engineers new slet t er volume 38–3 • providing insights for today’s hvac system designer peanut butter and jelly Series Chillers and VPF Chiller Plants Dram at ically rising energy cost s, wat er short ages, and em erging carbon footprint concerns all equat e t o t he need for m ore efficient building operat ion. Many owners are looking for innovat ions in HVAC syst em designs t o help m eet t hat need. This newslet t er discusses t hree design concept s and explores how t heir m erger can m ake t he syst em bot h m ore efficient and m ore reliable. The Three Concepts Series Chillers. An early Engineers New sletter, "Control Systems That Save Energy," contained a discussion of parallel versus series chillers. It included an evaluation stating: Variable Primary Flow (VPF). Enabled by the advances in today's intelligent chiller controls and driven by the promise of significant pump energy savings at a lower first cost, VPF chilled-water systems are currently experiencing explosive growth. [2] supply temperature. Several points become apparent after studying the data: • Both the capacity and the efficiency of the chiller pair increase in the series chiller configuration. • Even at 1.5 gpm/ton, a series chiller system appears to suffer from a prohibitively high chiller pressure drop. Quantifying the Benefits Series Chillers and Low Flow. Table 1 provides a comparison of the efficiency advantage of series versus parallel configurations for a two-chiller system. The chiller selections represent efficiencies for chillers selected for 1.5 gpm/ton chilled-water flow w ith a 40°F It is this last point that bears further examination. Could it be that the apparent pressure drop penalty is not prohibitive but actually beneficial in a VPF system? Table 1. Two-chiller system comparison data (1.5 gpm/ ton flow @ 40ºF supply water) Instead of piping chillers in parallel so that each must produce the coldest water in the system, they are piped in series. The upstream chiller requires less kW input per ton output, thus improving system efficiency. Low Flow Systems. There is grow ing realization that system parasitic losses can be reduced and efficiency improved by reducing the energy used to transport cooling and heating throughout a building. It is unusual to find a cooling system w hose optimal design flow rates are the ARI standard rating conditions of 2.4 gpm/ton and 3.0 gpm/ton for chilled-water and condenser-water systems, respectively. [1] Chiller model System capacity (tons) Combined chiller efficiency (EER) Chiller design flow (gpm) Design PD (ft) M in flow (gpm) Flow turn dow n ratio (design/min.) Parallel RTAC 200 Hi Eff 2-pass 380 9.8 284 5.1 241 1.2 Parallel RTAC 200 Hi Eff 3-pass 388 9.9 290 17.6 161 1.8 Series RTAC 200 Hi Eff 2-pass 404 10.1 580 37.1 241 2.4 Series RTAC 185 Hi Eff 2-pass 372 10.2 555 40.8 117 4.7 The ASHRAE GreenGuide recom m ends designing wit h: 12- 20°F chilled wat er ΔT and 12- 18°F condenser wat er ΔT This equat es flow rat es of: 1.2 - 2.0 gpm / t on chilled wat er and 1.6 - 2.3 gpm / t on condenser wat er © 2009 Trane All rights reserved ● 1 Two come to mind. One would be the dreaded "Low Delta T" syndrome. One of the commonly touted advantages of VPF is that chillers can be "overpumped" in VPF systems to prevent the premature operation of additional chillers just to satisfy system flow. This compensates for low delta T from the chiller-sequencing point-ofview. However, the system pumps still expend extra energy moving additional water through the system. VPF cannot compensate for the pumping energy penalty of low delta T. Figure 1. Ideal VPF flow relationship 2500 flow 2000 1500 2 chiller 1 chiller 0 10 20 30 40 50 60 80 90 100 90 100 90 100 Figure 2. Actual VPF flow relationship: 2.5 TD ratio 2500 2000 Chiller 1 capacity (tons) design flow (gpm) min. flow (gpm) turndown 500 1000 400 2.5:1 1500 two chiller min. flow 1000 one chiller min. flow 2 chiller 1 chiller 500 0 10 20 30 40 50 60 70 80 system load Figure 3. Actual VPF flow relationship: 1.4 TD ratio 2500 flow 2000 Chiller 1 capacity (tons) design flow (gpm) min. flow (gpm) turndown 500 1000 700 1.4 :1 two chiller min. flow 1500 1000 one chiller min. flow 2 chiller 1 chiller 500 Figure 3 show s the same system's actual pump flow (including the chiller's minimum flow ) but this time based on chillers w ith a 1.4:1 flow TD ratio. 70 system load In a VPF system, the pumps must provide enough flow to meet the greater of system flow or the chiller’s minimum flow requirements. Figure 2 show s the same system's actual pump flow (including the chiller's minimum flow ) based on chillers w ith a 2.5:1 flow turndow n (TD) ratio. 