Tag: refrigerant oil

First a quick summary of the role of oil in the refrigerant circuit –

The compressor requires oil for lubrication of the moving parts in the compressor. If we could, we would keep 100% of the oil in the compressor but since that is generally unrealistic we need to utilize oils and oil strategies that will circulate the oil through the system and return it back to the compressor where it belongs on a regular and continuous basis.

There are components called oil separators that can strip most of the oil from the discharge gas and return the oil to the compressor, these are often used on larger systems and they are still less than 100% effective by themselves.

In VERY large systems such as chillers, we are beginning to see oilless technologies with magnetic bearings like TurboCor from Danfoss (shown above), but these are still pretty rare in the field.

So we are left with circulating oil through the system and returning it to the compressor as a regular part of A/C and refrigeration system operation under normal running conditions.

First, let’s cover the oil considerations a service tech can easily diagnose and impact.

Technician Considerations

We are tasked with preventing liquid refrigerant from entering the compressor which can cause more rapid and potentially catastrophic oil loss. This is called “flooding” and it can occur while the system is running when the refrigerant superheat is allowed to stay at zero as it enters the compressor which indicates the presence of liquid refrigerant mixed with the suction vapor.

A flooded start is flooding that occurs during startup when liquid refrigerant was allowed to collect in the compressor, in the suction line or even in the evaporator. Both of these conditions can cause oil loss from the compressor as well as oil dilution which can result in rapid compressor wear.

Preventing flooding is a significant part of oil management and involves setting superheat properly and using other strategies such as crankcase heaters, non-bleed expansion valves and pump down on the off cycle to help keep liquid refrigerant out of the compressor.

Another factor in lubrication is oil breakdown that can occur at high temperatures. We should consistently monitor discharge temperatures exiting the compressor to ensure it doesn’t exceed 225° which equates to around 300° at the compressor discharge valves (on a reciprocating compressor). This helps to ensure that the oil doesn’t break down and “carbonize”. Now, this does vary based on the compressor type, system and oil type, but is a generally accepted rule in the absence of a more detailed guideline. On a properly functioning compressor, the mass flow rate (amount of refrigerant moving through the compressor) and the suction gas temperature are the primary factors that impact compressor discharge temperature. Often high discharge temperatures occur when the suction pressure is low, superheat is low or compression ratio is high (high head pressure, low suction pressure) or some combination of these issues.

Oil Return 

Once the oil has left the compressor it must circulate through the system and return to the compressor crankcase and there are a few key factors that impact oil return-

  1.  Oil/refrigerant miscibility (how well the oil mixes and moves with the refrigerant)
  2. Oil Viscosity (oil thickness)
  3. Refrigerant velocity throughout the circuit

The oil being utilized should be suited to the refrigerant type and of the proper viscosity for the compressor and the temperature application. Refrigerant velocity should be maintained according to manufacturer recommendations and low velocity will primarily be an issue in evaporator coils and suction lines when the suction pressure is lower than design due to improper tubing sizing, low evaporator load, metering device underfeeding or undercharge.

Oil Quantity 

Keep in mind that the longer the refrigerant lines are, the larger the evaporator(s) the more oil will be out in the “circuit” and the more total oil the system will need to contain. Technicians who work in “built up” systems like market refrigeration are very aware of this and take an active approach to manage oil. Residential and light commercial HVAC techs may take the approach that it’s the “manufacturers job” to ship the system with the correct amount of oil but may fail to read long line guidelines that call for more oil to be added.

On the other hand, too much oil can also lead to compressor issues and poor system performance. This often occurs when a new compressor is installed with a new oil charge on a system that previously had oil return issues. The new compressor will only add more oil to the circuit making the situation worse and again leading to a reoccurring failure. This is why diagnosing fully and finding WHY the old compressor failed is a huge part of the process so that you can make some oil adjustments if an oil return issue was found and rectified.

 

Important Oil Terms

Miscibility – the ability of the oil to mix with and move with the refrigerant.

