NFT systems for commercial greenhouse vegetable crops consist of a number of components:
- gullies in which the plants grow,
- a collecting system which collects flowing nutrients from the gullies,
- a sump, or reservoir for the recirculating nutrient solution
- pumps and associated headworks piping
- fresh water supply
- nutrient stock solution tanks
- nutrient solution distribution system to the gullies.
All components and materials used in NFT systems must resistant to corrosion
by the nutrient solutions and must not contaminate the nutrient solutions.
Minimum requirements for these system components are:
1. Gullies. Gullies can be folded black & white polythene film or rigid plastic gullies. Film plastic gullies are
usual for the crops with large plants (tomatoes, peppers,cucumbers, eggplants)
and rigid gullies for small plants such as lettuce and herbs. In either case the gullies must be laid with
an accurate and consistent fall of between 1 in 40 and 1 in 100 between the inlet and outlet end of the gully.
Any fall at right angles to the slope of the gullies should be less than in 1 in 250.
The root mat in a gully folded from 600 mm wide panda film. The tomato plants were propagated in peat pots. The gully is normally closed by stapling the two edges of the panda film together.
2. Collecting systems consist of collecting pipes running across the ends of the gullies and return mains running back to the sump. The pipes are usually unplasticised PVC piping with glued or rubber ring socketed joints. Gully tails can simply discharge into the collecting pipe through a 25 mm wide and 250 mm long slot cut in the top of the pipe, or better be fitted to a tee in the pipe making a water proof joint between the gully and the pipe. Collecting pipes with slots must have sufficient bore that the depth of flowing solution is less than half the pipe diameter, but collecting with gullies fitted to tees can run at full bore. The collecting pipes and return mains must be laid with a fall of at least 1 in 200. The end of the return pipe should discharge into the sump through a fine strainer. In smaller systems this strainer is often simply a nylon stocking.
3. The sump. Sumps are usually installed below ground level and may be stainless steel, polythene or concrete tanks. Concrete tanks are best painted or coated internally to prevent erosion by the nutrient solution using either bituminous paints or epoxy coating suitable for use with drinking water. The sump should be large enough to hold the total volume of solution in the system when intermittent flow is being used and between 30% and 50% of the maximum volume in the system when the crop is fully grown. The following guidelines are suggested for tomatoes and similar crops:
The plywood deck is over the sump. Two pumps deliver nutrient to the white manifold pipe above the desktop.Flow mains to the header pipes run from each end of the manifold.A & B stock tanks and acid tank are on the left.
||Greenhouse minimum sump
||recommended sump |
||operating volume m3
4. Pumps and headworks piping. Pumps must not be corroded by nutrient solution nor must the pumps contaminate the nutrient solutions with metals or other toxins. Moulded plastic centrifugal pumps meet this requirement and are most commonly used, but all stainless steel pumps would also be suitable. Pumps are ideally, but not essentially, self priming and are mounted above or close to the sump. Suction pipes with all plastic or stainless steel foot valve and strainer assemblies are desirable in both cases. Many submersible pumps are unsuitable for this duty. Installation of two pumps is strongly recommended as insurance against pump failures. Pumps should be selected with more capacity than is needed. Flow rates of at least 2 litres per minute per gully for tomatoes and similar crops are required, and higher flow rates are sometimes used (up to 6 l/minute in the UK).Pump capacity in smaller systems needs to be up to 50% greater than the total flow through the gullies, and in large systems 10-20% greater. Both pumps should discharge into a common header pipe. The header pipe must have the following outlets, each controlled by a valve: outlet to the flow mains to the header pipes and gullies, outlet to waste (for dumping), outlet returning solution to the sump through a sparge pipe, a small bore outlet (4-6 mm) supplying solution to a sampling cup, with overflow back into the sump, outlet to a heat exchanger for solution heating (if required).
