SUSTAINABLE CHILLED WATER SYSTEMS
Making Our Chilled Water Plants More Efficient
July 10, 2007
by
Thomas Hartman, P.E., Principal
The Hartman Company
755 County Road 247
Georgetown, Texas 78628
Until recently, few designers or manufacturers thought variable speed belonged in chiller plants.
Adding VFDs to chillers and permitting water flows to change were not usually recommended.
Now that has all changed. Nearly all in the industry embrace the concept of chiller plants in
which all the chillers, pumps and tower fans are variable speed. Such plants are called "allvariable speed" plants. Whatever the reasons for the change in attitude, simply configuring a
plant as all-variable speed without careful consideration of equipment selection and operation
may not yield energy and cost savings. Both designers and plant managers involved in chiller
plant design and operation need to consider how to most effectively integrate these new
technologies into their plants and distribution systems because they offer enormous cost
reductions but only when correctly designed and operated. Integrating an all-electric, all-variable
speed chilled water system utilizing configuring and control technologies derived from new
principles developed specifically for variable speed can cut the total plant energy nearly in half
compared to a state of the art conventional plant. Furthermore electric all-variable speed
configurations are less costly, simpler to operate and maintain, and save even more energy when
compared to hybrid plant alternatives. Since energy accounts for more than half the life cycle cost
of a typical chiller plant, new all-variable speed technologies offer a very substantial reduction in
overall plant life cycle cost.
A schematic of a typical all-variable speed chiller plant is shown in Figure 1 below. There are no
substantial differences in plant equipment (chillers, condenser pumps and cooling tower) layout
between an all-variable speed chiller plant and a typical conventional plant. Although it is not
shown, a header pumping arrangement on chilled and condenser water systems works equally as
well with an all-variable speed chiller plant just as with constant speed plants. Notice in this
figure that there is only one stage of pumping and no decoupling of the distribution from the
chilled water generation circuit. Incorporating determinant flows is an important element of new
all-variable speed technologies so decoupling lines are not employed. Also, the relative sizing
and selection of the equipment in the plant is likely to be different than in conventional plant
configurations. But the real differences between a constant speed plant and an all-variable speed
plant are the use of variable speed drives to operate all equipment and the DDC optimization
control network that automatically coordinates the operation of the equipment under all operating
conditions. Such automatic network control may or may not be employed in a conventional plant,
but effective automatic control is a necessary ingredient for the success of an all-variable speed
chiller plant.
Optional Network Extension to Loads
Served by Plant
All-Variable Speed Plant Optimization Network
VFD
VFD
VFD
VFD
VFD
VFD
COND
COND
VFD
EVAP
COMP
Chiller #1
VFD
COND
VFD
EVAP
Plant Operations
Workstation
COMP
Chiller #2
VFD
VFD
EVAP
COMP
Chiller #3
VFD
CHWR
CHWS
Figure 1: All-Variable Speed Chiller Plant Configuration.
Evaluating Benefits of All-Variable Speed Chiller Plants
To evaluate this new approach to chiller plant design and operation, designers or plant managers
must first get their arms around the potential benefits possible by building or converting their
existing chiller plants or new plant designs to all-variable speed. Figure 2 below shows the
average annual operating efficiency for plants with electric motor driven centrifugal chillers,
which make up the majority of larger chiller plants. The efficiencies shown in Figure 2 are based
on the total energy use of the plant including chillers, condenser water pumps, tower fans, and all
chilled water pumping. Most existing plants have been shown to operate at or above 1.0 kW/ton.
The most modern conventional plants that keep their equipment updated and are well maintained
may operate in the "Fair" range from 0.85 to 1.0 kW/ton average annual efficiency. Those that
also have effective efficiency monitoring and automatic optimized control systems may dip into
the "Good" range, but a conventional constant speed plant that shows an annual average
operating efficiency less than 0.8 kW/ton is a rarity, limited to those plants with exceptionally
efficient equipment and good automatic optimization controls.
