In Victorian times the environmental requirements of the plants were not well understood, and it was considered that temperature and ventilation were correct when it felt ‘buoyant’ and when it felt comfortable for the gardener. The temperature was read twice a day from a single unscreened thermometer placed in the centre of the. In modern glasshouses temperature, irrigation, and ventilation are controlled by computer systems working to pre-determined programmes. The more sophisticated automatic controls now fully integrate air temperature, light intensity levels and carbon dioxide concentrations by constantly measuring the temperature and wind speed, etc.
Temperatures are often measured by aspirated thermostats in which air is drawn across thermal rods giving an average rather than fluctuating measurement. Instead of measuring still air in one small part of the glasshouse it is constantly sampling the whole air mass.
Ventilation is the means by which glasshouses are COOLED by the natural exchange of air inside the glasshouse with cooler air from outside, through openings called VENTILATORS. or by the use of EXTRACTOR FANS.
Cooling. Heat radiation from the sun has little effect when passing through glass and air, but when it falls on SOLID objects such as crops and soil it transfers most of its heat. The crop temperature increases and heat is transferred to the air surrounding it. As the air heats up, and its temperature increases it is more difficult for the crop to lose heat so the crop tends to get hotter. It is necessary therefore for this hot air to be removed and replaced by cooler air from outside the glasshouse so that the crop can lose its heat to the surrounding cooler air. As the air must be replaced by cooler air, in any form of ventilation there must be inlets to let cool air in, as well as outlets to let hot air out. Also, since the air enters at the same temperature as that outside, andat a higher temperature because it has taken up some heat, the air temperature inside the glasshouse will in general always be higher on average than that outside. The amount by which the temperature inside is higher than outside depends on the rate at which the air is changed, and if this is fast enough, the temperature inside the glasshouse will be only slightly above that outside. Because the heat radiation entering the glasshouse is absorbed by the crop and the soil, the amount of cooling required to maintain a particular temperature is proportional to the area of the house.
Cooling is measured by the air extracted in CUBIC METRES PER MINUTE (cubic feet per minute), and the COOLING RATE is stated in terms of CUBIC METRES PER MINUTE PER SQUARE METRE OF SURFACE (cubic feet per minute per square foot of surface): SURFACE is taken here to be the actual AREA OF GROUND WHICH THE GLASSHOUSE COVERS. The actual ventilation rate to be used is determined primarily by economic reasons, and varies with the type of crop, type of glasshouse, and the location of the nursery. However, there is obviously a ventilation rate which gives the best CROP RETURN for the outlay, and experience has shown that a RATE OF 0.21 CUBIC METRE PER MINUTE. PER SQUARE METRE OF GLASSHOUSE FLOOR AREA (this can be abbreviated to 0.21 crnm/sa.nO [rate of 7 cubic feet per minute, per square foot of glasshouse floor area (this can be abbreviated to 7 cfm/sq.ft.)] has usually proved to be satisfactory but that under certain conditions, different values should be used.
Some more information about ventilation
The amount of ventilation, and hence cooling, depends on wind speed and direction, and on the MAXIMUM AREA OF OPEN VENTILATORS WHICH CAN BE OBTAINED. The most efficient ventilators are those capable of giving a maximum open area of about 15% of the floor area. For example, if the total floor area of a glasshouse was 500 square metres, then the maximum vent area required to ventilate the glasshouse efficiently would be 15% of this figure, i.e:
15% of 500 square metres = 15, x 500 = 75 square metres.
Therefore the optimum vent area for this particular glasshouse would be 75 square metres.
Fan ventilation is particularly useful for large blocks of glasshouse, also called multi-spans. They cover a much greater floor area than single spans, and the centre of the house will not benefit from the presence of side-ventilators. The fans help to suck the air through the glasshouse so ensuring that ALL the air in the centre of the block is removed and replaced with air from outside.
Side ventilators increase the chimney effect, and produce more efficient ventilation than ridge ventilators alone. In very hot spells of weather in the summer, panes of glass can be removed to supplement ventilation or to act as side ventilators and improve the efficiency of the ventilation rate.
Traditionally, ventilators have been operated manually, either by levers, or by chains which rotate a wheel. Where the temperature changes are rapid and the crop sensitive, it is clear that this can become time consuming. To overcome this problem, the ventilation system can be automated by the use of electrical control systems operating hydraulic rams, or electric motors operating rack and pinion systems. These sorts of automated systems save money, and give the grower an opportunity to give plants a better and more accurately controlled environment. In older glasshouses, it is often not practical to install side and ridge ventilators, so by way of compromise fan ventilators can be used; fans can be installed much more easily than rebuilding the structure to include side and ridge vents. Older glasshouse units can often be modernised by fitting automatic ventilation and irrigation systems, the costs being justified by savings made on labour. Consider how much time is taken each day to continually alter manually operated ventilators when the weather is changeable especially in the spring, and the hours spent ona range of glasshouses with only a hose-pipe or watering can. It will often be found when the actual hours and costs are calculated, that the outlay on a new irrigation system can be recouped in just a few months against the savings made on reducing hand watering.
Monotronic controllers work so that if the temperature needs adjustment so the switch is made.
and COMPUTER control of the glasshouse environment has meant that a much more accurate control of the internal environment has now become possible. Accurate control of humidity is very useful because by this means it is possible to reduce the incidence of diseases such as BOTRYTIS fGREY MOULDS the development of which is largely dependent on high humidity levels.
