7/25/2019 ASHRAE-D-AJ11July01-20110705 http://slidepdf.com/reader/full/ashrae-d-aj11july01-20110705 1/12 14 ASHRAE Journal ashrae.org July 2011 T his is the rst of a series of articles discussing how to optimize the design and control of chilled water plants. The series will summarize ASHRAE’s Self-Directed Learning (SDL) course called Fundamentals of De- sign and Control of Central Chilled Water Plants and the research that was performed to support its development (see sidebar [Page 20] for 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 signicantly reduced plant life-cycle costs. A procedure was developed to provide near-optimum plantdesignfor mostchill- er plants including thefollowing steps: 1. Select chilled water distribution system; 2. Select chilled water temperatures, flow rate, and primary pipesizes; 3. Select condenser water distribution system; 4. Select condenser water tempera- tures, flow rate, and primary pipesizes; 5. Select cooling tower type, speed control option, efficiency, approach temperature, and make cooling tower selection; 6. Select chillers; 7. Finalize piping system design, cal- culate pump head, and select pumps; and 8. Develop and optimize control se- quences. Each of these steps is discussed in this series of five articles. This article discusses Step 1: Selecting chilled water distribution systemtype. Table 1 lists recommendations for the “life-cycle cost optimum” distri- bution system based on the size and number of loads served and the num- ber of chillers. “Life-cycle cost opti- mum” is in quotes because these rec- ommendations are generalizations that should apply to the majority of typical HVAC applications, but they may not prove to be optimum for every appli- cation and have not been rigorously proven as the best choice. They are based on the author’s design and com- missioning experience, the analysis that was done in conjunction with de- velopment of theASHRAE SDL , work done on an earlier chilled water plant design manual, 1 and the prescriptive About the Author Steven T. Taylor, P.E., is a principal at Taylor Engineering in Alameda, Calif. By Steven T. Taylor, P.E., Fellow ASHRAE Optimizing Design & Control Of Chilled Water PlantsPart 1: Chilled Water Distribution System Selection This article was published in ASHRAE Journal, July 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.
1 4 A S H R A E J o u r n a l a s h r a e . o r g J u l y 2 0 1 1
T
his is the rst of a series of articles discussing how to optimize the
design and control of chilled water plants. The series will summarize
ASHRAE’s Self-Directed Learning (SDL) course called Fundamentals of De-
sign and Control of Central Chilled Water Plants and the research that was
performed to support its development (see sidebar [Page 20] for 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 signicantly reduced plant life-cycle costs.
A procedure was developed to providenear-optimum plant design for most chill-er plants including the following steps:
1. Select chilled water distributionsystem;
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, speedcontrol option, efficiency, approachtemperature, and make cooling towerselection;
6. Select chillers;7. Finalize piping system design, cal-
culate pump head, and select pumps; and
8. Develop and optimize control se-quences.
Each of these steps is discussed in
this series of five articles. This articlediscusses Step 1: Selecting chilled waterdistribution system type.Table 1 lists recommendations for
the “life-cycle cost optimum” distri-bution system based on the size andnumber of loads served and the num-ber of chillers. “Life-cycle cost opti-mum” is in quotes because these rec-ommendations are generalizations thatshould apply to the majority of typicalHVAC applications, but they may not
prove to be optimum for every appli-cation and have not been rigorouslyproven as the best choice. They arebased on the author’s design and com-missioning experience, the analysisthat was done in conjunction with de-velopment of the ASHRAE SDL, workdone on an earlier chilled water plantdesign manual,1 and the prescriptive
About the Author
Steven T. Taylor, P.E., is a principal at Taylor
Engineering in Alameda, Calif.
By Steven T. Taylor, P.E., Fellow ASHRAE
Optimizing Design & Control
Of Chilled Water Plants Part 1: Chilled Water Distribution System Selection
This article was published in ASHRAE Journal, July 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 permissionof ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.
requirements of ASHRAE/IESNA Standard 90.1.2 The in-tent is to allow designers to select the system that is mostoften the best choice from a life-cycle cost perspective fora given application without having to perform any lengthyanalyses.
