Trane SYS-APM001-EN - Manuals

SYS-APM001-EN Chiller System Design and Control 1 Primary System Components Chilled-water systems consist of these functional parts: • Chillers that cool the water or fluid • Loads, often satisfied by coils, that transfer heat from air to water • Chilled-water distribution pumps and pipes that send ...

2 Chiller System Design and Control SYS-APM001-EN Primary System Components Figure 1. Typical vapor-compression chiller Water-cooled chillers are typically installed indoors; air-cooled chillers are typically installed outdoors—either on the roof or next to the building. In cold climates, air-cooled...

Primary System Components SYS-APM001-EN Chiller System Design and Control 3 • In a direct-expansion (DX) shell-and-tube evaporator (Figure 3), warmer water fills the shell while the cool, lower-pressure liquid refrigerant flows through the tubes. Figure 3. Direct-expansion evaporator cut-away In eit...

4 Chiller System Design and Control SYS-APM001-EN Primary System Components Some chiller controls can accommodate very little flow variation during machine operation. 2 Other, more sophisticated, chiller controls allow some flow variation. Some chillers can tolerate flow-rate variations—as much as 5...

Primary System Components SYS-APM001-EN Chiller System Design and Control 5 While they found that some of the internally - enhanced tubes fouled in the long term, they concluded: Because of the high hardness and low water velocity used in these tests, we do not believe that the fouling experienced i...

6 Chiller System Design and Control SYS-APM001-EN Primary System Components Low-ambient operation Air-cooled chillers are often selected for use in systems with year-round cooling requirements that cannot be met with an airside economizer. Air-cooled condensers have the ability to operate in below-f...

Primary System Components SYS-APM001-EN Chiller System Design and Control 7 Figure 4. Air-cooled or water-cooled efficiency Another advantage of an air-cooled chiller is its delivery as a “packaged system.” Reduced design time, simplified installation, higher reliability, and single-source responsib...

8 Chiller System Design and Control SYS-APM001-EN Primary System Components Heat transferred from the loads can be controlled in a number of ways: • Three-way valve • Two-way valve • Variable-speed pump • Face-and-bypass dampers Three-way valve load control A three-way control valve (Figure 5) regul...

Primary System Components SYS-APM001-EN Chiller System Design and Control 9 Figure 6. Two-way valve Variable-speed pump load control By using a pump for each coil (Figure 7), the flow may be controlled by varying the pump speed. In such systems, there may be no control valves at the coil. This can r...

10 Chiller System Design and Control SYS-APM001-EN Primary System Components pumped all the time; however, in systems with very small water pressure drops, this system arrangement may work economically. Figure 8. Uncontrolled water flow with bypass damper Chilled-Water Distribution System Chilled wa...

Primary System Components SYS-APM001-EN Chiller System Design and Control 11 • accommodates the total pressure (static head plus dynamic head) on system components such as the chiller’s evaporator, valves, etc. Note that the pump heat is added to the water and must be absorbed by the chiller. Genera...

12 Chiller System Design and Control SYS-APM001-EN Primary System Components valves may be either three-way or two-way. As previously discussed, three-way valves require constant water flow, while two-way valves allow the water flow in the system to vary. As flow varies, the pump may simply ride its...

Primary System Components SYS-APM001-EN Chiller System Design and Control 13 the series, or another pumping arrangement can be considered. Reducing the flow rate affects this system type’s energy use all the time, so careful attention to flow rates and temperature is critical (refer to “System Desig...

14 Chiller System Design and Control SYS-APM001-EN Primary System Components water entering and leaving the cooling tower is the range. The temperature difference between the leaving water temperature and the entering wet-bulb temperature is the approach. Effect of load on cooling tower performance ...

Primary System Components SYS-APM001-EN Chiller System Design and Control 15 Unit-Level Controls The chilled-water supply temperature is usually controlled by the chiller. Most commonly, supply water temperature is used as the sensed variable to permit control of chiller capacity to meet system load...

16 Chiller System Design and Control SYS-APM001-EN Primary System Components In addition to monitoring data, it is vital that the chiller controls alert operators to possible problems. Diagnostic messages are necessary for the operator to respond to safety issues and data points that are outside nor...

