Understanding the life cycle costs of a chiller installation requires a clear view of the full picture. Wayne Perrins of Walter Meier (Climate UK) explores the features of chillers that can make all the difference
When considering the energy consumption of an air conditioned building, it's vital to take a close look at the chiller plant - not just its capital cost but also its performance during the life of the installation. Here, the devil is very much in the detail; simply assessing the capital costs and co-efficient of performance (COP) at maximum load provides a partial picture that will do the end client no favours.
This cost of ownership approach is, of course, a well established principle that many industry pundits talk about at length; and quite rightly so. Unfortunately, the enthusiasm for this principle that is prevalent in seminars doesn't always manifest itself in real projects.
However, all of the talk has certainly made end users more aware of these issues and there is now far more pressure on the building services industry to walk that talk. So we need to get away from sizing chillers on the basis of maximum ambient temperatures that may or may not occur for a few days every year and start to think about whole life costs. And that means giving higher priority to criteria such as COP at part-load and the system's ability to respond accurately and efficiently to variable loads.
A case in point is the inclusion of a slide valve in the compressor circuit which, when combined with the right controls, will enable continuous capacity control so that the required cooling capacity can be matched very accurately. The chiller also controls outlet water temperature, which is important when meeting the performance of the selected terminal units.
Chillers exhibit a much higher COP when operating at part load. For example, a chiller operating in an ambient of 20°C and around 70% capacity will provide a coefficient of performance of almost 200%, compared to that provided by the same chiller operating at 35°C at 100% capacity. This means that a COP of 3 (cooling capacity divided by power input) becomes a COP of 6. This effectively reduces the power input by half and so the running costs of the unit are similarly reduced.
With chillers that have more than one compressor, the typical control configuration will wait for the first compressor to reach load before starting the second - and so on. However, if each compressor is started unloaded and then all compressors modulate together to meet the cooling load this will use less power at start-up and avoid the need to oversize cables to allow for peak currents.
In this respect, Star Delta motors also offer benefits as they have the same peak current as a soft start but the peak is instantaneous rather than over a set time period. Also, if the chillers have a high power factor it will not be necessary to install a series of capacitors to compensate for the reactive energy consumed by the motors.
As noted earlier, the key to assessing the real cost of ownership is to get to grips with the nitty gritty. For example, the efficiency of oil separation in the refrigeration circuit can make a significant difference to life cycle costs. In particular, if too much oil is discharged to the refrigerant cycle this will increase the maintenance requirements of the chiller and reduce performance. Ideally, technologies such as more advanced cyclone oil separators should be used to ensure the oil is no more than 1% of the refrigerant weight.
The minimum water volume that must be maintained to prevent excessive stops and starts is another relatively 'minor' consideration that shouldn't be ignored, because this has an effect on the overall efficiency of the system. The same is true for the refrigerant volume, which can be minimised by the use of a compact plate heat exchanger that requires lower refrigerant volumes than conventional shell and tube type heat exchangers.
Paying attention to other chiller components can also make a difference. Use of DC fan motors, for example, will enable finer, linear control with up to 10% reduction of power losses compared to AC motors.
These are some of the key factors that ought to be considered when selecting chiller plant on the basis of whole life costs. Combined with these, it's important to have a good understanding of the characteristics of the chillers being used, generally by working closely with a knowledgeable supplier.
With the support of specialist expertise it's possible to ensure not only that the system is designed to give optimum performance but also to devise a control strategy that takes account of the chiller's characteristics to maintain optimum performance on a day to day basis.
All of these issues can be addressed properly simply by taking a step back so the full picture comes into view, and fine-tuning designs to suit the characteristics of the chillers being used.
Please don't hesitate to contact us on our free phone number 0800 801 819 or contact our Applied Products Division Manager on the email address given below.
Wayne Perrins
E-mail: >>
This cost of ownership approach is, of course, a well established principle that many industry pundits talk about at length; and quite rightly so. Unfortunately, the enthusiasm for this principle that is prevalent in seminars doesn't always manifest itself in real projects.
However, all of the talk has certainly made end users more aware of these issues and there is now far more pressure on the building services industry to walk that talk. So we need to get away from sizing chillers on the basis of maximum ambient temperatures that may or may not occur for a few days every year and start to think about whole life costs. And that means giving higher priority to criteria such as COP at part-load and the system's ability to respond accurately and efficiently to variable loads.
A case in point is the inclusion of a slide valve in the compressor circuit which, when combined with the right controls, will enable continuous capacity control so that the required cooling capacity can be matched very accurately. The chiller also controls outlet water temperature, which is important when meeting the performance of the selected terminal units.
Chillers exhibit a much higher COP when operating at part load. For example, a chiller operating in an ambient of 20°C and around 70% capacity will provide a coefficient of performance of almost 200%, compared to that provided by the same chiller operating at 35°C at 100% capacity. This means that a COP of 3 (cooling capacity divided by power input) becomes a COP of 6. This effectively reduces the power input by half and so the running costs of the unit are similarly reduced.
With chillers that have more than one compressor, the typical control configuration will wait for the first compressor to reach load before starting the second - and so on. However, if each compressor is started unloaded and then all compressors modulate together to meet the cooling load this will use less power at start-up and avoid the need to oversize cables to allow for peak currents.
In this respect, Star Delta motors also offer benefits as they have the same peak current as a soft start but the peak is instantaneous rather than over a set time period. Also, if the chillers have a high power factor it will not be necessary to install a series of capacitors to compensate for the reactive energy consumed by the motors.
As noted earlier, the key to assessing the real cost of ownership is to get to grips with the nitty gritty. For example, the efficiency of oil separation in the refrigeration circuit can make a significant difference to life cycle costs. In particular, if too much oil is discharged to the refrigerant cycle this will increase the maintenance requirements of the chiller and reduce performance. Ideally, technologies such as more advanced cyclone oil separators should be used to ensure the oil is no more than 1% of the refrigerant weight.
The minimum water volume that must be maintained to prevent excessive stops and starts is another relatively 'minor' consideration that shouldn't be ignored, because this has an effect on the overall efficiency of the system. The same is true for the refrigerant volume, which can be minimised by the use of a compact plate heat exchanger that requires lower refrigerant volumes than conventional shell and tube type heat exchangers.
Paying attention to other chiller components can also make a difference. Use of DC fan motors, for example, will enable finer, linear control with up to 10% reduction of power losses compared to AC motors.
These are some of the key factors that ought to be considered when selecting chiller plant on the basis of whole life costs. Combined with these, it's important to have a good understanding of the characteristics of the chillers being used, generally by working closely with a knowledgeable supplier.
With the support of specialist expertise it's possible to ensure not only that the system is designed to give optimum performance but also to devise a control strategy that takes account of the chiller's characteristics to maintain optimum performance on a day to day basis.
All of these issues can be addressed properly simply by taking a step back so the full picture comes into view, and fine-tuning designs to suit the characteristics of the chillers being used.
Please don't hesitate to contact us on our free phone number 0800 801 819 or contact our Applied Products Division Manager on the email address given below.
Wayne Perrins
E-mail: >>