60 ASHRAE Journal ashrae.org March 2012 T his is the fourth 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 Design and Control of Central Chilled Water Plants and the research that was performed to support its development. See sidebar, Page 69, 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 chiller plants including the following steps: 1. Select chilled water distribution sys- tem; 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 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 dis- cusses steps 5 and 6. Cooling Tower Selection Cooling tower characteristics the de- signer must select and define are de- scribed in the following paragraphs. Each is briefly discussed, but this article focusses on the last two variables, effi- ciency and approach temperature. • Open vs. closed circuit. This discus- sion is limited to open circuit towers. Closed circuit towers are seldom used 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 4: Chiller & Cooling Tower Selection This article was published in ASHRAE Journal, March 2012. Copyright 2012 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.
Transcript
60 AS HRAE Jou rna l ash rae .o rg M a r c h 2 0 1 2
This is the fourth 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
Design and Control of Central Chilled Water Plants and the research
that was performed to support its development. See sidebar, Page 69,
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 chiller plants including the following steps:
1. Select chilled water distribution sys-tem;
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 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; and8. Develop and optimize control se-
quences.Each of these steps is discussed in this
series of five articles. This article dis-cusses steps 5 and 6.
Cooling Tower SelectionCooling tower characteristics the de-
signer must select and define are de-scribed in the following paragraphs. Each is briefly discussed, but this article focusses on the last two variables, effi-ciency and approach temperature.
• Open vs. closed circuit. This discus-sion is limited to open circuit towers. Closed circuit towers are seldom used
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 4: Chiller & Cooling Tower Selection
This article was published in ASHRAE Journal, March 2012. Copyright 2012 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.
March 2012 ASHRAE Jou rna l 61
for chiller plants due to higher costs and reduced ef-ficiency due to the added approach of the heat ex-changer and higher tower fan energy due to the heat exchanger pressure drop.
• Propeller vs. centrifugal fans. Propeller fans are almost always preferred due to much higher efficien-cy (they use about half the power of centrifugal fans) and lower costs. Centrifugal fans are also limited to systems smaller than about 1,100 gpm (69 L/s) per ASHRAE/IESNA Standard 90.1 prescriptive re-quirements.
• Draw-through vs. blow-through fan arrangement. Most propeller-fan towers are draw-through with top discharge. This results in a high exit velocity, which reduces the possibility of recirculation of tower ef-fluent, and the tower mass below the fan reduces fan sound transmission into the occupied space that is often below the tower.
• Cross-flow vs. counter-flow arrangement. The flow arrangement describes whether airflow through
95°F (35°C) to 85°F (29.4°C) at 75°F (23.9°C) ambient wet-bulb temperature divided by the tower fan motor horsepower (kW). Optimum tower efficiency is discussed further below.
• Approach temperature. The tower approach temperature is the difference between the temperature of the water leaving the tower and the ambient wet-bulb temperature. Optimum ap-proach is discussed further below.
To determine optimum DT, efficiency, and approach tem-perature, a large office building chilled water plant was ana-lyzed as part of the ASHRAE self-directed learning course that is the basis of this series of articles. Utility costs and life-cycle cost assumptions are those used in the evaluation of energy conservation measures for Standard 90.1-2010 ($0.094/kWh average electricity costs and 14 scalar ratio1 [the scalar ratio is essentially the maximum simple payback period]). The plant was modeled in great detail (including real equipment and piping costs) for three climates: Oak-land, Calif., Albuquerque, N.M., and Chicago. Additional analyses for optimum approach temperature were made for Miami, Las Vegas, and Atlanta. The condenser water system was designed, cost estimated, and modeled at all permuta-tions of the following design parameters:
• Condenser water DTs of approximately 9°F, 12.5°F and 15°F (5°C, 6.9°C and 8.3°C). In Part 3 of this series, a 15°F (8.3°C) condenser water DT was found to be the life-cycle cost optimum for all climate zones, tower sizes, and tower ef-ficiencies analyzed.
