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A vacuum pump alone won't cure blockages in refrigerator systems

vacuum-pumpUnless other actions are also taken, using a vacuum pump to evacuate a R134a refrigeration system will almost certainly not result in a permanent resolution to particle or moisture blockages. This is true no matter how deep the vacuum or for how long the vacuum pump is run.

Moisture issues - Back in the day, when refrigeration systems used good old ozone-gulping refrigerant R12 and mineral oil in the compressor, moisture could be removed from the entire system, including the oil, simply by hooking up a vacuum pump and taking the system down into a pretty deep vacuum. Any moisture would evaporate out of the oil at this low pressure, and find its way out of the system as a vapor via the vacuum pump. Skip forward to the present day, and we now use more environmentally friendly refrigerants such as R134a which requires a synthetic oil for the compressor.

The Danfoss/Secop BD 35 and BD 50 compressors are supplied filled with a polyolester (POE) oil, which is extremely hygroscopic i.e. it will grab hold of any passing spec of moisture and won't let it go easily. In fact, the oil forms a molecular bond with the moisture, and no matter how deep a vacuum is applied, or for how long, those pesky moisture molecules will remain trapped in the oil unless another element is brought into play, and that element is heat.

A certain amount of oil is always flowing around the system with the refrigerant, and if any moisture molecules are present at an expansion valve orifice or the evaporator end of a cap tube, they may freeze and block the flow of refrigerant. Heating those areas is the only way to release the moisture molecules from the oil.

Standard refrigeration practice requires a process referred to as a Triple Evacuation to eliminate moisture from a refrigeration or air conditioning system containing POE oil, and is done as follows:

  1. Recover the refrigerant and evacuate the system down to a good vacuum. Add nitrogen to a zero pressure.
  2. Change or add liquid line Filter/Drier.
  3. Flood nitrogen through the system while heating relevant areas, i.e. expansion valve or end of cap tube at evaporator.
  4. Evacuate system then repeat step 3.
  5. Evacuate system then repeat step 3 again.
  6. Evacuate system and recharge with refrigerant.

If the above is not possible or practical, some have claimed successful resolutions simply by changing or installing a liquid line Filter/Drier and then repeatedly heating the problem area until normal operation returns.

Particle Blockages - It takes only a tiny particle of debris to cause mayhem when it finds its way to the entrance of the cap tube and blocks it. All the time the compressor is running it will remain lodged there, pinned down by high pressure from the compressor on one side and suction on the other.

Simply turning the system off for a while and letting the pressures equalize may be all it takes to dislodge the particle, and wiggling and tapping that section of tubing, along with re-orientating it, just might send it off to sit in a corner where it will not cause further trouble ..... for a while anyway.

It may be hours, weeks, months or even years before the right set of circumstances causes the particle to find itself back at the cap tube entrance and cause more trouble, and often a boisterous passage will be the catalyst. Subjecting the system to a long, deep vacuum may provide short-term relief and much jubilation by re-locating the debris, but it is extremely unlikely that any particle will have been sucked out of the system by the vacuum. A far more likely result is that the particle remains in the system as a sleeping gremlin that will strike again at the worst possible time, typically when far from civilization.

A particle blockage can be eliminated in the field by one of two methods:

  1. Replace the evaporator, or
  2. Install a cap tube filter.

Replacing the entire evaporator and tube-set, which typically also includes the cap tube, can be accomplished by a boat operator without any special tools or refrigeration know-how. Adding a liquid line Filter/Drier just before the cap tube section, or changing one if already present, will prevent any further particles upstream from getting to the end of the cap tube.

If the evaporator and Filter/Drier are supplied pre-charged with refrigerant and with self-sealing couplings, there will usually be no need to mess with the rest of the system or the refrigerant, although a small adjustment to the charge level may be required at some point.

The other sure-fire way to eliminate a cap tube particle blockage is to replace the short section of tubing containing the transition from liquid line to cap tube with a Cap Tube Filter. This requires good access to the appropriate section of tubing, and must be done by a technician with the appropriate tools and skills. The filter is installed with the system empty of refrigerant, and will require an evacuation and the addition or change of a liquid line Filter/Drier before recharging.

Both of the above methods will eliminate particle blockages while preventing further occurrences, whereas evacuating the system alone will only provide temporary relief.

No matter what type of blockage you are bedeviled with, beware any technician that says that his magic vacuum pump alone will permanently eliminate the problem.

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