Air Conditioning Compressor Basics

Air Conditioning Compressor Basics Types of AC compressor

There is a lot of different types of compressors in air conditioning refrigeration systems. The two most common compressors are reciprocating, which is traditionally one of the most common rotary compressors and generally used in smaller units. The other is that scroll compressors are more common nowadays in refrigeration and air conditioning. The other Large compressors, known as screw compressors and some tropical compressors are used in extensive applications. 

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There are typically five types, but they all work in similar ways, and their job is to compress the vapor refrigerant, and vapor means it is not in the liquid state; instead, it is in the vapor state, so its full vapor. All of these compressors are designed to compress complete vapor. All compressors decrease the volume that the refrigerant is in to increase the pressure. It reduces the volume and the space that the refrigerant is inside that compressor, which then increases the pressure and causes it to move. We know high pressure goes to low pressure and compressors create that difference.

Now, From where does the compressor get the energy? Most compressors nowadays have electrical plugs right on them and are generally a material called fuse. Fuse is like a glass plug with which we make these electrical connections, and the electricity is to pass through the shell. Once it gets inside the shell, the refrigerant and the motor and the compressor are inside. That is the most common type.

Types of AC compressors

Some compressors can be accessed through a series of bolts that can be removed, and those are called semi-hermetic compressors. Some compressors are entirely sealed that not designed to be accessed at all, and the only way to get into them would be to cut them open, and those are called hermetic compressors.

An older type of compressor that’s still out there in specific applications is called an open drive compressor and in an open drive compressor rather than introducing energy into the compressor via electricity through a Fuse type plug. Instead, we drive a shaft, and so the compressor is a separate opponent from the motor, so we have to connect a separate motor to the compressor. Open drive compressor is rare because there is a shaft that’s coming out of the compressor, and that is very likely to have leaked on it, so it was a prevalent issue with open drive compressors when they were familiar. So now Open Drive Compressors have been replaced by mostly sealed weather semi-hermetic or hermetic compressors.

Working of AC Compressor

So, as we mentioned, all the compressors function by pumping vapor, but an essential thing with compressors is that when we set up a system, we are only feeding it with vapor, so this is why we test something that we call superheat. It is a term that we hear a lot in air conditioning. It is one of the reasons we measure it so that we can ensure that all of the refrigerants entering the compressor are entirely vapor. If we get liquid refrigerant into that compressor, it can damage if it gets up to the head, which is the part that does the pumping, so that is very critical. 

Another critical thing with compressors is that we get the temperature right, so the temperature of that suction gas coming into the compressor needs to be the right temperature and at the right mass flow rate to keep that compressor cool. If the compressor is refrigerant cooled, which is the most common, there are two types of compressors air-cooled and refrigerant cooled. However, most of the industry is typically using refrigerant cooled compressors means that the refrigerant that enters the compressor is doing the cooling of the compressor and the motor cooling in those internal parts. So if the refrigerant coming into the compressor is too hot. However, the temperature is too high of the refrigerant or the mass flow rate of the refrigerant, the amount of it that’s moving through the compressor is too low, then it cannot cool the compressor. 

We have identified a couple of things here- first is compressors pump vapors, so we cannot bring liquid into the compressor. Secondly, we have to make sure to control the temperature, and the mass flow rate the amount of refrigerant coming down that suction line entering the compressor to make sure that it stays cold. There is another thing that we talk about a lot with compressors: the compression ratio and the compression ratio is just the absolute discharge pressure divided by the full suction. All that means is is how much is it needing to increase the pressure when that compressor has to increase the pressure to a higher degree there is more waste because there is more re-expansion of the high-pressure gas coming out of it. When a compressor has more work to do, it has to increase that pressure for lower base pressure to a higher pressure than differential is more significant then that compressor is going to do less work. It is going to run hotter so higher temperatures. We throw that compression ratio factor in there as well oil control is tremendous with compressors because the compressor has oil inside that crankcase. At least in most cases, it is going to have that oil inside that crankcase. 

There is a compressor that uses some magnetic bearings out there, which is pretty cool, but most compressors use standard bearings, and they need to have oil lubrication. So a small amount of oil is circulated through the system with the refrigerant, but its bulk should be in that compressor shell. However, there is a couple of different factors that can cause problems with that oil one. If we have liquid refrigerant coming down the suction line into that compressor, it costs foaming and loss of oil, loss of viscosity of the oil. 

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Another factor is if the compressor overheats, it can break down the oil; that is another thing that we look for and another reason why we have got to make sure to control that compressor temperature. However, there is another factor which is what we call flooded starts. That is where liquid refrigerant migrates to that compressor during the off-cycle, especially if the compressors in a cold environment. Say it is Split AC system, and it is located outside. That liquid refrigerant gathers in the oil, and then when that thing finally turns on, it creates that little mini-explosion in there, and a lot of the oil is lost as that liquid refrigerant begins to boil. That is another thing that we look for as we do want to make sure that we do not have flooded starts. We use things like solenoids and pump down solenoids and compressor crankcase heaters and hard shutoff TXV. Those are all strategies that are used by manufacturers in order to prevent flooded starts.

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Let us get down into the nitty-gritty of what the compressor does and how it fails. We talked about how it pumps vapor, but it takes vapor from the suction line down that suction line it enters the compressor and then pumps it up, and it goes in the discharge line and then goes into the condenser. Let us think of how to think about this because we will notice that the suction line coming into the compressor is low-temperature, and the discharge line leaving the compressor is high-temperature if we touch one. Then the other we are going to notice there is a big difference in temperature. I do not suggest to touching a discharge line because it can burn the hand. However, there is going to be a significant disparity in temperatures there a piece of standard air-conditioning equipment running. 

Under no conditions, We usually get to see something like a 50-degree suction line temperature entering the compressor on that long line and then coming out of the discharge line.

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