Barrel Compressor Resources

The main advantage is pressure. A barrel machine can handle a much higher pressure due to the way it seals off. A big, long, horizontal split is harder to seal. This is especially true in hydrogen service. Hydrogen is hard to seal at any pressure, but high pressure hydrogen in a refinery is even harder. A barrel machine has advantages in this. Essentially in a barrel machine you have a horizontal split shoved inside a barrel. This makes for better sealing.

Barrel machines are also used in high pressure re-injection machines in the oil field.

Modern barrel machines are not really more labor intensive, they just need the room to pull the bundle. If there is stuff in the way, then yes there is more labor involved moving things. Advantage over a top connected horizontal split is that you don't have to pull piping.

The other advantage of a barrel machine is that in a re-rate, you can leave the barrel on the deck as is, pop out the old bundle, pop in a new re-rated bundle (including a different number of impellers and different stationary gas path) and you are back in business with essentially a brand new machine. On a horizontal split you would need to pull the case and at least machine it, if not replace it to do the same.