ASHRAE Journal - July 2009 - 27

1 2 Bypass Valve to Maintain Minimum Flow in Chiller Coil Coil Coil Chiller Coil Coil Chiller Balance Valves Three-Way Coil Control Valves Two-Way Coil Control Valves Figure 1 (left): Constant flow system with balance valves. Figure 2 (right): Variable flow system without balance valves. This selection of larger chillers results in design safety factors (DSF) of around 11% for the 200 ton (703 kW) chiller and 25% for the 225 ton (791 kW) chiller. Evaluation of many existing systems have indicated DSFs as high as 50%, resulting in flows and pump heads much higher than the actual operational flow and head. A study of a hotel in California2 showed that a 350 ton (1231 kW) chilled-water system was installed on a 250 ton (879 kW) load for a DSF of 40%. Added capacity is needed for reasons of redundancy and future growth. One of the great advantages of a properly designed, variable flow chilled-water system is that the system operates at the actual load on the system, regardless of DSF or allowances for future expansion. In this case, as indicated previously, the actual design flow for the variable flow system is 360 gpm (23 L/s) compared to 400 gpm (25 L/s) for the constant volume system with a DSF of 11% and a second system with a DSF of 25% and a flow of 450 gpm (28 L/s). The overall DSF is a summation of all other safety factors such as diversity, load, and friction. Constant Flow and Variable Flow Systems Operation 10 ft (30 kPa). This is the head required for the branch with the highest pressure drop. The path of operation is on the uniform system head curve for the variable flow system. The system will never reach the design points for the constant flow systems because no requirement exists in the system for these high flows and heads. Energy Waste of Balance Valves The energy waste of balance valves is due to their use being based upon constant flow in a system that has a variable load. The system operates at 360 gpm (23 L/s), regardless of the cooling load using much more energy than the variable flow system shown in Figures 3 and 4. The following energy calculations are based upon a chilled-water system that operates from March to October for an annual operation of 5,880 hours. The same equation for brake horsepower is used for constant and variable speed pumps. The basic equation is: gpm × ft of head pump = horsepower 3,960 × pump efficiency as a decimal Pump kW for a constant speed pump pump horsepower × 0.746 motor efficiency as a decimal Pump kW for a variable speed pump pump horsepower × 0.746 wire-to-shaft efficiency as a decimal (3) (2) (1) Most existing constant flow systems use three-way coil control valves as shown in Figure 1. This is the system that is used in this evaluation of constant flow systems. Figures 3 and 4 show a pump flow-head curve, a system head curve for a variable flow system, and a point of operation for a constant flow system. The design points are shown as 400 gpm (25 L/s) at 70 ft (209 kPa) head and 450 gpm (28 L/s) at 90 ft (269 kPa), respectively. The point of operation for all three systems is 360 gpm (23 L/s) because that is the actual maximum flow required for each. The difference between the point of operation and the design flow is the flow reduction achieved through the use of balance valves on the constant flow systems. Compare these points of operation for the constant flow system to the path of operation of a properly designed variable flow, variable speed system. In this case, the setpoint required at the controlling differential pressure transmitter is July 2009 The data in this article assumes that the chiller plant is cooled by this chilled water. If not, then the efficiency of the motors and variable speed drives must be deducted from the calculations. The calculation of the energy consumption for the constant flow systems is simple since the pump always operates at one point, 360 gpm (23 L/s) at 73.5 ft (220 kPa) head for the system ASHRAE Journal 27 Coil

ASHRAE Journal - July 2009

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