Author: Mycond Technical Department
The most common mistake in designing air dehumidification systems is engineers focusing solely on the mechanical equipment while ignoring architectural features of the space and operational decisions that directly affect the system’s moisture load. A systems approach to design requires accounting for all factors that influence indoor humidity and developing an optimal solution considering both technical and economic aspects.
Stage one: Defining the project objective
Why this is critically important
Without understanding the fundamental reason for humidity control, it is impossible to make the right decisions on control accuracy, equipment type and budget. The project’s objective directly determines all subsequent design stages and the parameters of the chosen air dehumidification system.
Practical example: different aims — different solutions
Case 1: Corn storage, where it is sufficient to maintain humidity not above 60% RH without condensation. In this case, the system can be as simple and economical as possible.
Case 2: Lithium battery production, where lithium reacts with water vapour releasing explosive hydrogen already at 2% RH. Here a controller with ±5% RH accuracy is absolutely unacceptable; specialised equipment is required regardless of cost.
A real case of poor design
At a military ammunition depot, the specification required “maintaining a maximum of 40% RH”. The system met this requirement, yet the ammunition still corroded. The cause turned out to be condensation on the metal roof, which cooled below the dew point at night. If the objective had been formulated as “prevent corrosion of ammunition”, the engineer would have focused on condensation on cold surfaces.
Practical recommendations
When formulating the project objective, answer: 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 specified parameters.
Stage two: Establishing control levels and tolerances
Defining absolute humidity
A common mistake is specifying humidity only in relative terms (% RH) without considering temperature. For example, 30% RH at 21°C corresponds to an absolute humidity of 4.6 g/kg, while 30% RH at 10°C is only 2.3 g/kg. Rule: always define humidity in absolute units or specify RH together with a temperature range.
Example: In pharmaceutical production, it is necessary to maintain 10% RH at 21°C for tabletting. Temperature fluctuates within ±1.5°C, which leads to variation in absolute humidity from 1.4 g/kg at 19.5°C to 1.7 g/kg at 22.5°C. The engineer sets control by dew point at -7°C (which corresponds to 1.6 g/kg) regardless of temperature fluctuations.
Internal vs external conditions
When designing a dehumidification system, it is necessary to clearly distinguish between two sets of design conditions: the internal parameters to be maintained, and the external (design) weather conditions that impose the load on the system.
Choosing the design weather conditions
ASHRAE data for Europe offer 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 a pharmaceutical plant with downtime over €40,000 per day, 0.4% is used, while for a low-criticality warehouse — 2%.
Setting tolerances
Wide tolerances (±3–5% RH or ±1.5°C dew point) allow simpler, lower-cost system designs. Tight tolerances (±1% RH or ±0.5°C dew point) require high-accuracy sensors, more complex control algorithms, equipment redundancy, and lead to significantly higher system cost.
Stage three: Calculating moisture loads
Main sources of moisture
Main sources include: permeation through enclosures, evaporation from people, desorption from materials and products, evaporation from open surfaces, combustion products, infiltration through leaks, moisture in supply air.
Formulae for calculating the main loads
Permeation through walls: W = A × μ × Δpᵥ, where A — surface area, μ — vapour permeability coefficient, Δpᵥ — difference in partial water vapour pressure. For a 200 mm concrete wall with vapour-barrier paint (μ = 0.054 g/(m²·h·Pa)), with a humidity difference of 16–4 g/kg and an area of 100 m², we have: Δpᵥ = 12 × 133 = 1596 Pa, W = 100 × 0.054 × 1596 = 8.6 g/h. This load is negligible compared to other sources.
Moisture emission from people: W = n × wₚ, where typical values of wₚ for sedentary work are 40–50 g/h, for light physical work 90–120 g/h, for heavy physical work 150–200 g/h.
Infiltration through open doors: W = ρ × V × n × t × (wₑₓₜ - wᵢₙₜ). For doors 2×2.5 m (V = 10 m³), with 15 openings per hour of 30 seconds, external humidity 16 g/kg and internal 4 g/kg: W = 1.2 × 10 × 15 × 0.0083 × 12 = 18 g/h. If the doors are open for 3 minutes: W = 108 g/h. This demonstrates the criticality of opening time — reducing from 3 to 0.5 minutes reduces the load sixfold.
Moisture in supply air: W = Q × ρ × (wₑₓₜ - wᵢₙₜ). With ventilation at 400 m³/h: W = 400 × 1.2 × 12 = 5760 g/h = 5.76 kg/h. This is usually the largest load in most systems.
Practical example: refrigerated warehouse
Consider a cold store measuring 75×23×4.3 m with internal conditions of +2°C at a dew point of -9°C (2.0 g/kg) and external conditions of +28°C at a dew point of +16°C (11.4 g/kg). Two 3×3 m gates open 15 times per hour for 1 minute. The calculation shows: permeation through walls ~100 g/h, infiltration (V = 18 m³) W = 1.2 × 18 × 15 × (1/60) × 9.4 = 61 g/h. If the gates were open for 3 minutes, the load would be 152 g/h. Reducing opening time lowers the load by 60% and allows a system with half the capacity to be used.
