Trane SYS-APM001-EN manual System Design Options Selecting flow rates

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.

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			Entering	54.0	[12.2]	57.0	[13.9]
			Leaving	44.0	[6.7]	41.0	[5.0]
temperature	°	°
		F [ C]
Condenser flow rate, gpm [L/s]				1,350 [85.2]		900 [56.8]
Condenser water			Entering	85.0	[29.4]	85.0	[29.4]
			Leaving	94.3	[34.6]	99.1	[37.3]
temperature	°	°
		F [ C]
Chiller power, kW				256.0		292.0

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

30 Chiller System Design and Control SYS-APM001-EN