Bearing selection is an interesting concept. Can you really select the bearing – or does the bearing select you? It seems that we have come to understand where to use bearings by where they have been used successfully in the past. For instance, we’ve come to know the turbocharger bearing is a floating ring bearing, foil bearings are on the aircraft air-cycle machines, and the standard bearing for midrange industrial pumps is the preloaded pair of ball bearings in the back and a deep groove bearing up front. We also know that the big industrial compressors and turbines use tilt pads, canned pumps and mag drives use carbon and ceramic sleeves, and if you look far enough back, you might find a whole family of machines using bronze sleeves with pick-up rings. Those old sleeves got us through the industrial revolution. Recently, I saw a vendor with a bearing offering that has a wet sump under the bearing and they use the thrust collar to pump oil up into the sleeves. What was old is new again, but better!
Over time, there have been various combinations of bearings with seals, and bearings with no seals. We know that oil is not welcome in silicon chip manufacturing cells, so the turbomolecular pumps are hermetic machines, operating on magnetic bearings. The gas pipeline folks also don’t want oil in the pipe, so the mag bearing and gas seal are used in some very large pipeline compressors. These bearing selections are good design fits, and have become our reference point, when we think of how a bearing is applied. Where it gets interesting is the case where there is no good reference point for the challenge at hand.
Where we embark into the great unknown is when the application is a bit of an “oddball”. What is the best bearing to use? Chances are someone, somewhere, took a chance on something – and maybe it fell flat. Sadly, most people do not publish their failures – which is a bit of a shame since then we would all know not to try that again, or if we do, to try it with some improvements. And this is the area where things get exciting. One of the most interesting areas is oil-free bearings, including foil bearings and air bearings. They provide the means to eliminate the oil, and they are a lot cheaper than a magnetic bearing. But these bearings have limits. And when we push past the limit with a bearing, the entire machine, and possibly the entire process comes to a screeching halt, literally.
From a technical viewpoint, bearings don’t have much gray area. They work, or they don’t. You can sometimes catch them as they begin to fail, and you can make a maintenance cycle to prevent the failures, but they need to work. Other components of machines can underperform, maybe get hotter than we want, or less efficient, but the machine still runs, and the process stays up and running too. But if a bearing fails, someone is making a service call. In some cases, that service call can get expensive. A great example would be a power plant turbine or generator bearing. These bearings need to be reliable.
A recent opportunity for a new bearing application has found its way into the power generation business. A power cycle using supercritical CO2 has great promise for improved thermal efficiency. However, the supercritical fluid is operating at about 300 bar and 900°C! We are not going to seal this with a laby! These conditions place real challenges on the turbomachinery designers. For the smaller sCO2 pilot plants, designers avoided the seal problems by using an integral motor with hermetic pressure containment. This type of construction is acceptable for small pilot plants, because the turbines and motors are small. A significant benefit of the sCO2 cycle is that the turbomachines are much smaller than for other types of power cycles. As the power is increased, the small size benefit leads to cost reduction. The generator, however, is no smaller than those used in conventional power utilities. As the power level of the machine increases and the generator grows in size, the pressure containment around a hermetic machine becomes very thick. In addition, the cooling requirements of the motor must be managed in the high-temperature environment. As a result, the more economical design for the larger machines is not the hermetic design. A conventional machine design using seals, and a commercially available, air-cooled, atmospheric pressure motor or generator becomes the low-cost solution, assuming you can seal the turbine.
Concepts NREC is currently working with the Supercritical CO2 power cycle. One approach we are investigating uses the magnetic bearing in an interesting and innovative way. I could tell you more, but we would have to use poison ink! But once we get through it, we hope to publish the results. Stay tuned!