OILS and Refrigeration Controls


4-OILS :
There are 3 types of oil generally used in Refrigeration Systems. You need to know why it is
important to have the correct oil for the refrigerant that is in the system. The main reason is
that in all cooling systems, oil circulates in the system. With the introduction of the new
refrigerants, getting the oil to return to the compressor was a problem simply because the new
refrigerant and the old oils would not mix. Having said that, the only reason to match the oil to
the refrigerants is to get it to return to the compressor.
The first and oldest oil is Mineral Oil. This oil has a good lubrication property and was great with
old refrigerant (those with chlorine). This oil would foam somewhat so the introduction of
Alkybenzene, a synthetic oil, was introduced. It has the same lubrication qualities as Mineral Oil
but would not foam aiding in the lubrication factor. These oils will both mix with one another
and not cause a problem with the system. In fact, some manufacturers found that a blend of up
to 50 – 50 mix of these oils would give better lubrications while keeping foaming to a minimum
would actually quiet the Compressors operation.
However, these oils should only be used with the old CFC refrigerants such as R-12, R-22, R-502
and their drop end replacements. NEVER use it in the new HFC Refrigerants such as R134A,
R404A, R410A, etc.
The last and newest oil is Poly Ester or POE. This oil can be used with most all refrigerants, both
new and old oil. However, I have many reservations and would only recommend using it in new
systems because this oil is made up of alcohol and acid. In plain English, alcohol and soap! It will
take an old system and clean it like Mister Clean. Soon the oil becomes jet black with
contaminants and must be changed more than once. Also, it will attack any rubber seal and O
Ring that is not compatible causing leaks which may not be easily repaired.
Therefore, it is advisable to use the Drop in Refrigerants in older systems with Mineral or
Alkybenzene oils, POE oils should only be used in new systems with R134A, R404A etc.

This can become a problem in many systems. When any refrigerant runs a low head pressure
there is not enough pressure to force the oil through the cold evaporator so it accumulates
thus robbing the system of BTU’s and then coming back all at once resulting in a damaged
The key is to regulate the head pressure with a fan cycle switch or a head master valve. My
rule of thumb is to have a warm liquid line. This will get your oil back as long as the
Evaporator fans are doing their job.
A second consideration is adding a P trap. P Traps will also aid in getting oil to return. These
are needed when the condensing unit is installed higher than 12’ above the evaporator.
Refrigeration Controls

-5 Refrigeration Controls:
There are only a few controls on a normal Refrigeration system. To be a good service technician
you must know What they are, What they do and When they do it!! You need to know this in
order to know where to set them with different Refrigeration and Box Temperatures.
The most common control is the Thermostat. Sounds simple, but does it just cut the
compressor off? Or does it cut a solenoid valve off and the Pressure Control cuts the
compressor off after the system pumps down? Does it wire through a time clock or does it have
a standing defrost cycle where if you set it too cold it will ice the Box up? Does the thermostat
have an adjustable differential so you can spread the off cycle so it will keep the box off long
enough to defrost?
Next is the Pressure Controls. This is usually a Dual Pressure Control where both high and low
temperatures are read. The high side part of the control is a safety control in cases of a dirty
condenser, a bad fan motor, over charged system and several other reasons that could occur. In
those cases, you would want the control to switch the unit off in the case of high discharge
Pressure that can damage the valves or head gaskets of the compressor.
The other side, or Low Pressure side of this control, also serves as a safety device that shuts the
compressor off in the case of a loss of refrigerant. You always want the control to do this
without letting the system run into a vacuum. The reason for this is that in a vacuum air will be
sucked in introducing moisture into the system. Moisture is the #1 enemy of a refrigeration
system. A good rule of thumb to remember is to set this control off at 2 lbs of pressure and on
at 30lbs of pressure. This setting will also work fine for a pump down off cycle.
The Oil Safety Control is the least understood control in the system because it normally has
three wires and everyone panics. The two wires on the marked terminals L and M are only
Control Circuit wires meaning they are usually the same wires that go to or from the low, high
pressure control. They just series through the oil control again on terminals L & M. This also
powers one side of the oil switch. The third wire is the one no one seems to understand what to
do with. This wire is either on the 120V terminal or On the 230V terminal depending on the
control voltage of the unit. The tricky part is that this wire MUST BE wired to the LOAD side of
the contactor with the wires that feed the compressor (230V only) or through a Auxiliary
Contact or Relay because it must shut off power to the Oil Control when the compressor is not
The oil safety control usually has a 9lb differential. This means that the oil pressure must be
9lbs over the suction pressure when the compressor is operating. Your oil pressure is always
Pump Pressure minus Suction Pressure and it has a 120 second delay usually before the
control will shut the system down. All Oil Controls are a manual reset so as not to damage the
I have found that a minimum of 20lbs of oil pressure is needed to have a dependable system.
Less that 20lbs and you will find you will have a lot of nuisance service calls to rest this control.

I have categorized oil safety trips into three categories:

1. Lubrication problem such as a bad oil pump, low oil and other problems internal to
the compressor
2. Zero Super Heat or at the same time in the cycle a low load or an out of adjusted
TEV or Icing up accrues during the cycle flooding the compressor with liquid
refrigerant which thins the oil to the point where the Oil pressure is below 9 lbs of
suction pressure. This is the number 1 cause of compressor failure.
3. Less common and only found in compressors with line voltage over loads usually
inside the compressor. When it opens to stop the compressor, the power is still to
the compressor but because it is not running no oil pressure is produced and the oil
safety is the next thing to trip. Few OEM’s install a current sensing relay that can
stop this from happening by reading amp draw of the compressor and dropping out
the power supplied to 120 or 230 terminals of the oil switch.
The last two trip types can be very frustrating because by the time you get to the site and reset
the oil switch the liquid has boiled out of the oil or the compressor has cooled off and
everything is running fine with plenty of oil pressure.
Always look for these causes and remember “Every time you reset an oil safety switch is one
compressor you didn’t have to change”.

                                 abo bahaa eddine


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