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In past issues, we traced the key roles played by all essential elements of the modern industrial compressor. Now we take a look at just howcompressors can be designed and built in this decade leading to 2020. Computer Numerical Control (CNC) machining has changed the world of possibilities for such machines, as suggested in Figures 1 and 2.
Figure 1 shows an open impeller produced by flank milling. Such impellers cost about the same price today as they did 20 years ago, even though the value of currency has dropped sharply. And the time required to cut them has decreased just as much. At the same time, the availability of very fast CNC machines has increased greatly.
Not surprisingly, many turbomachinery components aremachined today versus cast, as in earlier generations. For critical impellers, even fully covered designsmay be machined from a solid block, as shown in the Figure 2. This process, which is becoming common, gives the owner the greatest strength for the specific design and the highest confidence in mechanical integrity. Whether the needs are for a single-stage or a multi-stage machine, as shown in Figure 3, CNC machining can be cost effective.
Figures 4 and 5 are industrial applications. Figure 4 is a turbocharger-like aircycle chiller compressor/expander. Figure 5 is a gear-driven compressor/turbine. The larger cast volutes are, by far, the longest lead time elements. Today, machined components, while not inexpensive, can bemade competitively and offer the advantage of comparatively short lead times.
In the 1970s, a machined impeller could take as long as nine months to procure due to the limited number of machining centers. Designing a complete machine and procuring all necessary parts could easily take two or three years.
Today, time spans are much shorter. The design cycle, depending on the starting point and the detailed information available, can be as little as fourmonths, ormay spread out to 10 to 20 months if complex design constraint issues need to be resolved. The last two examples each had moderate design cycles (typically about four to eightmonths), and critical parts could all be made on tight schedules. The scheduled lead items are the supply of gearboxes, motors and generators, and cast housing components. Such items can easily take up to a year to procure and should be identified and ordered as early in a project design as practical.
Industrial turbomachinery today differs only slightly from aerospace machinery, mostly in its outward appearance. As it does not have to fly, the housings are usually heavy, rugged, and somewhat simpler in form. In addition, they are usually cast. On the inside, however, many precision CNC machined parts are utilized, and extremely tight tolerance clearances are often held. Greater schedule flexibility, some cost savings, and greater component life have often been achieved by using modern machining methods on even the most traditional product applications.
David Japikse is Chairman of the Board, founder and CEO of Concepts NREC. Japikse has written or co-authored six books: Introduction to Turbomachinery, Centrifugal Compressor Design and Performance, Centrifugal Pump Design and Performance, Axial and Radial Turbines, Advanced Experimental Techniques in Turbomachinery, and Diffuser Performance.