Primary-Only – Single Coil
With one or more chillers serving a single cooling coil (Fig- ure 1 ), the simplest design strategy is to not use any controlvalves at the coil. Instead, a constant-volume pump circulates
water between the chiller and the coil and supply air tempera-ture is controlled by resetting the temperature of the chilledwater leaving the chiller. While constant chilled water flowresults in constant pump energy, chiller performance is im-proved when the leaving chilled water temperature is resetto be as high as possible. A variable frequency drive (VFD)could also be added to the pump to make the system vari-able flow, but that adds cost and complexity. VFDs are sel-dom cost effective since pump power is typically small in asingle-coil plant because the chiller and coil are usually close-coupled (physically close together), and it is more efficient toincrease chilled water temperature than to reduce pump speed
and pump energy. (Part 5 of this series will further discussthe trade-off between resetting chilled water temperature andpump energy.)
Many engineers are concerned about causing high spacehumidity with this design because chilled water temperaturesmust be aggressively reset to maintain supply air temperatureat setpoint under low load conditions. But, in fact, that is nevera concern because the leaving supply air condition is about the
* In all of the figures shown in this article, multiple chillers are shown in parallel. For most applications, the chillers could alternatively be pipedin series. This results in lower chiller energy use, partly offset by higher pump energy use. However, first costs are almost always higher with
series piping due to larger piping and pumps and bypass piping typically provided to allow one chiller to operate while the other is down formaintenance. Because of limited funding, we did not evaluate the life-cycle costs of series piping and so we have not included it in our recom-
mendations. This option will be evaluated for cost effectiveness in future versions of the ASHRAE SDL.
same regardless of chilled water temperature. For example, ifthe supply air temperature leaving the coil is 55°F (13°C), theair leaving the coil is close to saturated whether the chilledwater supply temperature is 42°F (6°C) or 50°F (10°C). It is
the supply air temperature setpoint that determines space hu-midity conditions, not the chilled water temperature.Figure 1 shows a single chiller, but any number of chillers
can be used. When two chillers are used, this is a good applica-tion for piping chillers in series* rather than in parallel.Figure 1 also shows an optional storage tank. Chilled water
systems must have a sufficient volume of water in the pip-ing system to prevent unstable temperature swings, possibly
ApplicationCoils/Loads
ServedChillers Size of Coils/Loads Served Control Valves
Recommended
Distribution Type
1 One Any Any NoneFigure 1
Primary-Only – Single Coil
2 More Than One One Small (< ~100 gpm) 2-Way and 3-Way
Figure 2
Primary-Only – Single Chiller
3Few Coils Serving
Similar LoadsMore Than One Small (< ~100 gpm) 3-Way
Figure 3
Primary-Only – Multiple Chillers –
Few Coils With Similar Loads
4
Many Coils Serving
Similar Loads or
Any Serving
Dissimilar Loads
More Than One Small (< ~100 gpm) 2-Way
Figure 4
Primary-Only
or
Figure 5
Primary-Secondary
5 More Than One Any Large Campus 2-WayFigure 6
Primary – Distributed Secondary
6 More Than One Any Large Coils (> ~100 gpm) NoneFigure 7
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causing chiller short-cycling. This isa potential problem with single-coilsystems since they are typically close-coupled with only short piping runs. Tocompensate for the small water volumein piping, a small storage tank is often
required. The minimum water volumeshould be verified with the chiller manu-facturer.
Primary-only – Single Chiller
Small chilled water plants com-monly have a single chiller, typicallyair-cooled. Single chiller plants donot have to deal with flow and stagingproblems common to multiple chillerplants and thus can have a simpledistribution and control system. The
recommended design is shown inFig- ure 2 . I t is the simplest variable flowprimary-only system. Two-way valvesare installed at most coils with justenough three-way valves installedto maintain the minimum flow re-quired by the chiller. This minimumrate, which can be obtained from themanufacturer, will vary with designchilled water flow rate and the chillertype, size, and manufacturer but istypically 25% to 50% of the design
flow. A VFD is shown in Figure 2 ;
VFDs are typically cost effectiveexcept on very small systems. Notethat Standard 90.1 requires VFDs onchilled water pumps exceeding 5 hp(3.7 kW). The VFD is controlled bya differential pressure (DP) sensor
located near the most remote coil sothat the DP setpoint can be as low aspossible; this is also a requirementof Standard 90.1. Locating the sen-sor near the pump requires a high DPsetpoint and eliminates most of theenergy savings from the VFD. The three-way valves should be lo-
cated near the chiller if the pump hasa VFD to minimize pump energy. Lo-cating them remotely increases flow tothe extremes of the system, which in-
creases the pump pressure and powerrequired. The one exception to this ruleis that three-way valves must be locatedin a manner that engages enough watervolume to maintain the minimum wa-ter volume required to minimize shortcycling as discussed previously. Thereis usually no benefit to locating three-way valves remotely to keep the systemcold so that chilled water is instantlyavailable to coils; it typically takes justseconds or perhaps minutes for water
to travel from the chiller to the most re-
Figure 2: Primary-only – single chiller.
