Balancing and Control Valve Sizing for Direct-Return,.pdf

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Technology Report November, 2006 Pressure-Independent Control Valves vs. High-Performance Control Valves Balancing and Control Valve Sizing for Direct-Return, Variable-Flow Hydronic Systems This technology report is primarily directed to the consulting engineering community and other
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    Technology Report  November, 2006  Siemens Industry, Inc. Page 1 of 26 Pressure-Independent Control Valves vs. High-Performance Control Valves Balancing and Control Valve Sizing for Direct-Return, Variable-Flow Hydronic Systems This technology report is primarily directed to the consulting engineering community and other technical advisors representing building owners. It is intended to be an objective  summary of the findings from research conducted by Siemens Building Technologies on balancing and valve sizing for variable-flow hydronic systems. This report examines the balancing and valve sizing issues of pressure-independent control valves versus high-performance, pressure-dependent control valves. Self-Balancing Hydronic Systems There is a growing consensus in the HVAC systems design community that if a hydronic system is properly designed, and control valves and actuators are properly sized and selected, then the system is self-balancing   and does not require mechanical balancing valves. Elimination of mechanical balancing valves enhances controllability of the heat transfer coil and reduces pumping energy costs. Properly sized and controlled two-way, pressure-dependent, high-performance  control valves will adequately balance the system such that at least 80% of the valve stroke is available for coil control  . Further, if only the maximum flow and pump head are known, a high-performance, pressure-dependent control valve can be effectively sized such that it will properly balance the system and have sufficient stroke available for precise coil control. Pressure-independent valves are relatively easy to size properly, and if installed on every circuit of a water distribution system, will eliminate the need to know the distribution system branch design flows and pressure losses needed to most accurately size a high-performance control valve. However, the first cost of these valves is as much as four to five times greater than that of a comparably-sized pressure-dependent control valve. Balancing Variable-flow Hydronic Systems How, and indeed whether, to balance variable-flow hydronic systems with two-way valves has been debated in the industry for many years (Avery, et. al. 1990,  Aver y 1993, Hegberg 1997, Taylor 2002). 1,2,3,4  The  ASHRAE 2003 HVAC Applications Handbook  5   continues to recommend two basic methods for balancing hydronic systems, including those that are variable-flow:   Balancing by temperature difference between supply and return water at the terminals.   Balancing the circuits in the system proportionately according to the actual flow rate divided by the design flow rate through the circuit. However, after a careful review of the literature, there appears to be a growing consensus among HVAC system designers that if a hydronic system is properly designed, and control valves and actuators are properly selected, and the sensor controlling the coil valve is always in control of the waterflow through the coil, then the system is self-balancing   and does not require additional balancing valves (Avery 1990, 1991, Bell & Gossett 1965, Haines 1991, Hansen 1991, Stethem 1990, Taylor 2002, 1  Avery, G., Stethem, W. C., Coad, William J., Hegberg, Richard  A., Brown, F. L., Petitjean, Robert. 1990. “The pros and cons of balancing a variable flow water system”.    ASHRAE Journal  , 32(10), pp. 30-59. 2  Avery, G. 1993. “Designing and commissioning variable flow hydronic systems”.    ASHRAE Journal  , 35(7). 3  Hegberg, Richard A.. 1997. “Selecting control and balancing valves in a variable flow system”.  ASHRAE Journal  , 39(6), pp. 53-62. 4  Taylor, Steven T. and Stein, Jeff.   2002.   “Balancing variable flow hydronic systems”.    ASHRAE Journal  , 44(10), pp.17-24. 5  See Chapter 37, pp. 37.8 through 37.10. Document No. 149-990    Utesch 1991). 6  Avery (1990) 7  goes so far as to say that balancing a variable-flow hydronic system will “ruin the control system.” One of the great concerns with the use of balance valves for balancing water circuits is the pressure drop across these valves, and the resultant additional pump energy consumption. This extra pump energy consumption can be calculated approximately as: wwevlvbal  pGPM  pumpP   3960))((     (1)  P  is the additional pump horsepower.   p bal vlv  is the pressure drop across the balance valve in feet of head.   wwe  is the wire-to-water efficiency of the pump. 8  In addition to the pumping energy saved, chilled water  T and effective plant cooling capacity increases when balancing valves are eliminated.  Anecdotal evidence (Rishel 1997) 9  indicates that these savings can be substantial. System waterflow balance is affected by the following system components:   Piping circuit   Pump or pumps   Control valves   Coil pressure drops and design  Ts   Chiller or boiler pressure drops and design  Ts Page 2 of 26 Siemens Industry, Inc. 6  The 1965 reference is Carlson, Gilbert F. 1965.   “Flow balance in hydronic systems”.    Air Conditioning, Heating, and Ventilating  .   September. The 1990 reference is Op. Cit., Avery et. al. 1990. The 1991 reference is: Utesch, A.L., Rosenfeld, Alan, Kettler, John P., Hansen, Erwin G.,Corll, James A., Hegberg, Richard  A., Haines, Roger W. 1991. “Point/counterpoint – Journal readers continue the debate on how to balance variable-flow water systems”.    ASHRAE Journal  , 33(4), pp. 50-54. The 2002 reference is Op. Cit., Taylor and Stein.   2002. 7  Op. Cit., Avery. 1990. 8  Pump wire-to-water efficiency,   wwe , can be written as the following equation: vm pwwe         , where    p  is the pump efficiency,   m  is the pump motor efficiency, and   v   is the pump speed drive efficiency. 9  Rishel, James B. (Burt). 1997. “Use of balance valves in chilled water systems”.    ASHRAE Journal  , 39(6), pp. 45-51.   Balance valves (if used) The hydronic system designer should select these components carefully so that when assembled into a working system, they work together to provide an adequately balanced system. Evaluations should be made for pressure drops of all of the above components. Balance Valves in Hydronic Systems Rishel (1997) 10  states that certain types of chilled water systems should use balance valves, while other types do not require them, and that the  pressure gradient diagram demonstrates which chilled water systems should use balance valves . The pressure gradient diagram is constructed by plotting the pump head in feet on the vertical scale and the distance from the pump on the horizontal scale. The horizontal scale on the pressure gradient diagram is used to separate the various pressure loss elements in the circuit. Figure 1 illustrates a pressure gradient diagram for a variable-primary-flow chilled water system. If the pressure drop designated as “chiller loss” in the figure is removed, this figure could also apply to a secondary chilled water loop of a primary-secondary chilled water system. Balance valves were used extensively in the past on hydronic systems with constant-speed distribution pumping and three-way control valves on the coils. This was a proper use of balance valves for these systems. However, due to their energy waste, three-way control valves should not   be used in today’s chilled water systems; they should be replaced with two-way control valves and the distribution pumping should be variable speed whenever economically possible. 10  Op. Cit., Rishel. 1997. Document No. 149-990    507090110130150170190 Distance    S  y  s   t  e  m   P  r  e  s  s  u  r  e   (   f   t   ) furthest circuitpump discharge loss chiller loss pump headpump suction lossclosest circuits   Courtesy: Flow Control Industries, Inc. Figure 1. Pressure Gradient Diagram for a Variable-Primary-Flow Chilled Water System. Rishel (1997) 11  demonstrates that it is impossible to adjust manual balance valves on variable-volume, direct-return chilled water circuits with modulating-type coil control valves. Therefore, manual balancing valves should not be used on these circuits. Rishel recommends manual balance valves be used on reverse-return systems with two-position  control valves while automatic   balance valves be used on direct-return  systems with two-position  control valves. Using balancing valves on variable-flow hydronic systems is detrimental to the control system performance and energy use because it reduces the authority 12  of the control valve and adds a permanent restriction on every branch circuit (Avery 1993). 13  Therefore, balancing valves should not   be used on direct-return variable-flow hydronic systems with two-way control valves. Balancing Options Most recently, Taylor (2002 14  and 2003 15 ) analyzed eight of the most commonly used methods for balancing hydronic systems. Two hydronic distribution systems were analyzed, one chilled Siemens Industry, Inc. Page 3 of 26 11  Ibid. 12  Valve authority will be defined and discussed later in this report. 13  Op. Cit., Avery. 1993. 14  Op. Cit., Taylor. 2002. 15  Taylor, Steven T. 2003. “Balancing variable flow hydronic systems”. ASHRAE, San Jose Chapter seminar presentation, October 14. water system and one hot water system, both of which were based on a real building. For each of the balancing options, flow through the system was analyzed using a commercially available pipe network analysis program that modeled how the flow and pressure vary throughout the system. Both first costs and balancing costs of each option were estimated. The eight balancing options studied were: 1. No balancing. The rationale behind this option is that if the coils can achieve their discharge air setpoint at or below the design flow of the coil, then the control valves themselves will automatically and dynamically balance the system. Under this option, neither balancing valves nor balancing labor is required. 2. Manual balance using calibrated balancing valves. Calibrated balancing valves have flow measurement capabilities integrated into the design of the valve. Flow is measured and valves are adjusted by the test and balance contractor to achieve design coil flow rates. 