• Three ranges of tower efficiencies: “low” was the least ef-ficient available for the cross-flow propeller fan tower series analyzed with efficiencies ranging from 45 to 60 gpm/hp (3.8 to 5.1 L/s·kW); “medium” with efficiencies ranging from 65 to 75 gpm/hp (5.5 to 6.4 L/s·kW); and “high” with efficiencies ranging from 80 to 100 gpm/hp (6.8 to 8.5 L/s·kW). Note that even the “low” efficiency towers are significantly more effi-
the tower is sideways across the water flowing downward through the tower fill or upwards, counter to the water flow direction. Counter-flow towers are usually a bit less expen-sive, but both arrangements are effective. The selection is often driven by the physical constraints of the tower location: cross-flow towers tend to have a low profile but a large foot-print, while counter-flow towers are the opposite.
• Single-speed vs. two-speed vs. pony motors vs. variable speed motors. Variable speed drives (VSDs) are the pre-ferred approach to fan control since they minimize energy costs, reduce belt wear due to soft start, and provide the most stable condenser water temperature control compared to other methods. VSD costs are now low enough that they are clearly cost effective, and often even lower cost, than alterna-tives such as two-speed and pony motors.
• Gear vs. belt drive. Gear drives cost more but reduce maintenance frequency and may reduce maintenance costs compared to belt drives. But the increasing popularity of VSDs has also increased the popularity of belt drives; the belts last longer due to the soft start feature of VSDs, and belt drives allow near zero minimum speeds while gear drives require a minimum speed of approximately 20% to ensure adequate lubrication. Lower minimum speed reduc-es the wear-and-tear of fan cycling, reduces noise levels and abrupt changes in noise levels, and improves energy efficiency.
• Temperature range. Tower range is the difference be-tween the temperature of the water entering and leaving the tower, also known as condenser water DT. Optimum con-denser water DT was discussed in Part 3 of this series of articles.
• Efficiency. Cooling tower efficiency, expressed in gpm/hp (L/s·kW), is defined by ASHRAE Standard 90.1 as the maximum flow rate in gpm (L/s) that the tower can cool from
High Medium Low
3 Million
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Life-Cycle Energy Cost Chillers Towers & VFDs
CW Piping CW Pumps & VFDs
Figure 1: Life-cycle costs of 1,000 ton (3157 kW) chilled plant serving a Miami office building as a function of tower efficiency range.
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cient than the Standard 90.1 minimum of 38.2 gpm/hp (3.2 L/s·kW).
• Tower approach temperatures ranging from 2.5°F to 11°F (1.4°C to 6.1°C) based on actual tow-er selections for a cross-flow propeller fan tower series.
Tower costs were based on manufacturer’s price plus sales tax and contractor markup, plus a 50% premium that is intended to estimate the secondary installed cost impact of larger towers. Tower size (both footprint and height) and weight increase with increasing efficiency and decreasing approach. Both can impact tower installed costs depending on tower location. The actual premium can vary from close to nothing for a tower located on grade to a significant premium if the tower is located on the roof and re-quires architectural screening to hide the tower from view since larger towers will require additional struc-tural work and larger screens. The 50% premium is probably conservative; in most cases we believe the premium will be lower.
Figure 1, Page 61, shows life-cycle costs for the three ranges of cooling tower efficiency for an office building
in Miami with a tower range of 15°F (8.3°C); life-cycle costs were minimized with the high efficiency towers. This
300,000
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0To
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$)0 2 4 6 8 10 12 14
Approach (°F)
Oakland Albuquerque Chicago × Miami
Figure 2: Cooling tower installed costs for a 1,000 ton (3157 kW) chilled plant as a function of tower approach.
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March 2012 ASHRAE Jou rna l 63
was true for all climate zones, range, and approach temperatures analyzed. The analyses were based on a fairly aggressive scalar ratio (maximum sim-ple payback period) of 14 but high-efficiency tow-ers were found to be cost effective down to a scalar ratio of about five, i.e. they will have a simple pay-back of five years compared to the next best tower option, even in the mildest climates. The reason is that the net cost premium for increasing tower efficiency is relatively small; physical size and fill area of the tower increase but motor and VSD size and cost decrease, partially offsetting the tower cost increase. For example, the net installed first cost add for the high efficiency tower vs. the low efficiency tower for the 1,000 ton (3517 kW) plant in Miami was only about $9,000, a 6% increase, while annual energy savings were about $5,500 and life-cycle energy savings were $7,700. The magnitude of the savings is smaller in milder cli-mates, but the high efficiency towers were found to be cost effective in all climates analyzed.
study), the same cannot be said for reducing tower approach. As shown in Figure 2, the cost of a tower that provides a low
While increasing tower efficiency from low to high is rel-atively inexpensive (about 6% to 12% of tower costs in this
Figure 3: Life-cycle cost optimum approach plus range as a function of cooling degree-days with a base of 50°F (10°C).
