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Applications Engineer ing Manual Chiller S yst em Design and Contr ol May 2009 SYS - A P M 0 01- E N[...]

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Chiller S yst em Design and Contr ol Susanna Hanson, appl ications engineer Mick Sc hwedler , applications manag er Beth Bakkum, infor mation designer[...]

© 20 09 T rane All rights reserved Chiller Syste m Design and Control SYS-APM00 1 -EN T ran e, in proposing these syst em design and application con cepts, assumes no responsibility for the performance or desir ability of any resulting syst em design. Design of the HVAC system is the prerogative and resp onsibility of the engineering professional.[...]

SYS-APM00 1 -EN Chiller System Design and Control iii Cont ents Pr ef ace .... ................ ................ ................ .................... ................ .......... i Pr imary Syst em Components .............. ................ ................ ..... 1 Chiller ................ ................ ................ ................ ........[...]

iv Chiller System Design and Control S YS-APM001 -EN S ystem Contr ols ................. .................... ................ ................ ...... 87 Chilled-W ater S ystem Control ..................... ................ ................... 87 Condenser -W ater System Control ..... .................... ................ ......... 89 Failure Reco [...]

SYS-APM00 1 -EN Chiller System Design and Control 1 Pr imary S yst em Components Chilled-water systems consist of these functional par ts: • Chillers that cool the water or fluid • Loads, often satisfied by coils, that transfer heat from air to water • Chilled-w ater distribution pu mps and pipes that s end c hilled water to the loads • Con[...]

2 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components Figure 1. Typical vapor-compression chiller W ater -cooled c hiller s are typically inst alled indoors; air -cooled c hillers are typically installed outdoors—either on the ro of or next to the building. I n cold climates, air -cooled c hiller s may have a remote evap[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 3 • In a direct-expansion (DX) shell-an d-tube evaporator (F igure 3), warmer water fills the shell while the cool, lo wer -pressure liquid refrigerant flows through the tubes. Figure 3. Direct-expansion evaporator cut-away In either design, there is an approac h te mpe[...]

4 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components Some c hiller controls can accom modate very little flow variation during mac h ine operation. 2 Other , more sophisticated, c hiller controls allow some flow variation. S ome c hillers can tolera te flow-rate variations—as muc h as 50 percent per minute or greater—[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 5 While they found that so me of the internally - enhanced tubes fouled in the long term, they c oncluded: Because of the high hardness and low water velocity used in these tests, we do not believe that the fo uling experienced is typical of that expected in commercial in[...]

6 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components Low -ambient operation Air -cooled c hillers are often selected for use in systems with year -round cooling require ments that cannot be met with an air side econom izer . Air - cooled condenser s have the ability to operate in below -freezing weather , and can do so wi[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 7 Figure 4. Air-cooled or water-cooled efficien cy Another advantage of an air -cooled c h iller is its delivery as a “pac k aged system. ” Reduced design ti me, simp lified installatio n, higher reliability , and single-source responsi bility are all factors that mak[...]

8 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components Heat transferred fro m the loads can be controlled in a number of ways: • Three-way valve •T w o - w a y v a l v e • V ariable-speed pump • Face-and-bypass damper s Thr ee-w ay v alv e load contr ol A three-way control valve (Figure 5) re gulates the amount of w[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 9 Figure 6. Two-way valve V ariable-speed pump load contr ol By using a pump for eac h coil (Figure 7), the flow may be controlled b y varying the pump speed. In suc h system s, there may be no control valves at the coil. This can reduce both the valve and the valve insta[...]

10 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components pumped all the time; however , in syst ems with very small water pressure drops, this syst em arrangem ent may work economically . Figure 8. Uncontrolled water flow with bypass damper Chilled-W ater Distr ibution S ystem Chilled w ater is circulated through fixed pipin[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 11 • accommodates the total pressure (sta tic head plus dynamic head) on system components suc h as the c hiller ’ s evaporator , valves, etc. Note that the pump heat is added to the w ater and must be absorbed by the c hil ler . Gen erally , th is represents a v ery [...]