500 100 500 A second "corrupter" of the system load/flow relationship can be the selected chiller's required minimum flow. Figure 1 show s the idealized pump flow for a two-chiller VPF system— ignoring the chiller's minimum flow. The pump flow is proportional to the system load under all conditions. Chiller 1 capacity (tons) design flow (gpm) 1000 flow VPF: Flow Considerations. VPF systems save considerable system energy primarily because the flow varies proportionally to the system load. If something were to corrupt this relationship it would adversely impact the expected energy savings. What are some corrupting influences? 0 10 20 30 40 50 60 70 80 system load With this in mind, if we look back at the Table 1 chiller selections, another conclusion becomes evident. • 2 The extremely low-flow TD ratio for the parallel chillers w ith 2-pass evaporators in the first row would not work well in a VPF system. ● Trane Engineers New sletter volume 38–3 providing insights for today’s HVAC system designer It's clear: a low turndow n ratio significantly impacts VPF pump operation. But is lack of adequate chiller evaporator flow turndow n a real concern? It depends on the chiller type, but the general answer is yes and it w ill only get worse due to the need for more efficient systems. M anufacturers are being driven to produce more and more efficient chillers to meet code or customer requirements. A common way to improve chiller efficiency is to increase the heat exchanger’s surface area— add more tubes to the evaporator and/ or condenser. M ore tubes result in a greater flow area and a lower design fluid velocity. A higher minimum flow is required to maintain sufficient fluid velocity to prevent laminar flow conditions. Laminar operation can lead to unstable chilled-water temperature control that can be hazardous to the chiller. Additionally, engineers are follow ing industry "best practices" and designing more efficient systems through the use of lower system and chiller design flow rates. Systems at 2.0, 1.7 or even 1.5 gpm/ton are becoming the norm, rather than the exception. While lowering the design flow is good for system efficiency, it reduces the available TD ratio for a given chiller. Applied centrifugal chillers typically have many heat exchanger options so that an adequate TD ratio can be selected. However, it's becoming common to see packaged chiller selections w ith very low TD ratios. This can make them difficult to apply in parallel chiller/VPF systems. • Include the requirement to submit chiller minimum and maximum rated flow s as a line item in the bid package. • Don't specify chillers w ith excessive capacity safety factor. • Consider applying the chillers in a series configuration. • Remember that series chiller system flow rate-of-change limitations are set by the chiller’s limitations just as in parallel VPF systems. Impact Impact of Low Turndow n and Bypass Selection and Control. In addition to the pumping energy impact, a high bypass flow requirement such as show n in Figure 3 (due to a low TD ratio) forces the selection of a relatively large VPF bypass line and control valve. The required range of control for both flow and pressure makes stable control more challenging. When a second chiller is added, the bypass valve must open quickly at a relatively low system differential pressure to allow sufficient flow to keep the operating chiller above its minimum flow. The same valve must also stably control flow at higher system pressures w hen only a small amount of bypass is required to keep multiple chillers operating above their minimum flow requirements. VPF Chiller Requirements. How can a designer ensure that the chillers applied in a VPF have sufficient flow TD to work well in a VPF design? There are several steps that can be taken. • Evaluate the bypass flow requirement w ith different chillers running, across the full system load spectrum. • Include the requirement for a minimum TD ratio as part of the chiller specification. VPF Flow w ith Series Chiller. Figure 4 plots the pump flow for the same chillers show n in Figure 3 but in a series configuration. In the series configuration, the chillers have an effective 2.8:1 TD ratio w hen both chillers are operating. We see that the operating curve looks quite different. Some bypass flow is still required during periods of very low load w hen a single chiller is operating. However, all bypass flow is eliminated w hen two are operating. System Power for VPF w ith Series Chillers. In systems w ith constant flow through the chillers' evaporators, designers often specify maximum acceptable pressure drops. The design flow pressure drop through a pair of chillers in series is likely to be much higher than w hat is considered acceptable in a parallel system. Figure 4. Series chiller VPF flow relationship: 1.4 TD ratio 2500 2000 flow However, this might be a perfect selection in a constant-flow system for reduced design pump power. Chiller 1 500 capacity (tons) design flow (gpm) 1000 700 min. flow (gpm) 2.8 :1 turndown 1500 1000 2 chiller 1 chiller 500 0 10 20 30 40 50 60 70 80 90 100 system load providing insights for today’s HVAC system designer Trane Engineers New sletter volume 38–3 ● 3 Table 2 compares the system energy use at different load points