64 AS HRAE Jou rna l ash rae .o rg M a r c h 2 0 1 2
Figure 4: Example system is an all-variable speed plant with three chillers.
VSD
Cooling Tower 5
Cooling Tower 1
VSD
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Cooling Tower 2
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Cooling Tower 4
Chiller 3
Chiller 2
Chiller 1
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approaches will be cost effective for occupancies with longer annual operating hours and higher loads, such as data centers. Higher efficiency and lower approaches may also be cost ef-fective for systems with water-side economizers; this study and resulting recommendations are based on systems with air-side economizers.
Chiller SelectionIn all other parts of this series of articles and the ASHRAE
SDL upon which it is based, the design guidance has been geared to providing near-optimum plant design from a life-cycle cost perspective with minimal or no added engineering time. Unfortunately, chiller selection is one area where this is not possible. To make an optimum selection, significantly more analysis is required than is typical of standard practice. This is because large water-cooled chillers are custom manu-factured products, not off-the-shelf items with a fixed design, which makes selection very difficult. Not only are there of-ten major design options such as centrifugal vs. screw com-pressors, refrigerants R-123 vs. R-134a, constant vs. variable speed, and conventional vs. magnetic bearings, there can be literally hundreds of combinations of evaporator, condenser, and compressor options for a given capacity that can radically affect chiller price and performance. To make the best choice among these options, the following approach is recommended:
1. Calculate the required plant total capacity and design temperatures and flow rates.
approach of 2°F to 3°F (1.1°C to 1.6°C) can be 60% higher than a tower providing a 10°F to 12°F (5.5°C to 6.6°C) approach.
A reasonable correlation was found between the sum of the life-cycle cost optimum tower approach (TA) (again using ASHRAE/IESNA Standard 90.1 energy costs and scalar ra-tio) and tower range (aka condenser water temperature differ-ence, DTCW) and cooling degree-days base 50 (CDD50). This is shown in Figure 3, Page 63, for each of the climate zones tested. The straight line curve fit is approximately:
TA+DTCW = 27 – 0.001CDD50 (1)
Solving for approach: TA = 27 – DTCW – 0.001CDD50 (2)
Based on the limited data, the approach determined from this equation should be limited to no more than 9.5°F (5.3°C) and no less than 2.5°F (1.4°C).
In summary, based on this analysis, the following design criteria are recommended for selecting cooling towers for of-fice buildings and buildings with similar load profiles:
1. Tower efficiency should be 80 gpm/hp (6.8 L/s·kW) or greater.
2. Tower approach should be selected using Equation 2, with a minimum of 2.5°F (1.4°C) and a maximum of 9.5°F (5.3°C).
While occupancy types other than offices were not explic-itly analyzed, we expect that even higher efficiency and lower
March 2012 ASHRAE Jou rna l 65
Example: Chiller and Cooling Tower SelectionThe recommended performance bid procedure was ap-
plied to the project shown in Figure 4 (Page 64). The all-vari-able speed plant includes three 775 ton (2,726 kW) chillers and serves a large office complex and data center. On past projects, chillers have been bid without specifying individual size, just overall plant size and redundancy requirements, if any. In this case, the 24/7 data center loads were such that three equally sized chillers fit well and thus were stipulated in the bid. Bidders were allowed to choose the condenser water flow rate and temperatures that associated with three differ-ent pipe sizes and provided roughly a 9.2°F, 12°F, and 19°F (5.1°C, 6.7°C, and 10.5°C) tower range. Condenser water temperatures were specified based on the performance of the previously selected cooling towers. The three range op-tions required that adjustments had to be made to the bids to reflect the different piping and pump sizes. While not re-quired by the specifications, variable speed drives were in-cluded in all proposals. Simulations of the various bids were run in a special version of DOE-2.2 that accurately modeled chilled water temperature reset and integrated waterside economizer operation, features that were not available in the standard version of DOE-2.2 at the time.
