Pumps, Fans and Turbines - Horsepower

Definition of British Horse Power for pumps, fans and turbines - how to convert to other common units

Horsepower

Horsepower is the imperial (British) unit of power. A horsepower is the ability to do work at the rate of

  • 33,000 ft.lb per min or
  • 550 ft.lb per second

Note that Power is "Work per unit time" and work is "Force through distance". In gravity systems Force is Weight - mass multiplied with gravity.

The total horsepower developed by water falling from a given height is the product of the mass flow rate in pounds per minute times the falling height in feet divided by 33,000. It can be expressed as:

Php = mmin h g / 33000         (1)

where

Php = power (horsepower, hp)

mmin = mass flow rate per minute (lbm/min)

h = head or height (ft)

g = acceleration of gravity (32 ft/s2)

(1) can alternatively be expressed as:

Php = msec h g / 550         (1b)

where

msec = mass flow rate per second (lbm/s)

(1) can also be expressed as:

Php = γ Q h / 33000         (1c)

where

Q = volume flow rate (ft3/min)

γ = specific weight (lbf/ft3) (weight is force)

Water Horsepower for Flow in gal/min

Water horsepower for flow in gal/min can be expressed as:

Pwhp = SG Qgal h / 3960         (1d)

where

Q = volume flow rate (gpm)

SG = specific gravity

h = head (ft)

SG for water is 1.001 at 32oF and 0.948 at 240oF.

SG - Specific Gravity

Qgal - Volume Flow (imp gpm)

h - head (ft)

Power to Pump Water

Required power (hp) to pump 1 cubic foot of water per minute (ft3/min) - efficiency 85% - is indicated in the diagram below:

horsepower due head water

  • 1 hp (English horse power) = 745.7 W
  • 1 ft (foot) = 0.3048 m

Example - Required Power to lift 10 ft3/min of Water 600 ft

According the diagram above 1 hp is required to lift 1 ft3/min of water 600 ft. Required power to pump 10 ft3/min can be calculated as

(10 ft3/min) (1 hp) / (1 ft3/min) = 10 hp

Shaft or Brake Horsepower

The brake horsepower is the amount of real horsepower going to the pump, not the horsepower used by the motor. In the metric system kilowatts (kW) is used.

Due to hydraulic, mechanical and volumetric losses in a pump or turbine the actual horsepower available for work on or from the fluid is less than the total horsepower supplied.

Shaft or Brake Horsepower for a Pump or Fan

The brake horse power - bhp - for a pump or fan can be expressed as:

Pbhp = ( γ Q h / 33000 ) / η         (2)

where

Pbhp = brake horse power (horsepower, hp)

Q = volume flow rate (ft3/min, cfm)

η = overall efficiency

Shaft or Brake Horsepower for a Turbine

The brake horse power - bhp - for a turbine can be expressed as:

Pbhp = η ( γ Q h / 33000 )         (2b)

Input Horsepower to the Electrical Motor

The input horsepower to the electrical motor for a pump or fan can be expressed as:

Php_el = Pbhp / ηe         (3)

or

Php_el = (m h g / 33000) / (η ηe)         (3b)

where

Php_el = input power to the electrical motor

ηe = mechanical efficiency of the electrical motor

Horsepower in Kilowatts and other Units

Horsepower can be converted to other common units as:

  • 1 hp (English horse power) = 745.7 W = 0.746 kW = 550 ft.lb/s = 2545 Btu/h = 33000 ft.lb/m = 1.0139 metric horse power

Transforming horsepower to kiloWatt can be expressed as:

PkW = 0.746 Php         (4)

where

PkW = power (kW)

Together with the equations above it's possible to express (4) in many common combinations as:

PkW = 0.746 (m h / 33000) / η ηe         (5)

Example - Pump Power

The horsepower required to pump 50 lbm/min water a head of 10 ft with a pump with overall efficiency 0.7 - can be calculated with eq. (3) as

Php = ((50 lbm/min)(10 ft) (32 ft/s2) / 33000) / 0.8

       = 0.6 hp

The power in kW can be calculated as 

PkW = 0.746 (0.6 hp)

       = 0.45 kW

Related Topics

  • Basics - Basic Information as SI-system, Unit converters, Physical constants
  • Pumps - Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more

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