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Seeing the bigger picture

Chiller plant represents a sizable investment, so it makes sense to take sensible measures to ensure equipment is protected. Mike Slattery of Axair Climate provides some pointers on the main things to consider
Seeing the bigger picture
CHILLERS are among the most expensive items of building services plant. So it makes sense to take a few precautions, beyond the installation of the chiller itself, to provide extra protection from damage and facilitate pre-commission cleaning, commissioning and maintenance.

Not only does this approach optimise both performance and plant life, it will also minimise the risk of the plant going wrong .

This is not a new idea, of course. All of the precautions described here are already accepted as best practice. But we all know that best practice and real practice are not always the same. A greater appreciation of the reasons behind best practice is the first step in achieving it on site.

For example, something as simple as fitting a strainer as close as possible to the water inlet will prevent debris from entering the chiller where it could cause blockages. But it is just as important to provide a means of local isolation to make it easier to clean the mesh - or it may not get done as often as it should be.

Similarly, a strainer should be fitted as close as possible to the water inlet connection to the pump and should be provided with a means of local isolation, again to facilitate cleaning of the mesh. This strainer is required to prevent debris entering the pump where it could cause impeller damage or seal wear.

Easier maintenance
During pre-commission cleaning, it will be necessary to bypass the chiller with a line-size flushing bypass. This will avoid dirty water passing through the chiller during cleaning, and will help to maximise flushing flow velocities in the pipework.

For multiple chillers, the isolating valve in the by-pass should be replaced with a double regulating valve. Regulating this valve so that its resistance matches that of the chiller will mean it can be isolated for maintenance purposes without disrupting the flows through the other chillers. This will make maintenance much easier during normal working hours.

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. 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.

To help with flushing or draining of the chiller and pipework, drain-off cocks with a line size of no less than 25mm should be located on both sides of the heat exchanger. A chemical dosing pot should be located across the pump for the addition of water treatment chemicals. This should be isolated during normal operation.

During maintenance, it will also be necessary to carry out visual inspection of the inner surfaces of the heat exchanger. This can be made easier by ensuring that the pipework immediately next to the chiller is easily demountable.

Under pressure
For all but the smallest of systems, 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. Pressurisation units are preferable to open-vented tanks since they allow less dissolved oxygen into the water, and hence reduce the risk of corrosion to steel pipes.

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. The pressurisation unit should be located on the inlet to the pumps since this makes it easier to confirm that the water pressure is high enough to avoid cavitation-induced noise or damage.

All of these things are straightforward and may strike readers as obvious. But time and again, problems initially attributed to a fault in the chiller are because one of these details are missed.

It may be a detail that will affect the performance of the chiller from the start, or it may be something that will have a later effect. Many of the things mentioned above are designed to facilitate maintenance, so clearly not doing them will impede or possibly
prevent effective maintenance.

All of this reinforces the point that there are very few items of building services equipment that can be dealt with in isolation.

We all need to be aware of the bigger picture and share specialist knowledge as much as possible.
1 October 2007


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