FIGURE 1: Circular vs Rolling Circulation DesignAs you can see, the image on the left represents a typical pool circular circulation paradigm where the thought is that we are pushing the debris using the inlets towards the skimmer. On the right, we have a rolling paradigm and in this case, we are creating individual circulation "pairing" with each inlet/skimmer combination. Any additional inlets are used to stir up dead spots. As well, the inlets could be on the floor, or low wall, but in this approach you don't get as much attraction toward the skimmer. For rolling to work, each skimmer must have a return under it (laterally with in +/- 12" of center is probably close enough. As well, commercial builders are probably quite familiar with gutters (typically stainless) that go around the entire pool that have holes around the entire perimeter that act as angled inlets down into the pool. This has the same effect on the gutter as a skimmer.So what is happening here? Let's focus in on the detail show in Figure 2
FIGURE 2: Circular vs Rolling Circulation DetailIn this case we have to compare what is happening at the surface and under the surface. This is the part of circulation that is not very easy to visualize. During my days at NASA as a material scientist, we looked at the effects of the weightless environment on materials processes like gelation, crystal growth, and alloy solidification. One of the nice advantages of weightlessness is there is no up or down. Gravity gets temporarily turned off. This allowed us to eliminate buoyancy driven convection - i.e. hot air (or liquid) does not rise in space and creates a quiescent environment for doing science on solutions. You can see this effect in a very visually presentation by looking at this dye test. Here is another great visual demonstration on convection by a student.Now, the take away from this is three things: first, that the hot water rising and cool water is falling due to thermal convection - heating the water. The second point is to note that we can also drive these same flows by using the inlets of a pool strategically located. Third, is we can use this rolling pattern to our advantage to drive debris directly into a skimmer.As a related side note, It is common for our industry to believe that heating a pool from the bottom somehow is an advantage. Realize that these convection flows are miniscule compared to what we find in the velocity of the inlets, however; a pool with radiant heat will continue to flow this way even when the circulation system is off. Reverse flow works (heat, chemical distribution, or otherwise) because it distributes conditioned water (heated, cleaned, sanitized) to the bottom, not because of these convective currents). A inlet has an effective area of at least 15 feet (the entire pool when one considers the back stroke) and when pointed toward the deepest end has no problem propelling conditioned water into the deepest pools. Similarly, in-floor systems work in a slightly different manner in that they sweep debris to an outlet and randomize the circulation pattern: every time the valve turns, you get an entirely new pool current produced - i.e no eddies stay around long. We can discuss this issue later in a future blog, but for now, just register this rolling pattern in your mind vs the merry go round pattern of the circular paradigm.Now, let's take a look at what is happening at the skimmer in the traditional circular approach. Look at FIgure 3
FIGURE 3: Circular Circulation - close up of what goes on at the skimmerOk, this is where I am going to lose some of you, but hang in there. In science we often use little arrows to show various resulting water currents. Look at closely at the diagram. For simplicity, I aimed the inlet directly toward the skimmer as it creates a much more dramatic example of what is happening. These results are skewed when we aim it at an angle across the pool. Water that enters the pool has a high velocity (like a pool cue). It hits the other water like in balls in billiards and they all go crashing to the other side. Note the fan pattern on the illustration. Now, look at what happens when it gets to the skimmer. On the other side of the pool (maybe this is why we call billiards - Pool? LOL), we have a much larger amount of water moving slower in comparison to what is moving at the inlet. In other words, when water enters the pool there exists a little moving fast and when it exits, it is transformed into a lot moving slow. By the time all of this reaches the other side, much of the moving mass is below the skimmer opening, so it must also hit the wall and move up/down - it cannot stop. This water contributes to the "water curtain" created on the pool surface in front of the skimmer.We all will agree that water hitting the opening of the skimmer goes directly back to the filtration system. But what happens to the majority of moving water that hits the wall on either side of the opening?It bounces off - just like in billiards. As you can see in the illustration, this creates an invisible "force field" that you can actually see on a calm day or early in the morning at low sun angle. Debris being carried along the wall gets pushed out into the center of the pool just as it approaches the skimmer - attempts to aim the inlet more towards the skimmer just makes the issue worse. What you will also find is that when the circular force is enough to dominate and overcome this outward surface current, the debris will more often fall to the bottom in the center of the pool and eventually becomes a "vacuuming event." This is no different than creating a whirlpool in an above ground pool. The edges are whipped around and the center is still. Heavier material moves to the edges at high enough flow, but we typically don't have circular pools.With in-floor cleaning systems they are typically rolling the water up the wall and across the surface, so statistically any time the nozzles are pointed away from the skimmer (more often than not) the drive debris into the skimmer. For the short time that they are rotated toward the skimmer, the nozzles create a some of the same describe effect seen in a circular-type pattern, but this is temporary. Btw, if you install in-floor and have the skimmer volume for your pump (most do), try turning off the drain for a while and watch what happens with grass clipping on the surface of the water. This is also very visual in the spring with pollen or using a dusting of DE.Finally, let's look at the Rolling Paradigm in Figure 4. We have to look under the surface to see what is really happening in this example.
