Fundamentals in Steamback Design 11.08.2010Arun base
SUNMAXX SOLAR HOT WATER SOLUTIONS
FUNDAMENTALS IN STEAMBACK DESIGN
Ok, Good Afternoon everybody. This is another one of our Sunmaxx Solar webinar series. Today I’d like to talk about the fundamentals in steamback design. This is a relatively new concept for the most part in the U.S., although there are many installers that have been doing it for quite a while. It’s much more common and useful in European solar thermal systems. So, I want to touch base a little bit on some of these design strategies, to introduce it to you, for those of you who are not familiar with steamback design. I think it’s going to become much more common in the U.S. as an alternative to drainbacks. Ok, so as always I’d like to encourage you to ask me questions in the chat box. We do have a relatively small class, small group today, so it makes it a little bit easier to address questions immediately.
Ok, I’ll get right into it now. So, generally when you design a combisystem, you’re going to produce more energy than you need in the summertime. So, the options are really to either, traditional options are to, reduce the production, or to dump your heat. Heat dumps are very, very common in the U.S. market. In the European market, dumps are relatively strange. They don’t design too many systems using a heat dump, instead they use the steamback designs. Now, not to say that they never design systems, and when I say they I mean the European installers, heat dumps are really only designed when steamback system will be limited. There are long runs of copper, convoluted runs, but I’ll get into that in a minute. So traditionally we either don’t make it or we have to do something with it.
Now with a steamback, looking at this simple design here, what I want you to focus on is the location of the expansion tank. So, in most pump stations, the expansion tanks are located above the pump, ok. The location of that is critical in allowing for steambacks to occur. So, for those of you in attendance that have installed systems, you probably have allowed for steamback to occur without even knowing it. So, steamback is, essentially, a time during stagnation when the pump is off and the collectors begin to overheat and boil the heat transcript fluid. Now, the heat transcript fluid in a closed loop system is usually 50% glycol and 50% water. Water has a lower boiling point than glycol okay. SRCC shows us that most collectors can stagnate at upper 300s low 400 degrees Fahrenheit with no danger. So, stagnation is not really a problem for collectors, stagnation is a problem for glycol. Much of the glycol in the market has a buffer and those buffers will break down when they approach temperatures of usually 250 and upwards of 360.
So some of the best glycols on the market are going to be able to stagnate up to 360 without turning acidic. But we can’t rely on that alone. We have to allow for this condition we call stagnation. So, when the pumps shut off and the collectors stagnate during the solar day the boiling points of the two fluids is going to cause water to boil, creating the steam, and that steam pressure is going to force the remaining fluid, or the remaining solution which would be mostly glycol, into the expansion tank. Okay, so imagine collectors at high noon stagnating, alright and they’re producing temperatures exceeding 390 degrees. There’s no danger to the collectors, remember, the only danger is to the glycol. So if the water boils at a lower temperature and forces the glycol out of the collectors, then what remains is a collector full of water steam. Which we know from SRCC that that presents no danger to system life or longevity. So simply not necessarily creating a steamback system or designing a steamback system but small differences in installation allow us to allow for steambacks to occur. So steamback, basically the laws of physics tell us that water will boil and increase the pressure and that pressure will be used to force the remaining solution out of the collector.
Now steamback happens in five phases. I would like to remind you we’ve been 6 minutes so far and I haven’t had a question so far. For those of you who know me, I do enjoy questions. So get your thinking caps on and see if you can ask me some questions. So there’s five phases to steambacks, okay. As I just described to you, the whole process. The first thing that’s going to happen is the liquid, the heat transfer fluid, is going to expand. And that expansion increases the pressure. What we don’t want to have happen is our safety valve to open, okay. Most safety valves in pump stations, I know for the Unimaxx pump station, it’s 87 psi. So, we want to be sure that we do not approach that pressure. So the first thing that happens is the liquid is going to expand, and once the fluid, the water, reaches the boiling point, the expansion is going to increase by a factor of 240. Okay, so that increasing in pressure is going to push the liquid out of the collector, and this is when we enter phase two. This can happen in a matter of 3 to 4 minutes, it might take up to an hour to work all of the remaining fluid out of the collector, and as you know the remaining fluid should be close to 100% glycol. Now, the third phase is when the collectors will push the remainder of the fluid out and the water will boil and this boiling of the water, as I mentioned, can increase the volume by 240%. So, it’s going to push every last bit of glycol out of the collectors. Now we have super saturated steam, or super heated steam. Again we are still less than 87 psi, but it’s extremely saturated water steam. Finally, once the solar day ends collectors cool down, pressure drops, and the expansion tank will refill the collector with the, not only the condensate from the steam, but also from the glycol that had been separated. And then that solution will very easily be mixed again and we have a 50% glycol concentration at the end of the day, and we haven’t boiled our glycol.
