Centrifugal Compressor Reverse Engineering and Digital Twin Development

Centrifugal Compressors are the turbomachines also known as turbo-compressors, and belong to the roto-dynamic class of compressors. In these compressors the required pressure rise takes place due to the continuous conversion of angular momentum imparted to the working fluid by a high-speed impeller into pressure. These compressors are used in small gas-turbines, turbochargers, chiller units, in the process and paper industries, oil & gas industries and others.

The design and manufacturing of such compressors are always challenging because of its 3-dimensional shapes, high rotational speeds that interact with different loss mechanisms, and stringent working environments. In many circumstances, it is necessary to analyze an existing compressor, with the end goal being to redesign it, enhance its performance, or to use it in completely different applications. In order to meet such requirements, reverse engineering is a viable option. With reverse engineering, one can review competitor’s design to remain in market competition.

Reverse Engineering

Reverse engineering allows us to collect incomplete or non-existing design data and manufacture an accurate recreation, safely, of the original product or component.

Sometimes, it is also referred to as back engineering, in which centrifugal compressors or any other product are deconstructed to extract design information from them. Oftentimes, reverse engineering involves deconstructing individual components like the impeller or diffuser of larger compressors. End-users often use this approach when purchasing a replacement impeller or any other compressor part from an OEM is not an option. In some cases, where older impellers that have not been manufactured for 20 years or more, the original 2D drawings are no longer available.  When this is the case, the only way to obtain the design of an original compressor is through reverse engineering.

Reverse engineering requires a series of steps to gather precise information on a product’s dimensions. Once collected, the data can be stored in digital archives. Figure 1 (left) shows the typical process of reverse engineering. In figure 1 (right), one can see the scanning process of the centrifugal impeller using a laser scanner.

Figure 1 (left) Reverse Engineering Process (right) Scanning of impeller
Figure 1 (left) Reverse Engineering Process (right) Scanning of Impeller. Source

To reverse engineer an impeller or any other part of compressor, an organization will typically acquire the component and take it apart to examine its internal mechanisms. This way, engineers can unveil information about the original design and construction of the product. One can start by analyzing the dimensions and attributes of the impeller and make measurements of the blade widths, diameters and angles, as these dimensions often relate to the compressor’s performance. Read More

Considerations in Industrial Pump Selection

Pumps are machines that transfer liquids from suction to discharge by converting mechanical energy from a rotating impeller into what is known as head. The pressure applied to the liquid forces the fluid to flow at the required rate and to overcome frictional losses in piping, valves, fittings, and process equipment.

When it comes to pump selection, reliability and efficiency go hand-in-hand. Generally, a pump that has been selected and controlled properly for its normal operating points will operate near its best efficiency point (BEP) flow, with low forces exerted on the mechanical components and low vibration — all of which result in optimal reliability.

There are several factors like process fluid properties, end use requirements, environmental conditions, pump material, inlet conditions, and others which should be considered while selecting pumps for industrial applications. Selecting the right pump type and sizing it correctly are critical to the success of any pump application. Pumping applications include constant or variable flow rate requirements, serving single or networked loads, and consisting of open loops (nonreturn or liquid delivery) or closed loops (return systems).

Some crucial factors considered while pump selections include:

Fluid Properties: The pumping fluid properties can significantly affect the choice of pump. Key considerations include:

  • Acidity/alkalinity and chemical composition. Corrosive and acidic fluids can degrade pumps and should be considered when selecting pump materials.
  • Operating temperature: Pump materials and expansion, mechanical seal components, and packing materials need to be considered with pumped fluids that are hotter than 200°F.
  • Solids concentrations/particle sizes: When pumping abrasive liquids such as industrial slurries, selecting a pump that will not clog or fail prematurely depends on particle size, hardness, and the volumetric percentage of solids.
  • Specific gravity: It affects the energy required to lift and move the fluid and must be considered when determining pump power requirements.
  • Vapor pressure and Viscosity: Proper consideration of the fluid’s vapor pressure will help to minimize the risk of cavitation. High viscosity fluids result in reduced centrifugal pump performance and increased power requirements. It is particularly important to consider pump suction-side line losses when pumping viscous fluids.

