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Trane Engineers Newsletter, vol 31-4

Engineers newsletter providing insights for today's hvac system designer the saga continues . Variable-Primary-Flow Systems Revisited from the editor Perhaps this explains the increased pressure drops and efficiency of the Obtaining reliable, cost-effective HVAC interest in chilled water systems A VPF design displaces the performance remains a priority for with variable primary flow (VPF). small, inefficient, low-head primary building owners, facility managers, and VPF designs use fewer pumps and pumps used in primary secondary system designers alike. A 1999 fewer piping connections than primary systems. The pressure drops Engineers newsletter (volume 23, secondary systems, which means previously satisfied by the primary number 3) discussed one avenue for fewer electrical lines and a smaller pumps are instead satisfied by the achieving that goal in a chilled water footprint for the plant.

4 Trane Engineers Newsletter — Vol. 31, No. 4 2 percent per minute and would need 30 minutes to adapt to a flow-rate reduction of 50 percent. Fluctuations of

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Transcription of Trane Engineers Newsletter, vol 31-4

1 Engineers newsletter providing insights for today's hvac system designer the saga continues . Variable-Primary-Flow Systems Revisited from the editor Perhaps this explains the increased pressure drops and efficiency of the Obtaining reliable, cost-effective HVAC interest in chilled water systems A VPF design displaces the performance remains a priority for with variable primary flow (VPF). small, inefficient, low-head primary building owners, facility managers, and VPF designs use fewer pumps and pumps used in primary secondary system designers alike. A 1999 fewer piping connections than primary systems. The pressure drops Engineers newsletter (volume 23, secondary systems, which means previously satisfied by the primary number 3) discussed one avenue for fewer electrical lines and a smaller pumps are instead satisfied by the achieving that goal in a chilled water footprint for the plant.

2 These factors distribution pumps, permitting selection system: variable primary flow. reduce the initial cost of the chilled of larger, more efficient pumps (with water efficiencies similar to those of the The growing number of successful secondary pumps in a primary . VPF installations has broadened their As for operating costs, how much a secondary system). acceptance, as well as our experience VPF design saves depends on the with them. Yet the inherent complexity makes many building professionals 2 Bahnfleth and Peyer discuss the operational reluctant to implement variable primary 1 The savings may be partially offset by additional savings of VPF designs in Comparative Analysis costs for flow-monitoring and bypass flow of Variable and Constant Primary-Flow Chilled- flow.

3 If this describes you, perhaps this (bypass line and control valve). VPF designs may Water-Plant Performance (HPAC Engineering, EN will allay your fears. In it, we provide also require more programming for system April 2001). For most common systems, specific guidelines for designing and control than other designs. however, the primary pump power on which they base their assessment may be too high. operating a VPF plant, and identify key equipment characteristics that are particularly critical for reliable operation. Figure 1. Variable-primary-flow system (chilled water loop). The desire to make or save money lies at the heart of many of our decisions.

4 In the context of HVAC design, decisions made to save money often involve a tradeoff between acquisition expense and operating cost. If you can realize savings on both fronts, so much the better. 2002 American Standard Inc. All rights reserved Volume 31, No. 4 . Table 1. Flow-rate changes that result from isolation-valve operation What Makes Number of operating chillers VPF Systems Different 1 2 3 4 5. Before we identify the ingredients that Flow-rate reduction when an isolation valve opens a 50% 33% 25% 20% 17%. go into the successful application of variable primary flow, let's review the a Flow-rate reduction is expressed as a percentage of the actual chilled water flow rate prior to transition: traits that distinguish this type of number of chillers operating % flow-rate reduction = 1 ---------------------------------------- ---------------------------------------- --------------------- number of chillers operating + 1.

5 System from the more familiar primary secondary design, which hydraulically decouples the constant- flow production side of the chilled A less obvious difference between system, the pump controller increases water loop from the variable-flow variable and constant primary flow lies the speed of the pump. Conversely, distribution side. in system operation. In a primary when the air-handler control valves secondary system, a chiller and its close in response to decreased coil The VPF design eliminates the primary pump typically operate in loads, the pump controller slows the constant-flow chiller pumps and uses tandem. The VPF design can separate pump speed to maintain the target P.

6 The variable-flow pumps to circulate pump control (delivering enough water). water throughout the entire chilled from chiller sequencing (making the Meanwhile, the plant controller water loop (Figure 1, p. 1). Both water cold enough). stages the chillers on and off to match systems include a bypass line. cooling capacity with system load. If However, notice that the VPF design Like the secondary pump in a primary the air handlers operate properly, the adds a modulating control valve in the secondary system, the pumps in a difference between the return- and bypass line. At low loads, the bypass typical VPF system operate to maintain supply-water temperatures, T, valve delivers the water necessary to a target differential pressure, P, at a remains nearly constant.

7 Therefore, maintain the minimum evaporator-flow specific point in the system (Figure 1). increasing the water flow through the limit of each operating chiller. By This pressure difference tends to chiller evaporators increases the load contrast, the bypass line in a primary decrease when the air-handler control on the operating chillers. secondary system ensures constant valves open in response to increasing chiller flow at all times. loads. To restore the P across the Challenges of Evaporator flow limits Variable Primary Flow Chiller manufacturers specify minimum machines with cooling capacities of Varying the water-flow rate through and maximum limits for evaporator water 150 tons or more).

8 Granted, more passes the chiller evaporator poses two control flow. Their objective? mean a higher evaporator pressure drop challenges for those who design and and more pumping power. However, each operate VPF systems: To promote good heat transfer and reduction of the system flow rate also stable control (minimum flow limit) reduces the evaporator pressure drop by approximately the square of the flow rate. Maintain the chiller flow rate To deter vibration and tube erosion (maximum flow limit). Therefore, the pump will require less extra between the minimum and power to work against the pressure drop maximum limits of the evaporator.

9 As recently as three years ago, the typical as the system flow rate drops below the range was 3 ft/sec to 11 ft/sec. Since then, design value. Manage transient flows without manufacturer-conducted testing shows compromising stable operation, that specific chillers may accommodate Small packaged chillers typically offer less especially in multichiller plants. evaporator flow rates as low as ft/sec, design flexibility than larger machines. It depending on tube type. This is good news may not be possible to select one for a for VPF systems because it extends the minimum flow rate of less than 60 percent Transient flows refer to the hydraulic chiller's ability to operate effectively of the design system flow But don't let effects caused by an isolation valve without the addition of bypass flow.

10 This deter you from designing a VPF plant that includes small packaged chillers. Just when it opens (before the associated You can lower the minimum flow limit for be sure to devise a sequencing strategy chiller starts) or closes (after the chiller a chiller by selecting an evaporator with that accommodates the chiller's minimum stops). To illustrate what happens, let's more passes (a common option for evaporator-flow limit. look at an example. 2 Trane Engineers newsletter Vol. 31, No. 4. Assume that the two-chiller VPF Our experience with actual VPF plants system in Figure 1 (p. 1) is designed for Holistic plant operation indicates that a minimum evaporator- a 16 F T and that it delivers 40 F Some practitioners believe that operating flow limit of 60 percent for packaged chilled water.)


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