for the 3pass chillers in parallel compared to the 2-pass chillers in a series configuration. Note: These specific 2-pass chillers should not be used in parallel in a VPF system w ith a 1.5 gpm/ton design flow because of poor flow TD, and the resultant high bypass flow requirement. This comparison demonstrates that the series chiller configuration has a better system COP at all load points. Even w ith a 20 ft higher system pressure drop at design load, it uses less energy! This is a direct result of the chiller's greater combined efficiency as well as the decreased bypass flow at part-load conditions. Sequencing lag chillers on and off to match the building load can use the identical logic that a parallel VPF system uses. Deviation in system chilled-water supply temperature is a simple and robust way to decide w hen to add a chiller. Chiller load, as measured by chiller RLA or kW, is a reliable and repeatable indication of the point to subtract a chiller from operation. 1. If one chiller is operating, it is given the system setpoint. 2. If both chillers are operating: (a) the dow nstream chiller is given the system setpoint. (b) the upstream chiller is given a setpoint that results in each chiller carrying one half the instantaneous load. The equation for the upstream chiller's setpoint is based on chilled-water return temperature and desired chilledwater supply temperature and calculated simply: CHSP up ⎛ CHRT − CHSP sys = CHRT + ⎜⎜ 2 ⎝ ⎞ ⎟ ⎟ ⎠ W h e n N OT t o u se se r ie s ch ille r s. Alt hough a series chiller configurat ion saves energy and m akes sense in m any cases, t here are t im es when it should not be applied. The setpoint is periodically recalculated and sent to the chiller. The chiller controls its ow n loading. 1. Syst em s wit h design flow rat es great er t han 1.5 gpm / t on are probably not good candidat es because of chiller pressure drop. I t 's best t o st art wit h a high- efficiency low- flow syst em design t o opt im ize pum ping power. CHSPup Upstream chilled-water setpoint Control of Series/ VPF Chiller Plants CHRT Some ow ners and engineers shy away from series chiller plants because they are unsure of the system control requirements. In fact, the only additional series plant control requirement is the reset of the upstream chiller's leaving chilled-water setpoint to equalize chiller loading. The three rules for chiller setpoint are actually quite straightforward. Actual chilled-water system return temperature 2. The cont rol int eract ion bet w een chillers wit h st ep- loading com pressors ( m ult iple scrolls) can result in undesirable com pressor cycling. St andard st ep- loading chillers should not be applied in series. CHSPsys Chilled-water system supply setpoint 3. If there is a failure of a chiller or controller, the operating chiller(s) defaults to the system setpoint. 3. Const ant - flow syst em s are not t ypically good candidat es for series chillers. Table 2. System energy use comparison Load Parallel RTAC 200 3-pass chiller Tons 4 ● Series RTAC 200 2-pass chiller Pumping PD Pump kW Chiller Total Sys kW kW COP Tons Pumping PD Pump kW Chiller kW Total kW Sys COP COP Increase % 100 388 70 10 472 482 2.83 404 90 14 482 496 2.87 1.3 90 349 60 8 396 404 3.04 364 76 10 400 411 3.11 2.6 80 310 51 6 326 332 3.29 323 63 8 323 331 3.44 4.4 70 272 43 4 260 264 3.62 283 52 6 265 271 3.67 1.6 60 233 36 3 208 211 3.88 242 43 4 207 211 4.04 4.0 50 194 31 3 160 163 4.19 202 35 3 164 167 4.26 1.9 40 155 27 2 123 124 4.39 162 29 2 121 123 4.62 5.3 30 116 24 1 99 100 4.09 121 25 1 98 99 4.31 5.3 20 78 21 1 85 86 3.18 81 22 1 84 85 3.36 5.7 10 39 20 1 71 72 1.90 40 20 1 70 71 2.01 5.7 Trane Engineers New sletter volume 38–3 providing insights for today’s HVAC system designer See the Engineers New sletter on VPF systems for more details on chiller sequencing. [2] Figure 5. Series chiller pairs Figure 6. Series plants passing a non-operating chiller w ill provide additional pump savings, the added piping and control complexity may not justify the savings. Figure 7. Series chiller pair w ith service bypass M ore Than Two Chillers? Odd numbers of chillers are difficult to deal w ith in a series configuration. Except for some very low flow process applications, the system pressure drop for three chillers in series becomes untenable. The solution is to resize the chillers so that an even number can be applied. If a system requires four, six or more chillers, one possible system configuration is series "chiller pairs" situated in parallel as show n in Figure 5. An alternate, and more versatile, approach of "series plants," show n in Figure 6, should be considered. A "series-plant" configuration provides several benefits: • One chiller out of service doesn't affect the operation of other chillers. • The operation of upstream and dow nstream chillers can be mixmatched for better flexibility and reliability. • Chiller Bypass Piping. Discussions of series chillers often include the issue