12 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components valves may be either three-way or two -w ay . As previousl y discussed, t hree- way valves require constant w ater flow , while two-w ay valves allow the w ater flow in the system to vary . As flow var ies, the pump may simply ride its curve or use a method of flow con[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 13 the series, or another pu mping arrangement can be considered. Reducing the flow rate af fects this system type’ s energy use all the time, so careful attention to flow rates an d temperature is critical (refer to “System Design Options” on page 27). Pr imary -se[...]

14 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components water entering and leaving the cooling tower is th e range. The temperature difference between the leaving w ater temperature and the ente ring wet-bulb temperature is the approac h. Eff ect of load on cooling to wer perf ormance As the building load—or he at rejecti[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 15 Unit-Lev el Contr o ls The c hilled-water supp ly temperature is usually controlled by the c hiller . Most commonly , supply water temperatur e is used as the sensed variable to permit contro l of c hiller capacity to meet syst em load demand. Sup ply- temperature cont[...]

16 Chiller System Design and Control S YS-APM001 -EN Pr imary Syst em Components In addition to m onitoring data, it is vital that the c hiller controls aler t operators to possible problem s. Diagno stic messages are necessary for the operator to respond to safe ty issues and data points that are outside normal operating rang es. While communicati[...]

Pr imary Syst em Components SYS-APM00 1 -EN Chiller System Design and Control 17 by reducing m otor speed at “low-lift” conditions, when cooler condenser water is available. Certain system c haracteristics fav or th e application of an AFD, including: • A substantial number of part-load opera ting hour s (for exampl e, when an air - or water [...]

18 Chiller System Design and Control S YS-APM001 -EN Application Consider ations Chiller system siz e affects design and control cons id erations. Eac h size comes with its own set of advantages and challenges. Small Chilled-W a t er Syst ems (1 -2 c hillers) Figure 17. Small chilled-water system schematic A common design goal for the sma ll c h il[...]

Application Considerations SYS-APM00 1 -EN Chiller System Design and Control 19 Constant flo w Constant flow is simple and often applied to small systems up to 20 0 tons— as long as the system pres sure drop is fairly l ow and a wider  T is applied to reduce the system flow rate. In consta nt flow systems, appropriate c hilled - water reset re[...]

20 Chiller System Design and Control S YS-APM001 -EN Application Considerations part of those jobs. S e e “Energy and ec onomic analy sis of alternatives” on page 26. Number of c hillers The number of c hillers to install is a functio n of redundancy requirements and first cost. In general, the more c hillers installed, the higher the initi al [...]

Application Considerations SYS-APM00 1 -EN Chiller System Design and Control 21 than at full load. V ariable frequenc y d rives for unloading tower fans and c hilled-water pumps may p rovide benefi ts, depending on the costs, system operating hours, system type, and outdoor air condit ions. (S ee “System Controls” on page 87 .) Mid-Siz ed Chill[...]

22 Chiller System Design and Control S YS-APM001 -EN Application Considerations but use it sparingl y due to its low er efficiency . Or , a c hiller may have a different fuel source, used as a hedge against either high demand or high energy consumption c harge s for other energy sources. (See “ Alternati ve Energy Sources” on page 82.) Some c h[...]

Application Considerations SYS-APM00 1 -EN Chiller System Design and Control 23 Creating one centraliz ed c hilled-wa ter system takes signif icant foresight, initial in vestment, and building devel opm ent with a multi-year master plan. If the initial pl ant is built to acco mmodate man y future buildings or load s, the early c hallenge is operati[...]

24 Chiller System Design and Control S YS-APM001 -EN Application Considerations T o minimiz e power , large systems must be very effi cient. Th e upside of a large system is the am plification of energy savin gs. A relatively small percentage of energy saved becomes more valua ble. For this reason, the highly efficient series-counterflow ar ra ngem[...]

Application Considerations SYS-APM00 1 -EN Chiller System Design and Control 25 Guidelines for syst em efficiency monit or ing ASHRAE Guideline 22 Instrumentation for Mon itoring Central Chille d-W ater Plant Effic iency 6 w as first published in June 20 08. It states: Guideline 22 was developed by A S HRAE to provide a source of information on th [...]

