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Chillers – Monsters on the roof

Chillers often waste considerable of amounts energy, both directly and through their interaction with heating. Yet in most cases this is not identified or measured and substantial inefficiencies go unaddressed – whilst the chillers themselves are out of sight and out of mind on the roof or somewhere equally hidden.

This article explains some of the reasons why chillers waste energy and what you can do to make yours more efficient.

It covers the following topics:

  • Chillers
  • Oversizing and part load performance
  • Isolation and sequencing
  • Maintenance

You might also like to read these related articles:

Air Conditioning Split Systems
Is your BMS Wasting Energy?
Managing Energy for Comfort

Chillers

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Many buildings require cooling, particularly if they are very large, accommodate energy intensive processes or have a requirement for humidity control. A common cooling medium is chilled water, usually provided from “packaged” chillers which are often located on the roof of the building. This may result in them being almost forgotten. These chillers, which may have cooling capacities of 1,000 kW or more (equivalent to an electrical input of over 250 kW), incorporate compressors, evaporators and condensers and quite often the primary circulating pump, all within a single package. Most sites have more than one chiller in the same way that they have multiple boilers, and many of the resulting problems are very similar.

Oversizing and part load performance

In the vast majority of cases, packaged chillers are installed as part of the ventilation and air-conditioning system, and in the UK the demands for cooling are seasonal. In the winter months there should be no requirement for cooling and some organisations not only switch off their chillers throughout the winter, but actually drain-down the chilled water systems as well. This is recommended as it can save a fortune just in the reduced use of energy for frost protection, aside from other benefits. However, the variable nature of the cooling load means that chillers really need to be able to operate effectively at part load. The requirement for this is exacerbated by the fact that most chiller installations are massively over-sized for their duty. Very often a single chiller will provide sufficient cooling in all conditions – with two additional units merely complicating the control process. It is therefore very important that the chiller has the capacity to operate at a variety of loads, and this will usually be through the incorporation of a number of separate compressors. However, some big chillers will have only two or three, and in this case part load performance is likely to be poor. The result will be frequent short-cycling of the compressors, which is both inefficient and detrimental to the longevity of the plant.

Isolation and sequencing

Chillers are often “plumbed” in parallel, usually in a “reverse return” configuration designed to equalise the flow through them. However, even in a correctly sized multiple-chiller installation, there will not be a requirement for more than one chiller to operate for the vast majority of the time, and this results in the problem of “dilution”. In a three-chiller system only one third of the circulation is actually being cooled. The installation of automatic isolating valves is a pre-requisite for optimal operation, and there is no better time to fit them than during the winter. The BMS must then be programmed to sequence the chillers according to the cooling load. On many sites, all chillers bar one could be manually isolated for much of the year, and the set point for the remaining chiller may be increased by several degrees, resulting in much more efficient chiller operation.

Maintenance

A comprehensive maintenance regime is essential to the preservation and optimisation of assets such as chillers that are both expensive to buy and operate. Refrigeration plant is extremely robust and, as it is often considerably over-sized, it will continue to do the job even when it is actually in an awful condition. Any money saved through skimping on maintenance will, in the longer term, be lost many times over through additional expenditure on energy, replacement compressors and other components, and eventually new plant. In particular, any refrigerant leaks that occur should be dealt with immediately as the energy use of this plant will increase dramatically if a circuit is undercharged. The corresponding compressor may end up running continuously, achieving very little except extended running hours and considerable electricity use.

Your Independent RISK FREE Solutions

A Green Consultancy Investment Grade Energy Efficiency Audit will identify and prioritise all cost-effective energy saving opportunities in cooling and all other aspects of your energy consumption. For an investigation of all equipment controlled by your BMS you might like to consider a BMS Health Check or a BMS Audit.

Put us to the test NOW! To discuss your requirements please call John Treble on 01761 419081 or email John@GreenConsultancy.com

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