>>> By William P. Bahnfleth, PhD, PE & Eric B. Peyer Varying Views on Variable-Primary Flow C H I L L E D - W A T E R S Y S T E M S Variable -primary flow survey of designers, chiller manufacturers, and system users Secondary pumps Variable-primary flow chilled-water systems are of much current interest. It has been suggested that the primary/secondary chilled-water system concept has outlived its lifespan and that variable-primary flow is the answer to the problems that have led to its demise. 1 This view is not universally held and certainly deserves close scrutiny before it is accepted. As part of a recent research project investigating variable-primary flow systems, 2 the authors summarized available literature and conducted surveys and interviews of designers (43), chiller manufacturers (4), and system owner/operators (8). This article presents selected findings of this state-of-the-art review. WHAT IS VARIABLE-PRIMARY FLOW? For several decades, most large chilled water systems have used primary/secondary pumping. A highly simplified schematic of a primary/secondary system is shown in Figure 1. Hydraulically, the system is comprised of two independent circuits separated by a decoupling bypass pipe, through which water may flow freely in either direction. On the primary (plant) side of the system, Chiller Chiller Load Load Primary pumps Decoupling bypass FIGURE 1. Schematic of primary/secondary system. A member of HPAC Engineering s Editorial Advisory Board, William P. Bahnfleth PhD, PE, is an associate professor and director of the Indoor Environment Center in the Department of Architectural Engineering at Penn State University in University Park, Pa. He can be reached at wbahnfleth@psu.edu. Eric Peyer, an engineer with Grumman/Butkus Assoc. in Evanston, IL., can be reached at EBP@grummanbutkus.com March 2004 CHILLED WATER ENGINEERING S5
Chiller 1 Chiller 2 Load 1 Load 2 FIGURE 2. Schematic of a primary-only, variable-primary flow system. System pump(s) Low-flow bypass Percent difference, KW per ton pumps typically operating at constant speed are matched to design evaporator flow rates of the chillers. Primary flow occurs in steps as chillers are staged on and off. On the secondary (load) side of the system, pumps typically operating at variable speed maintain a differential pressure set point at a hydraulically remote location in the distribution system, while control valves modulate the flow of water as required to meet the cooling load. Variable-primary flow chilledwater plants permit variation in evaporator-water flow rate to match the demand for flow of a variable-flow distribution system. Consequently, it is not necessary to have separate primary and secondary circuits, and only a single set of pumps is required. A simple primaryonly, variable-primary flow system is shown schematically in Figure 2. As in the primary/secondary system, chilled water flow rate is controlled to meet the cooling load, and variable-speed pumps control differential pressure at a remote location in the distribution system. A variable-primary flow system should have a bypass as shown. This bypass is normally closed and opens only under low load conditions to ensure that minimum flow is maintained through the evaporators of operating chillers. BENEFITS AND COSTS Proponents of variable-primary flow systems point to three main potential benefits of this system type relative to the primary/secondary system: energy and operating cost savings, first cost savings, and better ability to tolerate below-design chilled water temperature differentials. Energy savings are possible whenever the secondary flow is 2.0 1.5 1.0 0.5 0.0-0.5-1.0-1.5-2.0 Percent design load less than the sum of the design flows of operating chillers. In a primary/secondary system, this scenario would result in excess primary flow and recirculation from supply to return through the bypass. This excess primary flow does not occur in a variableprimary flow system except when cooling load is so low that the bypass must open. Energy savings are also possible when conditions permit flow to one or more chillers to exceed design flow. This is possible when the system flow is within the sum of the maximum flows of the active chillers and cooling load is less than or equal to the sum of their capacities. In this case, chilled water pumping energy may not be saved, but auxiliary energy consumption of cooling tower fans and condenser water pumps is saved if variable flow prevents the starting of an additional chiller. The first cost of a primaryonly variable-primary flow plant is likely to be lower than that of a primary/secondary plant simply because two sets of pumps are replaced with one. The capacity produced by a chiller is proportional to the product of the evaporator flow rate and the temperature difference of entering and leaving chilled water. Low T syn- Percent design flow rate 120 80 60 40 100 80 60 40 20 FIGURE 3. Chiller part-load performance (KW per ton) for various combinations of flow rate and chilled-water temperature difference. 4 S6 CHILLED WATER ENGINEERING March 2004
