Variable Speed Pump Control: Area Control Versus Curve Control
/By Chad Edmondson
In our last blog, we learned that ASHRAE significantly ramped up the efficiency requirements on chilled water pumping systems with the 2010 release of ASHRAE 90.1. In summary, all but very low capacity commercial systems will need to employ some sort of variable speed chilled water distribution. In addition, ASHRAE specifically calls out differential pressure control as a way to maximize efficiency. Simply put, ASHRAE doesn’t want facilities pumping any more chilled water than is takes to keep the most critical coil sufficiency pressurized.
We also left you with two new terms, “Control Curve” and “Control Area,” indicating that any variable speed control strategy will be based on one or the other. What are these strategies and how do they differ?
Before we pull out the curves, schematics and calculations, let’s start with a simple analogy: A road trip.
The Fastest Route to Grandma’s House
In a few short weeks, many of us will hit the highway to celebrate Thanksgiving with family and friends. Fortunately, most of us have some sort of GPS to help us get where we are going on time. Some of us are even lucky enough to have an app on our car or phone that will redirect us around the holiday traffic and keep us on the most efficient course. Conceptually at least, that’s a very good way to think of variable speed pump control.
A strategy based on the control curve of a system (i.e. the theoretical resistance of the piping system) is like a standard GPS; it doesn’t provide alternate routes.
A strategy based on control area is more flexible. Like the way a more advanced GPS can sense traffic conditions and provide an alternate route to keep you on the fastest track to grandma’s house, area control uses information from out in the system to determine the most efficient speed for the pump at any given time.
With this analogy in mind, let’s dig a little deeper.
Curve Control
Curve Control is purely based on math and theory. The pump is controlled according to the laws of physics, specifically the Pump Affinity Laws. As such, the pump is controlled to operate only at points along the system’s control curve. We will review what the control curve is in our next blog. For now, simply think of it as a curve that represents the theoretical head loss in the system from zero to maximum pump system pump flow. Wherever the selected pump curve intersects the (theoretical) control curve is where the pump will operate. These theoretical values are part of an algorithm that are literally programmed into the variable speed drive, and will determine how fast the pump will operate under any given set of circumstances.
Variable speed curve control will adjust the pump speed to keep the pump operating on the control curve at all times. But keep in mind, this control curve is theoretical. It is based on drawings, values, and calculations that are made long before the pump is installed or even selected. So, while that curve may look great on paper, using it as a singular guide to control pump operation in a real system is somewhat limited.
Area Control
Area Control takes a more dynamic approach to variable speed control, one that can adjust pump speed based on actual system load in real time. Because of this, pump operation isn’t tied to a predetermined control curve, but a multitude of curves that reflect actual load conditions, i.e. changing differential pressures as loads fluctuate. Thus, the pump may operate at many points above and below the theoretical control curve. This more flexible approach to pump control allows the pump to adjust to fluctuating load conditions throughout the day.
Both control strategies are illustrated on the pump curve below.
The dark blue curve at the bottom represents the theoretical system load. If we base our control on this curve, we will only operate our variable speed pump at points along this curve, even if actual demand falls above or below that line.
The purple lines represent “real world” system conditions – that is the resistance in the system as loads change due to time of day, occupancy, and even how the building heats up as the sun rises and sets. These are the control curves that might be generated from data taken from monitoring an actual operating system throughout an entire day.
The area enclosed within the red lines represents the control area, which encompasses nearly all the system operating points during any given day. We’d like to be able to operate the pump at any point inside the red lines, because this gives us the ability to adjust our pump speed to closely match the system demand at any time during the day.
It may come as no surprise to you that area control, while a bit more complex to execute, will save owners more money. It also keeps engineers well within ASHRAE 90.1 standards.
We’ll be covering how all this works very soon. Before that, we want to review some essential system fundamentals and terminology. Stay with us.
