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26 A SHRA E Jou rna l ash rae .o rg S e p t e m b e r 2 0 1
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This is the second of a series of articles discussing how to
optimize the design and control of chilled water plants. The series
will summarize ASHRAEs Self Directed Learning (SDL) course called
Fundamentals of
Design and Control of Central Chilled Water Plants and the
research
that was performed to support its development. See sidebar, Page
36
for a summary of the topics to be discussed. The articles, and
the SDL
course upon which it is based, are intended to provide
techniques for
plant design and control that require little or no added
engineering
time compared to standard practice but at the same time result
in sig-
nificantly reduced plant life-cycle costs.
A procedure was developed to provide near-optimum plant design
for most chill-er plants including the following steps:
1. Select chilled water distribution system.
2. Select chilled water temperatures, flow rate, and primary
pipe sizes.
3. Select condenser water distribution system.
4. Select condenser water tempera-tures, flow rate, and primary
pipe sizes.
5. Select cooling tower type, speed con-trol option, efficiency,
approach tempera-ture, and make cooling tower selection.
6. Select chillers.7. Finalize piping system design, calcu-
late pump head, and select pumps. 8. Develop and optimize
control se-
quences.Each of these steps is discussed in this
series of five articles. This article dis-cusses Step 3:
designing the condenser water distribution system. Steps 2 and 4
will be discussed in the next article.
Three common piping arrangements for condenser water pumps
are:
Option A: Dedicate a pump for each condenser (Figure 1a);
Option B: Provide a common header at the pump discharge and
two-way au-tomatic isolation valves for each con-denser (Figure
1b); and
Option C: Provide a common head-er with normally closed (NC)
manual isolation valves in the header between pumps (Figure
1c).
The advantages of dedicated pumps for each condenser (Option A)
include:
About the AuthorSteven T. Taylor, P.E., is a principal at Taylor
Engineering in Alameda, Calif.
By Steven T. Taylor, P.E., Fellow ASHRAE
Optimizing Design & ControlOf Chilled Water Plants Part 2:
Condenser Water System Design
This article was published in ASHRAE Journal, September 2011.
Copyright 2011 American Society of Heating, Refrigerating and
Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This
article may not be copied and/or distributed electronically or in
paper form without permission of ASHRAE. For more information about
ASHRAE Journal, visit www.ashrae.org.
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Sep t ember 2011 ASHRAE Jou rna l 27
1. The pump can be custom-selected for the condenser it serves.
Pump selection can then account for variations in condenser
pressure drop and flow rates when chillers are not identical. This
can reduce pump energy compared to Option B where the head of each
pump must be the same and sized for the condenser with the highest
pressure drop; balance valves at the other condensers must be
throttled to generate this same pressure drop.
2. Controls are a bit simpler because the pump can be
con-trolled using the contact provided with the chiller controller.
This ensures that the pump starts and stops when the chiller wants
it to. With Option B, the control of the isolation valves and pumps
is by the direct digital control (DDC) system and must be
coordinated with the needs of the chiller controller to avoid
nuisance trips. For instance, the pumps generally must run for
several minutes after the command for the chiller to stop so that
the chiller can pump down the refrigerant.
3. Pump failures do not cause multiple chiller trips. With
dedicated pumps, if a pump fails, only the chiller it serves will
see a flow disruption and trip. With Option B, all operating
chillers will see a flow reduction when a pump fails, possibly
causing more than one chiller to trip due to low flow or high
refrigerant head. However if there is a lag or standby pump with
Option B that can be started quickly, trips can usually be avoided
because it takes some time for refrigerant head to rise.
The advantages of headered (manifolded) pumps (Option B)
include:
1. Redundancy is improved. With Option A, if a pump fails and a
chiller other than the one it serves also fails (albeit a rare
event), then two chillers will be inoperative. With Option B, any
pump can serve any chiller and under many conditions one pump can
provide enough flow for two chillers to operate near full
capacity.
2. Including a standby pump is much simpler. Adding a standby
pump to Option A is cumbersome and expensive because it requires
extensive piping and manual or automatic isolation valves. If
standby pumps are desired, Option B is the best option.
3. Isolation valves can double as head pressure control valves.
See discussion on head pressure control later. For Option A, head
pressure control would require the addition of variable speed
drives on condenser water pumps or tower bypass valves.
4. It is easier to integrate a water-side economizer. See
discussion on waterside economizers below. Since waterside
economizers are only operational in cold weather when loads are
generally low, the condenser water side can use one (or more) of
the condenser water pumps serving chillers rather than providing a
dedicated pump. This reduces first costs.