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Materials of Construction: It is always required to check the compatibility of materials of construction with the process liquid or any other liquids the pump might encounter. The initial cost of these materials is normally the first consideration. The operational costs, replacement costs and longevity of service and repair costs will, however, determine the actual cost of the pump during its lifetime. Charts are available to check the chemical compatibility and identify the most appropriate materials of construction for the pump.

The impact of the impeller material on the life of a pump under cavitation conditions is shown in Figure 1. As an example, changing from mild steel (reliability factor of 1.0) to stainless steel (reliability factor of 4.0) would increase the impeller life from cavitation damage by a factor of four. Hard coatings, such as certain ceramics, can also increase the impeller life under cavitating conditions.

Material Cavitation Life Factors
Figure 1 Material Cavitation life factors

Pump Sizing and Performance Specifications: The desired pump discharge is needed to accurately size the piping system, determine friction head losses, construct a system curve, and select a pump and drive motor. Process requirements can be achieved by providing a constant flow rate, or by using a throttling valve or variable speed drives. Read More

Initial Sizing of Centrifugal Fans

Centrifugal fans are a type of turbomachine equipment widely used in all kinds of modern and domestic life. Centrifugal fans were developed as highly efficient machines, and the design is still based on various empirical and semi empirical rules proposed by fan designers. Due to these various rules, there are different methodologies used to design impellers and other components.

Centrifugal fans consist of an impeller in a casing with a spirally shaped contour, shown in Figure 1 (left side). The air enters the impeller in an axial direction and is discharged at the impeller outer periphery. The air flow moves along the centrifugal direction (or radial direction). Centrifugal fans can generate relatively high pressures, as compared with axial flow fans. For axial flow fans, the pressure rise is small, about be few inches of water.

Radial Fan and Static Pressure
Figure 1 Radial Fan and Static Pressure, Shaft Power V/s Volume Flow Curves for Different
Types of Blades

Generally centrifugal fans have three types of blade: forward blade, backward blade and radial blade. The characteristic curve of these three kinds of centrifugal fans is shown on right side in Figure 1.

Sizing Using Cordier Diagram

Centrifugal fans (most turbomachines) can be classified based on specific speed (Ns) and specific diameters (Ds) as shown in Figure 2. Specific speed is a criterion at which a fan of unspecified diameter would run to give unit volume flow and pressure. The correlation for specific speed and specific diameter can be seen here:

Pump Formula

where, ‘N’ is rotational speed (RPM), ‘Q’ is flow rate (ft3/sec), ‘H’ is head (ft), ‘D’ is diameter (ft) Read More

Centrifugal Compressors for Fuel Cells

The development of fuel cell technologies and improvements in fuel cells power densities combine to make the use of fuel cells possible in different power sectors as primary or secondary power sources for commercial purpose, residential power requirements, and automobiles, etc. The fuel cell harnesses the chemical energy of a fuel along with an oxidizing agent by converting it into electrical energy through a pair of reactions. For example, in a hydrogen fuel cell, as shown in Figure 1, the hydrogen combines with oxygen from the air to produce electricity and releases water.

Fuel Cell System
Figure 1 Fuel Cell System [1]
The design of a fuel cell system is quite complex and depends on fuel cell types and their applications. With so many possible combinations of fuel cells, this article will not focus on different type of fuel cells, but on Air Management Systems which may significantly affect the overall performance of a fuel cell system.

Air Management Systems

Key sub-systems of any fuel cell system are the fuel processor, fuel cell stack, air management and power management systems. The air management system strongly affects the fuel cell stack efficiency and the power loss of the fuel cells. Therefore, it is necessary to develop a clean, reliable, cost-effective oil-free air system [2].

