HYDRONICS BY JOHN SIEGENTHALER

Transcription

1 HYDRONICS BY JOHN SIEGENTHALER ALTERNATIVES TO PRIMARY/SECONDARY PIPING Most people who have been involved with modern hydronic heating have heard of primary/secondary (P/S) piping. For many it is now the backbone upon which modern multi- temperature/multi-load hydronic systems are built. The key concept in P/S piping is hydraulic separation that is the ability of two interconnected piping circuits, each with their own circulator, to operate so that flow in one circuit does not induce flow in the other. Each circuit behaves according to the flow dynamics established by its own piping components and its own circulator. The most popular method of achieving hydraulic separation in North America today is through closely spaced tees as shown in Figure 1. Because the tees are so close, the pressure drop between them along the primary circuit is almost zero. Hence the pressure at the centre of each tee is essentially the same. Since there is virtually no differential pressure across the beginning and end of the secondary circuit there is no motivation for flow to develop in the secondary circuit until the secondary circulator operates. The most common type of P/S piping uses a series primary loop as shown in Figure 2. Each secondary circuit connects to a common primary loop in sequence. The FIGURE 1 HYDRAULIC SEPARATION USING CLOSELY SPACED TEES secondary circuit virtually no induced flow (flow only occurs when secondary circulator operates) secondary circulator closely spaced tees A B primary circuit first secondary circuit receives the highest temperature water. Remaining secondary circuits receive progressively lower supply temperatures. FIGURE 2 COMMON PRIMARY/SECONDARY PIPING high temperature secondary circuit closely spaced tees series primary loop closely spaced tees medium temperature secondary circuit closely spaced tees low temperature secondary circuit These types of systems are good for situations when the supply temperature requirements of the loads vary over a wide range. Higher temperature loads are connected near the beginning of the primary loop while those that can operate at a lower temperature are placed farther downstream on the primary loop. The designer can calculate the temperature drop from one secondary circuit to another at design load conditions and compensate for the declining temperature when necessary. Another type of P/S system uses a parallel primary loop and is shown in Figure 3. Unlike the simpler series primary loop a parallel primary loop supplies the same water temperature to each secondary circuit while also providing hydraulic separation between all the circuits. However, these benefits come at the expense of more complicated and expensive piping. Balancing valves are required in each crossover of the parallel primary loop. Besides adding to the initial cost, these valves are often not properly adjusted at system startup. CONTINUED ON PAGE HPAC January/february 2007

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3 HYDRONICS c o n t i n u e d f r o m P A G E 2 4 FIGURE 3 PARALLEL PRIMARY LOOP SYSTEM crossover bridge closely spaced tees parallel primary loop secondary circuits balancing valves In the piping system shown in Figure 5 the boiler has been interfaced to a large diameter vertical header with a relatively compact space. The header s large diameter and close spacing between all supply and return tees brings the pressure drop between points A and B pretty close to zero, so close that interference between adjacent circulators will not be a problem. This is the concept of a low loss header. It provides the same supply temperature and hydraulic separation for all circuits. FIGURE 5 LOW LOSS HEADER SYSTEM short, wide diameter header supply temperature sensor for boiler reset controller closely spaced tees (for hydraulic separation) air vent A equal supply temperature to all loads distribution circuits B purging valves drain valve AN ALTERNATIVE Parallel P/S systems make sense when the primary loop needs to extend around a substantial part of the building as depicted in Figure 4. However, in situations where the distribution system is divided into separate circuits within the mechanical room it is simply not necessary to build a P/S system to achieve both hydraulic separation and equal supply temperature to all loads. FIGURE 4 EXTENSIVE PRIMARY LOOP return main supply main Designing the header for a flow velocity in the range of two feet per second under full design load flow conditions (e.g. when all distribution circulators are running) keeps the pressure drop along the header low enough that the slight length of the header from A to B is of no consequence. Under such conditions both variable speed and fixed speed circulators can be used side by side provided all branch circuits are equipped with spring-load check valves as shown. Thus, one of the branch circuits could serve a variable speed injection pump, another could serve a variable speed distribution circulator, and still another could serve a fixed speed circulator. ONE LESS CIRCULATOR A significant advantage of this approach over a parallel P/S system is that it eliminates the need for a primary circulator. Not only does this reduce installation cost, it also reduces operating cost. For example, a 200-watt primary circulator operating 3,000 hours per year in a location where electricity costs CONTINUED ON PAGE HPAC January/february 2007

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5 HYDRONICS c o n t i n u e d f r o m P A G E 2 6 $0.10 per kwh would have a present operating cost of about $60 per year. If you assumed a four per cent inflation rate on electricity, its total operating cost over the next 20 years would be over $1,500. This far exceeds the circulator s installation cost and shows the importance of life cycle cost comparison rather than fixating on the initial cost. Eliminating a primary circulator, even a small one, can potentially save the owner hundreds, if not thousands of dollars, over the life of the system. FIGURE 6 SYSTEM UTILIZING A HYDRAULIC SEPARATOR supply temperature sensor distribution circuits READY TO GO The physical principles at work in the low loss header drain shown in Figure 5 are also at work in a single device called valve a hydraulic separator (see Figure 6). The relatively large diameter of the body on the hydraulic separator reduces vertical flow velocity to the point where air bubbles can easily rise to the top and fine sediment particles settle to the bottom. The relatively short distance between the upper and lower connections on each side of the separator provide a hydraulic uncoupling effect equivalent to closely spaced tees in a P/S system. CONTINUED ON PAGE 30 boiler air vent hydraulic separator equal supply temperature to all loads purging valves short, wide diameter header (supply & return) Weil-McLain s NEW Ultra Oil Boiler delivers ultra-clean high performance and quiet operation in a compact design powered exclusively by Beckett s NX Oil Burner. Visit to see the entire family of Weil-McLain Canada products. Tel: Fax: info@weil-mclain.ca North American ingenuity.ultra results. 28 HPAC January/february 2007 Circle #16

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7 HYDRONICS c o n t i n u e d f r o m P A G E 2 8 Parallel P/S piping should be limited to situations where the primary loop must extend around an entire building. Use of a hydraulic separator eliminates the need for P/S piping and the requisite primary circulator. It is ideal for systems where multiple distribution circuits requiring the same supply temperature, as well as hydraulic separation, begin and end within the mechanical room. To reiterate, parallel P/S piping should be limited to situations where the primary loop must extend around an entire building. In systems where the distribution system is divided into sub-circuits within the mechanical room it simply is not necessary to create a parallel primary loop. Doing so adds installation cost and complexity to the system. It also significantly increases the operating cost of the system over its life. As energy costs escalate it is up to us as heating pros to adapt the best technical solutions available to keep system operating costs as low as possible while still providing superior comfort. This is routinely done with new boiler technology. Why not apply the same thinking to the hydraulic efficiency of our systems? n John Siegenthaler, P.E. is the author of Modern Hydronic Heating. Visit his website at for reference information and software to assist in hydronic system design. He can be reached at siggy@dreamscape.com. RATE THE ARTICLE! Will this information be useful? Please circle the appropriate number on the Reader Postcard. Thank you. VERY USEFUL USEFUL NOT USEFUL Circle #18 30 HPAC January/february 2007

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