26 Chiller System Design and Control S YS-APM001 -EN Application Considerations Ener gy and economic analysis of alt er natives The process of making decisi ons between multiple, com peting alte rnatives is simplified with the assist ance of simulation sof tware. Many pac k ages are available for this purpose (see sidebar) . While not every analysi[...]

SYS-APM00 1 -EN Chiller System Design and Control 27 Sy s te m D e s i g n O p t i o n s There are many c hilled-water -system design options; however , in a basic sense, eac h option is a function o f fl ow, temperature, system configuration, and control. This section discusses the ef fect of flow rate and temperature decisions. It is impor tant t[...]

28 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options recommends a design metho d that star ts with condenser -wat er temperat ure difference of 1 2°F to 18°F [7°C to 1 0°C]. Standar d rating t emperatur es Currently , the standard rating condition temperatures in ARI 550/590 5 and ARI 560 9 are: • Evaporator leaving wat[...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 29 Condenser -W at er T emper atur es T oday’ s c hillers can run at various en tering condenser -wa ter temperatures, from design temperature to the lowe st-allow able temperature for that par ticular c hiller design. However , many existing older c hillers are limited in t[...]

30 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options Selecting flow r ates Designers may use the standard rating conditions to compare manufacturer s’ performances at exactl y the sam e conditions. However , these standards allow any flow rates to be used and certified comparisons to be made at a wider range of conditions. [...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 31 1 .90 power , respecti vely . The examples here use the more conservati ve 1 . 85 power: DP2/DP1 = (Flow2)/(Flow1) 1. 8 5 Give n dif ferent flow rates and enteri ng water temperatures, a different cooling tower can be se lected for the lo w-flo w condition (T able 6): * Lo [...]

32 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options The total system powe r is now as foll ows: * Lo w -flow conditions represe nted in T able 5 through T able 8 are 1 .5 gpm/ton [0.0 27 L/s/kW] c hilled water and 2.0 gpm/ton [0.036 L/s/ kW] condenser water . Figure 20. System summary at full load It becomes clear that flow [...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 33 Figure 21. Chilled water system performance at part load While the magnitud e of the benefit of low -flow c hanges depends on the c hiller type used (centrifugal, absorpti on, helical-rotary , scroll), all c hilled- water systems can benefit from judicious use of reduced fl[...]

34 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options performance of this coi l when it is selected with a 44°F [6.7°C] entering fluid temperature and a 1 0°F [5.6°C ] fluid temper ature rise (  T). T o provide the required 525 MBh [1 54 kW] of cooling capa city , the coil require s 1 05 gpm [6.6 L/s] of water . The rig[...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 35 Q = U x A 1 x  T 1 , where A = area, U = coefficient of heat transfer , and  T = temperature difference so, for a roughly equival ent heat rejection, U x A 1 x  T 1 = U x A 2  x  T 2 and for a constant coef ficient of heat transfer , A 1 x  T 1 = A 2 x [...]

36 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options Same to w er , lar ger c hiller One retrofit option that benefi ts many building owners is instal ling a new, larger c hiller selected for a lower flow rating an d re-using the existing coolin g tower , condenser -wa ter pu mp, and condenser -water pi pes. In man y cases, t[...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 37 It quic kly becomes ev iden t that the same cooling towe r and flow rat e are adequate to re ject more heat—in this case, a pproxim ately 50 p ercent more heat. Figure 22. Cooling tower re-selection with different chiller capacities Retr ofit opport unities The low -flow [...]

38 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options In both cases, either reusin g an exist ing tower , or reusing exi sting c hil led water pi ping, the design engineer can of ten help reduce total proj ect costs using the existing infrastructure by sel ecting a c hiller with a higher temperature differential. Cost Implicat[...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 39 Figure 23. Annual system operating costs (absorption chillers) Ke lly and Chan 10 compare the operational cost s of c hilled-water system designs in site locations. Their summary states: In conclusion, there are times you can ’have your cake and eat it too. ’ In most ca[...]

40 Chiller System Design and Control S YS-APM001 -EN S ystem Design Options and a more conservative zero condense r -water -pipe pressure drop, we c an examine the effect of reducin g flow rates. Figure 24. System energy consumption (no pipes) Energy consumptio n for the c hiller , condenser -water pump, an d cooling- tower fans is show n in Figure[...]