Chiller
Supply WaterTemperature
CHW
Pump
3-Way ValveCoil
DP Sensor
2-Way Valve
Chiller No. 1
Supply WaterTemperature
CHW
Pumps
3-WayValve
Coil
Chiller No. 2
Figure 3: Primary-only – multiple-chill-
ers – few coils with similar loads.
VFD
mote coil, and the load will not be lost inthat short time.
Primary-Only – Multiple Chillers:
Few Coils With Similar Loads
When systems have multiple chillers,chiller staging can be a problem whenflow and load do not track, and theygenerally do not when three-way valvesare used. Consider the system shownin Figure 3. When the system operatesnear full load, performance is satisfac-tory since both chillers and pumps areoperating. However, the system canhave problems during part-load condi-tions depending how coil loads vary.For example, suppose the system shownin Figure 3 had two equally sized chill-ers and served two equally sized coils,each serving a hotel ballroom. If therewere functions in both rooms and bothrooms were above 50% load, the systemoperates well; both chillers with their as-sociated pumps will run and each func-tion space will receive its design flow.But when only one of the two functionspaces is occupied and the other is va-cant, the system, as a whole, will bebelow half load, so in theory only onechiller and pump could satisfy the load.
However, the coil serving the unoccu-pied room will still use its design flow,bypassing it around the coil to the re-turn. If the plant operates with only onechiller and pump, it has sufficient chillercapacity to meet the load, but it cannotmeet the flow demands; the coil serv-ing the occupied meeting room will bestarved of flow. To avoid this problem,both pumps will have to operate, so bothchillers will have to operate at or below50% load. This problem is one of the
reasons designers have migrated to vari-able flow designs, discussed later.
But this system can work well as longas all coil loads tend to vary in the sameproportion, as they might if all coilsserved similar occupancies (e.g., allserve offices on the same schedule). Forinstance, if the coils served are belowhalf load and only one chiller and pumpare operating, all coils will be capable ofmeeting their loads. The system is thus aquasi-variable-flow system in that pumps
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the loads vary similarly, chilled watertemperature may be reset aggressively,which allows the plant to be about asefficient as one of the true variable flowsystems discussed later. This system is a
reasonable choice for small applicationswith only a few coils serving similarloads; it is simple and inexpensive andavoids all of the complexities of vari-able flow systems. Also, small systemslike this are typically close-coupled, sothere is not much pump energy to save.Note that Standard 90.1 only allows thisapproach for systems with three coils orfewer or a total chilled water pump sys-tem power of 10 hp (10.7 kW) and less.
Primary-Only Variable Flow and
Primary-Secondary Systems
Application 4 in Table 1 is probablythe most common. It applies to systemsserving many small coils (or a few coilswith dissimilar loads) and more thanone chiller. In this case, either of twosystems is recommended, primary-onlyvariable flow (Figure 4 ) or primary-sec-ondary (Figure 5 ). Both systems haveplusses and minuses, discussed in detailin an article by Taylor3 and summarizedinTable 2 .
Primary-only systems always cost lessand take up less space than primary-secondary systems, and with variablespeed drives, primary-only systems alsoalways use less pump energy than tradi-tional primary-secondary systems. The pump energy savingsare due to:
• Reduced system head as a result of the elimination of theextra set of pumps and related piping and devices (shut-offvalves, strainers, suction diffusers, check valves, etc.).
• More efficient pumps. The primary pumps in the primary-secondary system will be inherently less efficient due to their
high flow and low head. This can be partially mitigated byusing larger pumps running at lower speed, but at an increasein first costs.
