Maximising chiller performance

Understanding the operating characteristics of chillers enables the energy efficiency of a multi-chiller installation to be maximised. In this installation of three chillers, questions to consider are would it be more efficient to operate one chiller at full or part load, two at a lower part load or all three at an even lower part load?
Many chiller systems fall far short of delivering their optimum efficiency, even when linked to control systems. JIM KILCOYNE explains how fine-tuning a system can make a significant difference to performance.With a general acceptance that higher energy costs are here to stay, the emphasis for many cost-conscious organisations is switching from negotiating keen tariffs to looking more closely at the actual consumption of energy. To that end, chiller systems are coming under increasing scrutiny because they account for a significant proportion of the overall energy costs of air-conditioned buildings. However, despite the many measures that are taken to optimise efficiency, many chiller installations still fall short of maximum efficiency. Even systems that are linked to a Building Management System (BMS) can often be made considerably more efficient by understanding the chillers and the way they are configured. Unfortunately, most control systems limit the information relayed from the chiller to just status signals and alarms — insufficient for an effective control strategy. Armed with more detailed information, it is possible to determine the most efficient operating conditions for each chiller and control them accordingly. For example, many chillers do not give maximum efficiency at full capacity, so running two chillers at part-load may be more efficient than running one at 100% and leaving the other off — except during periods of peak demand. Conversely, if you have two chillers that are most efficient at 60 to 80% of full load, it makes no sense to run them both at low load, it would be better to run one at 60% and leave the other off. In these cases, even greater efficiencies and system stability will be achieved by simultaneously adjusting setpoints to make the best use of the chiller’s characteristics. This is where detailed understanding of chillers and systems is so important, because each installation will vary. There is no set formula that will provide maximum performance for all chillers. A switch in time The key objective is always to reduce the number of times chillers are switched on and off, as this wastes energy and reduces the life of the compressors. As well as understanding chiller COPs, the ways they are brought into operation and unloaded are also vital. As cooling requirements increase, it is important to monitor the temperature for a while to ensure a change is not a blip before switching another chiller on. Equally, it may be better to work the chiller harder until it has reached full load before bringing in another machine. In other cases it may be more efficient to bring in another chiller when the lead chiller reaches 80%. Switching chillers on is relatively easy — compared to knowing when to switch them off. Switching them off prematurely often leads to frequent stop/starts if the remaining chiller(s) cannot serve the load. Waiting too long before switching them off will waste energy. In most cases, as the load declines, it is best to back both chillers down and monitor conditions until it is certain that just one will maintain the load efficiently. One chiller can then be ramped up while the other is turned off — secure in the knowledge that it will remain off for a long time, and switching will be minimised. Installations with a mixture of different chillers present more challenges. For example, with three chillers of different efficiencies it does not make sense to use them in rotation. It is far more sensible to meet the base load with the most efficient chiller, use the least efficient chiller at peak times only and swing the third in and out while the other chillers are ramping up to full output. When the power goes off Power cuts can be very disruptive, particularly if the space being cooled is mission-critical to the organisation, so it is important to handle these events efficiently. The conventional approach is to restart the sequence, switching one chiller on and then waiting for 20 minutes to see what happens before switching another on. However, downtime could be minimised by starting one machine at a time, as quickly as feasible and then considering the loads and deciding whether more or fewer machines are needed. Conclusion With such a wide range of parameters to take account of, it is clear why so many control systems are failing to deliver optimum performance — and why it pays to use specialised chiller-management systems to provide standard reports and other detailed information. At the same time, such systems should not operate in isolation; they still need to interface to other systems — ideally using open standard network protocols. This article can only provide a flavour of the complexity of chiller management, but it does highlight the need to take a closer look at the chiller system and take measures to maximise performance. Jim Kilcoyne is with Trane (UK) Ltd, Northern Cross, Basing View, Basingstoke, Hants RG21 4HH.
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