Installation of soil and air heating systems in a greenhouse

• cover the installed wire with a 50 mm layer of sand or pour over it a 30 mm layer of cement solution;
• on top of the sand layer (or cement screed), spread a 250-300 mm layer of nutrient-rich soil.

The strands of the heating wire, as well as its connection points to the supply cable, must not touch each other.

When installing a heating wire for air heating, it is necessary to make and attach supporting hooks to the walls of the greenhouse and lay the wire on them. The distance between the hooks is as follows:

For electric heating of air and soil in hotbeds and greenhouses, heating wires of the POSKhV, POSKhP, and POSKhVT types are widely used. The maximum permissible heating temperature of POSKhV and POSKhP wires is 70°, and for POSKhVT it is 105 °C.

Recently, film heaters with heating elements have been very widely used. They consist of a steel casing covered with insulating enamel, onto which a paste-like mass of resistor material is applied by pneumatic spraying. The top of the electrically conductive film is covered with heat-resistant electrical insulating varnish or epoxy resin.

Heating elements operate under high-temperature conditions, so their resistance to high temperature determines the service life of the device.

Special chromium-nickel alloys (nichromes), iron-chromium-aluminum alloys, and non-metallic heaters (graphite, carbon, carbide, etc.) meet these requirements best.

At operating temperatures of up to 300-360 °C, galvanized steel wire is used, which is a cheap and accessible material. However, heating elements made from it have significant disadvantages: they are prone to significant oxidation and rust, have a high temperature coefficient of resistance, and lack consistency in electrical properties even within the same grade of wire. Elements made of galvanized steel wire are used for soil heating and air in hotbeds and greenhouses.

To insulate heating elements, materials are used that, in addition to electrical insulation properties, have good thermal conductivity, which ensures minimal heat drop between the heating resistance and the working surface of the element. These materials must have high insulating qualities both in a cold state and at high operating temperatures and high humidity.

Electric heating elements are insulated with mica, asbestos, porcelain, or quartz sand. For the insulation of open heating elements, shaped ceramics are used, which simultaneously serve as a frame for the heater.

In electrode heating, the materials from which they are made are of great importance. Iron electrodes are used only when heating water for heating systems.

The industry produces electric heating elements that are sealed, enclosed, and open. Sealed heaters do not oxidize or get dirty; they are protected from mechanical damage; they are electrically safe; and they transfer heat without sharp temperature drops through convection. The most common are tubular electric heaters (TEHs). They are used in water heaters and electric air heaters.

A tubular heater consists of a metal tube, a nichrome coil, filler, terminal studs, and sealing bushings, and has nuts for attaching the heater. Magnesium oxide is usually used as a filler, as it conducts heat well and is a reliable insulator. The coil in such a heater hardly oxidizes, which ensures a service life of up to 10,000 hours. The tubes of the heaters are made of ordinary and stainless steel and brass. TEHs with tubes made of ordinary steel are used for heating air, and those made of stainless steel and brass are used for heating water.

Tubular heaters should only operate in the environment for which they are intended. If an electric heater is designed to work in water, then its entire active part must be submerged in water so that they do not touch each other. The terminals should be insulated from the effects of heat radiation.

TEHs are designed for nominal voltages of 12, 24, 36, 48, 55, 60, 127, 220, and 380 V; have a nominal power of 50, 65, 80, 100, 125, 160, 200, 250, 315, 500, 630, 800, 1000, 1250, 1600, and 2000 W; and have outer tube diameters of 7.9, 12.5, and 15.0 mm.

Enclosed electric heating elements are placed in a protective shell that protects them from mechanical damage but does not obstruct air access. Heat transfer is carried out by convection.

Open electric heating elements release heat through convection and infrared radiation.

A simple water-based electric heater can be made by oneself from the casing of a retired fire extinguisher. The top of the casing is removed, and a 1 kW TEH from an electric samovar is installed near the bottom. A removable lid is installed in place of the removed top. Two water pipes are connected to the casing, linking it to a radiator. When installing the pipes, rubber sealing gaskets and nuts from water pipe unions are used.

Electric heating is convenient because it is easy to automate. Control schemes have been developed for the heating of electrified greenhouses and hotbeds.

Using a simple electrical circuit and a temperature sensor, the heater will automatically turn on when the temperature in the hotbed drops to the set level.

Automatic electric heating: installation of devices and circuit adjustment

Automated heating of the soil and air allows for the maintenance of a stable microclimate in a greenhouse without constant human involvement. The system operates based on temperature sensors that control heating elements (tubular heating elements) depending on current conditions. When installing such automation, it is critical to correctly select the switching equipment to avoid burning contacts and failures in the power circuit.

