Look after your Vacuum Pumps
SPENDING so much time working with the vacuum environment, and in particular servicing vacuum pumps, has taught us a very simple lesson: looking after your vacuum pumps can save you a lot of time and money. This sounds incredibly obvious, but, while we usually see only those pumps sent in to our workshop for service due to apparent malfunction, more than half of the initial testing we do before servicing shows some form of vacuum leak in the fitting or pipework attached to the pump housing. This could also apply to the average manifold set used in refrigeration service.
The water vapour pressure chart shows that to remove moisture from any system at 20C, the vacuum needs to reach at least 23.36 mbar to start any water vapour activity and thereby removing it from the system. In practice, somewhere below 50 mbar will often do the same job, but there will be no guarantee that moisture removal has been effective. The vacuum pumps used by most refrigeration service technicians should be capable of at least getting below 6 mbar, and to 0.1 mbar in ideal conditions.
Difficulties can arise with the gas ballast valves that are usually fitted to oil lubricated high vacuum pumps. With this valve open, greater volumes of condensable vapours can be pumped before condensation begins to occur in the oil reservoir, and the lubricating properties of the oil can be maintained for longer. Water in the oil reduces the maximum vacuum achievable and may also contribute to insufficient preparation before commissioning a refrigeration system. However, often the pumps we receive for service have the gas ballast valve closed.
The average vacuum pump in good condition will achieve an adequate vacuum level with the ballast valve almost fully open. But be aware, this condition can also use greater quantities of vacuum pump oil.
Another common problem occurs when vacuum pumps are left to run continually (often overnight or at weekends) at rough or low vacuum pressures. This means they are usually running at a considerably higher temperature and using more oil. Furthermore, where the ballast valve has been forgotten, the pumps accumulate water in the pump oil reservoir, eventually displacing the oil and causing expensive damage.
With more careful leak testing and moisture control with polyolester oils, no doubt these problems have been reduced over the years. But how many times have we all connected a vacuum pump to a system late in the afternoon, after some service or installation work, expecting to find a moisture and air free system in the morning? The proper procedure would be to assess the pump's capabilities by pulling a quick vacuum to the nearest (closed) valve on the system, with an adequate measuring device in the line to ensure a vacuum within the specified range. Once the pump's capabilities are known, .we can determine when the system under vacuum is moisture free, as any water vapour remaining in the system will prevent the pump reaching the validated vacuum pressure.
Having one foot in both the refrigeration and vacuum industries, we also service refrigeration recovery units for the general refrigeration and motor vehicle trades. Exactly the same principle applies, with the most likely breakdown of vacuum integrity at the fittings between the recovery unit and the work.
So remember - look after your vacuum pumps!
Water Vapour vs Vacuum Pressure Table
