Evolution of Piping Design

1. Evolution of Piping Design The Advantages of Primary-Variable Piping Design, Especially in Modern Condensing Hydronic Boiler Systems Tom Heckbert Commercial Boiler Sales Specialist

2. The Evolution of the Hot Water BoilerThe Early Days In the earliest hot water systems, fuel cost was usually not a consideration. Plants could be installed far larger than they actually needed to be, and run hot, with the products of combustion being vented out of the boiler at temperatures much higher than the water being supplied. Being essentially just converted steam engines, straightforward boiler and piping designs reigned supreme. These large boiler pressure vessels had correspondingly huge water volumes (and by extension, massive heat exchangers), meaning temperature changes in the return water could easily be absorbed into the thermal mass. System control was equally basic, with on/off burner control being cutting edge technology. All equipment shared common piping. Efficiency was low, energy loss tremendous. Inevitably, fuel costs rose and boiler sizes began to more accurately match actual system load requirements. The energy wasted in high exhaust temperatures also had to be contained, so boiler heat exchangers capable of reducing flue gas temperatures close to the dew point began to be designed. But the market trend toward boiler downsizing was beginning to cause issues. Boiler pressure vessels began failing prematurely, fracturing due to metal fatigue caused by “thermal shock,” a contraction process seen when colder than desired water would enter the boiler in large enough volumes. The less thermal mass a boiler pressure vessel holds to mix incoming water with, the more the temperature will drop in that section nearest the return inlet, contracting as a result. With the area near the supply outlet remaining the same size it had been, the boiler will begin to change shape, putting stress on the area in between. Even a small dimensional difference creates huge lateral strain: the boilers simply hadn’t been designed for these new operating conditions.

3. The Evolution of the Hot Water BoilerThe Solution: Primary-Secondary Piping While boiler design engineers worked to create products that suited these new operational realities, mechanical engineers had to develop a way to continue using existing products in these new, more efficient systems. Their solution was what would become known as “Primary-Secondary” (PS) piping, which revolves around the hydraulic separation of the boilers from the system “loop,” by providing separate flow-inducing pumps for each. The objective with PS piping was to prevent large volumes of especially cold system water (for example, that had cooled down while sunlight was providing natural heat to a section of the building) from being injected directly into the boilers. So long as the total boiler-side flow at a given load condition meets or exceeds the total system-side flow, some water will tend to flow backwards through the “decoupler” and increase the boiler return temperature. Using Delta-T as the defining control parameter, this desirable flow characteristic tends to reveal itself in PS systems naturally: as return temperature drops due to load increases, more boilers and pumps are activated, increasing the flow through the decoupler. And finally, in the event the system side temperature simply drops too far, too fast, the boiler pumps can be deactivated, saving the boilers. PS piping had resolved the issue! (A popular design that has its roots in PS principles is the use of a “30% bypass” return conditioning pump: a partial injection of heated supply water into the return line. The true objective is to create a design that uses the system pumps to induce flow through the boilers, using the much smaller bypass pumps for boiler return temperature protection only, to save on costs.)

4. The Evolution of the Hot Water BoilerState Change: Condensing Technology Since then, many new piping designs have been developed to deal with the threat of thermal shock, including mixing valve control options, and “variable injection” loops placed between the boiler and system sides. There have also been a variety of boiler heat exchanger innovations, creating pressure vessels that can accept extreme return temperature reductions without lateral stresses being created, like flexible tube steel boilers, or top-return eutectic cast iron boilers. However, none of these innovations were able to improve upon the maximum theoretical efficiency of classic boiler heat exchangers. While system losses could be minimized through the use of intelligent controls and smart design principles, boiler thermal efficiency had to this point been limited to a function of the Delta-T between flame and flue gas temperature. For natural gas (NG) boilers, this equated to a temperature decrease from roughly 1,800°F (982°C) to 260°F (126°C), or 85.5% efficiency. (For a $200,000 annual fuel bill, even a 2% improvement would save $4,500/year.) To the boiler design engineers, the place to look for the desired efficiency improvements was obvious, but not easily capitalized on. The heat exchanger designs that were available already operated with low stack and return temperatures, and to push any further would require dropping below the NG dew point, something previously only permitted in stainless steel vents. What was required was designing both systems and boilers to operate at return temperatures below 131°F (55°C): the lower the better, as the potential efficiency benefits increase as the temperature drops (see chart). Two things would happen simultaneously. First, exhaust gases would be allowed cool well below the existing 260°F (126°C) lower limit, meaning more sensible heat was being extracted. Second, and more importantly, condensation would take place inside the boiler heat exchangers. This presented a massive design challenge, because unlike the condensation you see on a glass of cold water, this condensation would be acidic, enough to destroy any conventional boiler.