Primary System Components SYS-APM001-EN Chiller System Design and Control 17 by reducing motor speed at “low-lift” conditions, when cooler condenser water is available. Certain system characteristics favor the application of an AFD, including: • A substantial number of part-load operating hours (for...

18 Chiller System Design and Control SYS-APM001-EN Application Considerations Chiller system size affects design and control considerations. Each size comes with its own set of advantages and challenges. Small Chilled-Water Systems (1-2 chillers) Figure 17. Small chilled-water system schematic A com...

Application Considerations SYS-APM001-EN Chiller System Design and Control 19 Constant flow Constant flow is simple and often applied to small systems up to 200 tons—as long as the system pressure drop is fairly low and a wider  T is applied to reduce the system flow rate. In constant flow systems,...

20 Chiller System Design and Control SYS-APM001-EN Application Considerations part of those jobs. See “Energy and economic analysis of alternatives” on page 26. Number of chillers The number of chillers to install is a function of redundancy requirements and first cost. In general, the more chillers...

Application Considerations SYS-APM001-EN Chiller System Design and Control 21 than at full load. Variable frequency drives for unloading tower fans and chilled-water pumps may provide benefits, depending on the costs, system operating hours, system type, and outdoor air conditions. (See “System Cont...

Application Considerations SYS-APM001-EN Chiller System Design and Control 23 Creating one centralized chilled-water system takes significant foresight, initial investment, and building development with a multi-year master plan. If the initial plant is built to accommodate many future buildings or l...

24 Chiller System Design and Control SYS-APM001-EN Application Considerations To minimize power, large systems must be very efficient. The upside of a large system is the amplification of energy savings. A relatively small percentage of energy saved becomes more valuable. For this reason, the highly...

Application Considerations SYS-APM001-EN Chiller System Design and Control 25 Guidelines for system efficiency monitoring ASHRAE Guideline 22 Instrumentation for Monitoring Central Chilled-Water Plant Efficiency 6 was first published in June 2008. It states: Guideline 22 was developed by ASHRAE to p...

26 Chiller System Design and Control SYS-APM001-EN Application Considerations Energy and economic analysis of alternatives The process of making decisions between multiple, competing alternatives is simplified with the assistance of simulation software. Many packages are available for this purpose (...

SYS-APM001-EN Chiller System Design and Control 27 System Design Options There are many chilled-water-system design options; however, in a basic sense, each option is a function of flow, temperature, system configuration, and control. This section discusses the effect of flow rate and temperature de...

28 Chiller System Design and Control SYS-APM001-EN System Design Options recommends a design method that starts with condenser-water temperature difference of 12°F to 18°F [7°C to 10°C]. Standard rating temperatures Currently, the standard rating condition temperatures in ARI 550/590 5 and ARI 560 9...

System Design Options SYS-APM001-EN Chiller System Design and Control 29 Condenser-Water Temperatures Today’s chillers can run at various entering condenser-water temperatures, from design temperature to the lowest-allowable temperature for that particular chiller design. However, many existing olde...

30 Chiller System Design and Control SYS-APM001-EN System Design Options Selecting flow rates Designers may use the standard rating conditions to compare manufacturers’ performances at exactly the same conditions. However, these standards allow any flow rates to be used and certified comparisons to ...

32 Chiller System Design and Control SYS-APM001-EN System Design Options The total system power is now as follows: * Low-flow conditions represented in Table 5 through Table 8 are 1.5 gpm/ton [0.027 L/s/kW] chilled water and 2.0 gpm/ton [0.036 L/s/kW] condenser water. Figure 20. System summary at fu...

System Design Options SYS-APM001-EN Chiller System Design and Control 33 Figure 21. Chilled water system performance at part load While the magnitude of the benefit of low-flow changes depends on the chiller type used (centrifugal, absorption, helical-rotary, scroll), all chilled-water systems can b...