S ystem Design Options SYS-APM00 1 -EN Chiller System Design and Control 41 Misconception 2—Low flo w only wor ks for specific manuf actur ers’ chillers. Demirc hian and Marag areci 12 , Eley 13 , and Sc hwedler and Nordeen 11 independently showed that system energy consum ption can be reduced b y reducing flow rates. It is interesting to note [...]

42 Chiller System Design and Control S YS-APM001 -EN S y st em Configurations Multiple c hilled-w ater systems are more common than sing le c hilled-water systems for the same reason that most commercial airplanes have more than one engine—th e balance of reliab ilit y and cost. The most typical system configuration , by far , has two c hiller s.[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 43 Alternatively , the operating c hiller ca n be reset to produ ce a lower supply temperature at this co ndition. In this way , the mixed system supply-w ater temperature may be maintained at a more acceptable tempe rature. This complicates the control sy stem and presents t[...]

44 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Ser ies Chillers If c hillers are piped in series, as in Figure 27, the mixing problem disappear s and the starving coils problem (when one of the pumps in a par allel arrangem ent is not running) is resolved. S eries flow presents a new set of temperature and flow cont ro[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 45 percent of the system load. At system loads greater than 50 percent, the upstream c hiller is preferential ly loaded because it will attempt to produce the design leaving c hilled-water temper ature. Any por tion of the load that remains is directed to the downstream c hil[...]

46 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Figure 28. Decoup led arrangement The unrestricted bypass line hydraulica lly decouples, or separates, the production a nd distribution pumps so that they cannot operate in a series coupled pumping ar rangement. Although the two pumping sys tems are in dependent, they ha v[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 47 Pr oduction An individual production (c hiller) pump need only pump w ater from the return bypass tee (point A in F igure 29), through it s c hiller , and into the tee at the supply-end of the bypass line (point B in Figure 29). Thi s represents a relatively sma ll pressur[...]

48 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Distr ibution Distribution pumps take water from the su pply water tee (point B in Figure 29), p ush it through all the d istribution piping and load terminal s, and then on to the return water tee (poin t A in Figure 29). This pu mp can (and should) allow variable flow . [...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 49 Elev ated r etur n-wat er temper atur es. Because unused c hilled water does no t bypass the cooling coils (two-way , rather than three-wa y , co ntrol valves), all water that is returned a ccomplishes some c ooling. Theoretically , the return- water temperature will alway[...]

50 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Figure 33. Tertiary pumping arrangement Decoupled system–pr inc iple o f operation At the tee connecting th e supply and bypass lines, a supply–demand relationship exists, as shown in Figure 34. Think of the total flow rate from all operating pump–c hiller pairs as s[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 51 show a deficit an d the pump will be cy cled on again. The amount of surplus flow necessary depends o n th e size of the c hiller to be shut of f . The surplus flow must exceed a certain quantity befo re shuttin g off a c hiller–pump p air . If all c hiller s are equal i[...]

52 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Chiller sequencing in decoupled systems Give n the amount and directi on of flow in the bypass line , c hill ers can be added or subtracte d. Adding a chiller When there is deficit flow in the bypass line, the system is receiving water at a temperature above the desired su[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 53 Figure 36. Double-ended decoupled system One of the benefits o f decoupled w ater systems is that they are simple to control. The distr ibution pump flow is determ ined by a pressure transducer located at the fur thest load. Flow in th e decoupler indicates when to star t [...]

54 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations When more than o ne c hiller plant is operating, finding the right location for the differential pressure sensor ca n be dif ficult. The poi nt of lowest pressure in the system sh if ts depending on which loads are using the most water . It will probably be necessary to ha[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 55 with a surplus th at may , o r may not, be large en ough to indicate stopping a c hiller in that plant. Other plant designs There are many other ways to connect c hillers to distributed loo ps and eac h provid es its own c hallenges and oppor tunities. The adve nt of varia[...]

56 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations • The bypass can be positi oned either upstream or downstream o f the cooling coils. • A control valve in the b ypass ensures that the am ount of flow throug h the operating c hiller(s) never falls below the minim um limit, but remains closed most of the time. Notice t[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 57 secondary systems. The pressure drops previou sly satisfied by the distribution pumps are instead satisfied by the now larger p rimary -only pumps, permit ting selection of larger , more efficient pumps (with ef ficiencies similar to those of the secondary pu mps in a prim[...]

