Oil-free compressor benefits Medical Gas System

On page 71 of HTM 02-01 there is a quiet statement that ‘oil-free compressors may be beneficial in reducing filter requirements’. This little statement, delivered, as it were, sotto voce, means what, exactly? Perhaps a good place to start would be to compare other world standards where this comment is not made quietly at all, but is central to the ethos of producing medical air. Examples include the NFPA for the US, the AS standard in Australia, the JIS in Japan, and the CSA in Canada. The CSA may be the most relevant, since, like the HTM, it is a ‘flavour’ of the ISO 7396-1, deriving from (and sharing the number of) the parent ISO document. Under the CSA, the only permitted compressors are oil-free and oil-less. Three particular differences with the HTM immediately appear: there is no requirement to remove the oil from the air; the filtration required is less complicated, but the air must be taken from the best available outdoor source. These requirements recognize an important point: for most of the world, most of the time, clean outdoor air already meets the pharmacopeia. Most of the filtration we add to air systems is not there because the air was ‘bad’ to begin with – after all, we breathe it every minute of every day. All that filtration is there because the compressor contaminates the air, and we naturally cannot allow that ‘mess’ to reach the patient.

Oil-lubricated compressors remain the most used technology for any general air compression application, and tens of thousands of units are installed every year. Only in applications where oil is unacceptable, or the need for clean air is paramount, has the oil-free machine dethroned them. In less demanding applications, oil may be present, but filtration is usually considered sufficient. It is useful to understand filters in the context of how we deal with the inevitable oil from a lubricated machine for medical air. When we use lubricated machines, we naturally cannot simply accept oil passing to the patients; we rely on the filters we add to prevent this. Liquid oil is typically captured with a centrifugal separator, aerosols with a coalescing filter, and vapour with an activated charcoal filter or canister. HTM 02-01 does not specifically require any of these, but discusses all of them. In order to preserve the functioning of these elements, we must also control their operating conditions. Their efficacy is heavily dependent on operating temperature, loading, and ageing. Heat reduces the efficacy of all three filtration stages (they are typically designed for a 20°C ambient environment), and ageing will reduce the efficacy of the coalescing filter and of the charcoal. The charcoal also has the uncomfortable characteristic that we never know when it is full. Like a sponge, it can only absorb a fixed amount. Once full, it no longer provides any protection, but there is no indicator that lets you know. Normally, we change them on a calendar basis, and must assume that they have enough capacity for that period.

In reality, this is not as much of a problem as it might seem, and perhaps we should confess to a little cheating here. It is usual to put this absorber after the desiccant dryer. This means that the desiccant dryer acts as an oil filter, and thus protects the activated charcoal from overloading. Anyone who has serviced a desiccant dryer and changed the desiccant will have observed the consequence of this in the brown discoloration of the desiccant. It is a risky business, since one of the things that will prevent a dryer from working is oil on the desiccant. However, since the HTM requires that we monitor the dew point, a dryer failure is observable where an absorber failure is not. This does mean that desiccant coupled to lubricated machines has a very short life compared with oil-free systems – two years versus five is a common difference, but it can be much greater.

Overloading is typically not a problem in medical systems, since the components usually have a significant safety factor in their design. However, oil-lubricated compressors can also have problems which make a nonsense of all our calculations. The most common arises when the compressor is not run correctly. Probably the worst case is when a screw compressor is stopped, and quickly started. Under some circumstances, the compressor can ‘burp’ oil, which can, of course, overwhelm the separator, and pass significant quantities of oil downstream. All of this is the background to the statement in the HTM about filters. With an oil-free or oil-less compressor, some of this becomes unnecessary, and it all becomes less critical. Since there is no oil in the compression chamber, we can do away with these multiple filtration stages. Filters remain on the system, and even perhaps the charcoal absorber may be present, but now they are there to capture particulates, and to remove residual odour and taste that may derive from the outside air (for instance if there is a kitchen exhaust too close). Their failure no longer endangers the patient.

Oil-free compressor technology has now been around for decades – pumping seals, permanently sealed bearings, and engineered plastics, with high natural lubricity and excellent wear-resistance. These technologies are how air is compressed for the most demanding, high purity applications. These applications fortunately include most things we eat or drink. Pharmaceuticals, food processing, and laboratories, are examples of areas of activity where a little oil is a huge problem. Breweries and other bioreactors insist on oil-free air to ensure their process and product is not compromised. Curiously, medicine – human respiration – remains one of the few human-consumption applications where oil-lubricated machinery is even tolerated. Given that that reliably clean air is essential in these applications, a standard has been developed which allows the air from any compressor to be evaluated, and compressors compared based on the cleanliness of their output. This is the standard, ISO 8573-1 ‘Compressed Air, Contaminants and Purity Classes’. Class 1 in the standard, referred to as ‘technically oil-free’, calls for the amount of oil in any form to be at, or below, 0.01 mg/m3. This will seem a very small number, but it is still easily high enough to add odour and taste to the air when it is breathed. Clearly, if you are made to breathe this air through a ventilator or mask and cannot escape, even a little odour or taste could be miserably unpleasant.

Oil-free compressors are available with one certification for air cleanliness higher than Class One, termed Class Zero. At first read, Class Zero seems to be somewhat vague. The rule is “as specified by the user or supplier and more stringent than Class 1”. All this says is that the exact specification for Class Zero needs to be defined up front, but it must be more clean than Class 1. As applied by Atlas Copco – which is also the first company to obtain full Class Zero certification for its oil-free and oil-less compressors, and can thus be said to have set the benchmark – this means that the air from the compressor must be absolutely free of any oil, liquid, aerosol, or vapour, (i.e. 0 mg/m3) down to the limit of measurement. Since the measuring technique is specified by the standard (in Parts 2 and 5), this all becomes meaningful. Users of oil-free compressors gain another advantage. When run at very low utilisation (not uncommon in medical situations), there can be a problem with the condensation of water vapour in the first stage of the compressor as the compressor cools between starts. This water will enter the lubricating oil and become mixed with the oil as the compressor runs. This is termed emulsification, and can get bad enough to cause total failure of the compressor. It is a particular problem in tropical countries, but is potentially a problem anywhere that humid air is present at the intake. There are ‘fixes’ for this, but they offer another possible failure, and a troubleshooting mystery to solve. Oil-free and oil-less machines by their nature do not have the need for this.

BeaconMedæs and Atlas Copco are offering oil-free technology options fully compliant with HTM 02-01 now, and the number of available technologies will be increasing throughout 2014, until all the possible oil-free technologies are available for HTM 02-01 installations by 2015. On every terminal in the hospital there is the statement ‘Use No Oil’, but down in the basement we still use it. It is a strange anachronism that ‘Medical Air, EuPharm’ is almost alone among human-consumption applications in still permitting the use of lubricated compressors, even in the face of the solid technical advantages pertaining to oil-free compression. Perhaps it is time to pay more attention to that quiet clause on page 71 of HTM 02-01?

Mark Allen has been working in the medical gas industry since 1978, he has been involved ‘in virtually all aspects of medical gas work’, including regulatory, testing and verification, marketing, and product development, over the past 35 years.