Heating Systems for Greenhouses
Heat production units for pipe systems
A very wide range of ‘boilers’ exists, extending from simple sectional boilers to sophisticated steel-packaged units complete with controls. The basic purpose of any boiler is to burn the selected fuel and then transfer the heat to the water contained in it. This is achieved with widely varying efficiency, resulting in a combustion loss in all cases, except where an electric boiler or immersion heater is used, as electricity operates at 100 per cent efficiency. To bring about combustion of oil, gas or solid fuel of various types, air is necessary, but this must be supplied in the correct amount to bring about efficient combustion.
Where solid fuel is burned, a great deal of the generated heat will be transferred to the water contained in the jacket of the boiler by radiation and conduction. In addition the hot gases given off by the burning fuel pass through the flueways en route to the chimney and in doing so further warm the water by the process of conduction. It is obvious that the more heat which can be ‘extracted’ from the hot gas before it reaches the chimney, the greater the efficiency, although too much extraction of heat and the discharge of too cool gases below around 204°C (400°F) can result in condensation in the chimney, especially if an unlagged metal chimney is involved.
Combustion results in the production of the gas carbon dioxide (CO2) and the efficiency of the combustion is directly related to the percentage of CO2 contained in the flue gas. When this is measured with special instruments, in conjunction with flue temperature, it is a means of determining the percentage efficiency of any boiler or appliance. With the high cost of fuel the various modern types of boilers take this into account and are designed accordingly with balanced flues.
A simple and cheap boiler equipped merely with a grate for burning fuel allows the gases to pass directly into the flue at high temperature and can obviously never be as efficient as a boiler in which the flue gas must travel as far as possible within the limit of design, often through tubes jacketed by water. This is applicable no matter what fuel is being burned, although there is little radiant heat where oil or gas are used.
Where there is an excessive ‘pull’ or draught in a chimney because it is of too large a diameter or too high, this can also be a frequent cause of excessive heat loss and low efficiency, as the gases are pulled out before their heat can be passed on.
There can be no overall formula for setting up a boiler for maximum heat transfer and combustion efficiency, as this depends on the precise requirements of each boiler. The same is true of chimney height; the outlet should of course be above the ridge height of the greenhouse. It is also advisable to site the boiler so that the prevailing wind takes flue gases away from the greenhouse. Draught diverters or flue balancers are essential particularly for oil- or gas-fired boilers as they break the solid column of air in the chimney and enable a stable draught or chimney ‘pull’ irrespective of wind speed out of doors.
Solid fuel boilers
These are available for hand-firing or automatic firing in a wide range of forms. The smaller inexpensive types used for smallerare not noted for their high efficiency, being of very simple design. More expensive boilers are of course available which have a hopper feed and shaker grate and are remarkably efficient and reduce stoking and maintenance labour to the minimum. All solid fuel boilers are available in a very wide range of sizes and forms.
Natural draught oil burners installed in boilers
There are different ways of achieving ‘automatic’ heating in small heating units to avoid the constant necessity of hand-firing and cleaning. Natural draught burners are frequently installed in boilers intended for solid fuel burning with the fire grates removed, but the boiler can of course be bought complete with the burner. Natural draught burners are exceedingly simple in design and take several different forms, all of which rely on the natural intake of air for combustion. The burner must be installed completely level and the level of the oil control valve must be such that it cannot be over-filled. The simplest types must be manually lit and the level of heating controlled by a hand operated valve. For high efficiencies the flames should be baffled on to the sides of the boiler and a draught diverter must be fitted to reduce draught, as any excessive chimney ‘pull’ will extract the hot gases before they have effected heat transfer to the boiler surface. Natural draught burners are available in a range of sizes to suit the smaller boiler unit for heating small greenhouses, ie 3-9kW (10, 000 — 30,000Btu).
Pressure jet and wall flame oil-fired burners
A wide range of these exists and generally speaking the design of boiler is more sophisticated, as a pressure jet burner injects oil vapour and air under pressure which, on combustion, results in a surge of hot gases. Wall flame burners operate on a rotating system and are very efficient. Unless these hot gases can be directed through an efficient heat exchanging system they are expelled before they have given off a high proportion of their heat.
