Optimizing Design & Control Of Chilled Water Plants Part 1: Chilled Water Distribution System Selection

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1 This article was published in ASHRAE Journal, July Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit Optimizing Design & Control Of Chilled Water Plants Part 1: Chilled Water Distribution System Selection By Steven T. Taylor, P.E., Fellow ASHRAE This is the first of a series of articles discussing how to optimize the design and control of chilled water plants. The series will summarize ASHRAE s Self-Directed Learning (SDL) course called Fundamentals of Design and Control of Central Chilled Water Plants and the research that was performed to support its development (see sidebar [Page 20] for topics to be discussed). The articles, and the SDL course upon which it is based, are intended to provide techniques for plant design and control that require little or no added engineering time compared to standard practice, but at the same time result in significantly reduced plant life-cycle costs. A procedure was developed to provide near-optimum plant design for most chiller plants including the following steps: 1. Select chilled water distribution system; 2. Select chilled water temperatures, flow rate, and primary pipe sizes; 3. Select condenser water distribution system; 4. Select condenser water temperatures, flow rate, and primary pipe sizes; 5. Select cooling tower type, speed control option, efficiency, approach temperature, and make cooling tower selection; 6. Select chillers; 7. Finalize piping system design, calculate pump head, and select pumps; and 8. Develop and optimize control sequences. Each of these steps is discussed in this series of five articles. This article discusses Step 1: Selecting chilled water distribution system type. Table 1 lists recommendations for the life-cycle cost optimum distribution system based on the size and number of loads served and the number of chillers. Life-cycle cost optimum is in quotes because these recommendations are generalizations that should apply to the majority of typical HVAC applications, but they may not prove to be optimum for every application and have not been rigorously proven as the best choice. They are based on the author s design and commissioning experience, the analysis that was done in conjunction with development of the ASHRAE SDL, work done on an earlier chilled water plant design manual, 1 and the prescriptive About the Author Steven T. Taylor, P.E., is a principal at Taylor Engineering in Alameda, Calif. 14 ASHRAE Journal ashrae.org July 2011

2 Application s/loads Served Chillers Size of s/loads Served Control Valves Recommended Distribution Type 1 One Any Any None 2 More Than One One Small (< ~100 gpm) 2-Way and 3-Way 3 4 Few s Serving Similar Loads Many s Serving Similar Loads or Any Serving Dissimilar Loads More Than One Small (< ~100 gpm) 3-Way More Than One Small (< ~100 gpm) 2-Way 5 More Than One Any Large Campus 2-Way 6 More Than One Any Large s (> ~100 gpm) None Table 1: Chilled water distribution system. Figure 1 Primary-Only Single Figure 2 Primary-Only Single Chiller Figure 3 Primary-Only Multiple Chillers Few s With Similar Loads Figure 4 Primary-Only or Figure 5 Primary-Secondary Figure 6 Primary Distributed Secondary Figure 7 Primary Secondary requirements of ASHRAE/IESNA Standard The intent is to allow designers to select the system that is most often the best choice from a life-cycle cost perspective for a given application without having to perform any lengthy analyses. CHW Pump Chiller Supply Water Temperature Primary-Only Single With one or more chillers serving a single cooling coil (Figure 1), the simplest design strategy is to not use any control valves at the coil. Instead, a constant-volume pump circulates water between the chiller and the coil and supply air temperature is controlled by resetting the temperature of the chilled water leaving the chiller. While constant chilled water flow results in constant pump energy, chiller performance is improved when the leaving chilled water temperature is reset to be as high as possible. A variable frequency drive () could also be added to the pump to make the system variable flow, but that adds cost and complexity. s are seldom cost effective since pump power is typically small in a single-coil plant because the chiller and coil are usually closecoupled (physically close together), and it is more efficient to increase chilled water temperature than to reduce pump speed and pump energy. (Part 5 of this series will further discuss the trade-off between resetting chilled water temperature and pump energy.) Many engineers are concerned about causing high space humidity with this design because chilled water temperatures must be aggressively reset to maintain supply air temperature at setpoint under low load conditions. But, in fact, that is never a concern because the leaving supply air condition is about the Optional Storage Tank Figure 1: Primary-only single coil. Supply Air Temperature same regardless of chilled water temperature. For example, if the supply air temperature leaving the coil is 55 F (13 C), the air leaving the coil is close to saturated whether the chilled water supply temperature is 42 F (6 C) or 50 F (10 C). It is the supply air temperature setpoint that determines space humidity conditions, not the chilled water temperature. Figure 1 shows a single chiller, but any number of chillers can be used. When two chillers are used, this is a good application for piping chillers in series * rather than in parallel. Figure 1 also shows an optional storage tank. Chilled water systems must have a sufficient volume of water in the piping system to prevent unstable temperature swings, possibly * In all of the figures shown in this article, multiple chillers are shown in parallel. For most applications, the chillers could alternatively be piped in series. This results in lower chiller energy use, partly offset by higher pump energy use. However, first costs are almost always higher with series piping due to larger piping and pumps and bypass piping typically provided to allow one chiller to operate while the other is down for maintenance. Because of limited funding, we did not evaluate the life-cycle costs of series piping and so we have not included it in our recommendations. This option will be evaluated for cost effectiveness in future versions of the ASHRAE SDL. July 2011 ASHRAE Journal 15

