Compressor Basics


There are a lot of different types of compressors for air conditions systems. The five typical types of compressors are reciprocating, rotary, scroll, screw, and centrifugal. Reciprocating compressors are traditionally one of the most commonly used types. Rotary compressors are used for smaller units. Scroll compressors are becoming more and more common in air conditioning systems nowadays. Screw compressors and centrifugal compressors tend to be used in very large applications.

Regardless of the design, compressors all work in a similar manner to do the same job, compress vapor refrigerant. The compressor takes the vaporized refrigerant and compresses it by decreasing the volume of space that the refrigerant is in. This increases the pressure and causes the vapor to move as it creates a difference between high pressure and low pressure.

Of course, the compressor needs energy and most compressors used today have electrical plugs built-in. The plugs use a material called Fusite which uses glass to enable electrical power transmission through the compressor shell while protecting the electrical conductors from high pressure. Once the electricity gets inside the shell the refrigerant, motor and compressor are all inside.

Semi-hermetic compressors have a series of bolts that can be removed to allow access to the interior components. Completely sealed compressors, called hermetic compressors, are not designed to be accessed at all. An open drive compressor does not use a Fusite plug, instead has a separate motor that connects to the compressor with a shaft. As these compressors are likely to have leaks they are rarely used. Most compressors nowadays use semi-hermetic or hermetic designs.

When setting up an air conditioning system it is important to be sure that only vapor is being brought into the compressor. We test to ensure that the refrigerant is “superheated” and is in a complete vapor form. Should any liquid refrigerant get into the compressor it could cause damage to the pumping mechanism.

The compressor and the motor both need to be cooled and this cooling is done by either air cooling or refrigerant cooled. Typically, using refrigerant cooled compressors is the industry standard. The refrigerant coming into the compressor does the cooling for the compressor, motor, and other internal parts so it is critical that the temperature and flow rate are right.

The temperature of the gas being suctioned into the compressor is too high (too hot) or the mass flow rate (the amount of refrigerant moving through) is too low then the compressor can't cool correctly.

Compression Ratio

The compression ratio is the absolute discharge pressure divided by the absolute suction. Basically how much does the compressor need if it has to increase the pressure? When the compressor has to increase the pressure to a higher degree there is more waste because there is more re-expansion of the gas. In other words, when the compressor has more work to do it has to increase the pressure so the differential between the lower and higher pressure is greater and it will run hotter at a higher temperature.

Controlling oil is a huge deal with compressors. Most compressors use typical bearing and they need to have oil lubrication. The majority of this oil is inside the crankcase but a small amount does circulate with the refrigerant. There are a couple of problems that can occur related to oil. If any liquid refrigerant comes down the suction line and goes into the compressor it can cause foaming and loss of viscosity of the oil. If the compressor overheats it can break down the oil as well.

Another problem to avoid is flooded starts. Liquid refrigerant can migrate into the compressor during off cycles, especially in cold environments. The liquid refrigerant gathers in the oil and when the system turns on the next time a little mini-explosion can cause the oil to be lost as the refrigerant begins to boil. Pump down solenoids, compressor crankcase heaters, and hard shutoff TXV are just some of the strategies manufacturers use to prevent flooded starts.

The vapor comes down the suction line and enters the compressor at a relatively low temperature, around 50°F, but leaves the compressor through the discharge line at a much higher temperature of about 165F. Some of this increase comes from picking up heat as the refrigerant cools the compressor and motor.

The majority of the heat comes from compressing the mass of the vapor. Compressing the mass causes the molecules to bounce around more quickly. The higher temperature is the average molecular velocity, or average kinetic energy, within a substance. When you compress all this energy from the molecules closer and closer together they start bouncing off each other as well as the sides of the container.

The temperature goes up because the velocity of the molecules has increased but the overall heat content has not increased much. Heat from inside the compressor and the electrical windings in the motor case a little increase in heat. The temperature increases from the compression, the heat is absorbed in the evaporator coil and maybe a little bit in the suction line which is why we see the 50 degrees or so on that incoming line.

Forcing all that vapor together inside the compressor gives us the sudden sky-high temperatures without an increase in the overall heat content. Decreasing the volume increases the pressure and the density increases the temperature.

There is a lot of misconception out there that the compressor “compresses” the vapor into a liquid but this is not the case at all. The vapor coming into the compressor is already past the boiling point of the refrigerant and then the compression raises the temperature even higher. It is not possible for the vapor to return to a liquid state at these high temperatures.

After leaving the compressor the vapor needs to pass through the condenser coils. As the vapor passes through the condenser coil the heat will exchange with the ambient air. Since hot always goes toward cold, the heat from the very hot refrigerant moves out towards the cooler air. As the refrigerant rejects the heat the vapor condenses down to a liquid state by the time it reaches the bottom.