of including extra bypass piping around each chiller. An example of such piping is show n in Figure 7. The cooling system in question serves a critical load that cannot tolerate a short-term scheduled shutdow n of both chillers in a series pair. • One chiller must be available for comfort conditioning at all times. There are two potential reasons to include chiller bypass piping. The first is to eliminate a non-operating chiller's pressure drop from a series pair of chillers. However, as show n previously in Table 2, the pumping energy penalty at such part load conditions is minimal. While by- providing insights for today’s HVAC system designer Second, including chiller bypass piping w ith manual isolation valves to enable serviceability may be desired for the follow ing reasons: Chiller service external to the refrigeration system may be performed w ith chilled water flow ing through the evaporator heat exchanger. However, the refrigeration system must never be exposed to ambient atmosphere w ith active chilled-water flow. M oisture can enter the chiller's exposed refrigeration system and rapidly cause corrosion or contamination of hygroscopic oils used w ith many current refrigerants. Also, Trane Engineers New sletter volume 38–3 ● 5 proper evacuation of a refrigeration system is practically impossible w ith active chilled-water flow. Figure 8 - Free-cooling heat exchanger in upstream position. The key to answering the question of w hether to apply a chiller w ith a heatrecovery condenser or a completely separate heat-recovery chiller is a building hourly energy analysis w ith a program such as Trane TRACE™. Such an energy analysis w ill reveal if the heating and cooling load magnitude and occurrence are favorable to the application of a double-bundle condenser chiller. Relatively similar loads work well w ith double-bundle heat-recovery chillers. However, if the Series Unlocks Other System Efficiency Options There are several energy-saving system options that can work very well in conjunction w ith a series/VPF system. Series and "Free" Cooling. Use of cold condenser water, available during periods of low wet-bulb temperatures, to produce chilled water via plate-andframe heat exchangers or refrigerant migration w ithin a chiller are methods to significantly reduce system energy use. An excellent strategy for applying a refrigerant-migration, free-cooling chiller in a series system is to locate it in the upstream position. See reference [3] for more information. A free-cooling chiller can provide 30 to 40 percent of its design tonnage, depending on the operating conditions. When the refrigerant-migration chiller cannot meet the full building load, the dow nstream chiller can be started to augment the system cooling capacity. This coincident free and mechanical cooling greatly extends the freecooling operating hours and energy savings. Since the free cooling is provided through an option on a chiller, there is no additional space required in the equipment room and minimal increase in maintenance. Another free-cooling option is the application of a dedicated free-cooling heat exchanger in parallel w ith the upstream chiller as show n in Figure 8. While this option requires additional equipment room space and maintenance, it can be designed for greater free-cooling capacity than a refrigerant-migration chiller can provide. It also may be the only waterside free-cooling option for systems w ith chiller types that do not offer a 6 ● Trane Engineers New sletter volume 38–3 dow nstream chiller to carry any leftover cooling load. [4] Figure 9. Heat-recovery chiller w ith doublebundle condensers free-cooling option. Because of its upstream position, the heat exchanger also provides for coincident free, mechanical cooling, increasing the free-cooling energy savings. Series and Heat Recovery. A chiller w ith a dedicated heat-recovery condenser (sometimes called a doublebundle condenser) or an additional, properly sized, heat-recovery chiller works very well in a series/VPF plant in the upstream position (see Figure 9). The upstream positioning benefits from the warmest system return-water temperature for more efficient heatrecovery chiller operation. There is a grow ing synergy between the application of chiller heat-recovery and high-efficiency heating systems. Condensing boilers require lower heating system water temperatures to achieve their efficiency potential. As a result, heating system design temperatures of 180°F are being replaced w ith 105°F to 130°F. M any chiller types can provide efficient heat recovery in conjunction w ith lower heating temperatures. Se r ie s cou n t e r flow . The nat ural progression for increasing t he efficiency of wat er- cooled series/ VPF syst em s is configuring t he condensers in series as well as t he evaporat ors. This is a concept know n as " Series- Count er." For a det ailed analysis of t he perform ance of a series- count erflow chiller plant , see t he ASHRAE