3 CHW Pump 3-Way Valve 2-Way Valve Chiller DP Sensor Supply Water Temperature Figure 2: Primary-only single chiller. causing chiller short-cycling. This is a potential problem with single-coil systems since they are typically closecoupled with only short piping runs. To compensate for the small water volume in piping, a small storage tank is often required. The minimum water volume should be verified with the chiller manufacturer. Primary-only Single Chiller Small chilled water plants commonly have a single chiller, typically air-cooled. Single chiller plants do not have to deal with flow and staging problems common to multiple chiller plants and thus can have a simple distribution and control system. The recommended design is shown in Figure 2. It is the simplest variable flow primary-only system. Two-way valves are installed at most coils with just enough three-way valves installed to maintain the minimum flow required by the chiller. This minimum rate, which can be obtained from the manufacturer, will vary with design chilled water flow rate and the chiller type, size, and manufacturer but is typically 25% to 50% of the design flow. A is shown in Figure 2; CHW Pumps 3-Way Valve Chiller No. 1 Chiller No. 2 Supply Water Temperature Figure 3: Primary-only multiple-chillers few coils with similar loads. s are typically cost effective except on very small systems. Note that Standard 90.1 requires s on chilled water pumps exceeding 5 hp (3.7 kw). The is controlled by a differential pressure (DP) sensor located near the most remote coil so that the DP setpoint can be as low as possible; this is also a requirement of Standard Locating the sensor near the pump requires a high DP setpoint and eliminates most of the energy savings from the. The three-way valves should be located near the chiller if the pump has a to minimize pump energy. Locating them remotely increases flow to the extremes of the system, which increases the pump pressure and power required. The one exception to this rule is that three-way valves must be located in a manner that engages enough water volume to maintain the minimum water volume required to minimize short cycling as discussed previously. There is usually no benefit to locating threeway valves remotely to keep the system cold so that chilled water is instantly available to coils; it typically takes just seconds or perhaps minutes for water to travel from the chiller to the most re- mote coil, and the load will not be lost in that short time. Primary-Only Multiple Chillers: Few s With Similar Loads When systems have multiple chillers, chiller staging can be a problem when flow and load do not track, and they generally do not when three-way valves are used. Consider the system shown in Figure 3. When the system operates near full load, performance is satisfactory since both chillers and pumps are operating. However, the system can have problems during part-load conditions depending how coil loads vary. For example, suppose the system shown in Figure 3 had two equally sized chillers and served two equally sized coils, each serving a hotel ballroom. If there were functions in both rooms and both rooms were above 50% load, the system operates well; both chillers with their associated pumps will run and each function space will receive its design flow. But when only one of the two function spaces is occupied and the other is vacant, the system, as a whole, will be below half load, so in theory only one chiller and pump could satisfy the load. However, the coil serving the unoccupied room will still use its design flow, bypassing it around the coil to the return. If the plant operates with only one chiller and pump, it has sufficient chiller capacity to meet the load, but it cannot meet the flow demands; the coil serving the occupied meeting room will be starved of flow. To avoid this problem, both pumps will have to operate, so both chillers will have to operate at or below 50% load. This problem is one of the reasons designers have migrated to variable flow designs, discussed later. But this system can work well as long as all coil loads tend to vary in the same proportion, as they might if all coils served similar occupancies (e.g., all serve offices on the same schedule). For instance, if the coils served are below half load and only one chiller and pump are operating, all coils will be capable of meeting their loads. The system is thus a quasi-variable-flow system in that pumps and chillers can be staged. Also, because 16 ASHRAE Journal ashrae.org July 2011

