When specifying air conditioning equipment for a large commercial project, should you opt for chillers or one of their alternatives? Each system has its merits, but chillers certainly have significant strengths, as James Henley explains
Advantages and disadvantages, pros and cons, strengths and weaknesses, positives and negatives, upsides and downsides; however you couch your terms, it's safe to assume that most technologies are like the proverbial curate's egg - good in parts.

Chillers are no exception although a brief analysis of them does throw up some interesting contradictions. For example, given the sheer size of a typical chiller installation, it sounds extraordinary to claim that they are more compact than their main competitors - variable refrigerant flow (VRF) and split systems.

However, arguably, chillers offer the best heat exchange efficiency so fewer units are required and, consequently, they take up less space. On top of this, the efficiency of VRF can be compromised through poor design, for example if banks of condensing units are set up in such a way that the airflow is compromised because the units are placed too close together.

Of course, the configuration of chillers is also important, but a typical commercial air conditioning system only requires one or two of them. This, and the fact that they are centralised, make them simpler and faster to set up and easier to maintain.

All sorts of shapes and sizes

Chillers are also flexible - there are all sorts of shapes, sizes and conformations to suit most applications. For water-based systems, there are no limits on how much water is used or how far you can pipe it, or how large or small the system can be. Air-cooled chillers, meanwhile, can be located in open spaces like car parks, roofs or ground level areas and have the advantages of a relatively low installation cost and low maintenance. They can also do without a plant room, cooling tower or condenser pumps.

However, the flexibility of chillers is not limited to their design. It also extends later into the life of the building. For example, say you have a number of water terminals such as fan coil units.

If you later need to rearrange the floors and add fan coils, this would be allowed by a water system. In the same way, having a chilled water loop system will allow you to replace the chiller at a different date from the water terminals because the chiller is separate from the rest of the system and the terminals can be replaced floor by floor, terminal by terminal.

However, although they are flexible, chillers also tend to be less complex than the alternatives - the technology is well understood and they can be maintained by refrigeration specialists.

It is also possible to argue that a chiller looks better than VRF when viewed through the F-gas regulations because they oblige owners to have their systems regularly inspected to make sure they are not leaking. This is relatively easy on a chiller because it is all in one place and contained in a single accessible machine. That means it is reasonable quick to conduct a leakage inspection.

The alternative - VRF - has pipework spread throughout the building and running through occupied spaces so the potential for disruption is higher. After all, if you can only get to it out of hours, there are clear workload implications.

However, what about the weight of a chiller? If it is to be sited on a roof, surely this will need to be strengthened? In fact, although they are undoubtedly large and heavy, the extra weight of chillers is insignificant compared with the structural loads imposed on most commercial buildings.

The big chill

The key issues in the UK chiller sector are higher energy efficiencies and lower noise levels. The latest innovations in chillers are all about energy. It has been estimated that chillers spend just 3 per cent of the time at full load. It therefore pays to make the chillers more efficient at part load. That is why there has been a focus on variable speed drives (VSDs) and inverter (VSD) technology for chillers.

The incorporation of VSD technology, electronic expansion valves and optimised part load control results in high energy efficiency ratios which, in turn, lead to economical operating costs.

Manufacturers are also examining ways to improve the efficiency of heat exchangers and evaporators, as well as reducing the electrical usage of the compressor motor and fans.

And enlightened manufacturers are also including add-ons - enhancements to the basic machine - such as free cooling and heat recovery. There has been a move for manufacturers to work more closely with consultants to try and find appropriate system design compromises to meet energy efficiency targets. This means manufacturers getting involved earlier in the project and getting involved more than they may have done in the past when they might have just selected and priced the chillers to meet a certain performance.

Now, we as manufacturers might find ourselves doing a full energy consumption study for the chillers that comprise the system. So we are more involved in providing a service as well as the product. With commercial and residential dwellings being sited ever closer together, the need for quieter chillers is pressing. That is why noise mitigation measures such as sound barrier walls make sense. But manufacturers are also doing all they can to reduce noise and vibration around and within the chillers themselves. For example, many chillers feature compressors inside sound attenuating enclosures and manufacturers often put additional attenuation on or around the chillers. They also try to run the fans at slower speeds or cycle them to keep the noise down.

McQuay launches ATS chiller range
McQuay UK has unveiled what it describes as the world's most advanced air-cooled chiller featuring a single screw compressor with built-in inverter and variable volume ratio to maximise performance at both part and full loads.

The ATS chiller range is available in three versions from 170 to 730kW and is said to boast the highest efficiencies and smallest footprint of any chiller on the market.

The chillers, which run on R134a refrigerant, have a capacity range three times as wide as the units they replace. On top of this, full load efficiency (EER) is 35 per cent better, seasonal efficiency (ESEER) is 26 per cent higher, and the unit is 30 per cent smaller.

Other claimed features and benefits of the ATS chiller include:

Highest efficiency at both full load and partial load at a low cost with no compromise in reliability.

An integrated sub-cooling circuit to ensure perfect uniformity of temperature distribution.

Economised refrigerant circuits.

The latest shell and tube evaporators to get the best heat transfer and highest capacity, and to grant the lowest range of pressure drops values.

Axial condenser fans with wing-profile blades with superior aerodynamic performances and acoustic efficiency for the greatest possible comfort.

Inverter-driven and brushless fans are used to achieve the best efficiencies.

// The author is chiller technical manager at McQuay UK //