Specifications

Catalog WSC/WDC-4 53
Mixing Single and Dual Compressor Chillers
Dual compressor chillers excel at part load operation, while single compressor chillers usually
have better full load
efficiency. A good chiller plant strategy is to install one dual and one or more
single compressor units. Run the dual until it is fully loaded, then switch to the single compressor
unit and run it only at full load, using the dual to trim the load.
Series Counterflow and Series Parallel Chillers
The design of piping systems can greatly impact chiller performance. A popular system is to place
the evaporators in series with the chilled water flowing from one evaporator to the next as shown
in Figure 16 and Figure 17. Two different condenser water piping arrangements can be used.
Parallel flow (Figure 16) divides the total condenser flow between the two condensers. The
counterflow system (Figure 17) puts all of the condenser water through the condenser of the lag
chiller (chiller producing the coldest evaporator leaving water) and then through the lead chiller
(chiller seeing the warmest evaporator water temperatures).
For the evaporator design, it is a good design philosophy to use two identical machines. Since the
lead machine will see the warmest evaporator water, the lead machine will have the greater
capacity and larger portion of the total system evaporator temperature drop. Again referring to
Figure 16 and Figure 17, the lead machine has an 8.4 degree drop (56.0°F-47.6°F) and the lag
machine has a 5.6 degree drop (47.6°F – 42.0°F).
Condenser water flow is important to overall system efficiency. With parallel flow (Figure 16),
the condensers have identical flow conditions (95 to 85 degrees in this example) with the
compressor lift shown. With counterflow arrangement the lift on the lead machine is significantly
lower, reducing compressor work and making the overall system efficiency about 2% better. Even
though the chiller performance is different, it is good practice to use the same chiller models.
Both the WSC and WDC chillers are suitable for series counterflow arrangement and include
controls specifically designed for series chillers. For more information, please refer to
Application guide AG –31-003: Chiller plant design.
Figure 16, Series Parallel Flow
Figure 17, Series Counterflow Flow