The first cost, energy costs, and life-cycle cost results are shown in Table 1. The chiller option with the lowest life-cycle
costs was Chiller A-1. It also had the lowest energy costs, but it was the most expensive of the 11 options evaluat-ed. Chiller B-1 also performed very well and, in this case, was considered equal to Chiller A-1 from a life-cycle cost standpoint; it could be made to rank best with reasonable changes to assumptions such as energy escalation rates, dis-count rates, and data center load. With these two chillers considered equal with respect to life-cycle costs, the selec-tion came down to soft factors. Ultimately, Chiller B-1 was selected due to:
• Chiller B-1 had lower first costs by about $150,000 compared to Chiller A-1. Future energy savings pro-jections are just that, projections, but first costs are very precise and very real. It is therefore not uncom-mon for owners to select the lowest first cost option among two options that have nearly equal life-cycle costs.
• Chiller B-1 required less space than Chiller A-1 both for the chillers themselves and also for condenser wa-ter pumps and piping since Chiller B-1 was selected based on the 12°F (6.7°C) DT option while Chiller A-1 used the 9.2°F (5.1°C) option. The mechanical room was space-limited so the smaller size was a significant factor in the selection.
5. Estimate energy usage of options with a detailed computer model of the building/plant. Commonly used modeling pro-grams are EnergyPlus and DOE-2.
6. Estimate maintenance cost differences between options.7. Calculate life-cycle costs.8. Select the chiller option with the lowest life-cycle cost.9. Hard-spec the selected chiller (no substitutions) and in-
clude contractor price in specifications. This performance-based bid approach should take place
once plant capacity and the building load profile are well de-fined, typically at the end of design development phase. It has been successfully used on dozens of projects including those that require competitive bidding, such as most state and fed-eral government projects. This is because most of the statutes mandate only that competitive bids occur, not that they occur at the traditional bid time when the design is complete, and most allow metrics such as low life-cycle cost (not low first costs) to be the basis of selection provided the selection crite-ria are well defined.
The recommended approach has many advantages: • The owner generally benefits from lower life-cycle costs. • Arbitrary selection of chiller vendor and model is elimi-
nated, potentially lowering chiller costs due to a more com-petitive bid process.
• The procedure generally results in a more energy-efficient chiller selection. The traditional approach often leads to a low cost mentality where the least expensive, and often least ef-ficient, chiller is selected. The opposite also can be true: the selection might be the top-of-the-line chiller that is efficient but too expensive to be life-cycle cost justified.
• Chiller vendors can make proposals that take advantage of their systems’ strengths or “sweet spots” both for cost and ef-ficiency. The conventional approach, where size and efficiency are more arbitrarily selected, usually favors one vendor who happens to have a “sweet spot” for the selected criteria.
• Chiller selections are finalized in the design stage. Selec-tion at this time allows the designer to customize the design of the plant, including physical layout and chiller-specific design parameters such as minimum flow (which affects minimum flow bypass line and valve size on primary variable flow sys-tems) knowing that the chiller selection will not be changed at project bid time. The traditional approach can result in sub-stitutions at bid time that can lead to coordination issues (and costs) due to changes in size, weight, peak power, minimum flow rates, piping connections (even pass vs. odd pass), etc.
There are also disadvantages to this approach: • It takes more time, both for the engineer and the equip-
• Assumptions made in the energy calculations may prove to be incorrect. For instance, utility rates, internal loads, and occupancy patterns assumed in the analysis may not be correct or may change over time. Energy escalation and inflation rates used in the analysis also have consider-able uncertainty.
• The computer model used to calculate energy usage is im-perfect in the way it models building and system loads, the chiller plant and pumping systems, and plant control strate-gies. Optimum plant control strategies are often complex and can vary from one chiller option to another. These strategies cannot always be modeled accurately by existing energy simu-lation tools.
• The benefits of long-term product reliability and vendor support are seldom included in the life-cycle cost calculations because their cost benefits are difficult to estimate, although they may be significant.
Details of how to implement this approach can be found in the ASHRAE SDL course upon which this series of articles is based and in the CoolTools Chilled Water Plant Design Guide.2 This article includes only a summary of the key items.
