Impeller Design Challenges on Integrally Geared Centrifugal Compressors

The integrally geared compressor, also known as a multi-shaft compressor, is a technology that has been around since the 1960s, but remains underdeveloped.  Usually seen in applications in the industrial gases industry, integrally geared compressors (IGCs) can range in size from small product machines to steam turbine driven high-horsepower, high-flow compressors for air separation plants.  These compressors modular construction principle, consisting of as many as eight different stages, allows for implementation in a large number of varied customer processes.  The main advantages of IGCs in the industrial gases industry is the compact design and smaller installation footprint, efficiency increases due to the use of multiple speeds for separate impellers, and overall lower operational and installation costs.

semi-open-impeller
Figure 1 – Semi-Open Impeller

One of the key design differences between the standard inline compressors and the IGCs is that the integrally geared compressor makes use of both closed AND semi-open impellers.  The reason for the use of open impellers in IGCs are the higher strengths due to better manufacturing techniques, speed of manufacture, and the inherent lower costs.  However, the main drawback to having an open impeller in your system is that in the event of impeller rub, the damage to the compressor would be significantly worse than with a closed impeller.

Engineers have introduced several techniques in order to minimize this risk.  Axial position monitoring of the pinions as well as the use of high-speed thrust bearings can result in an improvement against this rub phenomenon.  However, even with these innovative solutions, open impellers still have a much larger tip clearance (clearance / blade height) then its closed impeller counterparts.  It is very important to realize the efficiency decrease present in compressors with varying tip clearances.

In SoftInWay’s latest update for its turbomachinery design and analysis software, AxSTREAM™, calculating models were improved for centrifugal compressors with large tip clearances.  By taking into account the additional flow deviation from the direction of the blade’s tail as well as the flow recirculation in the meridional direction, better estimates for the flow models can be identified on compressors with tip clearances greater than 0.1.

large-tip-clearance
Figure 2 – Flow Characteristic on a Compressor with Large Tip Clearance

Although integrally geared compressors present several elegant solutions in the industrial gases industry and overall show higher efficiencies than single rotor compressors, many different challenges arise when working with them.  Among them are the impeller blade design challenges mentioned above as well as numerous additional challenges to rotor dynamics that these intricate gearing designs introduce.  SoftInWay has worked alongside MAN Diesel & Turbo since 2005 to solve these problems and many others. To learn more about designing integrally geared compressors or for an in-depth look into our rotor dynamics software please explore the links below.

Integrally Geared Compressors 

AxSTREAM for Rotor Dynamics

 

 

References:

http://turbolab.tamu.edu/proc/turboproc/T20/T20131-144.pdf

http://turbolab.tamu.edu/proc/turboproc/T32/t32-21.pdf

http://turbomachinery.man.eu/products/compressors/integrally-geared

The Future of Turbocharger Technology

­One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio.  Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1).  This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.

Ratio visualization
Figure 1 – A/R Ratio Visualization

For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds.  This results in a more responsive engine and overall higher boosts at lower RPMs.  However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system.  This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine.  The same effect is present for larger turbochargers, only in reverse.  They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.

In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo.  One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series.  The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.

Other technologies are very common for overcoming this problem without the use of two separate turbos.  One design is the twin-scroll turbocharger, which works like a pair of turbochargers connected in parallel.  Instead of using two separately sized turbochargers, the twin-scroll design has two exhaust-gas inlets and two nozzles feeding into a single turbocharger.  Another design, the variable geometry turbocharger, uses internal vanes within the turbocharger to dynamically alter the A/R ratio of the system at different rev levels.  The variable geometry turbocharger (VGT) displays exceptional performance at the low and high RPM ranges, however, it requires the use several moving parts and exotic materials in order to handle the high exhaust temperatures present in petroleum engines.  For this reason, the variable geometry turbocharger is generally only used in diesel engines due to the relatively low exhaust temperatures in comparison.  Twin-scroll turbochargers, while more commonly used in gasoline applications, do not have the variability of the VGT and are not as efficient over the full rev range of the vehicle.

turbocharger
Figure 2 – BorgWarner Twin-Scroll Variable Geometry Turbocharger Concept

BorgWarner has recently showcased a new turbocharger concept that makes use of both technologies discussed above.  The “Twin-Scroll Variable Geometry Turbocharger” has a valve that can redirect airflow from just a single scroll to an airflow completely split between two scrolls.  It allows for the any range in between these extremes much like the VGT; however, it only uses only one valve as opposed to the several vanes needed in the VGT.  It derives the same exhaust separation idea as the twin-scroll while allowing for the variability of the VGT.  With all the new emissions regulations calling for increases in efficiency, it will be interesting to see if this new technology can find its way into the market.

References:

http://www.economist.com/news/science-and-technology/21638917-why-turbocharged-four-cylinder-engines-now-rule-road-little-engine-could

https://www.carthrottle.com/post/engineering-explained-6-different-types-of-turbocharger-and-the-advantages-of-each-setup/

Performance Effects of Axial Turbines & Compressors Due to Roughness Variations

As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate.  Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness.  Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.

gas turbine blade
Figure 1 – Gas Turbine Blade and Annulus Surface Wear (Source PowerMag)

There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces.  Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow.  Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces.  Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.

Some of these mechanisms, however, are more prevalent in higher temperature systems.  Hot corrosion, for example, occurs at a temperature range of around 730 to 950°C due to the chemical interaction of molten salts on the component’s surface.  In this case, high temperature gas turbines are most notably affected by this phenomenon.  Even with protective cooling flow systems, most gas turbines start to lose efficiency early in their life cycles.  For this reason, it is important to analyze the future performance of the machine before it is put into operation.  Attributing the necessary roughness factors can lead to a more accurate description of the performance of a machine over time.  Using the AxSTREAM software platform, these roughness factors can be implemented into a design to compare what a turbomachine might experience 5 or 10 years into its lifecycle as opposed to at its birth.  Furthermore, a user could model the effect of particular surface coating and polishing techniques and analyze whether the increases in efficiency outweighs the investment costs.

Streamline - Axial Turbine
Figure 2 – Streamline Calculation Stage for Axial Turbine in AxSTREAM

By assigning different surface roughness grades in AxSTREAM, one can analyze a number of parameters of interest such as efficiency and power output.  Using the streamline/meanline calculation module, (seen to the left), the user is able to seamlessly overlay a number of different performance factors and curves due to the prescribed changes in material roughness.  With increases in surface roughness, laminar to turbulent transitions will occur much sooner and will result in substantial losses in the machine.  Because of this, it is crucial to investigate what kind of investments and preventative measures need to be made to ensure proper treatment of a machine’s surface.

If you’re interested in learning more about our integrated software platform for turbomachinery applications, please read here.

Reference:

http://turbolab.tamu.edu/proc/turboproc/T43/TurboTutorial1.pdf