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Air Conditioning World - Get it right first time

Designing chilled water circuits to aid commissioning is the first step in achieving an efficient,
responsive system, says Wayne Perrins of Axair Climate
Air Conditioning World - Get it right first time
ONE of the beneficial results of the current focus on improving energy efficiency and reducing carbon footprint is that more attention is now being paid to the overall efficiencies of systems, rather than the headline figures.

Chillers are a case in point, where the emphasis on efficiency for a few days a year is gradually being replaced by a broader-ranging assessment of whole-life efficiencies. For instance, seasonal energy efficiency ratio has now been adopted by many specifiers as being more meaningful than coefficient of performance at full load.

When adopting this more holistic approach, it is important to look at every detail. You should take in the obvious factors, such as the capacity control of the chiller, but also look at how the chiller interacts with the rest of the system. To that end, there are a number of things that can be done with the design, installation and commissioning of the system to optimise performance.

This is an important consideration because, in recent years, as controls have become more sophisticated, there has been a tendency in some projects to rely on this cleverness to compensate for any inadequacies in the design of the system.

Efficient by design

We believe that the first step is to make the chilled water system efficient by design, and then use controls to monitor and fine tune the performance. This means that the quality of the commissioning is essential in establishing the ongoing efficiency of the system. Efficient by design, therefore, needs to encompass the commissioning process and facilitate it.

But the huge variation in chiller configurations - single, twin, multiple etc - means there is no one size fits all solution. But there is an approach that can be applied to any installation, perhaps calling on the chiller supplier's experience of the units to clarify the considerations and arrive at the best solutions.

Given the many permutations, there is not space here to look at all the options, but a few examples will serve to illustrate the concept.
For example, in some installations, a fixed orifice double regulating valve should be installed on return pipes from chillers to enable the chiller design flow rate to be established and measured. But, if there is a possibility that the overall flow rate could vary under various part-load operating conditions, then consideration should be given to replacing this valve with a constant flow regulator. This will ensure constant flow regardless of changes in pressure elsewhere in the system.

Similarly, a pressurisation unit is required as a means of filling the system, keeping it topped up and accommodating any expansion or contraction of the water volume. A water meter on the inlet to the pressurisation unit is a useful means of checking system volume, and for identifying whether water is draining from the system.

Clearly, a single chiller installation is the simplest configuration and this also provides some examples of how a design can be optimised to aid commissioning.

For instance, the use of three- or four-port control valves in the distribution system enables the cooling output to be reduced by diverting flow away from terminals, without reducing the overall flow rate through the chiller. These constant conditions ensure the chiller flow rate can never decrease to a condition where it might trip out on low-flow protection (to avoid the risk of freezing) - or increase to a condition where velocity damage to evaporator tubes or plates might occur.

The design flow rate can be set using the fixed-orifice double regulating valve in the chiller branch. Alternatively, if the pump is variable speed, design flow rates can be set using the pump itself. But, once set, the flow rate should remain constant under all conditions; that is to say the variable speed feature of the pump is only being used as an aid to commissioning.

In other situations, there may be a possibility that the downstream resistance will change in the future.

For example, small-sized four-port control valves can have significantly increased resistances when in bypass (diverting) mode. Here, a constant flow regulator can be used as an alternative to a fixed orifice double regulating valve in order to ensure constant flow through the chiller in systems.

In thinking about details such as these, the designer is effectively anticipating the conditions that may occur in operation, and building in safeguards against such conditions reducing efficiency or even causing a breakdown. Once all of these are in place, then the integral controls of the chiller begin to play their role in fine-tuning.

This level of forethought should be rudimentary, of course but it does not do any harm to revisit the basics, take a fresh look at them, and see if they can be improved.

Today's chillers are getting ever-more efficient but they can only be as efficient as the system allows them to be. A design that facilitates effective commissioning is the best way to enable the chillers to give their best.
1 December 2007

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