Stage four: Equipment selection
Selecting the system type
There are two main types of dehumidification systems. Refrigeration systems are effective at temperatures above 15°C and high humidity. Their practical dew-point limit is around +4...+7°C, as lower temperatures lead to condensate freezing. Desiccant systems are effective at low dew points below +5°C, operate at any temperatures and can achieve dew points of -40°C and below.
Combined systems
The optimal solution is often a combined system: pre-cooling from +16°C to +7°C with a refrigeration unit, followed by a desiccant to reduce from +7°C to -7°C. This allows each system to operate in its optimal range, reducing overall energy consumption by 30–40%.
Calculating the required dry-air flow rate
The calculation uses the formula: Q = W / [ρ × (wᵣₑₜᵤᵣₙ - wₛᵤₚₚₗᵧ)]. For example, with a load of 200 g/h, a requirement to maintain 4 g/kg, and a dehumidifier capable of delivering 0.7 g/kg: Q = 200 / [1.2 × 3.3] = 50.5 m³/h.
Selecting dehumidifier capacity
For a desiccant dehumidifier, key parameters are: air velocity through the desiccant (ideally 400–600 m/min); regeneration temperature (120–250°C); process/regeneration ratio (from 3:1 to 5:1). The 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 the heat load
Adsorption of water vapour releases heat: Q = W × (hᵥ + Δhₐ), where hᵥ = 2500 kJ/kg, Δhₐ ≈ 200 kJ/kg. When 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 control system must maintain the set parameters, modulate capacity under variable loads, minimise energy consumption, and protect equipment from abnormal operating conditions.
Types of humidity controllers
An on/off hygrostat provides ±3–5% RH accuracy and is suitable for non-critical spaces; a dew-point controller provides ±0.5–1.0°C accuracy, is independent of air temperature, and is recommended for dew points below +5°C; a PID controller with modulation provides ±1% RH or ±0.3°C dew-point accuracy and is required for critical applications.
Modulating the power of a desiccant dehumidifier
There are two main methods: process-air bypass (simple and low cost, but regeneration energy does not decrease, Qₑff = Qₘₐₓ × (1-k)) and modulation of regeneration temperature (a sensor controls 120–130°C at the outlet of the regeneration sector; as the load decreases the temperature rises, signalling the need to reduce heater power, ΔE = Pₙₒₘ × (1 - Tₐcₜᵤₐₗ/Tₙₒₘ) × τ).
Sensor placement
Humidity sensors should be located in a zone of good air mixing, at least 3 m from discharge grilles, at a height of 1.5–2 m from the floor, avoiding local moisture sources and zones with extreme temperatures. For multi-zone rooms, several sensors should be installed in parallel, and the system should respond to the highest reading.
Condensation protection
Surface dew-point sensors operate on the principle: if Tₛᵤᵣfₐcₑ Tdₑw + ΔT, then dehumidification switches on, where ΔT = 2–3°C is the safety margin.
Optimising the system to minimise costs
Reducing capital expenditure
Key optimisation measures: minimise moisture loads through building sealing (payback 3–12 months), manage door openings, install air curtains or lobbies; optimise control levels (each degree of dew-point reduction increases cost by 8–12%); implement combined systems that provide 20–35% savings compared with single systems.
Reducing operating costs
Methods to lower operating costs: regeneration heat recovery (an air-to-air heat exchanger returns 60–80% of energy, Qᵣₑcₒᵥₑᵣᵧ = ṁ × cₚ × (Tₑₓₕₐᵤₛₜ - Tᵢₙₗₑₜ) × η, typical savings 15,000–40,000 kWh/year); use of low-temperature energy sources (cogeneration, geothermal sources, rejected heat from refrigeration plants); seasonal optimisation (in winter, outdoor air is often drier than indoor, allowing a 40–70% reduction in load).
Typical design mistakes
Mistake 1 — underestimating infiltration. Example: a project with a calculated load of 3 kg/h proved insufficient because the actual load was 8 kg/h due to unplanned gate openings. Solution — allow a 25–40% margin for industrial spaces.
Mistake 2 — ignoring initial drying. New buildings contain significant moisture in the structures. Concrete and plasterboard release 100–500 kg of moisture over 2–6 months. Solution — provide an intensive drying mode or temporary additional capacity.
Mistake 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 air mixing. Solution — airflow modelling or installation of circulation fans.
Conclusions
The five-stage methodology for designing air dehumidification systems enables optimal solutions: a clear objective underpins all decisions; correct control levels balance requirements and cost; accurate load calculation is the key to proper equipment selection; optimal equipment choice considers the entire lifecycle; smart control minimises operating costs.
A successful project is not the most complex system, but the simplest system that reliably performs the task at minimal cost over its entire service life. The average payback of a well-designed air dehumidification system is 1.5–4 years.
