ASHRAE Journal - July 2009 - 28

3 Point of Operation–Constant Flow With Balance Valves 90 80 Pump and System Head (ft) 70 60 50 40 30 20 10 0 0 100 Path of Operation for Variable Flow System Without Balance Valves Minimum Flow Through Chiller Pump Flow–Head Curve Design Point: 400 gpm at 70 ft 4 120 110 100 90 Pump and System Head (ft) 80 70 60 50 40 30 20 10 Pump Flow–Head Curve Design Point: 450 gpm at 90 ft Point of Operation–Constant Flow With Balance Valves Path of Operation for Variable Flow System Without Balance Valves Minimum Flow Through Chiller 100 200 300 Pump and System Flow (gpm) 400 500 200 300 Pump and System Flow (gpm) 400 500 0 0 Figure 3 (left): System operation with 11% design safety factor. Figure 4 (right): System operation with 25% design safety factor. with an 11% DSF and 360 gpm (23 L/s) at 100 ft (299 kPa) head for the system with a 25% DSF. The annual energy consumption for the constant flow system with an 11% DSF is 40,219 kWh and 57,211 kWh for the system with a 25% DSF. These values were determined using high efficiency motors with 0.8 kW (1.07 kW/kW) per brake horsepower or 92% efficiency. The calculations for the energy consumption of the variable flow system are much more complex since they must be made at 10 points on the system head curve along with the extra energy that is consumed by minimum flow through the chiller. Tables 1 and 2 describe these calculations with data secured from the pump flow-head curves of Figures 3 and 4. The wireto-shaft efficiencies and percent of load in Columns E and G are from the authors’ experience with variable primary pumping systems. The pump energy for maintaining minimum flow in the chillers for the values up to 108 gpm (7 L/s) must be added to the total kW in Tables 1 and 2. This energy consumption is not great since the flow and head are at minimums. The calculation for this energy reveals that the energy to be added is 264 kWh for the 11% DSF system and 325 kWh for the 25% DSF system. The energy totals are 12,142 kWh for the variable flow system with the 11% DSF and 13,528 kWh for the system with a 25% DSF. If you subtract these totals from those for the constant flow systems, you get 27,813 kWh saving or 68% for the 11% DSF system and 43,358 kWh or 76% for the 25% DSF system. A summary of actual annual measurement and verification on a large 24/7 hotel facility in California is available in Reference 2. Interestingly, the net pump energy savings by eliminating balance valves and incorporating variable speed pumps was 80%. The earlier figures are for pumping energy alone and do not include the thermal equivalent of the pumping. An estimate for this energy is achieved by multiplying the energy lost in kWh 28 ASHRAE Journal by 3,413 Btu (3600 kJ), the heat content of a kWh and dividing by 12,000 Btu (12 660 kJ), the heat content of a ton of cooling. If the overall production of a ton of cooling requires 0.9 kW/ton (0.256 kW/kW) in the chiller plant, the added savings for the 11% DSF system would be 7,119 kWh for a total of 37,408 kWh and 11,099 kWh for a total of 54,457 kWh for the 25% DSF system. These totals are almost unbelievable, since a combination of energy saved in pumping and in the chiller plant is almost equal to the total energy of the pumping for the constant flow system. Since these savings are for a 180 ton (633 kW) system operating 5,880 hours per year, if you divide the savings by these two figures, you achieve a kW/ton figure of 0.035 and 0.052 (0.01 kW/kW and 0.015 kW/kW), respectively. This provides us with a simple number that can be used to estimate quickly whether a system should be evaluated for replacing three-way valves and constant speed pumps with a variable volume system. If we round off these numbers to 0.04, we can proceed to make a quick estimate for a larger system. For example, a 5,000 ton (17 585 kW) system operating 7,000 hours/year, the rough savings is 1.4 million kWh per year. If the electrical rate is $0.12/kWh, this amounts to $168,000 per year. This is a hypothetical case, but the procedures used here can be used to evaluate any existing chilled-water system. Assume that 20,000 chiller plants were in the U.S. at the end of 2008 and that these plants had a capacity of 16 million tons (5 627 200 kW). If we also assume that half of these plants are still constant flow with three-way coil valves and balance valves operating around 5,000 hours per year, with an overall savings factor of 0.04 kWh per ton/hour, the savings in electric generating plant capacity would be 320 MW and 1.6 million MWh in energy consumption. These are just gross estimates. However, are they not huge enough to encourage us to (1) stop designing variable flow systems with balance valves and constant speed ashrae.org July 2009

ASHRAE Journal - July 2009

Table of Contents for the Digital Edition of ASHRAE Journal - July 2009