58 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Experience with actual VPF plants in dicates that a minimum evaporator -flow limit of 60 per cent for pac kaged c hiller s and 40 percent or less for configured c hiller s work well. Chiller manu facturers specify minimum and maximum limits for evaporator water flow . Thei[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 59 Small pac kaged c hillers typically of fe r less design fl exibility than larg er mac hines. It may not be possible to se lect a small pac k aged c hiller with a minimum flow rate of less than 60 percent of the design system flow… but don’t let this de ter you from des[...]

60 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations rate c hanges (T able 1 4 ). S electing c hille rs with these c harac teristics improves the likelihood of stable, u ninterr upted operation. Estimate the expected flow-rate c h anges and make sure that the c hillers you select can adapt to them . F or example, one of the [...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 61 evaporator because its selectio n pressure drop is lower than that of Chiller 2. Load is proportional to flow rate and temperature difference, tons = (gpm ×  T) / 24. Because Chiller 1 is asked to sa tisfy a load that e xceeds its capa city , it cannot satisfy the c hi[...]

62 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Accur ate flo w measurement The success of a variable-primary-flow in stallation depend s on the quality of the flow-measuring device that controls the system bypass valve (and perhaps also indicates the plant load). Some practitioners use a flow meter 16 to directly detec[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 63 • Locate a bypass line and valve near th e end of the pipin g run. The bypass control valve sees a lower operating pressure and may provide more stable control. Some operating cost savings may be sacrificed to maintain the pump-operating pressure at a higher leve l with [...]

64 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations flow nears the maximum limit for the operating c hiller(s), another mac hine must be brought o nline. Similarl y , as the sy stem load and flow decrease, c hiller s must be shut down to re duce the need for bypass w ater flow. Adding a chiller in a VPF system The simplest [...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 65 Contr olling tran sient flows is mand at ory , reg a rdless of plant size. The number of c hillers in the plant will not alter th e degree of care needed to properly manage transient flow-rate c hange s because the transitio n from one operating c hiller to two is inevit a[...]

66 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations A more conservative approac h might be to wait to turn of f the c hiller until it would result in no higher than 80 percent capacit y for the remaining operating c hillers. Going bac k to the example, if the desired (n-1) c hiller capacity were 80 pe rcent, it would not be[...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 67 Plant configuration Consider a ser ies ar ra ngement f or small VPF applications. When the plant c onsists of only two c hillers and expansion is un likely , you can simplify control by pi ping the evaporator s in seri es. Doing so a voids flow transitions because the wate[...]

68 Chiller System Design and Control S YS-APM001 -EN S ystem Configur ations Figure 39. Example of operating-cost sa vings for a VPF, single-chiller plant Analysis results are based on a 50-ton scroll chiller and a 5-hp chilled water pump for two-story office bui lding in St. Louis, Missouri. Mo de ra te “ l ow  T syndrome ” by manif olding [...]

S ystem Configur ations SYS-APM00 1 -EN Chiller System Design and Control 69 • Understand the specific loading/unloading c haracteristics of the c hiller controller Bypass flow • Select a high-quality control valve with lin ear -flow c haracteristics • Select flow -se nsing devices that deliver precise, repeata ble measurements • Minimiz e [...]

70 Chiller System Design and Control S YS-APM001 -EN Chilled-W at er Syst em V ar iations A number of c hilled-water system variations can and should be used when appropriat e. Eac h con figuration offer s specific advantage s to solve problems and add value to the system. Heat Reco v ery ASHRAE/IESNA Standard 90.1–2007 20 requires heat recovery [...]

Chilled-W at er Syst em V ar iations SYS-APM00 1 -EN Chiller System Design and Control 71 load. The details of operation are di scussed in “Sidestream plate-and-fra me heat exc hanger” on page 7 4. Plate-and-frame heat exc hanger s isolat e the building loop from the wa ter in the open cool ing tower loop, but they must be cleaned, typ ically a[...]

