Chapter 13

Wells and Pumps for Greenhouses

Power for Pumping

Most irrigation pumps are powered by either electric motors or internal combustion engines. The source of power that is best suited for a specific installation depends on certain physical, environmental, and cost factors.

Electric Motors

Many irrigation-pumping plants use electric motors. They have a long expected life, require minimal maintenance, and are very reliable. An electric motor, properly selected and protected, can be expected to supply many years of trouble free power if properly designed and operated, including correct mounting, rodent protection, good ventilation, adequate shelter from the elements, and safety devices against overloading, under-voltage, and excessive heating. Many irrigators choose an electric motor because it is runs at the push of a button.

Advantages of the electric power are relatively long life of the motor, low maintenance costs, dependability, and ease of operation. An electric motor also will deliver full power throughout its life and can be operated from no load to full load without damage. A major disadvantage of the electric motor is the possible lack of availability of power in the area where the motor is to be located. Smaller electric motors, up to approximately 10 horsepower can operate on 120 to 240 volts, single phase, and 60-Hertz, power, but the larger motors, will probably require 230 to 460 volts, 3-phase, 60-Hertz power.

Time-of-Use Electric Rates

Time-of-use (TOU) electric rates are designed to encourage pumping during off-peak electric periods and to discourage pumping during periods of peak use. The incentives offered are low electric rates for off-peak pumping and penalties in the form of very high electric rates and high demand charges for on-peak pumping.  Demand charge (kW/hr) is the rate at which the greenhouse operation consumes electricity—or the amount needed to power the operation at any given point in time. 

Internal Combustion Engines

There are a number of factors in selecting an internal combustion engine for a pumping application, including portable or stationary, air cooled or water cooled, fuel type, speed, size, efficiency, emissions, maintenance costs, expected life, and initial cost (See Figure 13.9).

The best operating load for an internal combustion engine is at or near the continuous bhp curve. Running an engine under lighter loads usually results in poor fuel economy for the water pumped, because a proportionally larger amount of horsepower is used in overcoming engine friction and throttling losses. Running at wide-open throttle invites engine trouble as well as excessive fuel consumption. The main object in irrigation pumping is to pump as much water as possible for the fuel used. Therefore, operating the engine near its highest possible load on an economy fuel mixture is the best way to accomplish this objective.

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