New Technology
All-Variable Speed
Chiller Plants
EXCELLENT
High-efficiency Conventional
Older Chiller
Optimized
Code Based
Plants
Chiller Plants Chiller Plants
GOOD
FAIR
Chiller Plants with
Correctable Design or
Operational Problems
NEEDS IMPROVEMENT
kW/ton 0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
C.O.P. (7.0) (5.9) (5.0) (4.4) (3.9) (3.5) (3.2) (2.9)
AVERAGE ANNUAL CHILLER PLANT EFFICIENCY IN KW/TON (C.O.P.)
(Input energy includes chillers, condenser pumps, tower fans and chilled water pumping)
Based on electrically driven centrifugal chiller plants in comfort conditioning applications with
42F (5.6C) nominal chilled water supply temperature and open cooling towers sized for 85F
(29.4C) maximum entering condenser water temperature and 20% excess capacity.
Local Climate adjustment for North American climates is +/- 0.05 kW/ton
Figure 2: Chiller Plant Total Energy Use Spectrum
Now consider the range of annual efficiencies for an optimized all-variable speed chiller plant
composed of the same basic equipment but operated entirely with network optimized variable
speed control. Such all-variable speed plants generally operate at an average annual energy
efficiency of from less than 0.5 to 0.6 kW/ton, depending on the climate, the equipment
employed, the configuration, and the load profile of the load served. This means that the potential
reductions for converting a well maintained and operated chiller plant to an all-variable speed
chiller plant can be expected to amount to between 25% and 50% or more of the current plant
energy use. The resulting annual dollar energy savings amounts to a low of $30 to over $100 per
installed ton, depending on the length of the cooling season, the load profile and the electric rate.
For those considering hybrid plants containing absorption or engine driven chillers, consider that
all-variable speed electric cooling will nearly always be vastly more economical to operate and
much less costly to maintain. Unless waste heat is available (and in many cases, even if it is),
optimized all-variable speed chiller plant technologies make hybrid chiller plants obsolete. If
demand limiting is mandated by rate structure or emergency cooling is needed in the event of a
power failure, these can be accomplished far more cost effectively with a standby or emergency
electric generator to power some of the cooling equipment. Such an approach offers better
economy and greater load shaping flexibility than incorporating non-electric chillers in the plant.
All-Variable Speed for New Chiller Plants
For those with new plants in the planning or design stage, these potential energy reductions make
consideration of switching to an optimized all-variable speed configuration an imperative. The
marginal costs for incorporating optimized all-variable speed technologies into a new plant are
very small. Variable speed chillers cost very little more than their constant speed alternatives.
Applying variable speed drives to tower fans is cheaper than implementing two speed motors,
and locating variable speed drives at the pumps can be less expensive than using regular starters
housed in motor control centers. Payback on any extra investment required to configure a new
plant as an all-variable speed chiller plant will usually be well under a year.
Conversion of Existing Plants to All-Variable Speed
Although the substantial energy reduction from all-variable speed technologies always justifies
incorporating all-variable speed into a new plant, some analysis may be required to see if it can
be justified as an upgrade for existing plants in good condition and efficient operation. For these
plants, there are two paths to such a conversion that need to be considered. First, if the life cycle
savings do justify it, an immediate retrofit involving adding variable speed drives to all the
chillers, pumps and tower fans and implementing optimization controls should be considered. If
not, a second path to consider is replacing chillers with variable speed chillers as each
approaches the end of its useful life. In this path an upgrade of variable speed condenser pumps
and tower fans along with network controls may be initiated immediately or as the new variable
speed chillers come on-line.
The second approach is a fallback. It usually carries the same low marginal cost as that of
planning a new plant because this path waits until the chillers reach the end of their useful life
and have to be replaced anyway. If an immediate retrofit conversion to all-variable speed cannot
be accomplished due to budget limitations, then the "conversion at time of replacement" should
become the upgrade plan for the chiller plant.
