Before you buy: is the problem supply, or distribution?

The most common expansion mistake — and I've seen it many times in the field — is adding a compressor to fix a pressure-drop problem on the other side of the factory. Almost always, the real issue is the piping, not the supply. Adding hardware doesn't fix piping; it just creates a control problem on top of the piping problem. Before buying another compressor, find out whether you actually need more air, or whether you have enough air that just can't get where it needs to go.

The injection-molding plant story

A real example from my field service days: a plastic injection molding plant making food packaging — plastic cups, food delivery containers, that sort of thing. Original compressor room had 3 x GA75 oil-injected screws with separate dryers. Proper setup. Running 24/7 except Sundays.

They expanded production, added a new line on the other side of the factory. Air demand went up. They bought a GA90 VSD with integrated dryer ("Workplace" style — bare compressor with dryer built in) and dropped it into a converted storage room near the new line.

Result: constant pressure fluctuations across the whole plant. The interconnect piping between the two compressor rooms wasn't sized for the new flow. Pressure in the two halves of the factory wouldn't equalize. Control was a nightmare — two compressor rooms each thinking they were responsible for the whole plant, talking to each other through a pipe that was too small. The four compressors fought each other.

The fix would have been a bigger header pipe between the rooms, not another compressor. They didn't realize until after.

This pattern is everywhere. Diagnose before you buy.

How to diagnose: supply or distribution?

Before ordering anything, do this:

  1. Measure pressure at the compressor discharge (where the compressor leaves the receiver) during peak demand.
  2. Measure pressure at the equipment that's complaining about low pressure — the actual point of use, at the same time.
  3. Look at the gap:
    • If discharge pressure is fine (7+ bar) but the equipment is seeing 5-6 bar → it's piping. Adding a compressor won't fix it. Fix the pipes — bigger headers, eliminate restrictions, fix leaks, possibly a loop.
    • If discharge pressure can't keep up (drops below setpoint during demand peaks) → it's actually a capacity problem. Now you might genuinely need more compressor.

A 50€ pressure gauge tells you whether to spend 50,000€ on the right thing or the wrong thing.

When you DO need more capacity: three approaches

Option A: Same size as existing (the rotation play)

Most of the time, I see people buy the same type or capacity of compressor as the current one. Which is a good idea. It allows you to easily use the two compressors alongside each other, and use one as the backup for the other. Another idea is to buy a bigger compressor and use the old one only as an emergency backup.

This is the cleanest play if your existing compressor is reasonably sized and from a good brand. You get:

  • Identical service contracts and spare parts inventory
  • Easy weekend rotation (more on this below)
  • Mechanically simpler control logic
  • Redundancy — either one can handle the other's load if needed

Option B: Bigger than existing (new becomes primary)

If demand has clearly grown beyond what the existing can handle even with help, buy bigger. The old unit becomes emergency backup or supplementary. More flexible if demand keeps growing.

Downside: two different machines, two service contracts, two spare parts inventories.

Option C: A VSD trim compressor

You might think about getting a variable speed drive (VSD) compressor to take care of the compressed air need when your first compressor can't keep up.

This is the most efficient setup IF your demand actually varies through the day. The existing fixed-speed runs the base load, the new VSD modulates to handle the variation. Energy economics are excellent in this scenario.

But — vendors push VSD aggressively even when it doesn't pay off. See VSD vs fixed speed for when VSD is genuinely the right move and when it's just a sales pitch.

The weekend rotation pattern (this is how most factories actually run)

If you're going from one compressor to two, plan for the standard industrial rotation pattern from the start:

  • 2 compressors, 1 standby + 1 running — they swap priority every week
  • 1 compressor handles weekend / aux loads (no production, just bits and pieces — building services, instrument air for monitoring, occasional maintenance use)
  • Both run during the week when demand is up

This is how most well-run mid-size plants actually operate. It means:

  • Buying matched compressors (so either can do the work alone if the other is offline)
  • Buying a sequencing controller (Atlas Copco ES series, Ingersoll Rand X-Series equivalent, etc.) to manage the rotation automatically
  • Plumbing both into the same receiver / header system properly — the injection-molding plant story above is what happens when you don't

Going beyond two: real multi-compressor setups

If you're scaling up to a 4-6 compressor setup (proper mid-size production facility territory):

  • Build a real compressor room — separate dryers per compressor (or at least redundant dryers), proper ventilation, dedicated electrical supply
  • Multi-compressor sequencing controller is mandatory
  • N+1 redundancy for production-critical air (if you need 3 compressors to meet peak demand, install 4)
  • Properly sized headers between compressors and the end use — don't repeat the injection-molding plant's mistake

This is a system design project, not a compressor purchase. Worth bringing in someone who's designed plants of this scale before.

Where to next