Natural borne chillers

During the last 20 years we have seen significant changes in the use of refrigerants for air conditioning and refrigeration. The old chlorofluorocarbons like CFC 11 and CFC 12 are largely gone from European installations and the transitional hydrochlorofluorocarbons such as HCFC 22 are now on the decline. HCFC 22 (commonly referred to as R22) now accounts for under 50% of the installed refrigerant in the UK, having largely been replaced by ozone friendlier hydrofluorocarbons.

A few years ago, many people felt that the switch to hfcs was the end of the long refrigerant road the industry has travelled, but concerns are now being voiced about hfcs and there are moves to phase these out as well. Although hfcs have done a great job of freeing the industry from ozone depleting refrigerants, there is still a global warming penalty. HFCs have up to 3000 times the global warming potential of carbon dioxide and it is this property that is focusing the minds of environmentalists.

The UK Government has stated that hfcs are not sustainable and countries such as Denmark have already embarked on an hfc phase-out programme. "The balance of evidence within the hfc debate has shifted – it is now clear that there are a range of alternatives available and that the burden of proof is now on the chemical industry to prove that hfcs are necessary," commented Peterborough MP and environmentalist Helen Brinton.

However, it is important to keep this issue in perspective: "If all of the hfcs in Europe were vented to atmosphere they would still only account for around 2·5% of Europe's global warming emissions," points out Norman Mitchell of Mitchells of Gloucester. "The majority of emissions come from other sources such as internal combustion engines and power stations but the refrigeration and air conditioning industry is an easy target," he adds.

Alternative solutions
The most promising contenders for the replacement of hfcs are hydrocarbons and, to some extent, ammonia. But all of these also face limitations on their use. There are also changes in the hfcs preferred by manufacturers.

In particular, within the air conditioning industry we are seeing a sea change from HFC 407C to HFC 410A. There are a number of reasons for this. HFC 407C offers a similar performance to HCFC 22 but has always been considered to be an interim blend. This is partly because it fractionates to its component gases, particularly if used in a system that has not been optimised for use with HFC 407C.

HFC 407C is made up of three gases – HFC 134a (approximately 50%), HFC 32 and HFC 25 (around 25% each) – and HFC 134a has a higher boiling point than the other two. This means that if the refrigerant is allowed to settle as a liquid anywhere in the system it is possible that HFCs 32 and 125 will boil off first, leaving the HFC 134a as a liquid, thus changing the characteristics of the system.

HFC 410A, on the other hand, consists of almost equal proportions of HFCs 32 and 125. These have very similar properties, so the problems of HFC 407C are avoided. HFC 410A operates at higher pressures, though this shouldn't be a problem with good installation, and it does offer higher performance. "HFC 410A has a better heat transfer coefficient so it can be used with smaller heat exchangers and lower volumes of refrigerant in the system," notes Alan Treacey of consulting engineer Hoare Lea.

This enhanced performance has additional benefits, says Norman Mitchell: "The higher efficiency of HFC 410A means that smaller equipment, pipework and motors can be used and, with the use of inverters, this means that start-up currents can be reduced to eliminate major power surges."

Nevertheless, HFC 410A is on the hit list of those who would prefer to see hfcs phased out, and many people are now promoting the use of hcs such as ethane, propane, isobutene and propylene instead. Ironically, all of these substances have been widely used as refrigerants since the 1900s but were superseded by cfcs.

However, because of their flammability, hcs are also limited in their potential use, as BS EN 378 places upper limits on the amount of hc that can be used in a system. "Hydrocarbons are excellent refrigerants and are particularly suitable for fully hermetic systems that are manufactured, charged and sealed in a factory," says Mitchell.

For that reason, much current use of hcs is limited to domestic refrigerators' ice machines, drinks dispensers, portable air conditioners and similar sealed products, where the charge does not exceed 1·5 kg. They can also be used with small, split system air conditioners, offering up to 5 kW of cooling. They cannot be used with variable refrigerant flow (vrf) or other multi-split systems because of the large amount of refrigerant in the conditioned space.

When used with chilled water systems, a charge of up to 25 kg is allowed in each chiller, assuming the chiller is located appropriately. "The location is critical for propane chillers and rooftops are ideal so that any leakage will vent to atmosphere and the flammable gas will be diluted quickly," says Alan Treacey. "It's also important to ensure they are not near any air intakes," he warns.

With a charge of 25 kg, a propane chiller will provide up to 600 kW of cooling. This is sufficient for about 4000 m2 of office space, though the chillers can be used in parallel to increase capacity.

They also offer higher efficiencies than hfcs – on average 10% according to a study carried out at University College London. "Users report massive savings, adding a persuasive business case to the obvious environmental benefits," explains Loretta Powell of Calor Gas Refrigeration.

Neil Yates of Maplin Mechanical Services feels the future is bright for hydrocarbons: "The environmental argument for hcs has been made and won," he suggests. "Over the next ten years we will see an enormous growth in hc installations. In most straightforward chiller jobs, hcs are ideal in terms of efficiency and performance.

"Barclays Bank carried out an efficiency test with us by setting up a chilled water circuit served by two chillers – one using HFC 407C and the other the hydrocarbon CARE 50. They were so impressed with the running efficiency of the hydrocarbon chiller that they told us to convert the other one over to CARE too," he adds.

With any refrigerant though, there wouldn't be any problem at all if they were contained and not allowed to enter the atmosphere. Consequently, many people feel that preventing emissions through competent installation, maintenance and decommissioning is the most important consideration. To that end, the F Gas regulations are currently working their way through the European parliament, though this has been delayed by the European elections.

"Member states will be required to establish regulatory bodies to ensure that people will only be able to buy refrigerants if they can prove their competence in refrigerant handling," notes Mitchell. "We are hopeful this will be administered through REFCOM in the UK," he adds.

So, if you thought the refrigerants issue was all sorted, you were probably being over-optimistic. There's no doubt that the progress so far has been impressive, but there are many chapters yet to be written in this long-running saga.