Description Energy savings/ reduced operating costs 12 Lower first cost 11 Less space required Minimize number of chillers on line and KW/ton TABLE 1. Survey reasons for using variableprimary flow. drome can be a serious problem for primary/secondary systems with constant primary flow because it is not possible to reach full chiller capacity, unless design T is available. Variable-primary flow controls permit flow through evaporators to be increased above design value, making it possible to adjust to less than ideal chilled-water return temperatures. The major challenges of variable-primary flow are control complexity and stability. Primary/secondary system chiller staging sequences are relatively simple, well understood, and do not pose a serious challenge to the controls of packaged chillers. Variable-primary flow chiller staging requires more care so that chillers are not inadvertently shut down during simultaneous flow and load changes. The designer must also understand and properly apply low flow bypass controls. HARDWARE ISSUES The key component performance issue for variable-primary flow systems is the ability of chillers to respond to changes in flow. The minimum and maximum chilled water flow rates and maximum rate of change of 8 Simplicity 4 Owner preference 6 2 flow are important application considerations. Flow rate ranges are determined by maximum and minimum tube velocity limitations. A typical range is 3 to 12 ft per sec, although newer tube designs may be capable of even lower minimum velocities in the 1.5-ft-per-sec range. Maximum and minimum flows for a particular machine depend on where the design tube velocity is selected within this range. Clearly, maximum turndown cannot be lower than roughly 25 percent. In a plant with two or more chillers, this allows primary flow to vary over most of the hours of a typical cooling season. Manufacturers differ greatly in their allowances for maximum rate of change. These varied from not recommended for some models to as much as 30 percent of design per minute for others. One manufacturer recommends slower flow modulation for systems with shorter turnover time (i.e., the time required to circulate the entire volume of the piping system through the primary pumps). Absorption chillers can be used in variable-primary flow applications, although their ability to accommodate high flow rate variations is not as good as that of vapor-compression chillers. The effect of evaporator-flow variation on the energy consumption of chillers is also of interest to those considering the use of variable-primary flow. Published research and data provided by manufacturers indicates that to a first approximation, the energy use characteristics of constant speed drive vapor compression chillers are not affected by flow variation. 3,4 Figure 3 shows variation in energy consumption (KW per ton) of a chiller as both load and evaporator flow rate are varied. Energy use is within ±2 percent of design in all cases and within ±1 percent in most cases. SYSTEM CONTROL Successful system control in a variable-primary flow system requires working within the limitations of system components to adjust flow and chiller capacity to changing load conditions. The greatest risk to a variableprimary flow system is a sudden drop in flow rate when a new chiller is started and flow is diverted from the evaporators of active chillers to the evaporator of the starting chiller. For example, suppose that a variable-primary flow plant is just meeting a 50-percent cooling load with one of two identical chillers. Further increase of the load will require the addition of the second chiller. If its isolation valve is opened, there will be a rapid reduction of flow through the active chiller by as much as 50 percent that is likely to cause the active chiller to shut down. To prevent this problem, the capacity of active chillers should be limited prior to staging and the isolation valve for the added chiller should open slowly. Eppelheimer 5 gives a detailed discussion of this issue that includes a comparison with chiller staging in primary/secondary systems. He notes that the decoupling of primary and secondary circuits makes it possible to maintain design flow in the active chiller during staging, with excess primary flow recirculating through the bypass. Control of the low flow bypass is also a potential concern. The bypass should operate when one chiller is online and system flow is lower than its minimum flow. Modulation of the bypass valve is controlled by the flow or Description Lack of guidance/support from manufacturers and literature Recent technology /unproven Concerned about chiller performance Have not found right client/ application 13 7 5 Complexity 4 Unfamiliarity with the system TABLE 2. Survey reasons for not using variableprimary flow. 4 3 March 2004 CHILLED WATER ENGINEERING S7
