Refrigerant and the Compressor

Refrigerant and the Compressor

Every day technicians put their gauges on air conditioners and measure the high and low side pressures. Why do they do that? We know that we need the pressure information to make decisions about how well the unit is running and if there is anything we need to do to help it run better.  But did you ever stop and think about why you measure those pressures, and exactly which pressures you are measuring?

There are only few reasons to care about the refrigerant pressures in the system. One is to estimate the saturation temperature of the refrigerant.  This is because most of the calculations we make to determine the operational state of the refrigeration cycle compares the saturation temperature to the pipe temperatures or the ambient temperature. We need to measure the high and low side pressures in order to know the saturation temperatures, however once we have that, the measurements, calculations and rules will be in temperature and the pressure information has little further use.  There is another slight complication, the high-side pressure we need is the liquid pressure.  Sometimes in package equipment there may not be a convenient liquid port and you will have to use discharge pressure.

Important- The liquid pressure and the discharge pressure are not the same.  There is a pressure drop across the condenser coil.  When you have discharge pressure, you have to estimate the condenser pressure drop to arrive at an approximation of liquid pressure.  This guess will add an error to the subcooling estimate.  That error may be small or it may be great but it will be there.  We will discuss this further when discussing the condenser.

Saturation Temperature

What is saturation temperature?  It’s the temperature a pure stable mixture of vapor and liquid refrigerant will be at a given pressure. That relationship is documented on the TP chart that is used to characterize the refrigerants.  In an air conditioner, the low side saturation temperature is called the evaporating temperature. It can be thought of as the temperature at which the liquid refrigerant evaporates into a vapor. The high side saturation temperature is called the condensing temperature and it’s the temperature at which the refrigerant vapor returns to its liquid state.

The Temperature-Pressure Chart

What does the TP chart tell us? The T-P chart tells us the saturation temperature of the various refrigerants over the range of refrigerant pressures the technician is likely to see. You will often find temperature scales printed on the refrigerant gauge face; those are a version of the T-P chart.

What does saturation mean?  It means that there is both liquid and vapor present. It can be thought of as the temperature at which the refrigerant is changing state in an air conditioner. The refrigerant is flashing off at the evaporating temperature deduced from the low side pressure. And the vapor condenses back into a liquid at the condensing temperature that is derived from the liquid pressure.

What other criteria must be met for the T-P chart to be reliable? Aside from the presence of both liquid and vapor refrigerant together, the system must be at a stable temperature and contain it must contain pure refrigerant. Nearly all substances that contaminate refrigerant will have a higher vapor pressure than pure refrigerant, so the pressure of contaminated refrigerant will nearly always be higher than the T-P chart’s value at a given temperature.

Again, saturation temperature is interesting because the calculations we will make to determine whether or not the unit is running correctly have us comparing the saturation temperature to the pipe temperatures and to the ambient temperature. There are no pressures in any of these calculations.

For more information about using T-P charts, see:

http://www.achrnews.com/articles/print/using-the-p-t-card-as-a-service-tool-1

Thinking in Temperature

I found that when I stopped thinking in terms of head and suction pressures, and started thinking in evaporating temperature and condensing temperature, I started to “get“ the refrigeration cycle. I never really did before that. One advantage of thinking in temperature becomes clear when working with different refrigerants.  There is no important difference between a 40°F evaporator with R22 and a 40°F evaporator with R410A, or any other refrigerant, other than the trivia of what the pressure measurement is.

What is Superheat?

Superheat is any temperature over the saturation temperature. It means that if we are measuring a suction pressure that converts on the T-P chart to a 40°F evaporator and the temperature of our suction line near the compressor is 60°F then we have 20°F of total superheat entering the compressor. Superheat is how much warmer the refrigerant is than when it evaporated.  When you have sufficient superheat, you expect that the refrigerant entering the compressor is all vapor. That’s important because when liquid refrigerant enters the compressor, it can damage it.

Why Do We Care About Superheat?

In comfort cooling, the primary reason to care about superheat is to protect the compressor from harm.  High and low superheat leads directly to premature compressor failures.  It’s not possible to predict how long a compressor with high or low superheat will run, but if you keep your superheat at an acceptable level, your rate of compressor failures will be reduced.

Causes of Compressor Failure

Let’s review the ways that compressors can fail. There are mechanical failures and there are electrical failures. Mechanical failures are inefficient compressors and locked rotor compressors. Electrical failures include open and shorted motor windings, with and without acid. There are other types of compressor failures (an example might be leaking motor terminals) but these are, by far, the most common ways compressors fail. Did you know that the manufacturers tell us that between a third and a half of all compressors returned under warranty have no detectable fault? Those are big numbers.  It shows us that technicians, on average are not perfect at condemning compressors.

Locked Rotor Compressors

A common problem diagnosis is a locked rotor compressor.  What would you do to diagnose a locked rotor compressor? It would probably start with a no-cooling call, a locked rotor compressor won’t start and run when it’s energized and it gets hot and it hums. Measure the current drawn by the compressor with a current meter and if it is very high, many technicians would assume they have a locked rotor compressor.

But does it prove you have a locked rotor compressor? No, what you really have to do is to see what the LRA (locked rotor amperage) rating is if you suspect a locked rotor compressor. The compressor nameplate says what the LRA is, that is the current that would be drawn by the compressor if it has a locked rotor. If we are measuring LRA with a current meter, we have a locked rotor compressor.  If not, there is probably another problem.

