The Variation of Heating Load Coefficient for a Greenhouse and Overall Heat-Transfer Coefficient through Plastic Film

Abstract

The heating load coefficients were measured using a practical plastic greenhouse in 1974 and 1975. In the first year a fuel capacity of 89, 000kcal/hr for heating was used. A large variation of measured values appeared. However, the heating load coefficient can be expressed statistically by equation (3). In the next year a capacity of 26, 400kcal/hr was used to accurately determine the heating load coefficient. It was of insufficient capacity so that the inside temperature dropped slightly below the design temperature. The results were that the variation of each measured point increased over that of the test in the first year.
For investigating the variation of the heating load coefficient, model tests were performed using polyvinyl chloride (p.v.c.) film. In the model tests only the overall heat-transfer coefficient, which greatly affected the heating load coefficient, was able to be investigated. A large variation in the values of the overall heat-transfer coefficient from the model tests was shown when they were plotted against wind velocity. However, when the overall heat-transfer coefficient was plotted against the ratio of the convective heat-transfer to the radiative heat-transfer, a regular curve was able to express with sufficient accuracy the overall heat-transfer coefficient. Moreover, in tests using the various materials the same results were obtained. According to them, a large overall heat-transfer coefficient appeared when the radiative heat-transfer was large in comparison to the convective heat-transfer. On the contrary, the overall heat-transfer coefficient was nearly constant, if the convective heat-transfer was large.