GLASSHOUSE HEATING SYSTEMS EXPLAINED

To achieve some control of the growing temperatures inside the glasshouse, some form of heating can be introduced. There are many different systems for heating a glasshouse, below are a few examples found in practice.

1. Pipes containing hot water.

2. Pipes containing steam (usually only on very large glasshouse units).

3. The circulation of warm air; the air can be heated by oil, electricity, gas, or even steam, and is distributed around the glasshouse by the use of fans and polythene tube ducts about 300mm in diameter.

4. Paraffin heaters (useful for frost protection only situations).

5. Special metal tubes containing electrical heaters, or electric cables.

Traditionally almost the oldest form of glasshouse heating was effected by coke-fired boilers to heat water which circulated in pipes on a THERMAL SYPHON SYSTEM.

For this system, large bore pipes of either 100mm or 150mm were used, made of cast iron. The boiler was at the lowest point of the system (often in a sunken boiler room), and the pipes were constructed to rise away from the boiler so that at the highest point they were about 3m above it.

The movement of water in the pipes of this system is dependent on the large BORE which produces a natural SYPHON: as hot water is less dense and lighter in weight than cold water the warmed water from the boiler rises in the pipes slowly, being replaced by cold water from the other end of the pipes which enters the boiler for reheating. So a natural circulation is produced;

More complex systems accelerate the water flow rate, use small bore pipes with gills to increase the surface area and may use very hot water in large pressurised systems.

CARBON DIOXIDE LEVELS

The air usually contains about three hundred parts per million of carbon dioxide, some of this enters the leaf via the stomata and is used in photosynthesis. As the plant uses carbon dioxide during the daytime the concentration is reduced and for a glasshouse which has a full crop, the situation is often reached due to high light intensity and adequate water supply with nutrients, the only factor which reduces growth and photosynthesis is the C02 levels. This limiting factor can be overcome by using a C02 burner to increase levels during daylight hours. Photosynthesis is a process in which the rate is limited by the factor in least abundant supply. That can be water, nutrients, carbon dioxide or light intensity.

Photosynthesis occurs in good light at suitable temperatures within the green parts of plants (principally the leaves) as is explained elsewhere in the course. The result in a glasshouse is that if the vents are closed the levels of carbon dioxide present (0.03% naturally) are quickly depleted and growth is dramatically but not obviously restricted.

Hence some ventilation during daylight hours is virtually an essential for all growing and leafy plants under glass. (Carbon dioxide enrichment is one way out of the need to ventilate. One of the snags of ventilation has been that the heat supplied may be lost along with the extra humidity. As heat has become an increasingly costly commodity the efforts to conserve it have also become the subject of much scrutiny and research.)

Extra C02 is usually added to raise the levels to about 0.1% which may increase growth rates of heated crops like winter lettuce quite dramatically.

Duotronic controllers take into account other factors like wind speed, relative humidity, C02 emission from C02 burners or from bulk C02 gas supplies. More sophisticated controllers can integrate a range of parameters required by the grower.

Alarm systems are frequently built into sophisticated controllers so that a designated telephone is rung with a recorded message which might advise that a block of glass had for example a temperature above the agreed alarm level. It is a useful approach because crops lose quality and condition if they are Stressed’. With the large scale use of automatic controllers there may not be any competent staff on hand who would recognise that the optimum growing conditions were no longer prevailing.

REASONS FOR VENTILATION – REMINDER LIST

1. To limit the rise in air temperature die to solar heat.

2. To limit the rise in humidity due to transpiration.

3. To replenish carbon dioxide for use in photosynthesis.

4. To increase air movement over soil or growing media and assist the rate of exchange of gases between soil air and the atmosphere.

TEMPERATURE

The effect of the sunshine is to heat up the air and the objects under glass. This can be fairly dramatic for the smaller glasshouse because the change can be so speedy – from a chilly 10°C to maybe 32°C within an hour. Plants wilt and may scorch. If the temperature continues to rise to say 65°C (and I have known this happen) most plants do so badly that they are virtually dead. If the temperature continues to rise too high many plants will continually wilt and eventually collapse or die, as plants cannot take up water fast enough, nor can transpiration rates keep apace with the overall water losses. Pests such as aphids, red spider mite, or mealy bug breed much faster in higher temperatures and without regular checks to the crop on at least a daily basis, outbreaks will start quickly and soon proliferate throughout the glasshouse.

RELATIVE HUMIDITY

Ventilation has 3 major effects – on temperature, on relative humidity, on C02. It exchanges the inside and outside air – the latter is cooler. It is also often less humid (a lower relative humidity) and this helps to fight off the potential for BOTRYTIS and other disease invasions which thrive in damp air.

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