Author: Mycond Technical Department
The most common mistake when designing air dehumidification systems is focusing solely on mechanical equipment. Engineers often ignore architectural features and operational decisions that directly impact the system’s moisture load. Only a balanced approach that accounts for all project components can deliver an effective and economical humidity control solution.
Stage One: Defining the Project Objective
Why this is critical
Without understanding the fundamental reason for humidity control, it is impossible to make the right decisions about control accuracy, equipment type, and budget. The dehumidification system must solve a specific problem, not merely meet abstract technical specifications.
Practical example: different goals — different solutions
Case 1: Corn storage where it is sufficient to maintain humidity not above 60% RH without condensation. The system can be as simple as possible.
Case 2: Lithium battery manufacturing, where lithium reacts with water vapor releasing explosive hydrogen already at 2% RH. Here a controller with ±5% RH accuracy is unacceptable; specialized equipment is needed regardless of cost.
Real case of poor design
A military ammunition warehouse had a technical specification to “maintain a maximum of 40% RH.” The system met the requirement, but the ammunition corroded due to condensate on the metal roof, which cooled at night below the dew point. If the goal had been “prevent ammunition corrosion,” the engineer would have focused on condensation on cold surfaces.
Practical recommendations
When defining the project objective, ask: what fundamental problem must be solved; what are the consequences of insufficient humidity control; are there alternative causes of the problem other than high humidity; how critical are deviations from the set parameters.
Stage Two: Setting Control Levels and Tolerances
Defining absolute humidity
It is a mistake to specify only %RH without temperature. For example, 30% RH at 21°C = 4.6 g/kg, while 30% RH at 10°C = only 2.3 g/kg. Rule: always define humidity in absolute units or specify RH together with a temperature range.
Example: pharmaceutical manufacturing requires tableting at 10% RH and 21°C. Temperature fluctuates ±1.5°C. Absolute humidity varies from 1.4 g/kg at 19.5°C to 1.7 g/kg at 22.5°C. Therefore, the engineer sets dew-point control at -7°C (1.6 g/kg) regardless of temperature swings.
Indoor vs outdoor conditions
Proper design requires defining two sets of design conditions: the indoor parameters to be maintained and the outdoor conditions that impose loads on the system.
Selecting design weather conditions
ASHRAE data for Europe include three exceedance levels: 0.4% (exceeded 35 hours per year), 1.0% (88 hours), 2.0% (175 hours). For example, for Vienna the extreme dew point at 1% exceedance is +16°C at a temperature of +30°C. For pharmaceuticals with downtime over €40,000 per day, use 0.4%; for a low-criticality warehouse — 2%.
Setting tolerances
Wide tolerances of ±3–5% RH or ±1.5°C dew point yield simpler systems and lower cost. Tight tolerances of ±1% RH or ±0.5°C dew point require high-precision sensors, more sophisticated algorithms, equipment redundancy, and significantly higher cost.
Stage Three: Calculating Moisture Loads
Primary sources of moisture
Penetration through enclosures, evaporation from people, desorption from materials and products, evaporation from open surfaces, combustion products, infiltration through leakages, humidity of supply air.
Formulas for calculating the main loads
Penetration through walls: W = A × μ × Δpᵥ. Example: 200 mm concrete wall with vapor-barrier paint μ = 0.054 g/(m²·h·Pa), humidity difference 16–4 g/kg, area 100 m², Δpᵥ = 12 × 133 = 1596 Pa, W = 100 × 0.054 × 1596 = 8.6 g/h — negligible compared with other sources.
Moisture emission from people: W = n × wₚ. Typical wₚ values: seated work 40–50 g/h, light physical 90–120 g/h, heavy physical 150–200 g/h.
Infiltration through open doors: W = ρ × V × n × t × (wₑₓₜ - wᵢₙₜ). Example: doors 2×2.5 m (V=10 m³), 15 openings per hour for 30 seconds, outdoor 16 g/kg, indoor 4 g/kg: W = 1.2 × 10 × 15 × 0.0083 × 12 = 18 g/h; if for 3 minutes: W = 108 g/h. Opening time is critical: cutting it from 3 minutes to 0.5 minute reduces the load by a factor of 6.
Supply air humidity: W = Q × ρ × (wₑₓₜ - wᵢₙₜ). Example: ventilation 400 m³/h: W = 400 × 1.2 × 12 = 5760 g/h = 5.76 kg/h. This is the largest load in most systems.
Practical example: refrigerated warehouse
Warehouse 75×23×4.3 m, indoor conditions +2°C with dew point -9°C (2.0 g/kg), outdoor +28°C with dew point +16°C (11.4 g/kg), two 3×3 m gates, 15 shipments/hour, opening time 1 minute. Calculation: penetration ~100 g/h, infiltration V=18 m³, W = 1.2 × 18 × 15 × (1/60) × 9.4 = 61 g/h. If opening time were 3 minutes, this would be 152 g/h. A 60% load reduction allows using a system with half the capacity.
