Modern axial compressors: Complex, efficient and sensitive

Axial compressors are modern and compact turbo-compressors for applications with very large flows. They are used in air applications (in air compression units or as an air compressor in a gas turbine), natural gas services and mixed refrigerant applications (in some large LNG plants). However, they are delicate and fragile machines.

From the aerodynamic, surge and operational flexibility points of view, the axial compressor is probably the most crucial piece of turbomachinery.

Axial compressor design still remains a very complex task, where aeromechanical, surge, stall, structural, material, manufacturing, noise-related concerns and many other technical aspects should be taken into account simultaneously.

In some applications, an axial compressor is requested that can operate at a defined operating condition (a rated condition) with very limited changes (quasi-constant mass flow-rate, rotational speed and pressure ratio). In modern plants, axial compressors are required to fulfill many different operating conditions (a relatively vast operating capacity and pressure range). Considering the steep nature of an axial compressor curve, this is a great challenge.

Additional flexibility

Sometimes, a plant requires an axial compressor to operate far away from its nominal conditions. In design, it usually results in contradicting technical objectives. Inlet guide vanes (IGV) and variable stator vanes (VSV) are used in modern axial compressors to provide additional flexibility.

On the other hand, more sophisticated design procedures attempt to identify the nominal condition by weighing all compressor operating points along the desirable operating map. Considering the modern required operational flexibility (speed variation, variable IGV, etc.), many issues should be evaluated including structural, vibrational, weight, cost, manufacturability, accessibility and reliability. As a result, the axial compressor design and selection can be considered a multi-disciplinary approach which needs co-operation between specialists in different fields.

New designs have been developed for axial compressors (such as modern optimized 3D designs, new blade shapes, new profiles, etc.) for improved efficiencies at normal and alternative operating points. They provide more flexibility, end-wall contouring and casing treatments for an enhanced stall margin.

Careful optimization

Generally, a careful optimization should be done for a modern axial compressor. For instance, maximizing the adiabatic efficiency demands a deep understanding of the physics governing stage losses, which have to be minimized both in rated and alternative conditions. This, in turn, will have an important impact on the choice of stage geometrical and functional variables.

On the other hand, optimizing surge (or stall) margins involves acquiring a proper insight of surge/stall physics and various instability issues. Minimizing axial compressor weight (and achieving a compact design) implies reducing the number of stages and increasing individual stage loading, a fact which ultimately affects the choice of the blade shape, particularly cascade parameters.

The operating Mach number is usually less than 0.8 for a subsonic cascade, but can go up to 2 and more at the tip of a transonic blade. Some modern subsonic axial stages can develop pressure ratios in the order of 1.5-1.8.The transonic stages operate with 2 and more while maintaining acceptable efficiency and aerodynamic design. In a well-designed subsonic stage, polytropic efficiency could reach around “0.9”.

The polytropic efficiency for transonic bladings is a bit lower (around “0.84” – “0.89”). High peripheral mean stage rotor velocities, around 300-340 m/s for subsonic rotors and up to about 580m/s for transonic ones, could be achieved. An annulus radius ratio (R

hub

/R

tip

), usually chosen between around 0.45 (front stages) and approximately 0.9 (rear stages). The hub-to-tip ratios come from a careful optimization considering the aerodynamic, technical, mechanical and economic constraints. For inlet stages, annulus radius ratio values between 0.45 and 0.65 can be assigned, while outlet stages often are given a higher value (often from 0.8 to 0.9) in order to achieve a relatively high Mach number.

The Mach number distribution

The diffusion factor should be constrained to avoid designs involving a flow separation. The axial Mach number distribution along the different axial stages should be calculated. The Mach number distribution needs to follow an acceptable pattern and variations should not exceed a certain level. The ultimate goal of an axial compressor cascade design is to create a blade arrangement with the maximum pressure rise and the minimum total pressure loss (a relatively high efficiency) along with an acceptable operating range.

The shapes of the different blade and component profile play an important role because profiles can affect the nature of the boundary layers and therefore the amount of profile losses (and operating margins). On the other hand, a high solidity cascade is a popular option in order to decrease the aerodynamic loading, when reaching the maximum pressure rise. Also the design should minimize the friction losses for a required (prescribed) pressure rise. All these choices involve a complex optimization (and selection) process that can make the axial compressor design a great challenge. The stage arrangement is critical and the stage stacking procedure is intrinsically iterative.

The availability of advanced materials for axial compressor blade and component construction (and high-quality production methods) makes it possible to reach levels of aerodynamic loading never experienced in traditional axial compressors, while preserving high levels of efficiency for normal and alternative-operation cases. This is true both for high-speed-subsonic and ultra-high-speed-transonic blades.

However, it is difficult to give a general rule for the selection between a very large centrifugal compressor (with a rugged and massive design) and a compact, relatively-more-efficient, properly-optimized more-economic axial compressor (but a delicate, special and sensitive machine with a relatively-limited operation range, which could be vulnerable against surge events). The reality is that for very large capacities, the axial compressor could be the only option available.

 (The author is a Chartered Professional Engineer in Australia, Queensland and U.K. An M.Sc. and B.Sc. graduate in mechanical engineering, he is a senior consultant specializing in rotating equipment, condition monitoring and reliability).