture (COP 2-4), while for adsorption dehumidifiers it is 0.7-1.5 kWh/kg (COP 0.7-1.4).
Condensation systems are more energy efficient at higher room temperatures, while adsorption systems have stable consumption regardless of temperature regime. Annual energy consumption is calculated as the product of dehumidification capacity, operating hours, and specific energy consumption.
For example, for a museum hall of 200 m² with a dehumidification capacity of 2 kg/h, operating 4000 hours per year with a specific energy consumption of 0.5 kWh/kg, the annual consumption will be approximately 4000 kWh.
Recovering condensation heat can reduce heating costs by 20-40% by using the heat to warm the supply air. Using inverter compressors provides smooth capacity control and reduces consumption by 20-30% compared to ON/OFF control.
Common design mistakes when choosing dehumidification systems for museums
Analysis of implemented projects reveals typical mistakes in designing dehumidification systems for museums:
- Using condensation dehumidifiers in cold archives (below 15°C), leading to a sharp performance drop, evaporator icing, and emergency shutdowns
- Underestimating moisture emissions from visitors in high-traffic exhibition halls
- Ignoring infiltration through doors and windows, especially critical for historic buildings with leaky envelopes
- Lack of on-site measurements of parameters before design
- No zoning by exhibit types
- Incorrect dehumidifier placement
- No system redundancy for critical archives
- Ignoring the heat balance and lack of coordination with air-conditioning systems
- No microclimate parameter monitoring system
Consequences of insufficient system capacity include elevated humidity above the norm, risk of mold growth, and condensation on cold surfaces. Oversized systems can overdry air below 40% RH, causing cracks in wooden exhibits and increased energy costs.
Implementation results of dehumidification systems: efficiency analysis
Evaluating the effectiveness of a dehumidification system is based on comparing actual parameters with design values after commissioning. Key monitoring parameters include stability of temperature and humidity maintenance, the frequency of parameter excursions, and recovery time after deviations.
Typical results in exhibition halls show a reduction in humidity fluctuations from ±10-15% to ±3-5% and maintaining a target level of 50±3% RH throughout the year. For archive storage, parameters stabilize around 18°C and 45±2% RH without condensation on envelopes.
A properly designed dehumidification system reduces the aging rate of organic materials (paper, textiles) by 2-3 times due to stable parameters and suppresses mold and bacterial growth by maintaining humidity below 60% RH.
Economic efficiency is assessed through payback period, reduced restoration costs, and avoidance of emergencies. Based on monitoring results, operating modes, controller setpoints, and work schedules can be optimized.
Limits of applicability of calculation methods for museum systems
When designing museum systems, it is important to understand the limitations of calculation methods:
- Temperature limits: condensation dehumidification is ineffective below 15°C, and below 5°C exclusively adsorption systems are required
- Target humidity limits: condensation systems are inefficient for achieving humidity below 35-40% RH
- Facility scale: for rooms up to 500-1000 m³, stand-alone dehumidifiers are effective; above 1000 m³ — centralized systems
- Infiltration uncertainty: in historic buildings, air change rates of 0.3-1.5 h⁻¹ are possible depending on envelope condition, requiring on-site measurements
- Operational specifics: unpredictable door openings and sudden visitor influxes require a safety factor of 1.2-1.3
Before design, on-site temperature and humidity measurements for at least a week are mandatory to determine actual parameters. Despite higher energy consumption, adsorption systems are not advisable at temperatures above 20°C if condensation solutions are available.
Frequently asked questions
What is the target relative humidity for different exhibit types, and why is a single value for the entire museum impossible?
Different materials have different optimal ranges: metal (35-45% RH) to prevent corrosion, wood (45-55% RH) to avoid cracking, paper (50-55% RH) to preserve fiber flexibility. A single value is impossible due to conflicting requirements, so zoning by exhibit type with separate control systems for each zone is recommended.
How to accurately account for visitor moisture emissions when calculating dehumidification capacity?
Determine the average number of visitors per hour based on statistics, multiply by the dwell time in the hall (typically 0.5-1.5 hours) and by specific moisture emissions (40-80 g/h per person, depending on temperature and activity). For example, 50 people × 1 hour × 60 g/h = 3 kg/h of moisture.
Why are condensation dehumidifiers ineffective in cold archive storage, and when are adsorption systems mandatory?
Below 15°C, condensation dehumidifier performance drops due to the lower saturation vapor pressure, and below 5°C the evaporator ices up. Adsorption systems maintain stable performance at any temperature thanks to the physico-chemical moisture uptake process. The temperature threshold at which adsorption systems gain advantage is 12-15°C.
How to determine whether dehumidification must be integrated with air conditioning and when can they operate separately?
Integration is mandatory if the total heat load from dehumidification exceeds 3-5 kW, as excess heat must be removed from the room. Separate operation is possible in cold archives (15-18°C) and in shoulder seasons with moderate temperatures. Criterion: if dehumidifier operation raises the room temperature by more than 1-2°C above the target, cooling is required.
What specific consequences for exhibits arise from insufficient and excessive dehumidification capacity?
Insufficient capacity leads to humidity above 60-65% RH, creating conditions for mold growth, biological corrosion, condensation on cold surfaces, and swelling of wood and paper. Excessive capacity lowers humidity below 35-40% RH, causing paper brittleness, wood cracking, and delamination of paint layers on artworks. Both errors significantly shorten exhibit preservation time.
Conclusions
Designing a dehumidification system for a museum or archive requires a comprehensive approach that includes analysis of regulatory requirements, detailed calculation of moisture and heat balance, and zoning by exhibit type.
The choice of system type (condensation or adsorption) critically depends on room temperature: the threshold of 12-15°C is the limit of condensation systems’ effectiveness. Capacity calculations must be based on a detailed analysis of all moisture balance components with a mandatory safety factor of 1.15-1.25.
The room’s heat balance must not be ignored, as the heat of condensation and compressor operation creates a 5-10 kW load for a medium-sized hall, requiring coordination with air-conditioning systems.
Equipment placement and air distribution directly affect system effectiveness. Control and monitoring systems are integral to modern museum systems, providing continuous parameter logging to detect deviations and optimize operating modes.
Implementation results confirm the effectiveness of dehumidification systems: stabilizing relative humidity to ±3-5% RH instead of ±10-15% RH reduces exhibit aging rates by 2-3 times.
For successful design, engineers are advised to follow this sequence: define target parameters, verify results with mandatory on-site measurements, consider alternative technical solutions, and provide redundancy for critical systems.
