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