|
OPTIMIZATION OPTIONS - HOW
TO CHOOSE YOUR CHILLER WHILE
KEEPING YOUR COOL
Why
is there a need to address
chiller optimization? Aren’t
all of the critical factors
addressed during design? Why
is a system that may have
been “state-of-the-art” in
the past, now considered an
energy hog?
All good questions, these. The answers, while not
difficult, may not be the
same for all applications.
Depending on what one is
trying to accomplish,
“optimize” may be likened to
the elephant being examined
by multiple, blindfolded,
individuals appearing to be
a different thing to
different people. A
developer may want to build
as quickly and frugally as
possible; the owner wants to
minimize operating costs;
the occupants of a facility
want to be comfortable and
not experience drastic
swings in temperature and
humidity.
Why are there so many chiller installations which
appear today to be terribly
energy inefficient, which
were once considered, if not
top-of-the-line, at least
quite acceptable? Times
change….energy costs
change….users’ needs and
expectations change. The key
word here is change. If you
always do, what you always
did, you will always get
what you always got!
In years past, when systems were designed using slide
rules, nomographs and
“rules-of-thumb”, it was
normal to oversize rather
than undersize as one did
not want to have a system
installed which could not
sufficiently cool….and who
knows what additional
cooling loads may appear
from the time of design to
the time of construction. If
all of the capacity was not
needed, it would be
“throttled” back a bit,
energy was cheap (or at
least cheaper than it is
today) and all would
consider your design a
success. These “brute force”
designs were also,
generally, the lowest
first-cost compared to ones
which, in the “long-ago”,
would be considered pushing
the envelope (Note:
“long-ago” may denote fifty
years to a few months,
depending on how
dissatisfied the
developer/owner/occupant -
choose one - is with the
installed system and what
technological gem has
appeared on the horizon).
All of the items we will discuss apply to new and
existing installations;
however, existing ones
generally have more
con-straints than when
starting from a clean sheet
of paper. Economics is a
powerful consideration,
making it difficult to start
from scratch when one
already has significant
investment in physical
plant. So, where do you
start? It is said that you
can’t control what you can’t
measure. Similarly, you
cannot update and optimize a
system unless you can
identify what you have and
what is wrong.
Loads
– Identify the types and
sizes of the cooling loads
needed to be satisfied. If
the loads are of a type that
should have their own
dedicated cooling systems
(e.g. air compressors and
hydraulic pumps), they
should be removed from the
chiller loop. If the size
and occurrence of some or
all of the loads are such
that they can be satisfied
by waterside economizers,
consideration should be
given to adding that
capability.
Age of
Chiller/ Piping and type of
Refrigerants Used –
It is generally not
economical to replace
recently installed equipment
and piping purely from an
economical analysis.
However, if the physical
plant is getting on in years
and/or uses refrigerants no
longer readily available or
desirable from an
environmental perspective,
the opportunity for
replacement with more
optimum designs will be much
easier to accomplish with a
reasonable payback period.
Typically a chiller more
than 10 years old may be a
candidate for replacement,
especially if the building
load profile has
significantly changed since
installation. If piping is
in need of replacement, it
is the ideal time to rethink
the configuration with
regard to system losses.
Frequently, older
installations have been
modified and/or increased
without thought as to the
most efficient lay-out, just
building upon the existing
layout.
Ancillary Equipment –
In addition to chiller and
piping replacement, consider
the valves and controls
presently installed. Are
they capable of more precise
control and quicker response
time? Are cooling towers,
where used, capable of
effi-cient operation? Would
variable speed fans maintain
cooling load, while reducing
energy usage? As with
chillers, there is no
one-size-fits-all answer.
Depending on cooling load
the cooling tower
characteristics dictate may
dictate the liquid flow and
air flow based on minimum
acceptable turndown
(typically 3:1) to avoid
scaling issues. One should
keep try to size valves to
maintain the ability to
accurately control flow,
without introducing
excessive losses.