Determining which upgrade path to follow requires an analysis of the savings potential of the
conversion and the costs involved for an immediate retrofit. The most effective means of
calculating the savings potential is to determine the present energy costs for the plant, estimate
the present plant efficiency, and then apply the estimated efficiency improvement a conversion
would bring. Such an evaluation can become more involved if the electric rate structure is
dominated by a high demand charge. It may take some time to assemble the data necessary for
this analysis, but it is not really complicated. It is also surprising how many resources are
available to assist your efforts.
Once this energy savings estimate has been made, the next step is to estimate the costs involved
in implementing a retrofit that will achieve the estimated savings. For plant owners or operators,
the most direct route may be to call in manufacturers and contractors knowledgeable in the new
all-variable speed technologies. With a frank discussions it is possible to get reliable budget
pricing for adding the variable speed drives and necessary controls and making any other changes
that may be necessary for a retrofit to all-variable speed operation. It should be noted that
effective optimization control software modules are packaged technologies that are readily
available at low cost. The packages include complete control sequences, performance
verification, and the support necessary to ensure the plant performs as expected. With some
effort, it is not too difficult to arrive at a total upgrade cost estimate by assembling the cost of
upgrading the chillers, pumps and tower fans to variable speed and adding a digital control
system, if none presently exists, and the necessary control software and support.
Another approach is to have outside help to assist in this analysis. However, caution is advised.
Because all-variable speed technology is relatively new, many engineers are not knowledgeable
about all-variable speed plant operation, but few are eager to acknowledge this reality. It takes an
experienced firm to do justice to such an analysis. Chiller manufacturers are now developing
computer programs that can help with the savings portion of the analysis, and a manufacturer
may be able to recommend a firm to assist with other aspects of the analysis.
Distribution System Issues
A critical part of any chilled water system is the chilled water distribution system. Though chilled
water distribution systems vary widely in size and configuration, problems associated with these
systems are quite universal: low delta T, inability to fully load chillers, inadequate flow in
sections of the distribution system, and excessive pumping pressure requirements at peak cooling
demand conditions. One or more of these problems plague nearly all chilled water distribution
systems. Figure 3 below shows a typical primary-secondary variable flow distribution system. In
smaller systems the primary loop and the secondary distribution pumps may all be located in the
plant. In a large building complex, the secondary loop may extend throughout the campus and
tertiary distribution pumps may be located in the individual buildings served by the distribution
system. Actual configurations may have more or less distribution circuits and usually will have
multiple pumps at each pumping station. However, the lessons discussed here are generally
scalable and are easy to apply to a wide variety of distribution systems.
C
DP
DP
CHILLER #3
PCHWP3
C
DP
Load n
Load n
Load n
Load 3
Load 3
Load 3
Load 2
Load 2
Load 2
CHILLER #2
PCHWP2
C
CHILLER #1
PCHWP1
Load 1
Load 1
Load 1
Secondary
Distribution
Circuit #1
Secondary
Distribution
Circuit #2
Secondary
Distribution
Circuit #3
VFD
VFD
VFD
SCHWP1
SCHWP2
SCHWP3
Decoupler Line
Primary Distribution
Header
Figure 3: Typical Chilled Water Distribution System Configuration.
In Figure 3, the primary chilled water pumps (PCHWP1 through 3) are typically constant speed
pumps and the secondary chilled water pumps (SCHWP1 - 3) are variable speed pumps. The
primary pumps are cycled on and off with the chiller each serves, and the speed of the secondary
pumps is modulated to meet a differential pressure setpoint as measured at the end of the
distribution circuit each serves. A decoupling line shown in the figure permits flow in either
direction at the end of the primary circuit since the "stepped" primary flow will nearly always be
different than the continuously variable secondary flow. This system is widely employed, but has
two inherent problems that lead to low delta T and poor performance:
1. When primary flow is greater than secondary flow, low delta T in the primary circuit results
from the recirculating primary chilled water through the decoupling line and directly back to the
chillers. The lower than expected return chilled water temperature makes it impossible to fully
load the on-line chillers because the primary pumps are fixed flow. This wastes energy and if it
occurs at peak conditions, it robs the plant of capacity.
