Quick answer: A greenhouse moisture load is a water balance for a stated period: water vapour added (crop transpiration plus evaporation from substrate, floors and wet surfaces) minus water vapour carried out by air exchange (leakage and ventilation × the difference in humidity ratio between inside and outside air). What remains is what a dehumidifier must remove to hold the target. Calculate it for the periods that matter, such as night, dusk and screen-closed hours, not as a daily average. The same greenhouse can need almost nothing on a cold, dry night and a lot on a mild, humid one.
Quick answer
- Load to remove = transpiration + evaporation − air-exchange removal.
- Air-exchange removal = dry-air flow (kg/h) × (x_inside − x_outside).
- Calculate by period (night, dusk), and for the humid-season worst case.
The calculation
- Boundary and period: whole house or one compartment; e.g. 20:00–06:00 in the humid season.
- Target: indoor temperature and RH → humidity ratio x_in (g/kg).
- Sources (kg/h): transpiration and evaporation (ET) for the crop and stage at night. Use measured values, crop-advisor values or a crop model. Don't add evaporation twice if ET already includes it.
- Air exchange: leakage plus any ventilation, in m³/h → dry air in kg/h (× about 1.2 kg/m³).
- Outdoor air: x_out (g/kg) for the same period.
- Removal by exchange = dry air × (x_in − x_out) ÷ 1000 kg/h. This is negative if outdoor air is wetter.
- Remaining load = sources − removal by exchange. This is the dehumidification requirement at the target.
- Water volume: 1 kg ≈ 1 L of condensate.
Worked example (illustrative)
Greenhouse 5,000 m², 5 m average height (25,000 m³); target 18 °C / 85 % RH (x_in ≈ 11.0 g/kg); night ET assumed 15 g/m²·h → 75 kg/h; leakage 0.5 air changes per hour ≈ 15,000 kg/h dry air.
| Night | Outdoor | x_out (g/kg) | Removed by leakage | Remaining load |
|---|---|---|---|---|
| Cold, dry | 10 °C / 90 % | 6.9 | about 62 kg/h | about 13 kg/h (≈ 320 L per 24 h) |
| Mild, humid | 16 °C / 95 % | 10.8 | about 3 kg/h | about 72 kg/h (≈ 1,740 L per 24 h) |
(Sea-level pressure; humidity ratios from standard psychrometric relations; ET and leakage are assumptions to replace with site data.)
The mild, humid night sets the equipment size. Venting more on that night barely helps, because outdoor air is almost as wet as the greenhouse air.
Latent heat is not input power
Condensing 72 kg/h releases about 72 × 0.68 ≈ 49 kW of latent heat. That's the thermal effect of the phase change, not the dehumidifier's electrical input. Electrical input depends on the unit's efficiency at the operating condition.
Process airflow is a different quantity
Moisture load (kg/h) sets the dehumidifier's capacity. The airflow through the dehumidifier and the air distribution decide whether dry air reaches the crop. Check both; see greenhouse psychrometric calculation and how many dehumidifiers for a greenhouse.
What this method can and cannot conclude
- It gives a removal requirement for a stated boundary, period and set of assumptions.
- It doesn't give a model selection: that needs capacity at the operating condition (here about 18 °C / 85 %), distribution and controls.
- Accuracy depends on ET and leakage; measure them where possible (night water balance, tracer gas or decay tests).
Calculation record template
| Item | Value | Source |
|---|---|---|
| Boundary and period | ||
| Target T / RH → x_in | ||
| ET (kg/h) | measured / advisor / model | |
| Leakage and ventilation (m³/h) | ||
| Outdoor T / RH → x_out | weather data | |
| Removal by exchange (kg/h) | ||
| Remaining load (kg/h) | ||
| Assumptions and unknowns |
Bring this to a greenhouse dehumidifier review.
FAQ
Frequently asked questions
How do you calculate a greenhouse moisture load?
Add transpiration and evaporation, subtract the moisture carried out by air exchange, for the period that matters.
How much water does a greenhouse crop transpire at night?
It varies widely with crop, stage, heating and humidity; use measured or advisor values for your crop.
Why does the dehumidifier need to be bigger on mild nights?
Because outdoor air is almost as wet as indoor air, so leakage and venting remove very little.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- FAO Irrigation and Drainage Paper 56, Crop evapotranspiration (for ET concepts).
