Editor's note: This is the fourth article in a five-article series on central-chiller-plant modeling. The first three articles - "Primary/Secondary vs. Primary-Only Pumping" (http://bit.ly/Nelson_1), "Efficient Control of a Primary/Secondary Plant" (http://bit.ly/Nelson_2), and "Efficient Control of a Primary-Only Plant" (http://bit.ly/Nelson_3) - appeared in the April, May, and July 2011 issues of HPAC Engineering, respectively.

Low-load delta-T is said1 to exist in nearly every large distributed chilled-water system. The first three articles in this series2,3,4 described strategies for coping with low-load delta-T in plants with primary/secondary (P/S) and primary-only (P-only) distribution pumping. This article will discuss why low-load delta-T is so pervasive in central chiller plants and introduce a means of forcing load delta-T to 10°F.

Non-uniform Secondary Flow
The top chart in Figure 1 illustrates a concept5 regarding the changing characteristics of secondary head as a function of loads active close to or far from a plant. The analysis is with P/S pumping and a load delta-T of 10°F. The general equation5 is:

H = Ch + Hd (Q ÷ Qd)n
where:
Ch = static head or constant pressure (60 ft assumed for model)
Hd = system head at design flow (245 ft - see Figure 4 of Article 1 in this series2)
Q = intermediate (secondary) flow in the system, gallons per minute
Qd = design flow of the system (12,193 gpm - see Figure 4 of Article 1 in this series2)
n = system-friction coefficient (2 for all analysis in the first three articles in this series2,3,4)

This equation was applied to distribution piping and an air-handler coil.

System-friction coefficient can vary from about 0.37 to 3.5,5 generating a system head area (Figure 1). System curves are given5 as system head vs. flow, as shown by the second horizontal axis in the top chart of Figure 1. With flow for each condition of n nearly identical, model data will be presented here as a function of site load, as shown by the primary horizontal axis in the top chart of Figure 1, to be consistent with most other charts in this series of articles.2,3,4

The bottom chart in Figure 1 is the same curve, with the axis reversed to agree with all other charts in this series of articles.2,3,4 Figure 1 shows system head can more than double as a function of the changing flow characteristics of a load. Each system, however, has its own characteristics of system head area,5 which may be more or less than presented in Figure 1.

Effect of Non-uniform Flow
Figure 2 illustrates the effect of non-uniform secondary flow on pump and plant performance. The top chart gives the secondary-pump power required to maintain a load delta-T of 10°F. At a site load of 3,380 tons, secondary-pump power varies from about 190 to 470 kw as the system head curve changes. If secondary-pump power were controlled with a differential-pressure transmitter (DPT), operators likely would set the DPT to more or less match the secondary-pump-power requirements of the n≥2 curve; when the system operated within the system head curve at a lower value of n, low-load delta-T would occur.

The bottom chart in Figure 2 illustrates the variation in plant power per ton of site load for the system head curves of Figure 1. At a site load of 3,380 tons, plant performance varies from about 0.575 to 0.66 kw per ton. This shows DPT control of a secondary pump to be questionable and the definition of plant performance before a plant is built and operating impractical.

Potential Secondary-Pump Power
Figure 3 illustrates the potential for inefficient plant performance attributed to the assumed 700-kw installed capacity of the secondary pump (see Figure 4 of the first article2 in this series). The top chart illustrates the magnitude of the additional pump power available to drive load delta-T to below 10°F. If the secondary pump were operating at full capacity, load delta-T would be as shown by the second horizontal axis in the top chart, varying from 10°F at design conditions to 2.1°F at 492 site tons. At 3,380 site tons, about 310 kw of secondary-pump power would be required at n=2 conditions for a delta-T of 10°F to be provided; however, the DPT or operators potentially could call for 700 kw, driving load delta-T to 7.3°F. A lower delta-T would increase bypass flow and, thus, decrease plant efficiency, as shown by the bottom chart in Figure 3.

The bottom chart in Figure 3 illustrates plant power per ton of site load for the conditions in the top chart. Clearly, secondary-pump power can degrade plant performance significantly; therefore, DPT control of a pump is a potential source of low-load delta-T and inefficient plant performance. The bottom chart in Figure 2 illustrates plant performance for the three conditions of n for a delta-T of 10°F.

The bottom chart in Figure 3 also shows the performance of the P/S plant of the second article3 in this series (see Figure 6 of Article 23) with low-load delta-T and full loading of a chiller before another chiller is turned on (air-handler entering-water-temperature [(ewt)AH] control).

Secondary-Pump Power and Flow
According to the top chart in Figure 4, the pump power of the Article 23 plant falls between the 700-kw maximum and the power required to achieve 10°F delta-T for n=2 flow distribution. Load delta-T is shown by the secondary horizontal axis.

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