What if the compressor is not pumping, it’s getting hot, and it’s pulling a lot of current but less that LRA? If it’s a three-phase compressor, it might be single phasing and if it’s a single phase compressor it might have a bad capacitor or potential relay.

It’s important to be careful when condemning a compressor. On 3 phase I check current on all three legs, if one is pulling close to zero amps, it single phasing.  You may also check for voltage on all three phases at the compressor motor terminals before condemning a compressor as locked rotor. It might make more sense to replace a contactor or a wire or a fuse instead of the compressor.  On single phase equipment, checking the capacitor and starting components and the wiring to the compressor makes sense.

It’s important to know what causes compressors to fail because it can help you make a more accurate diagnosis and because, if you know the kinds of things that cause particular compressor failures you can take steps to make sure the new compressor doesn't fail the same way.

What is a Locked Rotor Compressor? 

The rotor is the part of the compressor/motor pair that turns. A locked rotor compressor would be one where those parts will not turn.

What Caused Locked Rotor Compressors? 

“Slugging” or the introduction of liquid refrigerant into the crankcase of the compressor is a common cause of locked rotor compressors. To think of it another way, it is often caused by compressors being forced to run with low superheat.

Liquid refrigerant is a highly effective degreasing solvent. When liquid refrigerant is in the crankcase of a compressor, it dissolves the oil. When the oil pump picks up the refrigerant-laden oil, sometimes the heat of compression in the pump is enough to flash off the liquid. That creates a bubble of gas that forces the oil down the dip tube and starves the pump for oil. The main bearings then run dry until they seize from lack of lubrication.

Another scenario is when the liquid refrigerant is actually pumped into the bearings. There it cleans away any lubrication and the compressor locks up. In either case, low superheat is the cause.

Inefficient compressors

What is an inefficient compressor? Generally when we think about efficiency we think about the capacity of something over the power used to achieve that capacity. That is not the kind of efficiency we are talking about when calling a compressor inefficient. A compressor is a pump. When engineers think about the compressor in a refrigeration cycle design, they are thinking about Cubic Feet per Minute (CFM) of pumping capacity. That capacity is equal to the volume of the cylinders multiplied by the number of cylinders working together further multiplied by the number of times the motor turns the crankshaft each minute. When the compressor delivers fewer CFM in pumping capacity than it is rated for it is considered volumetrically inefficient. Technicians are quick point out that volumetric inefficiency is the result of broken valves, pistons and piston rods.

What causes an inefficient compressor? 

Liquid refrigerant that enters the crankcase can get pulled into the cylinder heads along with the vapor. Liquid refrigerant is uncompressible in refrigeration compressors; the materials that the valves and pistons are made of were not designed to stand up to the kind of force needed to compress a liquid. When they are subjected to that condition they fail and break into pieces. A compressor without its pistons or valves pumps less than it is rated to with all its pistons and valves intact.

Liquid refrigerant in the compressor causes inefficient compressors, low superheat risks liquid refrigerant entering the compressor.

Open or Shorted With Acid

Electrical failures in a compressor are often caused by mechanical failures in that compressor, but without an intact motor there is no way to see it.

What Causes Electrical Failures With Acid? 

A common answer from technicians is that moisture in the oil causes acid. That is not the case.

What causes acid in refrigeration systems? In systems with R22 and mineral oil, acid formation is primarily due to the thermal degradation, or overheating, of the refrigerant; this results in the formation of hydrochloric and hydrofluoric acids. These are strong mineral acids. The oil lubricants used in the older systems were relatively stable and were not prone to hydrolytic degradation due to the low solubility of water in oil.

Excessive heat inside a compressor can come from external power problems like single phasing and low voltage. It can come from internal problems like a locked rotor compressor cycling on the thermal overload for an extended period of time or a system without a head pressure switch internally bypassing hot gas into the crankcase when operating with extremely high head pressure.

The refrigeration cycle maintenance problem that causes high temperatures is running with high superheat. In a hermetic or semi-hermetic compressor the motor often counts on there being enough cooling capacity in the returning suction line gas to cool the motor windings. When there isn't enough cooling capacity in the suction gas, the motor temperatures rise to the point where the high temperatures degrade the refrigerant.  The high temperatures also degrade the insulation on the motor winding causing them to short out.  The two effects are usually coincidental; one does not usually cause the other. An exception to that might be that acidic refrigerant will also degrade motor winding insulation. That may be the cause of a second burnout if effective acid neutralization of the oil is not performed as part of the replacement procedure.

When a burnout occurs, the system needs to be cleaned up or the acid will destroy the new compressor. It’s equally true that the high temperature condition that caused the original burnout must also be corrected for the system to continue in operation.

Open or Shorted Without Acid

What Causes Electrical Failures Without Acid?

Motor winding failure without acid is traceable to physical damage or an intentional disruption of continuity through the motor windings, like an open thermal overload. Running with low superheat may result in broken pistons and valves.  Pieces of broken pistons or valves may damage motor windings. High temperatures caused by locked rotor or single phasing or running with high superheat. This may cause the thermal overload to open

Compressors generally don’t wear out. They are often killed by being forced to run under conditions that inevitably lead to their failure. Those conditions are high or low superheat. Controlling superheat is an important part of the service technicians’ job. Controlling superheat is almost all about saving the compressor in comfort cooling applications.