Understanding the life-cycle costs of a chiller installation requires a clear view of the full picture. Rob Rhodes of Axair Climate explores the features of chillers that can make all the difference.
A PREVIOUS issue of HVR highlighted the need for future air conditioning installations to be more efficient, to meet the conflicting demands of enhanced comfort conditions and reduced environmental impact. In order to achieve this, it's important to take a look at the full picture, rather than being blinkered by up-front equipment costs and co-efficient of performance (COP) at maximum load.

This, in turn, requires a detailed look at every aspect of the system and understanding how it operates throughout the year. Chillers provide a case in point.

One obvious example of this is the requirement to size chiller plant on the maximum ambient temperatures that are likely to be experienced during the year - even if that is just for a few days a year, without looking at the total system characteristics. The actuality is that the chillers will be running at part load for most of the year - so the COP at maximum load is almost meaningless in relation to life-cycle costs. The COP at part-load and the chiller's ability to respond accurately and efficiently to variable loads are far more meaningful criteria.

For instance, a compressor circuit that uses a sliding valve, combined with the sophisticated controls that are now becoming the norm for modern chillers, allows continuous capacity control to match the required cooling capacity very accurately. Furthermore, it enables the chiller to provide that control on the basis of outlet water temperature rather than inlet temperature, providing far more precise matching of water temperature to cooling load.

The result is a much higher COP at part-load. For example, a chiller using this technology and operating in an ambient of 20°C and around 70% capacity will provide a coefficient of performance of almost 200%, compared with 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.

Historically, this level of modulation was usually achieved through inverter control of the compressor but, in some cases, this has led to problems with harmonics. A more satisfactory approach is to modulate the compressors on the basis of oil pressure, so that potential harmonics problems are avoided.

Even the fine details of the compressor, such as the efficiency of oil separation, all have an effect on life-cycle costs. If too much oil is discharged to the refrigerant cycle this will increase the maintenance requirements of the chiller and reduce performance. Ideally, this should be minimised to approximately 1% of the refrigerant weight, as can be achieved by advanced cyclone oil separator designs.

Similarly, the minimum water volume that must be maintained to prevent excessive stops and starts has an effect on the overall efficiency of the system. So too does 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.

However, the full picture extends beyond the compressors to encompass the other components of the chiller system, so these should also be considered. Fan motors, for example, can make a big difference to the overall performance of the chiller and the use of DC motors enables finer, linear control with up to 10% reduction of power losses compared with AC motors.

Making a good start

It's not uncommon for electrical wiring for chillers to be oversized to allow for the peak currents caused by many chillers during start up.

This extra expense can be avoided by considering the starting characteristics of the compressor. In this respect, Star Delta motors offer particular benefits as they have the same peak current as a soft start but the peak is instantaneous rather than over a set time period.

On systems with more than one compressor, motors with a low peak current should be combined with a staged start-up process. In this scenario, the start-up begins with the compressor that has run the least hours, running at minimum load to minimise the power load on the installation.

After a one minute delay, the second compressor is started, so that both then run at minimum load as the next compressor comes into operation - repeating this pattern until all the compressors are running. After a 30 second safety delay the machine then increases to normal power.

This arrangement has the advantage of using less power at start up than if all the compressors were started simultaneously, as well as avoiding the oversizing of cables mentioned earlier. And 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.

It is only by looking at all of these aspects, ranging from COP at part-load to power factor and start-up characteristics, that the full picture will become visible. And without the full picture, you can't assess the overall performance.


Axair Climate T: 0121 705 7601