Temperature conditions of hothouses during a warm season

On the basis of justification of the heat and mass transfer processes inside the hothouses during a warm season authors developed methods and means that control dynamics of temperature and humidity parameters and air conditions with the help of complex systems of removal of overheat in the hothouses during the all year round and diurnal operations at minimum power inputs.


Introduction
Under the climatic conditions of sharply continental climate in most regions of Russia growing vegetables in the cold season is only in winter hothouses. Exploitation of winter hothouses in the warm season is difficult due to overheating of the air inside of them because of the increased intensity of solar radiation. Crop losses in this period can reach 50... 80 %, and sometimes there is a destruction of plants.
The authors have developed a complex system of removal of overheat in the hothouses in the warm season. Given technique consists in use for the longest period of natural (passive) systems providing microclimate parameters (transoms, technological openings, aeration shaft) and for short-term inclusion of active systems, their key elements are the systems of water aerosol cooling (SWAC). While the system of water aerosol cooling the water is spraying in the whole volume of the hothouse through the nozzles, and during the adiabatic evaporation water lowers the temperature of the internal air.
We studied six typical modes of a complex system of heat removal to ensure acceptable temperature parameters of air in hothouses: mode I -an organized natural ventilation; mode II -co-operation of the systems of organized natural ventilation and active aeration; mode III -co-operation of the mechanical ventilation and natural ventilation; mode IV -cooperation of the SWAC and natural ventilation; mode V -co-operation of the SWAC, natural ventilation and active aeration; mode VI -co-operation of the SWAC, natural ventilation and mechanical ventilation.
Dynamics of changes in air temperature in an unventilated, with closed openings and transoms, hothouse with plants in the warm season is shown on the i-d-diagram of humid air ( fig. 1).   So the area, bounded by the points  Concurrently moisture content in the hothouse increases from d В to d А according to the relative humidity in the point A (97 ... 98 %). If the amount of the heat of solar radiation in the hothouse exceeds the amount of heat absorbed during evaporation of water aerosol droplets, so process of the air cooling is terminated at the point C during the temperature t C , the relative humidity φ C and the moisture content of air d C .
The air temperature after its processing by SWAC to the point A or to the point C ( fig.  2) may be reduced to a range of technologically necessary temperatures in the hothouse (beam B-A) or be above them (the beam B-C). Specific parameters of the air in the hothouse with volume V h , m 3 , during the ventilation rate n, h −1 , are determined by increment of its water content Δd, which depends on the flow rate of sprayed water G s.w , kg/h, and air density ρ int , kg/m 3 : The reducing of intensity of solar radiation in the afternoon is accompanied by a decrease of internal air temperature. SWAC switched off, but the exhaust fan built into the shaft continues to operate, transoms and technological openings are opened. The parameters of internal air are transformed into the state of the point B, corresponding to the parameters before the activation the adiabatic cooling system. The relative humidity and moisture content of the air decrease and the temperature increases. It is this thermodynamic process is inevitable, because the heat capacity of the equipment in hothouses is negligible and plants can self-regulate the air parameters around themselves in the process of adaptive reactions during transpiration of moisture.
The process of removing the intensity of solar radiation in the day period after switching off SWAC and further regulation of the air parameters in the hothouses within a standardized technological values will be carried out by changing the position of the point As an indicator of the efficiency of removal of overheat for a day (N = 24 h) during the period of the field studies was adopted a coefficient of probability of operation of cooling systems d pr K . Its value shows the share of the total number of hours for a day, which does not allow to exceed the internal air temperature in a hothouse relative to the design temperature: where m -the number of hours of temperature rises for a day. Figures 3 and 4 show generalized experimentally obtained temperatures in the hottest period of sunny day. The shaded part indicates the area of optimal temperatures of internal air for growing crops. Mode IV of operation of complex system of removal of overheat is not typical in practical terms and, therefore, has not been studied.  Table 1. temperature in the hothouse during operation of complex system is 23…25 ºС. It is almost always provides the necessary temperature regime for all the phenological phases of vegetables, including the fruiting stage (April -July).
Therefore, the value an pr K ≈ 1 -(120 − 120)/120 ≈ 1,0. The results of determining a coefficient of probability of the operating mode for the annual cycle of fruiting period are summarized in Table 2.

Conclusion
Graphic-analytical and field studies of the thermodynamic processes in the hothouses showed that the internal air temperature during the period of maximum heat gain from solar radiation can be reduced by 23 ... 25 о С. The authors obtained quantitative values of coefficients of air temperature parameters in the hothouses during the warm period of the year for the climate of central part of Russia in the daily and annual cycles of operation of hothouses.