It doesn’t have to be perfect to be effective.

Figure 1.


Many hydronic systems contain multiple, independently controlled circulators. These circulators can vary significantly in their flow and head characteristics. Some may operate at fixed speeds while others will operate at variable speeds.

When two or more circulators operate simultaneously in the same system, they each attempt to establish differential pressures based on their own pump curves. Ideally, each circulator in a system will establish a differential pressure and flow rate that are unaffected by the presence of another operating circulator within the system. When this desirable condition is established, the circulators are said to be hydraulically separated from each other.

Conversely, the lack of hydraulic separation can create very undesirable operating conditions in which circulators interfere with each other. The resulting flows and rates of heat transport within the system can be affected greatly by such interference, often to the detriment of proper heat delivery.

The degree to which two or more operating circulators interact with each other depends on the head loss of the piping path they have in common. This piping path is called the common piping, since it is shared by both circuits. The lower the head loss of the common piping, the less the circulators will interfere with each other. Figure 1 illustrates this concept for a system with two independently operated circuits.

In this system, both circuits share common piping. The “spacious” geometry of this common piping creates very low flow velocity through it. As a result, very little head loss can occur across it.

Assume that Circulator 1 is operating but that Circulator 2 is off. The lower (blue) system head loss curve in Figure 2 applies to Circuit 1.

The point where this lower-system head loss curve crosses the circulator’s pump curve establishes the flow rate in Circuit 1.

Next, assume Circulator 2 is turned on and Circulator 1 continues to operate. The flow rate through the common piping increases and so does the head loss across it. However, because of its spacious geometry, the increase in head loss across the common piping will be very slight. The system head loss curve  now “seen” by Circulator 1 has very slightly steepened. It is the upper (green) curve shown in Figure 2. The operating point of Circuit 1 has moved very slightly to the left and, as a result, the flow rate through Circuit 1 has decreased very slightly.

Such a small change in the flow rate through Circuit 1 will have virtually no effect on its ability to deliver heat. Thus, the interference created when Circulator 2 was turned on is of no consequence. We could say this situation represents almost perfect hydraulic separation between the two circulators.

Figure 2.

Almost perfect is good enough

One could imagine a hypothetical situation in which the head loss across the common piping is zero, even with both circuits operating. Because no head loss occurs across the common piping, it would be impossible for either circulator to have any affect on the other circulator. Such a condition would represent “perfect” hydraulic separation and would be ideal.

Fortunately, perfect hydraulic separation is not required to ensure that the flow rates through independently operated circuits, each with its own circulator and each sharing the same low-head-loss common piping, remain reasonably stable and thus capable of delivering consist heat transfer.

With good hydraulic separation, the simultaneously operating circulators can barely detect each other’s presence within the system. They operate as if they were essentially each in an independent circuit. For all practical purposes, one can think of (and design) circuits that are hydraulically separated as if they were completely separate of each other, as illustrated in Figure 3.

Figure 3.

Plenty of options

Any component, or combination of components, that has very low head loss and is common to two or more hydronic circuits can provide hydraulic separation between those circuits.

One way to create low head loss is to keep the flow path through the common piping very short. Another way to create low head loss is to greatly slow the flow velocity through the common piping.

Examples of devices that use these principles include:
  • Closely spaced tees;

  • A tank (which also might serve other purposes in the system); and

  • A hydraulic separator.

Let’s take a look at each of these methods.

The common piping in Figure 4 consists of the closely spaced tees and “generously sized” headers. Because they are positioned as close to each other as possible, there is virtually no head loss between the tees. These tees form the common piping between the heat source circuit and the distribution circuits, and thus provide hydraulic separation between these circuits.

The generously sized headers create low flow velocity and low head loss, and thus provide hydraulic separation between the five distribution circulators. These headers should be sized so that the maximum flow velocity - when all circulators served by the header are on - is no more than 2 ft. per second. Together, these details create hydraulic separation between all six circulators in the system.

The closely spaced tees allow the heat-source circulator to only “see” the flow resistance of the heat source and piping between the boiler and closely spaced tees. The heat-source circulator doesn’t help move water through the distribution circuits. Likewise, the five distribution circulators are only responsible for circulation through their respective circuits and do not assist in moving flow through the heat source.

Notice also that fixed-speed and variable-speed circulators, perhaps of different sizes, can be combined onto the same generously sized header system. Interaction between these circulators will be very minimal because of the generously sized (e.g., low head loss) headers.

Figure 4.

Beyond closely spaced tees

Figure 5 shows a buffer tank and generously sized headers serving as the low-flow-resistance common component that provides hydraulic separation between the heat-source circulator and each of the distribution circulators. This demonstrates that hydraulic separation can sometimes be accomplished as an ancillary function to the main purpose of the device (e.g., hydraulic separation is not the main function of the buffer tank).

Still another method of providing hydraulic separation is using a device appropriately called a hydraulic separator. Although relatively new in North America, hydraulic separators have been used in Europe for many years. Figure 6 shows a hydraulic separator installed in place of the buffer tank of Figure 5. Note the similarity of the piping connections between the buffer tank and hydraulic separator.

Figure 5.

Hydraulic separators, which are sometimes called low-loss headers, create a zone of low-flow velocity within their vertical body. The diameter of the body is typically three times the diameter of the connected piping. This causes the vertical-flow velocity in the vertical body to be approximately 1/9th that of the connecting piping. Such low velocity creates very little head loss and very little dynamic pressure drop between the upper and lower connections. Thus, a hydraulic separator provides hydraulic separation in a manner similar to a buffer tank, only smaller.

The reduced flow velocity within a hydraulic separator allows it to perform two additional functions. First, air bubbles can rise upward within the vertical body and be captured in the upper chamber. When sufficient air collects at the top of the unit, the float-type air vent allows it to be ejected from the system. Thus, a hydraulic separator can replace the need for a high-performance air separator.

Secondly, the reduced flow velocity allows dirt particles to drop into a collection chamber at the bottom of the vertical body. A valve at the bottom can be periodically opened to flush out the accumulated dirt. Thus, the hydraulic separator serves as a dirt-removal device.

The ability of modern hydraulic separators to provide three functions - hydraulic separation, air removal and dirt removal - makes them well-suited for a variety of systems, especially systems in which an older distribution system, one that may have some accumulated sludge, is connected to a new heat source.

Figure 6.

A matter of degree

Designers should think of hydraulic separation as a physical condition that can be accomplished to various degrees, rather than an all-or-nothing condition. “Perfect” hydraulic separation is never possible because no real common piping assembly can ever have zero head loss. Thus, there always will be some very slight interaction between simultaneously operating circuits within the same system. However, the methods just discussed, when properly applied, can keep such interaction to a level where it is insignificant in terms of detrimental operating conditions.

The ability to prevent such detrimental interaction between simultaneously operating circulators is critically important to modern hydronic system design.

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