2. Whenever secondary flow exceeds primary flow, flow reverses in the decoupling line and is
mixed to degrade the supply water temperature. This reduces the cooling capacity of the loads in
distribution circuits closest to the decoupling line. The result is greatly increased flow in those
circuits and reduced delta T, which also robs the system of its full capacity capabilities.
Because primary and secondary flow is almost never exactly balanced and actual delta T always
varies somewhat from design, one of the two problems is almost always at play in such systems,
both of which can reduce the design delta T of the system and both of which make it difficult to
operate the system effectively at full capacity. A number of solutions have been proposed to
correct this problem, but such "cures" often destroy the system's ability to meet the cooling load
requirements. One popular method of correcting low secondary circuit delta T problems is shown
in Figure 4.
Primary CHW Header
SCHWP
CWS
Secondary CHW
Header to/from Loads
CWR
CWR CWS
Figure 4: Diagram of a Typical Delta T "Enhancement"
While the Figure 4 diagram or some variation of it is often touted as a cure for low delta T, it
much more often has disastrous effects on system operation. The idea is that the diverting valve
on each secondary (or tertiary in some cases) circuit return (sometimes a mixing valve is used on
the chilled water supply) will modulate some return water back to the pump anytime the return
temperature is below design. It is reasoned that the elevated supply temperature will raise the
return temperature and ensure that the design delta T from the circuit is maintained at all times.
However, this fix rarely has the desired results. When air is the medium being cooled, return
chilled water temperature is much more affected by entering air temperature than chilled water
supply temperature. Raising the chilled water supply temperature thus has little effect on return
chilled water temperature, but it does profoundly reduce cooling coil capacity, especially latent
cooling capacity. As the supply chilled water temperature rises, load valves open further and flow
in the circuit increases dramatically, often without a significant increase in the return water
temperature and usually with a reduction in cooling effect. Thus, when the scheme shown in
Figure 4 is installed on a distribution circuit, one poorly operating load in the circuit can severely
compromise the capacity of all loads in the circuit. In large systems it is also possible at times to
have the flow reversal such that return chilled water from the mains travels to the supply header
through the diverting or mixing valve. Thus the Figure 4 "fix", and the many schemes that are
similar to it, do not fix system operation at all. Instead, it is a "poison pill" to chilled water
distribution systems.
Configuring the Distribution Solution
To configure a successful distribution system we must recognize what helps and hinders delta T.
Delta T problems are sometimes caused by the designs themselves. These may include added
bypasses and three way valves scattered through the system to keep water moving at low load
conditions. Solutions that involve mixing return water with supply water undermine the
thermodynamic efficiency of the system, destroy the capacity of the coils to meet their loads, and
add further to low delta T problems. To solve the types of distribution problems that lead to low
delta T, Figure 5 shows a system that when operated properly ensures every load will be satisfied
and ensures that the design delta T is maintained at all times.
