I spent practically all of last year's brutally hot summer crawling through 120 degrees F steam tunnels (cue the violin), evaluating the performance of a chilled water distribution system for a client in Boston. The assignment was to identify measures that could potentially reduce chilled water flow (not necessarily load) and increase the overall campus chilled water Delta T. The client purchases chilled water from a remote plant and there were significant penalty charges resulting from returning water that is far below the minimum return temperature required by the contract. Reducing flow demand and increasing the Delta T would free up capacity in the overloaded piping system and reduce penalty charges at the same time.
Low Delta T=High GPMWe found many of the typical control problems that you would find in an old system that has been poked and prodded for 50 years. But the survey also uncovered many AHUs on campus that were installed with low Delta T chilled water coils, some as low as 8 degrees to 10 degrees.
In addition, there were hundreds of fancoil units with one or two row coils adding to the problem. Low Delta T means low chilled water return temperature and high gpm. For example, a 100-ton cooling coil with a 10 degrees Delta T requires 240 gpm (gpm= tons x 12,000/500/Delta T). If the entering water is 40 degrees the leaving water would be 50 degrees, well below the minimum required by the plant. On the other hand, a 100-ton coil with more rows and fins capable of a 20 degrees Delta T would cut the flow in half to 120 gpm and the leaving water temperature would then be 60 degrees. One the more obvious recommendations was to deal with all of the low Delta T coils.
Low Velocity=Poor PerformanceThere were also some very large chilled water coils on campus that were designed for 60 degrees leaving water temperature but were simply not delivering. Figure 2 is a three-day trend showing the total building cfm vs. gpm for one of the newer lab buildings on campus. The building is served by 100% outside air, VAV AHUs with 18 degrees Delta T coils. Note that the chilled water return temperature leaving the building is consistently in the high 40s to low 50s and never close to the design of 60 degrees. The data also shows that the supply fans ran between 40% and 60% of the design airflow on average.
Additionally, the trend shows the total building chilled water gpm continually reaching 100% of the building design flow even though the units are only partly loaded. So why is the chilled water demand at 100% design when the load on the building is more like 50% most of the timeDelta Perhaps velocity has something to do with it.
Engineers will often select the coil with the lowest air and water pressure drops to minimize pump and fan energy costs. But low-pressure drop means low velocity. The coils mentioned above were double circuit coils with only a 1.3-psi pressure drop at design conditions (tube velocity = 2.1 fps). The minimum ARI standard 410 rating condition for water velocity through a coil is 1.0 fps. Below that value, the heat transfer capability of the coil is diminished.
When a chilled water control valve starts to close in response to a reduction in load, there is a reduction in velocity through the coil tubes. As 1.0 fps is approached, the control valve will want to hang open to maintain turbulent flow and meet the load. The result is higher than expected flow and lower than expected Delta T at less than design loads. In addition, the trend showed that the VAV AHUs are oversized for the actual load, resulting in low air velocities. The air velocity should always be maintained well above the minimum ARI-410 condition of 200 fpm at all times.
All of these issues could easily have been avoided with good planning and coordination. Ideally, there should have been a chilled water coil selection standard requiring minimum Delta T's and velocities enforced for all projects from day one. Also, it's always a good idea to go back to the client at least five times with the same question: Is the expected equipment cooling load in that room really 500W/sq ftDelta ES
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