Definition
The total amount of moisture entering a space in a given time period (g/h or kg/h). Includes all sources such as outdoor air infiltration, personnel, product processes, door openings, and moisture transfer through building elements. It is the fundamental parameter in determining dehumidifier capacity.
Detailed Explanation
Moisture load is the fundamental engineering parameter that determines the dehumidifier capacity required for a space. The total load comes from four independent sources:
1. Infiltration — load from outdoor air entering through door/window openings, building leaks, and ventilation imbalances. 2. Personnel — moisture from respiration and perspiration; 50–100 g/h per person at moderate activity. 3. Process sources — fermentation, wet product surfaces, open baths, vegetable washing, livestock, etc. 4. Building diffusion — slow but continuous moisture transfer through building materials (concrete, wood, plaster).
For accurate capacity selection, these four components must be calculated separately, peak load determined under summer outdoor design conditions (DOC), and a 15–25% safety factor added. An underestimated moisture load causes not only loss of comfort or product quality, but also structural mold growth and equipment corrosion.
Calculation
Total load: Mtotal = Minf + Mppl + Mproc + Mdif
Infiltration load: Minf = ACH × V × ρa × (Wout − Win)
ACH: air changes per hour (1/h) V: space volume (m³) ρa: dry air density (≈ 1.2 kg/m³ at standard conditions) Wout: outdoor specific humidity (kg/kg dry air) Win: indoor target specific humidity (kg/kg dry air)
Personnel load: Mppl = N × mperson N: number of people, mperson: moisture generation per person (≈ 80 g/h at moderate activity)
Practical Example
Consider a 500 m² meat processing facility with a 4 m ceiling height.
Inputs: • Space volume: 500 × 4 = 2000 m³ • ACH: 0.5/h (moderate door circulation) • Summer outdoor: 32°C, 65% RH → Wout = 19.5 g/kg • Indoor target: 8°C, 85% RH → Win = 5.7 g/kg • Personnel: 8 people × 100 g/h (heavy physical work) = 800 g/h • Process (surface evaporation from meat): approximately 1500 g/h
Infiltration load: Minf = 0.5 × 2000 × 1.2 × (19.5 − 5.7) g/kg = 16,560 g/h ≈ 16.6 kg/h
Total load: 16.6 + 0.8 + 1.5 = 18.9 kg/h With a 20% safety factor → 22.7 kg/h
The dew point of the 8°C / 85% RH indoor target is ≈ 5.6°C, which is at the practical lower limit of condensing units; TFT CD series condensing dehumidifiers are intended for room temperatures above +10°C. For this capacity at an 8°C room temperature, the selection is therefore made from the TFT AD series silica gel rotor dehumidifiers.
Engineering Note
In cold storage and freezer applications, door infiltration can account for 40–60% of the total moisture load. For this reason, the payback period for air curtains, high-speed industrial doors, or airlock solutions is typically 1–3 years.
Other points to consider:
• Peak load ≠ average load — shift changes, production peaks, and outdoor extremes must be included in the design load. • For intermittent processes (seasonal, campaign production), equipment selection is based on peak conditions, but energy simulation uses average load. • Building diffusion is negligible in most HVAC designs, but can be the dominant load in museum, archive, library, and storage applications. • ASHRAE 0.4% annual exceedance frequency is the accepted standard for outdoor design conditions, representing the extreme exceeded for 35 hours per year.
The NKT energy simulation tool generates an hourly moisture load profile from annual outdoor weather data and provides both peak and annual energy consumption estimates.