72 Chiller System Design and Control S YS-APM001 -EN Chilled-W at er Syst em V ar iations One option is to equip one or more parallel c hillers with the refrigerant migration cycle (Figure 43) . This essentially turns the c hiller into a shell-and- tube heat exc hanger run-around loop. Figure 43. W ater economizer pipe d in parallel with chill ers [...]

Chilled-W at er Syst em V ar iations SYS-APM00 1 -EN Chiller System Design and Control 73 body of water . Flow rates need to be carefully select ed to balance the economic and en vironm ental re quirements. Pr ef er ential Loading Preferential loading is desirable for sy stems that use heat recovery or free cooling to allow the equipment used in th[...]

74 Chiller System Design and Control S YS-APM001 -EN Chilled-W at er Syst em V ar iations One caveat when applying this arrangemen t is that c h illers on the producti on side of the bypass line will run more often at low par t-load conditio ns. Older c hil lers or newer c hiller s with a high cycle point may not have this capabil ity . Pr ef er en[...]

Chilled-W at er Syst em V ar iations SYS-APM00 1 -EN Chiller System Design and Control 75 Sidestream heat r ecovery A similar situation occu r s if a heat-reco very c hiller is placed in this sidestream position 24 (see Figure 46). This c hille r may be equipped with a heat recovery condenser or it could be a standard, single-condenser c hiller ope[...]

76 Chiller System Design and Control S YS-APM001 -EN Chilled-W at er Syst em V ar iations Sidestr eam system contr o l The flexibility of sidestream applicatio ns is increased by th e fact that the devices are used to pre-coo l return wa ter , not to produce the system c hilled- water temperature. This means that they may be loaded by a dif ferent [...]

Chilled-W at er Syst em V ar iations SYS-APM00 1 -EN Chiller System Design and Control 77 Ser ies–Count erflo w Application Another system configuration that can be very energy efficient incorporates the previously descri bed series applicat io n, but does so fo r both the c hilled wat er and condenser water . Fi gure 48 shows suc h a configurati[...]

78 Chiller System Design and Control S YS-APM001 -EN Chilled-W at er Syst em V ar iations series. The lef t half of Figure 50 shows a modularized configuration where series c hiller modules are placed in pa rallel with eac h other , so that any upstream c hiller ’ s valves could be “p aired” with virtually an y downstream c hil ler by opening[...]

SYS-APM00 1 -EN Chiller System Design and Control 79 S ystem Issues and Challeng es Lo w  T S yndr ome For man y years the “low  T sy ndrome” debate has raged. 27, 28 The symptom of the problem is that, in large systems, return-w ater temperature is too low, thus not allowi ng the c hillers to fully load. Man y system operato rs simply tu[...]

80 Chiller System Design and Control S YS-APM001 -EN S ystem Issues and Challeng es • Flow R ate = the system f low rate, in gpm [L/s] • Loop Time = the time it takes for flui d to leave the c hiller , move through the system, and return to the c hiller , allowing for stable system operation , in minutes [seconds] Chiller r esponse to changing [...]

S ystem Issues and Challeng es SYS-APM00 1 -EN Chiller System Design and Control 81 Conting ency T oday , many organizations have contin ge ncy plans for critic al area s of their business. Some deal with natural disasters and others with the loss of power in critical areas. However , few have ac tually taken the ti me to think about what a loss of[...]

82 Chiller System Design and Control S YS-APM001 -EN S ystem Issues and Challeng es Location of equipment Location can be a major factor in co ntingency pl anning. When selectin g the location of the tempo rary equipmen t, it is impor tant to consider: • W ater and electrical connections location • Soun d sensitive areas in the facility • Loc[...]

S ystem Issues and Challeng es SYS-APM00 1 -EN Chiller System Design and Control 83 situation. Electric al generation ca n be outsourced to avoid internal capitalization. A variation of electrical generation us es an engine indirectly - or directly- coupled to a c hiller . Either variation produ ces c hilled water using an alternative fue l suc h a[...]

84 Chiller System Design and Control S YS-APM001 -EN S ystem Issues and Challeng es Retr ofit Opport unities A tremendous retrofit oppor tunity can be realized i f the low -flow concepts discussed in the c hapter “System Design Options” on page 27 are ut ilized. Building owners may need to increase th e capacity of an ex isting system, for exam[...]

