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There are details to watch out for when lubricating a compressor.
At the end of the January TLT Lubrication Fundamentals article, we discussed various compressor designs and how some compressors utilize multiple stages to get to higher pressures. We also noted that we needed to cool the air between the stages for efficiency and safety reasons.
1 started to explain the relationship between gas pressure, volume and temperature using the various gas laws of Boyle's, Charles' and Gay-Lussac's leading to the Ideal Gas Law: PV=nRT (pressure times volume equals number of molecules times R the gas constant times temperature). Two things happened-I filled up a whole page without actually concluding my explanation of the need for cooling and as I reread the page, I nearly fell asleep. So, I'm going back to my Missouri farm boy experience to understand multi-stage and single-stage compressor system design needs.
I remember inflating a wagon tire in the field with a hand driven tire pump. By the time the tire was inflated properly, the cylinder on the tire pump was so hot I could not hold onto it with my bare hands. Compressing air not only increases the pressure but also raises the temperature - significantly! In the field I was only raising the tire pressure to 35 psig (Pounds per Square Inch Gauge pressure, or pressure above atmospheric pressure). In many industrial applications, air pressure is raised to between 90 and 110 psig, and that raises the high-pressure air temperature to around 500 °F, if not cooled.
However, if the air is compressed to 110 psig in two stages, the final discharge temperature from the second stage will be around 245 °F, with the air being cooled between the stages. This is a more reasonable temperature for the compressor seals, lubricants and even the metallurgy to handle.
Compressing air not only increases the pressure but also raises the temperature-significantly!
Figure 1 shows a two-stage reciprocating compressor where you can see the inter-cooler connecting the two cylinders. Also note the cylinders' sizes are different. The first stage is the larger cylinder on the right. The compressed volume of pressurized air from the first stage is cooled by the inter-cooler before it goes to the smaller second stage cylinder...