5. The Evolution of the Hot Water BoilerThe Physics: Why Condensing Is Important Yet it is the condensation of water vapour, naturally entrained in the products of combustion, that really pushes the boundaries of what is achievable in boiler thermal efficiency. This is true because the combustion of natural gas (202 + CH4) results in the production of roughly 10 USgal of water for every 1,000 SCFM (roughly 1,030 Mbtu/h) of fuel input. Because of the temperature of a NG flame, this inevitably begins as steam. When the exhaust temperature decrease takes place inside the heat exchanger, useful heat is extracted from this water vapour and the flue gas that carries it. This is sensible heat, measurable with a thermometer. But even more energy is collected when the flue gas passes below the dew point of this entrained water vapour, which happens when the return water temperature dips below 131°F (55°C). The “state change” of water from gas (steam) to liquid (condensate) can release as much 8.15 Mbtu/USgal of latent heat (see chart), a potential efficiency increase of 8%. Condensation rate increases as return temperatures drop, beginning at 1% of potential at dew point, up to roughly 95% at around 86°F (30°C). While the energy gains are immediate, the condensed water vapour pulls nitrogen based chemicals (undesirable byproducts of combustion) from the flue gases, ultimately forming a liquid as corrosive as citric acid. In order to stand up to the punishment this presents, boiler designers began to use stainless steel and aluminum as construction materials, instead of steel and cast iron, and designing for the expedient draining of condensate from boiler flue passages. But interestingly, due to a combination of market factors, this technological reformation did not begin in the commercial boiler sector, but rather in the residential market. Here, customers were accustomed to purchasing products with shorter lifecycles, and this multi-generational environment made an ideal crucible for testing this new technology.

6. The Evolution of the Hot Water BoilerThe Product: Where Condensing Boilers Began By the nature of designing for the residential market, these early condensing boilers were built small, so they could be easily replaced at the end of their lifecycle (circa 2005, the worldwide average lifecycle of residential condensing heat exchangers was estimated to be 12-15 years). This compact design principle also meant less material and freight costs involved in manufacturing, making the product competitive with conventional residential equipment. (Pictured right is one of the earliest designs ever available, built by Nefit in the Netherlands.) The tremendous success of condensing boilers globally is exemplified by the number of countries where the sale of non-condensing technology is now illegal, as it is in the United Kingdom. As the reliability and efficiency of this technology caught the attention of engineers worldwide, efforts were quickly made to scale up to commercial size applications. The design principles already being deployed, of being small mass and of watertube design, were extrapolated and expanded into higher Btu input units. If the combustion chamber is designed to create sufficient fireside turbulence, with a large surface area upon which condensation can take place, then very high input values can be achieved in impressively small footprints. Since then, commercial condensing equipment has been well received, not least because fuel costs continue to rise across the globe. But while these systems offer significant efficiency gains over conventional boilers, their roots in residential design have brought two notable disadvantages with them. First, the nature of low mass design means these products risk potentially shorter lifecycles (many manufacturers design well enough to eliminate this concern). Second, and much more importantly, these boilers were built with a specific design assumption in mind, a minor problem in a residential setting, but increasingly problematic when scaled up: they require PS piping for heat exchanger protection.

7. The Evolution of the Hot Water BoilerThe Problem: Why Low Mass Needs Primary-Secondary Primarily this is due to the high flow resistance in a small mass heat exchanger: it causes an increase in the pressure differential across common piping, which can cause problems like in the example shown here. The smaller zone is unable to create a large enough Delta-P to create flow in it’s loop, and will ultimately pump against a seated check valve. Besides the wear and tear caused, the primary concern is that the zone in question is not seeing any heat. Even near this condition, at 4 PSI Delta-P for instance, the flow through the zone will be greatly reduced, and comfort issues will arise. The solution chosen by engineers was to reduce the pressure differential in the common piping, and the best way to achieve that was to capitalize on the principles of separation present in existing PS piping systems. The market had no trouble in accepting this solution: PS was already being used on a variety of installations with great success, so why not here as well? In many ways this situation reflects the current dominant position in the commercial hydronic market. But as fuel costs still continued to rise, with little signs of slowing down, some designers went looking for even higher thermal efficiencies. And they found them where we started: primary-only (PO) systems. These drawings are from materials produced by John Siegenthaler.

