Sat Mar 16, 2019 12:38 pm
I understand your reasoning, but it seems to me that the heat produced by compressing air at such high ratios pretty much eliminates the practicality of performing that in a single stage of compression. High pressure compressors compress the air in stages, which keeps the compression ratios much lower in each stage.
Consider a 3 stage compressor for example of keeping compression ratios managable. A normal range for compression to 3000 psi varies some among given compressors, but let's say the atmospheric air is compressed to 80 psig in the first stage. Then the higher temperature of that air is moderated by passing through cooling coils on the way to the second stage. The second stage may compress that somewhat cooler 80 psig air to say 600 psig before going through cooling coils and expanding for additional cooling as it enters the moisture separator/condensor. The third stage then takes the cooled air to 3000 psi, which can create head and valve temperatures well in excess of 300 F before going through the after cooling coils and expansion in the final separator. An simplified calculation of the compression ratios (using 15 psia for atmospheric pressure) would give estimates of compression ratios of 6.33, 6.47, and 4.9 in the successive stages. Much less heat of compression is produced overall.
The compression ratio of the air being compressed to 3000 psig in a single cylinder by taking air in at atmospheric pressure is about 200:1,
( a simplification without figuring all other minor factors that might be involved). The heat produced would likely be enough to ignite any lubricants and orings at least, introducing hydrocarbons into the expelled air. I don't know how to calculate the potential temperature in the proper manner, but consider that a Diesel engine generates enough heat of compression (Adiabatic Heating) to ignite diesel fuel vapor at compression ratios from about 15:1 to 20:1.
I hope all our engineer friends will please forgive my simplified and possibly inaccurate drug-store physics calculations. In fact I would love to be enlightened on the temperatures potentially created at 200:1. I don't want to try to wade through those formulas myself!
The older I get the better I was.