Headered pumps with manual isolation valves (Option C) can have
the advantages of Option A (although it works best with identical
chillers) and it overcomes the redundancy dis-advantage of Option A
but accommodating a pump failure requires manual manipulation of
valves vs. the automatic response in Option B. Including a standby
pump is possible with Option C but it only works (depending on
which pump fails) with the header isolation valves open and
chillers must be staged by manually opening and closing their
isolation valves.
First costs are usually lowest with Option A if the chiller and
pump pairs are close-coupled and the manual isolation valves
between the two are eliminated (each chiller-pump pair is iso-lated
for service as a pair). Option C is usually less expensive than
Option B, but Option B is usually the best choice where head
pressure control and standby pumps are required.
Refrigerant Head Pressure ControlAll chillers will require a
minimum refrigerant head (lift)
between the evaporator and condenser. This can be quite high
Figure 1: Condenser water pump piping options. Option A (left):
Dedicated pumps. Option B (center): Headered pumps with con-denser
auto-isolation valves. Option C (right): Headered pumps with manual
isolation valves.
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
Chiller No. 1
Chiller No. 2
Chiller No. 3
CHW Pump No. 1
CHW Pump No. 2
CHW Pump No. 3
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
Chiller No. 1
Chiller No. 2
Chiller No. 3
CHW Pump No. 1
CHW Pump No. 2
CHW Pump No. 3
Optional Standby Pump
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
Chiller No. 1
Chiller No. 2
Chiller No. 3
CHW Pump No. 1
CHW Pump No. 2
CHW Pump No. 3
N.C.
N.C.
A B C
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for most screw chillers and some hermetic centrifugal chill-ers,
and very low for magnetic bearing chillers, which have no oil
return considerations. There are two common reasons why low
refrigerant head pressure can occur:
At start-up when water temperature in the cooling tower basins
is cold. Some chillers can operate for a short period of time with
low start-up head while others will trip on low head pressure
safeties almost immediately. To determine if head pressure control
is required, for cold starts, consult with the chiller
manufacturer.
When integrated waterside economizers are used (dis-cussed
later). Head pressure control is almost always manda-tory since
cooling tower water temperatures are deliberately kept very cold
for long periods.
Options to avoid low head pressure problem include: Tower
three-way bypass valves. The bypass water is di-
verted around the tower fill into the cooling tower sump or into
the suction piping, thus avoiding natural cooling that oc-curs
across the tower fill even when tower fans are off. Piping the
bypass to the suction line also avoids the mass of water in the
basin for an even faster warm-up, but the design can be
problematic: unless the bypass line is balanced to create a
pressure drop equal to the height of the cooling tower, air will be
drawn into the system backwards from the spray nozzles since piping
above the basin will fall below atmospheric pres-sure. For staged
or variable condenser water flow systems, the bypass must be
balanced at the lowest expected flow rate. This creates a high
pressure drop and reduced flow if more pumps operate, but reduced
flow is acceptable when the intent of the bypass is to raise head
pressure. The bypass valve is controlled by supply water
temperature typically with a low limit setpoint well below the
normal setpoint used to control tower fan on/off and speed. Tower
bypass is most commonly used where towers must operate in very cold
weather to avoid freezing in the fill. The following two options
are less expensive and, therefore, preferred in other
applications.
For systems with dedicated condenser water pumps (Op-tion A or
C, Figure 1), variable speed drives on the pumps can be used to
reduce water flow to the chiller. Head pressure can be maintained
even with very cold supply water as long as the flow rate can be
reduced so that the condenser refrigerant pressure can be high
enough (head pressure depends on the
condenser water temperature leaving the chiller, not entering
the chiller). Pump speed can be controlled by the temperature
leaving the condenser at a setpoint that corresponds to mini-mum
condenser pressure, or (preferably) by a signal from the chiller
controller indicating head pressure needs; most chiller controllers
have an analog output dedicated for this purpose.
For systems with headered pumps (Option B, Figure 1), the
isolation valves can double as head pressure control valves by
converting them from two-position to modulating. Valve position is
typically controlled by the chiller controller head pressure
con-trol analog output, either directly or through the DDC system.
This signal will close the valve when the chiller shuts off.
The second two options mentioned previously reduce flow through
the condenser. Many engineers are concerned that low condenser
water flow will contribute to fouling of the con-denser tubes, but
there is little definitive evidence to support the concept that
high velocity keeps tubes clean; strainers and sidestream filters
that prevent particles from entering the con-denser in the first
place are preferred. But even if this is an is-sue, for most head
pressure control applications there are few hours at reduced
flowonly during cold startsso the impact on tube fouling should not
be significant. Low flow through the cooling tower may also be an
issue (see discussion later) but, again, it should not be given the
short duration.