S ystem Issues and Challeng es SYS-APM00 1 -EN Chiller System Design and Control 85 T emperatur es out of rang e A laboratory load requires 120 gpm [7 .6 L/s] of water entering the process at 85°F [29.4°C] and returning at 95°F [3 5°C]. The accuracy required is more precise than the cooling tower can provid e. The selected c hiller has adequate[...]

86 Chiller System Design and Control S YS-APM001 -EN S ystem Issues and Challeng es Figure 53. P recise temperature control, multiple chillers Process Load Va r i a b l e - S p e e d Pump Chiller 2 Chiller 1 Control Va l v e s Bypass[...]

SYS-APM00 1 -EN Chiller System Design and Control 87 S ystem Contr ols Chilled-W ater S ystem Contr ol Chilled wat er reset—r a ising and lo wer ing Many c hilled-water plants use c hilled w ater reset, that is, the c hiller ’ s leaving- water temperature setpoint, in an ef fort to reduce c hiller energy consumption . This can either be accomp [...]

88 Chiller System Design and Control S YS-APM001 -EN S ystem Contr ols high. The control po int is selected to minimize over -pressu rizing the system and to assure adequate flow at all critical load s. Cr itical v alve r eset (pump pr essure optimization) Oft en, pumps are c ontrolled to maintain a constant-pressure dif ferential at a remote coil.[...]

S ystem Contr ols SYS-APM00 1 -EN Chiller System Design and Control 89 If the c hiller and tower capa bilities are conduci ve to th is strateg y , the location and load profile d etermine if, wh en, and for how lo ng the right cond itions might occur . Determine the opti mu m control sequence for the entire plant by performing a detailed en ergy an[...]

90 Chiller System Design and Control S YS-APM001 -EN S ystem Contr ols The flow reduction opti ons include: • Cooling tower bypass • Chiller bypass • One or two thrott ling valves in the condenser -w ater pipe with the pump riding its curve • A variable-speed condenser water pump After the minimum-pressure differenti al is reac hed, the flo[...]

S ystem Contr ols SYS-APM00 1 -EN Chiller System Design and Control 91 prevalent than either two-speed fans or pony motors. Using variable-speed driv es on cooling-tower fans offers tw o distinct benefits. First, the tower - water -temperature control is extre mely good. S econd, the fan power varies with the cube of the spee d, so there is great p[...]

92 Chiller System Design and Control S YS-APM001 -EN S ystem Contr ols Var iable condenser wat er flow Chiller -t ow er -pum p balance There are times when a system design er may c hoose to vary the condenser water flow in addition to, or instead of, the coo ling- tower fan speed. This may be beneficial in syst ems with high pum ping power . If a v[...]

S ystem Contr ols SYS-APM00 1 -EN Chiller System Design and Control 93 These three energy consumer s must be balanced to minimize overall energy use. This mak es varying condenser wa ter flow complex, but the strategy below has been implemented o n projects. Control of the condenser w ater pumps and coolin g tower fans is based on c hiller load, wh[...]

94 Chiller System Design and Control S YS-APM001 -EN S ystem Contr ols separate wetting (recircu lation) pump provides a constant flow of w ater through the tower . This is a l ow-energy p ump, as very lit tle lif t is required. Dedicated, variable-flow con denser pumps (CDWP -1 and CDWP -2) permit a reduction in the pump ing energy when ev er the [...]

S ystem Contr ols SYS-APM00 1 -EN Chiller System Design and Control 95 F ailur e Recov ery With all the varied approa c hes available to potential customers, it sometim es seems that the main idea ge ts lost. People purchase c hilled-water plants to reliably produce c hilled w ater to satisf y another need, suc h as comfort or process cooling. Th e[...]

96 Chiller System Design and Control S YS-APM001 -EN Conclusion It is vital to have a cl ear understandin g of c hilled-water system concepts and their application. There is not hing parti cularly complex about the principles inv olved. Instead, sys tem design is simply a mat ter of exercising a few key rules of applied physics. A myriad of c hoice[...]

