Editor's note: This is the second 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.

The first article in this series discussed a strategy for operating a primary/secondary (P/S) central chiller plant with bypass control: Turn on chillers to keep bypass flow greater than secondary flow so water off of the evaporator is not degraded by mixing with water in the bypass. This article will discuss a strategy for operating a P/S plant based on full loading of a chiller1 before another chiller is turned on, a strategy that often results in secondary flow being greater than primary flow, meaning water off of an evaporator must be less than 44°F for 44°F water to be supplied to site air handlers.

P/S-Plant Performance With Bypass Control
Figure 1 illustrates the performance of a P/S plant operating with bypass control. The top chart illustrates plant performance and chiller loading as site load and wet-bulb temperature drop. The number of chillers that must be on to achieve bypass flow is shown by the second horizontal axis. Loading on the chillers decreases as site load drops, with 50-percent loading occurring at a site load of 2,392 tons. The middle chart provides site supply- and return-water temperatures, the number of chillers on, and the load delta-T. The bottom chart gives the secondary flow and head and the pumping-plus-fan power.

The top chart in Figure 2 gives the bypass flow, which always is negative, except at design conditions, at which a small positive bypass flow results for the plant at energy equilibrium. The top chart also shows the temperature of water leaving the air handler, illustrating the effect of low-load delta-T. The bottom chart shows the temperature of water leaving the evaporator to be about 44°F because the bypass flow is maintained negative to prevent mixing of water supplied to the site. The bottom chart also illustrates chiller performance (kilowatts per evaporator ton). The greater the number of chillers/towers on—and the lower the wet-bulb temperature, as shown by the bottom chart—the better the chiller performance.

Two Control Strategies With System at 1,012 Tons
Figures 3 and 4 illustrate the system at energy equilibrium, with a site load of 1,012 tons, with bypass control and the control of the temperature of water entering the air-handler coil, respectively. In Figure 3, three chillers/towers are on, while in Figure 4, one chiller/tower is on. The one-chiller/tower plant performs better than the three-chiller/tower plant; however, chiller power is better with the three-chiller/tower plant, as shown by the system-performance indices in the middle of the figures.

P/S-Plant Performance—Two Control Strategies
The top chart in Figure 5 shows chiller loading with the two control strategies. Chiller loading increases significantly with the control of the temperature of water entering the air-handler coil because the number of chillers/towers on is reduced, as shown by the middle and bottom charts. The bottom chart illustrates the difference in the temperature of water delivered by the chiller. The bypass-control plant always delivers about 44°F water off of the evaporator as a result of the bypass flow being controlled by the number of chillers on. With the control of the temperature of water entering the air-handler coil, the chiller almost always is required to provide sub-44°F water, which mixes with bypass water to provide 44°F to the site air handlers.

The middle chart shows how bypass flow is reversed with the control of the temperature of water entering the air-handler coil, while the bottom chart shows how the temperature of water leaving the evaporator must decrease for 44°F water to be provided to the site. This colder water from the evaporator increases the lift on the chiller and, therefore, the chiller power required.

Figure 6 provides additional performance curves for the two control strategies. The top chart illustrates an advantage and a disadvantage of the control of the temperature of water entering an air-handler coil. The advantage: total pumping-plus-fan power because fewer chillers/towers and associated pumps and fans are required. The disadvantage: The temperature of entering condenser water is higher, resulting in additional chiller lift and, therefore, kilowatts because fewer chillers/towers are on.

The middle chart in Figure 6 illustrates chiller performance, showing the control of the temperature of water entering an air-handler coil decreases chiller performance, one reason being the greater evaporator load resulting from fewer chillers/towers being on.

The bottom chart in Figure 6 shows control of the temperature of water entering the air-handler coil performs equal to or better than bypass control for all of the operating conditions considered for this article. With a central chiller plant being as complex as it is, that is not to say there are not operating conditions for which bypass control provides an advantage. A more detailed analysis is required and can be performed with a model of this type; however, as a general rule, control of the temperature of water entering an air-handler coil yields better results than bypass control.

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Comparison With Primary-Only Plant
Figure 7 compares the plants considered thus far in this series of articles. The primary-only (P-only) plant of Part 1, in which only the chillers necessary to meet a load are turned on, is shown. The P/S plant with control of the temperature of water entering the air-handler coil performs better than the P-only plant.

Summary
Figures 1 and 2 show the performance of a five-chiller plant utilizing bypass control. Simply stated, this control strategy involves turning on enough chillers—if available—to ensure the temperature of water leaving the evaporator is not degraded by flow in the bypass. This strategy requires a greater number of chillers to be on as load delta-T decreases.

Figures 3 and 4 show the two plant-control strategies for a site load of 1,012 tons. Control of the temperature of water entering the air-handler coil has a significant advantage over bypass control for this particular plant and the conditions considered.

Figures 5 and 6 compare several plant parameters for the two control strategies. The bottom chart of Figure 6 shows control of the temperature of water entering the air-handler coil has an advantage over bypass control for the plant and conditions studied here.

Figure 7 compares the P/S plant with the P-only plant of Part 1 of this series of articles, showing that the P/S plant with control of the temperature of water entering the air-handler coil is more efficient than the P-only plant at low-load conditions and the minimum number of chillers/towers on. The next article in this series will discuss how to improve the performance of the P-only plant.

Reference
1) Reed, M.A., & Davis, C. (2007, July). Chilled water plant savings at no cost. ASHRAE Journal, pp. 36-44.

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at scott.arnold@penton.com.

SUGGESTED READING
To learn more, the author recommends:

  1. Nelson, K. (2010, July). Central-chiller-plant modeling. HPAC Engineering.
  2. Kirsner, W., & Rishel, J.B. (1998, January). A check valve in the chiller bypass line? Two views on this question. HPAC, pp. 128, 129, 131, 132, 134.
  3. Avery, G. (2009, July). Converting from primary/secondary to all-variable flow. HPAC Engineering.

Kirby Nelson, PE, has been involved in the modeling of HVAC systems since the oil embargo of 1973—first as corporate energy manager for Texas Instruments Inc., then as a consultant. Models he has used include DOE-2, E Cube, and models developed on an analog/digital computer, including models of cleanrooms. A life member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, he has presented numerous papers, led an energy engineering delegation to China, and more recently developed models for district cooling systems, thermal-storage systems, and central plants.