Ashrae Journal - October 2008 - (Page 26) 120% Ride Pump Curve Variable Speed Fixed 1/3 Head Setpoint Variable Speed Reset Setpoint Normal Operation, h/yr 1,500 2,000 3,000 4,000 6,000 Water Velocity, fps 15 14 13 12 10 100% Percent Pump kW 80% Typical Average Flow 60% Table 1: Maximum velocity to limit erosion.5 40% 10 20% Water Velocity (fps) 1 1 Pipe Size (in.) 10 0% 0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Percent gpm Figure 2: Typical pump power versus flow. Flow control system types included are: • Constant flow and speed; • Variable flow, constant speed (ride pump curve); Figure 3: Maximum velocity for piping adjacent to noise sensitive spaces.7 • Variable flow, variable speed with fixed differential setpoint (assumed to be ~one-third of design head); and • Variable flow, variable speed with differential setpoint reset by valve position. Annual hours of operation are input by the user. Part-load performance assumed for the variable flow systems is shown in Figure 2. The spreadsheet assumes the annual average flow rate is 55%, which was developed from generic DOE-2 models of several California office buildings. The resulting average percent power value for each system type can be overridden by the user. The addition of heat to the central plant load due to pump energy is taken into account since it significantly affects overall energy and life-cycle costs. For instance, chilled water pump energy is added to the chiller load. Hot water pump energy, on the other hand, reduces boiler load. Chiller annual average efficiency is assumed to be 0.6 kW/ton (0.17 kW) while boiler annual average efficiency is assumed to be 75%; both values are user adjustable. Velocity Limits www.info.hotims.com/16018-46 Erosion velocity limits are based on the rules-of-thumb shown in Table 1.5 These limits can be enabled and disabled for each piping section and manually changed by the user. Noise from piping results from turbulence, cavitation, entrained air, and water hammer. Of these, only turbulence associated with velocity can be addressed in the spreadsheet, but there are few studies showing a consistent relationship.6 The ashrae.org October 2008 26 ASHRAE Journal http://www.rawal.com http://www.rawal.com http://www.info.hotims.com/16018-46
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