The compressor is the heart of generator. Literally, this is the pump that pressurizes and drives the gas through the coils. Depending on the desired output, one needs to carefully consider what to get and how much to spend on this important piece of equipment.
This article will later deal with the thermodynamic theory involved with the expansion of gases. For now, accept that fact for now that a non-ideal gas follows the following formula derived by Joule and Thomson
This means that if we start at a temperature, T, of 0C (273K), we can expect a drop of close to 0.25C for each atmospheric drop of pressure. If one uses a refrigerator compressor that goes to 40 atmospheres, the first-pass temperature drop at the throttle is close to 10C. By using a scuba tank compressor that delivers 3500 psi (240 Atm) one can get a temperature drop close to 60C.
The compressor needs to have oil-free pistons. This means there are teflon rings that make the seal, and will need replacement after a suitable period of time. The compressor needs to deliver a sufficient rate of flow, or there will not be enough cooled air mass to achieve the appropriate temperature.
The RIX Oil-free compressor seemed a good choice. The SA-3E model could deliver 3 scfm at a pressure of 3500 psi. I modified sheaves and motor so I could deliver 4 scfm. I also added a 10 micron vacuum filter and water trap to protect the valves.
I believe I paid $1500 for the used compressor.
A significantly higher efficiency is possible if the expanded gas does work by moving a piston or rotating a turbine. This is called a turbine expander. This comes with many difficulties for the amateur. First of all, they are very expensive. I could not even find one for a small scale and I had no desire to fabricate one. One also needs to consider that the workmanship must be perfect as the cold temperatures could cause the locking of the moveable parts. Thus, after careful consideration, I decided to go with the lower efficiency Joule-Thomson throttle, with no moving parts, and a significantly lower cost.