34 Chiller System Design and Control SYS-APM001-EN System Design Options performance of this coil when it is selected with a 44°F [6.7°C] entering fluid temperature and a 10°F [5.6°C] fluid temperature rise (  T). To provide the required 525 MBh [154 kW] of cooling capacity, the coil requires 105 g...

System Design Options SYS-APM001-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 equivalent heat rejection, U x A 1 x  T 1 = U x A 2  x  T 2 and for a constant coefficient of heat tra...

36 Chiller System Design and Control SYS-APM001-EN System Design Options Same tower, larger chiller One retrofit option that benefits many building owners is installing a new, larger chiller selected for a lower flow rating and re-using the existing cooling tower, condenser-water pump, and condenser...

System Design Options SYS-APM001-EN Chiller System Design and Control 37 It quickly becomes evident that the same cooling tower and flow rate are adequate to reject more heat—in this case, approximately 50 percent more heat. Figure 22. Cooling tower re-selection with different chiller capacities Ret...

38 Chiller System Design and Control SYS-APM001-EN System Design Options In both cases, either reusing an existing tower, or reusing existing chilled water piping, the design engineer can often help reduce total project costs using the existing infrastructure by selecting a chiller with a higher tem...

System Design Options SYS-APM001-EN Chiller System Design and Control 39 Figure 23. Annual system operating costs (absorption chillers) Kelly and Chan 10 compare the operational costs of chilled-water system designs in site locations. Their summary states: In conclusion, there are times you can ’hav...

40 Chiller System Design and Control SYS-APM001-EN System Design Options and a more conservative zero condenser-water-pipe pressure drop, we can examine the effect of reducing flow rates. Figure 24. System energy consumption (no pipes) Energy consumption for the chiller, condenser-water pump, and co...

System Design Options SYS-APM001-EN Chiller System Design and Control 41 Misconception 2—Low flow only works for specific manufacturers’ chillers. Demirchian and Maragareci 12 , Eley 13 , and Schwedler and Nordeen 11 independently showed that system energy consumption can be reduced by reducing flow...

42 Chiller System Design and Control SYS-APM001-EN System Configurations Multiple chilled-water systems are more common than single chilled-water systems for the same reason that most commercial airplanes have more than one engine—the balance of reliability and cost. The most typical system configur...

44 Chiller System Design and Control SYS-APM001-EN System Configurations Series Chillers If chillers are piped in series, as in Figure 27, the mixing problem disappears and the starving coils problem (when one of the pumps in a parallel arrangement is not running) is resolved. Series flow presents a...

System Configurations SYS-APM001-EN Chiller System Design and Control 45 percent of the system load. At system loads greater than 50 percent, the upstream chiller is preferentially loaded because it will attempt to produce the design leaving chilled-water temperature. Any portion of the load that re...

System Configurations SYS-APM001-EN Chiller System Design and Control 47 Production An individual production (chiller) pump need only pump water from the return bypass tee (point A in Figure 29), through its chiller, and into the tee at the supply-end of the bypass line (point B in Figure 29). This ...

48 Chiller System Design and Control SYS-APM001-EN System Configurations Distribution Distribution pumps take water from the supply water tee (point B in Figure 29), push it through all the distribution piping and load terminals, and then on to the return water tee (point A in Figure 29). This pump ...

System Configurations SYS-APM001-EN Chiller System Design and Control 49 Elevated return-water temperatures. Because unused chilled water does not bypass the cooling coils (two-way, rather than three-way, control valves), all water that is returned accomplishes some cooling. Theoretically, the retur...

50 Chiller System Design and Control SYS-APM001-EN System Configurations Figure 33. Tertiary pumping arrangement Decoupled system–principle of operation At the tee connecting the supply and bypass lines, a supply–demand relationship exists, as shown in Figure 34. Think of the total flow rate from al...

System Configurations SYS-APM001-EN Chiller System Design and Control 51 show a deficit and the pump will be cycled on again. The amount of surplus flow necessary depends on the size of the chiller to be shut off. The surplus flow must exceed a certain quantity before shutting off a chiller–pump pai...

52 Chiller System Design and Control SYS-APM001-EN System Configurations Chiller sequencing in decoupled systems Given the amount and direction of flow in the bypass line, chillers can be added or subtracted. Adding a chiller When there is deficit flow in the bypass line, the system is receiving wat...