pressure difference measured across the evaporator of the chiller. The valve, however, can be located almost anywhere in the system, so it must be capable of functioning under the range of differential pressure that it will experience. 6 The most critical constraint is that the bypass valve be able to pass its maximum flow at the lowest possible pressure differential, which is the minimum control head of the system. An alternative to this approach is to use a limited number of three-way valves in the system to ensure that system flow is always at or above the chiller minimum. ATTITUDES Surveys of chilled-water-system designers, owner/operators, and manufacturers revealed a wide variety of experience and opinion regarding the use of variable-primary flow. Roughly half of the designers surveyed had some level of variable-primary flow experience. This should not be taken as an indication of the proportion of all designers who are currently applying variable-primary flow concepts, but it does show that a variety of engineers are utilizing this approach. Reasons given for survey decisions to use or not use variable-primary flow are shown in tables 1 and 2, respectively. It is interesting to note that simplicity was identified as a reason for using variable-primary flow by those with experience, while complexity was cited as a negative factor by those without experience. Those who favored variable-primary flow focused on the benefits to the owner, while those who were not using it focused on the lack of detailed information on how to apply it and its relatively short track record. Claims that technical support for variable-primary flow from manufacturers and in published literature is lacking are somewhat contradicted by a growing body of bulletins and articles (e.g., Schwedler and Bradley 7,8 ) that provide specific guidance on variable-primary flow system design. Well-documented case studies, however, are still in short supply. By far, most of the variable-primary flow systems designed by survey respon- Chiller selection criteria Velocity or flow limits Evaporator water-side pressure drop Internal chiller controls capable of VPF Rate of change for flow rates No particular criteria specific to VPF 14 6 4 2 1 Circle 180 TABLE 3. Variable primary flow chiller selection criteria. S8 CHILLED WATER ENGINEERING March 2004
dents were primary-only systems because of the first-cost advantage of that type of system. Respondents reported having successfully applied variable-primary flow to systems with water-cooled and air-cooled vapor-compression chillers, as well as with absorption chillers. When asked which chiller selection issues were of the greatest importance, they gave the responses summarized in table 3. Owners and operators of systems who participated in the survey had few negative comments to offer on their experiences. Owners claimed that conversion of their systems to variableprimary flow had resolved low T problems and reduced energy use. Most of those surveyed would choose variableprimary flow systems in the future. This does not mean that variable-primary flow systems have not experienced problems, but it does provide evidence that successful variable-primary flow systems are being built. PERFORMANCE Documentation of the energy- and cost-saving potential of variable-primary flow systems is not plentiful and based mainly on simulation. These studies show significant reductions in pump energy savings and good economic performance. 7,9,10 Simulations have also demonstrated some key characteristics of variable-primary flow system behavior. Of particular importance is the rapidly diminishing pump energy savings for variable-primary flow as the number of parallel chillers in a plant increases because the increments of flow possible in a primary/secondary plant become smaller and more nearly continuous. However, the promise of simulated performance needs to be confirmed by more and more rigorous, detailed measurements taken from actual systems. CONCLUSION Variable-primary flow chilled water systems may, in appropriate applications, reduce both first cost and operating cost relative to primary/secondary systems. Improved packaged chiller controls make many current model chillers suitable for variable-primary flow application. Designers of variable-primary flow systems are developing a record of success and experience with this system type that should lead to continued growth in its use in the future. ACKNOWLEDGMENT Survey data presented in this article were collected as part of research project 611-20070 sponsored by the Air-Conditioning and Refrigeration Technology Institute (ARTI) under its HVAC&R Research for the 21st Century (21CR) program. REFERENCES 1) Kirsner, W. (1996, November). The demise of the primary-secondary pumping paradigm for chilled water plant design. HPAC Engineering, pp 73-79. 2) Bahnfleth, W., & Peyer, E. (2004). Variable-primary flow chilled water systems: potential benefits and application issues. Final Report to the Air-Conditioning and Refrigeration Technology Institute (under review). Arlington, VA.: ARI 3) Redden, G. H. (1996). Effect of variable flow on centrifugal chiller performance. ASHRAE Transactions, 102 (2), 684-687. 4) Berry, F. (2000). Personal communication. 5) Eppelheimer, D. (1996). Variable flow the quest for system energy efficiency. ASHRAE Transactions, 102 (2), 673-678. 6) Taylor, S. (2002, July). Piping chillers to variable volume chilled water systems. ASHRAE Journal, 36 (7), 43-45. 7) Schwedler, M. & Bradley, B. (2000, April). Variable-primary-flow systems. HPAC Engineering, pp 41-44. 8) Schwedler, M. & Bradley, B. (2003, March). Variable-primary flow in chilled-water systems. HPAC Engineering, pp 37-45. 9) Bahnfleth, W. & Peyer, E. (2001, May). Comparative analysis of variable and constant primary-flow chilled-water-plant performance. HPAC Engineering, pp 41-50. 10) Bellenger, L. (2003, September). Revisiting chiller retrofits to replace constant volume pumps. HPAC Engineering, pp 75-76, 78-80, 82, 84, 86, 116. Circle 152 March 2004 CHILLED WATER ENGINEERING S9