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period two
Chilled-Water System Design
notes
Varying Distribution Flow | |
| | B | pump |
| | |
| pressurehead | | curve |
| | A |
| | |
pressure | 0 | 50 | 100 |
difference | | percent flow | |
variable-speed control riding the pump curve
Figure 53
The distribution pump is typically equipped with a variable-speed drive that is controlled to maintain a certain pressure difference between the supply- and return-water piping. In response to a reduced cooling load, the two-way valve modulates closed, restricting the flow of water through the coil. This causes an increase in system differential pressure, which can be measured and used to signal a reduction in the speed of the distribution pump.
An alternative is to allow the pump to “ride its pump curve.” As the two-way valves modulate closed, the increase in system pressure causes the pump to “ride up” its performance curve (A to B), resulting in a reduction to 50 percent of design flow in this example. This method, however, generally results in less energy savings than a pump with a variable-speed drive. Also, proper pump selection is important and part-load operating conditions must be considered.
In variable-flow systems, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.1) requires the use of a modulation device, such as a variable-speed drive, on pump motors larger than 50 hp [37 kW] that have a pump head greater than 100 ft H2O [300 kPa].