Variable Speed Pump Control: A Quick Review of Fundamentals and Terminology

By Chad Edmondson

We are now three blogs into our series on Variable Speed Pump Control and we are going to hit the pause button and review some fundamentals. If sequentially that seems like a strange thing to do, there is a method to our madness.

First, we wanted to impress upon our readers why this is such an essential topic right now--the increasingly widespread acceptance of ASHRAE 90.1-2010 and its focus on variable speed pumping and control to save energy. Second, we wanted to introduce you to the concept of curve control versus area control, because any variable speed control strategy you choose will ultimately fall into one or the other category. Hopefully you will recall our very elementary analogy of a GPS that provides the shortest route versus one that gives the fastest route based on traffic conditions. If not, we encourage you to go back and read the last blog.

Now, before we start painting in the detail of these two control strategies, we need to review some hydronic fundamentals. These are things we will be referring to in upcoming blogs, and we want to make sure you are up-to-speed.

Pump Affinity Laws.  These laws define the mathematical relationships between flow (GPM), pump speed (RPM/change in impeller diameter), head and brake horsepower (BHP). They are:

  1. Pump GPM capacity varies DIRECTLY as the speed (RPM) or impeller diameter ratio change.

  2. Total pump head varies directly as the SQUARE of the speed (RPM) or impeller ratio change.

  3. BHP varies directly as the CUBE of the speed (RPM) or impeller diameter ratio change.

These relationships provide the basis for variable speed pump control, because if we know a couple of the values, we can always determine the others. Keep in mind that a variable speed pump drive is the magical equivalent of switching out various size impellers on the same pump.

System Curve. This is a simple curve that represents the friction loss in a system as the flow changes.  Because of the quadratic relationship between flow and friction loss, if we know the head and flow for our system at any point in time, we can calculate the head for each corresponding flow point and draw a curve.  In a closed loop system this curve takes us all the way from design flow to zero flow and zero head.

Control Curve. The control curve is similar to the system curve except for one very important detail. It begins with a minimum head at zero flow. Why? Because in distributed HVAC flow there are coils and a certain amount of head must be present in order to establish satisfactory flow. The control curve represents a theoretical calculation of where a variable speed pump will operate at part load.

Control Head. This is the minimum amount of head that must be present in the system at all times to establish full flow through the critical coil(s). a variable-speed pump will produce in a closed loop.     It can be calculated differently depending on whether we are using curve or area control.  It’s like a monthly car or mortgage payment in your monthly budget. 

Variable Head.  Variable head, as the term suggest, is indeed variable. It is the piping head loss in the system, and because of the pump affinity laws, it will vary as we reduce system flow. It is calculated by subtracting control head from design pump head. The higher the variable head is compared to the control head, the more energy we will save.

Figure 1 and Figure 2 illustrate system curve, control curve, control head, and variable head in an example system:

Figure 1

Figure 1

Figure 2

Figure 2

 Note that in this system, we have 80 feet of variable head. That’s where the potential for savings with variable speed pumping lives.  The sum of the variable head and the control head equals the total pump head.  To continue with our financial analogy, variable head is where we can tighten our belt by spending less on dining out, playing golf, or purchasing lottery tickets.