A handy rule of thumb to retain for compression troubleshooting jobs is that the theoretical temperature increase of gas due to compression is linearly proportional to compression horsepower. An extremely useful application of this rule of thumb is the following approximation:
It is not too much to say that this relationship is the most important concept in this book in that it is the most useful. Note that the anticipated temperature rise is independent of compressor speed, unloader configuration or gas volume; it is only a function of the compression ratio—and of course compression inefficiency. While Figure 7—2 can be used to calculate the theoretical temperature increase for compressing natural gas, I used the concept in a more direct manner at El Gringo.
Figure 7—1 An unloading pocket reduces engine load and volumetric capacity.
Table 7-1 shows that the temperature rise for the individual cylinder compression varied from 28°F for the No. 1 cylinder crank end to 42°F for the No. 2 cylinder crank end. The key point of this table is that compression efficiency varies inversely with temperature rise. As both the suction and discharge pressures were the same for all cylinder ends, the only reason for the variable temperature rise were different efficiencies of compression. Since the work performed by the piston at each cylinder end was about the same, (except for No. 2 cylinder head end, which had the bad unloader) the observed temperature increases were inversely proportional to the gas flows. This means that if the No. 1 cylinder crank end was moving 30 MM scfd of gas, then the No. 2 cylinder crank end was moving only 20 MM scfd and the No. 1 cylinder head end was moving 23 MM scfd.
