Articles
THE VARIATION OF HEATING LOAD COEFFICIENT FOR THE GREENHOUSE
For determining the capacity of heaters the so-called heating load coefficient is of considerable importance.
Many investigators have studied this subject and the determination of its standard values for designing the heating system was tried by the Society of Agricultural Meteorology of Japan.
We also measured the heating load coefficient for a practical plastic greenhouse in 1974 and 1975. In the first year a fuel capacity of 89000 kcal/hr for heating was used.
A large variation of measured values appeared.
However, the heating load coefficient can be expressed statistically by equation U=5.7+0.16V. In the following year a capacity of 26400 kcal/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 pvc 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 the overall heat-transfer coefficient in the case of low wind velocity.
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.