System Configurations SYS-APM001-EN Chiller System Design and Control 53 Figure 36. Double-ended decoupled system One of the benefits of decoupled water systems is that they are simple to control. The distribution pump flow is determined by a pressure transducer located at the furthest load. Flow in...

54 Chiller System Design and Control SYS-APM001-EN System Configurations When more than one chiller plant is operating, finding the right location for the differential pressure sensor can be difficult. The point of lowest pressure in the system shifts depending on which loads are using the most wate...

System Configurations SYS-APM001-EN Chiller System Design and Control 55 with a surplus that may, or may not, be large enough to indicate stopping a chiller in that plant. Other plant designs There are many other ways to connect chillers to distributed loops and each provides its own challenges and ...

56 Chiller System Design and Control SYS-APM001-EN System Configurations • The bypass can be positioned either upstream or downstream of the cooling coils. • A control valve in the bypass ensures that the amount of flow through the operating chiller(s) never falls below the minimum limit, but remain...

System Configurations SYS-APM001-EN Chiller System Design and Control 57 secondary systems. The pressure drops previously satisfied by the distribution pumps are instead satisfied by the now larger primary-only pumps, permitting selection of larger, more efficient pumps (with efficiencies similar to...

58 Chiller System Design and Control SYS-APM001-EN System Configurations Experience with actual VPF plants indicates that a minimum evaporator-flow limit of 60 percent for packaged chillers and 40 percent or less for configured chillers work well. Chiller manufacturers specify minimum and maximum li...

System Configurations SYS-APM001-EN Chiller System Design and Control 59 Small packaged chillers typically offer less design flexibility than larger machines. It may not be possible to select a small packaged chiller with a minimum flow rate of less than 60 percent of the design system flow… but don...

60 Chiller System Design and Control SYS-APM001-EN System Configurations rate changes (Table 14). Selecting chillers with these characteristics improves the likelihood of stable, uninterrupted operation. Estimate the expected flow-rate changes and make sure that the chillers you select can adapt to ...

System Configurations SYS-APM001-EN Chiller System Design and Control 61 evaporator because its selection 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 satisfy a load that exceeds it...

62 Chiller System Design and Control SYS-APM001-EN System Configurations Accurate flow measurement The success of a variable-primary-flow installation depends on the quality of the flow-measuring device that controls the system bypass valve (and perhaps also indicates the plant load). Some practitio...

System Configurations SYS-APM001-EN Chiller System Design and Control 63 • Locate a bypass line and valve near the end of the piping 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-o...

64 Chiller System Design and Control SYS-APM001-EN System Configurations flow nears the maximum limit for the operating chiller(s), another machine must be brought online. Similarly, as the system load and flow decrease, chillers must be shut down to reduce the need for bypass water flow. Adding a c...

System Configurations SYS-APM001-EN Chiller System Design and Control 65 Controlling transient flows is mandatory, regardless of plant size. The number of chillers in the plant will not alter the degree of care needed to properly manage transient flow-rate changes because the transition from one ope...

66 Chiller System Design and Control SYS-APM001-EN System Configurations A more conservative approach might be to wait to turn off the chiller until it would result in no higher than 80 percent capacity for the remaining operating chillers. Going back to the example, if the desired (n-1) chiller cap...

System Configurations SYS-APM001-EN Chiller System Design and Control 67 Plant configuration Consider a series arrangement for small VPF applications. When the plant consists of only two chillers and expansion is unlikely, you can simplify control by piping the evaporators in series. Doing so avoids...

68 Chiller System Design and Control SYS-APM001-EN System Configurations Figure 39. Example of operating-cost savings 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 building in St. Louis, Missouri. Moderate “lo...

System Configurations SYS-APM001-EN Chiller System Design and Control 69 • Understand the specific loading/unloading characteristics of the chiller controller Bypass flow • Select a high-quality control valve with linear-flow characteristics • Select flow-sensing devices that deliver precise, repeat...

