04/06/2026
The Importance of Correct Turbo Compressor Turndown Capacity
Last week, I wrote about the importance of selecting the correct turndown capacity when installing a turbo compressor. This is critical to maintaining proper air delivery and overall compressor performance.
There are specific engineering principles that must be followed, as an incorrect turndown capacity can cause the compressor to enter either surge or stonewall conditions. These topics deserve their own discussion in a future post.
A rotating shaft operates within certain critical speeds. When surge or stonewall occurs, it disrupts the shaft's normal rotational behaviour and can cause the pinion shaft to bend in various ways.
Even a perfectly balanced shaft must pass through its critical speeds as it accelerates from standstill to operating speed, regardless of its length or diameter.
In simple terms:
• First Critical Speed – The lowest speed at which the shaft begins to deflect, causing it to bend into a U-shaped form.
• Second Critical Speed – At a higher rotational speed, the shaft deflects into an S-shaped form.
• Third and Higher Critical Speeds – At progressively higher speeds, the shaft can form three or more bending loops.
(See attached illustration.)
In theory, a rotating shaft has an infinite number of critical speeds. Each critical speed corresponds to a unique natural frequency and bending mode. Operating at or repeatedly passing through these conditions can cause damage to impellers, shafts, air seals, and bearings.
Selecting the correct compressor and ensuring the appropriate turndown capacity significantly reduces the risk of these damaging conditions occurring.
As rotors accelerate from idle to operating speed, changing airflow conditions and the enormous forces generated by rotating components challenge the balance limits of the shaft. These forces create vibrations as the shaft passes through its critical speeds.
In a poorly planned turbo compressor installation, these shaft disturbances can occur repeatedly as the compressor loads, reaches pressure, unloads, and cycles again. Over time, this can lead to severe damage and eventual failure of the rotating internals.
Critical speeds are not limited to industrial machinery—they are part of everyday life.
For example, when driving a vehicle with poorly balanced wheels, you may experience a steering wheel shudder at a specific speed. That vibration is a form of critical speed resonance.
Similarly, when seated in a jet aircraft before take-off, you may feel vibrations as the engines spool up. What you are experiencing is the engine shafts passing through their respective critical speeds during acceleration.
If you don't feel those vibrations, you might want to ask the cabin crew if you can get off!
At CVA, we understand the importance of critical speeds and their impact on turbo compressor reliability and performance. Every turbo machine we supply is engineered, commissioned, and configured to suit the specific site operating conditions, ensuring optimum efficiency and long-term reliability.
We also consider environmental factors that are often overlooked during the design stage, including the elevated ambient temperatures increasingly experienced due to global warming. By considering these real-world operating conditions, CVA ensures that your compressor delivers reliable performance, avoids unnecessary cycling through critical operating zones, and achieves maximum service life from its rotating components.
