Understanding Horsepower and Water Horsepower: Efficiency and Fuel Consumption Costs for Irrigation Pumps


1 Understanding Horsepower and Water Horsepower Efficiency and Fuel Consumption Costs for Irrigation Pumps Guide M-227 1 Blair Stringam Cooperative Extension Service • College of Agricultural, Consumer and Environmental Sciences ORIGIN OF THE HORSEPOWER UNIT Before steam power became widespread during the Industrial Revolution, horses were the main source of energy for applications like pulling carts, turning grinding mill wheels, and provid - ing movement to industrial machines. As the availability of steam engines increased, a means of providing understandable power ratings be - came important. Comparing the power output of a steam engine to a corresponding number of horses was an easy way for prospective engine owners to understand and compare power rat - ings. James Watt, an engineer, inventor, and entrepreneur of the late 1700s, determined that by recording the distance a horse traveled in a specific time while pulling a known weight Figure 1 . Diagram demonstrating the definition of horsepower. against gravitational force, a measurement could be made of the power the horse produced. After several observations, Watt concluded that a strong horse The following formula can be used to calculate water horsepower (WHP). could provide 550 foot-pounds per second of power, or one horsepower (Figure 1). WHP = HQ / 3960 WATER AND PUMP HORSEPOWER Where H is the change in pressure measured in height of water in feet and Q is the water flow rate in To fully understand water horsepower, it is important gallons per minute. to understand the terminology involved in deriving This equation was derived knowing that one horse such a unit. The term “energy” is defined as the capacity - power is equal to 550 ft-lb/s. to do work. “Power” is the rate at which energy flows For example, if an irrigation pump located at ground or at which energy is used per unit of time; it is also level is pumping 460 gallons of water per minute from a the rate at which work is performed. In other words, well whose water level is 112 feet below the ground level power is the amount of energy that is used to do work or how quickly work can be done. Water horsepower is and discharging that water at ground level, the water horsepower that the pump is delivering is: the minimum power that is required to move the water (Figure 2). In other words, it is the power that the pump WHP = (112 x 460) / 3960 = 13.0 - would require if the pump were 100% efficient. The wa ter horsepower can be determined if the flow rate of the water and the force (pressure) required to produce that So the pump is providing 13.0 WHP when it lifts flow are known. water 112 feet at a rate of 460 gallons per minute. 1 College Associate Professor, Department of Plant and Environmental Sciences, New Mexico State University. To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at aces.nmsu.edu

2 If the pump in the last example uses 17.0 HP to provide 13.0 WHP, the pump efficiency is: η = 13 / 17 = 0.76 or 76% The pump is 76% efficient, and 24% of the input energy is lost to friction and other losses. Most modern pumps have an efficiency of 50 to 85%. When choosing a pump, it is important to consider the relationship between efficiency and overall cost. More efficient pumps tend to be more expensive. How - ever, with better efficiency comes lower fuel costs to run the pump. Although more efficient pumps usually come with an increase in capital cost, the overall fuel con - sumption will be lower, resulting in lower annual fuel or electricity costs. - It should also be noted that the discussion in the pre vious paragraphs was for a pump properly sized for the - application. If the pump does not match the applica tion, it may have to operate in an inefficient range, and fuel or electricity will be wasted. Consult with a Profes - - sional Engineer or a pump supplier if you have ques tions about a specific pump or application. REFERENCES - James Watt: Craftsman and engi Dickinson, H.W. 1936. . Cambridge: Cambridge University Press. neer Hart, I. 1949. James Watt and the history of steam power . New York: Henry Schuman, Inc. Fox and McDonald’s introduction to Pritchard, P. 2011. Figure 2 . Diagram illustrating water horsepower by , 8th ed. New York: Wiley. fluid mechanics moving a volume of water a specified distance per period of time. Blair Stringam - is a College Associ PUMP EFFICIENCY ate Professor in the Department of No pump can convert all of its mechanical power into Plant and Environmental Sciences at water power. Mechanical power is lost in the pumping NMSU. He earned a Ph.D. in ag - process due to friction and other physical losses. It is ricultural and biological engineering because of these losses that the horsepower going into at Utah State University. His work the pump must be greater than the water horsepower focuses on irrigation, water manage - leaving the pump. The efficiency of any given pump (η ) ment, water measurement, and rain - is a ratio defined as the water horsepower out divided by water harvesting. the mechanical horsepower into the pump. < 1 η = water hp out / hp into pump 0 < η Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact [email protected] or the authors listed on the publication. New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating. Las Cruces, NM August 2013 Guide M-227 • Page 2

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