Compressor Dissection: Lab Write-Up

Austin Kaiser

ME 379

Intro & Background

In this lab we dissected one of the components in the vapor compression cycle, the compressor. A compressor is a mechanical device that is used to increase the pressure of a gas by reducing its volume. These small, hermetically sealed compressors are used in residential refrigerators and can cost anywhere from $100-$300. I learned what a compressor did physically in my thermodynamics class, but I really didn’t really know how it worked mechanically until this dissection.

How It Works

There are three main parts of the compressor: the piston (Figure 1), the filter (Figure 2), and the reed valves (Figure 3). The rod at one end of the compressor rotates to move the piston up and down. The piston is what actually compresses the working fluid and outputs the higher pressure working fluid. The working fluids in this compressor are oil and R-134a. The filter helps to separate the two fluids. The mixture enters hole #1 and since the oil is more dense than the refrigerant, it exits out the bottom hole (#2). The refrigerant then exits out hole #3. The reed valve is similar to the check valve we saw in the hydraulic lift dissection. This piece is more effective than the check valve and is fitted at the intake tract of the piston.

One of the interesting design features I observed was the fact that the compressor was welded shut. This is to prevent leaks from ever occurring. The only disadvantage to this feature is that when the compressor fails, the entire device needs to be replaced. Another design feature can be seen in Figure 4. These coiled springs help to maintain a constant pressure and also help manage the vibrations that the compressor observes while operating. The exit tube is small and bent for the same reasons. The springs on the outside the compressor also add flex while in operation to prevent failure from vibrations.

Rod

Figure 1 Piston and surrounding parts. Piston

Figure 4 Coiled spring and exit tube.

Figure 2 Reed valve: top and bottom views.

Hole #1

Hole #2

Hole #3

Figure 2 Filter with numbered holes.

Materials, Manufacturing, & Assembly

There were a number of different materials used to make this compressor. First, the device was really heavy considering its size so I’m guessing most of the metal parts were made of some type of steel or even iron. The encasing of the compressor wasn’t as dull or as heavy as the inside components so that might be made of an aluminum alloy. The filter is some type of plastic component that must be compatible with the oil and R-134a. The tubing that the coiled spring surrounds is made of copper. Copper is one metal that is compatible with the working fluids.

I’m assuming a lot of the parts in this compressor were manufactured by metal casting. I’m sure they have a number of different molds for the various parts and various compressor models. As stated previously, welding was also a process in the assembly of this device. I’m guessing the rest of the compressor was assembled by hand.

Design Improvements

There were many interesting design features that I observed in this compressor. With my lack of knowledge of materials that are compatible with oil and refrigerants, I really couldn’t come up with any additional design improvements.

Assigned Questions

1. The spring on the outside of the small copper tube at the compressor exit has two purposes. The coiled shape helps to maintain the pressure at the inlet and outlet. The springs help to not reach residence frequency from the vibrations. The higher the spring constant, the higher the residence frequency.

2. The diameter of the tube at the exit is so small to maintain the compressed working fluid. The compressor obviously compresses the working fluid and the small diameter keeps it from expanding again. This tube bends so much so that it doesn’t break when the compressor is operating at a really high rpm.

3. The extra reservoirs and holes in the castings act as a built in upgrade for the compressor. This allows for the same casting mold to be used for multiple models. Also, it makes the compressor symmetric.