AS A refrigerant with zero ozone depletion and negligible global warming potential, CO2 is expected by many to be the refrigerant of choice for many future applications.
Although other refrigerants, such as ammonia and hydrocarbons, offer these characteristics, the retail food industry in particular is attracted by the possibility of a refrigerant that can safely be used in large volumes in public spaces.

'Whilst its green credentials cannot be denied,' says Derek Alway at HRP, 'Some concerns have been expressed about the higher pressures required in CO2 refrigeration systems and the lack of experienced technicians. However, given the potential environmental gain, there must be benefits from working through these problems and finding solutions to them.'

Derek Alway has been working on finding solutions to many of the practical problems that engineers may face in installing CO2 systems: 'There can be no doubt that we are going to have to get used to working with CO2 if developments in many other countries are a guide.'

Naturals favoured
In September the US Environmental Protection Agency approved the use of CO2 in retail refrigeration. Many countries are pushing for controls on the synthetic fluorocarbon refrigerants, talking of 'phase-downs' or at least legislation in favour of natural refrigerants.

'We have been working with refrigeration contractors and retail
end-users on a number of sites to trial components and systems specifically for CO2 applications,' says Derek Alway. 'Hopefully, this will have helped remove some of the barriers to the more widespread use of CO2.

'Whilst the pressures involved with CO2 may be more than today's refrigeration industry is used to, we have to remember that systems in other industries routinely use higher pressures, and we need to think outside the box when it comes to the selection of some of the components we have traditionally used in the commercial refrigeration field.

Stockist
HRP has stocked CO2 (R744) from supplier BOC since November 2008. The gas as a refrigerant differs from commercially available CO2 in its lower moisture content - less than 10ppm - which is essential to prevent corrosion and to avoid ice crystals forming inside system pipe work.

'Much of our investigation of pipe work has been based around the experience in Sweden where CO2 is more commonly used,' says Derek Alway. 'A lot of the Swedish work is with stainless steel pipe which has the strength to withstand higher pressures, but requires the use of skilled welders on site which could be a barrier to wider use in the UK. In order to be able to use existing skills we have worked with our suppliers to design copper tubing which can safely be specified at the higher pressures.'

Sub-critical or transcritical
Two operating conditions need to be considered: sub-critical applications at pressures up to 50bar cover most freezing applications; transcritical applications up to 75bar for chillers and high temperature cabinets.

HRP developed a range of copper tube at heavier gauges rated to 50bar some time ago, but the company is now trialling a new range of tube from 3⁄8in to 13⁄8in copper which is rated for 75bar operating pressures based on BS 1306 pressure rating calculations.

There has recently been some debate about the pressure rating standards that are acceptable to the UK, and whether the German DKI method of rating might be an alternative even though it will suggest thinner wall thickness. 'We are sure that most customers will demand the higher safety margin of the BS 1306 rated tube,' says Derek Alway, but the debate rumbles on.

'A minor point which should not be overlooked by refrigeration engineers is that the pipe cutters used for standard copper tube will probably not be adequate for the increased wall thicknesses. We have taken this into account and have added suitable heavy duty cutters to our range.
'As well as new tube, we have had to look at fittings for higher pressures. Safety standards require a burst pressure of 3x maximum operating pressure which means that commonly used copper fittings will not be adequate in transcritical applications.'

Compression fittings
HRP's solution is a range of aluminium compression-style fittings which have been installed at three sites and are being carefully monitored.

Other new components which have been introduced into the HRP portfolio that are needed for use at higher operating pressures, include vibration eliminators ranging from 1⁄2in up to 21⁄8in rated up to 50bar, flexible gauge and pressure switch lines and CO2 compatible pressure transducers.

The high pressures involved and the unique thermal properties of CO2 have also required development of a new rig for charging and topping up CO2 systems. Vapour needs to be injected at a minimum of 7bar or dry ice will form inside the system followed by a liquid charging phase.

As a result of work with a regulator manufacturer HRP is able to provide a specially developed charging rig for both vapour and liquid CO2.
'Once the system is up and running we are also able to provide choice of CO2 service manifolds for on-going diagnostic work.'

Leak detection
Finally, there needs to be a method of detecting leaks which must be considered with higher pressure systems. 'Leak detection is, of course, complicated by the fact that there is CO2 in the surrounding atmosphere. We have, however, sourced a hand-held electronic detector which will identify leaks from CO2 systems.

A great deal of work has been done by many manufacturers, contractors and users in recent years to make CO2 a practical refrigerant for today's applications. These last few developments will mean that, with suitable training, every refrigeration engineer will be in a position to deliver the much needed environmental benefits of CO2 systems.