How to use your vacuum pump correctly 10/3/18

An engineer’s vacuum pump is either his best friend or his worst enemy. When it’s a straight forward connection and the system is reasonably accessible then the job is pretty straight forward.

Effective dehydration prior to charging a system with refrigerant takes valuable time and time is money. As a manufacture of high vacuum pumps for over 40 years we find many problems experienced in the field are due to misunderstanding of vacuum principles. We are often asked to supply a big vacuum pump because the job is big. Vacuum pumps can be as big as chest freezers but this doesn’t mean they will dehydrate a system any quicker than a properly connected and maintained hand portable unit.

We often have enquiries for extra-long refrigeration hoses to connect up the new big pump because it can’t be located close enough it reach the job; this doesn’t work. There are three considerations which will determine whether a vacuum pump is suitable for the application.

Size is not as important as we might expect; a bigger pump is not a faster pump. The size of a pump is determined by swept volume, in other words the amount of free air that the pump can clear in a set time. The depth of vacuum is the best vacuum the pump will achieve, measured at the inlet when blanked off. The pump down curve is engineered in to the design of the pump and allows it achieve a deep dehydrating vacuum after clearing the initial volume.

These days two sage vacuum pumps specify figures of .001milibar equating to 1 micron, which is 1 millionth of an atmosphere. This is a pretty impressive outer-space vacuum and is more than adequate for dehydrating water vapour. The most important consideration is the pump down curve and it’s this design consideration that lets many cheaper pumps down. To dehydrate water vapour effectively a specific depth of vacuum must be achieved and if the pump does not quite reach this area of operation on a system it will not dehydrate efficiently.

Another issue is water condensing in the oil. Once the oil becomes milky the vacuum pump can only reach the vapour pressure of the water; we can’t dehydrate water with water in the pump. Opening the gas ballast valve is vital in the early stages of evacuation as this works to keep the oil water free by destroying the vacuum after the sliding vanes have swept the inlet port of the pump. The water is not allowed to condense in the pump and is driven out of the oil box in the form of vapour.

If an electronic vacuum gauge is attached to the farthest available point of a system it will provide information not available from a mechanical Torr gauge. A mechanical Torr gauge relies on pump running time and a pressure rise test, which obviously works but relies on the skill and experience of the engineer. The Pirani/digital gauge doesn’t require a pressure rise test. Pirani/digital gauges measure the weight of the various molecules collectively (partial pressures); Torr gauges measure the volume of molecules (total pressures). Once the Pirani/digital gauge reaches a good stable vacuum the system should be dry and can be charged, saving unnecessary vacuum time.

In order to be confident with our vacuum gauge we must be confident that our vacuum pump is performing to its optimum. Connecting a vacuum pump to a system is usually done through a refrigeration manifold; this may be fine on small systems but detrimental on larger systems. Vacuum starts at 14.5psi atmospheric pressure and reducing. Unlike positive pressure there is no potential under vacuum for the molecules to leave the system and exit the vacuum pump. Initially we will get a flow through the pump. If the system is leak tight it will stop after a time. By reducing the pressure we reduce the boiling point, allowing any moisture to gas off. The distance between the molecules as they thin out is the vacuum. As the molecules thin out there is no potential for them to leave the system. They tend to buzz about and collect in the furthest points of the system, which is why we see differing vacuums at localised points; the higher the collection of molecules the higher the pressure.

The best vacuums will be at the pump inlet which is why it is not useful to fit a vacuum gauge here. Eventually the molecules may equalise out but not if they have to navigate through a restrictive manifold and down a long thin hose, cluttered with obstructions like Schrader valves and depressors.

Increasing vacuum hose diameters and removing obstructions will reduce vacuum times by hours and in some cases days. Using the correct vacuum set up on a large system we have reduced an acceptable five day evacuation down to two days and achieved a much better vacuum.

So how do we make our vacuum pump our best friend and not our enemy?

  • Designate the pump and connections as a separate piece of equipment.
  • Keep it away from manifolds.
  • Use the gas ballast in the early stages until the oil stops emulsifying.
  • Change the oil regularly as it’s not just for lubricating the moving parts but creates the vacuum by sealing the sliding vanes.
  • Use short large diameter hoses and in-line Schrader valve removers.
  • Use an electronic/digital vacuum gauge and fit it as far away from the pump as practical.
  • Don’t leave emulsified oil in the pump over the weekend as it wrecks the internals, especially if the water vapour is slightly acidic.
  • Always ensure that the refrigerant is removed completely and not frozen in the back of the system, otherwise it will lift the oil out of the pump and cause a seizure if left unattended; invalidating the warranty.

Understanding how to connect a vacuum pump and the principle of efficient evacuation can save time and effort as well as extending the life of our equipment.

Any enquiries to [email protected] or call (01642) 232 880

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