Minimum Flow RatesWhen water enters the cooling tower, it is
distributed uni-
formly across the fill through spray nozzles via a piping
head-er or gravity distribution basin. Each cell has a minimum flow
rate to ensure that tower fill is fully wetted along the face of
the air entering the fill. If there are dry spots along this face,
air will bypass the wetted fill due to lower pressure drop and,
more importantly, cause scale to build up at the boundary be-tween
the wet and dry fill as water is evaporated and dissolved solids
remain. So it is important to maintain minimum tower cell flow
rates, particularly in areas with hard makeup water.
In plants with multiple cooling towers and chillers, it is
desirable to operate one condenser water pump at low loads, which
will reduce the flow rate through cooling towers. Op-tions for
maintaining minimum flow rates (Figure 2) include:
Option A: Select tower weir dams and/or nozzles to allow one
pump to serve all towers. For systems with two or three
Figure 2: Cooling tower cell isolation options. Option A (left):
Weir dams and/or low flow nozzles. Option B (center):
Auto-isolation valves on supply only. Option C (right):
Auto-isolation valves on supply and suction.
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
Cooling Tower No. 1
Cooling Tower No. 2
Cooling Tower No. 3
A B C
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Sep t ember 2011 ASHRAE Jou rna l 29
tower cells, this can eliminate the need for isolation valves,
which cost much more than the weir dams and nozzles. This option is
also the most efficient; tower energy use is mini-mized by
operating as many cells as possible, particularly when tower fans
are controlled by variable speed drives. This is because fan speed
is reduced (reducing fan power by almost the cube of the speed) and
cooling is achieved through tower cells even when fans are off.
With most man-ufacturers and tower types, nozzles and dams are
available to reduce flow by 50%, and many can go down to 33% or
even 25% depending on the selection and design flow rate. Because
of low cost and high efficiency, this option should always be the
first choice. When a plant has many tower cells and automatic
isolation valves are unavoidable, the dams and nozzles should still
be selected to allow as many cells to operate as possible.
Option B: Install automatic isolation valves on supply lines
only. This option uses the equalizer to keep basin levels between
overflow and fill lines and will require that equal-izers be
oversized from that required by normal duty. For example, assume
there are three tower cells, and only one is active; supply flow to
the others is shut off. But water is drawn out of all three cell
basins since the suction lines have no automatic isolation valves.
The water level in the basin
of the cell that is supplied will rise while the other two
ba-sin levels will fall. The difference in the two elevations must
provide enough head for water to transfer from the supplied cell to
the others through the equalizer. If the equalizer is undersized,
water will overflow in the supplied cell, and the others will be
drawn so low that makeup water valves open, wasting water and water
treatment chemicals. There are only a few inches of elevation
difference between the overflow and fill lines, so it is imperative
that the equalizer be properly sized for this option to work.
Another approach is to elimi-nate the basins at each tower and use
a common sump, often located indoors in cold climates. This avoids
the need for equalizer lines entirely but is much more
expensive.
Option C: Install automatic isolation valves on both sup-ply and
suction lines. This is usually the most expensive option since
automatic valves are expensive relative to an incremental increase
in equalizer size. This design also in-creases exposure to a valve
failure; an oversized equalizer line has no failure modes. It also
increases the risk of freez-ing (or increases the energy used by
basin heaters) in the basins of inactive cells in systems that must
operate in cold weather. But this is often the best option when
there are many tower cells that are not located close together
(long equalizer lines).
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stead, waterside economizers must use an integrated piping
arrangement shown in Figure 4 for a primary-secondary system and
Figure 5 for a primary-only system. Integrated systems, which cost
only slightly more than non-integrated
Piping for Waterside EconomizersWaterside economizers are an
alternative to airside econ-
omizers. Airside economizers are usually more energy ef-ficient,
but they are not always practical and can be much more expensive.
Applications where waterside economiz-ers are often preferred
include floor-by-floor air handlers in a high-rise office building
or computer room air handlers serving a large data center. A
waterside economizer uses cold water generated at the cooling tower
to produce chilled water without, or with reduced, mechanical
refrigeration. This is accomplished by running the cooling towers
to produce water temperatures typically 45F (7C) and less during
periods of low ambient wet-bulb temperatures. The cold water is
pumped through a high effectiveness water-to-water heat exchanger,
usually a plate and frame type, to produce chilled water at
temperatures of 50F (10C) or less. The heat exchanger protects the
chilled water system from the corrosion, dirt and debris typical of
open circuit condenser water.
