Compressors are instrumental pieces of equipment used in several industries to increase the pressure of a fluid. This fluid is most commonly a gas, such as oxygen, nitrogen, and methane. These versatile devices come in a variety of designs and can be scaled to meet the needs of numerous applications, both large and small. At the simplest level, compressors are a type of turbomachinery, and much of their functioning will be familiar to those involved with turbines. Further, a solid understanding of the operating principles of a compressor will aid in the comprehension of similar turbomachinery. In this blog, we will focus on the nuances associated with centrifugal compressors, including their design, operating principle, and applications.
The best way to understand the operating principles of a centrifugal compressor is by examining its several components. Much like a turbine engine, centrifugal compressors contain four distinct elements which gas must pass through. Gas enters through the inlet, which is a straight pipe for the most straightforward designs. Other inlets may include ports, ducts, or vanes to help guide and change the flow of gas as it enters the system. Meanwhile, different designs implement instrumentation to help measure certain characteristics of the gas before it enters the compressor, including pressure and temperature.
After entering the compressor, gas travels directly to the impeller, which is the most crucial part of the entire assembly. It is also this element that varies the most between compressor designs. Open impellers contain no protective shrouds surrounding the blades. Compared to other configurations, they are cheaper and less structurally sound. As such, their use is limited to applications with low strain and should still be regularly inspected for damage. Semi-open impellers have a single wall on the back end, which helps support structural stability and efficiency. Meanwhile, closed impellers contain blades that are fully enclosed by two support surfaces. Although they are more expensive, closed impellers provide the highest reliability and efficiency. However, they should not be used in applications with heavy suspended solids, as they are more prone to clogging.
Downstream of the impeller is the diffuser, which is a component that converts the high-velocity kinetic energy of the gas into potential energy by slowing it down. Although there are also design variations with diffusers, they all slow the gas using a similar mechanism. The continuity equation of flow states that Area1*Velocity1=Area2*Velocity2. As such, decreasing the area in a given space will increase velocity and vice versa. Diffusers take advantage of this principle by increasing the area in which the gas is flowing, thereby decreasing the velocity. The collector is the last component that gas passes through in the compressor, and its purpose is to deliver the media to the target component. It is common for the collector to contain valves that control the rest of the compressor, with some acting as feedback devices to help ensure efficiency.
Well-built centrifugal compressors pose several benefits compared to other compressor types. Namely, they are low maintenance, oil-free, and have a higher pressure ratio compared to axial compressors. Despite these advantages, centrifugal compressors are also cheaper on average compared to counterparts due to the limited number of stages. With unmatched scalability, these compressors find use in the natural gas industry, oil refineries, HVAC systems, and gas turbine engines.
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