Viscosity – a measure of the oil’s flow resistance (how thick it is). Two units of measure are used with refrigeration oil. The older measure is Saybolt Universal Seconds (SUS), the newer is ISO viscosity grade number (ISO VG). In both cases a higher number is a thicker oil, just don’t mix up the two standards.

Hygroscopic – Many modern oils are hygroscopic which means they attract and hold moisture. It is very important to keep moisture away from hygroscopic oils to keep them from becoming contaminated.

Hydrolysis – decomposition due to a reaction with water. For example, POE oil decomposes into acids and alcohol in the presence of water which means that the once it decomposes it cannot be reconstituted with line driers or evacuation.

Oil Types  

Mineral – is a product of gasoline production. Naphthene based mineral oils are suitable for refrigeration systems using CFC or HCFC refrigerants and has been the standard oil used for generations. Mineral oil worked well with refrigerants that contained Chlorine but is not miscible with modern HFC and HFO refrigerants.

Alkylbenzene (AB) – a synthetic oil suitable for refrigeration systems using CFC or HCFC refrigerants. It is compatible with mineral oil and compared to mineral oil, it has improved refrigerant miscibility at low-temperature conditions which is why it was and is often used with HCFC refrigerants in commercial refrigeration.

Polyolester (POE) – The most common oil utilized in refrigeration and air conditioning systems using HFC / HFO refrigerants. It is also suitable for systems using CFC, HCFC refrigerants.

Polyvinyl Ether (PVE) – a synthetic oil that is being used as an alternative to POE oil and is very common in ductless and VRF. It is more hygroscopic than POE oil, but PVE oil does not undergo hydrolysis in the presence of water. This means that while PVE will grab water more easily it is capable or being dehydrated again unlike POE.

Polyalkylene Glycol (PAG) – a synthetic oil primarily used in automotive air conditioning systems. It is more hygroscopic that either POE or PVE oils, but like PVE it does not undergo hydrolysis in the presence of water.

Refrigerant Piping for Oil Return 

When oil does not return properly to the compressor it can cause compressor wear but it can also decrease system performance by coating the inside of the evaporator tubing walls and inhibiting heat transfer and can even cause restrictions.

Especially with mineral and AB oils it was very important to employ proper trapping strategies according to the manufacturers and industry guidelines such as THIS ONE FROM RSES 

with newer oils like POE and PVE these trapping and oil return strategies have become less critical in high and medium temperature applications due to the strong miscibility of the oil in the refrigerant. As always, read and follow manufacturers piping guidelines as the lower the velocity the more likely the oil is to have issues returning especially in retrofit applications where some Mineral or AB may still be present with the POE or PVE oils.

One good practice to use when running long runs of horizontal piping is to pitch it back towards the compressors. This is a common-sense practice no matter the oil, refrigerant or application whenever possible.

Oil Mixing 

When POE oils first gained common use it was widely rumored that mixing POE and mineral oil would result in “sludge”. This has been proven to be a myth, and to some extent manufacturers of retrofit, refrigerants have been suggesting adding small amounts of POE to mineral oil to help carry it through the system. It is always better to move to POE or PVE oils from mineral when retrofitting to an HFC refrigerant but small amounts of mineral oil have proven to be rather inconsequential in most cases.

New Oil Considerations

One thing that has become clear with the advent of POE oil is the importance of proper brazing practices (flowing nitrogen), proper deep evacuation and keeping the oil away from air and moisture during storage. Many poor practices that techs could get away with when CFC/HCFC and mineral oil were in common use can result in DISASTER with modern refrigerants and oils.

Keep the system clean and dry and use the correct oil in the correct amounts. Keep the oil from overheating and keep the compressor from “throwing” oil by preventing flooding. Maintain proper oil return through proper pipe sizing, pitching and trapping (as required) and by maintaining appropriate deisgn velocity of the refrigerant.

Easy..

— Bryan

 

 

 

 

This is a subject that even many commercial guys don’t have to consider.  For the majority of equipment, even refrigeration equipment, all that is required for proper oil return is to size the suction line properly, trap the suction line as needed, and allow for proper slope towards the compressor.