For detailed design guidance on sizing waterside economiz-er
heat exchangers and flow rates, see Stein.1
Figure 3 shows a non-integrated waterside economizer where the
economizer heat exchanger is piped in parallel with the chiller
evaporators on the chilled water side. This design allows the
economizer to operate only if the chill-ers are not operating and
vice versa; they cannot operate together. This design was the most
common when water-side economizers first became popular in the 80s,
but it is not very efficient and is no longer allowed to be used by
energy standards such as ASHRAE Standard 90.1.2 In-
Figure 3: Waterside economizer, non-integrated.
Cooling Tower No. 1
Cooling Tower No. 2
Chiller No. 1
Chiller No. 2
Plate and Frame Heat Exchanger
Figure 4: Waterside economizer, integrated,
primary-secondary.
Figure 5: Waterside economizer, integrated, primary-only.
Cooling Tower No. 1
Cooling Tower No. 2
Chiller No. 1
Chiller No. 2
Plate and Frame Heat Exchanger
Cooling Tower No. 1
Cooling Tower No. 2
Chiller No. 1
Chiller No. 2
Plate and Frame Heat Exchanger
Either Pump Or Valve
(Not Both)
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32 A SHRA E Jou rna l S e p t e m b e r 2 0 1 1
systems, allow simultaneous operation of the chillers and the
economizer because the heat exchanger is piped in se-ries with the
chiller evaporators on the chilled water side. The economizer can
provide some pre-cooling of the return chilled water temperature
even if it cannot provide all of the cooling. This substantially
extends the number of hours the economizer can be operational.
Figure 4 shows two options for how to provide flow through the
heat exchanger. The least expensive option is to place a
two-position valve in the chilled water return line. The valve
closes when the economizer is enabled and is open otherwise. This
option requires that secondary pumps have variable speed drives so
that they can slow down when the heat exchanger is out of the
circuit and vice versa. The secondary pumps generally do not need
to be sized for the added head of the heat exchanger since the heat
exchanger will be in the loop only when the economizer is active
and cooling loads (and flows) are low. If secondary pumps are
constant speed (rarely true in modern plants) or if the design flow
rate through the heat exchanger is much lower than the expected
chilled water flow during economizer operation, a sidestream pump
should be used instead of the two-position valve. This sidestream
pump is sized with enough head to
Figure 6: All-variable speed primary-only chilled water
plant.
VSD
Cooling Tower No. 1
Chiller No. 1
Chiller No. 2
Cooling Tower No. 2
VSD
VSD
VSD
VSD
VSDVSD
VSD
draw water out of the secondary return, pump it through the heat
exchanger then back to the return.
In both the integrated and non-integrated designs, the heat
exchanger is generally not provided with its own con-denser water
pumps. Since the load will be low when the weather is cold enough
for the towers to deliver cold water, it should not be necessary to
run all chillers, so one or more of the chiller condenser water
pumps can serve the heat ex-changer. The heat exchanger should be
selected so that its pressure drop is similar to the pressure drop
across chiller condensers.
When using waterside economizers, refrigerant head pres-sure
control is required because of the cold water coming off the
cooling tower. See the earlier discussion regarding head pressure
control options.
Variable Speed Condenser Water PumpsWith the ever-increasing
drive to improve plant effi-
ciency, there is more interest in all-variable speed chilled
water plants,3 which refers to plants with variable speed drives on
all components, including condenser water pumps (Figure 6). It is
common to find variable speed drives on cooling towers and chilled
water pumps and, in fact, they are required with few exceptions by
energy stan-dards such as Standard 90.1. Using variable speed
drives on
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chiller compressors is also more and more common as the cost
premium vs. fixed speed continues to fall. But vari-able speed
drives on condenser water pumps are relatively rare, and for good
reason: it is not clear that they are cost effective and the
required control logic is not self-apparent. For instance, as
condenser water flow falls, both pump energy and cooling tower
energy (for the same condenser water supply tem-perature) will
fall, but chiller energy will rise as leaving condenser water
temperature rises. The condenser leav-ing water temperature is
indicative of chiller condensing temperature and, therefore,
chiller efficiency; efficiency will vary little with changes to
con-denser supply water temperature at the same leaving water
temperature. With variable speed drives on the chiller compressor,
the impact of condenser
Figure 7: Denver chilled water plant energy use using three
control strategies.
1.4 Million
1.2 Million
1 Million
800,000
600,000
400,000
200,000
0
Ann
ual C
hill
ed W
ater
Pla
nt E
nerg
y U
se (
kWh
)
Chiller Tower CHWP CWP Plant Total
TOPPSTDOAK
temperature is even stronger and, in fact, these drives will
save no energy at all if leaving condenser water tempera-tures are
not driven down at low loads.