SYS-APM00 1 -EN Chiller System Design and Control 97 Glossary ASHRAE. American S ociety of Heating, Re frigerating, a nd Air -Conditioning Engineers (www.ashrae.org). building aut omation system (BAS). A cent ralized control and monitoring system for a building. ch illed w at er . Also known as leaving-c hilled-wa ter or leaving-evaporator -water; [...]

98 Chiller System Design and Control S YS-APM001 -EN Glossary COP . Coef ficient of P erformance; coo ling ef fect divided by heat input (dimensionless); the reci procal of ef ficiency . direct digital control. Program ming used by build ing control systems to control variable outputs, su c h as valves or actuators. In the HV AC industry , DDC mean[...]

Glossary SYS-APM00 1 -EN Chiller System Design and Control 99 temper atur e, am bient. The tempera ture of the ai r surrounding the object under consid eration. temper atur e, wet-bulb. A m easure of the degree of moisture in the air . It is the temperature of evaporation for an air sample, measured with a thermometer that has its bulb co vered by [...]

10 0 Chiller System Design and Control S YS-APM001 -EN Ref er ences 1 W ebb, R.L. and W . Li. “Fouling in Enh anced T ubes Using Coolin g T ower W ater , P art I: Long-T erm Fouling Data. ” Internatio nal Journal of Heat and Mass T ransfer 4 3, no. 1 9 (October 2 0 0 0): 3567 -3578. 2 Sc hwedler , M. and B. Bradley . “ An Idea for Chilled-W a[...]

Ref erences SYS-APM00 1 -EN Chiller System Design and Control 101 15 Bahnfleth, W . and E. Peyer . “Compa rative Ana lysis of V ariable and Constant Primary -Flow Chilled-W ater -Plant P erformance. ” HP AC Engineerin g (April 20 0 1). 16 Houghton, D. “Know Y our Flow—A Market Survey of Liquid Flow Meters. ” E SOURCE Te c h U p d a t e TU[...]

10 2 Chiller System Design and Control SYS-APM00 1- EN Ref erences 32 T rane Applications Engineerin g Group. “Thermal Storage – Understanding the Choices. ” Ice Storage S ystems, Engineered Systems Clinics . T rane, 1 991 . ( ISS-CLC-2) 33 T rane Applications Engineerin g Group. “Thermal Storage – Understanding System Desig n. ” Ice St[...]

SYS-APM00 1 -EN Chiller System Design and Control 10 3 Index A absorption refriger ation 98 ASHRAE GreenGuide 27, 29, 33 Guideli ne 22 25 B bypass flow control 63 bypass lo cations 62 bypass valve 8 C campus pum pin g arrangements 49 centrifugal chiller capacity control 16 check valv es 46 chilled water flow rate 3, 27, 29 temperature 3, 27 chilled[...]

10 4 Chiller System Design and Control SYS-APM00 1- EN Index controls chilled-water system control 87 condenser- water system con trol 89 direct-digital 24 managing control complexit y 21 programmable-logic 24 cooling tower 13, 97 cooling water 97 cost implications 38 D dampers 9 decoupled system s 45 condenser water s ystem 93 double-ended 52 dist[...]

Index SYS-APM00 1 -EN Chiller System Design and Control 10 5 loads overview 7 low  T syndro me 79 low flow cooling-tower options 34 misconceptions 39 M manifolded pumps 11, 14 mechanical-compression refr igeration 98 mid-sized chilled-water systems 21 minimum pressure differential 89 monitoring system efficiency 25 motors pony 90 P parallel arra[...]

10 6 Chiller System Design and Control SYS-APM00 1- EN Index standard ratin g flow conditions 29 standard rating temper atures 28 system configur ations 42 system control 87 system controls 87, 89 system design options 27 system issues and challeng es 79 T temperature chilled water 3, 27, 28 condenser wate r 27, 29 control 85 out of range 85 standa[...]

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Tr a n e www .trane.com For more information, contact your local T r ane office or e-mail us at comfort@trane.com Literature O rder Number SYS-APM0 01 -EN Date May 2009 Super sedes SYS -APM0 01 -EN T rane has a policy of continuous product and pro duct data improvement and reserves the right to c hange design and specifications without notice.[...]