• Variable flow through the evaporator, which allows flow todrop below design flow down to some minimum flow rate pre-scribed by the chiller manufacturer. VFDs can be added to theprimary pumps of a primary-secondary system and controlledto track secondary flow down to the chiller minimum rate, butat an increase in first costs and control complexity. The lower energy costs and lower first costs of the prima-
ry-only system often make it an easy choice versus primary-secondary, but the system does have two significant disad-
A bypass valve (Figure 4 ) is required to ensure that mini-mum flow rates are maintained through operating chillers. Thevalve must be automatically controlled by flow, typically usinga flow meter in the primary circuit (as shown in Figure 4 ) ordifferential pressure sensors across chillers correlated to flow.
The flow meter is more costly but is more easily adapted into
plant load (Btu) calculations, which will be necessary for op-timum chiller staging (discussed in Part 5).
Selecting the bypass control valve and tuning the controlloop is sometimes difficult because of the widely rangingdifferential pressure across the valve caused by its locationnear the pumps. The valve must be large enough to bypass theminimum chiller flow through it with a pressure drop as low asthe differential pressure setpoint used to control chilled waterpump VSDs. This is because if only a few valves are openin the system, the pressure at the DP sensor location will bewhat is available at the plant as well since there is little pres-sure drop between the two points due to the low flow. But this
makes the valve oversized for other flow scenarios that can
Advantages of Primary-Only Disadvantages of Primary-Only
Lower First Costs
Less Plant Space Required
Reduced Pump Peak Power
Lower Pump Annual Energy Use
Complexity of Bypass Control
Complexity of Staging Chillers
Table 2: Advantages and disadvantages of primary-only vs. primary-secondary systems.
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occur, so tuning can be difficult. If the control loop is unstable,cold chilled water supply can be fed back into the return inter-mittently and cause chillers to cycle off due to low load or coldsupply water temperatures. But if the loop is too slow, it maynot respond quickly enough to sudden changes in flow (e.g.,
when a large number of air-handling units shut off at the sametime), causing insufficient flow through the chillers, causingthem to trip on low flow or low temperature.
Complex control systems are prone to failure, so at somepoint in the life of the plant, one can expect the bypass con-trol to fail. A failure of the bypass system can cause nuisancechiller trips, which generally require a manual reset. If an op-erator is not present to reset the chiller, the plant can be out ofservice for some time.
2. Staging Control Complexities
When one or more chillers are operating and another chilleris started by abruptly opening its isolation valve (or starting itspump for dedicated pumps), flow through the operating chill-ers will abruptly drop. The reason for this is simple: flow isdetermined by the demand of the chilled water coils as con-trolled by their control valves. Starting another chiller will notcreate an increase in required flow, so flow will be split amongthe active machines. If this occurs suddenly, the drop in flowwill cause operating chillers to trip. To stage the chillers without a trip, active chillers must first
be temporarily unloaded (demand-limited or setpoint raised),then flow must be slowly increased through the new chillerby slowly opening its isolation valve. Then, all chillers can beallowed to ramp up to the required load together. During the
staging sequence, chilled water temperatures will rise some-what. This is seldom a problem in comfort applications, butmay be an issue for some industrial applications.
Given these considerations, primary-only systems are mostappropriate for:
• Plants with many chillers (more than three) and with fairlyhigh base loads, as might be expected in an industrial or datacenter application. For these plants, the need for bypass isminimal or nil due to the high base loads, and flow fluctuationsduring staging are small due to the large number of chillers.
• Plants where design engineers and future on-site opera-tors understand the complexity of the controls and the need to
maintain them. The primary-secondary system may be a better choice for
buildings where fail-safe operation is essential or on-site op-erating staff is unsophisticated or nonexistent.
Primary – Distributed Secondary
For plants serving groups of large loads such as buildings ina college campus, terminals in an airport, etc., the primary-dis-tributed secondary system (Figure 6 ) is usually the best solu-tion. The secondary pumps at the central plant are deleted andvariable speed pumps are added at each building. The build-ing pumps are controlled by differential pressure sensors at the
most remote coil in each building. Building pump heads are
This series of articles will summarize the upcomingSelf-Directed Learning (SDL) course called Funda-
mentals of Design and Control of Central Chil led Water
Plants and the research that was performed to support itsdevelopment. The series will include five segments:
Chilled water distribution system selection. Thisarticle will discuss distribution system options, such asprimary-secondary and primary-only pumping, and pro-vide a simple application matrix to assist in selecting thebest system for the most common applications.
