How to dehumidify the greenhouse?
a. Combined used of heating and ventilation:
- A common dehumidification practice is simply to open the windows, allowing moist greenhouse air to be replaced by relatively dry outside air
- This method does not consume any energy when excess heat is available in the greenhouse and ventilation is needed to reduce the greenhouse temperature.
- However, when the ventilation required to reduce the temperature is less than that needed to remove moisture from the air, dehumidification consumes energy. Warm greenhouse air is replaced by cold dry outside air, lowering the temperature in the greenhouse.
b. Absorption using hygroscopic material
- During the process, moist greenhouse air comes into contact with the hygroscopic material, releasing the latent heat of vaporization as water vapour is absorbed.
c. Condensation on cold surfaces
- Wet humid air is forced to a cold surface located inside the greenhouse and different from the covering material.
- One meter of finned pipe used at a temperature of 5 °C can remove 54 g of vapors per hour from air at a temperature of 20 °C and with 80 percent relative humidity.
d. Forced ventilation usually with combined use of a heat exchanger
- Mechanical ventilation is applied to exchange dry outside air with moist greenhouse air, exchanging heat between the two airflows.
- A ventilator capacity of 0.01 m3 s-1 is sufficient for all crops.
e. Anti-drop covering materials
- “Anti-dripping” films contain special additives which eliminate droplets and form instead a continuous thin layer of water running down the sides.