Trane SYS-APM001-EN manual Series-Counterflow Application, Series-series counterflow

Chilled-Water System Variations

Series–Counterflow Application

The series–counterflow configuration can work with all types of chillers. Duplex™ chillers are shown in Figure 48. A single large chiller may be built as a Duplex machine, and would itself constitute a series-counterflow arrangement. The evaporator and condenser water circuits are common to both halves of the Duplex. Increased efficiency comes from the separation of the compressors and refrigerant circuits.

Another system configuration that can be very energy efficient incorporates the previously described series application, but does so for both the chilled water and condenser water. Figure 48 shows such a configuration. The chilled- and condenser-water flow directions are opposite, or counter, to one another (thus the name, series-counterflow).

Note that the downstream machine, in this case, produces 40°F [4.4°C] chilled water, while the upstream machine produces 50°F [10°C] chilled water. The downstream machine receives 85°F [29.4°C] condenser water, while the upstream machine receives 95°F [35°C] condenser water. Therefore, the pumping requirements are only 1.2 gpm/ton on the chilled water side and 1.5 gpm/ton on the condenser water side—greatly reducing pumping, piping, and cooling tower costs. The configuration has the effect of equalizing each chiller’s required lift, as shown in Figure 49.

Figure 48. Series-counterflow arrangement

Upstream Chiller		Downstream Chiller
105 °F [40.6°C]	95 °F [35°C]
		85 °F [29.4 °C]

		40 °F [4.4°C]
60 °F [15.5°C]	50 °F [10°C]

For more information, refer to the ASHRAE Journal article, “Series-Series Counterflow for Central Chilled Water Plants.” 19

Figure 49. Equal lift concept

	105°F			105°F
	[40.6°C]	95°F		[40.6°C]
		[35°C]
			Upstream Chiller
Single					Series-
	65°F [36.1°C]	Downstream Chiller
Compressor					Counterflow
			55°F [30.6°C]
Chiller					Arrangement
		55°F [30.6°C]		50°F
	40°F	40°F		[10°C]
	[4.4°C]	[4.4°C]

Series-series counterflow

Figure 48 shows two Duplex™ chillers in series. Chiller module (combination of two Duplex chillers) power savings can be as high as 19% compared with a single chiller operating at the same conditions. Because the Duplex chiller is actually two refrigerant circuits in series on a common water circuit, Figure 48 operates with the efficiency of four chiller circuits in a series- counterflow arrangement. But system control is simply for two chillers in

SYS-APM001-EN Chiller System Design and Control 77