Efficiency is a key parameter when discussing the performance of any industrial equipment including a centrifugal compressor. Although the goal of manufacturers is to keep efficiency values as high as possible, its value varies depending on some factors. In this article, we examine how a typical centrifugal compressor works, its efficiency formula, factors influencing its efficiency, and ways to increase it.

How a Centrifugal Compressor Works

Centrifugal compressors also go by the name dynamic compressors, radial compressors, or impeller compressors. This is because they have an impeller disk that rotates to drive the inflow and outflow of air/gas. The process begins with the rotating impeller sucking air into the compressor via its inlet, which is perpendicular to the disk’s center. 

Inflow of air is encouraged by the curved impeller vanes of the disk, with a volute casing and diffuser vane surrounding the impeller. When air enters the eye of the impeller, impeller vanes and inertia of rotation radially redirect it outwards. The rapidly flowing low pressure air gets forced to the disc’s edge to enter the volute casing through the diffuser. Due to the expanding diameter of the volute casing and the diffuser, the air slows down, thereby increasing the pressure.

In industrial applications, this usually happens in multiple stages to attain the desired pressure.

Compressor Efficiency Formula

Efficiency in centrifugal compressors highlights how effectively they convert input power into kinetic energy, and subsequently into pressure energy. It is commonly expressed as the ratio of the actual work (W_actual) done by the compressor to the theoretical maximum/isentropic work (W_isentropic), as the equation below shows.

Generally, centrifugal compressors operate within 70 to 90% efficiency, depending on factors like design, operating conditions, and maintenance.

Factors Influencing Efficiency of a Centrifugal Compressor

There are several factors that can affect the efficiency of a centrifugal compressor. The following sections highlight some of them.

Operating Conditions

The efficiency of a centrifugal compressor has a direct correlation with its operating conditions, especially at the inlet. Deviations in parameters such as inlet pressure, temperature, and inlet flow rate can impact efficiency significantly. Take the inlet flow rate for example. Typical performance curves of centrifugal compressors show that as flow rate increases, efficiency also increases. But at one point, the curve reaches the maximum, and there is a decline in performance. This maximum point is referred to as Best Efficiency Point (BEP), which often coincides with the least amount of vibrations in the system. The goal for operators is to always run the compressor at or near the BEP. In addition, filters and other inlet conditioning devices can serve in improving inlet air quality and enhance efficiency.

Design Parameters

The geometric design of compressor features such as impellers, diffusers, volute, and other components, greatly influence efficiency. Analyzing the influence of these features cannot be done separately, rather it entails how the interaction of all component geometries affect airflow through the device. Thus, making this a complex process. However, by using modern computational fluid dynamics (CFD) techniques, manufacturers can optimize geometric parameters of each component and enhance performance.

Another aspect of design parameters that affects compressor efficiency is clearances. Maintaining tight clearances between rotating and stationary components helps minimize internal leakages and improve efficiency.

Mechanical Losses

Like in most mechanical systems, frictional losses within bearings, seals, and other components reduce efficiency. The use of high quality materials, lubrication, and precision manufacturing can help mitigate these losses.

Strategies for Enhancing Centrifugal Compressor Efficiency

To maximize the efficiency of a centrifugal compressor entails improving on any of the conditions mentioned earlier, that influence efficiency values. There are several strategies to achieve this, but the following sections review a few.

The Right Installation

From the onset, getting the installation of the compressor system right is important in maximizing its efficiency. It is necessary to have a proper room ventilation plan and achieve the minimum free spacing for air circulation. Also, air treatment technology should match air quality requirements. On the outlet side, the air piping layout should be such that it minimizes pressure drops, and it must be free of leakages.

Variable Inlet Guide Vanes

In most applications, the demand for compressed air varies from time to time. Having variable inlet vanes helps to minimize compressed airflow at lower load capacity, as well as increase flow at full load. Thus, ensuring that only the required amount of power is utilized per time.

Variable Frequency Drives

Variable frequency drive alters the speed and torque of a compressor by adjusting the frequency of the electricity input. This helps to maintain high efficiency across a wide operating range as demand fluctuates. 

Condition Monitoring and Intelligent Controls

Implementing robust condition monitoring systems enables timely detection of performance deviations, which facilitates proactive maintenance and optimization. In addition, deploying intelligent controls offer quicker response to changing conditions, thereby eliminating dependence on human intervention.

Every project has specific needs, so it is necessary for operators to liaise with suppliers to obtain bespoke solutions. Petrotech’s compressor control solutions provide users with a wide range of flexible and economical compressor control and protection capabilities. By utilizing our multiple algorithms, we can easily install the most cost effective and technically appropriate solution for your compressor application and configuration.