Driving Turboexpander Technology

Turboexpanders are used in a number of applications, including floating LNG (liquefied natural gas), LPG (liquefied petroleum gas) / NGL (natural gas liquids), dew point control, and ethylene plants.  Used as a highly efficient system that takes advantage of high pressure, high-temperature flows, the turboexpander both produces cryogenic temperatures and simultaneously converts thermal energy into shaft power.  Essentially, a turboexpander is comprised of a radial inflow expansion turbine and a centrifugal compressor combined as a single unit on a rigid shaft. The process fluid from a plant stream will run through the expansion turbine to both provide low-temperature refrigeration and convert thermal energy to mechanical power as a byproduct.  First, the gas will radially enter the variable inlet nozzles (or guide vanes) of the turbine, which will allow for a localized increase in fluid velocity prior to entering the turbine wheel.  The turbine wheel will accept this high-temperature, high-pressure, accelerated gas and convert it into mechanical energy via shaft rotation. The primary product of a turboexpander manifests at the outflow of this turbine.  After the process gas passes through the turbine wheel, this gas has expanded so dramatically that it produces cryogenic temperatures colder than any other equipment in the plant.

turboexpander
Figure 1- Typical Turboexpander – Expander-Compressor Configuration

The useful mechanical energy converted from this system is generally used to drive a centrifugal compressor positioned on the opposite end of the shaft.  In the case of this expander-compressor setup, the mentioned turboexpander technology avoids the excessive use of fuel consumption seen in other systems, and significantly decreases the CO2 footprint of the overall design.  As well, there are various examples of turboexpanders that use an expander-generator setup, which converts the mechanical energy from the turbine into direct electrical power.  Turboexpanders have come a long way in the last 40 years.  With the advent of magnetic bearings and more advanced sealing systems, turboexpanders have been able to handle shaft speeds in large and small machines of up to 10,000 rpm and 120,000 rpm, respectively.  Moreover, innovations in specific CFD modules for turbomachinery have allowed turboexpander systems to achieve efficiencies upwards of 90%.

turboexpander-expander-generator
Figure 2- Turboexpander – Expander-Generator Configuration

All the liquefaction applications mentioned above use this technology due to the optimal efficiencies it can produce.  An accepted condensation method for gases in the past was a technique called Joule-Thomson cooling.  The Joule-Thomson effect describes the phenomenon that a temperature change can occur in a gas due to a sudden pressure change over a valve.  Although extremely important in the advancement of refrigeration systems, Joule-Thomson cooling exclusively required much higher pressure in order to remove the same amount of energy because no external work is done.  The introduction of mechanical load devices allowed the process gas to distribute its energy more rapidly, in turn allowing more optimal refrigeration conditions.

The flow of gas through an expansion turbine encounters numerous rapid transitions and requires an in depth look using CFD simulations for analysis and further optimization.  To learn more about the design of turboexpander components, please visit http://www.softinway.com/machine-type/centrifugal-compressor/

 

Reference:

http://turbolab.tamu.edu/proc/turboproc/T39/ch15_Jumonville.pdf

https://www.researchgate.net/file.PostFileLoader.html?id=5236f8abd039b1146f66ab16&assetKey=AS%3A272141839208458%401441895078987

https://www.geoilandgas.com/sites/geog.dev.local/files/ge-turboexpander-compressors-product-brochure.pdf

http://bit.ly/2fdhyrM 

Understanding the Characteristics of Varying Centrifugal Blower Designs

Many people speculate about the confusion on what is considered a compressor, a blower, or simply a fan.  In essence, each of these turbo-machines achieve a pressure rise by adding velocity to a continuous flow of fluid.  The distinctions between fans, blowers, and compressors are quite simply defined by one parameter, the specific pressure ratio.  Each machine type, however, utilizes a number of different design techniques specific to lower and higher-pressure applications.  As per the American Society of Mechanical Engineers (ASME), the specific pressure is defined as the ratio of the discharge pressure over the suction pressure (or inlet pressure).  The table shown below defines the range at which fans, blowers, and compressors are categorized.
table

Similarities between the design of fans and blowers occur near the lower end of a blower’s range.  As well, many design parallels exist between high-pressure blowers and compressors.  For the article, we will be investigating the different design characteristics of centrifugal blowers. Blower selection depends on a number of factors including operating range, efficiency, space limitations, and material handled.   Figure 1 shows a number of different impeller blade designs that are available for centrifugal blowers.

dans-doc
Figure 1 – Impeller Blading Arrangements for Centrifugal Blowers

Each of these blading arrangements have unique operating ranges limited by overload and stall conditions.  Forward curve and radial blowers exhibit a performance curve for horsepower needed that increases with the volume flow rate.  This means that a motor can be overloaded if discrepancies occur which bring the system to flow rates higher than at the operating point.  On the other hand, any backward arrangement blading exhibits a horsepower curve that increases to a maximum as airflow increases and then drops off again.  This allows the engineer to specify a motor to accommodate the peak horsepower, which in turn ensures that the system cannot overload and is therefore considered “non-overloading”.  The stable range of a blower is defined as the condition under which enough air flows through the fan wheel to fill the spaces in between the blades.  Below this range, the instability of the machine will cause one or several section of blades to stall.

In general, forward curved blading arrangements (or sirocco blowers) are better suited for high volume with lower pressure applications.  These blowers and fans operate at relatively low speeds and pressures, which permit lightweight and cost-effective construction.  Restricted stability ranges, overloading at higher flow rates, and low static efficiencies all limit the capabilities of the forward curved centrifugal blower.  Radial centrifugal blowers have the same problem with overloading at high flow rates; however, they are quite beneficial for moving air in dirty environments due to their flat geometry that prohibits dust or sticky materials from rapidly accumulating.  Composed of 6 to 12 rugged blades extending radially from the hub, these types of blowers run at medium speeds and deliver low air volumes at medium to high pressure.

blower-for-blog
Figure 2 – Centrifugal Blower Design with Airfoil Impeller Arrangement in AxSTREAM™

Perhaps the most important centrifugal fan impeller orientation, the backward orientation, comes in three standard shapes: backward inclined, backward curved, and backward inclined aerofoil (or airfoil). All of these designs exhibit most of the same characteristics, with some discrepancies in their efficiencies.  Generally, the “flat-bladed” back inclined design achieves an efficiency of about 82%, while the backward curved and airfoil designs near 88% and 90%, respectively.  In addition to high efficiencies, backward oriented blade designs allow for the highest operating speeds of all centrifugal blowers and are considered non-overloading.  The only drawbacks to these types of blowers would be the high cost of manufacturing as well as the inability to handle flows with high particulates due to the close running clearances and complex geometries.

If you would like to learn more about the design of centrifugal fans, blowers, and compressors, please click here (http://www.softinway.com/machine-type/centrifugal-compressor/).

References:

http://www.wesellfans.com/pdf/underStanding-Centrifugal-Fans.pdf

http://www.saylor.org/site/wp-content/uploads/2011/09/Chapter-3.5-Fans-Blowers.pdf

http://www.clarage.com/docs/fan-engineering-letters/fan-performance-characteristics-of-centrifugal-fans—fe-2400.pdf?sfvrsn=2

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