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Chapter 9

Q. 9.1

Consider the water distribution system with only one primary pump as depicted in Figure Ex 9.1. Determine the hp size of the primary pump and check the settings of the valves for various secondary loops to balance the system. Determine the best primary/secondary pumping arrangement for retrofitting the water distribution system. Estimate the total energy cost savings from the retrofit assuming the system is operated 5,000 hours per year and that the cost of energy is $0.08/kWh. Assume the pump efficiency to be 70 percent and the motor efficiency to be 90 percent.


Verified Solution

For the water distribution system presented by the network above, the heads for the three circuits are first estimated as shown below:

Head (circuit A) = 10 ft + 40 ft + 10 ft = 60 ft
Head (circuit B) = 10 ft + 15 ft + 100 ft + 18 ft + 10 ft = 153 ft
Head (circuit C) = 10 ft + 15 ft + 10 ft + 45 ft + 20 ft + 18 ft + 10 ft = 128 ft

Thus the total head for the system is 153 ft corresponding to the longest run for circuit B. The horsepower for the primary pump is obtained using Eq. (9.15):

Pump horsepower = (1825 gpm)(153 ft)/(.70 * 3,960) = 101 hp

We can check that the valve for circuit B should be open and the valves for circuits A and C should be set to 93 ft and 25 ft, respectively.

To minimize energy use for the water distribution system, a primary/secondary pumping system can be considered. The best primary/secondary pumping arrangement is shown in the Figure Ex 9.2 with secondary pumps in circuits B and C. The longest run for the primary pump becomes 83 ft (corresponding to loop C without the secondary loop of 45 ft). The horsepower sizes of the two secondary pumps and the new primary pump are:

Secondary pump B horsepower = (450 gpm)(100 ft)/(.70 * 3,960) = 16 hp
Secondary pump C horsepower = (575 gpm)(45 ft)/(.70 * 3,960) = 9 hp

Primary pump horsepower = (1,825 gpm)(83 ft)/(.70 * 3,960) = 55 hp

Thus the total hp requirement for the new primary/secondary distribution system is:

Total hp = 55 + 16 + 9 = 80 – hp

Thus, a total savings of 21 hp can be achieved by retrofitting the water system to a primary/secondary pumping arrangement as shown above. Assuming an operation of 5,000 hours/year, a motor efficiency of 0.90, and a cost of $0.0.08/kWh, the energy cost savings achieved by the retrofit of the water distribution system is estimated to be:

Cost Savings = ($0.08/kwh) * (5,000-hrs) * (21– hp * 0.746 – kW/hp/0.90) = $6,250/year