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5 the loads vary similarly, chilled water temperature may be reset aggressively, which allows the plant to be about as efficient as one of the true variable flow systems discussed later. This system is a reasonable choice for small applications with only a few coils serving similar loads; it is simple and inexpensive and avoids all of the complexities of variable flow systems. Also, small systems like this are typically close-coupled, so there is not much pump energy to save. Note that Standard 90.1 only allows this approach for systems with three coils or fewer or a total chilled water pump system power of 10 hp (10.7 kw) and less. Primary Pumps 2-Way Valve Bypass Primary-Only Variable Flow and Primary-Secondary Systems Application 4 in Table 1 is probably the most common. It applies to systems serving many small coils (or a few coils with dissimilar loads) and more than one chiller. In this case, either of two systems is recommended, primary-only variable flow (Figure 4) or primary-secondary (Figure 5). Both systems have plusses and minuses, discussed in detail in an article by Taylor 3 and summarized in Table 2. Primary-only systems always cost less and take up less space than primarysecondary systems, and with variable speed drives, primary-only systems also always use less pump energy than traditional primary-secondary systems. The pump energy savings are due to: Reduced system head as a result of the elimination of the extra set of pumps and related piping and devices (shut-off valves, strainers, suction diffusers, check valves, etc.). More efficient pumps. The primary pumps in the primarysecondary system will be inherently less efficient due to their high flow and low head. This can be partially mitigated by using larger pumps running at lower speed, but at an increase in first costs. Variable flow through the evaporator, which allows flow to drop below design flow down to some minimum flow rate prescribed by the chiller manufacturer. s can be added to the primary pumps of a primary-secondary system and controlled to track secondary flow down to the chiller minimum rate, but at an increase in first costs and control complexity. The lower energy costs and lower first costs of the primary-only system often make it an easy choice versus primarysecondary, but the system does have two significant disadvantages. Chiller 1 Chiller 2 DP Sensor Flow Meter Figure 4 (left): Primary-only variable flow. Figure 5 (right): Primary-secondary. 18 ASHRAE Journal ashrae.org July 2011 Advantages of Primary-Only Lower First Costs Less Plant Space Required Reduced Pump Peak Power Lower Pump Annual Energy Use Primary Pumps 2-Way Valve Chiller 1 Chiller 2 Common Leg (Decoupler) Secondary Pumps DP Sensor Disadvantages of Primary-Only Complexity of Bypass Control Complexity of Staging Chillers Table 2: Advantages and disadvantages of primary-only vs. primary-secondary systems. 1. Bypass Control Complexities A bypass valve (Figure 4) is required to ensure that minimum flow rates are maintained through operating chillers. The valve must be automatically controlled by flow, typically using a flow meter in the primary circuit (as shown in Figure 4) or differential pressure sensors across chillers correlated to flow. The flow meter is more costly but is more easily adapted into plant load (Btu) calculations, which will be necessary for optimum chiller staging (discussed in Part 5). Selecting the bypass control valve and tuning the control loop is sometimes difficult because of the widely ranging differential pressure across the valve caused by its location near the pumps. The valve must be large enough to bypass the minimum chiller flow through it with a pressure drop as low as the differential pressure setpoint used to control chilled water pump VSDs. This is because if only a few valves are open in the system, the pressure at the DP sensor location will be what is available at the plant as well since there is little pressure drop between the two points due to the low flow. But this makes the valve oversized for other flow scenarios that can

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7 occur, so tuning can be difficult. If the control loop is unstable, cold chilled water supply can be fed back into the return intermittently and cause chillers to cycle off due to low load or cold supply water temperatures. But if the loop is too slow, it may not respond quickly enough to sudden changes in flow (e.g., when a large number of air-handling units shut off at the same time), causing insufficient flow through the chillers, causing them to trip on low flow or low temperature. Complex control systems are prone to failure, so at some point in the life of the plant, one can expect the bypass control to fail. A failure of the bypass system can cause nuisance chiller trips, which generally require a manual reset. If an operator is not present to reset the chiller, the plant can be out of service for some time. 2. Staging Control Complexities When one or more chillers are operating and another chiller is started by abruptly opening its isolation valve (or starting its pump for dedicated pumps), flow through the operating chillers will abruptly drop. The reason for this is simple: flow is determined by the demand of the chilled water coils as controlled by their control valves. Starting another chiller will not create an increase in required flow, so flow will be split among the active machines. If this occurs suddenly, the drop in flow will cause operating chillers to trip. To stage the chillers without a trip, active chillers must first be temporarily unloaded (demand-limited or setpoint raised), then flow must be slowly increased through the new chiller by slowly opening its isolation valve. Then, all chillers can be allowed to ramp up to the required load together. During the staging sequence, chilled water temperatures will rise somewhat. This is seldom a