Problem 2 (50 points)

Consider the secondary loop of a chilled water pumping system, such as the one in the diagram below.

Suppose one of the secondary pumps is active and the other is an identical backup. The performance

data for each pump is provided on the next page. The provided operating conditions are the design

conditions for this system.

a) What is the approximate pump efficiency degradation (expressed in percentage points) due to

trimming the impeller from 14" to 11.6"? Assume the 14" pump impeller serves the same

system curve.

b) If the 14" impeller were kept in place and a throttle valve were used at the pump discharge to

regulate flow to the design condition flow, what would be the brake horsepower requirement of

the active pump? What percentage increase in shaft power (relative to the 11.6" impeller pump)

would this represent?

c) For the scenario in part (b), how

much head is lost across the

throttle valve? Provide a

calculation and explanation.

d) If the system has zero control

static head, and pump speed for

the 11.6" impeller is adjusted

from 100% to 70%, what would

be the new pump flow, head, and

brake horsepower? What

percentage pump power

reduction does this represent?

e) Suppose the system controls

pump speed to maintain a

differential pressure (DP) setpoint

and the DP sensor is located

between the supply and return

lines at one of the air handlers.

Also suppose that an operator

adjusts pump speed to produce

70% of the design flow. How

would this scenario change your

answers to part (d)? Provide

qualitative descriptions (e.g., up,

down, or stay the same) and

explanations for each aspect.

TWO

POSITION

ISOLATION

VALVE

5 5

CHILLER #1

CHILLER #2

VARIABLE

SPEED DRIVE

(OPTIONAL)

COMMON LEG

SUPPLY AIR

TEMPERATURE

HIH

PRIMARY

LOOP

VSD

COIL

VSD

SECONDARY

LOOP

Page 2 of 3 KP - Horizontal Split Case Pump

Head - ft

NPSHr - ft

250

225

200

175

150

125

100

75

50

25

0

30

15

14.00 in

11.60 in

10.00 in

50

100

Operating Conditions

Flow, rated

Differential head / pressure, rated

(requested)

Differential head / pressure, rated

(actual)

Efficiency

Speed, rated

NPSH required

Stages

Impeller diameter, rated

150

600.0 USgpm

120.0 ft

119.9 ft

69.85 %

1780 rpm

13.88 ft

1

11.60 in

200

250

50

300

58

350

400

Liquid

Liquid type

Temperature, max

Fluid density, rated / max

Viscosity, rated

64

PACO KP is a single-stage, between bearings, split case pump. The axially split design allows easy

removal of the top casing and access to the pump components without disturbing the motor or pipe

work. (PC29)

Benefits

• Double suction minimizes axial load, which extends the life of the wear rings, shaft seals and

bearings

• Double Volute Design for increased efficiency, lower life cycle costs, & prolonged seal and bearing

life

• Independent bearing housing design allows access to the pump components without removing

the top half of the casing

• Suction baffles reduce losses and improve NPSH-R by directing flow into the eye of the impeller

• High energy efficiency and low life cycle costs

68

450 500 550

Flow-USgpm

Cold Water

68.00 deg F

1.000 SG

1.00 CP

A

600

70

650

71

NPSHr

700

71

System Curve #1

750 800

Driver & Power Data

70

850

Motor sizing specification

Site Supply Frequency

Nameplate motor rating

Rated power (based on duty point)

Max power (non-overloading)

Frame Size

900

MCSF

60 Hz

950

Max power (non-

overloading)

30.00 hp/22.37 kW

26.02 hp

27.44 hp

286T

Page 3 of 3

1,000