Many chillers in the UK are falling short of providing optimum efficiency, largely through insufficient control. Jim Kilcoyne of Trane Controls provides a few pointers on how to get the most from the chiller plant
In any building where air conditioning or comfort cooling make use of a chilled water system, the chillers will be making a significant contribution to that building’s energy consumption. And despite the many measures that are taken to optimise efficiency, many chiller installations still fall short of maximum efficiency.

Given the rising costs of energy and the impending application of energy targets for buildings, taking a closer look at how chillers are controlled is becoming an imperative rather than an option. And even systems that are linked to a BMS can often be made considerably more efficient by understanding the chillers and the way they are configured.

Unfortunately, most controls treat chillers as an add-on to the system, rather than an integral part of it. In these cases, the information relayed from the chiller back to the control system is often limited to just status signals and alarms. This isn’t enough information to formulate an effective, efficient control strategy.

Know your chillers

When a wider range of system temperatures and parameters are monitored it is possible to determine the most efficient operating conditions for each chiller and schedule them to give the optimum performance. Fundamentally it’s a matter of knowing when to turn each chiller on or off, or when to turn them down. The real skill is in how this is done.

For example, in an installation with two chillers, it is quite common to have one chiller running at maximum capacity, while the other remains off except at times of peak demand. However, many chillers do not provide maximum efficiency at 100% capacity, so it may be more efficient to have both chillers running at part-load. In other circumstances, and with other chiller characteristics, the reverse might be true. For example, with two chillers that are most efficient at 60-80% capacity, running both chillers at low load to provide a flow temperature of 6°C will not be the most efficient strategy. Here, it makes more sense to switch one off and let the other run at a higher load.

In this case, even greater efficiencies and system stability will be achieved by adjusting the individual set points at the same time to maintain stable system supply temperature, whilst making the best use of the chiller’s characteristics. Other considerations might include control of the cooling tower and chiller subsystem by optimising the cooling tower leaving-water temperature on to the chiller.

A switch in time

In all cases the key objective is to reduce the number of times the chillers are switched on and off, as this doesn’t just waste energy, it also increases the wear on the compressors. It has been calculated that each on/off switching takes eight minutes off a chiller’s life. With many chillers switching on and off many times a day, these cumulative effects can be expensive.

These principles are applied through the ways that the chillers are brought into operation and unloaded when they are not needed. For example, when the control system registers an increase in cooling requirements it should monitor the temperature for a pre-set time before bringing another chiller into play in case it’s just a temporary ‘blip’ in demand.

Equally, the point at which a second chiller is brought into play will depend on the system. In some cases it is best to wait until the first chiller has reached 100% of capacity, in other cases it will be more efficient to bring the second chiller in when the first chiller reaches 80% of capacity, depending on the COP of the chiller.

In fact, switching chillers on is relatively easy, compared to knowing when to switch them off. Switching them off prematurely often leads to frequent stop/starts if the remaining chiller(s) can’t maintain the load, whereas waiting too long before switching them off will waste energy.

In most cases, as the load declines, it is best to back both chillers down and monitor conditions until it’s certain that just one will maintain the load efficiently. Then the one chiller can be ramped back up while the other is turned off, secure in the knowledge that it will then remain off for a long time and switching will be minimised.

When an installation has a mixture of different ages and makes of chillers, managing them efficiently becomes even more challenging. For example, if we consider a site that has three chillers with differing efficiencies it doesn’t make sense to simply use them in rotation – though this is often what happens.

It is far more sensible to run the most efficient chiller to meet the base load, use the least efficient chiller only at times of peak demand, with the third swinging in and out while the other chillers are ramping up to full output.

Of course, depending on the characteristics of the chillers, full output may not represent 100% capacity; it may be that 80% is considered as the optimum point to bring in more cooling capacity, depending on the COP curves of the machines. With such a wide range of parameters to take account of, it’s clear why so many control systems are failing to deliver optimum performance – and why it pays to use specialised chiller management systems.

Some chiller control experts now offer systems with integrated chiller plant management systems to provide standard reports and other detailed information about the plant. This makes commissioning sequencing a much more simple process.

Analysis of these and other complex control strategies allows end-users to ensure that their plant runs smoothly and efficiently, thereby making a significant contribution to the bottom-line profits of the company.

At the same time, such systems should not operate in isolation. They still need to interface to other systems ideally using open standard network protocols while also providing clear information on the system’s performance for on-site engineers to refer to and, if necessary, override.

The scope of this article can only provide a flavour of the complexity of chiller management but it does highlight the need to take a closer look at your chillers and take measures to squeeze the maximum performance out of them.

Trane (UK)

01256 306000