Mechanical horsepower is a unit of work, and is defined as 550 ft./lb. per second, or more commonly in pumping applications as 33,000 ft./lb. per minute. The work performed in moving a liquid is a function of the weight of the liquid and the total head or differential pressure output within in a given amount of time. The following calculations are made to determine the horsepower required for a pump selection.
Because we know that one horsepower is equal to 33,000 ft./lb. per minute, the theoretical horsepower or hydraulic horsepower (WHp) is calculated as such:
This hydraulic horsepower calculation will yield a result based on theoretical, or perfect conditions where no energy is lost due to friction, leakage, etc. inside the pump. Therefore we must take into account losses in efficiency, and find the brake horsepower (BHp).
Brake horsepower calculations are simplified if the weight of the liquid is expressed in terms of gpm and the differential pressure is expressed in terms of head in feet (H).
While brake horsepower tells us the horsepower requirements of the pump at the pump shaft, it does not take into account efficiency losses in the motor. Similar to brake horsepower, the motor input horsepower (EHp) is a function of the hydraulic horsepower and the pump and motor efficiencies.
It is important to remember that any other components, such as VFDs, which reduce the efficiency of the electrical system should be incorporated to find the total motor input horsepower. Rather than performing each calculation independently, it may be easier to perform one calculation to find the total required EHp. This calculation looks like this:
To reduce horsepower requirements, it is important to make multiple selections to find the most efficient pump for your needs. Innovations in pump design have made it possible to greatly reduce horsepower requirements in a variety of applications. Take some time to play around with these formulas as well as your favorite selection tool to become more familiar with your current energy usage and to learn how to potentially improve your system.
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