Trane TRG-TRC016-EN Varying Distribution Flow, notes, Chilled-WaterSystem Design, period two

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].

TRG-TRC016-EN 47