Condenser water distribution system selection. Thisarticle will discuss piping arrangements for chiller-con-densers and cooling towers, including the use of variablespeed condenser water pumps and water-side economiz-ers.
Pipe sizing and optimizingΔT . This article will dis-cuss how to size piping using life-cycle costs then how touse pipe sizing to drive the selection of chilled water andcondenser water temperature differences (ΔT s).
Chillers and cooling tower selection. This article willaddress how to select chillers using performance bids andhow to select cooling tower type, control devices, towerefficiency, and wet-bulb approach.
Optimized control sequences. The series will con-clude with a discussion of how to optimally control chilledwater 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 opera-tors of chilled water plants to optimize life-cycle costswithout having to perform rigorous and expensive life-cycle cost analyses for every plant. In preparing the SDL,a significant amount of simulation, cost estimating, andlife-cycle cost analysis was performed on the most com-mon water-cooled plant configurations to determine howbest to design and control them. The result is a set of im-proved design parameters and techniques that will pro-vide much higher performing chilled water plants thancommon rules-of-thumb and standard practice.
Central Chilled Water Plants Series
sized for the pressure drop of the loop from the plant, to thebuilding, through the building’s coils, then back to the plantthrough the common leg. Therefore, each pump has a differenthead customized for the building. The advantages of this design compared to conventional
primary-secondary and primary-secondary-tertiary systemsinclude:
• Overall pump horsepower is reduced. With the conven-tional system, secondary pump head must be sized for themost remote building (say 100 ft [299 kPa]) while the distrib-uted building pumps close to the central plant can have much
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• The system is self-balancing via thespeed controls on the secondary pumps.
There is no need to throttle pressure atclose buildings and flow self-adjustsover time as additional buildings areconnected to the system.
• Overpressurization of control valveslocated near the central plant is elimi-nated. With large, high head second-
ary systems, these valves must operateagainst excess differential pressure,which can reduce controllability andmay even force flow through the valve ifit does not have sufficient shut-off head.
• Pump energy is reduced because ofthe custom pump heads and the moreprecise control of the variable speeddrives. With a conventional primary-secondary system, the secondary pumpsare typically controlled to maintain dif-ferential pressure at the entry to the most
remote building. Therefore, the setpointmust be higher than for the distributedpump system, which is controlled bydifferential pressure at the most remotecoil in each building. At part load, thepumps therefore can operate at slowerspeeds.
• With primary-secondary-tertiary sys-tems, the tertiary pumps are generallypiped with a bridge and two-way controlvalve. Control of the bridge is alwaysdifficult and, if done incorrectly, often
the cause of degradingDT .
4
With this
Figure 6: Primary-distributed secondary.
Building A Building B
CHWSupply
CHWSupply
Primary
Pumps
Chiller N
Chiller 2
Chiller 1
Common Leg
SecondaryPump
VFD VFD
distributed pumping system, bridge con-nections are eliminated.
• The system will be less expensive,more energy efficient, and have lowermaintenance costs than primary-second-ary-tertiary systems.
Disadvantages include: • Expansion tank pressurization may
have to be increased to maintain posi-
tive suction pressure at building pumpsif the pumps are located at the top ofcampus buildings. This has only a mi-nor cost impact to the expansion tank.
Figure 7: Primary-coil secondary.
VFD
VFD
Chiller 1
Primary
Pumps
Coil
Chiller 2
Coil
Pump
AHU 1(Large Coil)
AHU 2(Large Coil)
AHU 3(Large Coil)
VFD VFD VFDChiller 1
Chiller 2
Common Leg
VFD VFD
Figure 8: Hybrid primary-coil secondary and primary-secondary system.
• Primary-distributed secondary sys-tems will usually cost more than con-
ventional primary-secondary systemsbecause there are more pumps andspace is required to house them in eachbuilding.