Clearly the optimum control logic will not be the same for all
plants. For instance, a plant with very efficient (high
gpm/horsepower) cooling towers will operate more effi-ciently by
driving condenser water temperatures down fur-ther than a plant
with inefficient towers. So what is the best control strategy? The
answer is it depends. A few authors have proposed theoretical
approaches to determining the optimum logic,4,5 but the techniques
are either difficult and time consuming to implement or require
proprietary con-trol logic.
As part of the development of the ASHRAE SDL refer-enced
earlier, studies were conducted to develop generalized optimum
control sequences for all-variable speed plants and to determine
life-cycle costs of various design alternatives. Our studies led to
two important conclusions about variable speed drives on condenser
water pumps:
1. They are life-cycle cost effective if optimum control
se-quences are used.
2. They can increase the energy use of the plant if not
opti-mally controlled.
The second conclusion is disturbing, in particular, because we
found that the difference was very subtle between the con-trol
logic that minimized energy use and that which increased use above
constant speed pumps. For example, Figure 7 shows energy use for a
plant serving an office building in Denver, using three control
strategies:
TOPP. This is the theoretical optimum plant performance of the
plant with variable speed condenser water pumps deter-mined using
the technique described in Reference 5. This is the theoretical
best performance possible.
STD. This is the performance of the plant with constant speed
condenser water pumps and cooling tower fans con-trolled to reset
supply water temperature per ARI Standard 550/5906 condenser water
relief curves. This is most indica-tive of conventional
practice.
OAK. This is the performance of the plant with variable speed
pumps controlled using control sequences that were found to provide
near-TOPP level performance for the same plant located in Oakland,
Calif., instead of Denver.
The figure shows that energy use is highest using control
sequences that provided near-ideal performance for the same plant
in another climate zone, significantly higher energy use than the
plant without pump variable speed drives. This dem-onstrates how
sensitive plant performance is to the details of the control logic.
So, variable speed drives should only be used on condenser water
pumps if the designer takes the time to en-sure that control
sequences are near-optimum. These sequenc-es will be discussed in
detail in Part 5 of this series of articles.
SummaryThis article is the second in a series of five that
summarize
chilled water plant design techniques intended to help
engi-neers optimize plant design and control with little or no
added engineering effort. In this article, condenser water system
pip-ing and distribution system options were discussed. In the next
article pipe sizing and optimizing T will be addressed.
References1. Stein, J. 2009. Waterside Economizing in Data
Centers:
Design and Control Considerations. ASHRAE Transactions
115(2):192 200.
2. ANSI/ASHRAE/IES Standard 90.1-2010, Energy Standard for
Buildings Except Low-Rise Residential Buildings.
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36 A SHRA E Jou rna l S e p t e m b e r 2 0 1 1
This series of articles will summarize the upcoming Self
Directed Learning (SDL) course called Fundamentals of Design and
Control of Central Chilled Water Plants and the research that was
performed to support its develop-ment. The series includes five
segments. Part One: Chilled Water Distribution System Selection was
published in July.
Pipe sizing and optimizing T. This article will discuss how to
size piping using life-cycle costs then how to use pipe sizing to
drive the selection of chilled water and con-denser water
temperature differences (Ts).
Chillers and cooling tower selection. This article will address
how to select chillers using performance bids and how to select
cooling tower type, control devices, tower efficiency, and wet-bulb
approach.
Central Chilled Water Plants Series Optimized control sequences.
The series will conclude with a discussion of how to optimally
control chilled water plants, focusing on all-variable speed
plants.
The intent of the SDL (and these articles) is to provide simple
yet accurate advice to help designers and oper-ators of chilled
water plants to optimize life-cycle costs without having to perform
rigorous and expensive life-cycle cost analyses for every plant. In
preparing the SDL, a significant amount of simulation, cost
estimating, and life-cycle cost analysis was performed on the most
common water-cooled plant configurations to determine how best to
design and control them. The result is a set of improved design
parameters and techniques that will provide much higher performing
chilled water plants than common rules-of-thumb and standard
practice.
3. Hartman, T. 2001. All-Variable Speed Centrifugal Chiller
Plants. ASHRAE Journal 43(9):43 51.
4. Hartman, T. 2005. Designing Efficient Systems with the Equal
Marginal Performance Principle. ASHRAE Journal 47(7):64 70.
5. Hydeman, M., G. Zhou. 2007. Optimizing Chilled Water Plant
Control. ASHRAE Journal 49(6):44 54.
6. ARI Standard 550/590-2003, Performance Rating of Water
Chill-ing Packages Using the Vapor Compression Cycle.