Fewer Bridges To Cross
November 1, 2006
Alternatives to parallel primary loops.
Virtually everyone working in the North American hydronic heating industry has heard of primary/secondary (P/S) piping. Many now use it as a routine “backbone” for their system designs. The most widely used form of this piping technique is called a seriesP/S system. Each secondary circuit connects to a common primary loop in sequence. The first secondary circuit receives the highest temperature water, the next secondary circuit downstream gets water that's slightly cooler and so on. Although this arrangement can work in systems having a wide range of supply temperature requirements, the sequential temperature drop effect is undesirable in situations where each load requires the same supply water temperature. Another variant of the concept is called aparallelP/S system (see Figure 1). Although not extensively used in residential and light commercial system applications, parallel P/S does eliminate the sequential temperature drop effect associated with series P/S systems. It does so by dividing the primary loop into two or more crossover bridges, each with its own set of closely spaced tees. This provides water at the same temperature to each of the secondary circuits, while at the same time ensuring hydraulic separation between the circuits through the “magic” of the closely spaced tees. The benefit of equal supply temperature in a parallel P/S system comes at the cost of more complex and expensive piping. Each crossover bridge is usually equipped with a balancing valve so that flow through that bridge can be set in proportion to the secondary circuit load it serves. The valves add cost, and may or may not be properly adjusted once the system is in service. Parallel primary loops also require more fittings and “wall space” than do series primary loops.
Parallel primary loops can extend well beyond the mechanical room. They can literally circle the interior perimeter of a large building as shown in Figure 2. However, in many situations it's not necessary to build a primary loop that encircles the building. Instead the hydraulic separation and division into zone circuits is all done within the mechanical room. Here is where the opportunity for some good value engineering exists. Take a look at the piping system shown in Figure 3. The boiler system has been interfaced to a large diameter vertical “header” within a relatively compact space. The generous diameter of the header and relatively 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. My suggestion is to design the header for a flow velocity in the range of 2 feet per second under full design load flow conditions. This keeps pressure drop along the header extremely low. Low enough that the slight length of the header from A to B is of no consequence. For example: Suppose the length of the header from A to B in Figure 3 is 4 feet. The design flow rate along the header with all branch circulators operating is 20 gallons per minute. If a 2-inch copper tube is used for this header, the flow velocity at 20 gallons per minute will be about 2 feet per second. The head loss along this header would be less than 0.03 feet of head. The associated pressure drop would be under 0.012 psi. The pressure drop is not zero, but it's pretty darn close! These numbers are conservative because they assume the 20 gallon per minute flow rate exists over the full 4-foot header length. This, of course, is not true since the flow rate along the header decreases toward both ends as flow moves into and out of the branch circuits. A pressure differential of say 0.015 psi is not enough to open a flow check or spring-loaded check valve in a branch circuit attached as shown in Figure 3. In my opinion, it's also low enough to allow both variable-speed and fixed-speed circulators to be supplied from the same header (provided all branch circuits are equipped with a spring-load check valve as shown).
Hence, one of the branch circuits could serve a variable-speed injection mixing pump, another could serve a variable-speed distribution circulator, and still another could serve a fixed-speed circulator. The key to keeping the peace between all circulators is a very low pressure drop along the header, just like in P/S systems. The validity of this approach has been tested and proven successful on a recent minitube injection system in which three independently controlled variable-speed injection circulators are supplied from a common low flow resistance header system. If the header is vertical, as shown in Figure 3, it would be a good idea to install a float-type air vent at the top. A drain valve at the bottom is also recommended.
If you estimated a 4 percent annual inflation rate on the cost of electricity, the total cost of operating this circulator over the next 20 years would theoretically be $1,800! This far exceeds the circulator's installation cost and shows the importance of “life-cycle” cost comparison rather than simply fixating on first cost. The lesson here is simple: 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. It's not something a true hydronic heating professional should ever ignore or dismiss as inconsequential. The “physics” of the piping arrangement shown in Figure 3 is now available as a single device called a hydraulic separator (see Figure 4). 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 the hydraulic “uncoupling” effect equivalent to closely spaced tees. Again, one of the components you don't see in this schematic is a primary circulator - you simply don't need it. The branch circuits are uncoupled from each other by the very low flow resistance through the hydraulic separator and short headers.