8. The Evolution of the Hot Water BoilerThe Future: Large Mass Condensing Boilers Take Us Full Circle What PO piping systems offered to the condensing boiler market is obvious, when you consider what PS piping was originally intended to do: boost and maintain higher return temperatures. Given that flue gas condensation requires the coldest return water possible be brought into the heat exchanger, PS piping is to some extent the natural enemy of condensing boiler systems. With mechanical engineers increasingly designing around low temperature return heating systems (i.e. radiant ceiling or wall panels, low temperature baseboard, or, best of all, radiant in-floor heating), boiler designers realized that a system with no tempering of return water will inevitably be best positioned to take advantage. However, the problems that had always existed in hydronic design did not go away. Flow resistance and thermal shock danger remained just as valid of concerns as ever. So designers went back to the only boilers in history that had been able to circumvent these problems: very high mass, low flow resistance boilers. By choosing construction materials suitable to flue gas condensation, and redesigning the structure of the heat exchanger itself, they aimed to build products small enough to be sized for real world building loads, but robust enough to handle diverse hydraulic conditions. As of today, boiler designers at several major manufactures have succeeded, creating high-mass, robust firetube boilers capable of extremely high efficiencies, up to 99% (which means very dry exhaust, at just above room temperature). The heat exchangers in these boilers have very low flow resistance, meaning they can be piped PO, using only “fail-open” valves to isolate inactive boilers. They are also designed in such a way that even extreme temperature changes will not cause the lateral stresses associated with thermal shock. So since this is about cost, what are the savings?

9. The Evolution of the Hot Water BoilerThe Costs: Large Mass Boilers Save Money The largest stumbling block for the adoption of this high-mass condensing technology has so far been the costs of the boilers themselves. When compared directly with the low-mass competition, these boilers do cost notably more per Btu input. If both are competing to be installed in PS systems, high-mass product will typically lose. But since these products were designed specifically to avoid PS piping, the initial place to look for savings is in the equipment that can be removed from the system, For a three boiler plant around 6.6 MMbtu/h, the equipment cost savings alone can easily reach the values shown here. But the most significant savings in high-mass systems reveal themselves in day-to-day operational. The majority of the savings arise because of the use of PO piping design and the associated reduction in return temperatures, which allows higher thermal efficiency and less fuel consumed per Btu delivered to the building. A second, less commonly thought of, savings can be found in electrical consumption, as high-mass systems do away with boilers pumps, which can be several horsepower each in PS systems, requiring a significant quantity of power to operate. These days, most buildings use variable speed system pumps instead of constant speed, on/off models, and turning a Primary-Only system in to “Primary-Variable” (PV), will improve these electrical savings even more. So let’s examine a hypothetical boiler plant, sized for a 6 MMbtu/h heating load, under varying load conditions, using both PS and PV piping designs. Afterwards we will come back to the question of system design, as high-mass boiler systems allowed one more design innovation, entirely new to hydronics, that offers the greatest potential efficiencies yet.

10. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 100% The above system uses a 300gpm, 5.7hp variable speed pump (with a duty-standby) for system flow. This pump will decrease speed as system loads decrease. The boiler loop side has three 100gpm, 1.6hp constant speed circulators. Under design conditions the system and boiler loop will match capacity, and flow through the decoupler will be 0gpm. System return and boiler return temperatures will necessarily be the same. (Many manufacturers are starting to include variable speed boiler pump controls in their product. While much of what follows in this example is negated by this innovation, these pumps do carry significantly higher up front costs. Moreover, these VS pumps are typically only allowed on mid to high-mass condensing boilers anyway, as low-mass boilers often have minimum flow requirements, requiring the use of constant speed pumps. For this reason, this example assumes constant speed pumping, as it more clearly illustrates the negative effects of PS piping on return water temperature.)

11. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design– 100% The PV variation of this boiler plant will, at design loads, have almost identical performance in terms of return water temperature at the boilers. This is because the system and boiler loop flow rates in the PS system were matching while at full capacity. One component in this system does see a decrease in performance, and this is the system pump. While the flow resistance in the high-mass boilers is very minimal it is not quite zero. As a consequence, the system pump size will have to be increased to overcome the added pressure drop. However, this increase will be less than 0.5hp, and as the pump horsepower drops during part load conditions, this electrical increase will rapidly disappear.

12. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 100% Now that we have the framework for the system, we will compare these two plants under part load conditions. (It is worth noting at this point that no heating plant will operate at or even near it’s design load conditions for much of it’s operational life. Most studies have shown that buildings spend the bulk of the year in North American climates at around 50% of design capacity. A good rule of thumb for designers, when trying to decide how much spare plant capacity is required and among how many boilers that capacity should be divided, is to assume the plant will operate below 70% capacity at least 90% of the year.)

13. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 70% At this stage we are operating at 70% of plant design capacity, or roughly 4,200 Mbtu/h heating load. Because this capacity exceeds the output achievable by only two boilers, all three remain operational. With all three boiler circulators also running, 90gpm must be diverted backwards through the decoupler, causing the boiler return temperature to increase to 132°F (55.5°C). (Note that thanks to the hydraulic separation of the decoupler, there has been no impact on the system flow rate of 210gpm as a result of this backflow. This is hydraulic separation in action.)

14. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 70% At 70% of plant design capacity, the high-mass boilers in the PV system also need all three boiler operating in unison to match the required plant capacity. However, because their flow rate is simply a factor of the system flow divided by the number of operational boilers, each unit is now seeing only 70gpm. This also means the boiler return temperature is the same as the system temperature, 120°F (48.8°C). Note that the Delta-T through the boilers at design load, 40°F (22.2°C), has not changed in this system.

15. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 70% Because the PS system is producing higher temperature boiler return water, more of the heat produced by combustion is finding it’s way out through the stack, largely in the form of uncondensed steam. As a result, more fuel has to be combusted to make up the load requirement. In other words, the 12°F (6.6°C) return temperature increase has pushed the boiler out of the “condensing” zone. Note that while the system pump horsepower requirement drops off fairly quickly in both systems, because the three boiler pumps are still running at nearly full power in the PS system, the electrical consumption gap has remained wide.

16. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 50% Continuing on, at this stage we are operating at 50% of plant design capacity, or roughly 3,000 Mbtu/h heating load. This has allowed the staging control to deactivate one of the three boilers and it’s associated circulator. However, because the boiler pumps combined flow rate is still higher than that of the system (the natural operational reality of PS piping), 50gpm is being diverted through the decoupler, resulting in a boiler return temperature increase to 120°F (48.8°C).

17. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 50% At 50% of plant design capacity, the staging control in the PV system no longer requires that all three boilers be operating in unison to match the building load. However, a smart control strategy with condensing boilers will leave all three units operational. This is because, unlike conventional boilers where the objective is to get the products out of combustion out of the heat exchanger before they cool below the dew point, in condensing systems the objective is the exact opposite. We instead want to delay the flue gas passage through the boilers as much as possible, to allow extra time for heat transfer and condensation to take place. As before, because there has been no mixing of flow before the boilers, the 110°F (43.3°C) system return water is brought directly in for maximum condensing. Once again, the Delta-T through the boilers at design load, 40°F (22.2°C), is still being maintained at this level.

18. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 50% At this point, the differing efficiency levels (caused by the different boiler return temperatures) mean that the PV system will consume about 100 Mbtu, about one therm, less every hour than a PS system, using about 3hp less electric power. Moreover, the PV system hasn’t yet cycled off any boilers, so if this load reduction turns out to be temporary, and the requirement moves back up to 70%, we’ll be able to increase the firing rate accordingly, responding immediately to the swing. The PS system will have to run through the boiler’s start-up procedure before returning the boiler to service. It is also worth noting that, during a load increase, when the system return temperature will begin to drop, the PS system must wait until the colder water cycles through the building and returns to the plant, or newly made heat will cycle back through the decoupler and boost the boiler loop temp. This means that for PS systems, the whole building must feel the load increase before responding, while PV systems can send out heat when the outdoor change is first measured.

19. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 50% Here we see what happens if a PS system attempts to spread the load through all three boilers while at 50%, in an attemptto delay the flue gas passage through the boilers as much as possible, the desirable operating mode for a condensing appliance. If a particular control were to leave the third boiler on, as we had done in the PV system, the additional circulator will further heat the return temperature, to 130°F (54.4°C), essentially stopping condensation from taking place, and reducing efficiency even further. The boilers firing rate would have to increase to compensate, consuming considerably more fuel. Efficiency gains from low firing rates can be capitalized on in PV systems only.

20. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 33% Now at 33% or 2,000 Mbtu/h load, two boilers are mixing their supply with the system return 1:1, which means a 100°F (37.7°C) return is boosted to 120°F (48.8°C), barely within condensing levels. (It would be possible at this stage to run only one boiler to match load, which would allow unmixed flow to the operating boiler, giving it the coldest possible return and maximum condensing efficiency. However, this would require the boiler to fire at 100% of it’s rated capacity, while the boilers in the PV system continue to run low. By spreading the same amount of fuel through three separate heat exchangers, the PV system will now have considerably more time and surface area for heat transfer and condensation to occur, meaning the PS system will still run less efficiently. Ultimately, at 33% of design load, the PS system efficiency is roughly equivalent whether one or two boiler are operating, so given that two boilers will respond to variable load demands more effectively, this is the suggested control strategy.)

21. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 33% The PV system boilers are now nearing the threshold where one of them will have to be deactivated (13% each boiler’s modulation range remains), but at the moment there is still no reason to deactivate a boiler. Because the fuel input is being combusted and processed through three heat exchangers simultaneously, the flue gas flow rate is as low as possible. And because the boilers are all still seeing direct injection of the coldest available water, condensation is promoted to it’s maximum level. (It should now be apparent that the Delta-T through the boilers at design load, 40°F (22.2°C), can be maintained under any part load condition. Even as units stage off and are isolated by their valves, the operational boilers can continue to modulate their firing rate to maintain the same Delta-T. It isn’t necessary to run this way, but the same temperature drop sent out more slowly into the system will result in fewer Btu delivered, which may simplify the outdoor reset scheduling.)

22. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – 33% At this point, it’s worth remembering that commercial heating systems spend about 90% of the year operating at 70% or less of their design load. This is the reason why outdoor reset supply temperature scheduling can produce such tremendous savings: setback is going to be used nearly all of the time. If the boiler piping system is able to work with the supply temperature setback schedule, to increase the amount of condensing they are able to do, then the maximum system efficiency is achieved. Below the 33% firing rate it is increasingly likely that the PS system control will chose to run on only one boiler, but if the building has multiple zones served by more than one pump, the system flow may continue to drop. easily as low as 10% of design. As this happens, more and more supply water will be diverted backwards through the decoupler, meaning that the 92% efficiency this PS system achieves at 33% of design load may well be the highest it will be able to accomplish.

23. The Evolution of the Hot Water BoilerCompare: PS versus PV Piping Design – Overview This is not so for the PV system. Assuming the lowest firing rate of each boiler is 20% of maximum (a 5:1 turndown), then that will equate to 6.6% of system capacity. If the system flow were to drop to something as low as 10% of its design total, a single boiler in the PV system would be able to produce the desired 40°F (22.2°C) Delta-T while firing at roughly 30% of it’s rated input. Because the two deactivated boilers would be hydraulically isolated by their valves, 100% of the now very low temperature water that is returning to the boiler will be directed through this single operational unit. With very cold return and a 30% firing rate, this boiler should be operating at or near 99% thermal efficiency!

24. The Evolution of the Hot Water BoilerThe Cutting Edge: Two-Return High-Mass Heat Exchangers The most recent advance in commercial boiler design was only made possible by the advent of high-mass condensing heat exchangers, which allowed an entirely new innovation, one that offers the greatest potential efficiencies yet: the two-return boiler. The essential insight was that just as we would prefer not to mix return water with the boiler supply before injecting it back into the boiler loop, so too should we try to avoid blending system returns of different temperature. Having gone to the trouble of mixing high temperature supply water down for use in low temperature heating loops, wouldn’t it be ideal to use the extremely cold water coming out of the low temperature zone to induce condensing? And so boilers with two separate return connections were developed, allowing water returning from different system loops (say air handling and radiant in-floor loops) to connect to different physical zones in the heat exchanger. The cooler return water is introduced closest to the exhaust connection, where the flue gases are at their coldest, ensuring all possible heat energy and condensate is extracted. The boiler isolation valves are placed on the supply connection, ensuring one valve is still able to isolate any idle boilers. In this configuration, it is now possible for condensing boilers to supply high temperature water for air handling units and domestic hot water indirect heaters, while simultaneously providing low temperature heating supply water to in-floor or radiant panel zones, all while achieving very high condensing efficiencies, and a 100°F (55.5°C) boiler Delta-T.

