The Overall Heat Transfer of Greenhouses Covered with PE and PVC Single Layer

Abstract

Overall heat transfer of polyethylene film (PE) and polyvinyl chloride film (PVC) were measured in the experimental greenhouses with hot-air heaters on the clear and on the cloudy nights during the period Nov. 1979 to Jan. 1980. Both films are 0.1mm thick and have different physical properties for long-wave radiation. The heat insulation efficiency of the greenhouses covered with PE and PVC single layer was investigated, and the ratio of floor area to covering area for the experimental greenhouses, which is one of the indices for the heat insulation efficiency of greenhouses, was also taken into consideration. The results are as follows:1. Using the ratio of the overall heat transfer and the overall heat transfer coefficients for the heat insulation efficiency, the PE-house revealed to be less efficient than the PVC-house. This can be due to PE being more transparent to long-wave radiation than PVC. The advantage in the PVC-house, however, decreased with the increasing of the inside-outside air temperature difference (Figs. 3 and 5).2. The overall heat transfer coefficients of both greenhouses depended on the inside-outside temperature difference. As the temperature difference increased, the overall heat transfer coefficients decreased (Fig. 5).3. The overall heat transfer coefficients of both greenhouses were smaller on the cloudy nights than that on the clear nights. When the condensation occurred at the interior film surface, the heat insulation efficiency of both greenhouses was increased, resulting in the decrease of the coefficient. The efficiency of the PE-house was more affected than the PVC-house when the condensation occurred (Figs. 6 and 7).4. When the inside-outside air temperature difference was small, convective heat transferred from the outside air to the outside cover surface. With an increase in the inside-outside air temperature difference, convective heat flow occurred from the outside cover surface to the outside air. This phenomenon was observed more clearly in the PVC-house, and more on the clear nights (Fig. 4).5. The ratio of floor area to covering area for the paticular type, such as those experimental greenhouses and trench houses, should be expressed as two different functions: One is the ratio for radiative heat transfer; the view factor of the covering area to the floor area and the wall area, the other is the ratio for convective heat transfer; (the floor area+the wall area)/the covering area.