EFFECTIVITY OF AIR AND SOIL HEATING IN PLASTIC GREENHOUSES
Since 1966, studies have been conducted in plastic greenhouses with air and soil heating by means of the heat generator TG-150-B ECX at the Mihajlovskoe TCCA experimental farm. A blocktype, 1000 sqm plastic greenhouse built according to the Latgiproselstroj typical plan, has been used for the purpose. The greenhouse air temperature is semi-automatically controlled.
The 200 1 capacity heater tank out of which the water is warmed up to a temperature of 25 to 40°C and then pumped through the distributor into the polyethylene pipeline, is used for heating the soil. The 20 mm dia high rigidity pipes are laid at a depth of 40 cm and spaced 1.5 m along the length of the greenhouse. Each one of them, having once reached the end of the greenhouse, turns at 180 degrees and heads back to the heating unit.
This lay-out of the plastic greenhouse pipeline is being tested for the first time in this country. It has shown the following advantages: simplicity of design, low prime cost, higher heating effectivety of the root system developing zone through the use of a low-temperature heat carrier, security against overheating, etc. It ought to be mentioned that according to J.H. Groenewegen this type of heating, under the conditions in Holland, is more effective than the electric heating or the hot water in metal pipeline heating.
Observations have shown that the greenhouse air temperature, measured in horizontal direction, is quite levelled. Even with an intensified air inflow, the difference in temperature between the ends and the middle part of the greenhouse does not exceed 1.5°C. Greater fluctuations are observed in vertical direction. Thus, the difference between the temperature at soil surface and at a height of 2 m in cold nights and clear-sky days reaches 10.5°C, while under ordinary conditions it is about 8°C.
The use of perforated polyethylene air-conducting sleeves helps towards equalizing the temperature differences in vertical direction as well. The air-conducting sleeves are disposed at 3 m distances immediately above the soil surface. They can be easily suspended at the end of the greenhouse and removed with the warming up of the weather.
As proved by practice, a heat generator TG-150 can ensure an increase in the temperature of a 1000 sqm plastic greenhouse by 16 to 17 degrees. This type of heating has another significant advantage, namely, the decrease in relative humidity. With the heat generator operating, it drops down to 50–60 per cent. Such a decrease in relative humidity is observed