By Grant Laidlaw
A common problem in the industry is that technicians are not evacuating systems and this results in the consequence of compressor failures.
Refilwe asks: Hi Grant, we have run into a problem with compressor failures on the increase. Our suppliers are giving the reason as incorrect installation procedures – in particular due to no vacuum having been drawn and moisture in the system. Why do they say this? What harm can a little moisture really do?
Hi Refilwe. The purpose of using a vacuum pump on a refrigeration system is to remove air (non-condensables), and any moisture from the system. This is very important as we shall see
I think firstly let us consider how moisture enters the system, and this may happen in many ways:
- Moisture can enter the system if charge lines are not purged.
- In a new installation, moisture could remain in a pressure vessel as part of the certification tests.
- Usage of refrigerant that is contaminated with moisture.
- Moisture could enter the system prior to evacuation, possibly when the system is open for repair.
- Usage of unsealed refrigerant tubing.
- Oils used in refrigeration systems are hydroscopic and can easily become contaminated with moisture, therefore charging a system with contaminated oil will add moisture to the system.
- On indirect systems: burst tubing in shell and tube heat exchangers.
- Finally, with air and moisture entering the refrigeration system, chemical reactions begin to break down the refrigerant, this can produce more water and acids; a very undesirable situation.
As a starting point, determine how moisture entered the system? Take the necessary remedial action.
There are two main areas of concern when moisture enters the system:
- It may freeze in expansion devices, causing partial blockage and system malfunction.
- It sets up chemical reactions which damage the system.
The first area of concern is moisture freezing in the expansion device. This is an issue for any system but arguably has a far greater impact for low temperature and smaller refrigeration systems. For example: in the case of a system utilising capillary tubing as an expansion device and running evaporator temperatures below zero degrees Celsius, moisture moving through the system can freeze in the capillary tube causing a blockage in the system.
Crankshaft journal with copper plating. Image credit: ACRA
Typically, these systems are controlled by a thermostat. When the system is no longer cooling, the thermostat will sense the increasing cabinet temperature and supply power to the compressor. The compressor now runs against the blockage causing high side pressures to increase.
As the pressure increases the compressor begins to draw higher amperage to the point where the overload kicks out and cuts off the power. The overload then gradually cools down and within a few minutes, resets and supplies power to the compressor.
The compressor now attempts to start but the blockage still exists. Whilst struggling to start the compressor again draws very high amperage causing the overload to activate again. This cycle will repeat itself, each time causing extreme strain for the compressor motor and ultimately failure will occur. All this for one drop of moisture.
Moving forward to the second area of concern, the chemical reactions and consequences thereof.
The chemical reactions are complex and dependent on many variables but what we do need to consider is that these reactions result in the formation of acids in the refrigeration system.
A chemical reaction can occur between the moisture, oil and refrigerant, bringing about a decomposition which results in hydrochloric and hydrofluoric acids being produced. These acids create a host of issues.
We know that it is almost certain that if acid formation has commenced, there will be some air or oxygen present in the system. This will allow corrosion to take place. The first issue is that products of corrosion will break away or be flushed away by the oil in circulation and will start forming a ‘grinding paste’ in the oil. This results in accelerated wear in the machine.
Refilwe, do you see that when you return a compressor to a supplier and they find acids, the finding will be that there was moisture and or air in the system indicating poor installation procedure and inadequate evacuation?
Manifold charging line not secured, loose on ground. Image credit: ACRA
The next aspect is copper plating. The crankshaft journal pictured below is not made of copper but has been copper plated in the system.Copper plating is a situation that arises when moisture and acids are present in a system which is operating at high temperature. Copper is dissolved from copper and brass components, and copper plates out as a layer of copper onto rubbing metal surfaces and steel or iron surfaces.
As the plating gets thicker clearances are reduced. Oil clearances being reduced creates lubrication issues. Friction then increases which results in the compressor starting to run hot. In addition, the copper plating may tear away in the system. These pieces of copper will easily cause more damage.
Copper plating has been known to seize compressors and to lock up oil pumps, causing their drive shafts to shear.
Yet another aspect that we need to consider is varnish and sludge formation. As we have increased temperatures caused by increased friction and possibly not condensable, these increases in temperatures can cause the formation of sludges and varnishes in the oil. Copper flakes from copper planting processes, corrosion products from system metals which have undergone corrosive attack, and fine metal particles torn from bearing surfaces all form a rough abrasive paste in the system.
Aside from increased wear, this can block screens and strainers and reduce lubrication to the point where oil is not getting to essential areas of the machine. This could be the cause of total failure.
Normal cooling of the motor of a hermetic compressor is by flow of cooler suction vapour across the windings. If the windings are coated with fouling materials, this too can result in a temperature increase.
Increased temperatures can cause carbonisation, in particular – at the hottest part of the system – the compressor discharge valves. A film of oil, being discharged with the hot gas continually coats these valves as it moves through the system. Excessive heat will break down the oil at the valves causing carbon deposits to form.
The carbon will build on the valves, preventing them from seating properly. Hot gas will move back past the valves on the down stroke of the piston reducing system capacity. Additionally, the stressed discharge valves can break and lodge in the minimal clearance space between the piston crown and cylinder head. This will, of course, damage the piston and valve plate and result in system failure.
All systems should have filter driers in their liquid lines. But never depend upon the filter drier to remove moisture which you allowed to enter into the system. That constitutes poor practice. View the drier rather as a backup should some moisture enter the system.
Refilwe, as you can see, moisture entering a refrigeration system is to be avoided as it can result in total system failure. Most of the time the entire situation can be avoided by following good practice.
Use a vacuum pump and follow all the correct evacuation procedures:
When using manifold gauge sets, purge the lines to remove air and moisture and when not in use do not allow the charging lines to hang loose, dragging on the ground. Seal/secure the loose ends by attaching them to the manifold blank off fittings and keep moisture and contaminants out.
Seal/reseal tubing ends and components.
Manufacturers dehydrate refrigerant tubing, keep sealed and reseal unused portions.
Keep refrigerant oil containers sealed until needed, do not leave open as the oil is hydroscopic and will attract moisture from the ambient air.
Refilwe, air in a refrigeration system is also very undesirable. Air may have entered the system during installation or it could have been introduced by poor service practice. Regardless, the air should have been removed through proper evacuation procedures.
Sealed tubing ends. Image credit: ACRA
Dalton’s Law tells us that when gases share a space, each behaves independently of the other, but their partial pressure are summed to provide the total pressure of the system. That having been said, it means that the condensing pressure will be further increased for any combination of load and condenser water or air temperature if air is present in the system.
As the air in the system causes increased discharge pressures the compressor now has to continuously work against the higher discharge pressure. This means that it will continuously draw more electrical power, and its working temperature will also be higher.
The higher working temperature can be damaging to the refrigerant and oil as it can cause carbonisation of oil and can start chemical reactions and breakdowns between these fluids.
As air cannot be condensed in a normal refrigerant plant, air is commonly termed a ‘non-condensable’.
In the system, the air moves into the condenser with the hot gas, becoming trapped there. The result is that all of air which may be present in the system is concentrated in the condenser. The presence of the air reduces the capacity of the condenser which further increases discharge temperatures.
Of course, we now know that this this may cause carbon build up on the compressor discharge valves with all of the consequences previously mentioned. As there is no such thing in nature as air which is moisture free, any problem of air in a system will mean there will also be a problem with moisture.
Refilwe, as you can see a little bit of moisture can in fact cause quite a problem. I hope that this assists you with your compressor failure rates.
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