The polythene header pipe in this photo is covered by the panda film floor. Each gully is supplied with nutrients by two 4mm bore leader tubes inserted into the header pipe through rubber grommets
5. Fresh water supply The volume of solution in the NFT system is kept constant by adding fresh water to the sump. The fresh water supply should pass through a water meter, and be controlled by a level control in the sump. The water level is easily controlled by using a Jobe full flow trough valve. The Jobe valve is all plastic and does not contaminate the solution.A ball cock is not suitable for controlling the water supply because of metal contamination and corrosion and because ball valves do not open to full flow with slightly low solution levels. Water meters do not measure properly at low flows. The Jobe valve is a full flow valve (either open or closed) and so the water meter registers properly in this system.
6. Nutrient solution stock tanks Stock tanks tanks range in size from 100 litres through to 1,000 litres depending on system size Stock tanks are usually polythene or stainless-steel. The stock tanks must either be fitted with calibrated sight glasses, or calibrated dipsticks must be used. Stock tanks can discharge directly into the sump through solenoid valves for automatic control, but it is better to use a small day tank holding only enough stock solution for one days use between the main tank and solenoid as an insurance against control failure.
7. Nutrient solution distribution system to the gullies. Pipes used for flow mains and headers must be opaque to all light, or algal growth inside the pipes is likely, and detached algal clumps may block the leader tubes. Many PVC pipes are not sufficiently opaque to prevent algal growth and for this reason it is best to use black polythene pipe for flow mains and headers. Nutrient solution is discharged from the header pipes into the gullies through 3 or 4 mm bore leader tubes. Leader tubes are connected to the header pipes by inserting one end through a rubber grommet in the wall of the header pipe.
8.Nutrient solution heating and cooling . Nutrient solutions in heated greenhouse stay close to greenhouse air temperature and solution heating is not necessary. In milder climates where greenhouse heating is often not available, solution heating can often extend the growing season through the colder months. A stainless steel plate heat exchanger supplied with hot water is the most effective way of heating NFT solutions. A small diesel or gas fired boiler is used to provide the hot water. Boilers cannot be used to heat the solution directly as nutrient solutions would quickly corrode most boilers with toxic contamination of the nutrient solution as a result. The plate heat exchanger prevents contamination and corrosion and provides a huge surface area for raid heat exchange between the boiler water and solution. The boiler usually operates under its own water temperature thermostat typically set at 65C to avoid condensation on the fire side of the boiler. A thermostat measuring NFT solution temperature controls the operation of a small pump circulating the boiler water through the plate heat exchanger whenever the NFT solution is too cool. NFT solution from the NFT pump header manifold circulates continuously through the plate heat exchanger and back into the NFT sump. Typical solution temperature set points are 16-18C for lettuce, 18-21C for tomatoes and over 20C for cucumbers. Other systems for solution heating, including long coils of pipe inside NFT sumps and electric immersion heaters in sump have been tried but the plate heat exchangers are the most efficient and effective system.
This small boiler supplies hot water through steel pipe to the plate heat at the top left. NFT solution flows continuously through the black polythene pipes through the heat exchanger and back into the sump at the lower left.
High NFT solution temperatures can sometimes be a problem. Lettuce do not grow well with solution temperatures over 26C. Summer planted vine crops (tomatoes, cucumbers and peppers) can suffer from burnt roots when when small plants are placed in polythene film gullies. Solar heating of the solution flowing through the gullies can heat the solution to temperatures approaching 40C. THis is a temporary problem which disappears when thecrop has sufficient leaves to shade the gullies, but the crop may be more susceptible to root disease. The problem could be reduced by shading the gullies with a reflective insulated cover such aluminised bubble packing film.
Cooling NFT solutions by passing the solution through a cooling tower is very effective, especially in arid climates. Refrigeration has been used for nutrient solution cooling, experimentally in Europe and commercially in Singapore, but is expensive for both capital and runnng costs.
Copyright ; R.A.J.White 26 June 2007