• Depth of sensor placement in the soil — 5–7 cm
• Minimum relay contact current — 5 A
• Capacitance of spark-suppression capacitors — 1000 pF
• Permissible contact power for a mercury thermometer — 2–4 W

To assemble the control unit, when an AC relay is available, a standard scheme based on MKU-48 is used. If such a relay is not available, the circuit is assembled according to an alternative scheme with D226 diodes and spark-suppression capacitors C1 and C2 with a capacitance of 1000 pF. In this case, a relay with contacts rated for a current of at least 5 A is used. The automatic control system operates according to a cyclic algorithm.

1. The electric contact thermometer (KT) is installed in the soil at a depth of 5–7 cm.
2. When the soil temperature drops to the specified limit, the contacts of the KT thermometer close.
3. The closing activates relay K1, which uses its K1.1 contacts to close the heater power circuit.
4. After the soil is heated to the required temperature, the thermostat breaks the K1 relay power circuit, and the heater turns off.

Manometric electric contact thermometers of the TPR-SK, TPP-SK, or TPG-SK types are installed on electrode heaters. They consist of a thermal bulb, a capillary, a manometric spring, and a needle with a movable contact. The entire system is sealed and filled with a substance having a high volumetric thermal expansion coefficient. As the temperature changes, the pressure of the filler changes, forcing the spring to rotate the needle and close the control circuit at the set value.

The following substances are used as fillers for the sealed system of manometric thermometers:

• argon;
• nitrogen;
• freon;
• methyl chloride;
• acetone.

For precise control of the temperature regime, metal and semiconductor sensors (thermistors) are used. An element made of thin copper or platinum wire is wound on an insulating frame and placed in a protective casing. Thermistors have high sensitivity and small dimensions, but their characteristics are less stable. Electronic circuits for these sensors are simple to assemble and can be easily built from standard radio components.

Alternative heating: preparation and placement of biofuel

If it is impossible to supply electricity to a greenhouse, heating is organized using biofuel. As it decomposes, it releases heat that warms the root system of plants and improves the soil structure. Any organic components capable of decomposition are used as fuel: manure, wood shavings, bark, straw, or household waste. Cow manure and dry fallen leaves from forest trees provide the highest heat output.

Preparation of forest leaves begins in the autumn, combining this process with the protection of garden plantings. The collected leaves are distributed under the canopy of fruit trees, where they protect the roots from freezing during winter. In the spring, this compacted biological material is used to create warm beds in the greenhouse. Work on laying biofuel begins in early spring as soon as the soil thaws.

1. In the spring, immediately after the snow melts, collect the leaves compacted under the trees.
2. Transfer the partially decomposed organic mass to the greenhouse.
3. Place the prepared biofuel in trenches under the beds and cover with a layer of soil.
4. Put the remaining collected leaves into compost bins to produce humus.

Installation of subsoil equipment and natural ventilation

To create an optimal microclimate in the root zone and the greenhouse air, precise installation of all systems is required. When installing subsoil devices, it is necessary to strictly comply with the design geometric parameters. This will ensure uniform distribution of heat and air in the root zone of the plants.

Properly designed natural ventilation helps regulate air temperature and humidity. Transoms and vents are located in the end walls (including curtain doors), side walls, or on the roof. In film greenhouses, you can simply roll up the side covering for ventilation using special reels.

The design of the vents must be airtight and easy to operate. They are attached to hinges suspended from the ridge or the upper tie of the side wall, necessarily using sealing gaskets to prevent heat loss. To reduce the effort required for manual or motorized opening, shift the rotation axis of the vent closer to its center.

• Ventilation opening area — 10–15% of the greenhouse area
• Forced ventilation air flow rate — 1–1.5 m³/min per 1 m² of area
• Domestic fan capacity — 20–30 m³/min
• Size of the vinyl plastic actuator plate — 1100x170 mm

If natural air inflow is insufficient, install a forced supply and exhaust ventilation system. For this purpose, an exhaust electric fan is installed at one end, and intake louvers at the opposite end. The louvers must remain closed at all times and open automatically due to air rarefaction when the fan is turned on.

Ventilation automation: electric actuators and thermostats

Automatic opening of vents eliminates the need to constantly monitor the temperature manually. Electromagnets or actuator mechanisms with an electric motor are used for automation. Industrial PR-1M actuators with a power of 50 W or domestic actuators for rotating Christmas trees are well-suited for this purpose.

It is much easier to automate ventilation using direct-acting thermostats, which combine a sensor and an actuator mechanism. Their operation is based on the thermal expansion of technical oil in a cylinder, which pushes out the piston rod when heated. The industrial "Tulip" regulator serves as an example.

Another option for an autonomous device is an original thermal actuator made of a two-layer plate (metal and vinyl plastic). Its operation is based on the difference in thermal expansion of these materials. The plate is hinged: at the bottom to the greenhouse base, and at the top to the transom rod in the side wall. As the temperature rises, the plate bends and opens the sash, and it returns to its original state upon cooling.