Abstract: A frost-proof shed was designed to prevent perennial frost disaster suffered by red plum apricots in the southern area of Ningxia. During the tests at night, the ambient temperature in the shed was always lower than that outside the shed, resulting in a "cold chamber effect”, and the desired effect was not achieved. Therefore, there was a test that needed to probe into the mechanism of the “cold chamber effect” in the frost-proof shed to provide a theoretical basis for improving the design of the frost-proof shed accurately and prevent frost disaster effectively. According to the principle of mass and heat balance, the test was researched the heat budget of the cover layer, the moist air in the shed, and the soil layer respectively then studied the heat accumulation in the whole system of the frost-proof shed during the period after the covering of the shed cloth to the time before its folding on the next day. Finally, the test explored the influence of relative humidity and the changes of moist air on phase transition latent heat by analyzing the dynamic process of main heat and humidity parameters in the shed. The study results show that: (1) when the “cold chamber effect” occurs at night in spring and autumn, both the soil layer and the cover layer become the part losing heat due to more heat loss, while the moist air in the shed becomes the part gaining heat due to more heat gain. (2) In spring and autumn, the accumulated heat gain is lower than the accumulated heat loss in the shed during the period after the covering of the shed cloth to the time before its folding on the next day, resulting in the heat imbalance in the shed. (3) There is a significant difference in the heat and humidity parameters of the moist air in the shed and those outside the shed (except the saturated water vapor pressure in the shed in spring); The dynamic changes of the water vapor density and the dew point temperature in the shed at night can reflect the change rules of latent heat caused by evaporation and condensation; The low temperature and higher water vapor density in the shed make the saturated water vapor pressure infinitely close to the actual water vapor pressure, resulting in the relative humidity in the shed persistently on the high side. The accumulated heat gain of the frost-proof shed for red plum apricots is always less than the accumulated heat loss at night, and the soil layer and the cover layer are the most important parts losing heat, and thus the “cold chamber effect” appears in the case of heat imbalance.

Key words: Red plum apricots, Cold chamber effect, Heat balance, Water vapor density, Dew point temperature, Water vapor pressure