Major tasks in air management system are Air Supply, Air Cleaning, Pressurization and Humidification.
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Preventing Choke and Surge in a Compressor

Turbo Compressors are used to increase the pressure of a gas, which are required in propulsion systems like a gas turbine, as well as many production processes in the energy sectors, and various other important industries such as the oil and gas, chemical industries, and many more.

Such compressors are highly specific to the working fluid used (gas) and the specific operating conditions of the processes for which they are designed. This makes them very expensive. Thus, such turbo compressors should be designed and operate with high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.

Axial Compressor and Centrifugal Compressor in AxSTREAM
Figure 1 (A) Axial Compressor (B) Centrifugal Compressor in AxSTREAM®

Turbo Compressor Characteristic Curves

The characteristic curves of any turbo compressor define the operating zone for the compressor at different speed lines and is limited by the two phenomenon called choke and surge. These two opposing constraints can be seen in Figure 2.

Choke conditions occurs when a compressor operates at the maximum mass flow rate. Maximum flow happens as the Mach number reaches to unity at some part of the compressor, i.e. as it reaches sonic velocity, the flow is said to be choked. In other words, the maximum volume flow rate in compressor passage is limited by limited size of the throat region.  Generally, this calculation is important for applications where high molecular weight fluids are involved in the compression process.

Surge is the characteristic behavior of a turbo compressor at low flow rate conditions where a complete breakdown of steady flow occurs. Due to a surge, the outlet pressure of the compressor is reduced drastically, and results in flow reversal from discharge to suction. It is an undesirable phenomenon that can create high vibrations, damage the rotor bearings, rotor seals, compressor driver and affect the entire cycle operation.

Compressor Performance Curve
Figure 2 Compressor Performance Curve

Preventing Choke and Surge Conditions

Both choke conditions and surge conditions are undesirable for optimal operation of a turbo compressor.  Each condition must be considered during design to ensure these conditions are prevented. Read More

When to Upgrade Your Pump

The typical life cycle cost of an industrial pump depends on its maintenance and energy consumption. Hence, it is necessary to keep track of the pump performance and do periodic maintenance to achieve performance level close to the performance predicted by the manufacturer. There are many instances in which maintenance becomes very costly to achieve the required performance. This is the point when owners must decide about whether to upgrading the system. Figure 1 shows the life cycle cost of typical industrial pumps.

Figure 1 Life Cycle Cost of Typical Industrial Pump
Figure 1 Life Cycle Cost of Typical Industrial Pump

In recent years, there have been many innovations in implementing newer materials as well as improvements in hydraulics. Improving pump designs is an ongoing process with designers looking for increasing performance by a few percentage points. The goal of the present pump manufacturers is to offer higher efficiency and reliability, but replacing an older pumps with newer pumps can mean higher costs. The focus for replacing the internals of the pumps with improved design has gained prominence since many of the components, like the casing and rotor, of the existing pumps can be reused. So instead of replacing the entire pump, it can be upgraded or retrofitted. When it comes to an upgrade, the first thing that should be considered is the return on investment which includes the initial investment, operating costs, and the reduction in energy consumption due to the improved pump performance.

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An Overview of Axial Fans

Axial fans have become indispensable in everyday applications starting from ceiling fans to industrial applications and aerospace fans.  The fan has become a part of every application where ventilation and cooling is required, like in a condenser, radiator, electronics etc., and they are available in the wide range of sizes from few millimeters to several meters. Fans generate pressure to move air/gases against the resistance caused by ducts, dampers, or other components in a fan system. Axial-flow fans are better suited for low-resistance, high-flow applications and can have widely varied operating characteristics depending on blade width and shape, number of blades, and tip speed.

Fan Types

The major types of axial flow fans are: propeller, tube axial, and vane axial.