Stage Four: Equipment Selection
Selecting the system type
Refrigeration systems are effective at temperatures >15°C and high humidity, with a practical dew-point limit of +4...+7°C (lower leads to condensate freezing).
Desiccant systems are effective at low dew points +5°C, operate at any temperatures, and reach dew points of -40°C and below. For more information on the differences between cooling-based and desiccant dehumidification methods, see our dedicated guide.
Combined systems
Scheme: pre-cooling from +16°C to +7°C with a refrigeration unit, then desiccant from +7°C to -7°C. Advantages: each system operates in its optimal range, overall energy consumption is 30–40% lower.
Calculating the required dry-air flow rate
Q = W / [ρ × (wᵣₑₜᵤᵣₙ - wₛᵤₚₚₗᵧ)]. Example: load 200 g/h, control 4 g/kg, dehumidifier down to 0.7 g/kg, Q = 200 / [1.2 × 3.3] = 50.5 m³/h.
Selecting dehumidifier capacity
Key parameters of a desiccant dehumidifier: air velocity through the desiccant of 400–600 m/min is optimal; regeneration temperature 120–250°C; process/regeneration ratio 3:1 to 5:1. Outlet dew point depends on velocity and temperature: at 400 m/min and 190°C, -15°C is achieved; at 250°C, -25°C; at 600 m/min and 190°C — -10°C, at 250°C — -18°C.
Calculating heat load
Adsorption releases heat: Q = W × (hᵥ + Δhₐ), where hᵥ = 2500 kJ/kg, Δhₐ ≈ 200 kJ/kg. Example: removing 5 kg/h of moisture, Q = (5/3600) × 2700 × 1000 = 3750 W = 3.75 kW — this heat must be removed by cooling.
Stage Five: Control System
Basic control principles
The system must maintain parameters, modulate capacity under variable loads, minimize energy consumption, and protect equipment.
Types of humidity controllers
On/off hygrostat with ±3–5% RH accuracy for non-critical spaces; dew-point controller with ±0.5–1.0°C accuracy, independent of air temperature, recommended for dew points below +5°C; PID controller with modulation, ±1% RH or ±0.3°C dew point accuracy, required for critical applications.
Modulating desiccant dehumidifier capacity
Two methods: process-air bypass — simple and low-cost, but regeneration energy does not decrease, Qₑff = Qₘₐₓ × (1-k); regeneration temperature modulation — a sensor controls 120–130°C at the regeneration sector outlet, savings formula ΔE = Pₙₒₘ × (1 - Tₐcₜᵤₐₗ/Tₙₒₘ) × τ.
Sensor placement
Critical rules: locate the sensor in a well-mixed air zone, at least 3 m from discharge grilles, 1.5–2 m above the floor; avoid local moisture sources and zones with extreme temperatures; for multi-zone spaces install several sensors in parallel — the system responds to the highest reading. See also practical advice on the correct placement of dehumidifiers and sensors.
Condensation protection
Surface dew-point sensors operate on the principle: if Tₛᵤᵣfₐcₑ Tdₑw + ΔT → enable dehumidification, where ΔT = 2–3°C safety margin.
System Optimization to Minimize Costs
Reducing capital expenditures
Minimize moisture loads by sealing the building (payback 3–12 months), managing door opening, using air curtains or airlocks; optimize control levels — each degree of dew-point reduction increases cost by 8–12%, avoid overly stringent requirements; combined systems deliver 20–35% savings compared to mono-systems.
Reducing operating costs
Regeneration heat recovery — an air-to-air heat exchanger returns 60–80% of the energy: Qᵣₑcₒᵥₑᵣᵧ = ṁ × cₚ × (Tₑₓₕₐᵤₛₜ - Tᵢₙₗₑₜ) × η, typical savings 15,000–40,000 kWh/year; low-temperature energy sources — cogeneration, geothermal sources, rejected heat from refrigeration units; seasonal optimization — in winter, outdoor air is drier than indoor, free dehumidification reduces the load by 40–70%.
Typical Design Mistakes
Error 1 — underestimating infiltration. Example: a project with a design load of 3 kg/h and an actual 8 kg/h due to unplanned gate openings. Solution — include a 25–40% margin for production spaces.
Error 2 — ignoring initial dry-out. New buildings contain moisture in structures; concrete and drywall release 100–500 kg of moisture over 2–6 months. Solution — intensive dry-out mode or temporary additional capacity.
Error 3 — incorrect sensor placement. Example: a sensor near the dehumidifier grille showed 5% RH while it was 35% RH in the working zone due to poor mixing. Solution — airflow modeling or circulation fans.
Conclusions
The five-stage methodology for designing air dehumidification systems provides a systematic approach to solving complex engineering tasks: a clear objective is the foundation of all decisions; proper control levels balance requirements and cost; accurate load calculation is the key to correct selection; optimal equipment choice considers life cycle; intelligent control minimizes operating costs.
A successful project is not the most complex system, but the simplest system that reliably performs the task with minimal life-cycle cost. The average payback of a well-designed payback period is 1.5–4 years.
