Wells and Pumps for Greenhouses
Pumps Used for Greenhouse Irrigation
Different types of applications require different types of pumps. Pumps are selected based on system requirements, discharge pressure required, flow capacity required, and availability of space. The two most common pumps used in greenhouse operations are: centrifugal pumps, deep-well vertical turbine pumps, submersible pumps, used most often in irrigation, and positive-displacement pumps, most commonly used in for chemical dosing (i.e., fertigation).
Centrifugal pumps, usually called booster pumps, have a circular “fan/turbine-shaped” structure called an impeller that is mounted on a centrally supporting structure called the shaft. Water enters at an opening in the center called the suction (See Figure 13.3). The rotating impeller imparts a high velocity to the water, and it is circulated and thrown outward by the rotation of the curved impeller blades. A circular-shaped covering called casing surrounds the impeller. The casing is shaped like a spiral so that the water slows down, and the velocity head is converted to pressure head as it flows out of the casing. The centrifugal pump is limited to pumping from reservoirs, lakes, streams, and shallow wells where the total suction lift is not more than approximately 20 feet.
Deep-well Vertical Turbine Pumps
Deep-well vertical turbine pumps are centrifugal pumps adapted for use in cased wells or where the water surface is below the practical limits of a centrifugal pump (See Figure 13.4). Successful installations have been made where the water surface was 500 feet below the ground. These pumps are generally used for high capacity, high head installations where the horsepower requirements exceed the capability of submersible motors. Turbine pumps are classified by the type of flow produced by the impeller. The centrifugal type pump discharges water at right angles to the axis of rotation. In the axial-flow type, water is given an upward thrust by the impeller similar to a boat propeller. Another type commonly used is a combination of axial-flow and centrifugal and is known as a mixed-flow turbine.
The submersible pump is simply a turbine pump close-coupled to a submersible electric motor attached to the lower side of the turbine. Both pump and motor are suspended in the water, thereby eliminating the long-line shaft and bearing retainers that are normally required for a conventional deep-well turbine pump. In this way the pump is always filled with water (primed) and ready to pump. The submersible pump consists of a pump and motor assembly, a head assembly, discharge column, and a submarine cable to furnish power to the motor, as shown in figure 13.5. Virtually all submersibles are “multi-stage” pumps. All of the impellers of the multi-stage submersible pump are mounted on a single shaft, and all rotate at the same speed.
Axial-Flow (Propeller) Pumps
Pumps using axial-flow types of impellers are designed for conditions where the capacity is high and the head requirements are low. Most axial-flow pumps operate on installations where suction lift is not required. They are installed in such a way that the impeller is submerged in the water. Generally, these pumps are mounted vertically or on an incline from vertical position, since it is necessary to submerge the impeller of an axial-flow pump. In some applications where high volumes of water are required and ample submergence above the pump is available, it is possible to mount an axial-flow pump in a horizontal position.
Positive-displacement pumps are used in water supply operations for feeding chemicals at various stages of the process. They displace a certain volume of water in each stroke as they operate. These pumps are not suitable for pumping large volumes of water they are more suited for high pressure and low flow service. There are two types of positive-displacement pumps: reciprocating pumps and rotary pumps.
Reciprocating pumps have a piston that moves back and forth in an enclosing cylinder (See Figure 13.6). The arrangement of rotating shaft, connecting rod, and joint pushes the volume of the fluid in the cylinder through an outlet valve.
Diaphragm Pumps. Diaphragm pumps are reciprocating positive displacement pumps that employ a flexible membrane instead of a piston or plunger to displace the pumped fluid. They are self-priming (can prime dry) and can run dry without damage.
In rotary pumps, the impeller rotates within an enclosing structure and imparts energy to the water (See Figure 13.7). The impeller can be in the shape of a gear, screw, or lobes. For rotary pumps, the output is continuous and smoother compared to reciprocating pumps.
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