problem in comfort applications, but may be an issue for some industrial applications. Given these considerations, primary-only systems are most appropriate for: Plants with many chillers (more than three) and with fairly high base loads, as might be expected in an industrial or data center application. For these plants, the need for bypass is minimal or nil due to the high base loads, and flow fluctuations during staging are small due to the large number of chillers. Plants where design engineers and future on-site operators understand the complexity of the controls and the need to maintain them. The primary-secondary system may be a better choice for buildings where fail-safe operation is essential or on-site operating staff is unsophisticated or nonexistent. Primary Distributed Secondary For plants serving groups of large loads such as buildings in a college campus, terminals in an airport, etc., the primary-distributed secondary system (Figure 6) is usually the best solution. The secondary pumps at the central plant are deleted and variable speed pumps are added at each building. The building pumps are controlled by differential pressure sensors at the most remote coil in each building. Building pump heads are Central Chilled Water Plants Series This series of articles will summarize the upcoming Self-Directed Learning (SDL) course called Fundamentals of Design and Control of Central Chilled Water Plants and the research that was performed to support its development. The series will include five segments: Chilled water distribution system selection. This article will discuss distribution system options, such as primary-secondary and primary-only pumping, and provide a simple application matrix to assist in selecting the best system for the most common applications. Condenser water distribution system selection. This article will discuss piping arrangements for chiller-condensers and cooling towers, including the use of variable speed condenser water pumps and water-side economizers. Pipe sizing and optimizing ΔT. This article will discuss how to size piping using life-cycle costs then how to use pipe sizing to drive the selection of chilled water and condenser water temperature differences (ΔTs). Chillers and cooling tower selection. This article will address how to select chillers using performance bids and how to select cooling tower type, control devices, tower efficiency, and wet-bulb approach. Optimized control sequences. The series will conclude with a discussion of how to optimally control chilled water plants, focusing on all-variable speed plants. The intent of the SDL (and these articles) is to provide simple yet accurate advice to help designers and operators of chilled water plants to optimize life-cycle costs without having to perform rigorous and expensive lifecycle cost analyses for every plant. In preparing the SDL, a significant amount of simulation, cost estimating, and life-cycle cost analysis was performed on the most common water-cooled plant configurations to determine how best to design and control them. The result is a set of improved design parameters and techniques that will provide much higher performing chilled water plants than common rules-of-thumb and standard practice. sized for the pressure drop of the loop from the plant, to the building, through the building s coils, then back to the plant through the common leg. Therefore, each pump has a different head customized for the building. The advantages of this design compared to conventional primary-secondary and primary-secondary-tertiary systems include: Overall pump horsepower is reduced. With the conventional system, secondary pump head must be sized for the most remote building (say 100 ft [299 kpa]) while the distributed building pumps close to the central plant can have much smaller heads (say 50 ft [150 kpa]). 20 ASHRAE Journal ashrae.org July 2011

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9 Building A Building B Chiller 1 Primary Pumps Primary Pumps Chiller N Chiller 2 Chiller 2 Secondary Pump Chiller 1 Common Leg CHW Supply CHW Supply Pump Figure 6: Primary-distributed secondary. The system is self-balancing via the speed controls on the secondary pumps. There is no need to throttle pressure at close buildings and flow self-adjusts over time as additional buildings are connected to the system. Overpressurization of control valves located near the central plant is eliminated. With large, high head secondary systems, these valves must operate against excess differential pressure, which can reduce controllability and may even force flow through the valve if it does not have sufficient shut-off head. Pump energy is reduced because of the custom pump heads and the more precise control of the variable speed drives. With a conventional primarysecondary system, the secondary pumps are typically controlled to maintain differential pressure at the entry to the most remote building. Therefore, the setpoint must be higher than for the distributed pump system, which is controlled by differential pressure at the most remote coil in each building. At part load, the pumps therefore can operate at slower speeds. With primary-secondary-tertiary systems, the tertiary pumps are generally piped with a bridge and two-way control valve. Control of the bridge is always difficult and, if done incorrectly, often the cause of degrading DT. 4 With this distributed pumping system, bridge connections are eliminated. The system will be less expensive, more energy efficient, and have lower maintenance costs than primary-secondary-tertiary systems. Disadvantages include: Expansion tank pressurization may have to be increased to maintain positive suction pressure at building pumps if the pumps are located at the top of campus buildings. This has only a minor cost impact to the expansion tank. Chiller 1 Chiller 2 Common Leg AHU 1 (Large ) Figure 7: Primary-coil secondary. AHU 2 (Large ) AHU 3 (Large ) Primary-distributed secondary systems will usually cost more than conventional primary-secondary systems because there are more pumps and space is required to house them in each building. Figure 8: Hybrid primary-coil secondary and primary-secondary system. 22 ASHRAE Journal ashrae.org July 2011

