John Siegenthaler, P.E. 2015-05-08 01:44:43
FOUR APPROACHES TO HYDRAULIC SEPARATION Integrate this very desirable characteristic into your systems where appropriate. Twelve years ago, the term hydraulic separation was very new to the vocabulary used in the North American hydronics market. Back then, the contemporary topic being discussed and applied was primary/secondary piping. The idea that several loads, each served by their own secondary circuit and associated circulator, could be connected to a common primary loop with its own circulator. The “magic” of closely spaced tees is what prevented the flow rate in any of these circuits from interfering with the flow rate in the others. That “magic” is the essence of hydraulic separation: The ability of two or more circulators within the same piping system to simultaneously operate without interfering with each other. Properly designed and installed primary/secondary piping can achieve hydraulic separation between all circulators. However, primary/secondary piping is not the only way to achieve hydraulic separation, as depicted in Figure 1 on page 49. Think of hydraulic separation as the broad topic, whereas primary/secondary piping is one of several subtopics. This article describes several other ways to achieve the same results delivered by classic primary/secondary piping, but in ways that often simplify the system and reduce its installation cost. GOOD AND BAD HEADERS Before detailing other methods of hydraulic separation, it’s important to understand the role of headers in a hydronic system. The “ideal” header in any hydronic system would simply split up the flow entering it, into the branch circuits attached to it, with zero head loss. The spherical header shown in Figure 2 on page 49 would be a close approximation of this ideal concept. Imagine piping coming out of this spherical header in all directions, such as a copper basketball with tubes coming out all over its surface. The water would be very “content” to flow through such a header, but imagine how this would look in a typical mechanical room! If you can’t image this, I’ll tell you how it would look: Terrible! It would take up lots of room and be very difficult to install using standard piping practices. In short, we don’t build headers like this, not because they wouldn’t work, but because of other practical and aesthetic reasons. Instead, we approximate the favorable fluid mechanics offered by the spherical header concept using standard hardware that looks good and lays up neatly in a mechanical room as depicted in Figure 3. I like to call these “short/fat” headers. Simply put, the shorter the header and the greater its diameter, the closer it comes to approximate the spherical header. Remember, the goal is to split up the flow into the branches with as little head loss as possible. So, here’s something that every hydronic system designer can easily remember: Short/fat headers are good and long/skinny headers are bad! It’s always good to keep the headers in your systems as short as practical and use a tube size that keeps the flow velocity low. I suggest sizing the header so the maximum flow velocity within it, when all branch circuits are operating, is no higher than 2 ft. Per second. Figure 4 (right) shows a table that lists the flow rates corresponding to flow velocities of 2 ft. Per second for type M copper tube. BEYOND THE HEADERS Short/fat headers provide hydraulic separation between the circulators connected to them. These circulators can be different sizes. Some may be variable-speed circulators while others operate at fixed speeds. Hydraulic separation occurs because the head loss (and thus the pressure drop) along the length of the header is very low. To maintain the hydraulic separation afforded by short/fat headers, it’s important that the headers connect to a piping assembly that itself creates very low head loss. One arrangement that achieves this is simply connecting the headers to a low-flow resistance heat source, such as a cast-iron boiler, or a “tank-type” hydronic heat source, as shown in Figure 5 (right). Both of these heat sources create very little head loss. When combined with short/fat headers, the “common piping” assembly, outlined with the dashed lines, creates very little head loss even when all the circulators are operating. The lack of any significant head loss in the common piping is what prevents circulators from “feeling” each other’s presence in the system. If the circulators can’t “feel” each other, they can’t interfere with each other and hydraulic separation is achieved. The piping in Figure 5 also provides the same supply water temperature to each load served by the header. This is not true with traditional primary/secondary piping where all sets of closely spaced tees are arranged in a series along a common primary loop. The latter arrangement creates decreasing supply water temperature in the downstream circuits. Furthermore, the extent of this temperature drop is not consistent. It varies depending on which secondary circuits are operating at any given time. If the heat source you want to use has higher flow resistance — as is typical of mod/con boilers with compact coil-type heat exchangers or the coaxial heat exchangers used in hydronic heat pumps — you can merge the headers to that heat source as shown in Figure 6 (right). The pair of closely spaced tees hydraulically separate the boiler circulator from the circulators on the header. Thus, the overall piping assembly within the dashed lines (e.g., the common piping) has low head loss. Viola! You have hydraulic separation between all circulators in the system. If your system needs added thermal mass to stabilize a low-mass heat source against the potential demands of a highly zoned distribution system, let a buffer tank, piped as shown in Figure 7 on page 50, work in partnership with the short/fat headers to provide the hydraulic separation. All these methods provide the same supply water temperature to the loads. Finally, it’s possible to use a component called a “hydraulic separator” to provide – you guessed it – hydraulic separation. The piping is shown in Figure 8 (left). Along with hydraulic separation, many hydraulic separators now contain internal screens called “coalescing media.” These inserts enhance the ability of the hydraulic separator to separate microbubbles of air passing through the upper portion of the separator. Using a hydraulic separator with a coalescing media eliminates the need to use a separate high-efficiency air separator in the system. A second coalescing media, built into the lower portion of the hydraulic separator, enhances its ability to trap dirt parts that might be riding along with the flow as it returns from the distribution system. With multiple passes, some coalescing media can separate out dirt particles as small as five microns. They drop out of the active flow zone and into the lower portion of the separator. A periodic opening of the valve at the bottom of the hydraulic separator can flush out this dirt. There you are: Four methods of achieving the very desirable characteristic called hydraulic separation in your hydronic systems. Notice that all these methods provide the same supply water temperature to the loads and that none require a dedicated primary loop circulator. In that respect, the methods shown, in my opinion, are improvements over traditional primary/ secondary piping. Integrate them into your systems where appropriate.
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