70 Chiller System Design and Control SYS-APM001-EN Chilled-Water System Variations A number of chilled-water system variations can and should be used when appropriate. Each configuration offers specific advantages to solve problems and add value to the system. Heat Recovery ASHRAE/IESNA Standard 90....

Chilled-Water System Variations SYS-APM001-EN Chiller System Design and Control 71 load. The details of operation are discussed in “Sidestream plate-and-frame heat exchanger” on page 74. Plate-and-frame heat exchangers isolate the building loop from the water in the open cooling tower loop, but they...

Chilled-Water System Variations SYS-APM001-EN Chiller System Design and Control 73 body of water. Flow rates need to be carefully selected to balance the economic and environmental requirements. Preferential Loading Preferential loading is desirable for systems that use heat recovery or free cooling...

74 Chiller System Design and Control SYS-APM001-EN Chilled-Water System Variations One caveat when applying this arrangement is that chillers on the production side of the bypass line will run more often at low part-load conditions. Older chillers or newer chillers with a high cycle point may not ha...

Chilled-Water System Variations SYS-APM001-EN Chiller System Design and Control 75 Sidestream heat recovery A similar situation occurs if a heat-recovery chiller is placed in this sidestream position 24 (see Figure 46). This chiller may be equipped with a heat recovery condenser or it could be a sta...

76 Chiller System Design and Control SYS-APM001-EN Chilled-Water System Variations Sidestream system control The flexibility of sidestream applications is increased by the fact that the devices are used to pre-cool return water, not to produce the system chilled-water temperature. This means that th...

Chilled-Water System Variations SYS-APM001-EN Chiller System Design and Control 77 Series–Counterflow Application Another system configuration that can be very energy efficient incorporates the previously described series application, but does so for both the chilled water and condenser water. Figur...

78 Chiller System Design and Control SYS-APM001-EN Chilled-Water System Variations series. The left half of Figure 50 shows a modularized configuration where series chiller modules are placed in parallel with each other, so that any upstream chiller’s valves could be “paired” with virtually any down...

SYS-APM001-EN Chiller System Design and Control 79 System Issues and Challenges Low  T Syndrome For many years the “low  T syndrome” debate has raged. 27, 28 The symptom of the problem is that, in large systems, return-water temperature is too low, thus not allowing the chillers to fully load. Man...

80 Chiller System Design and Control SYS-APM001-EN System Issues and Challenges • Flow Rate = the system flow rate, in gpm [L/s] • Loop Time = the time it takes for fluid to leave the chiller, move through the system, and return to the chiller, allowing for stable system operation , in minutes [seco...

System Issues and Challenges SYS-APM001-EN Chiller System Design and Control 81 Contingency Today, many organizations have contingency plans for critical areas of their business. Some deal with natural disasters and others with the loss of power in critical areas. However, few have actually taken th...

82 Chiller System Design and Control SYS-APM001-EN System Issues and Challenges Location of equipment Location can be a major factor in contingency planning. When selecting the location of the temporary equipment, it is important to consider: • Water and electrical connections location • Sound sensi...

System Issues and Challenges SYS-APM001-EN Chiller System Design and Control 83 situation. Electrical generation can be outsourced to avoid internal capitalization. A variation of electrical generation uses an engine indirectly- or directly-coupled to a chiller. Either variation produces chilled wat...

84 Chiller System Design and Control SYS-APM001-EN System Issues and Challenges Retrofit Opportunities A tremendous retrofit opportunity can be realized if the low-flow concepts discussed in the chapter “System Design Options” on page 27 are utilized. Building owners may need to increase the capacit...

System Issues and Challenges SYS-APM001-EN Chiller System Design and Control 85 Temperatures out of range 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 [35°C]. The accuracy required is more precise than the cooling tower can provide...

SYS-APM001-EN Chiller System Design and Control 87 System Controls Chilled-Water System Control Chilled water reset—raising and lowering Many chilled-water plants use chilled water reset, that is, the chiller’s leaving-water temperature setpoint, in an effort to reduce chiller energy consumption. Th...