So this steamback happens during stagnation, and it’s going to happen whether you like it or not. So what you really have to do is consider what types of design, and what types of collectors, will more easily allow for steamback to happen, because steamback is a physical process and it essentially protects your glycol. So by allowing for steamback, you have an added insurance. So the last step, to address the question, as the solar day ends the production by the collectors decreases, the steam is going to obviously condensate. As it condensates, it reduces the pressure inside the collectors. That pressure inside the collectors, once it reaches a point less than the pressure of the expansion tank, well then the expansion tank is going to force the glycol back into solution, back into the collectors. This happens just as a natural process.
In order for these five phases to occur, I’m going to go over a couple of points here on system design. Now this graph shows the different pressures at the expansion vessel, depending on the quality of the design and the quality of the collectors. Now if the ability of the collectors to effectively steam back. Now, the red line is a poor emptying behavior collector. So, what you see there, is during phase two, the transition between phase two and three, when the super heated steam is emptying the collector, it creates more steam power. So it takes more steam to force the remainder of the fluid out of the collector. So, although this does work, it takes longer for that steam to force the glycol out. Some of you may know that glycol buffers that protect the pH of the solution, they don’t break down immediately, once they – okay Nathan, good question I’m going to actually address that in just a couple of slides. So as I was saying, the pressure inside the collector doesn’t necessarily have to climb to four to four and half bars. One bar is one atmosphere by the way. If the, and I’m going to discuss the perimeters of the collector that allow for steamback to occur, there’s a couple of design strategies, not only in the collector but also in the piping. So what we want to have happen, as you can see the green line shows once we enter phase two, phase three occurs without any added pressure at all. So in other words, once we begin to empty the collector in phase two, phase three, the steaming of the water doesn’t do anything to increasing pressure because it is easy to empty the collector into the expansion tank, and I’m going to show you a couple of designs that will allow us to do that.
So what you see here, these four would not have very good steamback characteristics. In other words, these would take more steam pressure to remove the fluid from the collectors, than it would – if I go back for a moment, this green line is what we’re really after. We want to design a system with collectors that allows for the transition from phase two to phase four with no added pressure. These systems would represent the blue and the red line, because as you can see, if we are coming in on the left hand side, in order for the steam to force the fluid out it’s got to overcome a pressure rise and push it all the way down and then back up and out. And the second one, we’re basically moving in one direction, so the amount of pressure, these are actually in parallel but in an unbalanced, parallel arrangement. And the third one, this is very typical of a U-pipe evacuated tube system. You essentially have reverse return piping folded in half, so the third one is not effective in removing.
Now bear in mind all four of these styles will allow for steamback, just being very particular now as to which style requires less steam in order to steamback. Now the answer to that question is here, so this style piping arrangement allows for steamback to occur very easily because it can happen in two directions. So for example, the serpentine style on the left. The serpentine style that you see on the left-hand side is the single best design for steamback. Now when I say best, again I mean it requires less pressure to empty the collector. What we’re really after is emptying the collector of glycol. Most collectors are rated for about 10 bars, they are pressure tested for 10 bars. And if I go back up here, even the worst ones, so poor, good, and very good are just relative, so even the ones that say poor emptying behavior, that pressure that’s required of them to eliminate the glycol from the collector is still about half of the pressure the system can take. So most collectors are rated, and you should check with your manufacturer, speaking for SunMaxx the collectors are rated at 10 bars, that’s what they’re pressure tested to. So even though I say poor emptying behavior, it’s still less than the maximum allowable pressure of the collector. I hope that answers your pressure there, Nathan, in terms of parameters of collector that affect the performance.
