District Energy - 2005 Second Quarter

cooling coils is low, extra chilled water is circulated per ton of cooling. Building owners may suffer from rising supply-water temperature. The central plant may suffer from low return-water temperature.

Buildings that are directly connected may experience improved coil performance since return water is not permitted

the chilled-water supply temperature (CHWST) to the building is 45 F, but it rises in the building as high as 57 F. Supply air temperature rises to 62 F, resulting in excess air flow, humidity concerns and excess fan energy consumption. Delta T within the building is far below the 12 F design Delta T for the coils.

Figure 2. Effect of Rising Chilled-Water Supply Temperature (CHWST) on Coil Capacity, Flow Rate and Delta T.

need to deliver the chilled water required to serve the load.

Hydraulic gradient analysis is one technique that may be used to evaluate distribution system design and operation but is beyond the scope of this article. Using this technique enables an analyst or engineer to see in many systems that there is plenty of differential pressure in buildings close to the central plant to serve the load without running building pumps (provided that Delta T in excess of design is achieved). Return-water temperature control valves, when operating properly, consume all the pressure between the building return and distribution return headers but do little to provide accurate flow control at individual loads.

Flow Control Industries Inc.

to blend with supply at the building; however, if return-water temperature to the chilled-water plant is less than design, system capacity (pipes, pumps, chillers, and thermal storage) may be unnecessarily limited. Low Delta T across cooling coils in directly connected systems causes the excess flow to be processed at the central plant by overflowing equipment, permitting supply-water temperature to rise, or by running additional chillers.

Return-Water Temperature
Control

Return-water temperature control is a very common decoupling strategy employed in buildings served by large district cooling systems. By separating the building flow from the primary or secondary distribution flow, chilled water is re-circulated in buildings until enough heat has been gained to produce adequate return-water temperatures (and Delta T) for the distribution pipes and the central plant.

Figure 1 demonstrates the adverse effect of return-water temperature control in an actual system. In this example,

While this technique has been used as an intermediate step to generate the desired temperature rise at the plant or through the main distribution pipes, it is fraught with potential problems for customers. When return water is permitted to blend with supply, CHWST increases. This commonly leads to building performance issues including wasted pump and fan energy, lost coil capacity and poor humidity control. On a system level, pumps used in buildings close to the plant may starve buildings downstream of the differential pressure they

Factors Influencing Delta T

Through the cooling load range, the Delta T performance of a properly selected coil can decline for a number of reasons. Rising CHWST and poor control valve sizing and performance are among the factors that will adversely affect chilled-water Delta T performance (dirty fins and coils, low supply air temperature, improper piping and over-pumping are a few others).

Chilled-Water Supply Temperature

Figure 2 illustrates how Delta T, flow and capacity of a 45/55-split ARI-certified cooling coil vary with a change in the CHWST. At 100 percent design flow for the coil, capacity is dramatically reduced when the CHWST rises to 48 F. The coil is capable of producing a high Delta T when low CHWST is maintained.