Greenhouse Environmental Control
The ventilation ofis one of the most important facets of environmental control. It is necessary for the following reasons: to limit undue rise in temperature, to replace exhausted air with fresh, to maintain humidity at the required level.
Control of temperature
This is the most important consideration with many gardeners, as solar radiation between spring and autumn is very frequently in excess of needs on bright sunny days, necessitating frequent air changes to keep the greenhouse air temperature at an acceptable level for plants. When solar short-wave radiation reaches the plants in a greenhouse about three-quarters of it is absorbed by the leaves and the remaining quarter reflected back to the greenhouse as long-wave radiation. As all other objects in the greenhouse reflect back the radiation, absorbing only a small proportion, the net effect is therefore to cause a rise in temperature of the plants’ leaves and the surrounding area. The rise is reduced to a certain extent by the loss of heat by evaporation, caused by the process of transpiration. The total amount of foliage in any greenhouse varies considerably, and this in turn greatly affects the percentage of radiation absorbed and reflected, and in turn the amount of transpiration which takes place.
As air, when warm, becomes less dense and rises, an exit is ideally allowed for it at the highest point in the greenhouse along the ridge. Cool air also gains entry into the greenhouse through the same ‘exit’ by a complicated interchange mechanism, provided there is enough air movement out of doors to give sufficient impetus for this interchange. If, however, cool air is allowed entry through lower vents this speeds up the whole process of interchange.
Ventilator size and position
The size of ventilator on the ridge is obviously critical and an actual ventilation area of at least one-fifth to one-sixth of the actual floor area (irrespective of lower vents) is the accepted commercial norm, although if a greater area can be provided this is better still. Lower vents or louvres should be proportionate in size with ridge vents. It helps greatly if ridge ventilators are capable of opening to an angle of 55°— in line with the slope of the opposite roof and well above the horizontal — as in this position they scoop in the wind while also allowing easier exit for warm air.
Correct provision of ridge vents is a critical point. For greenhouses of a width module of over 6m (20ft) it is advisable to have continuous ventilation on both sides of the ridge which allows effective air change under a wide range of conditions. On smaller greenhouses (which will include most amateur sized structures) continuous ridge ventilation is seldom fitted, it being usual to rely on alternate ridge ventilators, with fixed glazing opposite each vent. In this case, when a wind is blowing, air comes in on one side and is discharged out the other. It is important to remember that the total area of these should still be at least one-fifth to one-sixth of the floor area. For a 3 x 2.4m (10 x 8ft) greenhouse with a ground area of 7.44m2 (80sq ft), one-fifth is 1.48m2 (16sq ft). This would allow for four 60 x 60cm (2 x 2ft) ridge vents, provided they are capable of opening fully to that area. Most manufacturers are quite happy to provide additional ventilation up to the required ratio. Amateur greenhouses do, in fact, because of a higher glass/ground ratio, need a higher ventilation percentage than commercial greenhouses, possibly up to 30-40% of the ground area. Some manufactures take the door into account to provide satisfactory ventilation.
With simple opening type vents on roofs, sides or doors, draughts are unavoidable. Louvred types, frequently used in sides or ends, are less liable to allow entry of draughts but it is doubtful whether any system of ventilation depending on the opening of a flap or louvre can be 100% draught proof. Attempts at air-conditioning to allow ‘perfect’ ventilation without draughts will be referred to later.
Operation of ventilators
Ventilators can be operated by simple levers or by pulley and cord. In recent years rack and pinion operation for commercial glasshouses has become almost standard. To ensure as accurate a temperature regime as possible ventilators should operate automatically, either by electric motor or, on an amateur scale, by expansion type lifts which are cheap to purchase yet highly dependable in practice.
The mechanism for automatic ventilators is varied in design and depends much on the type of greenhouse and all the various design factors involved. Step control to give a quarter, one-third, three-quarter and full opening is available, the most sophisticated types of ventilator being linked to a proportional or modulating controller which sets the position of the vents according to the rate of change required in the greenhouse temperature. In addition there can be a wind gauge to open or shut vents in one direction to avoid draughts and a rain gauge to shut vents up when it rains. The whole system can be integrated with computers.
Although this is still not widely used in amateur greenhouses, fan ventilation is widely accepted in commercial spheres. When designing a fan ventilation system it must be remembered that most plants dislike rapid air movement which, like excessive wind out of doors, causes them to transpire too rapidly. It would be easy to fit a high output fan in a greenhouse to move the air at high speed, but this would inevitably cause turbulence and air movement to the detriment of the plants. Research work over many years has shown quite clearly that plants will tolerate an air flow rate of 0.1982 m3/s (7cu ft per sq ft) maximum. On a practical level this involves using relatively slow speed fans which are capable of moving a large volume of air slowly, rather than high speed fans which suck air rapidly over a fairly localized area.