And this design, take a look at the third one, the third one is a harp-style or header riser style collector, and it does show arrow coming in, the supply to the collectors coming in on the bottom right, and it’s moving up and out to the top left. So, it’s coming in on the bottom right and it’s leaving on the top right. But this arrow very easily, in normal design we would have the supply going to the tank opposite the return coming from the collectors. So if you look at this third one, this arrow can just as well be over on the opposite side as it might be piped in for a reverse return method. The fourth diagram shows how easy it is to remove the glycol solution from this system because I have two possible exits.
Now, location of the expansion vessel is probably the single most important thing in allowing for steamback. I did mention the design of collectors will affect performance, however, even some of the worse poorly designed collectors still can empty the collector with less pressure than is allowed in the collector. But, if you do not install the expansion tank in the proper location, then regardless of your collector’s ability to create steam power, it’s simply not going to work. So, if you take a look at the design that’s in place, it says poor location of the check valve. Now many pumps Taco, for example, that has check valves that are integrated into the pump. It’s not a very strong check valve but it’s strong enough that it will slow down the movement of steam into the expansion tank. So if you take a look at the placement of the expansion tank in the diagram labeled poor location, why is it a poor location to put the expansion tank below the pump? If we read textbooks on hydraulics and pump performance, we talk to Taco and we talk to Vilo about the placement of the expansion tank relative to the pump, it’s often been argued that expansion tanks should go behind the pump so that the pump doesn’t have increased pressure that it has to overcome. If we put the expansion tank in front of the pump, while the pump turns on, the required pressure may be taken in by the expansion tank and the pump is just pumping and pumping and pumping but it’s not going anywhere because of the positive pressure on the push side of the pump is being taken in by the expansion tank and the column of water doesn’t get moving. So, I have been involved in a system where we had the expansion tank behind the pump like this, the pump did not have enough head to move the fluid, so we had to move the expansion tank behind the pump, and the pump then was able to move the fluid. Now, as you know, if you put the expansion tank behind the check valve, it’s really not a question as to where the expansion tank should go in relation to the pump, it’s a question as to where the expansion tank should go in relation to the check valve. So, the check valve is not going to allow for movement in the opposite direction. So, it’s going to actually block the expansion tank’s ability to take in any of the super heated fluid coming off the collectors.
So, just to the left of this, this is the good location of the check valve. So, if we put the expansion tank in front of the check valve, but behind the pump, now what we’ve done is we’ve eliminated the need for increased pressure by the pump because the expansion tank is behind the pump, but we’ve also increased the system’s ability to allow for steamback because we’ve put the check valve behind the expansion vessel. There is another consideration to make, however, most pumps and I know that Taco and Grundfos are rated at about 250 degrees Fahrenheit. So if we put the expansion tank behind the pump such as this, but in front of the check valve, which is what we recommend, then we also run a risk of overheating the pump, because if the collectors are stagnating and producing temperatures that are boiling, and we might have a 250 degree boiling point for water. If your system’s operating at 30 psi, for example you have 2 bars, the required temperature in order to boil the water may approach 250 or more, 260, so once that temperature is reached, then we have some steam production and we’re into phase three. Well phase three is going to send that glycol back down into the expansion tank, that’s exactly what we want, but it has to go through the pump before it gets to the expansion tank. So you do run a risk of endangering the pump, so you want to check what the high temperature limits of your pumps are, and I can tell you that the Taco 00 series, I believe it’s a 256 degree Fahrenheit number, I may be off by a little bit, but not much. So I’ve got poor, good, and I might recommend better. Better would be where we put the expansion tank in front of the pump altogether. Another thing I’d like to point out is very simply that the bladder, or diaphragm, in your expansion tank should always be wet. We don’t want to have 250 or 270 degree water or glycol or steam or anything slamming down into our epdm lining, most bladders are made out of epdm rubber, and that does have a melting point. So, if we flip our expansion tank upside down, which I’ve seen some installers do, then it’s going to cause the bladder to be dry. That would be a situation where we have 260 degree water being forced onto the collectors and strikes the epdm lining on the membrane directly, and if this happens everyday, all summer-long, you’re definitely going to have a short-lived expansion tank. So please be sure that your expansion vessel is always wet.