88 Chiller System Design and Control SYS-APM001-EN System Controls high. The control point is selected to minimize over-pressurizing the system and to assure adequate flow at all critical loads. Critical valve reset (pump pressure optimization) Often, pumps are controlled to maintain a constant-pres...

System Controls SYS-APM001-EN Chiller System Design and Control 89 If the chiller and tower capabilities are conducive to this strategy, the location and load profile determine if, when, and for how long the right conditions might occur. Determine the optimum control sequence for the entire plant by...

90 Chiller System Design and Control SYS-APM001-EN System Controls The flow reduction options include: • Cooling tower bypass • Chiller bypass • One or two throttling valves in the condenser-water pipe with the pump riding its curve • A variable-speed condenser water pump After the minimum-pressure ...

System Controls SYS-APM001-EN Chiller System Design and Control 91 prevalent than either two-speed fans or pony motors. Using variable-speed drives on cooling-tower fans offers two distinct benefits. First, the tower-water-temperature control is extremely good. Second, the fan power varies with the ...

92 Chiller System Design and Control SYS-APM001-EN System Controls Variable condenser water flow Chiller-tower-pump balance There are times when a system designer may choose to vary the condenser water flow in addition to, or instead of, the cooling-tower fan speed. This may be beneficial in systems...

System Controls SYS-APM001-EN Chiller System Design and Control 93 These three energy consumers must be balanced to minimize overall energy use. This makes varying condenser water flow complex, but the strategy below has been implemented on projects. Control of the condenser water pumps and cooling ...

System Controls SYS-APM001-EN Chiller System Design and Control 95 Failure Recovery With all the varied approaches available to potential customers, it sometimes seems that the main idea gets lost. People purchase chilled-water plants to reliably produce chilled water to satisfy another need, such a...

96 Chiller System Design and Control SYS-APM001-EN Conclusion It is vital to have a clear understanding of chilled-water system concepts and their application. There is nothing particularly complex about the principles involved. Instead, system design is simply a matter of exercising a few key rules...

SYS-APM001-EN Chiller System Design and Control 97 Glossary ASHRAE. American Society of Heating, Refrigerating, and Air-Conditioning Engineers (www.ashrae.org). building automation system (BAS). A centralized control and monitoring system for a building. chilled water. Also known as leaving-chilled-...

98 Chiller System Design and Control SYS-APM001-EN Glossary COP. Coefficient of Performance; cooling effect divided by heat input (dimensionless); the reciprocal of efficiency. direct digital control. Programming used by building control systems to control variable outputs, such as valves or actuato...

Glossary SYS-APM001-EN Chiller System Design and Control 99 temperature, ambient. The temperature of the air surrounding the object under consideration. temperature, wet-bulb. A measure of the degree of moisture in the air. It is the temperature of evaporation for an air sample, measured with a ther...

100 Chiller System Design and Control SYS-APM001-EN References 1 Webb, R.L. and W. Li. “Fouling in Enhanced Tubes Using Cooling Tower Water, Part I: Long-Term Fouling Data.” International Journal of Heat and Mass Transfer 43, no. 19 (October 2000): 3567-3578. 2 Schwedler, M. and B. Bradley. “An Idea...

References SYS-APM001-EN Chiller System Design and Control 101 15 Bahnfleth, W. and E. Peyer. “Comparative Analysis of Variable and Constant Primary-Flow Chilled-Water-Plant Performance.” HPAC Engineering (April 2001). 16 Houghton, D. “Know Your Flow—A Market Survey of Liquid Flow Meters.” E SOURCE ...

102 Chiller System Design and Control SYS-APM001-EN References 32 Trane Applications Engineering Group. “Thermal Storage – Understanding the Choices.” Ice Storage Systems, Engineered Systems Clinics . Trane, 1991. ( ISS-CLC-2) 33 Trane Applications Engineering Group. “Thermal Storage – Understanding...

SYS-APM001-EN Chiller System Design and Control 103 Index A absorption refrigeration 98 ASHRAE GreenGuide 27, 29, 33 Guideline 22 25 B bypass flow control 63 bypass locations 62 bypass valve 8 C campus pumping arrangements 49 centrifugal chiller capacity control 16 check valves 46 chilled water flow...