Designing a chilled water system that genuinely offers low cost of ownership requires a detailed look at every aspect. Dean Ward of Walter Meier outlines the key factors that can make a real difference.
WHEN specifying building services plant there's a temptation to focus on the performance of the main plant items, as they usually represent the highest energy consumption as well as the highest capital cost. However, to get the most from a system, it's also worth looking at how that plant interacts with other components of the system.
This principle is clearly exemplified by chilled water systems, where the chiller certainly represents a significant investment - and where the attention to detail described above can make a big difference to cost of ownership.
Very often, the additional details that would benefit from closer attention relate to ancillary elements of the system. For instance, selecting chillers with a high power factor can save on the overall project costs by eliminating the need for a series of capacitors to compensate for the reactive energy consumed by the motors.
Similarly, it's not uncommon for projects to allow for the peak currents generated when the compressors start up by using oversized cables, thus adding to the overall cost of materials. Where the chiller only has one compressor there's little that can be done about this.
But as modern chillers tend to incorporate multiple compressors there is scope to configure the compressors for a 'softer' start.
So, rather than waiting for each compressor to reach load before starting the next - which is the typical configuration - it makes more sense to start each compressor unloaded and then modulate them together to meet the cooling load.
In this way, less power is used at start up, enabling the use of smaller cables.
Another detail that is often overlooked is the minimum water volume that needs to be maintained to prevent excessive stops and starts, and this can vary considerably from one chiller to another.
Keeping compressor cycling to a minimum increases overall efficiency of the system by reducing electrical demand. And the same is true for the refrigerant volume, which can be minimised by the use of a compact plate heat exchanger to obtain the maximum efficiency with the least possible amount of refrigerant.
As emphasis moves away from initial cost to cost of ownership, it becomes increasingly important to consider the factors that affect both performance and maintenance costs through the life of the plant. A case in point is the efficiency of oil separation in the refrigeration circuit.
This may seem like a trivial consideration, but 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.
Making the most of recent technical developments can also make a difference, as has been proved by the increasing popularity of DC fan motors.
Not only will these enable finer linear control of operation; they will also reduce power losses by around 10% compared to AC motors. Of course, the overall efficiency of the chiller itself is not something that can be ignored, and to get a full picture it's important to think beyond the COP at maximum load. The seasonal energy efficiency ratio (SEER) provides a much more meaningful assessment, as this relates to performance on a day to day basis.
At the risk of stating the obvious, this is because chillers generally only operate at full load for a few days a year, beyond the initial start up when the design temperature in the space is being established. COP at part-load is therefore a better way of predicting whole life costs, not least because chillers exhibit a much higher COP when operating at part load.
However, it's also important to be aware that different makes of chiller vary significantly in their part-load efficiencies, so it's essential to understand the characteristics of each type of chiller being considered. This then needs to be matched to the anticipated operating conditions so that maximum efficiencies can be achieved for the majority of the run-time.
Given that cooling loads vary from day to day, and even throughout each day, the chiller's ability to respond accurately and efficiently to variable loads is also important.
For instance, compressors with continuous capacity control allow for a 15 to 20% energy saving over step control and when combined with appropriate electronic controls will enable the chiller to precisely maintain a constant outlet temperature independent of cooling load. This is beneficial for both air conditioning and industrial process uses.
Given that there are so many things to consider during the specification and selection process, it makes a lot of sense to call in specialist assistance when it's available.
If the chillers are being sourced from a company that offers technical expertise and support, then why not take advantage of it? Then, you can be sure nothing has been missed and that the end- client is getting the best value for money.
Walter Meier (Climate UK)
0871 663 0664