Abstract
Effects of dew condensation on plastic film on the air temperature in the greenhouse or cultivation tunnel were examined by a simple heat insulating experimental box. The box is 50×50cm and the depth is 15cm. In the first experiment the upper part of the box is covered by Air-Cap which is widely used for packing. An arbitrary amount of water was injected to air bubbles of Air-Cap and the emissivity change due to water injected was evaluated. In the second experiment polyethelene film was used. Changes in emissivity of the film cover and temperature inside the box due to dew condensation were obtained by field observation in clear nights and by using heat balance equations at the polyethelene film cover and the bottom of box.
The experimental results showed that the emissivity of Air-Cap linearly increased with increasing in the watered area when the water film thickness was enough. The change in emissivity of wet polyethelene film cover due to dew condensation in the field was expressed as follows, using dry film emissivity (ε0) and total dew amount (ΣWg/cm2).
ε=ε0+(1-ε0)exp{-(C3/ΣW)2}
where C3 is a constant and the value of 2.0×10-3g/cm2 was obtained. It is shown from the formula that the emissivity of film cover rapidly increases as the dew condensation exceeds about 3mg/cm2.
Following results were obtained from the observation and heat balance analysis. As the emissivity increases, the temperature of film decreases, while the temperature of the bottom of box increases because a wet film obstructs the radiation loss to the atmosphere. Then both temperatures gradually approach to the same value with the increase of the dew amount. When the emissivity of film cover becomes unity, the film and the bottom have the same temperature.
