Effects of Nonsteady Hot-water Greenhouse Heating on Heat Transfer and Microclimate

Abstract

Hot-water tubes are used for heating in many agricultural buildings. Owing to the thermostatic control of the heating system, the tubes usually operate under non-steady-state conditions. Experiments were carried out to determine the effects of non-steady heating on the heat transfer coefficients of the heating tubes, on the amount of energy supplied in the different phases of the heating cycle and on the temperature variation of the air and the crop. The experimental data show that the heat transfer coefficients decreased during the cooling-down phase of the heating cycle. In addition, there was a continuous change in the ratio between the heat lost by convection and that lost by radiation from the outer surface of the tubes. At the beginning of the cooling-down phase, convection contributed more than radiation to the total heat loss, while towards the end of this phase radiation made a slightly larger contribution than convection. From energy calculations, it appears that approximately 60–70% of the energy used was supplied to the greenhouse during the heating-up and quasi-steady heating phases; the remainder was supplied during the cooling down of the tubes. Owing to the non-steady-state heating, the temperatures of the greenhouse air and crop varied in a cyclic manner with respect to time. These temperatures were closely correlated and the difference between them also changed in a cyclic manner.