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30 Chiller System Design and Control SYS-APM001-EN
System Design Options
Selecting flow rates
Designers may use the standard rating conditions to compare
manufacturers’ performances at exactly the same conditions. However, these
standards allow any flow rates to be used and certified comparisons to be
made at a wider range of conditions.
For a given load, as flow rate decreases, the temperature differential
increases. Table 4 reflects a 450-ton [1,580-kW refrigeration] chilled-water
system, both as a base case and with low flow.
In this example, notice that the leaving chilled-water temperature decreases
and the leaving condenser-water temperature increases. This means that the
chiller’s compressor must provide more lift and use more power. At first
glance, the design team may decide the chiller power difference is too large
to be overcome by ancillary equipment savings. The key question is, How
does this impact system energy consumption? Using the following
assumptions, we can calculate system energy usage:
• 80 feet of water [239 kPa] pressure drop through chilled-water piping
• 30 feet of water [89.7 kPa] pressure drop through condenser-water piping
• 78°F [25.6°C] design wet bulb
• 93 percent motor efficiency for pumps and tower
• 75 percent pump efficiency
• Identical pipe size in chilled- and condenser-water loops (either a design
decision, or indicating changing flows in an existing system)
The pressure drop through the chiller will decrease due to the lower flow
rates. When using the same size pipe, the pressure drop falls by nearly the
square of the decreased flow rate. While this is true for straight piping, the
pressure drop does not follow this exact relationship for control valves or
branches serving loads of varying diversity.
Be sure to calculate the actual pressure drop throughout the system.
Hazen–Williams and Darcy–Weisbach calculate the change is to the 1.85 and
Table 4. Standard rating conditions for chilled-water systems
Chilled Water System Base Case Low Flow
Evaporator flow rate, gpm [L/s] 1,080 [68.1] 675 [42.6]
Chilled water
temperature F [C]
Entering 54.0 [12.2] 57.0 [13.9]
Leaving 44.0 [6.7] 41.0 [5.0]
Condenser flow rate, gpm [L/s] 1,350 [85.2] 900 [56.8]
Condenser water
temperature F [C]
Entering 85.0 [29.4] 85.0 [29.4]
Leaving 94.3 [34.6] 99.1 [37.3]
Chiller power, kW 256.0 292.0