25. The Evolution of the Hot Water BoilerThe Cutting Edge: Two-Return High-Mass Heat Exchangers If the low temperature return is able to receive 30% of the total flow through the boilers, then the plant will operate at the same efficiency that it would if 100% of the flow was coming through at that low temperature. In other words, the fact that the boilers are supplying high temperature water will have no impact on plant efficiency. Below 30% flow the efficiency will begin to decline, but as low as 10% flow condensation will still be taking place. If flow through the low temperature return were to stop under some particular system condition, there is no danger of boiler damage: the only consequence would be that condensing efficiency would be temporarily lost. In the example above, the high temperature loads total roughly 2,000 Mbtu/h (40°F (22.2°C)Delta-T @ 100gpm), and the low temperature loads total roughly 2,500 Mbtu/h (40°F (22.2°C)Delta-T @ 125gpm). Roughly 60% (75gpm) of the low temperature return water needs to be diverted back to the three-way mixing valve to produce the 120°F (48.8°C) desired supply temperature, leaving 50gpm to return directly to the boiler plant. This results in 33% of the flow through the boilers being received in the low temperature return connection, so this boiler plant will now operate exactly as efficiently as it would if 100% of the return flow was low temperature. And because the plant maintained a hot supply temperature, there is no issue serving the high temperature equipment in the building, overcoming the final strong criticism of condensing technology in commercial applications: that high temperature supply cannot be done away with, so condensing boilers won’t operate at condensing levels. With two-return boilers, they will.

26. The Evolution of the Hot Water BoilerThe Cutting Edge: Two-Return High-Mass Heat Exchangers High and low temperature zones may be controlled differently. High temperature equipment may want constant supply temperature in part load conditions, while low temperature zones may operate best when the flow remains constant and the supply temperature is scheduled back according to an outdoor reset schedule. With that in mind, let’s take a look at half load for this example. The high temperature zones are still being supplied with 180°F (82.2°C) water and producing a 40°F (22.2°C) Delta-T, while the low temperature zones have dropped their supply to 100°F (48.8°C) and the Delta-T to 20°F (11.1°C), to maintain the 80°F (26.6°C) return water. The Delta-T in the boiler plant is still 100°F (55.5°C). The high temperature zones, which are being supplied with the same supply water temperature, are now flowing at half their original rate. The assumption here is that this equipment is being controlled in an on/off fashion, with roughly half the equipment calling at any given time during 50% load conditions. However, if the flow rate through this equipment was being modulated or throttled down to result in a smaller Delta-T (say 180F in and 160F out) to reduce heat delivery, the effect on the boiler plant is the same: the flow through high temperature zones is reduced by 50%.

27. The Evolution of the Hot Water BoilerThe Cutting Edge: Two-Return High-Mass Heat Exchangers In the low temperature zones, the control reduced the loop supply temperature and maintained the flow rate. This would mean a smaller Delta-T through the system, achieved by looping a greater percentage of the low temperature return water back through the three-way mixing valve. For this boiler plant this means that the low temperature return flow rate has reduced by half, down to 25gpm. (Note that a 20°F (11.1°C) Delta-T @ 125gpm in the low temperature zone is 1,250 Mbtu/h, but only 25gpm of high temperature supply is needed to boost this zone back to it’s desired target.) The net result is that we are still running 33% of the water through the boiler at very low temperatures, meaning we get full condensing efficiency. The total flow through the boiler plant would be 75gpm, so with both boilers operating at roughly 50% capacity, producing 1,125 Mbtu/h gross each, they would each be receiving only 37.5gpm total. This would be very efficient, very low stress operation. This is technology on the cutting edge of hydronic design: the most advanced boilers systems on the market today.

28. Please contact JPI for help on your next boiler design! Tom Heckbert Commercial Boiler Sales Specialist theckbert@johnsonpaterson.com 1-416-436-4660