  • – Propellers usually run at low speeds and handle large volumes of gas at low pressure. Often used as exhaust fans these have an efficiency of around 50% or less.
  • – Tube-axial fans turn faster than propeller fans, enabling operation under high-pressures 2500 – 4000 Pa with an efficiency of up to 65%.
  • – Vane-axial fans have guide vanes that improve the efficiency and operate at pressures up to 5000 Pa. Efficiency is up to 85%.
Types of Fans
Figure 1 Different Types of Axial Fans
Aerodynamic Design of an Axial Fan

The aerodynamic design of an axial fan depends on its applications. For example, axial fans for industrial cooling applications operate at low speeds and require simple profile shapes. When it comes to aircraft applications however, the fan must operate at very high speeds, and the aerodynamic design requirements become significantly different from more traditional fan designs. Read More

Computational Fluid Dynamics for Centrifugal Pumps

Pumps are important for many common systems which deal with water, such as heating circulating flows, consumer or industrial water supply, fountains, and fire protection systems.

Pumps are classified into two major categories: Rotodynamic pumps and positive displacement pumps (piston pumps). Rotodynamic pumps can be further classified as axial pumps, centrifugal (radial) pumps, or mixed pumps.

Centrifugal pumps are the devices which impart energy to the fluid (liquid) by means of rotating impeller vanes, and the fluid exits radially from the pump impeller. Such pumps are simple, efficient, reliable, relatively inexpensive, and easily meet the pumping system requirements for filtration. This is a great pump choice for moving liquids from one place to another using pressure.

Types of Rotodynamic Pumps
Figure 1. Types of Rotodynamic Pumps

Centrifugal Pump Design

A centrifugal pump is a very common component in turbomachines, but as with any component, it still needs continual improvement in the design methodology, from conceptual level to the final product development including testing at different levels. The challenge is to design a pump with improved efficiency while minimizing the possibility of cavitation.

Need of Numerical Simulation

Years ago, engineers performed prototype testing at each level of design to check the performance (which was very costly and time consuming). Now with advancements in the computation technology and resources, it is comparatively easier to design high efficiency pumps within a short duration of time. These simulations can be done with a computer, so, the number of physical prototypes required is greatly reduced. The main advantage of numerical simulation is that it allows engineers to virtually test the CAD model early in the design process, and provides flexibility for engineers to iterate the design until getting the required performance.

Computational Fluid Dynamics for Centrifugal Pumps

Computational fluid dynamics (CFD) replaces the huge number of testing requirement. This not only shortens the design cycle time but also significantly reduces development cost.

In a CFD model, the region of interest, a pump impeller flow-path for example, is subdivided into a large number of cells which form the grid or mesh. The PDEs (partial differential equations) can be rewritten as algebraic equations that relate the velocity, pressure, temperature, etc. in a cell to those in all of the cell’s immediate neighbors. The resulting set of equations can then be solved iteratively, yielding a complete description of the flow throughout the domain.

To accomplish CFD simulations, there are several software programs available, but user must select a very well validated software that can provide and easy user interface, automatic mesh generation and flexibility to modify the geometry to perform optimization without needing to move to some other software platform.

In the current trend, automatic mesh generation tools like AxCFD™ are employed in the AxSTREAM® software platforms which reduces the turbomachines meshing complications and generate good quality mesh in considerably short timeframe which can capture the accurate flow features needed. Figure 2 shows the discretized impeller and pressure contour after CFD analysis.

Discretized Impeller and Pressure Contour After CFD Analysis
Figure 2. Discretized Impeller and Pressure Contour After CFD Analysis

AxCFD™, in AxSTREAM® platform, provides user an opportunity to perform CFD analysis by applying standard methods of full three-dimensional CFD, axisymmetric CFD (meridional), and blade-to-blade analysis. User can even perform optimization of the blade profiles and other geometrical parameters within the AxSTREAM® platform and perform CFD simulation without altering any CFD settings.

Computational Fluid Dynamics for Centrifugal Compressors

In today’s world where “time is money,” each and every industry involving turbomachinery wants to deliver their high performance products in the quickest time possible. Computational fluid dynamics (CFD) replaces the huge number of testing requirements thus not only shortening the design cycle time, but also reducing development costs.