Now location of the expansion tank is one thing, but size of the expansion tank is another. Now before I go over this formula, which I’m not going to do in too much detail, I want you to understand that SunMaxx sales reps, engineers will size your expansion tank for you. All we need to know from you is the size of your collector field, the pressure rating on your pressure relief valve, the target pressure (what you hope to be operating your system at) and that target pressure can be anywhere from 1 bar to 3 bars, generally anywhere from 15 to 50 psi. Now, the side note here, system pressure does not affect performance. So if you’re used to operating and you want to operate your system at a higher pressure, that’s not going to affect performance. It will, however, dramatically increase the size of the expansion tank required because you’ve increased the boiling point of both the water and the glycol, which means you’re going to have hotter fluid which will have expanded, as temperatures increase so does expansion and you’re going to be able to allow for an increased temperature because of an increased pressure. So although I make a recommendation of setting your target pressure anywhere between 15 and 50, I would recommend you set your target pressure in the low end of that range, as system pressure does not affect performance.
Okay, so the size of the expansion vessel, as we have very poignantly learned over the last 3 or 4 years, systems have been exploding, pressure relief valves are opening, customers have been scared. They see steam shooting out of their basement, they don’t know what’s going on and they think it’s dangerous. So we have to take it very seriously, this steamback phenomena. Steamback will happen, our job is to let it happen. And without a properly sized expansion vessel, we’re not going to let it happen. So it’s not as simple as saying, “well expansion vessel should be 20% of the system volume or 30%, or even 60% of the system volume.” We have to look every case by case, and what we need from you is the volume of the volume of fluid in your piping, total volume of fluid in the piping, and we can help calculate that if we know pipameter. We also need to know the target pressure of your system, the maximum rating of your safety valve, and the concentration of the glycol that you’re using. And if you’re using 50% glycol – ah yes, Vince, what we recommend is that you actually charge your expansion vessel at 5 psi greater. So as you take your expansion tank out of your kit, I believe they are charged at 37 psi, I’m pretty sure that’s what they are. So what you want to do, just like a tire tube, you can have a straighter valve, take a look at your needle get a tire gauge in there, see what the pressure of the tank is and charge that up to 5 psi greater than what your target pressure is.
So sizing up your expansion tank is something. Well Carl that’s a good question – expelling steam means a loss of water, how often do you need to replenish? You’re not really expelling the steam, this is a closed loop. So when you create the steam, the steam actually gets trapped inside your collectors, and it’s that steam pressure that forces the fluid out of the collectors. So there may be some losses if you have an air vent that might be open a little bit, but generally, there should be no losses of steam because if you’re losing the water, you’re not increasing the pressure, and it’s that pressure increase that we need to force the glycol out of the manifolds. So it is a closed loop and you will not be expelling the steam, rather you’re going to trap the steam, and that steam gets trapped inside the collectors.
Okay, so for those of you who are interested in sizing your expansion tanks by yourself, then this would be the formula to use. It’s very straightforward, I’m not going to take the time right now to go over each particular, I’m sure there’s a name, a step and process in algebra, but I don’t remember the steps involved. However, I’m not going to go over each variable, but you should know this, you need to know the volume of fluid, the concentration of fluid, the collector field, your target pressure and your maximum pressure rating for your pressure relief valve. Those factors we will use to size up your expansion vessel and you can have relatively quick turnaround. If you have systems out there that are about to enter their first winter, you probably don’t need to worry about it too much because stagnation rarely occurs in the winter time. Stagnation generally occurs in the summer, so if you had just installed a system and you want us to double-check the size of your expansion tank, get a hold of your sales rep and we can do that for you too. It’s better to be safe than surprised. Clients can be very surprised if steamback happens without a properly sized expansion tank. If they happen to be home during the time, it will scare them, I’ve seen it happen, so this is the number one thing we’ve got to be sure of, is the expansion tank properly sized.
