Consulting-Specifying Engineer - August 2008 - (Page 42) 3,600 0.6 3000 Annual operating hours 0.5 2,400 0.4 kW per Ton 1,800 New chiller part load performance is optimized for the part load condition with the most hours 0.3 1,200 0.2 600 0.1 0 150 175 250 350 Load in tons Anticipated annual operating hours in each load range Base bid with VFD 450 550 0 Figure 4: Load profile and chiller performance requirements developed from the data gathered by the team. Source: David Sellers original plan was to overhaul the pumps and use them to serve the new chiller. But as the project moved into construction, the MRCx team realized that the potential existed to pick up a 10% improvement in pumping efficiency by spending the overhaul dollars for one of the existing condenser pumps on a new pump selected to match the new machine’s flow rate. Rather than calculate the head, the team plans to perform a pump test to identify the exact requirements for the new point, allowing its operating point and peak efficiency to be matched to the new base load machine. This approach represents the true vision of Marriott’s MRCx process in particular and any retrocommissioning process in genera; that is, the evolution of retrocommissioning from a one-time event to the way a building is operated day in and day out. Marriott terms this process MCCx for Marriott Continual Commissioning. The bottom line is that if the MRCx team is right in their assessment of the improvement potential described above, the proposed test will allow the new chiller and the condenser system supporting it to tell them what pump they need to achieve the plant kW per ton profile associated with the purple line in Figure 2 continuing the effort to improve plant efficiency. The key to solving a problem like this is to have a good measure of the information available. The data available from the building can be a great resource if harnessed properly. Although it may seem that commissioning providers have sharper hearing when it comes to listening to the building, experience, a fundamental understanding of physics, and a passion Acknowledging limitations The technique illustrated in the preceding discussion can work well in many situations, but it is important to know its capabilities and limitations when applying it to an analysis. Because we did not know the flow in the pipe and made some assumptions regarding what it might have been, our solution was approximate rather than exact. But had we known the flow, either via a pump test or measured data, the solution would have been as exact as our measurements, perhaps even more exact than a number we might have developed using standard load calculation techniques given the assumptions that come into play. The accuracy of our analysis was directly linked to the accuracy of the temperature sensors we based it on and the credibility of our assumptions. Based on past experience with the facility, I knew the temperature sensors were accurate within a degree or two. In addition, they furnished readings were believable (the supply was hotter than the return) and in the ballpark of what we they should have been given the systems operating set points. With regard to our assumptions about flow rates, given a bit more time, we could have firmed them up with a pump test, assuming we had a curve for the pump. Lacking that, our assumption implied that the pipe had been properly sized, which is a reasonable assumption, but not as accurate as the measured temperatures. Bear in mind that all equipment will fully load under some operating condition; the key is to assess whether the fully loaded equipment keeps the load it serves under control if you are using the technique to assess capacity requirements. If the equipment is fully loaded but can not maintain the required conditions, then it is actually overloaded and using this technique to asses the real load on the facility will not provide the right answer. If the load is going to change, then you need to supplement what the building is telling you with other information. The building can’t tell you about something it hasn’t experienced yet. 42 Consulting-Specifying Engineer • AUGUST 2008
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