Today with advancements in computational resources, numerical methods, and the availability of commercial tools, CFD has become a major tool for the design phase of a project. With a large number of validations and bench markings available on the applicability of CFD for centrifugal compressors, it has become an indispensable tool for the aerodynamic designer to verify the design and understand the flow physics inside a compressor’s flow path. However, CFD is still computationally expensive and requires a high level of user-knowledge and experience to get meaningful results. CFD analysis can be performed with and without considering viscous effects of the flow. The inclusion of viscosity into the flow introduces additional complexities for choosing the most appropriate turbulence closure model. CFD however, has some limitations due to:

  • – Errors created during modeling where the true physics are not well-known and are very complex to model.
  • – Multiple approximation and model errors created during the calculation process (such as mesh resolution, steady flow assumption, turbulence closure, geometric approximation, unknown boundary profile etc.). These approximations impact the calculations of local values of vital parameters.

In CFD for example, if the 1D design is not accurate, (stage loading and blade diffusion factors etc.), then CFD cannot turn out a good design. It is critical to use a design tool such as AxSTREAM®  which can generate optimized designs with less time and effort starting from the specification.

The preliminary design modules of AxSTREAM® uses inverse design tasks to generate the initial flow path for the centrifugal compressor. By choosing the right combination of geometrical and design parameters from the start, AxSTREAM® reduces the number of design cycle iterations required in generating an accurate design.

This initial design obtained is further analyzed and optimized using throughflow solvers in AxSTREAM® which considers various operating conditions. The throughflow solvers in AxSTREAM®  predict the performance parameters at different sections and stations, and presents the blade loading, flow distribution along the flow path, etc.

The generation of 3D geometry for the impeller and diffuser is another complex activity which is greatly simplified by using the radial profiler and 3D blade design module in AxSTREAM®. The geometry generated in AxSTREAM® is fully parameterized with complete control for the user to modify as and when required. Figure 1 shows a parameterized impeller geometry generated using seven spanwise sections with contours of the curvature.

Parameterized Impeller Geometry
Figure 1. Parameterized Impeller Geometry

In CFD analysis of turbomachines, grid generation becomes a very challenging task due to the geometries of complicated, twisted blades. To achieve reliable CFD results, the grid must resolve the topology accurately to preserve this geometric information. The quality of the grid should be in an acceptable range especially the angle, aspect ratio, and skewness of the grid elements. Automatic mesh generation tools are employed to reduce the turbomachines meshing complications. The AxSTREAM® platform uses AxCFD™ to generate a high quality mesh in considerably short time which captures the accurate flow features.

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An Introduction to Centrifugal Pumps

In every modern cleaning system there exists at least one pumping unit. With this in mind, understanding how it works and how to use it efficiently is critical to the successful operation and maintenance of that cleaning system. This blog will discuss centrifugal pumps in this context and take a look at important attributes to bear in mind when working with these systems.

In general, pumps are devices which impart energy to a flow of liquid.  Although there are different types of pumps based on the flow direction, blade designs, and so on, centrifugal pumps are in the majority of those used in cleaning systems.  Centrifugal pumps are simple, efficient, reliable, relatively inexpensive, and easily meet the needs of most cleaning system requirements including spraying, overflow sparging, filtration, turbulation and the basic function of moving liquids from one place to another using pressure.

A centrifugal pump uses a combination of angular velocity and centrifugal force to pump liquids.  The below figure illustrates the working principle of the centrifugal pump.

Centrifugal Pump

The pump consists of a circular pump housing which is usually made up of metals, (stain steels etc.) solid plastic, or ceramics.  The outlet extends tangentially from the diameter of the pump housing.  Inside the pump housing there is a rotating component an “impeller” which rotates perpendicular to the central axis and is driven by a shaft secured to its center of rotation.  The shaft, powered by an electric motor, enters the pump housing through a liquid tight seal which prevents leaking.  Liquid entering the pump through the inlet is swirled in a circular motion and displaced from the rotation center of the impeller by centrifugal force.  The combination of the swirling action (angular velocity) and centrifugal force (radial velocity) pushes the liquid out of the pump through the outlet.

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