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Packing the right performance

Selecting the most appropriate packaged chiller for a project isn't as straightforward as it used to be as the number of specification parameters has grown. Dean Ward explains
Packing the right performance
In the last few years we've seen quite a change in the way that all building services plant is specified - or at least in the criteria that now drive the specification. For example, a few decades ago the cooling capacity would have been the main consideration on specifying a packaged chiller. Now, while still being a primary consideration, cooling capacity is just one of a number of parameters that form the basis of such decisions.

One of the main reasons for this, of course, is that building operators have become more energy and carbon conscious, and are subject to many more pressures to reduce their environmental impact. As a result, their requirements have matured from simply expecting reliable cooling to encompass total cost of ownership and carbon emissions.

Optimising the use of space
Furthermore, in the last few years of recession many organisations have been forced to rationalise their product portfolios and optimise their use of space. This has had two key effects. In some workspaces it has led to higher densities of people and office machinery, thus increasing internal heat gains. In other areas, workspaces have become more flexible with hot desks, touch down areas etc. These are therefore subject to more variable occupancy and more variable comfort cooling demands.

And, of course, these different types of space may well exist in the same building, resulting in considerable fluctuation in cooling loads through the course of the day.

Fortunately, packaged chillers have evolved considerably over this same time period and now offer many more options to help specifiers address these issues. So it's important that anyone involved in specifying packaged chillers is up to speed with the latest developments - especially as the rate of change has accelerated rapidly in the last few years.

Given that chillers now spend a great deal of time operating at part-load the Coefficient of Performance (CoP) has taken second place to the European Seasonal Energy Efficiency Ratio (ESEER) when considering overall efficiency. As a result, many newer packaged chillers incorporate features to improve the ESEER.

An obvious example is free-cooling, as it has been shown that chillers optimised for free cooling and operating 24 hours a day can reduce energy consumption by around 35 per cent over the course of a year, compared to conventional air cooled chillers. This is achieved through relative positioning of the condensing coil and the free cooling coil, so that some modern chillers can provide some free cooling at ambient temperatures as high as 15 deg C. The percentage of free cooling then increases in proportion to falling ambient temperatures, achieving 100 per cent free cooling at 0 deg C (subject to water design temperatures). Such chillers may be combined with higher flow and return temperatures to maximise free cooling.

When free cooling is not available and the fans are used, fan power is another consideration. To that end, many manufacturers are now making EC condenser fans available to reduce energy consumption, noise levels and inrush currents.

Chillers also offer a wider choice of compressor types and it's important to select the best compressor for the project, based on anticipated loads and load patterns. Companies that supply a wide range of chiller types should be able to help with compressor selection. For example, scroll compressors offer high part-load performance and the introduction of multiple scroll compressors has enabled these efficiencies to be achieved at higher capacities where semi-hermetic screw compressors might have been the first choice in the past. Indeed, there are circumstances where a screw compressor chiller will still be the best solution, so it really is a matter of understanding the application.

Another option is the Turbocor compressor, which is variable-speed-controlled and highly responsive to variations in cooling demand, so that an ESEER of up to 9.0 can be achieved. Furthermore, Turbocor compressors use magnetic bearings, so there is no physical contact between moving parts, which eliminates wear and tear and reduces noise and vibration. This characteristic greatly reduces maintenance requirements and overall life cycle costs.

Other considerations when selecting a chiller include the noise and footprint and, again, these criteria will be dictated by each project. Footprint versus capacity is often an issue where existing chiller plant is being replaced, and where access is limited there may even be a need to disassemble the chiller and reassemble it in situ. The latter capability often depends as much on the technical expertise of the supplier as the design of the chiller itself.

As refrigerant costs rise these also make a more significant contribution to cost of ownership so reducing refrigerant volumes, perhaps by specifying all-aluminium micro channel coils, is another cost-saving option.

The really important thing is that there is no 'one size fits all' solution with packaged chillers, because there are so many criteria to consider. As manufacturers, we need to offer sufficient choice so that specifiers are able to make the right choice.

//The author is product manager at Climaveneta UK //
18 June 2012

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