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

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Image 36
Trane SYS-APM001-EN manual System Design Options Selecting flow rates

SYS-APM001-EN specifications

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