The basic design of an extractor fan ventilation system is to extract the air at one end or side of a greenhouse and provide for the entry of air at the opposite end or side. Where large units of greenhouses are involved, several fans are required, spaced to allow even, complete air extraction with no stagnant areas. With large commercial type greenhouses it is usual to have 1.2m (4ft) fans operating at 360 — 470 rpm spaced evenly along the north or east side wall of the block, allowing for 5.58m2 (60sq ft) per fan of inlet area on the opposite wall. This results in air movement which is achieved whenever the fans are in operation, irrespective of wind speed or weather conditions out of doors, generally maintaining a satisfactory air flow. The inlet can either be manually or automatically operated, it being important to ensure that the fans cannot operate until the vents are open. Fans are best operated from an aspirated sensor or thermostats which can either operate all the fans simultaneously or in batches to come on at different temperatures. Time clocks or computers are considered necessary on commercial holdings to give short periods of night ventilation, otherwise humidity can build up to a dangerous level, although fan operation can also be controlled by a humidistat. Variable speed fans are also available, worked by thermostatically operated variable speed controllers, but special types of 3 phase fan motors are necessary, along with the sophisticated control gear for the inlet vents, which adds considerably to the cost of the installation.
While the installation height of a 1.2m (4ft) fan offers little difficulty as it invariably rests on the foundation wall, choosing the height for small fans is not entirely straightforward. Experience has shown that with high set 60cm (24in) fans on the side wall of smaller blocks of commercial greenhouses, the inlet being provided on the opposite wall, there is a tendency for the air to move over a dense crop such as, rather than through it.
Amateur gardeners will almost certainly be using the smaller type fans, the specification for which must be carefully calculated by a firm specializing in fan ventilation of greenhouses, many domestic type fans being incapable of giving the requisite number of air changes. All fans should ideally be louvred so that unwanted air cannot gain entry when the fans are not operating. It is convenient to install these fans fairly high in the greenhouse, a usual location being above the door.
Inlet for fans
The position of the inlet for fans has proved to be critical, experience showing that vents on the side wall rather than the roof are better for blocks of greenhouses. This allows the incoming air to make a quick loop before moving across the greenhouse. Single greenhouses lend themselves to the fitting of the fan in the gable end, either at both sides of the door if two are used, or above the door for one only. A further advantage in having side vent entry for fans is that heating pipes can partially warm the incoming air, a distinct advantage in cold weather as it tends to prevent any cold shock to the plants. Damp pads can also be used to wet incoming air, a technique practised in very hot, dry countries, and gaining some favour in Britain for certain crops.
The temperature gradient which can exist between inlet and outlet in fan-ventilated greenhouses causes concern in certain quarters, the exact difference in temperature depending on length of travel of the air. In practice this seems to be of negligible proportions, certainly if compared to the temperature gradients which exist in greenhouses where conventional ventilators are fitted.
The positive air movement induced by fans ensures that humidity is kept at a low level and stagnant pockets avoided, there being ample experimental and practical cropping evidence to support this. The positive intake of air, especially in hot, still conditions when conventional vents cannot induce a sufficient number of air changes, results in a constant supply of carbon dioxide being brought to the plants. The form of air intake can vary from top-hinge ventilators to louvres, in both cases ensuring that there is sufficient inlet to cope with the fan capacity. Use has been made of mesh material, which allows entry of small amounts of air all the time and, with the suction developed when fans are in operation, acts as an efficient inlet. Flap arrangements are also possible, using sheets of plastic dropped down over the back of an inlet, which lifts up when the fans are in operation but prevents draughts when the fans are not in operation. Collapsible poly thene tubes may also be used as inlets. A more recent development is polythene ducted fan in-takes to try and minimize temperature gradient between intake and fans.
In addition to conventional ventilation and extractor fans, a more recent aspect of environmental control is “air conditioning”. This can take several forms, the most elementary of which is to pull air into the greenhouse thus pressuring it to a limited degree. The air is exhausted out of counterbalanced louvres. Such systems, although not complex, have proved effective, although temperature gradients in the greenhouse can be excessive if the design is not correct. Warm air can be introduced by the same basic system and humidity control can also be effected to a certain degree by using a humidistat to inject water into the moving air stream when necessary. These matters are, however, better taken up with specialist suppliers of equipment, as most systems need to be designed specially for each individual greenhouse.
Desirability of environment control
Plants have their own inbuilt requirements in respect of leaf temperature, air temperature, humidity, carbon dioxide and light required for photosynthesis. While a greenhouse creates an artificial environment, it is necessary to try to control this within reasonable limits in sympathy with the needs of the plants being grown in it. Under commercial growing conditions, where only one specialist cropping system may be involved, every attempt is made to conform to a “blue-print” in respect of environment, level of water and feeding requirement. This is now done by computer. The object of this is to ensure the maximum production of quality crops or plants on what can be a very tight schedule. Amateur or professional gardeners are not perhaps so deeply concerned with their greenhouse gardening, but there is still considerable virtue in calling on modern aids to control the environment automatically as far as this is possible, simply to ease the load of look- ing after plants and at the same time help to ensure quality despite enforced absence at business or elsewhere.
The full control of the greenhouse environment is a complex matter with a great many inter-related factors to be taken into account. Much research is still likely to take place over the next decade to try to discover what exactly constitutes the ideal environment for different plants and thereafter set out to achieve this effectively without too much complicated and expensive equipment.