A couple of other considerations as we wrap it up, the position and size of the expansion tank, and proper concentration of glycol. I just want to point out that although some of you look at glycol as an antifreeze, that is what it is, it’s not necessarily any more or less of an antifreeze at 40% or 50% or 60% or 70% or 80%, the magic number for most places in the U.S. is 50%, and you can go down to 30%. However, please remember this, if I have 60% glycol or 70% glycol or 80% glycol, as I increase the concentration of glycol, water and glycol have different densities, so I’m going to encourage the separation of that solution. So, even though I have an 80% glycol, water is not going to be mixed in solution as it would be with a 50% glycol, so the water comes out of solution, and at night if the water comes out of solution then we have pockets of solution that are vulnerable to freezing and they’re going to end up in that collector manifolds. So 50% is actually better than 60%. There are lots of other fluids that are used, oils and ethylene, there are a lot of different heat transfer fluids and we’re playing around with some materials now, but currently for closed loop systems that need antifreeze protection, the best approach is to use a propane glycol because it is safer for the environment, it’s readily available, it does have some drawbacks with viscosity and the like, but for the most part, for the next couple of years anyway, propane glycol will be the heat transfer fluid of choice.
Nathan, the webinar slides will be posted on the site. As soon as I’m done here you’ll see a pdf on solar webinars that you can download and use that formula, that probably would appear in a pdf form much clearer than it has here. For references use Siegenthaler and Hausner and Fink.
So to wrap it up, I’d like to thank you for your attention once again. Stay tuned for next week, take a look at our schedule. If you have any suggestions or things that you’d like to see covered please don’t hesitate to send an email to me with that suggestion. I do encourage feedback, both positive and negative.
Amy, you want to position, normal installation of expansion tank is going to cause the bladder to do what? The point is don’t turn it on it’s side or flip it upside down because what’s going to happen is the steam is going to rise and come right up in direct contact with the bladder, so we generally recommend taking a look at the stickers on the expansion tank and installing them so you can read them like you normally would. If there are not stickers for proper orientation, then look at the installation manual and just be sure that any of the fluid that’s trapped in your expansion tank is resting on the membrane, so that in order for any steam to touch the membrane, it’s got to pass through fluid. That’s cooling it down, reducing the temperature and likelihood of melting. Yeah, it is a problem, Dave, and what it is, is little micro-explosions of steam that, the banging of pipes happens in situations that don’t have good emptying behavior. If you have convolutions in your pipe where you might get pockets of condensate, that condensate is going to be introduced to steam and you get little micro-explosions and those are the bangings that you hear. So, not only the collector requires good emptying behavior, but the entire pipe run requires good emptying behavior. So, in a sense, just as we have limitations for drainback systems, everyone of our pipes needs to be sloped down in order to drain. Steambacks require the same type of attention, we don’t want to have places where there are pockets of water before the expansion vessel. Not that it’s a problem, but you will end up with the interface of steam and condensate and little micro-explosions, and there’s not a whole lot you can do to reduce that at this point.
Well I do want to formally end it, and I’m happy to – okay I got another question… No, you don’t need a specific steam condenser, and I would be interested in email to speak with you more about that to see. I’m not exactly sure what you’re referring to, but as I mentioned before, steamback will happen whether we want it to happen or not, it’s going to happen it’s just a matter of have we designed it so that we allow for steamback to occur, is the expansion tank properly sized, does our piping represent proper emptying behavior, and do the collectors represent proper emptying behavior, and is our expansion tank located in the correct spot relative to the check valve. If those measures are in place, then we’re in good shape.
Again, email me any questions, this is just the beginning of our interactions and hopefully we’ll see you next week. Have a great solar day, as I look out my window here I don’t see the sun, but that’s not surprising, again we’re in upstate New York, hopefully we’ll see it here in the next couple of days. Okay, take care everybody, bye bye.