VFD
CHILLER #3
PCHWP3
VFD
Load n
Load n
Load n
Load 3
Load 3
Load 3
Load 2
Load 2
Load 2
CHILLER #2
PCHWP2
VFD
CHILLER #1
PCHWP1
Load 1
Load 1
Load 1
Booster
Distribution
Circuit #1
Booster
Distribution
Circuit #2
Booster
Distribution
Circuit #3
VFD
VFD
VFD
BCHWP1
BCHWP2
BCHWP3
DP
Primary Distribution
Header
DDC NETWORK
Figure 5: "All-Variable Speed" Chilled Water Distribution System Configuration with Network
Controls
Notice how similar Figure 5 is to Figure 3. Because there are no decoupling lines in Figure 5, it is
called an "all-variable speed series Primary/Booster system." Here’s how the rules listed above
have been implemented to convert the conventional Primary/Secondary system to a
Primary/Booster and solve the problems typically associated with distribution systems:
Eliminate all possibility of direct mixing between chilled water supply and
return: In Figure 5, the decoupling line in the primary header has been removed and
the primary pumps have been converted to variable speed control. With a DDC
network coordinating the primary and secondary (now called booster) pumps, the
pumping systems no longer need to be decoupled. Modern chillers easily
accommodate the varying flows over wide ranges so varying the flow throughout the
entire system as conditions change works very well. The primary pumps operate with
their respective chillers to coordinate with the operation of the booster pumps
described below.
Employ a direct coupled distribution system: The schematic in Figure 5 is now a
series distribution system with the booster circuit pumps directly in series with the
primary pumps. In addition to eliminating the possibility of mixing supply with return
chilled water, this direct coupled configuration can save capital cost when compared
to decoupled Primary/Secondary schemes because Primary/Booster configurations
accommodate built-in backup without the need for redundant equipment.
Focus delta T attention at each and every load: This is probably the most
important area of improvement. In Figure 5 control of the booster pumps has changed.
The network control system is employed to connect the system with loads served and
it enables a much more efficient and reliable method of making certain all loads in the
circuit are satisfied with a minimum of pumping power. Network control of the
booster pumps eliminates the need to maintain a fixed static head in the circuit at all
times. Instead direct service of the loads calling for cooling is accomplished with a
new network enabled control called "demand based control."
There are some hydronic issues that must be addressed with large series pumping systems. The
potential for water hammer can be mitigated with a variety of design features that include slow
acting valves and distributed expansion tanks. The need for minimum flow at low loads may be
resolved by a networked controlled valve as shown in Figure 5.
Implementing an Effective All-Variable Speed Chiller Plant
Whether the all-variable speed chiller plant is a new one under design or the result of a decision
to retrofit an existing plant, achieving a successful result requires close attention and cooperation
by those responsible for the design and operation of the plant. To be successful, an all-variable
speed chiller plant requires a method of automatic control that optimizes the equipment operation
under all loading conditions and a method to provide continuous measurement and verification of
plant performance so that any equipment or operation anomaly that may affect plant performance
is identified and corrected quickly.
To obtain optimum efficiency of the plant, the variable speed chillers, pumps and towers must be
sequenced so that they operate at part capacity whenever possible. The only time any equipment
should be operated at full capacity is when all plant equipment must operate at full capacity to
meet a peak load condition. When a plant is oversized for redundancy, this will only be necessary
in the event of a component failure. Although this means more equipment is generally on line at
any given time, manufacturers and operators agree that the maintenance of the equipment is
reduced in ultra-efficient all-variable speed chiller plant configurations because equipment is
started softer and less frequently, and the average loading of the equipment is much less. These
factors more than compensate for the longer operating hours.
Summary and Conclusion
All-variable speed chiller plants employ relatively new technologies that have the capacity to
reduce cooling energy use by 25% to more than 50% below well operated conventional plants.
Plant designers and managers need to consider this new approach to configuring and operating
chiller plants as they plan new plants or consider upgrading existing ones. Managers and
operating staff also need to understand the operations and maintenance implications of this
technology which rightfully will challenge traditional perspectives. In order to achieve the ultraefficient chiller plant operations it promises, new perspectives on chillers, towers, condenser
flow, and plant operations are required. However, this is a fundamentally straightforward and
easily applied technology. It may take some time to upgrade existing plants, but the results will
be well worth the effort.
Additional information on technologies discussed in this article is available at
www.hartmanco.com. Comments and questions may be addressed to Mr. Hartman at
tomh@hartmanco.com.