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11 Primary Secondary For plants serving large individual air-handling systems, using distributed variable speed driven coil secondary pumps (Figure 7, Page 22) is usually the best solution. The advantages of this design compared to a conventional primary-secondary system include: Connected pump motor hp is reduced. This is due in part because of the customized heads for each pump but also because the Advertisement formerly in this space. control valve is eliminated. Two-way control valves are typically selected for a wide-open pressure drop of 4 to 5 psi (27.5 kpa to 34.4 kpa), about 10 ft (29.9 kpa). This is a substantial savings. The system is self-balancing. There is no need for balancing valves of any kind nor are there any advantages to self-balancing designs such as reverse-return arrangements. Pump energy is significantly lower with this design. This is due mostly to the reduced pump heads but also because there is no need to maintain a minimum differential pressure in the system as there is with conventional secondary pumps. Because of this minimum DP and because of the throttling caused by partially closed control valves, conventional secondary pumps will not follow the theoretical parabolic system curve. Hence, pump efficiency will generally get worse, particularly at low load. With the variable speed coil pump design, there are no control valves or minimum DP, so pump efficiency will be nearly constant. Control of large control valves is inherently slow due to the size and slow responsiveness of the valve. With the coil pump design, flow can be controlled almost instantaneously with the, so control is precise. There is also no fear of over-pressurizing control valves, which reduces their controllability. Because of the eliminated control valves and lower pump HP, this system will generally have lower costs than a conventional primary-secondary system. It is usually a little more expensive than a primary-only system. Control valves can be thought of as brakes on a car while pumps are the car engines; from an energy perspective, it never makes sense to press both the brake and the accelerator pedals at the same time, but that is effectively what systems with control valves do. So, this system is actually ideal from a pumping perspective: it has no brakes. Unfortunately, there are a few disadvantages of this system. First, all coils must have a pump. If a coil were connected to the secondary circuit without a pump, flow through the coil will be backwards from the return to the supply. For a building that has a 24 ASHRAE Journal July 2011

12 mixture of small coils and large coils, pumps for the small coils will most likely have to be expensive multistage pumps. Another disadvantage is the increased exposure to equipment failure. A control valve is extremely reliable the pump and in this design are more likely to fail. Duplex pumps could be used to improve redundancy, but the cost is prohibitive in most situations. Our philosophy is to provide the same level of redundancy as the rest of the system served. For instance, if the air handler has only a single fan, then it makes sense to provide only a single pump. For more critical applications, redundant pumps, or an alternative distribution system design, should be considered. For this design to be energy efficient, coils must be large due to the inherent inefficiency of small pumps, particularly low flow/high head pumps. For instance, a typical pump at 60 ft of head (179 kpa) will have an efficiency on the order 30% at 20 gpm (1.3 L/s), 50% at 50 gpm (3.2 L/s), 60% at 100 gpm (6.3 L/s), and 70% at 200 gpm (12.6 L/s). That is why this system is recommended only for coils with flows greater than 100 gpm (6.3 L/s) in Table 1. This flow limit is obviously a rough rule-ofthumb since efficiency will vary over a range, not drop abruptly below 100 gpm (6.3 L/s). If a project includes both small and large coils, a hybrid system of both distributed coil pumps and conventional secondary pumps to serve small coils is possible. See Figure 8, Page 22, for an example hybrid plant. References 1. Energy Design Resources CoolTools Chilled Water Plant Design Guide. 2. ASHRAE Standard , Energy Standard for Buildings Except Low-Rise Residential Buildings. 3. Taylor, S Primary-only vs. primary-secondary variable flow systems. ASHRAE Journal 44(2). 4. Taylor, S Degrading chilled water plant delta-t: causes and mitigation. ASHRAE Transactions 108(1). Advertisement formerly in this space. Summary This article is the first in a series of five that summarize chilled water plant design techniques intended to help engineers optimize plant design and control with little or no added engineering effort. In this article, a simple look-up table is provided to allow designers to select a near optimum chilled water distribution system based on their application without having to do any rigorous life-cycle cost or system analysis. Next month, condenser water distribution system selection will be addressed. July 2011 ASHRAE Journal 25