Pumping Calculations - Basic Mathematics for Water and Wastewater Operators - page 203

From Equation 14.15, the specific equations to be used for motor, pump, and over-
all efficiency are

bhp)
Motor horsepower (mhp)

Brake horsepower (

Percentmotor efficiency = ×100 (14.16)

Water horsepower (w hp)

Brake horsepower (bhp)

Percent pumpefficiency = ×100 (14.17)

(whp)
Motor horsepower (mhp)

Water horsepower

Percent overall efficiency = ×100 (14.18)

■ Example 14.10
Problem: A pump has a water horsepower requirement of 8.5 whp. If the motor sup-
plies the pump with 12 hp, what is the efficiency of the pump?

Solution:

Water horsepower
Bra

8.5 whp
12 bhp

Percent pumpefficiency = ×= ×= 100 100 71%

ke horsepower

■ Example 14.11
Problem: What is the efficiency if an electric power equivalent to 25 hp is supplied
to the motor and 14 hp of work is accomplished by the pump?

Solution:

Water horsepower 14 whp

25 mhp

Percent overall efficiency = ×= ×= 100 100 56%

Motor horsepower

■ Example 14.12
Problem: Suppose 12 kW of power are supplied to a motor. If the brake horsepower
is 14 hp, what is the efficiency of the motor?

Solution : First convert the kilowatts power to horsepower. Based on the fact that 1 hp
= 0.746 kW, the equation becomes

12 kW ÷ 0.746 kW/hp = 16.09 hp

Now calculate the percent efficiency of the motor:

Brake horsepower
Mo

14 bhp
16.09 mhp

Percentmotor efficiency = ×= = 100 87%

tor horsepower