Moisture Contamination refers to a failure that results from conditions that can occur when moisture is introduced into the system.
The compressor is designed to perform efficiently under specific controlled conditions. If contaminants are introduced into the system, they act to reduce compressor efficiency, effectiveness and durability.
Moisture refers to water in any form (solid, liquid or gas). When moisture is introduced in the system, it may combine with the system refrigerant to form an acidic solution which can erode internal compressor components. In this case, moisture does not cause direct compressor failure; the failure results from the failure of a part which has been weakened as a result of the effects of rust and/or corrosion.
During normal compressor operation, the pistons compress refrigerant gas in the cylinders. Moisture in the system can lead to liquid slugging. Liquid slugging is a condition that occurs when liquid is allowed to enter one or more cylinders. Because the liquid is practically non-compressible, the compressor seals may be compromised when the Pistons attempt to compress the liquid. Liquid slugging can lead to permanent valve damage and reduced compressor efficiency.
Copper plating is a condition in which metal parts in the compressor become coated/plated with copper. This condition is often observed in compressors which have high moisture content in the system.
One possible cause of copper plating is that as moisture (water) combines with the refrigerant, it forms an acidic solution; this chemical may then dissolve or leach copper from other components in the system which are copper or contain copper-based alloys such as brass or bronze. The method by which the copper is deposited on metallic parts of the compressor (i.e. Races, Bearings, Centering ball, Fixed gear, etc.) is not known for certain, but this occurrence would have to be facilitated by the circulation of refrigerant, oil, and moisture in the system. Although copper plating alone does not cause a specific failure of the compressor, the conditions under which it is likely to occur are very detrimental to compressor durability.
Moisture Contamination occurs as a result of moisture being allowed to enter and remain in the system. This condition can be caused by the following:
A) System Leaks.
B) Improper Vacuuming of system.
C) Contaminated system components.
D) Contaminated refrigerant and/or oil.
E) Saturated or malfunctioning drier.
There are 4 primary identifiers of moisture contamination:
1. Copper Plating - The presence of copper plating generally occurs when there is high moisture content in the system.
2. Rust - Rust may occur on internal steel compressor components that are exposed to moisture for extended periods.
3. Slugged Valves - A Slugged valve is one that has been permanently deformed as a result of liquid slugging.
4. Contaminated Oil - Contaminated oil reflects a contaminated system.
Evacuation and Removal of Moisture
Moisture is in the atmosphere as vapour, ice or water and represents an unseen threat as it can affect the running of an air conditioning or refrigeration system and cause component failure e.g. through the formation of acidic residue and blockages due to ice formation in metering or expansion devices. Ingress of air into the pipework during installation should, therefore, be kept to a minimum. Just as there is a relationship between refrigerant pressure and temperature, the same relationships exist between pressure and the boiling point of water - i.e. at atmospheric pressure, the boiling point of water is 100°C, as the pressure reduces the boiling point of the water also reduces.
The picture below is an example of moisture damage-causing copper plating.
Pipework should be capped as much as possible during installation; the use of oxygen-free nitrogen (OFN). Purging during installation also reduces the amount of air introduced and therefore also moisture inside the pipework as well as preventing carbon deposits forming during brazing.
During installation, there will always be points where pipework is exposed to atmosphere i.e. cutting, fitting, etc.
Evacuation / Moisture removal method
Once you have proved the integrity of the pipework by carrying out a strength and tightness test using OFN in line with BS EN378:2016 (and/ or manufacturer's instructions), you need to evacuate the pipework to remove moisture and air from the system.
It is generally a good practice to evacuate to 2 Torr or manufacturers' instructions level to remove air and moisture in a system.
However, there may be circumstances when you need to carry out a calculation to ensure that you have achieved an effective vacuum for that system by following the process below:
1. First, measure the air temperature surrounding the refrigerant pipework both internally and externally, make a note of the coldest temperature measurement (usually outside), connect your gauges and vac pump as normal and set it to run.
2. As pressure starts to drop inside the pipework, so does the boiling point of water. During the evacuation, the pressure will reduce efficiently to allow any moisture inside the pipework to turn into steam and be drawn out by the vacuum pump.
3. In order to achieve moisture removal from the pipework, the pressure reading on the vacuum gauge must be lower than the boiling point of water from the coldest ambient temperature reading you took earlier.
You can use the table below to work out the vacuum pressure you need to achieve moisture removal. For example, at 10°C ambient, you would not remove any moisture until a vacuum of 9.3 Torr is achieved.
4. Moisture removal will only start once a vacuum below the equivalent temperature in the table level is achieved.
5. With the vacuum below the table figure, you need to periodically isolate the vacuum pump from the pipework that the vacuum is connected to - the service gauges are the easiest point. Check if the pressure increases, this is an indication of moisture still being present. The moisture will evaporate and expand inside the pipework resulting in loss of vacuum. In this case, continue evacuating the system and then check again periodically until the vacuum holds.
6. If after completing the above there is still a pressure increase, the pipework should be pressurized to 10psig (0.7 barg) using OFN. The nitrogen will aid moisture absorption which can then be safely vented. Before continuing with evacuation, it is good practice to change the oil in the vacuum pump. Once the oil has been changed, continue evacuation using the vacuum pump as many times as required to dehydrate the system.
7. There is no set time limit for dehydration, you have successfully removed moisture from the system when the system holds a vacuum. At this point, it is safe to introduce refrigerant and start the commissioning process.