3. Automatic flow limiting valves. Automatic flow limiting valves (also called automatic flow balance valves) are self-powered devices that limit flow to a preset value when the differential across the valve is within a certain range. Typically, the valves include a cartridge with specially shaped orifices controlled by a spring.  As the differential pressure across the valve depresses the spring, a varying amount of orifice area is opened. The area and shape of the orifices are designed to deliver a constant flow rate, within the limits of the spring. 4. Reverse-return piping. The effect of reverse-return piping causes the differential pressure across each terminal unit (coil and control valve combination) to remain constant. 5. Oversized main piping. This option attempts to equalize the differential pressure across each sub-circuit by reducing the pressure drop of the piping mains by keeping the mains the same size for the entire length of the system. 6. Undersized branch piping. This option is similar to option 2, except that pipe size is reduced to increase pressure losses in branches instead of adjusting a valve. 7. Undersized control valves. This option is similar to option 6, except control valves are undersized. 8. Pressure-independent control valves. Document No. 149-990    Page 4 of 26 Siemens Industry, Inc. Document No. 149-990 Option 1, no balancing, requires further explanation. No Balancing (Option 1) Two questions arise with this design: 1. What happens when control valves cannot meet setpoint and are wide open, as occurs during transients such as warm-up (heating systems) or cool-down (cooling systems) when setpoints are beyond attainable levels, or if the coils are simply undersized? 2. What is the impact on controllability caused by control valves having to be partly closed at design flow? The computer simulation of this option showed that when all the control valves are wide open, the coils hydraulically closest to the pump had a flow rate substantially above design (143% for chilled water and 212% for hot water), while those farthest from the pump had flow rates approximately 25% below design. For equal percentage valves selected with an authority 16  of 0.5, in the case for chilled water systems, for those coils closest to the pump, the valve would have to close to about 85% of full open to reduce the flow to design flow, and for hot water systems, the valve would have to reduce to 75% of full open to reach design flow. It is unlikely that reducing the effective control range of the valve by 15% to 25% will cause problems if the valves have been properly selected, particularly with today’s controllers and actuators. However, for systems larger than the ones studied, reducing excess flow for coils near the pump may require further reductions in valve stroke. The more pressure the control valve has to absorb to achieve design flow rates, or the larger the difference between the  pressure drop through the hydraulically farthest circuit and that through the closest circuit, the more likely control problems will result from direct-return systems using control valves for balancing  . Therefore, for large systems, or those zones (control valves) that experience a high degree of pressure variation, other balancing options described later should be considered. For those coils farthest from the pump, it is also unlikely that the reduced flow will cause problems. This is due to the inherently non-linear nature of coil heat transfer characteristic (Figure 2 and Figure 3). In the case of cooling coils, a reduction in flow to 16  Valve authority will be defined and discussed later in this report. 75% of design flow still results in a coil heat transfer rate that is 89% of design, while for heating coils, a reduction in flow to 75% of design results in a coil heat transfer rate that is 96% of design. Summary and Conclusions of Balancing Options Table 1 lists the advantages and disadvantages of each balancing option. Taylor (2002) 17  concludes that the “no balancing” option is the best, except for very large distribution systems, because it combines low first costs with minimal operational problems. In Taylor’s 2003 presentation on this topic, 18  he concludes with the following recommendations:   Automatic flow-limiting valves and calibrated manual balancing valves are not recommended on any variable-flow system with modulating, two-way control valves. There are few advantages and high first costs.   Reverse-return and oversize mains may have reasonable pump energy savings payback on 24/7 chilled water systems.   Undersized piping and valves near pumps improves balance and costs are reduced, but significant added engineering time is required.   Pressure-independent control valves should be considered on very large systems for coils near pumps. The valve first costs are high but are coming down now due to competition.   For other than very large distribution systems, no balancing   (option 1) appears to be the best option. 17  Op. Cit., Taylor, 2002. 18  Op. Cit., Taylor, 2003.
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