All oil-fired burners must be correctly installed with no air leaks and they must be set up so that the correct quantity of air is used for efficient combustion. Simultaneously there should be correct adjustment of the flue draught — both matters for heating engineers or fuel consultants with the correct instruments. Pressure jet burners are of course fully automatic, which is a great boon to the busy gardener.
Waste oil burners
There are several types of burners for using waste car engine oil, and one made in Scotland of 3,682 watts burns for 24 hours on 13.6 litres and has a rating of 3,682 watts heating 21m (69ft) of 10cm (4in) pipes. These burners do not require any electricity which is a point in their favour. The only real draw-back is handling the oil, which can be messy, and cleaning the burner which can also be dirty operation, but is essential since this type of waste fuel has a high waste-matter content and builds up carbon fairly quickly.
Basically gas boilers are similar in design to oil-fired boilers. Indeed the same design formula is frequently used for both, with the obvious difference that the gas requires a different burner. The availability of natural gas in many areas has led to an increased interest in gas firing for greenhouse heating, especially with the rise in oil prices over the years. Bulk gas is now readily available. Automatic control is more or less standard on all gas-fired boilers.
Solar panels are now available for domestic use in homes, installed on the sunny slope of the roof. They consist of a number of gilled pipes backed by reflectors. Their principal use is to provide domestic hot water. Several prototype solar units have been produced for greenhouses, but so far nothing impressive has appeared at an acceptable price for use in temperate zones. Basically speaking all solar heating units aim to heat water from direct solar radiation, the water then being stored in insulated tanks for recirculation through a pipe system when solar heat input is low or non-existent and the air temperature in the greenhouse drops sufficiently to demand the input of heat.
Large diameter, black, water-filled polythene pipes and PVC mattresses have been used a great deal in France for absorbing the sun’s heat during the day to be radiated back into the greenhouse or plastic structure in the evening and night. In temperate zones, including the UK, the weakness so far of all solar heating units lies in the low level of solar radiation from autumn to spring when there is likely to be most demand on a heating system for the greenhouse. Research into ways of transmitting solar radiation more effectively by microwaves from afar is still in the very early stages of development. At the moment such solar heating units which do exist are likely to be far too costly for small-type greenhouses and the more successful uses of solar radiation have combined the heating of living quarters and greenhouse-growing quarters, so justifying the high cost.
Heat from a wide range of domestic and industrial processes, which is normally allowed to ‘escape’ into the atmosphere, has been effectively collected and distributed and has a vital role in greenhouse heating. With the ever increasing cost of fuels, many temperate climate zones have started to look very closely at methods of utilizing waste energy. This involves a number of different approaches, including the use of flue gases and the installation of heat exchangers to ‘catch’ the heat, waste warm water from industrial processes and other techniques, many of which are of little interest to the average amateur gardener but which could in future years have a profound application on all forms of glasshouse growing. Several large commercial units have already been set up using waste industrial energy and more will undoubtedly follow.
Wind and water power
There is also considerable interest in using electrical power generated by wind or water. While at present the interest largely centres around larger installations suitable for the commercial grower, here again there will undoubtedly be application for smaller greenhouses before many years have passed.
Heat pumps are also being used for greenhouse heating, preferably and ideally when there is a fairly warm source of base heat available eg subterranean water, waste industrial heat etc.
Low temperature corrosion of boilers
The problem of low temperature corrosion of boilers caused by the sulphur and possible chlorine content of fuels in conjunction with the temperature of the boiler heating surface is a major one. In essence what happens is that acids are formed which corrode the boiler surface, this occurring at operating temperatures below about 65°C (150°F), and there will, in greenhouse heating, be many occasions when pipes are required no hotter than this. A simple way of overcoming this, and at the same time enabling the pipe to be regulated to give a range of temperatures, is to install a “mixing valve”. This allows a close loop of heat to circulate through the boiler in a ratio dependent on the heat demand as called for by thermostat or by manual adjustment.
Note that a pump is required for circulation before close loop systems will operate. Mixing valves are frequently installed in domestic systems not only to keep the boiler at a high operating temperature, but to ensure a constantly warm domestic hot water supply and supply heat as required for home heating.