Editor's note: This is the third article in a five-article series on central-chiller-plant modeling. Part 1, "Primary/Secondary vs. Primary-Only Pumping", appeared in the April 2011 issue of HPAC Engineering. Part 2, "Efficient Control of a Primary/Secondary Plant", appeared in the May 2011 issue of HPAC Engineering.

The second article in this series arrived at primary/secondary- (P/S-) plant performance better than primary-only- (P-only-) plant performance when a P-only plant is operated with the minimum number of chillers/towers required to meet load. This article will develop a rationale for controlling a P-only plant at a level of performance on par with that of a P/S plant operating at the low-load delta-T conditions defined by the previous two articles.

Chart key: click on image to view larger

P-Only-Plant Performance and Chiller Loading
The top chart in Figure 1 compares P/S- and P-only-plant performance, with the P/S plant having an advantage at site-load conditions of about 1,200 tons or less. The bottom chart shows chiller load in a P-only plant as load, wet-bulb temperature, and the number of chillers/towers in operation decreases. At a site load of 1,272 tons, the chiller is 49.5-percent loaded, suggesting the load might be met with one chiller/tower. One chiller/tower, however, cannot meet the load because when one chiller is on, the temperature of the water leaving the tower is higher, significantly increasing chiller lift.

P-Only-Plant Evaporator Flow and Velocity
In the P/S plant, flow in the evaporator is a constant 2,400 gpm; in the P-only plant, it varies, exceeding 2,400 gpm for virtually all site loads, as shown in the top chart in Figure 2. At 1,855 tons of site load, evaporator flow is approximately 3,500 gpm; at loads of 1,012 and 946 tons, it is more than 5,000 gpm.

The bottom chart in Figure 2 illustrates evaporator velocity. Evaporator velocity exceeds 10 fps at several site loads; at site loads of 1,012 and 946 tons, it exceeds 17 fps. Clearly, evaporator velocity must be controlled, if velocities of 10 to 12 fps are considered the maximum permissible, and control requires turning on another chiller. The design evaporator velocity for this chiller is 8.05 fps. The site loads were selected randomly; thus, there are other conditions at which evaporator velocity exceeds 12 fps. Generally, evaporator velocity reaches a maximum value at chiller loading of 90 percent or more and as a function of load delta-T.

System at 1,855 Tons
Figures 3 and 4 illustrate the modeled plant at energy equilibrium with two and three chillers/towers operating, respectively. Turning on a third chiller/tower reduced the load on each chiller from 82 percent to 51 percent and improved both chiller and plant performance. Plant power dropped from 1,271 to 1,249 kw, while evaporator velocity dropped from about 11.95 to 7.93 fps.