I needed an air compressor for various shop tasks, but as I don't use compressed air often, I did not want to spend money.... I'm also just a cheapskate. An air compressor is a tool that you don't always need, but when you need it, nothing else will suffice.
A big requirement of this project is that it is silent/quiet, as traditional piston pump and diaphragm compressors are extremely loud, and I have sensitive hearing. I then decided how I would go about building this compressor, what materials I would use to keep it under budget but still have some semblance of quality.
Build your silent compressor today as a sweet recycling project that helps keep perfectly good machinery out of the landfills!
Step 1: Basic Idea
My first question is how do we make it silent? The most popular method, and the method I chose is to re-purpose a refrigeration compressor, of which are typically copeland scroll compressors. Refrigeration compressors are encased in a steel container and filled with oil, and designed to operate quietly, as they will typically be operated in residential homes. Another method would be to take a unmodified piston pump compressor and build a sound deadening enclosure, however this needs to be cheap and recycled, and a properly designed enclosure is not what this is about.
Refrigeration compressors are not ideal however, especially if you need large volumes of air or a high CFM. Since I will not be operating air tools with this, that doesn't matter. Refrigeration compressor pumps are designed to pump a continuous loop of refrigerant and an oil lubricant in a sealed system, and since we will be using this compressor for something it is not designed to do, don't expect it to last forever. We can periodically add oil to the inlet to keep the pump lubricated, but keep in mind this will result in a compressed air stream with a fine oil mist. It is extremely fine however, so if you are not doing something stupidly sensitive, it shouldn't matter.
Another major note is to only use refrigeration compressors if you can acquire one legally. Deliberately opening a sealed refrigeration system is a crime, as these systems contain chlorinated fluorocarbons, which deplete the ozone layer. However, if you can either purchase a pump only, or find a system that has leaked and been discarded, it is safe to harvest the pump.
Step 2: Aqcuire All the Things
I collected the parts for this compressor over many months from various scrap, causing the total cost to come up to $5, as most of the parts were salvage. Of course, if you want to make a compressor quickly, expect to pay more.
I got my compressor pump from a broken down dehumidifier that had leaked and was on the way to the salvage yard. These sealed refrigeration systems are almost universally discarded instead of refilled and resealed since we live in such a disposable society, so do your part to combat this by building something and be a cool human! You can find these pumps in any small sealed system, such as refrigerators, dehumidifiers, window air conditioner units, or water coolers and fountains. If these systems are still sealed or you are unsure if they have leaked or not, have a service tech drain the refrigerant or learn how to properly remove refrigerant yourself. You seriously don't want to risk letting it out, as getting caught will result in a major fine, that, and even if you don't get caught, the rest of us will think you are a lazy polluter.
Now that we have the pump, we need to round up some sort of vessel to contain our air. This vessel MUST be rust free and capable of handling 350 PSI, compressed air tank explosions can be deadly. The easiest way to do this is to purchase a portable air compressor tank that is used as a containment vessel for transporting compressed air, or to buy a cheapo junk air compressor and replace the pump.
You will need to plumb the compressor pump output to the tank, which will require regular copper tubing. You will need to make strong air tight connections, and there are two ways to do this safely.
You can: Solder the pipe using sweat fittings, or purchase brass compression fittings rated for pressure. Brass compression fittings are more expensive, but much easier to work with.
You will also need a gauge to determine your pressure, and a high pressure rated hose to connect to the output. Optionally, you can also install an air drier to keep water and oil out of the air coming from the hose, and you must also have a pressure cut off switch or pressure relief valve to prevent over-pressurization and a possible explosion.
Step 3: Plumbing
I chose to use pressure rated brass compression fittings to connect the pressure lines, which are copper tubes bent into a shape that allows for slight flex in the pump assembly. The intake tube of this pump has a foreline filter drier installed on it from the dehumidifier, this is helpful, as it will somewhat protect your pump from debris. You can also choose to install an intake muffler to make it even quieter, however this is largely unnecessary as it is pretty quiet already.
On the output side, you will need a valve block with a relief valve and pressure gauge. A relief valve is essential, so if something happens and you leave the pump running, it won't over-pressurize and detonate. It is also preferable to install a cutoff switch, which cuts power to the pump when it reaches a certain pressure, this will allow you to leave the pump running, and it will stop pumping when it reaches final pressure and re-start automatically when you draw air from the tank. I chose not to have this as I almost exclusively use my compressor for blowing an air stream for dust cleaning, rather than operating tools. The gauge is simply an AC recharge gauge, which tops out at 100 PSI. Yay for recycling!
Step 4: Electrical
The electrical is fairly simple, your compressor pump will almost universally have a overheat cutoff switch, and a start capacitor. These pumps need a capacitor to start under pressure otherwise you will just burn out the motor. be EXTREMELY careful around these capacitors, the contain large amounts of energy, and WILL hurt or possibly kill you. My system has a rubber boot that covers the capacitor terminals, make sure you have something like this or some form of insulation to protect you from bare wire. Like, really, mains voltage hurts mmkay?
Step 5: Testing
Now that your compressor is a complete system, you need to seal and pressure test. For pressure testing put the compressor on an extension cord far away and let it run until the relief valve opens. You want to make sure your connections wont blow off under pressure, make sure to be far away or have something large and dense between you and the compressor while testing. You can make sure there are no leaks by spraying soapy water on any connection points where air might come out.
When you have verified the safety and operation of your system, you can now enjoy the beautiful silence, and never listen to the deafening rattles of an inferior non-hermetic piston pump again.