Journal June 2002 art icle: " Series- Series Count erflow for Cent ral Chilled- Wat er Plant s" by Groenke and Schwedler. As w ith the free-cooling application, it is a simple matter to optimize the system operation by controlling the upstream heat-recovery chiller for optimum operation and allow the providing insights for today’s HVAC system designer heating load magnitude and occurrence result in the available heating load to the chiller being a small fraction of its cooling capacity, then applying a properly sized, dedicated heat-recovery chiller may likely be a better option. M ore to Come... Peanut butter & jelly, Bacon & eggs, Table & chairs, Series chillers & VPF... • High efficiency due to the upstream chiller operating at an elevated temperature. • Simple control and loading of either chiller. • Ability to apply other energy-saving options such as "free cooling" or heat recovery. By Lee Cline, application engineer, and Jeanne Harshaw, Trane. You can find this and previous issues of the Engineers New sletter at w w w.trane.com/engineersnew sletter. To comment, e-mail us at comfort@trane.com The benefits of efficiency and controllability, along w ith the natural performance enhancing fit of free cooling or heat recovery, is resulting in rapid grow th and application of these system concepts. Systems using series chillers in conjunction w ith variable primary flow have the follow ing advantages: Air-Handling Systems, Energy and IAQ N ovem ber 4 References. Foot not es: There are many things that naturally complement each other. M any designers and contractors are finding this true of series chillers and variable primary flow w hen a high-efficiency, chilled-water system is the goal. Engineers N ew sletter LIV E! [1] M . Schw edler, 1997. "How Low-Flow Systems Can Help You Give Your Customers What They Want." Engineers New sletter, volume 26-2. mark your calendar! 2 010 Schedule Fans in Air-Handling Systems [2] M . Schw edler, 2002. "Variable-Primary-Flow Systems Revisited." Engineers New sletter, volume 31-4. M arch [3] S. Hanson, 2008. " ’Free’ Cooling Using Water Economizers." Engineers New sletter, volume 37-3. Central Geothermal Systems [4] M . Schw edler, 2007. "Waterside Heat Recovery." Engineers New sletter, volume 36-1. M ay ASHRAE 90.1-2010 October contact your local Trane office for details • Great fit w ith "reduced flow " systems as recommended by the ASHRAE GreenGuide. • Significant ability to accommodate reduced flow rates at part load conditions. • M aximized pump savings due to minimal requirement for bypass flow. providing insights for today’s HVAC system designer Trane Engineers New sletter volume 38–3 ● 7 educat ional resources www.Trane.com/bookstore application manuals are comprehensive reference guides that can increase your working know ledge of commercial HVAC systems. Topics range from component combinations and innovative design concepts to system control strategies, industry issues, and fundamentals. Visit w w w.trane.com/bookstore. Chiller System Design and Control examines chilled-watersystem components, configurations, options, and control strategies. The goal is to provide system designers w ith options they can use to satisfy the building ow ners’ desires. (SYS-APM 001-EN, M ay 2009) Chilled-Water VAV Systems focuses on chilled-water, variableair-volume (VAV) systems. These systems are used to provide comfort in a w ide range of building types and climates. To encourage proper design and application of a chilled-w ater VAV system, this guide discusses the advantages and draw backs of the system, review s the various components that make up the system, proposes solutions to common design challenges, explores several system variations, and discusses system-level control.(SYS-APM 008-EN, August 2009) air conditioning clinics are a series of educational presentations that educate readers about HVAC fundamentals, equipment, and systems. The recently revamped series now includes full-color student workbooks, w hich can purchased individually. engineers new sletters are published as a free service to building professionals and are archived at w w w.trane.com/ engineersnew sletter. Each issue tackles a timely topic related to the design, application and/or operation of commercial, applied HVAC systems. engineers new sletter live is a series of 90-minute recordings that presents technical and educational information on specific aspects of HVAC design and control. Visit w w w.trane.com/ENL for specific program details or to check out the 2010 schedule. Trane, A business of Ingersoll-Rand For m ore inform ation, contact your local Trane office or e-m ail us at com fort@trane.com 8 ● Trane Engineers New sletter volume 38–3 Trane believes the facts and suggestions presented here to be accurate. However, final design and application decisions are your responsibility. Trane disclaims any responsibility for actions taken on the material presented. ADM -APN033-EN (September 2009)