Hydronic Configuration
– Perhaps central to the
question of whether to
upgrade and how to do it is
how the system is configured
and how it can be changed to
be more energy efficient,
while supplying the required
cooling load. Most of this
effort will be directed to
maintaining the correct ΔT
conditions. It has been
noted that if design ΔT
conditions are not
maintained, chiller
efficiency can be decreased
by up to 40%. Further, in
many installations the
system does not operate at
the “design conditions” more
than 5% of the time. Since
chiller operation may
consume up to 35-40% of the
cooling system energy
budget, improvements in
chiller efficiency will pay
large dividends in
operational costs savings.
One needs to determine, for
the given installation,
which topology makes the
most sense:
Primary (fixed)/Secondary
(fixed)
Primary Only (fixed)
Primary (fixed)/Secondary
(variable)
Variable Primary (only)
Variable and Fixed Primary
(staged)
Each of these options need to be evaluated on what
benefit they may provide in
a particular application.
Fixed Primary/Secondary,
once the norm, has fallen
into disfavor (as has fixed
primary only) as better
controls and equipment are
available to implement
variable load schemes,
permitting the elimination
of bypass control valves and
throttling control valves,
which contribute to system
pumping losses.
Use of variable load chillers in staged applications
simplifies the staging
control and minimizes
chiller trips. Some
addi-tional instrumentation
and control will be
necessary to accomplish
this, however, the
elimination of the need to
maintain minimum flow in
“bypass” configurations will
somewhat offset this cost.
The Fixed primary/variable
secondary (distributed)
configuration may lend
itself to installations with
large air handling systems.
Benefits include elimination
of control valves with flow
restrictions, while costing
(first cost) the same as
variable primary (only).
Controls – Newer
control systems are
available which, harnessing
the computational power
available at a reasonable
cost, permit adaptive
control to result in lowest
kW per ton cooling, taking
into account changing load
conditions and am-bient to
vary flows, reset chiller
and condenser water
temperatures and stage
multiple chillers to
maintain operation at the
most efficient design point.
With the installation of the
proper instrumentation,
control software can monitor
system operation and, where
variances from the norm are
indicated, recommend
corrective action to be
taken. A side benefit to the
newer controls is, it is
claimed, easier start-up and
“tuning” in comparison to
older PID type systems,
although some of this
benefit may be due to the
decreasing availability of
resources who can “tame” the
older systems.
Cooling Towers – Much
of what has been said in
regard to chillers will also
apply to cooling towers.
That is, use of vari-able
speed pumps and fans, where
not prevented by the
operating limitations of the
cooling tower construction,
can have a beneficial effect
on overall system
efficiency. Proper
coordination of the variable
flow cooling towers and the
variable load chillers will
require the newer controls
noted above.
Measurements and Data
Gathering – The need
to measure system and
ambient parameters becomes
more critical, when trying
to optimize system
operation. Classical
application of “Bin”
climatic data may not be
sufficient to achieve an
optimum design, requiring
additional up-front
measurement and calculation.
However, the adaptive
control systems, once
operational, will continue
to gather data and
constantly finesse the
system settings to achieve
the best set points.
Conclusion – With all
of the above technology at
our fingertips, does this
mean that we will no longer
have to oversize systems to
account for the
unanticipated? No….but with
the use of variable load
systems, it will be possible
to operate “oversize”
systems at much more
efficient levels. While the
chiller equipment may not be
operating at its maximum
effi-ciency, the decrease in
pumping losses in oversize
piping and the elimination
of many three way valves
will contribute to an
overall acceptable
operation.
Postscript - Now that
we have come this far, and
know how to design more
efficient chiller systems,
we see that Intel has found
that Data Center cooling may
not be as necessary as
previously thought. They
have found that Data Centers
can be operated at up to
95-100 o F without damage to
the equipment (and maybe
only a little damage to the
people who have to service
the equipment) resulting in
an energy reduction of up to
65%. Intel’s results
indicate that Data Centers
could be cooled with outside
air in 95% of the world’s
locations. I guess that
means that in the near
future, our topic should be
filtering and
dehumidification of outside
air.
|
