Applications for small 'smart circulators' in hydronic heating systems

Already prospering Europe, a new generation of setpoint-controlled, variable-speed circulators are coming this way. Their agenda? Delivering constant differentia pressure control, protecting boilers from flue gas condensation or thermal shock, regulating temperature differential in heat exchangers, and ultimately delivering an attractive [DELTA]$. Pump this article for in-depth information regarding this stage of hydronics evolution.

Fixed-speed circulators have been used in residential and light commercial hydronic heating systems for decades. Although they hay, generally delivered reliable performance, they often use more electrical energy than necessary, especially when systems operate under partial-load conditions. The circulator in such a system is usually sized assuming all zone circuits are operating simultaneously. Under these conditions, the system resistance curve is relatively shallow, as shown in Figure 2.

[FIGURE 2 OMITTED]

However, as thermostats are satisfied and zone valves start to close, the system resistance curve gets progressively steeper. When this happens, the operating point shifts upward along the pump curve. Each time another zone valve closes, the remaining zone circuits "feel" an increase in differential pressure. In situations where only one zone circuit is open, the pressure differential may be high enough to create high flow velocities and flow noise. In some cases, the high differential pressure may even cause closed valve plugs to lift partially off their seats.

One way to manage the increase in differential pressure is to install a differential pressure bypass value (DPBV), as shown in Figure 3.

[FIGURE 3 OMITTED]

This valve "truncates" the upper portion of the pump curve above a set differential pressure, as depicted in Figure 4. Notice how the operating points (where the system resistance curves cross over the truncated pump curve) do not move upward any significant amount as the system resistance curves get progressively steeper. This is desirable because it minimizes changes in differential pressure across the other operating zone circuits, and thus helps maintain reasonably stable flow rates.

[FIGURE 4 OMITTED]

A differential pressure bypass valve is a "parasitic" device. It controls differential pressure by throttling away excess head energy imparted to the fluid by the circulator. This energy originated as electrical energy input to the circulator motor. Thus, there is an energy cost associated with operation of a differential pressure bypass valve.

THE 'IDEAL' CIRCULATOR

A circulator that could operate along a perfectly flat pump curve at some preset head could maintain a constant differential pressure on the distribution system regard less of how much flow is passing through it. Such a circulator would allow any given zone circuit to turn on or off without affecting flow in the other zone circuits. Although it's possible to build circulators with relatively flat pump curves, the curve of a centrifugal pump will always have some drop in head (and hence differential pressure) as flow rate increases.

There is a way to mimic the operating characteristic of a circulator with a flat pump curve. It requires the speed of the circulator to vary so that the intersection of the pump curve and the current system curve always occur at a fixed differential pressure. The concept is shown in Figure 5.

[FIGURE 5 OMITTED]

A differential pressure controller can provide the "intelligence" necessary to adjust the circulator speed. Such a device monitors the pressure difference across the piping mains, compares it to a "target" differential pressure, and generates an output signal based on the error that exists between the measured and target differential pressure. A common control signal for such an application is a variable voltage signal of 2-10 VAC, or a variable current signal of 4 20 mA. A typical variable-speed circulator is regulated by a closed loop/feedback PID (proportional-integral-derivative) control loop.

Although larger pumps have been operated at variable speeds using VFDs for several years, such technology has not been readily available for applications requiring smaller circulators. Fortunately, this is beginning to change. Small circulators are now available with integral VFDs. Two examples of such circulators are shown in Figure 6.

[FIGURE 6 OMITTED]

Such a circulator could be used to control differential pressure in a distribution system, provided a differential pressure controller with a 2-10 VAC/4-20 mA output is used to regulate pump speed. Although such devices are available, they are presently relatively expensive for small system applications.

However, the benefits offered by this concept have led to new approaches that eliminate the need for an external differential pres sure controller. Instead, control circuits within the pump referent the pump's own electrical operating characteristics as a basis for speed adjustment. Such circulators are currently in use in Europe and they will soon be introduced into the North American market.