Myth: Avoid a Gearbox at All Costs for Compressor Applications

Have you traveled in a modern, fuel-efficient midsize passenger plane lately? Given that these planes look rather conventional, you may wonder: Where does the fuel efficiency come from? There is a good possibility you were on a plane with a geared turbofan.

Turbofans with large bypass ratios have an issue. The large fan is difficult to speed match with the low-pressure turbine that drives it. That’s due to large differences in their diameters. The turbine wants to run at a much higher speed than the fan. The solution is to use a gearbox (rated between 16,000 and 32,000 hp with a ratio of 3:1) between the turbine and the fan. This allows both to operate at their optimal speeds, thereby increasing efficiency. It also allows an increase in fan diameter, improving propulsion efficiency.

New aircraft engines of this type can reduce fuel consumption by 15% compared to older engines. While the introduction of this engine, first used in commercial service in 2016, was a bit rocky, none of the teething problems were related to the gearbox.

Full disclosure: Neither author of this article works or has worked for the manufacturer of these aircraft engines. So why are we writing about them? Because there is proof a gearbox can be a good thing, allowing the match of aerodynamic components with different optimum speeds. Without a gearbox, the speed of the components—for example, a power turbine and a high-pressure gas compressor—is a compromise, reducing overall efficiency. Of course, this improvement is, to some extent, compromised by the fact that the gearbox itself creates losses.

We’re not advocating for a gearbox in every possible application. In many instances—for example, gas turbine-driven compressors in pipeline applications—there is no significant speed mismatch. The power turbine of the gas turbine and the pipeline compressor that’s sized to absorb the gas turbine power have their respective best efficiency at about the same speed.

On the other hand, consider the same gas turbine powering a high-pressure, gas-injection compressor. That is an example of a physically small compressor that absorbs a lot of power, and the compressor wants to run significantly faster than the power turbine. It may be possible to design a compressor for the same duty that works as a direct drive, but it would be bigger and most likely less efficient. In this situation, a gearbox may be a better solution.

Properly sized gearboxes rarely cause problems. Still, end users and turbomachinery experts often advocate against their use. This is interesting because gearboxes—either speed-reducing for generators or speed-increasing for electric compressor drives—are the norm rather than the exception for compressors driven by electric motors and generators driven by gas turbines. They operate to the satisfaction of most users.

In fact, the most challenging applications are not parallel shaft gearboxes used as speed increasers in gas compressor drive applications. The most challenging gearbox designs are used for integrally geared compressors where it’s often necessary to reach to the limits of possible pitch line velocity and where multiple pinions run off the same bull gear. Also, since gearboxes for industrial applications require oil lubrication, a properly designed oil supply and cooling system is imperative to achieve high system reliability and avoid forced outages.

There’s another non-intuitive gearbox preference. For compressor trains, parallel shaft gearboxes are the norm, in particular because API does not recognize epicyclic or planetary gearboxes for this duty. Interestingly, they are commonly used for gas turbine-driven generators, as well as in variable-speed hydraulic gearboxes.

For compressor applications, there are advantages to using epicyclic gears. They are more compact and often, especially for higher gearbox ratios, more efficient than parallel shaft gears. They also allow for an inline drive train, which helps significantly in offshore installations where space requirements, weight, and single-lift capability are important.

Rather than avoiding a gearbox on principle, it is a good strategy to consider if its use could lead to better performance, lower costs, and greater flexibility. Gearboxes are proven devices and, if sized and integrated properly, provide high reliability.

Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the Manager of Gas Compressor Engineering at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil and Gas Applications Committee.

Any views or opinions presented in this article are solely those of the authors and do not necessarily represent those of Solar Turbines Incorporated, Elliott Group, or any of their affiliates.