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Water-cooled chiller plants are traditionally designed with the evaporators and condensers piped in parallel, which makes the compressor in each machine have to do the entire cooling delta T and lift.  Lift is the difference between leaving evaporator pressure and leaving condenser pressure.  This is often approximated by the leaving temperatures.

Above is a chilled water system diagram illustrating a series counterflow arrangement

Configuring water-cooled chiller plants in a series counterflow (SCF) arrangement splits the lift between two machines to enhance the operating efficiency of the plant. In a SCF configuration the chiller making the coldest chilled water also gets the coldest condenser water.  Now one machine will cool the water from 56 F° to 49 F° and the second machine cools it from 49 F° to 42 F°.  Just like in life, splitting the work results in a more efficient outcome.  Our analysis has shown SCF configurations saving 15-20% compared to typical parallel configurations with full load efficiencies below 0.5 kW/ton (COP=7).

Annual chiller efficiency map of a single parallel chiller.
Annual chiller efficiency map of a series counterflow branch. Notice the drastic difference compared to the single parallel chiller. Please also note that the peak efficiency in the legend is 17 compared to 14 on the parallel chiller
A series counterflow chiller branch performance map plotted from custom bi-quadratic performance curves