The traditional way of ventilating glasshouses in this country is to open ROOF VENTS to let the hot air inside rise and escape to the outside. Only very occasionally are side vents used to let cool air in to lower the temperature of the crop. When conditions are very windy, only the vents on the LEEWARD side of the house should be opened; if vents on the WINDWARD side are opened, this can lead to excessive levels of turbulence inside the glasshouse. This feature – side vents – has in recent years been a more standard incorporation into glasshouse design. In older glasshouses the ‘leakiness’ assists in the process of allowing cool air in lower down to cool the crop. This ‘chimney effect’ occurs, but has little useful result since to produce any appreciable air movement, the air inside the house must be much hotter than outside which is precisely the result we wish to avoid. Even with the maximum size of vent now recommended, very little is achieved. In the presence of wind, roof vents can scoop air in and a reasonable wind can produce an acceptable temperature in the upper part of the house.
Reasons for ventilation
1. It limits the temperature rise in summer; lack of ventilation produces very high temperatures which can damage plant tissue.
2. Without ventilation, the air temperature inside the glasshouse could rise to about 20°C above AMBIENT temperature (outside air). Even a good ventilation system may not work too well on a still day.
3. To replenish the C02 content of the glasshouse.
4. In daytime, it needs about 10 air changes per hour in a fully cropped house to maintain the C02 level at 0.03% (300ppm).
5. The National Institute of Agricultural Engineering has shown that with a natural vent system with no side vents, the C02 level can be 10% down at the side of the house as compared with the middle and upper layers of air.
6. To reduce and.control HUMIDITY levels.
A positive movement of air has many advantages inside the glasshouse. The crop itself is cooler in a good airstream than where the air is stagnant, even when the air temperature is the same, since the airflow helps to remove heat from the plants. It must be emphasised that with cooling as well as heating, it is the crop temperature which is important. The temperature of the surrounding air only matters when it affects the crop, and the temperature of air well away from the crop (such as in the roof) is of no importance. This cooling by airflow is probably the reason why shading is unnecessary for most plants in a fan ventilated glasshouse.
Another very important consequence of good ventilation is that carbon dioxide (C02) is introduced into the glasshouse atmosphere by fresh air entering from outside. This helps to restore the C02 level to that outside, thus promoting optimum plant growth. C02 levels always tend to drop in the cropping house because plants use up the C02 in PHOTOSYNTHESIS. The natural concentration of C02 in the atmosphere is 300 parts per million (ppm), or 0.03%. Even when cooling is not needed, air movement in a glasshouse is very important. The air surrounding a crop tends to become depleted of C02 and also to become very humid, causing poor growth and disease. These adverse effects can be avoided by a good air movement which prevents these stagnant conditions developing. The air movement produced must be uniform so that ALL parts of the crop experience the same cooling effect. In areas which are stagnant, disease and poor growth will result and areas which are too windy can suffer other adverse effects depending on the crop.
Considerable thought and experience is employed to keep the TEMPERATURE & HUMIDITY to some set figures needed for a particular crop. To emphasise the importance of ventilation, it has been stated that a similar amount of thought and expense is justified economically to produce good ventilation and cooling. The advent of MICRO-PROCESSOR
The larger surface area would counter the benefit of the improved heat retention but the actual sum of the difference would depend upon the extra area involved and the thermal conductivity factors for each material.
The amount of INFRA-RED or LONG WAVE RADIATION transmitted by the materials differs. Polythene transmits the most, and glass the least. The amount of infra-red radiation which is maintained within the structure is important because this helps to maintain a higher temperature: I.e. without any heat input from a heating system. Thus after a sunny day in winter, there will be less chance of plants suffering frost damage inside a greenhouse than inside a polythene tunnel, because more heat will have been stored under the glass due a to greater amount of long wave radiation being retained. Notice also from the section on glazing materials that PVC retains more infra-red radiation than polythene, and so is better at providing frost protection than the latter.
A question to ponder – would the rigid corrugated PVC with its greater surface area be significantly better than the thin polythene tunnel cloche? See answer below.
Twin-skinned polythene tunnels are nearly as good as glass at retaining heat.
Rigid plastic sheeting is available, for example acrvHc sheeting. Although it has a slightly lower light transmission rate than glass, since it is lighter, it needs less structural support, and so this compensates for the lower light transmission. The real disadvantage is that it only lasts from 15 to 20 years. Polythene and PVC sheeting are available with ultra violet inhibitors, which extend the life of the material. Ultra violet light is responsible for the breakdown of these materials.
Formulations containing these chemicals are called ‘UV inhibited’, and are widely available.
Glass allows the admission of short-wave radiation from the sun. This radiation is absorbed by plants, the soil and the greenhouse structure itself, and then re-radiated as LONG WAVE RADIATION. This long wave radiation CANNOT pass through the glass, and is REFLECTED back into the greenhouse, thus raising the temperature inside the structure. This is called the greenhouse effect. Where heating pipes are fitted, the radiation given off by these is in the long wavelength, and so is itself kept inside the greenhouse by being reflected back into the greenhouse by the glass. As can be seen from the section on glazing materials, the