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19-07-2009, 02:04 PM #1
Optimising parallel water chillers for energy saving
Hello All,
My colleague and I are currently tasked with examining the energy consumption of the whole of our site.
One thing that I am questioning is the (short) cycling of a pair of TRANE RTAB213 water chillers. Each unit has 2 compressors A and B. Both chillers are connected in parallel on the same water circuit, and both set for similar operating parameters.
These are: Setpoint 7deg C, Differential to start 1.1deg C. The only difference appears to be the 'design delta temperature setpoint' which is 8.5deg C on Chiller #1 and 7.5deg C on Chiller #2. Both are set to Setpoint: 'Local' and Mode: 'Auto'.
They are controlled (?) by a TRANE Sequencer, and the operation has been observed and recorded thus:
(Note: Compressor B runs in both chillers rather than compressor A due to the difference in run hours of each compressor) Return temperature reaches 8.1deg C, Chiller #1 starts runs at 90%FLA, 2 mins later Chiller #2 starts running at 50% when its return temperature reaches 8.3deg C, 3 mins later Chiller #1 stops as output water reaches 4.9deg. Chiller #2 continues to run (at 50%FLA) for up to 45mins!
Consequently Chiller#1(B) has low running hours but a higher number of starts than Chiller#2(B).
This is one set of readings:
Ch#1(A): 21152 starts, 12123 run hrs.
Ch#1(B): 17588 starts, 7237 run hrs.
Ch#2(A): 20114 starts, 30177 run hrs.
Ch#2(B): 12167 starts, 15497 run hrs.
It seems that the sequencer might be working in the fact that it starts the one with the lowest run hrs, but it appears that the fact that both start and #1 then shuts down to be wrong and wasteful of power, to say nothing of motor/compressor wear.
The system is presumably a 6 / 12 deg c design, so should the setpoint(s) be raised and perhaps a higher differential to start be set on Chiller #2 ?
Your ideas are welcome, especially from the TRANE guys who might have come across similar problems before.
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