Chapter 20

Fertigation in Greenhouse Production

Positive Displacement Injectors

Positive Displacement Fertigation Injectors

Positive displacement pumps are piston or diaphragm pumps, which can be configured to handle a wider range of flows and are more easily controlled by a computer than other devices. Positive displacement pumps can be powered electrically, pneumatically, or hydraulically (water). The water-driven pumps can be installed in locations that lack power. Positive displacement injectors provide consistent injector ratios for their designed flow rates over wide variations in water pressure. Simple in design, the water enters one side of the unit, forcing up a piston. This action draws in the stock solution from a stock tank under the unit and into the chamber of water. The force of the water then pushes the piston back down, expelling the mixture of water and fertilizer out the other side always in proportion to water flow.

Diaphragm Pumps

Diaphragm pumps are typically powered by electric motors, but may also be belt-driven or powered by small gasoline engines. They have traditionally had the distinct advantage over piston pumps in that the flow rate is easy to manually adjust while the pump is operating. In addition, electric units can also be equipped with a variable frequency drive (VFD) to control the speed of the pumping.

Piston Pumps

Piston pumps have the same characteristics as diaphragm pumps, in terms of power supply and maintenance. An advantage of piston pumps is that as positive displacement pumps, the discharge flow rate will not change as the irrigation pipeline pressure varies. Their big disadvantage is that flow rates cannot be adjusted while the pump is operating, unless a variable frequency drive (VFD) is used.

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