Pumps & Systems, January 2009

Editor's Note: This is part three of a three part series about variable speed pump selection. To read the previous article, click here.

Last month we reviewed the pump selection criteria for a closed loop circulation system (Vp-Vf). This month we will take a look at a similar application that adds some elevation to the equation.

Open loop applications are similar to closed loop systems in that friction in the loop varies with flow. The major difference between the two is that the static heads are unbalanced in the up and down legs of the open loop system. Simply stated, the discharge elevation is usually greater than the supply elevation. Figure 1 is an example of a simple open loop application.

Figure 1 shows a pump circulating water through a piping system that includes a heat exchanger and a cooling tower. In this example, friction will arise from two sources-the flow in the piping and the flow through the heat exchanger passage. These values will be added, as in last month's application, to produce the frictional component of the system curve.

The elevation component is composed of Ha and Hb. Ha is the balanced elevation in the loop. The lower (solid blue) portion of the cooling tower is the pan or reservoir that collects the cooled process water. The distance from the surface of the water to the bottom of the down leg will produce the same elevation in the up leg. Therefore, no energy is required to produce the static elevation of Ha. Hb is the unbalanced elevation and is the distance from the surface of the water in the reservoir to the piping entrance in the upper (solid blue) portion of the tower. Consequently, Hb becomes the elevation head that the pump must produce.

Depending upon the tower design, another head component can be required. Some cooling towers have a collection pan in the upper portion of the tower that distributes the water to the cooling area via gravity. Others rely on nozzles to spray water into the cooling area. When nozzles are used, a certain amount of pressure is required to create a spray pattern. In our example, we will assume that gravity distributes the water. We will also ignore the potential siphon effect that can be produced by the so-called "down comer" that was often used on older cooling tower designs.

Figure 2 is a graphical representation of the components that comprise the system curve for the open loop system in Figure 1. The blue and green curves are the frictional components and the horizontal black line is the elevation component. The red system curve is the sum of the other three curves at each flow point. The total head required to operate the loop ranges from 13-ft at 1-gpm to about 58-ft at 560-gpm. The resulting system curve is similar to the one we saw last month in our closed loop example. The difference is the influence of the elevation component. We will see later that a combination of friction and elevation can affect the operating points of the H/Q curves produced at lower frequencies.

Figure 3 plots the H/Q curve of a pump, selected for this application, against the system curve we generated in Figure 2. The application calls for a flow range of 240- to 480-gpm. The data labels show that this pump will meet the maximum flow at a hydraulic efficiency of 75 percent and requires 7.5-hp. As expected, HP drops as flow is reduced. At minimum flow, efficiency drops to 69 percent, and the power required is reduced by about 2-hp from that required at maximum flow.

Figure 4 shows the same pump operating under VFD control. The data labels show a major reduction in HP at each of the flow points. For example, the power required at minimum flow (38-Hz) is reduced from 5.6 (constant speed) to 1.7-hp.

I mentioned earlier that the elevation component can affect the operating points of the lower frequency H/Q curves. In last month's closed loop example, the system curve (friction only) closely followed the H/Q isomer of the 60-hz flow point as speed was reduced. This occurred because friction changes as nearly the square of a change in flow and thus closely matches the change in head. In this example, the linear elevation component changes the shape of the system curve as flow is reduced. Thus, the system curve moves farther and farther to the left as pump speed is reduced. This is typical of systems composed of both elevation and friction.

In our example, it does not matter because the peak 60-hz efficiency occurs at 400-gpm. Therefore, the system curve moves toward and eventually through that efficiency isomer as speed is reduced. However, there are times when it can have an effect on lower speed efficiency and pump selection.