I enjoyed the recent Mythbusters column on hermetically sealed compressors. While the authors made many valid points, I would point out that old-fashioned gear-unit driven turbocompressors using seal and lubrication oil systems seem inefficient, unreliable, inflexible, and wasteful, especially for new project developments. Hermetically sealed direct-driven centrifugal compressors, on the other hand, use magnetic bearings without seals or lubrication oil systems.

Considering the hermetically sealed direct-coupled compressors, advantages include: High expected reliability; a simple, light-weight and compact design; and lower footprint and weight. As a result, they are the only available option for subsea and some offshore revamp projects.

It is true that hermetically sealed compressors are expensive compared to other compressor types. If a certain production volume can be achieved, however, they could be cheaper than conventional compressors because they are inherently simpler.

That said, the conservative nature of the turbomachinery industry may not allow widespread application in the near future. Over the long term though, this compressor type could replace conventional compressors. The elimination of the gear unit, oil system and seal is important in any compressor application. Also this design offers superior rotordynamics, torsional behavior and condition monitoring capabilities.

Amin Almasi, Rotating Machine Specialist, Australia

Dear Amin,

Thank you for your comments on our recent article on hermetically sealed compressors. It is letters like yours that lead to useful discussions, and for that we like to thank you. Now to the topic at hand: It is undisputed that hermetically sealed compressors are crucial for subsea applications. It is the specific requirements of subsea applications that allow us to overlook some of the shortfalls of the concept that we have highlighted in the article. I would, by the way, disagree with your statement that the industry is too conservative. Rather, for land based (and many offshore) applications, conventional units are selected because they offer important benefits to the end user, including attractive cost. To categorize conventional electric motor driven units as inefficient, unreliable, inflexible and wasteful does not reflect user experience. The issues that we brought up regarding efficiency penalties of high speed motors are well recognized. Limited load bearing capabilities of magnetic bearings impose additional limits. It is interesting to speculate about future developments, and I would agree that there is certainly market potential for hermetically sealed machines.

Thank you again for your interest.

The Myth Busters