Environmental Control in the Greenhouse
Control of heating systems
With simple solid fuel hand-fired boilers and large diameter pipes, where the water circulates entirely by gravity or thermosyphon, there is no real place for automatic control, and the usual control of temperature is to rely on the boiler damper and the level of firing in association with regulating air entry at the fire door. It may be possible to restrict the flow from the boiler by the use of a check valve situated on the flow pipe, although this could be dangerous and lead to overheating of the boiler. Solenoid valves, electrically operated by thermostat can also be used, but suffer the same drawbacks. In practice and with experience a fair degree of control of actual temperature can be exercised, but only with constant vigilance, this showing the defects of such systems where strict environmental control is sought.
Where circulation of the water relies largely on an accelerator pump, good temperature control can be achieved by linking the accelerator to an on/off thermostat set in the greenhouse, the amount of control which can be exercised varying according to the degree of dependence on pump circulation. Where the circulation of water depends entirely on the pump, there can be excellent temperature control, especially if the boiler is automatically fired by oil or gas.
Ideally there should be an independent on/off thermostat to control the boiler at a pre-set temperature (generally above 71°C/ 160°F) and an on/off or multi-step control (a combination of two or more on/off units arranged to operate in stages) sited in the greenhouse to call for heat. This operates the pump and sends water round the system. There are definite drawbacks to this system when large bore pipes are concerned, especially if these are of cast iron with cemented joints. Unless there can be arrangements to ‘bleed’ a small quantity of water into the pipes to keep them warm, especially adjacent to the boiler, the sudden rush of hot water when the pump is set in motion can split the pipes or loosen the cement joints. Arrangements to bleed in a small quantity of water usually consist of a pipe-fitted on/off thermostat, adjacent to the boiler on the greenhouse side of the pump and set to override the greenhouse thermostat when the temperature of the pipes falls below about 27°C (80°F).
Pump control is ideal for small mini bore systems of heating with their low thermal inertia and lack of complications from pipe damage. The methods of arranging a bypass to ensure a continuously warm boiler and simultaneously give a range of temperatures in the pipes, have been referred to earlier but an electrically operated modulating valve would ideally then require to be linked to a modulating controller. Modulating controllers can of course also be linked directly to pumps. Like proportional controllers they operate on the principle that the amount of heat supplied is related to the variance of the greenhouse air temperature from the previous level, rather than by successive periods of full heat input followed by no heat, as dictated by on/off control. Night setback of temperature can also be manipulated according to average day temperatures.
A still further development is the use of integrating photometers or light sensors which control the input of heat according to the prevailing light intensity and, still more sophisticated, relate this to the prevailing weather pattern in order to avoid sudden shocks to the plants’ physiology. Night setbacks according to light intensity and temperature can also be incorporated.
Ventilators or fans can conveniently be operated on the same range of instruments as heating systems — independently but perfectly integrated by computer.
What has been said about hot water heating systems applies equally well to other heating systems, whether these are steam or warm air supplied by direct firing or through heat exchange units. Reference has been made earlier to the complications of steam heating with regard to its on/off qualities, but in essence the same basic principles of automatic control apply.
Carbon dioxide enrichment and humidity control
The dispensing of carbon dioxide can be as simple as on/off and the shutting down of vents during the enrichment period to avoid wastage. In any case supplies of extra CO2 are now invariably provided by direct fired gas or paraffin heaters. Preferably, however, there must be integration between the two factors, so that ventilators are automatically closed and fans out of action when CO2 is being dispensed. The amount of CO2 supplied can also be related to light intensity.
Humidity control is not straightforward. When a rise in humidity is called for by a humidity sensor something of a dilemma may result, there being a choice of bringing into operation spraylines or mist jets and shutting the vents. Conversely, for lowering the humidity, are the vents to be opened, or the fans operated, or the heating called on to raise the temperature (warm air holding more moisture than cold air)?
It has previously been stated that allowance must be made in really sophisticated systems for damage to open vents caused by sudden winds, demanding a wind sensor, and a rain sensor to shut down the vents when it is raining heavily.
Automatic feeding and watering control
The supply of water and liquid feeding of plants and crops is also carried out automatically with sequence controllers operating solenoid valves.
Measuring and recording instruments
An accurate greenhouse thermometer is an essential piece of equipment and these can take several forms, but undoubtedly the most useful is a maximum/minimum model, especially a push button reset type. A humidity gauge or hygrometer is also a useful item. Ideally these instruments should be in an aspirated screen, failing which a metal foil shield is reasonably effective in shading them from direct sun.
However no instrument that is exposed freely to solar radiation, draughts, or radiation loss can be considered accurate. Solar radiation absorbed by the body of a thermometer or the case of a thermostat will result in a variance between the recorded and the actual conditions. For this reason the only way of ensuring accurate sensing is to use an aspirated screen for housing all instruments. This is an insulated box or section of large diameter PVC tubing fitted with a constantly operating small fan which pulls air gently through the box or tube over the thermostat or other instrument housed in the screen. This ensures that the actual air temperature or humidity is accurately recorded. Far more popular these days is the use of an electronic sensor, which gives highly accurate temperature control and is relatively unaffected by solar radiation and draughts.
Aspirated screens are usually provided with the instrument installed, and the unit generally sited in the centre of the greenhouse; for greater accuracy it can be placed in the actual area of crop growth.
Control equipment and the amateur
The above items of control equipment in one form or another, and in various degrees of refinement and sophistication, are fully operational in many commercial and local authority units. Their application in amateur spheres is obviously more limited, and gardeners should confine their horizons to reasonable levels, seeking perhaps the automatic control of ventilation and heating on a simple on/off basis and little more. In recent years, however, mist and drip watering systems on automatic control have made a big impact in amateur circles.
Electric heating units of all types readily lend themselves to thermostatic control and it would be foolish not to take advantage of this. Manually operated appliances of any kind, and similarly hand-operated heaters, require constant regulation and, more important, the presence of the gardener. One cannot, however, always get good results by relying entirely on manual management; many have achieved excellent results, especially those blessed with intuition and a good growing sense.