 

Then we get into larger equipment.   Due to what can be extreme swings in load that result in wide swings in suction line velocity, oil return isn’t always what we’d like to see, even with proper trapping and line slope, so rather than allowing that oil to load up the evaporators and affect heat transfer among other problems an oil logged evaporator can cause, we install systems to prevent the oil from ever leaving the mechanical room.

 

I’ll try to lay this out in a step-by-step manner, adding layers of complexity as needed.

 

Since oil will be entrained (mixed / carried) in the discharge gas leaving the compressors, we’ll first want to install an oil separator in the discharge line to capture this oil, then we’ll work on managing its return to the compressor crankcase where it belongs.

 

That’s step one:  separating any oil from the discharge gas leaving the compressor.   There are 3 basic methods used for this (in order of effectiveness)

 

Impingement .  In this method, all of the discharge gas passes through a screen where oil vapor gathers into larger droplets and drips off into a vessel where we can deal with it later.

 

Helical.   In this method, the discharge gas enters the vessel at an angle and swirls around plates within the vessel.   The oil droplets entrained in the discharge gas, being heavier than the vapor itself, are flung outward and hit these plates and drain to the bottom of the vessel as before.

 

Coalescing .  Here, the discharge gas is forced through a filter where the oil droplets are captured and, again, drain to the bottom of a vessel.

 

Now that we’ve captured the oil, half of the job is done.   We’ve prevented it from going out into the system, now we basically have a bucket full of oil under discharge pressure we’ve got to manage.

 

One thing to remember is that oil tends to accumulate the worst of the garbage in the system, so a quality oil filter is necessary.   To prevent problems with clogging fine orifices, needle valves and pressure regulators we’ll encounter in our oil management systems, that filter should be installed as close to the oil separator as possible.

 

Things start to get interesting from here, so I’m going to try to explore the simplest methods first and dig into more and more complicated oil management strategies as we build an understanding.

 

Probably the simplest management strategy is one of the most modern ones.  A direct oil level management system.  An electronic float at each compressor monitors level in each crankcase and, as that compressor pumps out the small amount of oil it normally pumps, the electronics package energizes a solenoid valve to let that oil back into the crankcase.  There will typically be a small orifice within this valve so that feed happens rather slowly but fast enough to prevent the level from dropping low enough to cause a problem.

 

Most equipment that is out there however, isn’t quite so simple, direct and easy to understand.

 

None of the other systems use electronics to manage oil flow, so from here on out, all controls are mechanical.

The next type of system uses mechanical float-type regulators bolted to each compressor to monitor the oil level in the crankcase.  As before, when the level drops, the regulator needs to add oil back into the compressor.   Much like a toilet tank or other float controlled device, the float opens a needle valve to allow oil into the regulator . The actual oil level within the regulator is adjustable within a fairly narrow range.

 

For this control to regulate properly, we need to reduce the oil pressure to a safe level. If we fed these regulators oil at discharge pressure, the high-pressure differential would force the small needle valve inside open and allow the regulator to overfeed and overfill the compressor.   Instead, we install a valve between the separator where the oil is at discharge pressure and the regulators on each compressor to reduce the pressure down to typically about 20# above crankcase pressure.

 

Adding a couple layers of complexity to the system,  we arrive at what is probably the most common type of oil system in use on parallel refrigeration equipment today.

 

Oil drains to the bottom of the separator vessel, as the oil level rises there, it opens a float valve. Oil passes through the float valve into a reservoir tank.   The reservoir tank serves two purposes.   First, it simply holds the oil until it’s needed.  Second, through a special check valve installed between this reservoir and the suction header, the oil pressure is lowered to that same 20# above suction pressure figure.  These check valves are available in different pressure differential settings, but 20# is the most common.

 

From the reservoir to the compressor, the system is the same.   An oil line sends oil from the reservoir out to the mechanical float devices that control the level of oil in each compressor.