For plants serving large individual air-handling systems,using distributed variable speed driven coil secondary pumps(Figure 7, Page 22) is usually the best solution. The advantages of this design compared to a conventional
primary-secondary system include: • Connected pump motor hp is reduced. This is due in part be-cause of the customized heads for each pump but also because the
control valve is eliminated. Two-way control valves are typicallyselected for a wide-open pressure drop of 4 to 5 psi (27.5 kPa to34.4 kPa), about 10 ft (29.9 kPa). This is a substantial savings.
• The system is self-balancing. There is no need for bal-ancing valves of any kind nor are there any advantages to
self-balancing designs such as reverse-return arrangements. • Pump energy is significantly lower with this design. This is due mostly to the reduced pump heads but also
because there is no need to maintaina minimum differential pressure inthe system as there is with conven-tional secondary pumps. Because ofthis minimum DP and because of thethrottling caused by partially closedcontrol valves, conventional second-ary pumps will not follow the theo-retical parabolic system curve. Hence,pump eff iciency will generally getworse, particularly at low load. Withthe variable speed coil pump design,there are no control valves or mini-mum DP, so pump eff iciency will benearly constant.
• Control of large control valvesis inherently slow due to the size andslow responsiveness of the valve. Withthe coil pump design, flow can be con-trolled almost instantaneously with theVFD, so control is precise. There isalso no fear of over-pressurizing con-
trol valves, which reduces their con-trollability.
• Because of the eliminated controlvalves and lower pump HP, this systemwill generally have lower costs than aconventional primary-secondary sys-tem. It is usually a little more expensivethan a primary-only system.
Control valves can be thought of asbrakes on a car while pumps are thecar engines; from an energy perspec-tive, it never makes sense to press both
the brake and the accelerator pedals atthe same time, but that is effectivelywhat systems with control valves do.So, this system is actually ideal froma pumping perspective: it has no“brakes.”
Unfortunately, there are a few dis-advantages of this system. First, allcoils must have a pump. If a coil wereconnected to the secondary circuitwithout a pump, flow through the coilwill be backwards from the return to
mixture of small coils and large coils, pumps for the smallcoils will most likely have to be expensive multistagepumps.
Another disadvantage is the increased exposure toequipment failure. A control valve is extremely reliable—
the pump and VFD in this design are more likely to fail.Duplex pumps could be used to improve redundancy, butthe cost is prohibitive in most situations. Our philosophyis to provide the same level of redun-dancy as the rest of the system served.For instance, if the air handler hasonly a single fan, then it makes senseto provide only a single pump. Formore critical applications, redundantpumps, or an alternative distributionsystem design, should be considered.
For this design to be energy eff i-cient, coils must be large due to theinherent ineff iciency of small pumps,particularly low flow/high headpumps. For instance, a typical pumpat 60 ft of head (179 kPa) will havean efficiency on the order 30% at 20gpm (1.3 L/s), 50% at 50 gpm (3.2L/s), 60% at 100 gpm (6.3 L/s), and70% at 200 gpm (12.6 L/s). That iswhy this system is recommended onlyfor coils with flows greater than 100gpm (6.3 L/s) in Table 1 . This flowlimit is obviously a rough rule-of-
thumb since efficiency wil l vary overa range, not drop abruptly below 100gpm (6.3 L/s).
If a project includes both small andlarge coils, a hybrid system of both dis-tributed coil pumps and conventionalsecondary pumps to serve small coils ispossible. See Figure 8, Page 22, for anexample hybrid plant.
Summary
This article is the first in a series of
five that summarize chilled water plantdesign techniques intended to help en-gineers optimize plant design and con-trol with little or no added engineeringeffort. In this article, a simple look-uptable is provided to allow designers toselect a “near optimum” chilled wa-ter distribution system based on theirapplication without having to do anyrigorous life-cycle cost or systemanalysis. Next month, condenser waterdistribution system selection will be
addressed.
References1. Energy Design Resources. 1999. CoolTools™ Chil led Water
Plant Design Guide.
2. ASHRAE Standard 90.1-2007, Energy Standard for Buil dingsExcept Low-Rise Residential Buil dings.
3. Taylor, S. 2002. “Primary-only vs. primary-secondary variable
flow systems.”ASHRAE Journal 44(2).4. Taylor, S. 2002. “Degrading chilled water plant delta-T: causes