In the conventional procurement approach, a chiller is typ-ically specified by capacity, efficiency, construction details,
etc. These details, however, are not usually appropriate for a performance specification. For instance, the specification should not specify chiller efficiency or whether the chiller must have a variable speed drive or magnetic bearings. Ven-dors should be encouraged to propose as many chiller op-tions as possible without constraints; the life-cycle cost anal-ysis will determine which option is the best so constraints are not necessary and may inadvertently eliminate better choices. In general, the specifications should list only mini-mally required details such as total capacity, evaporator and condenser design conditions, and any application constraints such as redundancy and space and noise limits. An example specification is included in the referenced manuals.
To fairly evaluate chiller performance, accurate models must be created of each proposed chiller. Chiller perfor-mance data must be collected over a wide range of operat-ing conditions so that an accurate regression model can be made for use in the energy model of the building and plant. A spreadsheet for data collection and creating EnergyPlus and DOE-2 models can be found in the referenced manu-als. The form includes entries for pricing and also generates full load and part load performance data request forms based on design conditions that the chiller vendor must complete.
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March 2012 ASHRAE Jou rna l 69
The performance data request forms are designed to ensure that all operating conditions seen in the model are within the bounds of the collected data so that regression models do not need to extrapolate outside the data range provided by the manufacturer.
The plant modeling software also must be capable of mod-eling the impact on overall plant energy use (including pumps
and towers) of chiller characteristics such as pressure drop across evaporators and condensers, minimum chilled water flow rates on variable flow systems, and minimum require-ments for lift (the difference between leaving condenser water temperature and leaving chilled water temperature). It also must be able to model various control strategies for operating cooling tower fans, staging chillers and pumps, and (for vari-
This series of articles summarizes the upcoming Self Directed Learning (SDL) course called Fundamentals of Design and Con-trol of Central Chilled Water Plants and the research that was performed to support its development. The series includes five segments. Part One: “Chilled Water Distribution System Selec-tion” (July 2011), Part 2: “Condenser Water System Design” (September 2011), and Part 3: “Pipe Sizing and Optimizing DT ” (December 2012).
Optimized control sequences. The series will conclude with a discussion of how to optimally control chilled water plants, focusing on all-variable speed plants.
Central Chilled Water Plants Series The intent of the SDL (and these articles) is to provide simple yet accurate advice to help designers and operators 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 simula-tion, cost estimating, and life-cycle cost analysis was performed on the most common water-cooled plant con-figurations 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 perform-ing chilled water plants than common rules-of-thumb and standard practice.
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able condenser water flow systems) condenser pump speed. Because of limitations in most commercial programs such as DOE-2.1 and 2.2, more advanced programs such as Energy-Plus, TRYNSYS, or custom models3 should be used for best results.
There is significant uncertainty in life-cycle cost analy-sis such as uncertain plant loads and future utility rates. The results will also vary depending on the life-cycle cost parameters selected such as discount rates and escalation rates. Once all of the results are in a spreadsheet, these variables can be adjusted within reasonable ranges to see how they affect life-cycle costs. Typically, there will one to three options that all result in similar life-cycle costs and each could be the “winner” depending on assumptions. This group should be considered equal from a life-cycle cost perspective, and the final selection based on “soft” fac-tors such as:
• Past experience with the chiller manufacturer. • Past experience with the local service company. • Refrigerant preferences with respect to their impact on
ozone or global warming. • Preferences for open vs. hermetic motors.
• Impact on meeting energy targets such as those in the LEED rating program.
• Impact on redundancy and serviceability of options that include more than one type or size of chiller.
• Impact on construction budget.
SummaryThis article is the fourth 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, optimum cooling tower and chiller selections were discussed. In the next and final article, optimized control logic will be addressed.
References1. McBride, M. 1995. “Development of Economic Scalar Ratios for
Standard 90.1.” Proceedings of Thermal Performance of the Exterior Envelopes of Buildings VI. ASHRAE.
2. Taylor, S., et al. 2009. CoolTools Chilled Water Plant Design Guide, Energy Design Resources. http://tinyurl.com/7ag9s24 and http://tinyurl.com/82yllad.
3. Hydeman, M., G. Zhou. 2007. “Optimized chilled water plant control.” ASHRAE Journal 49(6).