One other common feature in oil management systems like this is an equalizing line.   We all understand that 2 containers of any liquid will have the same level in them due to the self-leveling nature of liquids.  This equalizing line, in theory, connects the crankcases of all compressors together to create a self-leveling system.   It doesn’t always work quite as well as hoped for because there can be different pressures in the crankcase of a running and non-running compressor.  We’ll dive a little deeper into that as we move into troubleshooting oil problems.

 

Troubleshooting

 

While these systems seem complicated, and they have a lot of moving parts that can fail, they really boil down to oil level and pressure differential.   We need to maintain a level of oil in the compressors and in the reservoir or separator and maintain enough pressure differential to keep that oil moving.   Lose one or the other and you’re not going to stay running for long.

 

Let’s kind of walk step-by-step through a troubleshooting process until we find or eliminate all problems.   First, I look at all compressor levels and reservoir level.   If I’ve got a lot of oil in the compressors, I want to check equalization lines.  If they’re all consistently low, I’m going to start looking at the oil management system.

 

To evaluate the oil management system, start by checking the temperature leaving the oil separator.   The line leaving the separator should be warm to the touch (100F).  I like to put a thermometer that logs min/max temps and observe it for 10 or 15 minutes.  You’ll see the temperature climb and drop as the oil float inside feeds.  No feed?  Time to consider the float inside as a problem.

Next, let’s check the pressure in the reservoir or the outlet pressure of the unit’s pressure reducing valve against suction pressure.   20# above and we’ve got level in the reservoir?  OK. No level in the reservoir?  Lets try to find that oil before we go adding oil.   A system with too much oil can be as problematic as one with too little oil… If we don’t have differential in the reservoir, I’ll isolate the inlet and outlet of the reservoir and bleed some hot gas from discharge into the reservoir with my gauges to see if the differential check is faulty.   Since we’ve already demonstrated that the separator is feeding, we need to see if the differential check valve has failed.

 

Next step for me is to start checking each individual oil level regulator.   I’ll normally uncap the adjustment stem, turn it CCW to the top stop, counting the turns then adjust it down (CW) to a midpoint which is typically 5 turns.  If any are wildly out of adjustment, I single that compressor out for some additional attention.  It is very important to not adjust these controls more than 10 turns from the top stop.   The adjustment range is limited and adjusting beyond that limit will ruin the control, regardless of its condition before you worked on it.

I mentioned crankcase pressure earlier, and this is an issue that can be problematic with oil issues.   As the rings wear in a compressor, we can see some discharge blowby into the crankcase.  Not enough to warrant replacement of the compressor necessarily, but enough to sometimes cause issues with oil.   First, if we put additional pressure into one compressor, that unbalances the oil equalization system by pushing oil in unwanted directions.  Second, by increasing the pressure in the crankcase over the suction pressure, we reduce the net oil feed pressure, slowing the oil feed rate.

 

To check a compressor for pressurized crankcase, install a gauge on both the suction port and the crankcase.   If the pressure within the crankcase is more than about 2# higher than suction, you may have some problems.

 

A few other, kind of random thoughts on oil failure trips and trouble.

 

These are 3 phase motors.   A contactor with severely pitted points or a contact that doesn’t make good contact can cause a temporary single phase, prevent the compressor from running and create a situation where the oil control is energized but no pressure is created by the compressor.  Always check the contactor.

 

Screens and filters.   Since the oil system tends to collect all of the garbage in the system, oil systems tend to have a high concentration of filters and strainer screens.   From the impingement screen and coalescing element in a coalescing separator, screens are a huge problem.   Add to that the screens, an oil filter, the float at the bottom of the separator, the pressure reducing valve, the screens and valves in each individual oil level control and, often an oil pickup screen in the compressor itself and there are many points that can become obstructed by the debris in an oil system.   Regular oil filter maintenance is important for a reliable system.

— Jeremy Smith CM

P.S. – Henry Technologies has compiled everything that you can possibly need to know into a handy manual and, most importantly, a quick-reference chart with some basic diagnostic readings and measurements to take HERE

 

 

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