4458 (RP-992) -- Operating Characteristics of Five Stratified Chilled Water Thermal Storage Tanks ✓ Most Recent

An investigation of the operation of stratified chilled water storage tanks at five installations was conducted. The study confirmed that the thermocline thickness is dependent upon the flow rate of the inlet water into the relatively still tank water, reflected in the parameters of the Reynolds and Froude numbers. It was discovered that none of the five installations is operated in the expected manner of completely discharging the chilled water to meet a cooling load and charging the tank by completely filling the tank again with chilled water. The typical operation entailed discharging the tank as necessary to meet the load, which was less than the cooling capacity of the tank, and then recharging the partially discharged tank. One installation discharged the tank through multiple cycles, supplying cooling water to the load, returning the water to the tank, and then continuing to discharge the water to the cooling load but at continually increasing supply water temperatures. A similar process was used when charging the tank. Thermocline thicknesses were determined from operating data at the five sites. The thermoclines were relatively thin for all of the sites. The Reynolds numbers were below the limiting values recommended by ASHRAE guidelines for octagonal pipe diffusers. The results show that a Reynolds number of 6000 does not exceed a limit that would result in a significantly larger thermocline. Tank operations prevented the determination of tank efficiencies and figures of merit, though the parameters can be approximated. Unfortunately, the operating data did not reveal Reynolds or Froude numbers greater than those recommended by ASHRAE guidelines for octagonal pipe diffusers.

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