FIGURE 4: Rolling Circulation - close up of what goes on at the skimmer (top and side)Note that in this example, the water is directed into the pool and down toward the space between the wall and pool centerline. Also see the optional floor returns that are located along the long axis centerline for the pool. This creates a roll of the entire pool. Again, invoke our billiard analogy, but NOW also remember that as the water pushes away from the wall, the surface water fills in the "hole" created by pushing the water away from the wall. It works a lot like the wake on a boat, but the hole fills in so fast you cannot see the roll. It is sometimes noticed as small vortex above the inlet (especially on down jets). All surface is eventually headed toward, the inlets. Not what one expects - we want to believe that water is headed toward the outlets and yet, outlets do not cause any significant movement in the pool (with exception of whirlpools). It is the INLETS that move water and since water is moving away from the wall, the water has to come back on the surface. Conversely, if you point the jets all up to the surface, you are pulling a lot of water off the floor.I know, it is strange and uncomfortable, but it is what is happening - check it out for yourself with die.In the rolling case, the inlet just creates this nice racetrack loop. Clean water in and to the bottom, encounters water moving toward the center from the opposite side, and debris is forced back across the top surface to the wall. Notice that viewed from the top, this motion toward the inlet happens in ALL directions; once you build a pool inlet located in this fashion and you sprinkle grass/DE on the surface, you will see the debris heading into the skimmer from all directions. As important, no "force field" is sending water to the center of the pool. All surface flow should be toward a wall. For existing pools, you can sometimes dampen the circular motion by aiming all the returns down toward the floor, and then look at any one of them to observe the effect. While it may or may not work, at least you can see it happening. I have been able through trial and error to drive material in a skimmer with an inlet that was a foot or so away. It's not perfect, but can be done. I typically "break" the circular pattern first, be certain that inlets "shine" on the floor of the pool like spotlights, then try to make minor adjustments and attempt to push debris into the skimmer. It will not always work, which will mean a little more vacuuming, but you will typically see some improvement over circular flow.This should provoke a lot of discussion and thought. As well, it's pool season, so give it a try on next week's install. Put a return under a skimmer and you will see what happens. It is VERY visual and very impressive for customers. If you want to wow them with something the competition is NOT going to get, this is one of those great wow moments.Hope this LOOOOOOOONNGG blog spawned some good thought. Rathe than just blast the idea (there are many of us that KNOW it works) try a new approach, dare to do something different. Go do some tests and report back results. Take some video. Take some photos. Most of all, challenge those things that you thought before were unchallengeable and make sure that when you create a hypothesis about this or that, you find a way to test your hypothesis and confirm or deny it.Btw, these same basic prinicples play out in above ground pools (1 inlet 1 skimmer), Pool Genie, and Myrtha Pools. Each takes a very similar approach in one way or another and they all work.In close, one should ask, what was the origin of this circular motion in the first place? How did we get here? I have the answer for you and history once again left a very clean trail of how we started this whole circular circulation, but that will be held for a future blog.