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.

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%.

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Using 1D Models to Predict the Thermal Growth and Stresses During The Start up and Shutdown Phase of a Steam Turbine

Steam turbines are not just restricted to conventional or nuclear power plants, they are widely used in combined cycle power plants, concentrated solar thermal plants and also geothermal power plants. The operational requirements of a steam turbine in the combined cycle and CSP’s means that they operate under transient conditions. Even in conventional steam turbines, the market requirements are changing with requirements for faster and more frequent start-up which can result into faster deterioration of the equipment and reduced lifespan. During the startup phase, significant heat exchange takes place between the steam and the structural components that include the valves, rotor and casing. The accuracy of the life prediction is strongly affected and dependent on the accuracy of the transient thermal state prediction [1].

Though the expansion of steam takes place in the nozzles and blades, the influence of the leakage steam during the startup phase is significant with steam expanding through the labyrinths resulting in expansions, condensation, and increased velocities which may even reach supersonic levels. During cold start, the flow is minimal, the temperature of the metal is at room temperature and heat exchange happens between the steam and metal parts resulting in thermal stress.

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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.

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.

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Gaining Turbomachinery Insight Using a Fluid Structure Interaction Approach

Existing research studies for the corresponding flow-induced vibration analysis of centrifugal pumps are mainly carried out without considering the interaction between fluid and structure. The ignorance of fluid structure interaction (FSI) means that the energy transfer between fluid and structure is neglected. To some extent, the accuracy and reliability of unsteady flow and rotor deflection analysis should be affected by this interaction mechanism.

In recent years, more and more applications of FSI are found in the reliability research of turbomachinery. Most of them are about turbines, and a few of them address pumps. Kato [1] predicted the noise from a multi-stage centrifugal pump using one-way coupling method. This practical approach treats the fluid physics and the solid physics consecutively.

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A Reasonable Approach to Pump Design While Avoiding Resonance

For the majority of pump application, the growing use of variable speed operation has increased the likelihood of resonance conditions that can cause excessive vibration levels, which can negatively impact pump performance and reliability. Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations (external excitation source) matches the system’s natural frequency of vibration more than it does at other frequencies. To avoid vibration issues, potential complications must be properly addressed and mitigated during the design phase.

Some of the factors that may cause excitation of a natural frequency include rotational balance, impeller exit pressure pulsations, and gear couplings misalignment. The effect of the resonance can be determined by evaluating the pumping machinery construction. All aspects of the installation such as the discharge head, mounting structure, piping and drive system will affect lateral, torsional and structural frequencies of the pumping system. It is advised that the analysis be conducted during the initial design phase to reduce the probability of reliability problems and the time and expense associated.

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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.

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.

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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.

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Optimizing the Cooling Holes in Gas Turbine Blades

To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.

Figure 1: Variations of Thermal Efficiency with TIT [1]
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of  the gas turbine.

With this in mind, during  the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.

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Exotic Turbomachinery – Viscous Disc Pumps

Turbomachinery can be divided into two main groups. Group one consists of machines that perform work on the fluid, requiring energy and increasing its pressure, such as compressors, pumps, and fans. Group two consists of those that extracts energy from the fluid flowing through it – for example, wind, hydro, steam, and gas turbines.

Pumps specifically are devices whose purpose is to move fluid at a constant density, increasing its kinetic energy and its pressure while consuming energy in the process. We are quite used to seeing centrifugal and axial pumps, as they are the most common configurations.  However, more exotic designs have been tested and developed throughout the history of fluid machinery.

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Working with Geothermal Heat Pumps

A geothermal heat pump utilizes earth’s thermal energy as a way to manipulate temperature. This is seemingly attractive toward HVAC utilization due to the relatively high efficiency as well as economic benefit. Temperature fluctuations below ground are relatively low as earth absorbs solar energy all year round and insulates the heat underground. Taking advantage of this event, geothermal energy heat pump application for residential and commercial building uses the “underground” as a heat source/sink.

geothermal heat pumps
Source: http://tidewatermechanical.com/geothermal-heat-pumps/

How does geothermal heat pump work?

A heat pump system mainly consists of a heat-pump unit, a pipeline loop functioning as a heat exchanger for a desired area (it can be horizontal, vertical or installed to an aquatic medium), and a duct – to deliver the controlled temperature flow to the consumer.

Fluid is pumped through an installed pipeline loop which transfers heat based on the season. During the hotter season (summer), heat will be absorbed from the air in the building, transferred into the ground and then cooler air will be circulated to the designated area. The contrary happens during the winter. In colder months, heat will be transferred into the fluid from the ground and collected heat will be distributed.

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