Chiller Sequence

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Can someone help me with the proper inputs for the following chiller
sequence of operation. I have five 400-ton water cooled centrifugal
chillers, three of them have variable speed drives, the other two just inlet
vanes for capacity control. For loads up to 360 tons, one of the variable
speed chillers operates. For loads up to 720 tons, two of the variable speed
chillers operate, but one of them at constant full load and the other
responding to variations in loop loads. This sequence continues similarly at
greater loads, basically one chiller handles loads up to about 80% of its
capacity, at which time another chiller is added, but only the last one to
come on modulates to the load, while the others operate a full load. In
reading the program manuals for equipment control, it appears that you can
establish the appropriate sequence, but once additional chillers come on,
all modulate to handle the load in proportion to their capacities, which in
my case because all chillers are the same size, would handle equal portions
of the loads. I am modeling an existing chilled water plant that is operated
in this manner, and one of the things I'm looking at is the savings of
indeed operating them equally loaded when multiple chillers are needed. The
variable speed chillers in particular operate at their highest efficiency in
part load ranges, and it's intuitively obvious that operating two of them at
a mid-range part load will be more energy efficient than operating one at
full load and one at a small part load, but my client would like to quantify
this before changing his control sequences. Thanks in advance for any

Rick Routh

Routh Consulting Engineers's picture
Joined: 2011-09-30
Reputation: 0

Hi Rick,

You should be able to do this using a combination of EQUIP-CTRL and
LOAD-MANAGEMENT commands. If you Google Equip-Ctrl you will get a link to a
thread on the list-serv with some good examples.


cmg750's picture
Joined: 2010-10-05
Reputation: 0